Inspection for structural integrity of the Space Shuttle Solid Rocket Boosters (SRBs) is of paramount importance to mission safety. After every shuttle launch, the booster rockets are retrieved and an extensive inspection operation performed to detect any mission-related anomaly. If damage occurs to the structure during shuttle mission, Acoustic Emission (AE) from cracks could be monitored and used as a means for initial screening to identify potential damage locations for selective postlaunch inspection

Tsai, Chon L.

Yeh, James C.

English

Inspection for structural integrity of the Space Shuttle Solid Rocket Boosters (SRBs) is of paramount importance to mission safety. After every shuttle launch, the booster rockets are retrieved and an extensive inspection operation performed to detect any mission-related anomaly. If damage occurs to the structure during shuttle mission, Acoustic Emission (AE) from cracks could be monitored and used as a means for initial screening to identify potential damage locations for selective postlaunch inspection.</p

Yeh, James

Tsai, Chon

Digital Repository @ Iowa State University (ISU)

STUDY OF ACOUSTIC EMISSION CHARACTERISTICS FOR FRACTURE ASSESSMENT James C. Yeh United Technologies Corporation USB! Company P.O. Box 21212 Kennedy Space Center, FL 32815 Chon L. Tsai Ohio State University Welding Engineering Department 190 W. 19th Avenue Columbus, OH 43210 INTRODUCTION Inspection for structural integrity of the Space Shuttle Solid Rocket Boosters (SRBs) is of paramount importance to mission safety. After every shuttle launch, the booster rockets are retrieved and an extensive inspection operation performed to detect any mission-related anomaly. If damage occurs to the structure during shuttle mission, Acoustic Emission (AE) from cracks could be monitored and used as a means for initial screening to identify potential damage locations for selective postlaunch inspection. This study was organized to look for answers in these two separate and related concerns of understanding and quantifying of AE signal or AE energy counts, and employ-ment of the AE signal to assess fracture mechanics behaviors of a given structure. In addition, energy emission activity as related to both elastic and plastic behavior of the samples being tested is also a part of this study. [1-4] This study is organized into subtasks of specimen manufacture, AE experimental data acquisition, elastic-plastic J-integral analysis using finite element analysis, and acoustic emission wave propagation behavior study. Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1993 2207 SPECIMEN MANUFACTURE Dogbone-shaped simple tensile specimens with built-in center transverse fatigue crack were selected for this initial study. Inconel-718 alloy commonly used in the space shuttle operation was selected for its autogenous material property. The specimens were preloaded to 5,000 lbs before machining to dogbone shape to avoid emission from the pin-hole area during AE test. A tiny initiator hole was drilled from which two EDM notches were prepared to both sides of the hole to serve as fatigue crack starter. The specimens were then subject to low cycle fatigue under a prescribed tension load. During manufacturing process, the specimens were monitored using a travel micrometer to ensure uniformity of the final crack length. Maximum tensile loading of 5,000 lbs was followed for entire manufacturing operation. [5] ACOUSTIC EMISSION EXPERIMENTAL DATA ACQUISITION A total of sixteen (16) specimens were tested using tensile loading in a screw-driven Instron machine for a full load of 10,000 lbs. The samples were divided into four groups according to the overall EDM crack size of 0.1",0.2'; 0.4", and no cracks respectively. There were two specimens each of longitudinal or transverse material grain flow direction in each of the four specimen groups. The crosshead speed was 0.01 inches per minute and the load rose at about 201 lbs per second. The acoustic emission data acquisition system used for this test was a Physical Acoustic Corporation LOCAN-AT, utilizing three channels of AE input through 100-300 Khz band filters and 40 dB gain preamplifiers. Three model PAC R15 150 Khz piezoelectric resonant AE sensors were mounted on the samples as illustrated in Figure I. The two outer sensors were employed as guard sensors to reject any noise entering into test specimen from the Instron machine grip coupling! clevice device. [6] EDMcrack~ 004236 Loading Direction Fig. 1. Specimen orientation and sensor placement. 