U.S. patent application number 11/620047 was filed with the patent office on 2008-05-08 for methods and apparatus for stress relief using multiple energy sources.
Invention is credited to Donna Murray Walker.
Application Number | 20080105339 11/620047 |
Document ID | / |
Family ID | 31888310 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080105339 |
Kind Code |
A1 |
Walker; Donna Murray |
May 8, 2008 |
Methods and apparatus for stress relief using multiple energy
sources
Abstract
Methods are presented for modifying a physical property of a
structure, such as reducing or relieving remaining internal stress,
in which two or more energy types are concurrently applied to the
structure to change the physical property of interest in an
accelerated fashion. A first energy type, such as heat, is applied
according to time values and operational settings derived from a
first order rate relationship for the first energy type and from a
first order rate relationship for a second energy type. The second
energy type, such as vibration or other time-varying energy form,
is applied concurrently for the time value. Methods are also
provided for determining operational settings for concurrent
application of multiple energy types to a structure.
Inventors: |
Walker; Donna Murray; (Novi,
MI) |
Correspondence
Address: |
Donna M. Walker
40388 Ladene Lane
Novi
MI
48375
US
|
Family ID: |
31888310 |
Appl. No.: |
11/620047 |
Filed: |
January 5, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10632231 |
Jul 31, 2003 |
7175722 |
|
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11620047 |
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60404020 |
Aug 16, 2002 |
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Current U.S.
Class: |
148/558 |
Current CPC
Class: |
C21D 1/34 20130101; C21D
10/00 20130101; C21D 1/30 20130101; C21D 11/00 20130101; C22F 1/00
20130101; C22F 3/00 20130101; C21D 1/78 20130101; Y02P 10/25
20151101; C21D 1/42 20130101; C21D 7/00 20130101; C21D 10/005
20130101; C21D 1/04 20130101; Y02P 10/253 20151101 |
Class at
Publication: |
148/558 |
International
Class: |
C21D 10/00 20060101
C21D010/00 |
Claims
1. A method of changing a physical property of a structure,
comprising: providing energy of a first energy type to a structure
by performing a first energy process according to an operational
setting, at least one of the operational setting and a time value
being selected according to a first order rate relationship for the
first energy process, according to a first order rate relationship
for a second energy process, and according to a desired physical
property value; and providing energy of a second energy type to the
structure at an energy level above an activation energy for the
structure by performing the second energy process; wherein the
first and second energy processes are performed concurrently for at
least the time value; wherein the first order rate relationship for
the first energy process relates application of the first energy
type to the structure and a physical property of the structure; and
wherein the first order rate relationship for the second energy
process relates application of the second energy type to the
structure and the physical property.
2. The method of claim 1, wherein the first energy type is thermal
and wherein the second energy type is oscillatory.
3. The method of claim 2, wherein the operational setting is a
temperature setting, wherein one of the temperature setting and the
time value is selected according to the first order rate
relationship for the first energy process, according to the first
order rate relationship for the second energy process, according to
the desired physical property value, and according to the other one
of the temperature setting and the time value.
4. The method of claim 3, wherein the first order rate relationship
for the first energy process is a first Larson-Miller relationship
that relates application of thermal energy to the structure and the
physical property, and wherein the first order rate relationship
for the second energy process is a second Larson-Miller
relationship that relates application of oscillatory energy to the
structure and the physical property.
5. The method of claim 4, further comprising: determining a first
Larson-Miller parameter according the first Larson-Miller
relationship, the first Larson-Miller parameter corresponding to
the desired physical property value; determining a second
Larson-Miller parameter according to the second Larson-Miller
relationship, the second Larson-Miller parameter corresponding to
the desired physical property value; selecting a first one of the
temperature setting and the time value; selecting a second one of
the temperature setting and the time value according to the first
and second Larson-Miller parameters, according to the first
Larson-Miller relationship, and according to the first one of the
temperature setting and the time value.
6. The method of claim 5, further comprising determining a third
Larson-Miller parameter according to the first and second
Larson-Miller parameters, wherein the second one of the temperature
setting and the time value is selected according to the third
Larson-Miller parameter, according to the first Larson-Miller
relationship, and according to the first one of the temperature
setting and the time value.
7. The method of claim 6, wherein determining the third
Larson-Miller parameter comprises subtracting the second
Larson-Miller parameter from the first Larson-Miller parameter.
8. The method of claim 7, wherein selecting the second one of the
temperature setting and the time value comprises evaluating the
first Larson-Miller relationship using the third Larson-Miller
parameter and the first one of the temperature setting and the time
value to obtain the second one of the temperature setting and the
time value.
9. The method of claim 4, wherein the physical property is internal
stress, and wherein the desired physical property value is one of a
remaining internal stress value and an internal stress reduction
value.
10. The method of claim 1, wherein the physical property is
internal stress, and wherein the desired physical property value is
one of a remaining internal stress value and an internal stress
reduction value.
11. The method of claim 1, wherein the first order rate
relationship for the first energy process is a first Larson-Miller
relationship that relates application of the first energy type to
the structure and the physical property, and wherein the first
order rate relationship for the second energy process is a second
Larson-Miller relationship that relates application of the second
energy type to the structure and the physical property.
12. The method of claim 11, further comprising: determining a first
Larson-Miller parameter according the first Larson-Miller
relationship, the first Larson-Miller parameter corresponding to
the desired physical property value; determining a second
Larson-Miller parameter according to the second Larson-Miller
relationship, the second Larson-Miller parameter corresponding to
the desired physical property value; selecting a first one of the
operational setting and the time value; selecting a second one of
the operational setting and the time value according to the first
and second Larson-Miller parameters, according to the first
Larson-Miller relationship, and according to the first one of the
operational setting and the time value.
13. The method of claim 12, further comprising determining a third
Larson-Miller parameter by subtracting the second Larson-Miller
parameter from the first Larson-Miller parameter, wherein the
second one of the operational setting and the time value is
selected according to the third Larson-Miller parameter, according
to the first Larson-Miller relationship, and according to the first
one of the operational setting and the time value.
14. The method of claim 13, wherein selecting the second one of the
operational setting and the time value comprises evaluating the
first Larson-Miller relationship using the third Larson-Miller
parameter and the first one of the operational setting and the time
value to obtain the second one of the operational setting and the
time value.
15. The method of claim 1, wherein the second energy type is
oscillatory, wherein the second energy type is provided to the
structure at a frequency selected according to a resonant frequency
of a system in which the structure is mounted while performing the
first and second energy processes.
16. The method of claim 15, wherein the second energy type is
provided to the structure at a frequency at or near the resonant
frequency of the system.
17. The method of claim 1, wherein the second energy type is
selected from the group consisting of sonic, laser, electrical,
magnetic, mechanical vibration, and microwave.
18. A method of changing a physical property of a structure,
comprising: providing energy of a first energy type to a structure
by performing a first energy process according to an operational
setting; and providing energy of a second energy type to the
structure at an energy level above an activation energy for the
structure by performing a second energy process; wherein the first
and second energy processes are performed concurrently for at least
a time value; and wherein one of the operational setting and the
time value are selected according to a desired physical property
value and according to a first order rate relationship that relates
concurrent application of the first and second energy types to the
structure and a physical property of the structure.
19. The method of claim 18, further comprising determining the
Larson-Miller relationship that relates concurrent application of
the first and second energy types to the structure and the physical
property of the structure.
