U.S. patent application number 13/568039 was filed with the patent office on 2012-11-29 for method for automated testing of a material joint.
Invention is credited to Christoph DOTTINGER, Roman LOUBAN, Jurgen ZETTNER.
Application Number | 20120298870 13/568039 |
Document ID | / |
Family ID | 39186187 |
Filed Date | 2012-11-29 |
United States Patent
Application |
20120298870 |
Kind Code |
A1 |
LOUBAN; Roman ; et
al. |
November 29, 2012 |
METHOD FOR AUTOMATED TESTING OF A MATERIAL JOINT
Abstract
In a method for automated, contactless and non-destructive
testing of a material joint (4), a dynamic threshold value is
varied between a minimum threshold value and a maximum threshold
value, with regions of a heat flow dynamics through the material
joint (4) being determined which represent values of the heat flow
dynamics exceeding the dynamic threshold value. The regions of the
heat flow dynamics are examined with respect to an abrupt change in
perimeter. An Abrupt change in perimeter occurs if a boundary (7)
between a molten zone (5) and a non-molten but still adhering zone
(6) of the material joint (4) is being crossed.
Inventors: |
LOUBAN; Roman; (US) ;
ZETTNER; Jurgen; (US) ; DOTTINGER; Christoph;
(US) |
Family ID: |
39186187 |
Appl. No.: |
13/568039 |
Filed: |
August 6, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12520504 |
Jun 19, 2009 |
8235588 |
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PCT/EP2007/010800 |
Dec 11, 2007 |
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13568039 |
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Current U.S.
Class: |
250/341.6 |
Current CPC
Class: |
G01N 25/72 20130101;
B23K 31/12 20130101; B23K 11/36 20130101 |
Class at
Publication: |
250/341.6 |
International
Class: |
G01J 5/10 20060101
G01J005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2006 |
DE |
10 2006 061 794.0 |
Claims
1. A method for automated, contactless and non-destructive testing
of a material joint of at least two mating parts, wherein the
material joint being a two-section joint which consists of a molten
zone and a non-molten zone surrounding said molten zone;
comprising: obtaining infrared images by using at least one
excitation source to excite a test sample and at least one infrared
sensor to detect a developing heat flow in a sequence of thermal
images, such that result images are obtained from the sequence of
thermal images; examining the thermal images and the result images
to detect the molten zone from a result image, which illustrates a
heat flow dynamics (W) through the material joint, in such a way
that a minimum threshold value (W.sub.min) is determined which
exceeds a heat flow dynamics (W) of an image background (H); a
maximum threshold value (W.sub.max) is determined which corresponds
to a peak value of the heat flow dynamics (W) through the material
joint; a dynamic threshold value (W.sub.dyn) is varied between the
minimum threshold value (W.sub.min) and the maximum threshold value
(W.sub.max); a sequence of regions (B) of the heat flow dynamics
(W) through the material joint (4) is determined which represent
the values of the heat flow dynamics (W) exceeding the dynamic
threshold value (W.sub.dyn); the regions (B) of the heat flow
dynamics (W) are examined with respect to an abrupt change in
perimeter (.DELTA.U); the molten zone (5) is determined as a region
(B.sub.i) from the regions (B), the abrupt change in perimeter
(.DELTA.U) indicating that a boundary (7) between the molten zone
(5) and the non-molten zone (6) is being crossed; and a position
and a size of the molten zone (5) are evaluated.
2. A method according to claim 1, wherein the minimum threshold
value (W.sub.min) is determined from a reference region (R) of the
heat flow dynamics (W) of the image background (H).
3. A method according to claim 1, wherein the maximum threshold
value (W.sub.max) is determined from a test region (T), with the
test region (T) being located in the center of a region (S) which
represents the values of the heat flow dynamics (W) that exceed the
minimum threshold value (W.sub.min); and with the maximum threshold
value (W.sub.max) being an average value of the values of the heat
flow dynamics (W) from the test region (T).
