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Ultrasonic detection of fatigue damage
EngrnemnpFmctur~Mechnnm. 1972.Vol. 4,pp.577-583.Per~amon Press.
ULTRASONIC
DETECTION
NARAYAN
PrintedinGreat BriIain
OF FATIGUE
DAMAGE?
R. JOSH1 and ROBERT E. GREEN. Jr
Department of Mechanics and Materials Science. The Johns Hopkins University. Baltimore, Maryland 21218, U.S.A. .4bstract- Results are reported of recent experiments which used change in ultrasonic attenuation measurements as a continuous monitor of fatigue damage during cyclic testing of polycrystalline aluminum and steel specimens. Ultrasonic pulses were generated by an x-cut quartz transducer firmly attached to the clamped end of a specimen rod using Eastman 910 cement. The frequencies of these ultrasonic pulses were IO MHz for polycrystalline aluminum and 5 MHz for cold rolled steel. The specimen shape, transducer size, and frequency used insured that the entire specimen was completely filled with ultrasound in a guided wave mode. The specimen was fatigued as a cantilever beam in reverse bending at 1800 cycles per minute with vertical amplitude peak-to-peak set at a fixed value in the range 7.5-15 mm. In a typical test the ultrasonic attenuation initially remained constant, increased slowly, and then increased catastropically just prior to fracture of the test specimen. All experiments performed on both aluminum and steel specimens at various vibrational amplitudes yielded similar results in that ultrasonic attenuatton served as a very sensitive indicator of fatigue damage and in every case indicated that failure was eminent several hours before conventional ultrasonic testing could detect an additional echo caused by energy reflected from a crack. These results strongly suggest that ultrasonic attenuation measurements can be exploited successfully to predict earlier fatigue damage and perhaps fatigue life in practical applications.
INTRODUCTION though ultrasonic methods have found extensive application in various areas of practical non-destructive testing, the vast majority of these applications have been based on monitoring wave travel times and not on monitoring attenuation of the wave. Typically, the measured arrival time of an ultrasonic pulse reflected from the side of a test specimen opposite to that from which it was transmitted, coupled with a knowledge of the elastic wave speed in the test material, serves to measure the thickness of the test specimen. A similar method serves to detect a structural flaw in the test piece provided that the flaw is of sufficient size and correct orientation to reflect some of the ultrasonic energy back to a detecting transducer. As useful as such travel time measure ments have become, they yield only part of the practical information that can be obtained from ultrasonic tests. The amplitude of the returning pulse compared with the amplitude of the transmitted pulse can also yield useful information since the decrease in amplitude of the reflected pulse is a measure of the energy lost from the transmitted wave in propagating through the test specimen. It is customary to specify this energy loss in terms of ultrasonic attenuation. In metallic specimens various mechanisms can contribute to the energy loss or apparent energy loss experienced by the propagating wave [ 1,2]. Among these mechanisms are thermoelastic or heating effects, grain boundary scattering, acoustic diffraction, dislocation damping, interaction with ferromagnetic domain walls, and scattering by point defects. The multiplicity of these mechanisms makes absolute attenuation measurements very difficult to interpret. However, the only loss mechanism which is markedly altered by plastic deformation is dislocation damping. It has been well documented[3-51 for all types of fracture that nucleation of cracks occurs as a result of inhomogeneous plastic deformation in microscopic regions. This EVEN
*Presented at the Symposium on Fracture and Fatigue at the School of Engineering and Applied Science, George Washington University. Washington, D.C., May 3-5, 1972. 577
578
N. R. JOSHI
and
R. E. GREEN
inhomogeneous plastic deformation can be in the form of twinning, slip bands, deformation bands, or localized concentrations at inclusions and grain boundaries. Moreover, the mechanisms responsible for these regions of inhomogeneous plastic deformation are all based on dislocation interactions. In particular, dislocation interactions with point defects, with other dislocations, with stacking faults, with grain boundaries, and with inclusions are known to create regions of severe localized plastic deformation, which develop into microcracks, and these, in turn, grow to sufficient size to cause fracture. Therefore, since ultrasonic attenuation is extremely sensitive to dislocation motion and since dislocation motion is a prerequisite to fracture, measurements of change in ultrasonic attenuation permit monitoring of dislocation motion and subsequent crack formation. Moreover, ultrasonic attenuation measurements coupled with more conventional travel time measurements permit monitoring of crack growth. As early as 1956, True11 and co-workers[6] observed changes in ultrasonic attenuation in the early stages of fatigue cycling on a polycrystalline aluminum specimen and subsequently made similar measurements up to fracture. In every case the attenuation changed as a function of the number of cycles and for tests carried out to fracture there was a marked increase in attenuation just prior to failure. Although only a small number of fatigue tests were performed and all of these used aluminum specimens, the results indicated the usefulness of such measurements for prediction of fatigue failure. Other workers[7-121 continued these investigations until 1964, when despite encouraging results, such tests were stopped and, to the best of the present authors’ knowledge. no definitive effort has been made to develop a technique for detection of the onset of fatigue damage based on ultrasonic attenuation measurements since that time. In the present paper results are reported of recent experiments which used change in ultrasonic attenuation measurements as a continuous monitor of fatigue damage during cyclic testing of metal specimens. EXPERIMENTAL
PROCEDURE
schematic diagram of the experimental arrangement used is shown in Fig. 1. pulses were generated by an x-cut quartz transducer firmly attached to the clamped end of a polycrystalline aluminum or steel bar using Eastman 9 10 cement. A
Ultrasonic
Krouse
Fatigue
Machine
Qdzl Movie Camera
Polaroid Camera
Oscilloscope No I
Fig.