20. A method of stress-relieving a structure, comprising:
determining a first Larson-Miller relationship that relates
application of thermal energy to the structure and internal stress
in the structure; determining a second Larson-Miller relationship
that relates application of oscillatory energy to the structure and
the internal stress in the structure; determining a first
Larson-Miller parameter according the first Larson-Miller
relationship and according to a desired internal stress value for
the structure; determining a second Larson-Miller parameter
according to the second Larson-Miller relationship and according to
the desired internal stress value; determining a third
Larson-Miller parameter according to the first and second
Larson-Miller parameters by subtracting the second Larson-Miller
parameter from the first Larson-Miller parameter; selecting a first
one of a temperature setting and a time value; selecting a second
one of the temperature setting and the time value according to the
third Larson-Miller parameter, according to the first Larson-Miller
relationship, and according to the first one of the temperature
setting and the time value; selecting one or more oscillatory
operational settings according to a resonant frequency of a system
in which the structure is mounted; providing thermal energy to the
structure according the thermal operational settings; and
concurrently providing oscillatory energy to the structure
according to the oscillatory operational settings for a time
greater than or equal to the time value.
21. The method of claim 20, wherein selecting the second one of the
temperature setting and the time value comprises solving a first
Larson-Miller equation for the second one of the temperature
setting and the time value first one of the temperature setting and
the time value and the third Larson-Miller parameter, wherein the
first Larson-Miller equation represents the first Larson-Miller
relationship.
22. A method of determining operational settings and time values
for concurrent application of multiple energy types to a structure
to change a physical property of the structure, the method
comprising: determining a first parameter according to a desired
physical property value for the structure and according to a first
order rate relationship for a first energy process that relates
application of a first energy type to the structure and the
physical property; determining a second parameter according the
desired physical property value and according to a first order rate
relationship for a second energy process that relates application
of a second energy type to the structure and the physical property;
selecting a first one of a time value and an operational setting
for the first energy process; selecting a second one of the time
value and the operational setting according to the first and second
parameters, according to the first order rate relationship for the
first energy process, and according to the first one of the time
value and the operational setting.
23. The method of claim 22, further comprising: determining the
first order rate relationship for the first energy process; and
determining the first order rate relationship for the second energy
process.
24. The method of claim 22, wherein the first order rate
relationship for the first energy process is a first Larson-Miller
relationship that relates application of the first energy type to
the structure and the physical property, and wherein the first
order rate relationship for the second energy process is a second
Larson-Miller relationship that relates application of the second
energy type to the structure and the physical property.
25. The method of claim 22, further comprising determining a third
parameter according to the first and second, wherein the second one
of the time value and the operational setting is selected according
to the third parameter, according to the first order rate
relationship for the first energy process, and according to the
first one of the time value and the operational setting.
26. The method of claim 25, wherein determining the third parameter
comprises subtracting the second parameter from the first
parameter.
27. The method of claim 25, wherein selecting the second one of the
time value and the operational setting comprises evaluating the
first order rate relationship for the first energy process using
the third parameter and the first one of the time value and the
operational setting to obtain the second one of the time value and
the operational setting.
28. The method of claim 22, wherein the physical property is
internal stress, and wherein the desired physical property value is
one of a remaining internal stress value and an internal stress
reduction value.
29. A method of determining operational settings for concurrent
application of multiple energy types to a structure to change a
physical property of a structure, the method comprising:
determining a first order rate relationship that relates concurrent
application of first and second energy types to the structure and a
physical property of the structure; determining a first order rate
parameter according to the first order rate relationship, the first
order rate parameter corresponding to a desired physical property
value for the structure; selecting a first one of an operational
setting for the first energy process and a time value; and
selecting a second one of the operational setting and the time
value according to the first order rate parameter, according to the
first order rate relationship, and according to the first one of
the operational setting and the time value.
30. The method of claim 29, further comprising determining the
first order rate relationship that relates concurrent application
of first and second energy types to the structure and a physical
property of the structure.
31. A system for changing a property of a structure, comprising: a
thermal energy source; an oscillatory energy source; and a control
system that provides control signals to the thermal and oscillatory
energy sources to control concurrent delivery of thermal and
oscillatory energy from the thermal energy source and the
oscillatory energy source to a structure.
32. The system of claim 31, further comprising transducers
operatively coupled with the structure to provide feedback signals
to the control system 180.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of U.S.
Provisional Patent Application Ser. No. 60/404,020, which was filed
Aug. 16, 2002, entitled A PROCESS AND DEVICE FOR METAL OR METAL
ALLOY STRESS RELIEF, the entirety of which is hereby incorporated
by reference.
FIELD OF INVENTION
[0002] The invention is generally related to the field of changing
physical properties of structures and more particularly to methods
and processes for changing physical properties of structures using
two or more energy sources.
BACKGROUND OF THE INVENTION
[0003] Manufactured parts are often fabricated using a variety of
mechanical and thermal processing steps, such as heat treating,
welding, and others, that cause mechanical stress within the
materials. Residual stress remaining in a manufactured part has
been found to adversely affect fatigue life, corrosion
susceptibility, and strength, wherein areas near weld joints have
been found to be particularly susceptible to stress problems. In
many machines and structures, component parts may suffer early
degradation in load bearing capability, corrosion resistance,
and/or catastrophic failure due at least in part to internal stress
remaining after fabrication or welding. Furthermore, the repair
and/or replacement of components is costly in many situations.
[0004] Localized internal stresses may cause accelerated failures
due to stress corrosion, fatigue, and premature overload fractures.
These failures can occur in bridges, aircraft structures, ship
hulls, pipelines, liquid storage tanks, rails, and reactor vessels,
as well as in many other structures. Relieving or reducing internal
stress in large structures is sometimes difficult, particularly
where the structure is in a remote location. For example, stress
may occur as a result of welding pipes together in remote areas to
create an oil pipeline, or from welding, forming, and/or assembling
structural components in bridges, ships, or airplanes. For large
and small structures, premature degradation or failure of the
structure may result from remaining internal stresses. Durability
and performance of welded parts are affected by internal stresses
that can reduce fatigue life and corrosion resistance. Welding
involves providing high temperatures to melt a welding rod or other
filler metal used to join two sections of plates. The base metal
joining surfaces are also heated to melting temperatures during the
welding process. The presence of thermal gradients adjacent to the
weld line affects the microstructure of the plate. Thermal
gradients are the primary cause of residual stresses along the weld
lines and contribute to diminished mechanical properties and
reduced corrosion resistance in the heat-affected zone compared to
the base material. In addition, welding, especially when coupled
with variations in thickness, leaves significant internal stresses
as the material attempts to adjust to the varying thermal
gradients.
[0005] Accordingly, techniques have been developed for relieving
internal stresses in manufactured parts that may be employed during
or after fabrication or welding operations. However, conventional
stress-relief processes are typically time-intensive, requiring
application of energy to the stressed parts for long periods of
time. In a manufacturing setting, lengthy stress-relief processes
are costly in terms of total fabrication time, throughput, and
energy. Time and energy are also an important consideration in
stress-relieving structures in the field. For example, performing a
stress-relief operation on an aircraft in a commercial airline
fleet requires that the aircraft be grounded during the
stress-relief operation. For large structures, such as welded pipes
in a remote pipeline, ship or aircraft hulls, etc., the energy for
the stress-relief operation often needs to be brought to the
worksite, wherein time-intensive conventional stress-relief
techniques are particularly costly. Accordingly, there remains a
need for improved stress-relief techniques and systems for reducing
stress in manufactured parts and/or welded structures.
SUMMARY OF THE INVENTION
[0006] The following presents a simplified summary of the invention
in order to provide a basic understanding of some aspects of the
invention. This summary is not an extensive overview of the
invention. It is intended neither to identify key or critical
elements of the invention nor to delineate the scope of the
invention. Rather, the primary purpose of this summary is to
present some concepts of the invention in a simplified form as a
prelude to the more detailed description that is presented later.