4. A method according to claim 1, wherein the maximum threshold
value (W.sub.max) is determined from several test regions (T) of
the same size, with the test regions (T) being located in a region
(S) which represents the values of the heat flow dynamics (W)
exceeding the minimum threshold value (W.sub.min), with an average
value of the values of the heat flow dynamics (W) being determined
for each test region (T) from the test region (T), and the maximum
threshold value (W.sub.max) being a maximum value of the average
values.
5. A method according to claim 1, wherein the dynamic threshold
value (W.sub.dyn) is varied with an increment size
(.DELTA.W.sub.dyn), the increment size (.DELTA.W.sub.dyn) being
determined iteratively.
6. A method according to claim 1, wherein the material joint (4) is
a weld point, with the molten zone (5) being referred to as weld
nugget and the non-molten zone (6) being referred to as weld
glue.
7. A method according to claim 6, wherein the weld point is
evaluated by means of a material characteristic curve (K), with the
characteristic curve (K) being determined by means of reference
weld points which have different remaining material thicknesses (M)
and interconnect at least two mating parts (2, 3); the remaining
material thickness (M) being measured for each reference weld
point; a peak value of a heat flow dynamics (W) being measured for
each reference weld point; and the characteristic curve (K) being
generated from the peak values of the heat flow dynamics (W) and
the associated remaining material thicknesses (M).
8. A method according to claim 7, wherein the maximum threshold
value (W.sub.max) is compared with a first limiting value
(G.sub.1), with the presence of a hole in the weld point being
indicated if said maximum threshold value (W.sub.max) exceeds said
limiting value (G.sub.1).
9. A method according to claim 7, wherein the maximum threshold
value (W.sub.max) is compared with a second limiting value
(G.sub.2), with the presence of a cavity in the weld point being
indicated if said maximum threshold value (W.sub.max) is less than
the second limiting value (G.sub.2).
10. A method according to claim 6, wherein surface damages of the
weld point are detected by means of another image, with the image
being provided with a coordinate system which is identical to that
of the result image by means of which the weld nugget was detected;
and the detection and evaluation of surface damages taking place in
the detected region (B.sub.i) of the weld nugget.
Description
[0001] The invention relates to a method for automated, contactless
and non-destructive testing of a material joint of at least two
mating parts according to the preamble of claim 1.
[0002] Weld points are important material joints for industrial
applications. A weld point usually comprises a two-section joint
which consists of a molten and a non-molten zone. The molten zone
is located in an inner region of the weld point, thus forming the
so-called weld nugget. The non-molten zone surrounds the weld
nugget and is referred to as weld glue. In the non-molten zone, the
mating parts are not welded together. The strength of the joint
between the mating parts is therefore not sufficient in the
non-molten zone as there is only a certain amount of adherence. The
quality of the weld point is therefore substantially determined by
the weld nugget.
[0003] It is known to evaluate the quality of a weld point by means
of destructive testing or examination. An examination of this type
can however only be performed by random sampling. A more frequent
examination--up to an examination of 100 percent of all samples--is
only performable by means of non-destructive examination.
[0004] Heat flow thermography is a long-established contactless and
non-destructive examination method. According to this method, a
test sample is excited by at least one excitation source in order
to generate a heat flow. The thermal radiation emitted by the test
sample is recorded in a sequence of images by means of at least one
infrared sensor. In a computing unit, the recorded sequence of
images is developed into result images of various types. Result
images of these types are for instance an amplitude image and a
phase image which respectively illustrate the amplitude and the
travel time of the thermal waves at various points of a material
joint. By means of a phase image, local differences in heat
conductivity of a material joint can be made visible (Theory and
Practice of Infrared Technology for Nondestructive Testing, Xavier
P. V. Maldague, John Wiley and Sons, Inc., 2001).
[0005] A method for automated testing of weld points is disclosed
in WO 01/50116 A1 where the quality of a weld point is evaluated by
means of a half-value period of the heat flow. A low half-value
period of the heat flow at the individual image points indicates a
good-quality weld joint. The disadvantage of this method is that
the absolute half-value periods that are detected do not provide
any objective information as to the location of the boundary
between the weld nugget and the weld glue. With this method, an
automated determination of the size and position of the weld point
is therefore impossible on an industrial scale.