2208 As expected, the larger cracks gave more emission for any given applied load. The results also points out that the opening mode stress intensity factor is really the governing factor of acoustic emission activity. Larger emission (58-68 dBae or higher) from subcritical crack growth was also detected in the 0.4" crack length specimens. These 0.4" crack specimens emitted one or two dozen crack growth increments within the last 10-20 seconds before failure. This unique emission feature is typically used to provide a warning in the event that a specimen or component is unintentionally brought very close to its failure load. [6] In this study, a unique signal feature of "energy counts" is used to replace the older ringdown counts as a more acceptable measure of AE activity. AE signal energy is defined as the area of time-voltage integral under rectified signal waveform. The experimental acoustic emission test results are summarized both in AE energy counts versus loading and also in energy counts versus stress intensity factor (Figure 2). Notice in the first chart that emission activity is related to applied loading as well as crack length. However, from the second chart, opening mode stress-intensity factor governs emission activity as acoustic emission is caused by stress acting on the crack tip and the stress field at the crack tip is specified by stress intensity factor. [6, 7 J AE Emission Activity Below Previous Loading One interesting finding observed during the course of this study concerns acoustic emission activity of specimens at a loading that is below the maximum fatigue stress level recorded previously during specimen manufacturing operation. All specimens were subjected to a maximum initial fatigue cycle loading of 4,900 Ibs (or 56 Kips per square inch [ksi]) during specimen manufacturing. During experimental data acquisition task, there were emission activities at the first 2,000 lbs load holding period, corresponding to a stress level of 46 ksi for 0.4" crack specimens. Manufacturing cyclic fatigue loading, material recovery property, and perhaps time lapsed between specimen manufacturing and our AE experiment are all possible contributing factors to this interesting observation. It is beyond our study scope, but nevertheless, it warrants further investigation effort. 3,000 2,000 Experimental Data Experimental Average AE Energy Counts Data !l 2,500 Crack Size IJ) c E 1,500 Crack Size ::I 8 2,000 0.4 inch ::I 2a = 0.1 0 >- 0 Ol >-~ 1,500 Ol ~ 1,000 w w w w ct 1,000 <I CII CII 500 Ol Ol E '" CII 500 ~ > 0.1 inch > ct <I 0 0 0 2,000 4,000 6,000 8,000 10,000 12,000 a 20 40 60 80 100 Load (Ib) K,(KSI~ Fig. 2. AE energy counts versus load. AE energy counts versus stress-intensity factor. 2209 ELASTIC-PLASTIC J-INTEGRAL ANALYSIS USING FEA Strain energy release rate has been used for evaluation of elastic-plastic fracture behavior of structural materials. J-integral is a parameter to assess this material behavior. Since the amount of AE energy emission depends on growth of the crack-tip plastic volume due to increased load, there exists a correlation between the AE energy counts and the J-integral value for a given sample condition and load level. Therefore, AE energy counts monitored during loading of a structure component can be used to assess the elastic-plastic fracture behavior of the component. In this study, J-integral was calculated with increasing load for different crack sizes. Figure 3 shows the calculated J values versus applied load for various crack sizes. It also shows the relationship between J and K, which is no longer depend of crack size. By comparing Figure 3 with Figure 2, Figure 4 shows the relation between AE counts versus 1. A linear relationship, which can be expressed as AE Energy Counts = 3.9 J + 3.0. A threshold AE criterion for elastic-plastic fracture assessment of simple geometric samples were obtained using the critical strain energy release rate property of the sample material. [7] ACOUSTIC EMISSION WAVE PROPAGATION BEHAVIOR STUDY Although the linear relationship between AE energy counts and J was observed for simple tension specimens, the physical interpretation of the measure AE counts is qualita-tive. Effects of boundary reflection of AE waves in a complex geometry joint may influ-ence the measurement. Theoretical modeling studies using the finite element method can improve the understanding of the AE propagation characteristics. In this study, AE was modeled as a dipole at the crack-tip plastic zone. AE emissivity and wave propagation behaviors can then be simulated by the finite element method. 400~--------------------~ 300 z = 200 (J) a.. 100 ~ Theoretical -0- FEM, LEFM -0- FEM, EPFM Crack Size 0.4" 0.2" 140~----------------------~ 120 100 Z 80 en 60 a.. -, 40 FEM EPFM For all Three Crack Size 2a = 0.1", 0.2", 0.4" 0.1" 20 2210 o+-~~~~~--~~ o 2000 4000 6000 8000 1000012000 Load (Ib) J-Integral versus load, finite element analysis O+-~~~--r--r--~-.