The invention relates to techniques for changing physical
properties of a structure using concurrent application of multiple
energy types to the structure, and methodologies for determining
operational settings for concurrent application of multiple energy
sources to a structure.
[0007] The concurrent provision of multiple energy types in
accordance with the present invention may be advantageously
employed to significantly reduce the time and/or energy required to
change a physical property of interest, such as reducing remaining
internal stress in manufactured parts or other structures, compared
to previous techniques. The invention may find utility in
technology areas of solid diffusion, including but not limited to
heat treating and aging, surface diffusion treatments for metals
(e.g., oxynitriding, nitriding, carburizing, etc.), zone refining
of metals, battery manufacturing, doping semiconductors (e.g., to
speed processing or lower temperature in semiconductor
manufacturing), or other technologies where time is spent while
atoms diffuse through a material. The invention may also be
employed in areas of liquid diffusion, including but not limited to
osmotic membranes (e.g., water purification, chemicals separations,
etc.), liquid chromatography (e.g., chemicals separation, etc.),
and chemical mixing.
[0008] In accordance with one aspect of the invention, a method is
provided for changing a physical property of a structure, wherein
the physical property can be creep rate, creep, strain, stress,
residual stress, internal stress, aging, mixing, motion through a
membrane, or any property, such as those that may be controlled
according to an Arhennius-type first order rate equation. The
method comprises providing a first energy type to a structure by
performing a first energy process according to an operational
setting. The operational setting and/or a time value is selected
according to a first order rate relationship for the first energy
process, a first order rate relationship for a second energy
process, and according to a desired physical property value for the
structure. In one example, the physical property may be internal
stress, where the desired physical property value is one of a
remaining internal stress value and an internal stress reduction
value. The relationship for the first energy process relates
application of the first energy type to the structure and a
physical property of the structure, and the relationship for the
second energy process relates application of the second energy type
to the structure and the physical property. The method further
comprises providing energy of the second type to the structure at
an energy level above an activation energy for the structure, where
the first and second energy types are provided concurrently for at
least the time value.
[0009] The first and second energy types may individually be any
form of energy applied to a structure. In one example, the first
energy type is thermal and the second energy type is time varying,
such as oscillatory mechanical vibration. The first order rate
relationships may be Larson-Miller (L-M) relationship curves that
relate the application of thermal and oscillatory energy to the
structure and the physical property of interest. A first L-M
parameter is determined according the first L-M relationship,
corresponding to the desired physical property value, and a second
L-M parameter is determined according to the second L-M
relationship, also corresponding to the desired physical property
value. For example, the first L-M parameter may be determined
according a first L-M relationship (e.g., L-M curve, etc.), wherein
a desired remaining internal stress value is selected along the Y
axis of the first L-M curve, and the corresponding parameter ("P")
value is ascertained along the X axis (P.sub.1). A second L-M
parameter is determined for the desired physical property value
according to a second L-M relationship (e.g., a second L-M curve)
by locating the desired internal stress value on the Y axis of the
second L-M curve, and locating the corresponding second parameter
value (e.g., P.sub.2) along the X axis. A third L-M parameter
(e.g., P.sub.3) may optionally be determined according to the first
and second L-M parameters (P.sub.1 and P.sub.2), such as by
subtraction (e.g., P.sub.3 =P.sub.1- P.sub.2).
[0010] An operational setting and a time value are then selected
according to the third parameter P.sub.3 using the first order
relationship for applying the first energy type to the structure.
One of a temperature setting and a time value is selected for
applying the first energy type to the structure, for example, based
on structural, equipment, economic, or other considerations, or
randomly. The other value is then determined or selected according
to the third parameter P.sub.3 (e.g., based on P.sub.1 and
P.sub.2), the first L-M relationship, and according to the
previously selected value. For instance, a temperature value may be
selected based on thermal heating equipment limitations, structural
material properties, etc., and a time value is then determined by
solving a first order rate equation using the pre-selected
temperature value and the third parameter. In another
implementation, the time value may be selected first, and the
temperature setting is then determined according to the time value,
the first and second parameters, and the first order rate
equation.
[0011] In accordance with another aspect of the invention, a method
is provided for determining operational settings for concurrent
application of multiple energy types to a structure. The method
comprises determining a first parameter according to a desired
physical property value for the structure and according to a first
order rate relationship for a first energy process that relates
application of a first energy type to the structure and the
physical property. A second parameter is determined according the
desired physical property value and according to a first order rate
relationship for a second energy process that relates application
of a second energy type to the structure and the physical property.
A time value or an operational setting for the first energy process
is selected, for example, according to structural, equipment,
economic considerations, etc. The remaining time value or
operational setting is selected according to the first and second
parameters (e.g., or a third parameter that relates the first and
second parameters), according to the first order rate relationship
for the first energy process, and according to the previously
selected one of the time value and the operational setting. The
method may further comprise determining the first order rate
relationships for the first and second energy processes.
[0012] Other aspects of the invention provide methods for changing
a physical property of a structure by applying two or more energy
types concurrently, methods for determining operational settings
for concurrent application of multiple energy types to a structure,
which employ a combined first order rate relationship for multiple
energy types, and systems for concurrent application of multiple
energy types to change a structure property. In these methods, the
first order rate relationship characterizes a relationship between
concurrent application of the first and second energy types to the
structure and a physical property of the structure. Operational
settings are selected by determining a parameter corresponding to a
desired physical property value and deriving one or more settings
from the L-M parameter.
[0013] To the accomplishment of the foregoing and related ends, the
following description and annexed drawings set forth in detail
certain illustrative aspects and implementations of the invention.
These are indicative of but a few of the various ways in which the
principles of the invention may be employed. Other objects,
advantages and novel features of the invention will become apparent
from the following detailed description of the invention when
considered in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a flow diagram illustrating an exemplary method of
modifying or changing a physical property of a structure using
concurrent application of multiple energy types in accordance with
the present invention;
[0015] FIG. 2A is a plot illustrating an exemplary first order rate
relationship curve for a first energy process in accordance with
the invention;
[0016] FIG. 2B is a plot illustrating an exemplary first order rate
relationship curve for a second energy process in accordance with
the invention;
[0017] FIG. 3A is a flow diagram illustrating an exemplary method
of stress relieving a structure in accordance with the
invention;
[0018] FIG. 3B is a flow diagram illustrating further details of
the exemplary stress-relief method of FIG. 3A;
[0019] FIG. 4A is a plot illustrating an exemplary Larson-Miller
(L-M) relationship curve for application of thermal energy to
stress-relieve 7055-T7 aluminum structures in accordance with the
invention;
[0020] FIG. 4B is a plot illustrating an exemplary Larson-Miller
relationship curve for application of oscillatory vibration energy
to stress-relieve 7055-T7 aluminum structures in accordance with
the invention;
[0021] FIG. 4C is a schematic diagram illustrating an exemplary
system for stress-relieving a structure in which various aspects of
the invention may be carried out;
[0022] FIG. 4D is a plot illustrating comparative stress relief
results for sample structures processed in accordance with the
invention and samples processed according to conventional
techniques; and
[0023] FIG. 5 is a flow diagram illustrating another exemplary
method of changing a physical property of a structure using a
combined first order rate relationship in accordance with the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] One or more implementations of the present invention will
now be described with reference to the drawings, wherein like
reference numerals are used to refer to like elements throughout.
The invention relates to methods for stress-relieving or changing
other physical properties of a structure using concurrent
application of multiple energy types to the structure, as well as
to methods for determining operation settings for concurrent
application of multiple energy sources to a structure. The
invention finds particular utility in association with stress
relieving structures such as manufactured parts, assemblies of
multiple parts, welds, or other structures, wherein concurrent
application of a thermal energy type and one or more time varying
energy types can provide for temporal acceleration of the
stress-relief operation not previously attainable using
conventional stress-relief techniques. However, the invention may
also be utilized in association with any property changing
processes, such as those that may be modeled according to first
order rate equations, wherein a plurality of energy types or
sources are employed to accelerate the change in one or more
physical properties, such as remaining internal or residual stress.