[0006] DE 101 50 633 A1 discloses a method for automated testing of
a weld point where the quality of the weld point is evaluated by
means of a phase image. The phase image that is used is obtained by
means of given parameters which are defined before the sequence of
images to be examined is taken. The quality of the weld joint is
determined by means of clearly defined threshold values. The
disadvantage of this method is that the weld nugget to be evaluated
is not reliably detectable on an industrial scale by means of
clearly defined threshold values.
[0007] It is the object of the invention to provide a method for
automated, contactless and non-destructive testing of a material
joint which allows the molten zone of a material joint to be
reliably detected and evaluated.
[0008] This object is achieved by the features of claim 1. It has
been found according to the invention that the boundary between the
molten zone and the non-molten but still adhering zone is an
additional obstacle for the heat flow. This boundary causes an
abrupt decrease of the heat flow dynamics. Immediately behind this
boundary, the dynamics of the heat flow in the non-molten zone
increases again. In a result image in which the heat flow dynamics
through the material joint is illustrated as intensity values of
the result image, this is shown by an intensity bead which forms at
this point.
[0009] In a first step, this result image is used to determine a
heat flow dynamics of surroundings of the material joint to be
examined, said heat flow dynamics thus forming an image background.
The heat flow dynamics of the image background is for instance
determined by means of a histogram. A value which is lifted from
the heat flow dynamics of the image background is defined as a
minimum dynamic threshold value which defines a region where both
the non-molten zone as well as the molten zone of the material
joint may be located. In a next step, a peak value of the heat flow
dynamics through the material joint is determined in this region,
the peak value being a maximum dynamic threshold value. A dynamic
threshold value that is varied between the minimum threshold value
and the maximum threshold value defines a sequence of regions on
the result image, with each of these regions representing the
values of the heat flow dynamics through the material joint that
exceed the dynamic threshold value. These regions are examined with
respect to their perimeter. The intensity bead causes the outer
shape of these regions to increase abruptly when crossing the
boundary between the molten zone and the non-molten zone. The
perimeter of these regions is measured and represented by a feature
vector, the perimeter being a numerical illustration of the outer
shape of said regions. An abrupt increase in this feature vector
indicates that the associated region has enclosed a portion of the
non-molten zone. This abrupt increase of the feature vector may for
instance be detected by means of conventional methods such as curve
smoothing and curve examination. The boundary between the molten
zone and the non-molten zone of a material joint can thus be
localized dynamically and objectively. In this way the molten zone
is reliably detectable. The detected molten zone is then evaluated
in terms of its position and size.
[0010] If excitation and detection of the heat flow occur on the
same side of the material joint, the heat flow dynamics through the
material joint is reduced when crossing the boundary between the
molten zone and the non-molten zone. In this case, the intensity
bead causes the perimeter of the examined regions to decrease
abruptly when crossing the boundary, with the result that an abrupt
decrease can be observed in the feature vector.
[0011] The described method is generally identical for all mating
parts. Thus material joints of mating parts from identical or
different materials can be examined. Furthermore, the method allows
one to examine weld joints as well as solder joints. These material
joints show a boundary between a welded and a non-welded zone or a
boundary between a soldered and a non-soldered zone, respectively.
Accordingly, an intensity bead develops on the result image which
illustrates local differences in heat conductivity of the material
joint to be examined. Said intensity bead can be detected by means
of conventional signal and image processing methods in order to
detect and evaluate the weld and solder joint.