I, Schematic
of experimental
Oscilloscope No 2
apparatus.
579
Ultrasonic detection of fatigue damage
The specimen shape. transducer size and frequency used insured that the entire specimen was completely filled with ultrasound in a guided wave mode. The specimen was held in a horizontal plane and was displaced vertically at the opposite end by a fixture attached to cam of the fatigue machine. In this manner the specimen was fatigued in reverse bending as a cantilever beam to fracture at 1800 cycles per min. with a vertica1 amplitude peak-to-peak which was set at a fixed value in the range 7.5 mm to I5 mm. The specimens were either made from 606 1-T6 polycrystalline aluminum or from cold rolled IO 15 polycrystalline steel. The dimensions of the specimens used are shown in Fig. 2. The portion of the test specimen to the left of the dotted line in Fig. 2 was clamped rigidly in the fatigue machine fixed end fixture. The cyclic displacement was
U
Ii
I
f
12”
I
i
(b)
Fig. 2. Specimen geometry (a) aluminum (b) steel.
applied in all cases to the far right end. Ali specimens tested were made from the same lot of aluminum or steel in order to minimize differences in alloy composition, cold work, heat treatment, and surface finish. Prior to fatigue testing, each specimen was ultrasonically tested for internal flaws and microscopically examined for any surface defects or scratches. A Matec Model 6600 pulse modulator and receiver incorporating a Model 950-B radio frequency plug-in was used to generate and receive 5 MHz pulses for use with the steel specimens and IO MHz pulses for use with the aluminum specimens. A pulse width of 3 psec and a repetition rate of 200 per sec. were used in all cases. X-cut quartz transducers possessing resonance frequencies of either 5 MHz or 10 MHz were used to generate longitudinal waves in the steel and aluminum specimens, respectively. A Matec Automatic Attenuation Recorder Model 2470, with automatic gain control, was used to continuously record the ultrasonic attenuation as measured from the first two echoes in the pulse-echo pattern. Two oscilloscopes were used to display the pulseecho patterns. One oscilloscope was used to visually monitor the pulse-echo pattern during the test and permitted intermittent recording of the pattern using an oscilloscope camera with a Polaroid film cassette. The pulse-echo pattern displayed on the second oscilloscope was photographed in time lapse fashion ( 1 frame/l 5 set to I frame/5 min) using a 16 mm Cine-Kodak Special II motion picture camera actuated by a solenoid-plunger device controlled by a Model MC5E50 Samenco movie control
580
N. R. JOSH1
and
R. E. GREEN
unit. in this manner the pulse-echo pattern was recorded throughout the entire test and permitted comparison of attenuation values as obtained from analysis of the photographic record with that recorded by the automatic attenuation recorder. Moreover. examination of the movie film permitted accurate determination of the appearance of an additional pulse due to ultrasonic wave reflection from a crack. Each fatigue test at a given amplitude was repeated five times for aluminum and three times for steel. A total of 30 tests were run using aluminum specimens and 15 tests using steei specimens. EXPEWAL RESULTS Figure 3 shows the results of a typical experiment for an auburn specimen tested at a constant vibration amplitude peak-to-peak of 7,5 mm. It can be seen that the attenuation began to increase at about 24 million cycles and proceeded catastropically to fracture at about 3.5 million cycfes. The 2.8 million cycle mark occurred at about 26 hours and failure occurred after about 33 hr. Thus the ultrasonic attenuation measureTime (Units gf Hours) Scale Chcmge
0L
0.64 i
1.29 I
1.94 t
2.16 f
Number of Cycles Fi.