--~ o 10 20 30 40 50 60 70 KI (KSlffl l-Integral versus stress-intensity factor, finite element analysis Fig. 3. Result of elastic-plastic analysis. .a 3000 c:: ~ 0 () >. ~ 2000 Q) c:: W w AE Counts = 3.9J +3.0 « Q) 1000 Ol ~ ~ 0 0 100 200 300 J (PSI Inch) Fig. 4. AE energy counts versus strain energy release rate. Figure 5 shows the predicted Von Mises equivalent stress distributions along the centerline of the cracked sample at four instances. These distributions indicated the wave propagation along the centerline. Figure 6 shows the Von Mises equivalent stress variation with time at a point I" from the crack along the centerline. These figures demonstrated that sound propogation and attenuation behaviors on complex geometries could be studied. [7] 0.0000001 lIl e Q)(/) lila... ~';;;" c: 1Il o~ > ..... (/) Propagation at 2 x 1 0-7 S 0.0000000 +---"~-Il----o---Cl-r--o--o-~<>-.Q--,-<l 0.9 1.9 2.9 Distance from crack along center line (IN) 0.10 Propagation at Ci5 0.08 6.55 x 10-5 S !?;. III III ~ 0.06 en III Q) 0.04 III ~ c: 0 0.02 > 0.00 0.9 1.9 2.9 Distance from crack along center line (IN) 0.05-r---------------. g- 0.04 a... III ~ 0.03 en III ffi 0.02 ~ §0.Q1 > Propagation at 3.46 x 10-5 S o.oo+-------r------:::;:::O 0.9 1.9 2.9 Distance from crack along center line (IN) 0.05 g 0.04 a... III ~ 0.03 '-en ~ 0.02 III ~ § 0.01 > 0.00 0.9 1.9 Propagation at 1.66 x 10-4S 2.9 Distance from crack along center line (IN) Fig. 5. Von mises stress wave propogation 2211 0.2,...---________________ ---. 0.15 -C/) a.. 0.1 (/) (/) Q) ..... -C/) 0.05 0~L-_.--,_-_.--._~~--1r-_.-__4 o 2 3 4 Time (10-4 second) Fig. 6. Stress wave propogation at a single node point. SUMMARY The predicted relationship between strain energy release rate and applied load for various crack sizes follows the same trend as the experimentally measured AE energy counts versus load relationship. By comparing with the predicted or theoretical strain energy release rate, AE energy counts are physically related to strain energy release rate. AE energy counts versus strain energy release rate or J-integral has a linear relationship before the threshold, i.e., critical Je. For a given component geometry and applied load, crack size can then be related to the AE energy counts using theoretical relationship between crack size and strain energy release rate or stress-intensity factor. The threshold AE energy counts can be established from critical stress-intensity factor or critical J-integral based on experimental test results. AE behavior can therefore be used as an effective means to monitor structural integrity and the threshold condition can be used as an assessment criterion to select structural components for postlaunch inspection and evaluation. Theoretical modeling studies using finite element method have improved under-standing of the AE propogation characteristics in the test specimen. Finite element analysis relates AE energy counts to strain energy release rate caused by a crack in the test speci-men under load. The predicted displacement variations at AE sensor locations may be correlated to measure AE signal magnitude (number of AE event or AE energy counts). Sound propogation and attenuation behaviors on the test samples can then be assessed. AE emission activity was noted at a loading lower than the maximum stress level applied to the specimens during manufacture. This interesting observation warrants further study to determine its cause and for possible application for structures or specimens under fatigue cycle loading. 2212 ACKNOWLEDGEMENTS The authors wish to express their appreciation to Mr. Fritz E. Nelson and other members of USBI Company's Independent Research & Development Committee for their support and guidance. Thanks also to Metcut Research, Inc. for preparation of all samples, to Dr. A. Pollock and Mr. D. Pointer of Physical Acoustic Corporation for their technical assistance in AE data acquisition and guidance in testing approach, and to USBI-KSC Graphic Department for preparation of this manuscript. REFERENCES 1. Dunegan, H.L., Tatro, C.E. and Harris, D.O., "Acoustic Emission Research," Lawrence Radiation Lab. Rept. No. UCID-4868, Rev. 1 (1964) 2. Dunegan, H.L., Harris D.O. and Tatro, e.A., "Fracture Analysis by Use of Acoustic Emission," Engr. Fract. Mech., 1 (1), pp. 105-122 (June 1968) 3. Dunegan, H.L., Harris, D.O., and Tetelman, A.S., "Detection of Fatigue Crack Growth by Acoustic Emission Techniques," Mat!. Eva!., 28 (10), pp. 221-227 (Oct. 1970) 4. Pollock, A.A., "Quantitative Evaluation of Acoustic Emission from Plastic Zone Growth," Report No DE 76-8 for Tooele Army Base, DuneganlEndevco Corporation, Aug. 1976 5. Booker, MJ. and Arnold, T.E., "Manufacturing, EDM Notching and Fatigue Cracking of (32) Inconel 718 Acoustic Emission Test Samples," Metcut Project Report No. 3036-54141-1. Oct. 1991 6. Pollock, A.A. and Pointer, DJ., "Acoustic Emission Monitoring of Inconel 718 Specimens," Physical Acoustic Corporation Report R91-317, Dec. 1991 7. Tsai, C.L., Papritan, J.e., Beard, M.E., Richardson, M.G., and Zhai, D., "Sample Preparation and Acoustic Emission Study," an internal research report, The Ohio State University, Dec. 1991 2213 

Study of Acoustic Emisison Characteristics for Fracture Assessment

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1855&amp;context=qnde

Study of Acoustic Emisison Characteristics for Fracture Assessment

Abstract

Similar works

Full text

Available Versions

Digital Repository @ Iowa State University (ISU)