An analytical and experimental study is conducted to determine criticality of interlaminar disbands by NDE methods. Criticality of such flaws in a shear environment (action of shear near support) is defined in terms of crack propagation and is analyzed by principles and methods of fracture mechanics. Growth of disbands under cyclic loading is also being studied. Fajlure under compressive loading in presence of a disband is defined in terms of buckling and an elastic stability analysis is utilized for assessing criticality. Analytical predictions are compared with experimental results in both cases

Chatterjee, S. N.

Hashin, Z.

Pipes, R. B.

English

Iowa State University Computer Science Technical Reports

Nondestructive Evaluation of Flaw Criticality in Graphite-Epoxy Laminates

Chatterjee, S N

Hashin, Z

Pipes, R B

An analytical and experimental study is conducted to determine criticality of interlaminar disbands by NDE methods. Criticality of such flaws in a shear environment (action of shear near support) is defined in terms of crack propagation and is analyzed by principles and methods of fracture mechanics. Growth of disbands under cyclic loading is also being studied. Fajlure under compressive loading in presence of a disband is defined in terms of buckling and an elastic stability analysis is utilized for assessing criticality. Analytical predictions are compared with experimental results in both cases.</p

Chatterjee, S.

Pipes, R.

Digital Repository @ Iowa State University (ISU)