3.
2.52 /
3.02
329 1
3F58 8
388
( Units of IO61
Typicalptot of ukrasonicattenuationvs. number of cycbs for ~u~num.
ments gave a strong indication of the onset of fatigue failure at approximatefy 7 hr prior to the occurrence of the fracture itself. Conventions ultrasonic monitoring was unable to detect any additional echoes due to energy reflected from the crack until about 2 hr prior to failure. Thus, for this particular experiment, ultrasonic attenuation indicated that faiI_ was eminent 5 hr before conventions ultrasonic testing could indicate any crack formation. Figure 4 shows the results of a similar experiment for a steel specimen tested at a constant vibration amplitude peak-to-peak of 7~5mm. In this case the attenuation began to increase at about 6 x lo5 cycles and the increase proceeded catastropically to fracture at about 9 x 105 cycles. The 6 x lo5 cycle mark occurred at about 6 hr and failure occurred after about 8 hr. Conventional ultrasonic monitoring was able to detect an additional echo at the 7 hr mark. Thus, ultrasonic attenuation measurements gave warning 1 hr earlier than ultrasonic pulse detection in this case.
Ultrasonic Time (Units of Hours
b _-
i
581
detection of fatigue damage
)
I
4
6
:, 6 hrr ‘49min
I
ib
F 6hn:OSmm
I.’0
Number of Cycles (Units of 105)
Fig. 4. Typical plot of ultrasonic attenuation vs. number of cycles for steel.
Figure 5 summarizes the experimental results obtained from ultrasonic monitoring during the fatigue tests of the aluminum specimens. The ordinate axis in this figure corresponds to the maximum stress cr on the outermost fiber of the bar when bent to the maximum amplitude chosen for the particular test. It was obtained by substitution of the various maximum deflection values, y, into the linear elastic bending formula, 3Ed ==FY, where E is Young’s modulus, d is the thickness of the bar in the bending direction, and 1 is the length of the bar. The upper abscissa axis indicates the test time in hr. The Tiie
457
( Units of Hours
I
?
4
6
$
IO
I5
22
24
26
2%
30
32
-I
14.12 C&
c
3.66
oAD
OAO
3.20 .
5 2.74 L t
5r” E
A
0
0
2.29
II
A
0
‘ii
0.216
0.432
0.640
2592 Number
of Cycles (Units
I
0.864 3.024
1.06
1.296 3.456
of IO61
Fig. 5. Experimental results obtained with aluminum specimens 0 0.4 db ultrasonic attenuation change; A additional pulse observed; q fracture occurred.
582
N. R. JOSH1
and R. E. GREEN
portion of this scale ranging from 22-32 hr should be used with the tests run at a maximum fiber stress of 2.29 x IO4 psi; the scale ranging from O-12 hr should be used with all other tests. The lower abscissa in Fig. 5 indicates the number of fatigue cycles which elapsed before various events were detected. The portion of this scale ranging from 2.59 X lo6 cycles to 346 x lo6 cycles should be used with the single test run at a maximum fiber stress of 2.29 x lo4 psi; the scale ranging from O-l -296 x 10” cycles should be used withal1 other tests. The entries on Fig. 5 represented by circles indicate the average time elapsed in 5 similar tests before an O-4 db change in attenuation was observed. The criterion of O-4 db change was selected as an early warning signal since this amount of change could be clearly distinguished from background fluctuations on the *attenuation recording strip chart in all aluminum and all steel tests. The entries on Fig. 5 represented by triangles indicate the average time elapsed in 5 similar tests before detection of an additional echo in the pulse-echo pattern due to energy reflected from a crack in the test specimen. The entries on Fig. 5 represented by squares indicate the average time elapsed in 5 similar tests before fracture occurred. It can be clearly seen that in all cases the change in ultrasonic attenuation indicated that failure was eninent before conventional ultrasonic testing could detect an additional echo caused by energy reflected from a crack. Moreover, the smaller the amplitude of vibration and hence the longer the fatigue test. the more absolute time there is between the ultrasonic attenuation warning and fracture of the test