In this regard, the invention is not limited to the particular
examples set forth herein.
[0025] Referring initially to FIGS. 1, 2A, and 2B, an exemplary
method 2 is illustrated in FIG. 1 for modifying or changing a
physical property of a structure using concurrent application of
multiple energy types in accordance with the present invention.
Although the exemplary process or method 2 and other methods of the
invention are illustrated and described below as a series of acts
or events, it will be appreciated that the present invention is not
limited by the illustrated ordering of such acts or events. For
example, some acts may occur in different orders and/or
concurrently with other acts or events apart from those illustrated
and/or described herein, in accordance with the invention. In
addition, not all illustrated steps may be required to implement a
methodology in accordance with the present invention.
[0026] The method 2 illustrates characterization of two energy
application processes at 6 and 8, correlation of first order rate
relationships and first order rate parameters ("P") for different
energy types at 10-14, selection of operational and time values or
settings at 16-18 and application of multiple energy types at 22.
It is noted at the outset that certain aspects of the invention
provide methods including the characterization, correlation,
setting selection, and energy application features described
herein, while other methods of the present invention do not require
all these acts. As one example, the invention provides methods for
changing physical properties of structures that may require only
certain of the acts illustrated for the exemplary method 2. The
invention also provides methods for determining operational
settings for concurrent application of multiple energy types that
may not require all the acts illustrated in the exemplary method
2.
[0027] Beginning at 4, a first order rate relationship is
determined at 6 for a first energy process and another first order
rate relationship is determined at 8 for a second energy process.
The first order rate relationships determined at 6 and 8 may be
curves, mathematical expressions resulting from curve-fitting a
series of data points, or any characterization or expression of a
relationship between application of the corresponding energy type
to the structure and a physical property of interest, for example,
including but not limited to Larson-Miller relationships and/or
equivalents thereof. Thus, the first order rate relationship
determined at 6 relates application of the first energy type to the
structure and the physical property of the structure, and the first
order rate relationship determined at 8 characterizes the
relationship between application of the second energy type to the
structure and the physical property. FIG. 2A provides a plot 31
illustrating an exemplary first order rate relationship curve for a
first energy type, and FIG. 2B shows a plot 32 illustrating an
exemplary first order rate relationship curve for a second energy
type in accordance with the invention.
[0028] A first order rate relationship may be expressed as an
equation used to model or represent a process that relies upon
diffusion or dislocation motion over time. For example,
stress-relief may be expressed as a first order rate equation
representing passage of dislocations through a material. In one
implementation of the invention, L-M equations may be derived by
taking the log of the Arrhenius equation, and may be plotted as a
curve of a physical property such as remaining internal or residual
stress on a Y axis vs. a first order rate parameter (e.g., a
Larson-Miller parameter) "P" on the X axis. In the example of
stress-relief through application of thermal energy, a first order
rate relationship can be plotted as remaining internal stress vs. a
thermal first order rate parameter Pt, which incorporates both time
and temperature. Alternatively, actual structures may be stressed
and processed using thermal stress-relief operations at various
times and temperatures in order to create a plot of several
remaining internal stress vs. first order rate parameter
values.
[0029] It is noted at this point that the physical property or
characteristic of interest may be plotted on reverse scales along
the Y axis of such relationship curves or plots within the scope of
the invention. These and any other suitable techniques may be
employed at 6 and 8 to determine the first order rate relationships
for the first and second energy processes in accordance with the
present invention. It is noted that more than two energy types may
be provided to the structure in accordance with the invention and
the appended claims, wherein corresponding first order rate
relationships are employed to determine one or more operational
settings and/or time values for acceleration of the change in the
physical property of the structure. It is further noted that the
first order rate relationships may be determined using any samples
made of the same or similar materials as the structures to be
processed, wherein the samples may, but need not, be of the same
size, shape, etc., as the structures of interest. In this regard,
the first order rate relationships and the first order rate
parameters are applicable to any structure made of the material of
interest, whereby the first order rate relationships and the
parameters derived therefrom are universally applicable to any
structure made from that material.
[0030] At 10-14, first order rate parameters and first order rate
relationships are correlated for different energy types.
Alternatively, as discussed below with respect to FIG. 5, a
composite or combined first order rate relationship may be
determined for multiple energy sources, where the correlated effect
of concurrent application of the multiple energy types to a
structure is embedded or embodied in the combined first order rate
relationship and specific first order rate parameters thereof. In
the method 2, a first order rate parameter P.sub.1 is determined at
10 according the first order rate relationship for the first energy
process, wherein the first parameter P.sub.1 corresponds to the
desired physical property value for the structure.
[0031] In FIG. 2A, the desired physical property value is located
along the Y axis 31 y of the plot 31, and the corresponding point
on the first order rate curve is identified as P.sub.1 along the X
axis 31x. The same desired physical property value is used at 12 in
the method 2 to determine a second parameter P.sub.2 according to
the first order rate relationship for the second energy process.
For example, in FIG. 2B, the desired physical property value is
located along the Y axis 32y of the plot 32, and the corresponding
point on the first order rate curve is identified as the second
parameter P.sub.2 along the X axis 32x.
[0032] The operational settings for concurrently providing two or
more energy types to the structure are selected at 16 and 18
according to the first order rate parameters. One of a time value
and an operational setting for the first energy type are selected
according to one or more selection criteria, including but not
limited to structural, equipment, or economic considerations, or
arbitrarily. The other variable is then selected or determined
according to the first and second parameters P.sub.1 and P.sub.2
using the first order rate relationship associated with the first
energy process.
[0033] Toward that end, a third parameter P.sub.3 is optionally
determined at 14 according to the parameters P.sub.1 and P.sub.2,
such as by subtracting P.sub.2 from P.sub.1, wherein the time
and/or operational settings can then be selected or determined in
accordance with P.sub.3 at 18. The correlation of the first order
rate parameters P.sub.1 and P.sub.2 in determining the operational
and time settings for application of the first energy type results
in the third parameter P.sub.3 that reflects a temporal
acceleration achievable within the scope of the invention. This
acceleration, in turn, facilitating stress-relief or implementation
of other multiple energy type processes to change a physical
property of a structure in less time and using less energy than was
possible using previous methods. The inventor has appreciated that
dislocation motion within a structure (e.g., such as during stress
relief) occurs by a diffusion process and can be described by a
first order rate equation:
D = D 0 - Q RT , ( 1 ) ##EQU00001##
where
[0034] D=rate of diffusion at some time, t
[0035] D.sub.0=initial rate of diffusion
[0036] Q=activation energy for reaction to begin
[0037] R=universal gas constant
[0038] T=temperature in degrees K (Kelvin).
Generally, the rate of stress relief obtained using thermal methods
is driven by diffusion and can be described using the Arrhenius
first order rate equation (2), named after the chemist Svandte
Arrhenius:
[0039] r ( T ) = A .DELTA. H RT , ( 2 ) ##EQU00002##
where
[0040] r=rate of stress relief at some time, t
[0041] A=constant
[0042] H=free energy of the reaction
[0043] R=universal gas constant
[0044] T=temperature in degrees K.
The level of stress relief can be related to a first order rate
relationship or equation. In one example, a Larson-Miller (L-M)
equation is obtained by taking the log of the Arrhenius equation
(2).