NONDESTRUCTIVE EVALUATION OF FLAW CRITICALITY IN GRAPHITE-EPOXY LAMINATES S.N. Chatterjee and Z. Hashin Materials Sciences Corporation, Blue Bell, PA 19422 R.B. Pipes University of Delaware, Newark, DE 19711 ABSTRACT An analytical and experimental study is conducted to determine criticality of inter-laminar disbands by NDE methods. Criticality of such flaws in a shear environment (action of shear near support) is defined in terms of crack propagation and is analyzed by prin-ciples and methods of fracture mechanics. Growth of disbands under cyclic loading is also being studied. Fajlure under compressive loading in presence of a disband is defin-ed in terms of buckling and an elastic stability analysis is utilized for assessing cri-ticality. Analytical predictions are compared with experimental results in both cases. INTRODUCTION . Various kinds of defects can cause relevant strength and stiffness degradation in composite laminates, namely, (i) inter-laminar disbands, (ii) through-the-thick-ness defects, (iii) defects in bolted and bonded joints, (iv) impact damage, (v) fatigue damage. Different NDI techniques (active and passive), which are usually used for studying effects of such defects, are listed below: (i) Ultrasonics (modulus degradation meas-urement and damage detection) (ii) Accoustic emission (sequential record-ing of the damage growth process) (iii) x-ray and thermography (visual and real-time detection of defect growth) (iv) Structural vibrations (stiffness de-gradation measurement) (v) Penetrants (vi) Holography This work is mainly directed towards use of ultrasonics for detection and follow-ing growth of interlaminar disbands. At-tempts are also being made to use measure-ments from wave propagation studies for estimating possible modulus degradation due to damages induced by fatigue and moisture conditioning. Analytical studies are aimed at: (i) definition of required NDI observations, (ii) model physically realistic damage modes (iii) assess property degradation, and flaw criticality, and (iv) translation of results from test sys-tems to real world composite structures. ~echanical testings are conducted to relate Jbserved state of magnitude and geometry of jamage and/or degradation to estimate re-sidual performance capability. Correlations Jf data from NDI, mechanical tests and anal-yses are carried out to demonstrate feasi-bility of using fundamental approach at laminate level for development of quantita-tive NDE of flaw severity in composites. CRACK PROPAGATION IN SHEAR ENVIRONMENT Analytical methodology for stress anal-ysis of a laminated beam containing two symmetrically located disbands as shown in Figure 1 has been developed. The mixed boundary value problem of anisotropic elas-ticity has been reduced to the solution of a coupled pair of singular integral equa-tions. This method of reduction has been computerized. For numerical solutions these equations are converted to a discretized system. Evaluation of the unknown functions appearing in these equations yields the stress intensity factors and strain energy release rates. Methods of analysis are based on principles and mathematical tech-niques commonly used in crack problems of linear elastic homogenous media. Details of the methods are omitted here for brevity, but can be found in [1]. BUCKLING FAILURE UNDER COMPRESSION Buckling failures under compression near disbands can be expected in different types of structural configurations. One example is a sandwich construction under flexure with a disband in the compression flange (Figure 2a). For stability analysis the delaminated compression flange is model-ed as a system of four interconnected beams (Figure 2b) . Beams 3 and 4 are considered as semi-infinite beams. Beams 2, 3 and 4 are supported on an elastic foundation which can carry extensional and shear loads. The foundation elastic moduli (spring constants) are calculated from the properties of the sandwich construction. All beams are anal-yzed with laminated beam theory including the effects of shear deformations and pre-stresses. A stiffness formulation is employed and the critical value of prestress needed to cause buckling (instability in beam 1 i.s usually critical) is calculated by solving the resulting eigenvalue problem by a trial and error procedure. 219 THICK LAMINATE BEAMS WITH IMPLANTED DEFECTS Graphite-epoxy thick beam laminates with implanted defects of various geometries were fabricated using AS3501-6 prepreg ma-terial. The fiber orientation of the 10" long beams was [ (0/90)fio 12 J 5 .and the defect geometries were of square, d~amond and "hourglass" shapes. The square defects were fabricated by welding .001" thick tef-lon film into a tube. The seam in the tube was put in the center of the defect so that the edges of the defect have the same radius of curvature. The diamond and "hour-glass" shaped defects were fabr~cated ~y shearing two layers of teflon f~lm aga~nst an abrasive surface in a manner which causes the two layers of teflon to remain together after shearing. The defects were implanted between the center plies of the laminate at the quarter-span position of the beams. Ultrasonic C-scans were made of all laminates before and after machining. All beams are precracked prior to testing in order to obtain a sharp crack front. The method used to precrack the beams is to subject the beams to three point bending with clamps placed at the boundaries of the defects. The clamps are used only during the precracking operation to prohibit the defects from propagating completely through the beam. Attempts to precrack the 0.6" rectangular defects and the "hourglass" de-fects resulted in laminate flexure failure. Attempts to precrack the 1.0" rectangular defects were partially successful. C-scans of these specimens after precracking are shown in Figure 3. Beam No. 12 is the only specimen which exhibited the desired degree of precracking. Figure 4 shows the results of precracking the diamond defect beams. Precracking of these beams occurs at a load of approximately 400 kg as compared with 600 kg for the 1" rectangular defect beams. The results of precracking the diamond de-fect beams are the most satisfactory. After precracking, three beams with diamond de-fects and three beams with 1.0" rectangular defects were loaded to failure which occur-ed due to catastropic growth of disbonds. Figure 5 shows