specimen. Figure 6 summarizes the experimental results obtained from ultrasonic monitoring during the fatigue tests of the steel specimens. The ordinate axis corresponds to the maximum stress on the outermost fiber of the bent bar as given by equation (I). The upper abscissa indicates the test time in hours, while the lower abscissa indicates the number of fatigue cycles which elapsed before various events were detected. The entries on Fig. 6 represented by circles indicate the average time elapsed in 3 similar tests before an O-4 db change in attenuation was observed. The entries represented by triangles indicate the average time elapsed in 3 similar tests before detection of an additional pulse due to reflection of energy from a crack. The entries represented by squares indicate the average time elapsed in 3 similar tests before fracture occurred. The results are similar to those obtained with aluminum in that ultrasonic attenuation gave early warning of fatigue failure. f
Time
$
4
(Units of Hours) 4 2I
8I
6
IO
12 /
108
1.296
z _9$& m C s b
8.0*
!j -7.0
q
OAo
z z -6.0 .g E z ‘Z
s
OA
-50
0
cl 0.216
0.432 1
A
0.648
0
0 064
Number of Cycles (Units of IO? Fig. 6. Experimental results obtained with steel specimens ci 0.4 db ultrasonic attenuation change: A additional pulse observed: P fracture occurred.
Ultrasonic detection of fatigue damage
583
CONCLUSIONS The attenuation of an ultrasonic pulse propagating through an aluminum or steel bar subjected to cyclic loading initially remained constant, increased slowly, and then increased catastropically just prior to fracture of the test specimen. Ultrasonic attenuation served as a very sensitive indicator of fatigue damage and indicated that failure was eminent before conventional ultrasonic monitoring could detect an additional echo due to energy reflected from a crack. These results strongly suggest that ultrasonic attenuation measurements can be exploited successfully to predict early fatigue damage and perhaps fatigue life in practical applications. - This research was supported in part by the Air Force Office of Scientific Research and by the Middle Atlantic Power Research Committee. Special thanks in this regard are due to Dr. Jacob Pomerantz. Mr. Milton Rogers. and to Mr. Robert Fitzgerald. The authors would like to thank Mrs. Corinne Harness for typing the manuscript.
A cknon4edgements
REFERENCES and the Properries of Solids D. Van Nostrand. New York (1958). [2] R. True& C. Elbaum. and B. B. Chick. Ultrasonic Methods in Solid S:ate Physics. Academic Press.
[I] W. P. Mason,
Physical
Acoustics
New York t 1969). [3] G. Sines and J. L. Waisman. Metal Far&e. McGraw-Hill. New York (1959). [4] D. C. Drucker and J. J. Gilman (editors), Fracture of Solids. Gordon & Breach, New York (1963). [5] J. J. Burke. N. L. Reed and V. Weiss (editors). Fufigue-An Interdiscip/inav Approach. Syracuse University Press (I 964). [6] R. True11 and A. Hikata. Fatigue in 2S Aluminum as Observed by Ultrasonic Attenuation Methods, Watertown Arsenal Technical Reporf No. WAL 143114-47 (1956). [7] R. True11 and A. Hikata, Fatigue and Ultrasonic Attenuation. American Society for Testing Materials. Special Technical Publication No. 2 13 (1957). [8] R. Truell. B. Chick, A. Picker and G. Anderson, The Use of Ultrasonic Methods to Determine Fatigue Effects in Metals. WALK’ Technical Report 60-920 ( 196 1). [9] R. Truell. B. Chick. G. Anderson. C. Elbaum and W. Findley, Ultrasonic Methods for the Study of Stress Cycling Effects in Metals. WADD Technical Report60-920 ( I961 ). [IO] W. J. Bratina and D. Mills. Study of Fatigue in Metals Using Ultrasonic Technique. Cunudiun Mer. Quart. 1, 83-97
(1962).
[ 1 I ] B. Chick. A. Hikata. G. Anderson. W. Findley. C. Elbaum and R. True]], Ultrasonic Methods in the Study of Fatigue and Deformation in Single Crystals. Wright-Purrerson AFB Report No. ASD-TDR62-186 Pt. II, AD No. 408704(1963). [ I21 Z. Pawlowski. Ultrasonic Attenuation during Cyclic Straining. Proceedings Fourth International Conference on Nondesrructive Tesfing 1963. Butterworths. London. 192-195 ( 1964).