[0045] Assuming
r .varies. 1 t , ##EQU00003##
and rearranging, the L-M equation for thermal stress relief may be
expressed as:
P = .DELTA. H R = T ( C + log t ) , ( 3 ) ##EQU00004##
where P is the L-M first order rate parameter. Other specific forms
of first order rate expressions may be used, which are logarithmic
expressions of an Arrhenius equation, wherein Larson-Miller or
"L-M", as used herein, is intended to include all such first order
rate relationships and associated parameters which characterize a
logarithmic expression of the Arrhenius equation. The first order
rate equation (3) may be plotted as shown in FIG. 2A with the first
order rate parameter P on the X axis with the physical property of
interest (e.g., stress relief or remaining internal or internal
stress) on the Y axis. For example, such first order rate curves
may be obtained for a thermal energy process using three or more
data points determined by experimental procedures at different
times and temperatures, although other techniques may be employed
at 6 and 8 above for determining the first order rate relationships
within the scope of the invention. The thermal first order rate
curve (e.g., plot 31 in FIG. 2A) can be used to predict the
time/temperature combination that may be employed to achieve any
desired physical property value (e.g., stress relief level).
[0046] The inventor has appreciated that internal or residual
stress relief can primarily be achieved by moving dislocations and
reducing the overall dislocation density, and/or the density of
other lattice defects such as stacking faults. A component of the
overall stress state may be due to local lattice strains, but is
anticipated to be of absolute values. This component is pure
elastic strain and is local within the lattice. The formation of a
dislocation will result when the strain reaches a certain value
equivalent to the activation energy. No dislocation interaction
will occur unless it is energetically favorable to do so. When
energy, whether in the form of heat or any time-varying (e.g.,
oscillatory, periodic, pulsed) applied energy that can cause a
pressure wave to be generated, is applied to a crystalline solid,
energy is added to the dislocations that results in exciting their
motion. Since any system will move to the lower energy state, the
dislocations will try to attain a lower energy configuration by
combining or annihilating, thus reducing the internal stresses in
the material.
[0047] In addition, the inventor has appreciated that similar
mathematical relationships should hold for any stress relief or
other property changing method that causes dislocation motion using
a diffusion process, and can thus be described by the Arrhenius
first order rate equation (2) above. In accordance with the present
invention, an acceleration results in the diffusion process through
the concurrent application of multiple energy types to a structure
for changing a physical property (e.g., stress relief) where the
time and operational settings are determined as described herein.
To further illustrate the acceleration results, the following
derivation is provided for an example wherein thermal energy and
vibration energy are concurrently applied to stress-relieve a
structure.
[0048] Assuming that the matrix of the material or part in question
can be partitioned into strained and non-strained portions, each
with a concentration C, the total concentration may be expressed
as:
C.sub.(total)=C.sub.s(strained)+C.sub.n(non-strained) (4)
In the case of strain relief, the following relationship holds:
C.sub.s.fwdarw.C.sub.n (5)
The rate of change of concentration of the strained portion during
stress relief may be expressed as:
C s t = - k C s ( 6 ) ##EQU00005##
for the rate change of a reaction, where
k = A .DELTA. H RT . ##EQU00006##
Thus
[0049] C sv t = - k v C sv , ( 7 ) ##EQU00007##
where k.sub.v, is the Arrhenius equation for an oscillatory
(vibratory) second energy type, and:
C sT t = - k v C sT , ( 8 ) ##EQU00008##
where k.sub.T is the Arrhenius equation for a thermal energy
type.
[0050] The total for the combined method is:
C s t = C sv t + C sT t = - ( k v + k T ) C s . ( 9 )
##EQU00009##
Solving for C.sub.s (t):
[0051] C.sub.s(t)=A e.sup.-(k.sup.T.sup.+kv)t+B (10)
Applying boundary conditions for t=0 and t=.infin.:
[0052] C.sub.s(0)=A e.sup.-(k.sup.T.sup.+kv)0+B=A+B, and (11)
C.sub.s(t)=A0+B=B (12)
Combining equations (11) and (12):
[0053] A=C.sub.s(0)-C.sub.s(.infin.) (13)
Substituting into (10):
[0054]
C.sub.s(t)=[C.sub.s(0)-C.sub.s(.infin.)]e.sup.-(k.sup.T.sup.+kv)t+-
C.sub.s(.infin.), and (14)
C.sub.s(t)=C.sub.s(0)e.sup.-(k.sup.T.sup.+kv)t+C.sub.s(.infin.)[1-e.sup.-
-(k.sup.T.sup.+kv)t], (15)
from the boundary conditions of C.sub.s (0)=1 and C.sub.s
(.infin.)=0. Thus:
C.sub.s(t)=e.sup.-(k.sup.T.sup.+kv)t. (16)
Applying L'Hopital's rule and substituting for t at completion:
C.sub.s(t)=e.sup.-(k.sup.T.sup.+kv). (17)
For the case of two diffusion-controlled processes occurring
simultaneously then, where k=Arrhenius equation:
1 t = A - ( .DELTA. H T + .DELTA. H v RT ) . ( 18 )
##EQU00010##
Taking the natural log:
- ln t = ln A - ( .DELTA. H T + .DELTA. H v R ) ( 1 T ) . ( 19 )
##EQU00011##
Rearranging and converting to log:
( .DELTA. H T + .DELTA. H v R ) = T ( C + log t ) . ( 20 )
##EQU00012##
[0055] Thus, for any combination of simultaneously applied stress
relief processes, the sum of the stress relief process rates can be
related to the first order rate parameter P. The oscillatory
process, whether using sound, mechanical vibration, laser impulses,
or some other oscillatory or time-varying applied energy process,
is usually performed at a fixed temperature, where time, frequency,
and amplitude are the variables. The vibration frequency and
amplitude settings can be determined according to any suitable
techniques so as to provide energy at or above the activation
energy of the material of interest within the scope of the present
invention, for example, by identifying a resonant frequency for the
structure and the system in which the structure is to be mounted
during processing, and selecting a frequency at or near the
resonant frequency. In this regard, the frequency may be adjusted
during processing as the resonance point changes, wherein the
invention is not limited to fixed frequency or fixed amplitude
implementations for providing time varying energy to the structure.
The inventor has also appreciated that frequency may have a weak
dependence on temperature, that can be calculated as follows:
c=8f, (21)
where
[0056] c=speed of sound in a material
[0057] 8=f (specimen dimension)
[0058] f=frequency of harmonic.
Also:
[0059] c = [ Y .rho. ] 1 2 , ( 22 ) ##EQU00013##
where
[0060] Y=Young's modulus
[0061] .rho.=density of material.
The relationship between density and temperature is expressed
as:
.rho.=.rho..sub.0(1+.beta.T),
where .beta.=3 c.sub.te and c.sub.te is the coefficient of thermal
expansion. Solving for frequency:
.lamda. f = [ Y .rho. ] 1 2 , and ( 24 ) f = [ Y .rho. ] 1 2 ( 1
.lamda. ) . ( 25 ) ##EQU00014##
Including the variation with temperature, the frequency is
expressed as:
f = [ Y .rho. 0 ( 1 + .beta. T ) ] 1 2 ( 1 .lamda. ) ( 26 )
##EQU00015##
Thus, the working frequency f, is a function of 1/T.
[0062] Consequently, as the temperature T changes, the working
frequency will shift as well. Thus, for greatest efficiency of
stress relief, frequency can be adjusted to a final value once the
processing temperature is reached. For some oscillatory techniques,
this is believed to be a very weak relationship at best and so
adjustments in the oscillatory operational frequency setting may,
but need not be made in accordance with the invention. Because of
the cyclic lattice displacement by wave energy is proportional to
Young's modulus E and to Poisson's ratio within a single method,
the frequency/modulus relationship will be unique to an alloy and
product form and thus may be advantageously determined by
experimental procedures on the specific part in question.