[0012] Excitation of the material joint and detection of the heat
flow may generally occur on different sides or on identical sides
of the material joint. Accordingly, result images of various types
are evaluated, which illustrate a time and space resolved heat flow
in transmission and/or reflection. It must be ensured that the
measurement results of the heat flow are not considerably
influenced by intensity variations of the excitation source, the
state and properties of the material surface and the material
thickness of the mating parts. Therefore, a result image is used
which does not illustrate absolute values of the heat flow or of
its speed through the material joint to be examined but local speed
differences of the heat flow. A result image of this type may for
instance be a phase image which is obtained using infrared lock-in
thermography. (Theory and Practice of Infrared Technology for
Nondestructive Testing, Xavier P. V. Maldague, John Wiley and Sons,
Inc., 2001). A phase image illustrates the travel time of the
thermal waves when propagating through the material joint, with the
result that differences in heat conductivity of the material joint
occurring between the various image points of the result image
become visible. The local speed differences of the heat flow are
referred to as heat flow dynamics.
[0013] A reference region according to claim 2 ensures that the
minimum threshold value is obtained from the heat flow dynamics of
the image background.
[0014] The maximum threshold value is easily detectable by means of
a test region according to claim 3. The test region may for
instance be located in the center of the region to be examined. The
size of the test region can be determined experimentally, wherein
Shannon's sampling theorem needs to be observed (Industrial Image
Processing, Christian Demant, Bernd Streicher-Abel, Peter
Waszkewitz, Springer-Verlag, 1998). The size of the test region may
for instance be defined to be 3.times.3 pixels.
[0015] Determining the maximum threshold value by means of several
test regions of the same size according to claim 4 is reliable. The
test regions are generated in such a way that a test region with a
defined size is displaced in the region which represents the values
of the heat flow dynamics that exceed the minimum threshold value.
The size of the test regions may for instance be defined to be
3.times.3 pixels.
[0016] Determining the increment size by means of an iterative
process according to claim 5 allows one to find an optimum
increment size. In order to determine an optimum increment size, it
is for instance conceivable to start with an increment size of
1.
[0017] A weld point according to claim 6 which comprises a molten
zone referred to as weld nugget and a non-molten but still adhering
zone referred to as weld glue is an important material joint for
industrial applications; the advantages of the method according to
the invention are therefore particularly evident.
[0018] A material characteristic curve according to claim 7 allows
one to exactly determine the remaining material thickness of a weld
point and therefore of the indentation at the weld point that was
formed by the welding gun. In order to examine the material joint,
the material characteristic curve for the material combination of
the mating parts to be examined needs to be generated in advance.
The material characteristic curve illustrates a nonlinear
dependence of the peak value of the heat flow dynamics from the
remaining material thickness of a weld point. The data required for
this characteristic curve are obtained at various reference weld
points of the same material combination, the reference weld points
having different remaining material thicknesses. The transmitted
component and the dissipated component of the heat flow play
different roles in the determination of the remaining material
thickness. If the remaining material thickness of a weld point is
small, it is the transmitted component of the heat flow that
determines the peak value of the heat flow dynamics. In the case of
a greater remaining material thickness, it is the dissipated
component of the heat flow which is more important. The peak value
measured in the region of the weld nugget can therefore be used to
determine the remaining material thickness. The peak value of the
heat flow dynamics corresponds to the maximum threshold value which
is measured in the region of the weld nugget of the weld point. The
remaining material thicknesses of the reference weld points are
measured using an independent method.
[0019] A comparison with a first limiting value according to claim
8 allows one to detect holes. If the value of the heat flow
dynamics is too high, this indicates the presence of a hole in the
weld point. The first limiting value is determined empirically.
[0020] A comparison with a second limiting value according to claim
9 allows one to detect cavities. If the value of the heat flow
dynamics is too low, this indicates the presence of a cavity in the
weld nugget. A weld nugget of this type is referred to as a burned
out weld nugget. The second limiting value is determined
empirically.