(Received
3 April 1972)
ULTRASONIC
DETECTION
NARAYAN
PrintedinGreat BriIain
OF FATIGUE
DAMAGE?
R. JOSH1 and ROBERT E. GREEN. Jr
Department of Mechanics and Materials Science. The Johns Hopkins University. Baltimore, Maryland 21218, U.S.A. .4bstract- Results are reported of recent experiments which used change in ultrasonic attenuation measurements as a continuous monitor of fatigue damage during cyclic testing of polycrystalline aluminum and steel specimens. Ultrasonic pulses were generated by an x-cut quartz transducer firmly attached to the clamped end of a specimen rod using Eastman 910 cement. The frequencies of these ultrasonic pulses were IO MHz for polycrystalline aluminum and 5 MHz for cold rolled steel. The specimen shape, transducer size, and frequency used insured that the entire specimen was completely filled with ultrasound in a guided wave mode. The specimen was fatigued as a cantilever beam in reverse bending at 1800 cycles per minute with vertical amplitude peak-to-peak set at a fixed value in the range 7.5-15 mm. In a typical test the ultrasonic attenuation initially remained constant, increased slowly, and then increased catastropically just prior to fracture of the test specimen. All experiments performed on both aluminum and steel specimens at various vibrational amplitudes yielded similar results in that ultrasonic attenuatton served as a very sensitive indicator of fatigue damage and in every case indicated that failure was eminent several hours before conventional ultrasonic testing could detect an additional echo caused by energy reflected from a crack. These results strongly suggest that ultrasonic attenuation measurements can be exploited successfully to predict earlier fatigue damage and perhaps fatigue life in practical applications.
INTRODUCTION though ultrasonic methods have found extensive application in various areas of practical non-destructive testing, the vast majority of these applications have been based on monitoring wave travel times and not on monitoring attenuation of the wave. Typically, the measured arrival time of an ultrasonic pulse reflected from the side of a test specimen opposite to that from which it was transmitted, coupled with a knowledge of the elastic wave speed in the test material, serves to measure the thickness of the test specimen. A similar method serves to detect a structural flaw in the test piece provided that the flaw is of sufficient size and correct orientation to reflect some of the ultrasonic energy back to a detecting transducer. As useful as such travel time measure ments have become, they yield only part of the practical information that can be obtained from ultrasonic tests. The amplitude of the returning pulse compared with the amplitude of the transmitted pulse can also yield useful information since the decrease in amplitude of the reflected pulse is a measure of the energy lost from the transmitted wave in propagating through the test specimen. It is customary to specify this energy loss in terms of ultrasonic attenuation. In metallic specimens various mechanisms can contribute to the energy loss or apparent energy loss experienced by the propagating wave [ 1,2]. Among these mechanisms are thermoelastic or heating effects, grain boundary scattering, acoustic diffraction, dislocation damping, interaction with ferromagnetic domain walls, and scattering by point defects. The multiplicity of these mechanisms makes absolute attenuation measurements very difficult to interpret. However, the only loss mechanism which is markedly altered by plastic deformation is dislocation damping. It has been well documented[3-51 for all types of fracture that nucleation of cracks occurs as a result of inhomogeneous plastic deformation in microscopic regions. This EVEN
*Presented at the Symposium on Fracture and Fatigue at the School of Engineering and Applied Science, George Washington University. Washington, D.C., May 3-5, 1972. 577
578
N. R. JOSHI
and
R. E. GREEN
inhomogeneous plastic deformation can be in the form of twinning, slip bands, deformation bands, or localized concentrations at inclusions and grain boundaries. Moreover, the mechanisms responsible for these regions of inhomogeneous plastic deformation are all based on dislocation interactions. In particular, dislocation interactions with point defects, with other dislocations, with stacking faults, with grain boundaries, and with inclusions are known to create regions of severe localized plastic deformation, which develop into microcracks, and these, in turn, grow to sufficient size to cause fracture. Therefore, since ultrasonic attenuation is extremely sensitive to dislocation motion and since dislocation motion is a prerequisite to fracture, measurements of change in ultrasonic attenuation permit monitoring of dislocation motion and subsequent crack formation. Moreover, ultrasonic attenuation measurements coupled with more conventional travel time measurements permit monitoring of crack growth. As early as 1956, True11 and co-workers[6] observed changes in ultrasonic attenuation in the early stages of fatigue cycling on a polycrystalline aluminum specimen and subsequently made similar measurements up to fracture. In every case the attenuation changed as a function of the number of cycles and for tests carried out to fracture there was a marked increase in attenuation just prior to failure. Although only a small number of fatigue tests were performed and all of these used aluminum specimens, the results indicated the usefulness of such measurements for prediction of fatigue failure. Other workers[7-121 continued these investigations until 1964, when despite encouraging results, such tests were stopped and, to the best of the present authors’ knowledge. no definitive effort has been made to develop a technique for detection of the onset of fatigue damage based on ultrasonic attenuation measurements since that time. In the present paper results are reported of recent experiments which used change in ultrasonic attenuation measurements as a continuous monitor of fatigue damage during cyclic testing of metal specimens. EXPERIMENTAL
PROCEDURE
schematic diagram of the experimental arrangement used is shown in Fig. 1. pulses were generated by an x-cut quartz transducer firmly attached to the clamped end of a polycrystalline aluminum or steel bar using Eastman 9 10 cement. A
Ultrasonic
Krouse
Fatigue
Machine
Qdzl Movie Camera
Polaroid Camera
Oscilloscope No I
Fig.