[0063] Furthermore, the resonant frequency may change as a
structure is stress relieved, wherein adjustments to the frequency
of a time varying energy type may be made accordingly. In addition,
frequency may also be highly dependent on part dimension. In
general, though, specific frequency ranges for alloys can be
predetermined for a specific method of frequency generation,
whether sonic, laser, electrical, magnetic, mechanical, and
microwave, or some other type, wherein adjustments according to
specific structures and mounting systems are contemplated as
falling within the scope of the invention.
[0064] Returning to FIG. 1, once the first order rate parameters
P.sub.1-P.sub.3 have been determined at 10-14, the operational
setting and time value are selected in accordance therewith at 16
and 18. One operational setting for the first energy process or the
time value for concurrent processing is selected at 16, such as a
temperature setting or a concurrent processing time setting where
the first energy type is thermal. The first variable selected
(e.g., independent variable) may be chosen at 16, based on
structural, equipment, economic, or other considerations, or
randomly. In one example, processing equipment, such as thermal
energy sources, may have upper limits on temperature, and/or it may
be desired to maintain the structure at or below a safe temperature
to avoid changing the structure temper or melting the structure,
whereas a minimum temperature value would be a critical temperature
for the material to exceed the activation energy therefor. In
another example, a section of a ship hull, aircraft structure, or
bridge may be difficult to bring to a very high temperature, due to
ambient conditions and/or heat sinking from attached structures,
wherein such considerations may be taken into account in selecting
a temperature setting at 16.
[0065] At 18, the other one of the operational setting and the time
value is selected or determined according to the first order rate
parameter P.sub.3 (e.g., or simply in accordance with P.sub.1 and
P.sub.2), and also in accordance with the previously selected
(e.g., independent) variable. For the above example where a
temperature setting was selected at 16, the time value is
determined at 18 according to the temperature setting and the
parameter P.sub.3 for application of the first energy type to the
structure using the first order rate relationship for the thermal
energy process. The first order rate relationship is used, for
example, by evaluating a corresponding first order rate equation
(e.g., equation (3) above) for the first energy process, using the
parameter P.sub.3 and the temperature setting as independent
variables, and solving for the dependent variable, which is the
time value in this example.
[0066] Operation settings for the second energy process can be
selected according to other criteria for concurrent application of
energy together with the first energy process. For example,
frequency and/or amplitude settings may be selected for an
oscillatory second energy process, wherein the structure, together
with the system in which it is mounted for processing, is scanned
to determine a resonant frequency. The processing frequency may
then be chosen to be at or near the resonant frequency, wherein the
frequency may be adjusted during processing. The frequency and
amplitude of such an oscillatory second energy process are also
adjusted such that the energy provided thereby is above an
activation energy for the material of the structure.
[0067] It is noted in this regard, that whereas the first order
rate relationships and first order rate parameters determined at
6-14 above are generally material specific and structure
independent, that one or more of the operational settings for
application of the energy types to the structure may be chosen
according to material specific considerations (e.g., activation
energy, etc.), as well as according to the specifics of a
particular structure and/or a system in which the structure is
being processed, such as mass, size, shape, or other system or
structure characteristics. Once the operational and time settings
or values have been selected at 16 and 18, the method 2 proceeds to
22, where the first and second energy processes are performed to
apply the first and second energy types to the structure
concurrently for at least the selected time value to achieve
accelerated change in the physical property of the structure,
before the method 2 ends at 24. In certain implementations of
concurrent energy application in which one of the energy types is
thermal, the heat may preferably be turned off first, leaving the
oscillatory energy running until the part being processed (e.g.,
stress-relieved) achieves a minimum temperature for dislocation
mobility.
[0068] As exemplified in FIGS. 1, 2A, and 2B, the invention
provides methods for determining operational settings and time
values for concurrent application of multiple energy types to a
structure to change a physical property of the structure, that may
be employed alone or in conjunction with the actual processing of
structures to change a physical property of interest. The
determination of the first order rate relationships (e.g., in 6 and
8 above) may be done in a laboratory setting using sample specimens
of a given material type to determine, or appropriate pre-existing
first order rate relationships may be used. With these, the first
order rate parameters can be determined (e.g., at 10-14) for a
desired physical property value, and the operational settings and
time values can be selected (e.g., in 16 and 18 above). Thereafter,
the first and second energy processes may be applied to any
structure of the material for which the first order relationships
and parameters have been determined, wherein one or more
operational settings (e.g., such as vibration frequency) may be
selected according to processing system particulars (e.g., system
resonant frequency).
[0069] It is further noted that the various aspects of the
invention may be employed in concurrent application of more than
two energy types. Thus, the invention contemplates concurrent
application of any integer number N different energy types, where N
is a positive integer greater than 1, wherein first order rate
relationships (e.g., derived or pre-existing) may be employed in
determining N first order rate parameters (e.g., P.sub.1 . . .
P.sub.N) corresponding to a desired physical property value. One or
more operational settings and/or a time value may then be
determined according to these parameters, or the parameters may be
correlated using another parameter (e.g.,
P.sub.N+1=P.sub.1-(P.sub.2+P.sub.3+. . . +P.sub.N)). In addition,
as illustrated and described below with respect to FIG. 5, a single
first order rate relationship may be derived or obtained for
concurrent application of more than one energy type, wherein fewer
than N parameters may be needed in selecting the operational and
time settings and values for application of N energy types. In one
example, a single first order rate relationship may be employed
that relates application of N energy types to a structure and a
physical property of the structure, wherein a single parameter may
be used in selecting the operational and time settings and values.
In this regard, all such alternate implementations are contemplated
as falling within the scope of the invention and the appended
claims, wherein the illustrated implementations are merely
examples.
[0070] Referring now to FIGS. 3A, 3B, and 4A-4D, the invention has
been successfully implemented in reducing remaining internal (e.g.,
residual) stress in aluminum structures using concurrent
application of thermal and oscillatory energy (e.g., mechanical
vibration) to achieve a significant reduction in the time required
to obtain a desired remaining internal stress value (e.g., or a
desired amount of internal stress reduction). FIGS. 3A and 3B
illustrate an exemplary method 102 of stress-relief using two
energy processes (e.g., heat and mechanical vibration) in
accordance with the invention, FIGS. 4A and 4B illustrate exemplary
Larson-Miller first order rate relationship curves for thermal and
vibratory energy, respectfully. A Larson-Miller first order rate
relationship or relationship curve, as used herein, includes any
first order rate equation or logarithmic expression of the
Arrhenius equation. FIG. 4C illustrates an exemplary system in
which the stress relief techniques of the invention may be carried
out, and FIG. 4D illustrates a plot of comparative stress relief
results for aluminum structures processed in accordance with the
invention together with structures processed using conventional
stress-relief techniques.
[0071] In FIG. 3A, the method 102 begins at 104, wherein a thermal
Larson-Miller curve (first order rate relationship) is determined
for an induction heating stress-relief process, such as the
exemplary plot 151 in FIG. 4A. In this example, the plot 151
includes a number of data points corresponding to measurements of
7055-T7 aluminum specimens or samples subjected to varying amounts
of thermal stress-relief using different time and temperature
settings. The test specimens were obtained from a single lot so as
to have identical or nearly identical initial physical properties,
and the samples were stressed and separated into two halves. A
baseline stress measurement was taken on one of the halves to
establish a baseline value for remaining internal stress in units
of kpsi (thousand pounds per square inch). The samples were then
stress relieved at different time and temperature values using
thermal energy application, using induction heating equipment. The
remaining internal stress was then measured for the stress-relieved
samples, characterized in terms of percent remaining stress. A
Larson-Miller equation for 7055-T7 aluminum was then employed to
compute an L-M first order rate parameter value "Pt" for each of
the stress-relieved samples. For example, the above equation (3)
was used to calculate "Pt" for each sample, using the corresponding
time "t" and temperature "T" (degrees K) used, wherein the value 10
was used for the constant "C" corresponding to this alloy. It is
appreciated that different "C" values are used for different
materials, for example, 20 for steel, 10-15 for titanium, etc.,
wherein other values may be obtained for other materials of
interest. However, for a given structure, the same C value is used
to obtain, and later calculate conditions for, all the operational
values for a single stress relief procedure. The plot 151 was then
constructed by plotting the first order rate parameters (e.g., "Pt"
values) along the X axis 151x and the corresponding remaining
percent internal stress values along the Y axis 151y as shown in
FIG. 4A.
