Apparatus for automated crack growth rate measurement

Apparatus for monitoring and measuring the growth of a crack in an elastic specimen. The specimen is repeatedly flexed by a piston at a set frequency and periodically slowed to a substantially lower frequency during which time a line scan camera monitors the length of the crack. The camera is positioned such that the specimen is drawn through the scan line thereof. The camera is calibrated such that the output signal from the camera correlates directly to the cracklength. This output signal is digitized, received, and stored by a digital processor for use in determining the crack growth rate.

TECHNICAL FIELD 
The invention herein resides in the art of testing apparatus, and more 
particularly to such apparatus for testing elastomeric materials such as 
rubber whereby the fatigue crack propagation may be monitored and 
measured. 
BACKGROUND OF THE INVENTION 
In the past, it has been desirable to make life predictions on products 
made of rubber and other low modulus materials. Such materials may be 
tested to determine the resistance of the material to cracks or crack 
propagation. Typically, a strip of the material is placed into a 
reciprocating device such as a mechanical or servo- hydraulic testing 
machine. The strip is slit or precracked at an edge thereof and then 
repeatedly flexed or stretched a predetermined amount, with periodic 
measurements of the growth of the slit or crack being manually taken. From 
these measurements, the crack growth rate could be determined. 
In the past, the periodic measurements were taken visually by an operator 
through a microscope or other optical device appropriately equipped with a 
reticle. The growth of the crack was thereby monitored and recorded as a 
function of the number of flexing cycles imparted to the sample or 
specimen. The growth of the crack length as a function of the number of 
flexing cycles could then be plotted. The derivative of this curve is then 
the growth rate, from which the resistance of the material to crack growth 
may be determined in a well known fashion. 
In the prior art, the operation has been totally manual, relying upon an 
operator to physically start or stop the reciprocating device, visually 
observe the length of the crack in the specimen, determine the number of 
cycles between measurements, determine the crack growth from the last 
measurement, and ascertain the crack growth rate. Being totally manual, 
the prior art approach has been time consuming and given to inaccuracies 
resulting from the subjective operator readings with their inherent human 
error. The prior test could not be run continuously without the presence 
of operators over a number of sequential shifts. When the tests were run 
discontinuously, the results were suspect due to resultant viscoelastic 
transient effects. 
There have been attempts at automating methods of monitoring crack growth 
in a specimen. Applicant is aware of U.S. Pat. No. 4,175,447 which teaches 
the use of reflected light rather than transmitted light to characterize a 
center cracked specimen. Applicant is concerned with edge-cracked 
specimens, monitored with transmitted light, and with particular means for 
following both the apex and mouth of the crack. Such is absent in this 
reference. Similarly, applicant is aware of British Pat. No. 2,057,124 
which uses two-dimensional video mounted on a traveling base which is 
adapted to move parallel to the crack line. The reference fails, however, 
to teach a single dimensional stationary monitoring system and similarly 
fails to follow both the apex and mouth of the crack. 
U.S. Pat. No. 4,418,563 is of general interest, but it incorporates a test 
method using caustics rather than transmitted light and is not 
particularly adapted for automated crack growth measurement. Pat. No. 
3,918,299 presents a method of detecting cracks by utilizing eddy currents 
rather than optical techniques and, in that regard, is of general interest 
only. Pat. No. 3,983,745 uses displacement and force transducers to 
inferentially determine crack length, but is only functional for elastic 
materials, not the time dependent or viscoelastic materials of concern 
herein. Similarly, British Pat. No. 2,108,684 uses light reflected off an 
applied coating for testing cracks in stiff elastic materials and is not 
adapted for the concept presented herein. Finally, an article by Burnos, 
et al appearing on page 305 of Ind. Lab (U.S.A.), Vol. 3, No. 2 (Feb. 
1972) is of very general interest, teaching the production of maximum 
sharpness stress concentration notches in cylindrical specimens. The 
concepts presented in this article are not fatigue related, nor are they 
for monitoring time dependent crack growth. 
DISCLOSURE OF INVENTION 
In light of the foregoing, it is an aspect of the invention to provide an 
apparatus for automated crack growth rate measurement which reduces 
operator errors and bias in the test procedure. 
Another aspect of the invention is the provision of apparatus for automated 
crack growth rate measurement which is substantially unlimited with 
respect to data collection intervals. 