I, Schematic
of experimental
Oscilloscope No 2
apparatus.
579
Ultrasonic detection of fatigue damage
The specimen shape. transducer size and frequency used insured that the entire specimen was completely filled with ultrasound in a guided wave mode. The specimen was held in a horizontal plane and was displaced vertically at the opposite end by a fixture attached to cam of the fatigue machine. In this manner the specimen was fatigued in reverse bending as a cantilever beam to fracture at 1800 cycles per min. with a vertica1 amplitude peak-to-peak which was set at a fixed value in the range 7.5 mm to I5 mm. The specimens were either made from 606 1-T6 polycrystalline aluminum or from cold rolled IO 15 polycrystalline steel. The dimensions of the specimens used are shown in Fig. 2. The portion of the test specimen to the left of the dotted line in Fig. 2 was clamped rigidly in the fatigue machine fixed end fixture. The cyclic displacement was
U
Ii
I
f
12”
I
i
(b)
Fig. 2. Specimen geometry (a) aluminum (b) steel.
applied in all cases to the far right end. Ali specimens tested were made from the same lot of aluminum or steel in order to minimize differences in alloy composition, cold work, heat treatment, and surface finish. Prior to fatigue testing, each specimen was ultrasonically tested for internal flaws and microscopically examined for any surface defects or scratches. A Matec Model 6600 pulse modulator and receiver incorporating a Model 950-B radio frequency plug-in was used to generate and receive 5 MHz pulses for use with the steel specimens and IO MHz pulses for use with the aluminum specimens. A pulse width of 3 psec and a repetition rate of 200 per sec. were used in all cases. X-cut quartz transducers possessing resonance frequencies of either 5 MHz or 10 MHz were used to generate longitudinal waves in the steel and aluminum specimens, respectively. A Matec Automatic Attenuation Recorder Model 2470, with automatic gain control, was used to continuously record the ultrasonic attenuation as measured from the first two echoes in the pulse-echo pattern. Two oscilloscopes were used to display the pulseecho patterns. One oscilloscope was used to visually monitor the pulse-echo pattern during the test and permitted intermittent recording of the pattern using an oscilloscope camera with a Polaroid film cassette. The pulse-echo pattern displayed on the second oscilloscope was photographed in time lapse fashion ( 1 frame/l 5 set to I frame/5 min) using a 16 mm Cine-Kodak Special II motion picture camera actuated by a solenoid-plunger device controlled by a Model MC5E50 Samenco movie control
580
N. R. JOSH1
and
R. E. GREEN
unit. in this manner the pulse-echo pattern was recorded throughout the entire test and permitted comparison of attenuation values as obtained from analysis of the photographic record with that recorded by the automatic attenuation recorder. Moreover. examination of the movie film permitted accurate determination of the appearance of an additional pulse due to ultrasonic wave reflection from a crack. Each fatigue test at a given amplitude was repeated five times for aluminum and three times for steel. A total of 30 tests were run using aluminum specimens and 15 tests using steei specimens. EXPEWAL RESULTS Figure 3 shows the results of a typical experiment for an auburn specimen tested at a constant vibration amplitude peak-to-peak of 7,5 mm. It can be seen that the attenuation began to increase at about 24 million cycles and proceeded catastropically to fracture at about 3.5 million cycfes. The 2.8 million cycle mark occurred at about 26 hours and failure occurred after about 33 hr. Thus the ultrasonic attenuation measureTime (Units gf Hours) Scale Chcmge
0L
0.64 i
1.29 I
1.94 t
2.16 f
Number of Cycles Fi.