[0072] The following table 1 illustrates the data points used in
constructing the exemplary L-M curve plot 151 in FIG. 4A. Although
the plot 151 is illustrated in terms of percent stress relief (Y
axis), equivalent L-M curves can be plotted in terms of absolute
stress values, for example, in units of kpsi. In addition, reverse
scales may alternatively be employed for the X axis values and/or
for the Y axis values.
TABLE-US-00001 TABLE 1 T (degrees Stress Relief F.) T (minutes) L-M
parameter Pt (%) 275 0 14.9 25 275 2.5 14.99 31 275 10 15.44 26 275
24 15.71 34 290 0 14.9 25 290 5 15.52 31 290 16 15.9 34 290 24
16.03 37 300 0 14.9 25 300 5 15.73 33 300 16 16.12 40 300 24 16.25
54 325 0 14.9 25 325 5 16.25 41 325 16 16.65 77 325 24 16.78 97
[0073] The method 102 continues at 108, where an oscillatory
Larson-Miller curve (e.g., first order rate relationship) is
determined for a vibration stress-relief process, such as the
exemplary plot 152 in FIG. 4B. A similar technique was employed to
establish the four exemplary data points in the plot 152, wherein
samples were initially stressed, then subjected to vibratory energy
at or above the activation energy for 7055-T7 aluminum at ambient
temperature (e.g., 70 degrees F in this example) to reduce the
internal stress. The stress-relieved samples were then measured to
determine the remaining internal stress. The plot 152 of FIG. 4B
was then constructed by plotting the "Pv" values along the X axis
152x and the corresponding remaining internal stress values along
the Y axis 152y.
[0074] The following table 2 illustrates the data points used in
constructing the exemplary L-M curve 152 in FIG. 4B.
TABLE-US-00002 TABLE 2 T L-M parameter Stress Relief (hours) Pt (%)
0 9 0 0.2 10.33 26.67 0.5 10.54 80 1.0 10.7 93.33
[0075] With the L-M relationship curves 151 and 152 for the thermal
and oscillatory processes determined at 106 and 108, respectively,
a first thermal L-M parameter Pt is determined at 110 that
corresponds to a desired remaining internal stress value for the
structure. In the illustrated example, 3% remaining internal stress
was selected (e.g., 97% stress reduction) as the desired physical
property value. Using this value along the Y axis 151y in FIG. 4A,
a corresponding thermal L-M parameter Pt is identified along the X
axis 151x from the plot 151, having a value of 16,800. At 112, an
oscillatory L-M parameter Pv is determined from the oscillatory L-M
curve 152 shown in FIG. 4B, that also corresponds to the desired
remaining internal stress value. The desired remaining stress value
(e.g., 3%) is located along the Y axis 152y in FIG. 4B, and the
corresponding vibratory L-M parameter Pv is identified along the X
axis 152x having a value of 10,800.
[0076] Referring again to the thermal L-M curve plot 151 in FIG.
4A, a final L-M parameter Pf is determined at 114 by subtracting
the vibratory parameter Pv from the thermal parameter Pt (e.g.,
Pf=Pt-Pv=16,800-10,800=6,000). Using this Pf value of 6,000, one of
a temperature setting and a time value is selected at 116 for the
induction heating process, and the other is selected at 118 by
solving the L-M equation for the remaining dependent variable. For
instance, a temperature may be selected at 116, taking into account
one or more limitations, including but not limited to equipment
limitations (e.g., maximum temperatures possible with available
heating equipment), material limitations (e.g., keeping the
structure temperature below melting or other critical temperatures
for the structure material), and heating the structure to above a
temperature related to the activation energy of the material, etc.
In this example, 300 degrees F. was selected for stress-relieving
the aluminum structure without melting or altering the temper of
the material. In other situations, the process temperature may be
dictated by other considerations, such as an in-line process where
the structures are being heat treated, and the temperature profile
is fixed for the heating process, in which case the present
invention may be employed to determine the time value for
concurrent application of oscillatory energy or other second energy
type while the structure is at temperature, is being cooled, or
quenched.
[0077] At 120, the dependent variable is selected. In the
illustrated example, the processing time t is determined according
to the final L-M parameter Pf determined at 114 above (e.g.,
6,000). In this example, the L-M equation (3) is solved for the
time value t using the temperature T selected at 116 (e.g., 300
degrees F.), the constant (e.g., C=10 for aluminum), and the
parameter Pf (e.g., 6,000), to yield a process time t=about 28.25
seconds for concurrent application of thermal and vibrational
energy to achieve the desired 3% remaining internal stress value.
One or more vibrational settings may be selected, for example
frequency and/or amplitude, in order to provide oscillatory or
time-varying energy at or above an activation energy of the
aluminum material. One technique that may be employed involves
determining a resonant frequency for the structure and the system
in which the structure is to be processed, and selecting a
frequency at or near (e.g., slightly below) the resonant frequency,
wherein the frequency may be adjusted during processing. In this
example, it is noted that separate application of heat at this
temperature or of vibration alone for this time would not yield the
desired stress-relief goal. Thus, the invention facilitates
significant acceleration of stress-relief, as illustrated further
below with respect to FIG. 4D. At 122, the induction heating and
vibration processes are performed concurrently for at least the
selected time value and according to the selected operation
parameters for the induction heating and vibration processes, and
the method 102 ends at 124.
[0078] As illustrated further in FIG. 3B, the thermal and
oscillatory processes need not be exactly aligned in time, but need
only be applied concurrently for the minimum time value at 122 to
achieve the desired amount of internal stress relief within the
scope of the invention. The stress-relief processing 122 begins at
130 in FIG. 3B, wherein the vibration process is initially started
at 132 and continues until the selected amplitude and frequency are
achieved at 134. The thermal processing begins at 136 (while the
vibration continues) and the temperature is monitored at 138. Once
the selected temperature setting is reached at 138 (e.g., 300
degrees F in this example), the selected temperature and
vibrational settings are maintained at 140 until the concurrent
processing time t is determined at 142 to be at least the selected
time value t.sub.SEL. Thereafter, the application of thermal energy
is discontinued at 144 and the vibration is continued at 146 until
the structure temperature T is less than a critical temperature
TCRIT (e.g., YES at 148), whereafter the vibration is discontinued
at 150 and the method 102 ends at 124. Other implementations of
concurrent application of multiple energy types are contemplated
within the scope of the invention. For example, either processes
(e.g., heat or vibration) may be started before the other one, or
both may begin at the same time. Furthermore, either process may be
continued after the other is discontinued or both may end at the
same time. In order to prevent the buildup of new residual stresses
due to the presence of thermal gradients, it may be desirable that
the oscillatory energy level be maintained until the temperature
falls below the value required for activation of dislocation
motion. In this regard, the example illustrated in FIG. 3B is but
one possible implementation within the scope of the invention,
wherein the invention and the appended claims are not limited to
the exemplary implementations illustrated and described herein. In
one example, where thermal energy is already present, a temperature
value is selected at which the second energy form is to be applied.