[0021] A development of the method according to claim 10 allows one
to specifically search for surface damages in the region of the
detected weld nugget. The detection of damages is restricted to the
region of the weld nugget so that the method is protected from
producing false results. The further image may for instance be one
of the recorded thermal images or a further result image. The
result image may for instance be an amplitude image which is
obtained using infrared lock-in thermography. The amplitude image
shows the amplitude of the thermal waves when propagating through
the weld point. Depending on the origin, size and position as well
as the combination of the detected defects, the quality of the weld
point can be precisely classified and evaluated. Furthermore, it is
possible to identify and trace the causes of the damaged weld
point, thus allowing a statistical evaluation of the entire welding
process to be performed which may serve as a basis for quality
assurance.
[0022] Further features and advantages of the invention will become
apparent from the description of an embodiment by means of the
drawing in which
[0023] FIG. 1 shows a section through a material joint in the form
of a weld point;
[0024] FIG. 2 is a view of a one-dimensional distribution of a heat
flow dynamics through the weld point;
[0025] FIG. 3 is a view of a first feature vector by means of which
a perimeter of regions to be examined is represented as a function
of the heat flow dynamics;
[0026] FIG. 4 is a view of a second feature vector by means of
which a perimeter of regions to be examined is represented as a
function of the heat flow dynamics; and
[0027] FIG. 5 is a material characteristic curve which represents a
peak value of the heat flow dynamics as a function of a remaining
material thickness.
[0028] A test sample 1 comprises a first mating part 2 and a second
mating part 3 which are interconnected by a material joint 4. The
mating parts 2, 3 may be formed of identical or different materials
having identical or different material thicknesses. The material
joint 4 is a weld point. The following is a description of the
material joint in the form of a weld point 4.
[0029] The weld point 4 forms a two-section joint which consists of
a molten zone 5 and a non-molten zone 6 surrounding said molten
zone 5. Between the molten zone 5 and the non-molten zone 6, there
is a boundary 7 which delimits the molten zone 5 and separates it
from the non-molten but still adhering zone 6. The molten zone is
hereinafter referred to as weld nugget 5 while the non-molten zone
is referred to as weld glue 6. An excitation source 8 and an
infrared sensor 9 are arranged on opposite sides of the test sample
1.
[0030] The test sample 1 and the weld point 4 to be examined are
excited in pulses by means of the excitation source 8. A heat flow
10 is generated which consists of a transmitted component 11 and a
dissipated component 12. The dissipated component 12 is also
referred to as dissipative component. The transmitted component 11
of the heat flow 10 is recorded by means of the infrared sensor 9
in a sequence of thermal images taken one after the other.
[0031] Each of the mating parts 2, 3 of the weld point 4 has an
indentation 13. The indentations 13 are formed by a welding gun
which is used for producing the weld point 4. The indentations 13
define a remaining material thickness M.
[0032] A computing unit 14 is provided for evaluation of the
recorded sequence of thermal images, the computing unit 14 being
connected to the excitation source 8 and the infrared sensor 9. The
sequence of thermal images is developed into result images of
various types. A result image in the form of a phase image
illustrates the heat flow dynamics W through the weld point 4. The
heat flow dynamics W describes the local speed differences of the
heat flow 10 when propagating through the weld point 4, and
therefore the local differences in heat conductivity.
[0033] FIG. 2 shows a one-dimensional distribution 15 of the heat
flow dynamics W along a cross-section coordinate x. The heat flow
dynamics W generally shows a two-dimensional distribution. From a
qualitative point of view, said two-dimensional distribution
corresponds to the one-dimensional distribution 15, the
two-dimensional distribution may however have a non-symmetric and
irregular shape corresponding to the geometry of the weld point
4.
[0034] The two-dimensional distribution of the heat flow dynamics W
is substantially determined by the geometry of the weld point 4.
The geometry of the weld point 4 is impressed by the indentations
13 of the weld gun. The following is a more detailed description of
the one-dimensional distribution 15 of the heat flow dynamics W. In
order to be able to correctly detect the weld nugget, however, it
is the two-dimensional distribution of the heat flow dynamics W
that needs to be evaluated; the following descriptions therefore
apply correspondingly to the two-dimensional distribution.