3.
2.52 /
3.02
329 1
3F58 8
388
( Units of IO61
Typicalptot of ukrasonicattenuationvs. number of cycbs for ~u~num.
ments gave a strong indication of the onset of fatigue failure at approximatefy 7 hr prior to the occurrence of the fracture itself. Conventions ultrasonic monitoring was unable to detect any additional echoes due to energy reflected from the crack until about 2 hr prior to failure. Thus, for this particular experiment, ultrasonic attenuation indicated that faiI_ was eminent 5 hr before conventions ultrasonic testing could indicate any crack formation. Figure 4 shows the results of a similar experiment for a steel specimen tested at a constant vibration amplitude peak-to-peak of 7~5mm. In this case the attenuation began to increase at about 6 x lo5 cycles and the increase proceeded catastropically to fracture at about 9 x 105 cycles. The 6 x lo5 cycle mark occurred at about 6 hr and failure occurred after about 8 hr. Conventional ultrasonic monitoring was able to detect an additional echo at the 7 hr mark. Thus, ultrasonic attenuation measurements gave warning 1 hr earlier than ultrasonic pulse detection in this case.
Ultrasonic Time (Units of Hours
b _-
i
581
detection of fatigue damage
)
I
4
6
:, 6 hrr ‘49min
I
ib
F 6hn:OSmm
I.’0
Number of Cycles (Units of 105)
Fig. 4. Typical plot of ultrasonic attenuation vs. number of cycles for steel.
Figure 5 summarizes the experimental results obtained from ultrasonic monitoring during the fatigue tests of the aluminum specimens. The ordinate axis in this figure corresponds to the maximum stress cr on the outermost fiber of the bar when bent to the maximum amplitude chosen for the particular test. It was obtained by substitution of the various maximum deflection values, y, into the linear elastic bending formula, 3Ed ==FY, where E is Young’s modulus, d is the thickness of the bar in the bending direction, and 1 is the length of the bar. The upper abscissa axis indicates the test time in hr. The Tiie
457
( Units of Hours
I
?
4
6
$
IO
I5
22
24
26
2%
30
32
-I
14.12 C&
c
3.66
oAD
OAO
3.20 .
5 2.74 L t
5r” E
A
0
0
2.29
II
A
0
‘ii
0.216
0.432
0.640
2592 Number
of Cycles (Units
I
0.864 3.024
1.06
1.296 3.456
of IO61
Fig. 5. Experimental results obtained with aluminum specimens 0 0.4 db ultrasonic attenuation change; A additional pulse observed; q fracture occurred.
582
N. R. JOSH1
and R. E. GREEN
portion of this scale ranging from 22-32 hr should be used with the tests run at a maximum fiber stress of 2.29 x IO4 psi; the scale ranging from O-12 hr should be used with all other tests. The lower abscissa in Fig. 5 indicates the number of fatigue cycles which elapsed before various events were detected. The portion of this scale ranging from 2.59 X lo6 cycles to 346 x lo6 cycles should be used with the single test run at a maximum fiber stress of 2.29 x lo4 psi; the scale ranging from O-l -296 x 10” cycles should be used withal1 other tests. The entries on Fig. 5 represented by circles indicate the average time elapsed in 5 similar tests before an O-4 db change in attenuation was observed. The criterion of O-4 db change was selected as an early warning signal since this amount of change could be clearly distinguished from background fluctuations on the *attenuation recording strip chart in all aluminum and all steel tests. The entries on Fig. 5 represented by triangles indicate the average time elapsed in 5 similar tests before detection of an additional echo in the pulse-echo pattern due to energy reflected from a crack in the test specimen. The entries on Fig. 5 represented by squares indicate the average time elapsed in 5 similar tests before fracture occurred. It can be clearly seen that in all cases the change in ultrasonic attenuation indicated that failure was eninent before conventional ultrasonic testing could detect an additional echo caused by energy reflected from a crack. Moreover, the smaller the amplitude of vibration and hence the longer the fatigue test. the more absolute time there is between the ultrasonic attenuation warning and fracture of the test specimen. Figure 6 summarizes the experimental results obtained from ultrasonic monitoring during the fatigue tests of the steel specimens. The ordinate axis corresponds to the maximum stress on the outermost fiber of the bent bar as given by equation (I). The upper abscissa indicates the test time in hours, while the lower abscissa indicates the number of fatigue cycles which elapsed before various events were detected. The entries on Fig. 6 represented by circles indicate the average time elapsed in 3 similar tests before an O-4 db change in attenuation was observed. The entries represented by triangles indicate the average time elapsed in 3 similar tests before detection of an additional pulse due to reflection of energy from a crack. The entries represented by squares indicate the average time elapsed in 3 similar tests before fracture occurred. The results are similar to those obtained with aluminum in that ultrasonic attenuation gave early warning of fatigue failure. f
Time
$
4
(Units of Hours) 4 2I
8I
6
IO
12 /
108
1.296
z _9$& m C s b
8.0*
!j -7.0
q
OAo
z z -6.0 .g E z ‘Z
s
OA
-50
0
cl 0.216
0.432 1
A
0.648
0
0 064
Number of Cycles (Units of IO? Fig. 6. Experimental results obtained with steel specimens ci 0.4 db ultrasonic attenuation change: A additional pulse observed: P fracture occurred.