For example, the temperature value might be selected as the highest
operating temperature that a part will see in service, or the
temperature at which a stress-inducing process concludes, or the
temperature at which the part becomes solid after welding, which
may then be used as the appropriate time in the process and
temperature at which to apply the vibration or other oscillatory
process.
[0079] The above techniques can then be used to determine Pt, Pv,
and Pf using the first and second L-M relationships from either two
first order rate relationships or from a single combined first
order rate relationship characterizing concurrent application of
multiple energy types. For example, where a thermal L-M curve and
an oscillatory L-M curve are available, P3 may be calculated by
subtracting P2 from P1. Once P3 is obtained, the previously
selected temperature value may be used to determine the time for
the concurrent energy application. The first and second energy
types are applied concurrently for the appropriate time, beginning
once the structure has reached the temperature value selected
above, and then the thermal energy application is discontinued. The
second (e.g., oscillatory) energy type may be continued thereafter
until the structure temperature drops below activation energy for
dislocation motion for improved stress relief results. This
technique of continuing the second energy type after removal of the
thermal energy source may advantageously prevent new residual
stresses from being created by the thermal gradients inherently
present during cooldown.
[0080] FIG. 4C illustrates an exemplary system 160 in accordance
with another aspect of the invention, in which multiple energy
types may be concurrently provided to a structure 162 to change a
physical property thereof (e.g., internal stress, etc.) in
accordance with the present invention. The system 160 comprises an
induction heating system 164 operatively coupled with the structure
162 via cables or other coupling connections 166 to impart thermal
energy to the structure 1 66, wherein the connections 166 may
include coils for imparting fields in the structure 162 according
to standard inductive heating techniques. The system 160 further
includes a vibration system 170 coupled to impart oscillatory
energy (e.g., mechanical vibration) to the structure 162 via
connections 172, such as cables, mechanical actuators, etc. The
heating and vibration systems 164 and 170 are operated according to
control signals 174 and 176, respectively, from a control system
180, and feedback signals (e.g., vibration frequency, vibration
amplitude, structure temperature, etc.) are provided along
connections 182 from transducers (not shown) operatively coupled
with the structure to the control system 180.
[0081] FIG. 4D illustrates a plot 190 of internal stress (kpsi)
remaining in four 7055-T7 aluminum samples produced by an ASTM E8
tensile test, wherein all the tested specimens were selected from
the same heat-treat and test lot, and tested under the same
conditions to ensure that all the samples started with
approximately equivalent internal stress states. The resulting
internal stresses are illustrated following stress-relief
processing according to four techniques at different depths below
the sample surface, wherein the observed variation in stress with
depth from the surface was a result of the manufacturing process
and is typical for this product form. The stress results for a
first sample (#1) are illustrated by a curve 191, the results for a
second sample (#2) are illustrated by a curve 192, and the results
for a third sample (#3) are illustrated by a curve 193. The fourth
sample (#4) was tested according to the present invention using
heat and vibration, and the exemplary results for the fourth sample
are illustrated by a curve 194.
[0082] Sample #1 was stress-relieved using vibration only at a
frequency of 53 Hz for 4 minutes, yielding the remaining internal
stress shown in the curve 191. Sample #2 was stress-relieved using
vibration only under similar frequency conditions for 24 minutes,
resulting in the stress shown in curve 192. Sample #3 was
stress-relieved using a thermal only process (no vibration) at 300
degrees F. for 4 minutes.
[0083] The fourth sample (#4) was stress relieved using concurrent
application of thermal and vibratory energy for 4 minutes according
to the invention, wherein the stresses are seen in the curve 194 to
be approximately zero throughout the depth of the measurement. In
the illustrated example, the fourth sample was processed generally
according to the operational settings set for in the description of
FIGS. 3A-4B above, using a temperature setting of about 300 degrees
F. and vibrational settings selected to provide energy above the
activation energy at or near a resonant frequency of the system in
which the structure was mounted during processing. As the data
demonstrates in FIG. 4D, the inventive process, in four minutes,
stress-relieves the fourth sample (#4) sufficiently to meet the
desired stress reduction goal in less time than either the
vibration only or the thermal only techniques of the other
samples.
[0084] FIG. 5 illustrates another exemplary method 202 for changing
a physical property of a structure and for determining operational
settings for a concurrent multiple-energy process in accordance
with the invention, beginning at 204. At 206, a combined first
order rate relationship is determined for concurrent application of
two or more energy types to a structure for changing a physical
property thereof. In this method, the combined first order rate
relationship relates concurrent application of the first and second
energy types to the structure and a physical property of the
structure, which may be a single L-M of other first order rate
curve.
[0085] To develop a first order rate relationship (e.g., curve,
equation, model, etc.) that combines the effects of concurrent
application of two or more energy types for stress relief or other
property change, the following technique may be used at 206. For
the example of thermal and oscillatory energy types, the frequency
dependence on temperature is believed to be strongly dependent on
alloy and part configuration (e.g. including the mass, size, shape,
etc. of the structure being processed and the system in which the
structure is mounted during processing) and weakly dependent on
temperature. Therefore, a frequency may be selected that is
appropriate to the system and alloy. Fine-tuning may be done but
generally the frequency will not vary greatly over a wide range of
temperatures if the rest of the system is stable. Test coupons or
samples, preferably from a single lot of material (alloy, etc.) of
interest, can be used to derive test results for plotting a
combined first order rate relationship curve. For residual or
internal stress relief, generally tensile specimens may be produced
per ASTM E8 or some suitable test method and tested appropriately
to failure. One half of each broken tensile coupon is used for
testing (e.g., measuring) the baseline residual stress. The other
half of each tensile coupon is stress relieved using concurrent
application of thermal and oscillatory energy (e.g., using the
frequency setting determined above) at varying times and
temperatures chosen to yield a wide range of L-M parameters within
the limits of the heat source and time available. The resulting
(e.g., measured) remaining stress values (e.g., or calculated
amounts of stress relief) can then be plotted vs. the corresponding
L-M parameter to generate the combined relationship curve, wherein
at least five points are ideally used to determine the L-M curve
for the combined (e.g., concurrent) processes.
[0086] The resulting combined first order rate relationship curve
from 206 (e.g., or a suitable pre-existing combined first order
rate relationship) is then used to determine time and temperature
settings and parameters for process application. A combined first
order rate parameter Pc is determined at 208 from the combined
first order rate curve or relationship, corresponding to a desired
value of the physical property of interest. Time and operational
settings are selected at 210 for the concurrently applied energy
types according to the first order rate parameter Pc, and the first
and second energy types are concurrently provided to the structure
at 212 according to the selected operational and time settings, and
the method 202 ends at 214.
[0087] Although the invention has been illustrated and described
with respect to one or more implementations, alterations and/or
modifications may be made to the illustrated examples without
departing from the spirit and scope of the appended claims. In
particular regard to the various functions performed by the above
described components or structures (blocks, units, assemblies,
devices, circuits, systems, etc.), the terms used to describe such
components (including a reference to a "means") are intended to
correspond, unless otherwise indicated, to any component or
structure which performs the specified function of the described
component (e.g., that is functionally equivalent), even though not
structurally equivalent to the disclosed structure which performs
the function in the herein illustrated exemplary implementations of
the invention. In addition, while a particular feature of the
invention may have been disclosed with respect to only one of
several implementations, such feature may be combined with one or
more other features of the other implementations as may be desired
and advantageous for any given or particular application.
Furthermore, to the extent that the terms "including", "includes",
"having", "has", "with", or variants thereof are used in either the
detailed description and the claims, such terms are intended to be
inclusive in a manner similar to the term "comprising."
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