A further aspect of the invention is the provision of apparatus for 
automated crack growth rate measurement which requires less manpower and 
downtime of the test equipment than previously known devices and 
techniques. 
Still a further aspect of the invention is the provision of apparatus for 
automated crack growth rate measurement which allows greater and more 
accurate control of test variables than previously known devices. 
Yet an additional aspect of the invention is the provision of apparatus for 
automated crack growth rate measurement which is readily constructed from 
state of the art elements. 
The foregoing and other aspects of the invention which will become apparent 
as the detailed description proceeds are achieved by apparatus for 
monitoring the growth of a crack in the edge of an elastic specimen, 
comprising: first means for receiving and flexing the specimen; second 
means in juxtaposition to said first means for optically viewing the 
specimen in an area including the crack and generating an output signal; 
and third means connected to said second means for collecting and storing 
data obtained from said output signal. 
Other aspects of the invention are attained by apparatus for monitoring the 
growth of a crack propagating from the edge of an elastic specimen, 
comprising: reciprocating means for receiving and flexing the specimen; a 
line scan camera maintained opposite said reciprocating means and having a 
scan line traversing an edge of the specimen; and processing means 
connected to said reciprocating means and said camera for controlling the 
frequency at which the specimen is flexed and periodically receiving data 
from said camera.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawings and more particularly FIG. 1, it can be seen 
that the crack growth rate measurement system of the invention is 
designated generally by the numeral 10. As shown, the system 10 includes a 
load frame 12, such as MTS Model 810 or 831, which would typically 
comprise a servo-hydraulic testing machine as is presently well known in 
the art. The load frame 12 includes an actuator 14 adapted for controlling 
a reciprocating piston 16. The stroke and frequency of the piston 16 is 
thereby regulated. A load cell or tranducer 18 is axially aligned with the 
piston 16 and positioned thereabove by being secured to the crosshead or 
header 20. Support columns or posts 22 interconnect the header 20 and 
actuator portion 14. 
A sample or specimen of rubber or other elastomeric material is secured by 
clamps between the reciprocating piston 16 and the fixed load cell 18. The 
specimen 24 is characterized by an edge crack 26, the propagation of which 
is of concern under the test accomplished by the structure of the 
invention. The crack 26 is shown in greatly exaggerated proportions in the 
drawing of FIG. 1. In any event, a light source 28 is provided for 
backlighting the specimen 24 which, in the case of rubber, is black, 
giving a good contrast between the backlighting and the specimen. A line 
scan camera 30 is positioned on the opposite side of the specimen 24 from 
the light source 28 and collinear therewith. Preferably, the camera 30 may 
be a Model 300A as manufactured by Optron Corporation, having a resolution 
of 1024 pixels per scan. A camera controller 32, such as Model 300A as 
manufactured by Optron Corporation, converts the pixel information from 
the camera 30 into an analog signal proportional to the number of white 
pixels. 
With reference now to FIG. 2, a more detailed illustration of the crack 
growth rate measurement system 10 may be seen. An actuator controller 34 
interconnects with the actuator 14 to control the displacement and 
frequency of the stroke of the piston 16 to control the frequency and 
displacement throughout the test of the specimen 24. A programmable 
function generator 36 provides an analog signal to the controller 34 to 
achieve such control. 
The heart of the control of the system 10 is the processing unit 38, 
including a digital processor 40 such as the Hewlett Packard HP1000 
processor. The digital processor 40 interconnects through an input/output 
device 46 with the function generator 36, providing to the function 
generator 36 digital signals corresponding to the desired waveform, 
frequency and displacement of the stroke of the piston 16. The function 
generator 36 performs a digitial to analog conversion for application of 
the appropriate control signal to the controller 34. 
Also included as a portion of the processing unit 38 is a multiprogrammer 
42 such as the Hewlett Packard HP6942A. The multiprogrammer 42 includes an 
analog to digital converter to receive analog outputs of the line scan 
camera 30 via the camera controller 32, and subsequentially digitizing the 
same. This digitized output is then provided to the processor 40. Signal 
condition circuitry 44 is interposed between the load frame 12 and the 
multiprogrammer 42 for purposes of scaling the output signals from the 
load frame 12. The outputs from the load frame 12 consist principally of 
an output from a linear variable differential transformer (LVDT) 
indicating the displacement of the piston 16, and an output from the load 
cell 18 indicating the force imparted to the specimen 24 as it is flexed 
by the reciprocation of the piston 16. The signal condition circuitry 44 
includes amplifiers for the purpose of amplifying or attenuating the 
outputs of the load frame 12 to a usable level by the multiprogrammer 42. 