[0035] In a center of the weld point 4, the heat flow dynamics W
has a peak value, with the heat flow dynamics W decreasing from
this peak value towards the periphery of the weld point 4. It has
been found according to the invention that the boundary 7 between
the weld nugget 5 and the weld glue 6 leads to an additional local
decrease of the heat flow dynamics W, with the heat flow dynamics W
increasing again immediately behind this boundary 7. This effect
results in an intensity bead 16 which is shown in FIG. 2 for the
one-dimensional distribution 15 of the heat flow dynamics W. The
effect of the formation of an intensity bead 16 around the weld
nugget 5 may be non-symmetric and irregular; it is therefore the
two-dimensional distribution of the heat flow dynamics W that needs
to be evaluated in order to correctly detect the weld nugget 5.
[0036] In order to detect the weld nugget 5, a minimum threshold
value W.sub.min is determined in a first step. The minimum
threshold value W.sub.min exceeds the heat flow dynamics W of an
image background H. The minimum threshold value W.sub.min is
obtained from a reference region R of the heat flow dynamics W of
the image background H, the reference region R forming a part of
the image background H.
[0037] Furthermore, a maximum threshold value W.sub.max is
determined which corresponds to the peak value of the heat flow
dynamics W through the weld point 4. The maximum peak value
W.sub.max is obtained from a test region T which is located in the
center of a region S which represents the values of the heat flow
dynamics W exceeding the minimum threshold value W.sub.min. The
maximum threshold value W.sub.max is an average value of the values
of the heat flow dynamics W obtained from the test region T.
[0038] Alternatively, the maximum threshold value W.sub.max may be
obtained from several test regions T of the same size which are
offset relative to each other in the region S, with an average
value of the values of the heat flow dynamics W being obtained from
the test region T for each of the test regions T. The maximum
threshold value W.sub.max is the maximum value of these average
values.
[0039] In order to detect the weld nugget 5, a dynamic threshold
value W.sub.dyn is varied between the minimum threshold value
W.sub.min and the maximum threshold value W.sub.max. The varied
dynamic threshold values are referred to as W.sub.dyn, i, with i=1
to n. The dynamic threshold value W.sub.dyn is varied with an
increment size .DELTA.W.sub.dyn. This means that two subsequent
dynamic threshold values W.sub.dyn, i and W.sub.dyn, i+1 are spaced
from each other by the increment size .DELTA.W.sub.dyn. The optimum
increment size .DELTA.W.sub.dyn can be obtained iteratively.
[0040] An associated region B.sub.i of the heat flow dynamics W
through the weld point 4 is determined for each dynamic threshold
value W.sub.dyn, i, with the region B.sub.i representing the values
of the heat flow dynamics W exceeding the dynamic threshold value
W.sub.dyn, i. Each region B.sub.i has an associated perimeter
U.sub.i which is determined and represented by a feature vector.
All regions B are then examined with respect to an abrupt change in
perimeter .DELTA.U. If the dynamic threshold value W.sub.dyn is
varied between the maximum threshold value W.sub.max and the
minimum threshold value W.sub.min, an abrupt increase of the
perimeter U occurs if a region B to be examined crosses the
boundary 7 between the weld nugget 5 and the weld glue 6. FIG. 2
shows a region B.sub.i with a perimeter U.sub.i when the region
B.sub.i has not yet crossed the boundary 7. FIG. 2 further shows a
region B.sub.i+1 with a perimeter U.sub.i+1 where the region
B.sub.i+1 has already crossed the boundary 7. The region B.sub.i+1
thus encloses a portion of the weld glue 6. The enclosed portion of
the weld glue 6 shows a greater heat flow dynamics W than the
boundary 7. The region B.sub.i is thus the greatest region which
has not yet crossed the boundary 7 and does therefore not enclose a
portion of the weld glue 6. The region B.sub.i therefore
substantially corresponds to the weld nugget 5. The position and
the size of the weld nugget 5 can therefore be evaluated by means
of the region B.sub.i and its associated perimeter U.sub.i.