Ultrasonic detection of fatigue damage
583
CONCLUSIONS The attenuation of an ultrasonic pulse propagating through an aluminum or steel bar subjected to cyclic loading initially remained constant, increased slowly, and then increased catastropically just prior to fracture of the test specimen. Ultrasonic attenuation served as a very sensitive indicator of fatigue damage and indicated that failure was eminent before conventional ultrasonic monitoring could detect an additional echo due to energy reflected from a crack. These results strongly suggest that ultrasonic attenuation measurements can be exploited successfully to predict early fatigue damage and perhaps fatigue life in practical applications. - This research was supported in part by the Air Force Office of Scientific Research and by the Middle Atlantic Power Research Committee. Special thanks in this regard are due to Dr. Jacob Pomerantz. Mr. Milton Rogers. and to Mr. Robert Fitzgerald. The authors would like to thank Mrs. Corinne Harness for typing the manuscript.
A cknon4edgements
REFERENCES and the Properries of Solids D. Van Nostrand. New York (1958). [2] R. True& C. Elbaum. and B. B. Chick. Ultrasonic Methods in Solid S:ate Physics. Academic Press.
[I] W. P. Mason,
Physical
Acoustics
New York t 1969). [3] G. Sines and J. L. Waisman. Metal Far&e. McGraw-Hill. New York (1959). [4] D. C. Drucker and J. J. Gilman (editors), Fracture of Solids. Gordon & Breach, New York (1963). [5] J. J. Burke. N. L. Reed and V. Weiss (editors). Fufigue-An Interdiscip/inav Approach. Syracuse University Press (I 964). [6] R. True11 and A. Hikata. Fatigue in 2S Aluminum as Observed by Ultrasonic Attenuation Methods, Watertown Arsenal Technical Reporf No. WAL 143114-47 (1956). [7] R. True11 and A. Hikata, Fatigue and Ultrasonic Attenuation. American Society for Testing Materials. Special Technical Publication No. 2 13 (1957). [8] R. Truell. B. Chick, A. Picker and G. Anderson, The Use of Ultrasonic Methods to Determine Fatigue Effects in Metals. WALK’ Technical Report 60-920 ( 196 1). [9] R. Truell. B. Chick. G. Anderson. C. Elbaum and W. Findley, Ultrasonic Methods for the Study of Stress Cycling Effects in Metals. WADD Technical Report60-920 ( I961 ). [IO] W. J. Bratina and D. Mills. Study of Fatigue in Metals Using Ultrasonic Technique. Cunudiun Mer. Quart. 1, 83-97
(1962).
[ 1 I ] B. Chick. A. Hikata. G. Anderson. W. Findley. C. Elbaum and R. True]], Ultrasonic Methods in the Study of Fatigue and Deformation in Single Crystals. Wright-Purrerson AFB Report No. ASD-TDR62-186 Pt. II, AD No. 408704(1963). [ I21 Z. Pawlowski. Ultrasonic Attenuation during Cyclic Straining. Proceedings Fourth International Conference on Nondesrructive Tesfing 1963. Butterworths. London. 192-195 ( 1964).
(Received
3 April 1972)