As mentioned above, an input/output device 46 provides for communication 
between the processor 40 and the function generator 36. The device 46 also 
allows for communication between the processor 40 and a graphics terminal 
48 such as a Hewlett Packard HP2623A, consisting of a CRT on which may be 
displayed the output of the camera 30. 
As shown in FIG. 3, a rubber specimen 24, approximately 25 mm in width and 
200 mm in length, is secured by clamps or the like between the 
reciprocating piston 16 and the fixed load cell 18. The specimen 24 is 
precracked as at 50 by means of cutting or slitting the same with a sharp 
instrument such as a razor blade. Typically, the length of the precrack 50 
is approximately equal to the thickness of the specimen, for example 1 mm. 
The line scan camera 30 is positioned with the scan line 52 beneath the 
precrack 50. As can be seen, the scan line is of fixed length, extending 
from approximately the center of the specimen 24 to a point beyond the 
edge of the specimen 24 equivalent to approximately twenty percent of the 
total length of the scan line. As will be discussed hereinafter, as the 
crack 50 grows, the edges of the specimen 24 positioned on either side of 
the crack begin to deflect. This extension of the scan line 52 beyond the 
edge of the specimen 24 assures that such edge will always be within the 
scan line regardless of deflection. As will become further apparent, the 
scan line 52 is maintained below the precrack 50 such that the crack 50 is 
drawn through the scan line 52 when the specimen 24 is flexed as by 
reciprocating movement of the piston 16. 
In operation, the specimen 24 is repeatedly flexed at a fixed frequency and 
displacement of the piston 16, controlled as presented above by the 
controller 34. After a selected number of such cycles, the processor 40 
causes the actuator controller 34 to put out a signal by which the 
specimen 24 is extended very slowly, on the order of 0.1 hz, having a 
cycle period of 10 seconds. During this slow test cycle, the crack 26 is 
extended into and beyond the scan line 52 of the camera 30 and thence 
returned. During this test cycle, the processor 40 receives the output of 
the line scan camera 30 and measures the length of the crack 26 at that 
point in time in a manner to be discussed hereinafter. 
At the beginning of each cycle, the function generator 36 puts out a sync 
signal. Upon the beginning of the slow test cycle, the sync signal enables 
the processor 40 to receive the output of the camera 30 via the camera 
controller 32. This output is digitized by the A/D converter of the 
multiprogrammer 42. The camera is enabled for the entire test cycle to 
obtain the output data shown in FIG. 5, to be discussed hereinafter. After 
obtaining the data from the test cycle, the frequency of the piston 16 is 
reinstated to the excitation frequency for crack propagation. Typically, 
such frequency is on the order of 3 hz. The specimen 24 is then cycled at 
this frequency for a selected number of cycles which has been determined 
to be sufficient for meaningful crack growth since the last monitoring. At 
that time, a test cycle is again entered with the piston 16 being slowed 
to 0.1 hz, and the processor 40 enabled to receive digitized signals from 
the camera 30 via the controller 32 and multiprogrammer 42. Hence, the 
growth of the crack 26 is monitored as a function of the number of cycles 
of flexing imparted to the specimen 24. 
As mentioned above, FIG. 4 presents a view of the specimen 24 during the 
test cycle in which data is taken by the processor 40. It will be noted 
that as the specimen 24 is stretched between the piston 16 and the 
stationary load cell 18, the crack 26 passes across the scan line 52 of 
the stationary camera 30. During the test cycle, the movement of the 
piston 16 actually draws the crack 26 from the position as shown in FIG. 3 
below the crack to a substantially equal position above the crack and 
returns the same to the initial position. As shown in FIG. 4, the scan 
line 52 of the camera falls across the apex 54 of the crack 26. 
Peculiarities of the specimen 24 may be noted in FIG. 4. For example, the 
specimen 24 curves inwardly or experiences in-plane deflection as at 56, 
at the back of the specimen 24 opposite the crack 26. This deflection is 
of a concave nature. In the same manner, the flexing of the specimen 24 
results in convex deflection on each side of the crack 26 at the front of 
the specimen, forming lips as designated by the numeral 58. This in-plane 
deflection results from a weakening of the specimen 24 due to the presence 
of the crack 26. The actual length of the crack 26 at any point in time is 
accordingly the distance along the scan line from the apex 54 to the 
normal point on the scan lihe 52 taken from a maximum point of deflection 
58. In other words, the deflection 56, 58 results in the crack 26 actually 
moving to the right as shown in FIG. 4. Accordingly, if the crack length 
were taken merely as the distance from the apex 54 to the line of the 
right edge of the specimen 24, the measurement would be inaccurate for 
failing to take into account the deflection of the specimen at the crack. 