[0041] Depending on the state of the outer shape of the regions B
to be examined, the abrupt change in perimeter .DELTA.U may be
followed by a rapid decrease in perimeter as shown in FIG. 4, or by
another increase of the perimeter U as shown in FIG. 3. This is the
result of the Poisson effect (Gerthsen Physik, 23.sup.rd edition,
p. 130 ff., Springer Verlag, 2006) which states that an object with
a distinct outer shape tends to either eliminate or further
increase the deviation of its shape from an ideal circle when the
surface area of said object increases. If these deviations are
eliminated when the size of the object increases, this will result
in a reduction of the perimeter. As soon as the deviations are
substantially eliminated, the perimeter of the object will increase
continuously again.
[0042] The abrupt change in perimeter .DELTA.U of the regions B to
be examined is detectable by means of conventional signal and image
processing methods. They allow a non-symmetric and irregular
formation of the intensity bead 16 around the weld nugget 5 to be
detected. A dynamic and adaptive and therefore automated detection
of the weld nugget 5 is thus guaranteed.
[0043] The peak value of the heat flow dynamics W, which is the
maximum threshold value W.sub.max, allows a contactless and
non-destructive examination of the remaining material thickness M
of the weld point 4 to be performed. In order to determine the
remaining material thickness M, a material characteristic curve K
is generated in advance with respect to the material combination of
the mating parts 2, 3 to be examined. The characteristic curve K is
determined by means of reference weld points which have different
remaining material thicknesses, the reference weld points
interconnecting corresponding mating parts 2, 3 which are to be
examined later. For each reference weld point, a remaining material
thickness M is measured by means of an independent method.
Furthermore, a peak value of the heat flow dynamics W is measured
for each reference weld point. The characteristic curve K describes
the nonlinear dependence of the peak value of the heat flow
dynamics W from the remaining material thickness M.
[0044] The characteristic curve K allows one to examine the
remaining material thickness M of the weld point 4 to be examined
and of the indentations 13 at the weld point 4 that were formed by
the weld gun. The maximum peak value W.sub.max of the weld point 4
to be examined is compared with a first limiting value G.sub.1; if
this limiting value G.sub.1 is exceeded, this indicates the
presence of a hole in the weld point 4. Furthermore, the maximum
peak value W.sub.max of the weld point 4 to be examined is compared
with a second limiting value G.sub.2; if said maximum peak value
W.sub.max is less than this limiting value G.sub.2, this indicates
the presence of a cavity in the weld point 4. The limiting values
G.sub.1, G.sub.2 are determined empirically.
[0045] Furthermore, the method allows various surface damages of
the weld point 4 to be detected and evaluated. Detection and
evaluation of surface damages takes place in the detected region
B.sub.i of the weld nugget 5. Surface damages are detected by means
of another image which is provided with a coordinate system that is
identical to that of the result image used to detect the weld
nugget 5. An image of this type may for instance be a thermal image
or an amplitude image which is generated by means of infrared
lock-in thermography (Theory and Practice of Infrared Technology
for Nondestructive Testing, Xavier P. V. Maldague, John Wiley and
Sons, Inc., 2001). The detection of defects occurs exclusively in
the region B.sub.i of the weld nugget 5 so that false results are
avoided. The boundary 7 between the weld nugget 5 and the weld glue
6 is for instance not identified as a defect. The evaluation and
detection of defects may for instance be performed using
conventional signal and image processing methods.
[0046] The weld point 4 can be classified depending on the origin,
size and position as well as the combination of the detected weld
point defects. This allows one to precisely evaluate the quality of
the weld point 4.
[0047] The method according to the invention enables an automated,
contactless and non-destructive examination of a weld point 4 to be
performed on an industrial scale. The method provides for a
comprehensive detection of defects which ensures a precise and
reliable classification of the weld point 4 to be examined.
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