a magnified view of the in-terlaminar defects. Figure 6 depicts the growth pattern of such flaws under cyclic loading [2]. COMPOSITE SANDWICH BEAM WITH IMPLANTED DEFECTS Test specimens are sandwich beam con-struction consisting of graphite-epoxy laminate adhesively bonded to an aluminum honeycomb core. The graphite epoxy system is AS-3501-6 QF. Two 23" panels were fab-ricated in the following configurations, where D denotes the disbond; [0/+45/±45/0/D/0/+45/+45/0]; _ . - [0/+45/+45/0/0/+45/D/+45/0] Disbonds were-fabricated from 0.001 inch teflon film. The disbond lengths (L) were 0.5", 1,0", 1.5." Disbond strips were implanted in the laminates on the longitudi-nal mid-plane at the aforementioned locations in the laminate, The laminates were cured and subjected to ultrasonic C-scan to locate the exact position of the disbonds. Next the laminates were bonded to a 24 lb/ft3 aluminum honeycomb (Hexel Corp.), American Cyanamid's FM-300/BR 127 sheet adhesive system was the bonding cgent. The laminate with the near surface disbonds was oriented so that the disbonds were far-thest from the laminate-honeycomb inter-face. The laminate-adhesive-honeycomb sys-tem was cured together in an autoclave. Specimens of 1.0" width and 22.0" length were cut using a rotary diamond saw (fig-ure 7). The beams were instrument.ed with two strain gages (one on each .laminate sur-face) located at midspan parallel to the beam longitudinal axis. The specimens were tested in a tension-compression load frame employing a four point bending fixture with spans of four and 20" (see Figure 8). A strain rate of .05/ins/in/min was applied to the beams. The strain from the compressive face of the beam was plotted on the x-axis and ten-sile strain on the Y-axis using an X-Y re-corder. Load levels at which variations from 1 inear i ty occurred were noted. (Table 1: The laminates with 0.5" disbonds may have exhibited a catastophic buckling in-stability. The tensile and compressive strain increased linearly until failure oc-curred. The 1.0" specimens with disbonds at the laminate mid-plane exhibited the same type of behavior except the buckling of the laminate occurred in a stable manner which was shown by the abrupt change from linear to nonlinear behavior. The specimen! with 1.5" mid-plane disbonds exhibited the same gradual nonlinear response from the onset of loading followed by a quasi linear response. The initial compressive strain of the tensile face laminate was due to the initial out of plane deformation due to residual thermal stresses. The 1.5" near surface disbonds were prebuckled to such an extent that the initial tensile strain on the compressive face continued to increase throughout the test. The buckling failure mode is illustrated in a photograph (Fig-ure 9). CORRELATION STUDIES Correlation of experimental results for growth of delaminations in precracked [ (0/90) 6o12 Js laminated beams with defects located in mid-plane under static loading is given in table 2. Results from past studies on a different lai~inate (not pre-cracked) reported in [2] are also shown in this table. The critical values of strain energy release rate from two sets of data reported in table 2 are of the same order and show the same amount of scatter. It appears, therefore, that precracking does not have any significant influence on crack propagation. Experimental results from honeycomb sandwich beam tests, reported in table 1, are compared in figure 10 with the analyti-cal predictions. The results agree well with one another over a wide range of 220 disband length. MODULUS DEGRADATION DUE TO MOISTURE AND FATIGUE DAMAGE Wave propagation studies on neat resin and (~45) 2 s coupons are being conducted to measure effects of conditioning on storage and loss moduli. The objective is to de-termine the possibility of using ultrasonic techniques for assessing magnitude and cri-ticality of damages caused by such condi-tioning. Table 1 SAtlDWICH DISBOND DEFECT STUDY Defect Location Buckling Sample Size in Load No. (In.) Laminate (Lb.) lT 1/2 Center 1345 lOB 1/2 1477 3T 1 522 7B 1 661 12T 1 584 8B 1 661 6T 1.5 2 95 5T 1.5 309 llT 1.5 293 9B 1.5 364 3B 1 Near Surface 22 12B 1 22 7T 1 49 8T 1 29 NTERFACE 1 2 tZ=Zl 1+1 l X:Xt J--c--1 n ~ d -I + t ! i p ACKNOWLEDGEMENTS This work is being performed under NADC contract N62269-78-C-Olll-Dr. William Scott, Contract Monitor. REFERENCES 1. Chatterjee, S.N., "On Interlaminar De-fects in Laminated Composites," to be presented at the Winter Annual Meeting of ASME to be held December 2-7, 1979 New York. 2. Ramkumar, R.L., Kulkarni, S.V. and Pipes, R, B., "Definition and Modelling of Critical Flaws in Graphite Fiber Rein-forced Composites," NADC-76228-30, January 1978. Table 2 ENERGY RELEASE RATE~--l Laminate [ (04t!.452/ +45 2/0'l) sl 8 [(0/90)6 012] s DISsbND Specimen 1-1 1-2 1-3 1-4 1-1 1-3 1-6 D-2 D-5 D-3 LAYER 1 2 i-1 i n ~ Gc(lb/in) 5.63 6.34 2.94 3.74 Avg. 4. 66 4.37 6.53 5.02 5.23 4.14 2.93 Avg. 4.70 FIG. 1. LAMINATED BEAM CoNTAINING DisBaNDs 221 11 -FIG. 2A. SANDWICH BEAM . ..:: -~<-:· . -~--: ,,_ ~--~~ ·-;~!:!::,i..;;..:/_·· ·- ~·--- .- .• -:,o .. --.-_-;;.-._·~;. ..... "':"· 12 · .. ~ .. ·.·-. .-_ -;····;;::.:.·.::o. ._-: ~; _!· : "'t., ... :...::~: ~ ;;; ____ ... :-.-:. . . ---·· ·--.:i. '· 13 14 Core FIG. 2B. INTERCONNECTED BEAM SYSTEM ·, ' ,. 15 01 02 03 04 FIG. 3. PRECRACKING SQUARE DEFECTS FIG. 4. C SCAN OF DIAMOND DEFECTS FIG. 5. MAGNIFIED VIEW OF INTERLAMINAR DEFECTS 222 FATIGUED@ 50% !itt DEFECT 10,000 20,000 30,000 40,000 50,000 60,000 70,000 •"< 0 j .... FIG, 6, GROWTH OF DELAMINATION WITH FATIGUE CYCLING FIG, 7, C SCAN OF SANDWICH BEAM SPECIMENS 22) !ll,OOO 90,000 N .-I p/2 p/2 GRAPHITE EPOXY LAMINATE (0/~4!1/!4!1101 1 ALUMINUM HONEYCOMB FIG, 8, TEST ON SANDWICH BEAM WITH DISBOND FIG. 9. FAILURE oF CoMPRESSION SKIN 100 - -- ---- - - - - - - -80 60 40 20 0 .25 • • • • r STABIUTY ANALYSIS .5 .75 -• • 1.0 DEFECT SIZE , INCHES FIG. 10. CRITICAL STRESS VS, DEFECT SIZE 224 1.25 1.5 

Chatterjee, S

Pipes, R

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Nondestructive Evaluation of Flaw Criticality in Graphite-Epoxy Laminates

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Iowa State University Computer Science Technical Reports

Iowa State University Computer Science Technical Reports

Digital Repository @ Iowa State University (ISU)

Digital Repository @ Iowa State University (ISU)