Accordingly, the instant invention seeks to determine the crack length as 
the distance between the apex 54 and the point on the scan line 52 which 
would be intercepted by a line drawn between the points of maximum 
deflection 58 on each side of the mouth of the crack. Typically, this line 
would be normal to the scan line 52. 
It should also be understood, with reference to FIG. 4, that the scan line 
52 makes a transition from the totally dark field of the specimen 24 to 
the substantially light field resulting from the backlighting of the 
specimen by means of the light source 28. Accordingly, good resolution is 
achieved as to the point at which the specimen 24 ends and the crack 26 
begins. 
With reference now to FIG. 5, a trace of the data obtained by the processor 
40 during the 10 second cycle of the 0.1 hz test cycle is shown. It will 
be appreciated by those skilled in the art that the lens of the camera 30 
is calibrated such that the camera output voltage correlates to the number 
of white pixels on the scan line 52. Each pixel correlates to a specific 
length. Accordingly, the voltage output of the camera 30 via the 
controller 32 correlates to the specific length along the scan line 52 
which is characterized by white pixels. 
As shown in FIG. 5, the output of the camera 30 has a quiescent level of 
approximately 2.0 mm. This correlates to the extension of the scan line 52 
beyond the right edge of the specimen 24 as shown in FIG. 3. At the start 
of the cycle, as shown in FIG. 3, the number of black pixels is 
significant since the major portion of the scan line 52 is across the 
black specimen 24. As the specimen 24 is extended, the number of black 
pixels increases as at 62 due to the deflection of the specimen as at the 
lips 58, shown in FIG. 4. The number of black pixels hits the maximum as 
at 64 corresponding to the maximum point of deflection of the specimen 24 
which would be that edge of the specimen immediately adjacent the opening 
or mouth of the crack 26. As the crack 26 comes into the scan line the 
number of white pixels begins to increase dramatically as at 66 to a 
maximum at 68, corresponding to an in-line relationship between the scan 
line 52 and the apex 54. As the specimen is further extended by the piston 
16 beyond the apex 54, the number of light pixels decreases as at 70 to a 
mimimum as at 72, again due to the deflection at the upper lip 58 on the 
top side of the crack 26 as shown in FIG. 4. The number of black pixels 
then decreases as at 74, again due to the deflection 58, to the quiescent 
state 76, completing the first half of this test or monitoring cycle. On 
the upward stroke of the piston 16, the trace of FIG. 5 repeats itself for 
the reasons just described such that the second half of the cycle is a 
mirror image of the first half. 
The in-line deflection and resultant movement of the lips 58, as just 
described, effectively shifts the crack 26 to the right as shown in FIG. 
4. Accordingly, if one merely monitored the position of the apex 54 to 
determine cracklength and growth rate, the measurements would be in error 
by an amount dependent upon the degree of shift of the apex as a result of 
in-plane deflection. Such error has been found to be on the order of 
twenty percent. 
The processor 40 is set to take data from the spike defined by the points 
64-72, this spike corresponding to the length of the crack 26 at the time 
of measurement. To do this, the processor 40 defines a band 78 which 
encompasses the spike of interest. The processor 40 may establish the band 
78 by taking the difference of overlapping groups of data points, finding 
where the difference increases greatly such as at 64-72, and then setting 
the band on either side of these points. In other words, the location for 
the band may be determined by maximizing the sum of the absolute 
differences of the displacement data for adjacent points. Of course, the 
band 78 could also be established by taking the derivative of the curve 
established by the data points and determining where that derivative or 
slope greatly increases. It should be appreciated that the band 78 is 
established to preclude anomalous minimum points in the trace of FIG. 5 
resulting from buckling of the specimen 24 as by out-of-plane-deflection 
during flexure of the specimen. 
Having established the band 78, the processor 40 establishes the length of 
the crack 26 for that particular test by determining the absolute length 
thereof as the maximum amplitude distance of separation between data 
points in the band 78. For example, the length of the crack in FIG. 5 
would be determined on the basis of the measurement from the peak 68 to 
the trough 72. Typically, the points 64, 72 would lie on the same 
horizontal line, but signal noise may account for a slight difference 
between the two. In any event, such a measurement guarantees that the 
measurement of the length of the crack 26 is unaffected by the deflection 
58. 
The length of the crack monitored at each test is recorded, as is the 
number of cycles or time elapsed between monitoring tests, such that the 
growth of the crack can be readily determined. 
With reference now to FIG. 6, the control program of the processor 40 may 
be seen in flow chart form. As demonstrated, the operator begins by 
providing to the processor 40 the conditions of the test such as the 
identity of the material, the test temperature, the specimen number, a 
frequency at which the piston 16 is to reciprocate, and the waveform 
output of the function generator 36. Similarly, the files in which data is 
to be placed are named, and the output devices such as the graphics 
terminal 48 are identified. 
The test is then initiated by measuring the input energy to the specimen 
24. As is well known in the art, the input energy to the specimen is the 
area under the curve achieved by a plot of the load sensed by the load 
cell 18 and the displacement of the specimen 24 as determined by the 
stroke of the piston 16. The input energy is stored by the processor for 
use in graphically plotting the fatigue crack propagation resistance of 
the material by plotting the crack growth rate versus the energy release 
rate in a manner well known in the art. While the data acquired from the 
apparatus and method disclosed herein is used for determining the 
characteristic fatigue crack propagation resistance of the material, it is 
the method and apparatus used for acquiring the data which is of 
importance herein and not the actual calculation which is well known in 
the industry. In any event, it should be appreciated that to determine the 
fatigue crack propagation resistance of the material, the input energy to 
the material at the time of the measurement of the crack is important. 
The system then determines the number of flexing cycles imparted to the 
specimen 24 since the beginning of the test or since the last crack growth 
measurement was made. The system is then slowed to the monitoring 
frequency of approximately 0.1 hz. The lamp 28 is energized through a 
relay contact in the multiprogrammer 42 and the trace of the camera is 
digitized as by the controller 32 and multiprogrammer 42 discussed above. 
This digitized trace is stored in the processor 40 for the ten second 
duration of the monitoring cycle at 0.1 hz. The light or lamp 28 is then 
turned off. With the voltage output of the camera controller 32 being a 
function of length, the crack length is determined in the method described 
above with respect to FIG. 5. If desired, the crack contour may then be 
displayed on the graphics terminal 48. The processor 40 then updates its 
storage positions in memory with respect to crack length, energy density, 
and number of cycles of specimen flexing. This information may then be 
printed out, if desired. 
The test of the instant invention is set such that the test terminates when 
the length of the crack 26 exceeds 20% of the entire width of the specimen 
24. Of course, any desired criteria for terminating the test may be 
selected. If the crack length does not exceed the preset threshold, the 
width of the test band 78 is adjusted to accommodate the expanding width 
of the growing crack 26. This adjustment may be accomplished in any of 
several manners as discussed above. With the test band having been set, 
the processor 40 projects the number of cycles necessary to achieve a 
desired crack length increment. The actuator 14 is restarted to flex the 
specimen 24 such desired number of cycles. The time period of such flexing 
is measured and multiplied by the frequency to determine when the 
requisite number of cycles have been accomplished. At that time, the input 
energy is again measured, the number of cycles determined, and the 
monitoring cycle is entered into to obtain new information as to the crack 
growth resulting from the recent number of flexures. 
The device and apparatus just discussed is capable of automatically 
achieving crack growth measurements in a test specimen. The crack growth 
rate may then be determined in standard fashion and, having available the 
input energy to the specimen, the fatigue crack propagation resistance of 
the material may also be demonstrated. The foregoing test is accomplished 
with a minimum of operator interface and without the subjectivity of 
operator readings and control. The calibrated stationary camera 30 
achieves reliable test measurements which have been found to closely track 
those measurements obtained through the utilization of a microscope fitted 
with a reticle as first described herein. 
Thus it can be seen that the objects of the invention have been achieved by 
the structure and techniques presented hereinabove. While in accordance 
with the patent statutes only the best mode and preferred embodiment of 
the invention has been presented and described in detail, it is to be 
understood that the invention is not limited thereto or thereby. 
Accordingly, for an appreciation of the true scope and breadth of the 
invention reference should be had to the following claims.