Interlaminar and In-Plane Shear Strengths of a Unidirectional Carbon/Epoxy Laminated Composite under Impact Loading

Transcription

1 Interlaminar and In-Plane Shear Strengths of a Unidirectional Carbon/Epoxy Laminated Composite under Impact Loading T. Yokoyama and K. Nakai Department of Mechanical Engineering, Okayama University of Science 1-1 Ridai-cho, Okayama 7-5, Japan ABSTRACT The interlaminar (out-of-plane) and in-plane shear strengths of a unidirectional carbon/epoxy (T7/51) laminated composite with a thickness of 1.5 mm at high rates of deformation are evaluated using the standard split Hopkinson pressure bar. Two kinds of double-notch shear (DNS) specimens with lateral constraint from a supporting jig are used in the static and impact compressive-shear tests. It is shown that both shear strengths increase very slightly with increasing deformation rate at failure up to nearly f =3.m/s( = 6/s), and the interlaminar shear strength is about 3% lower than the in-plane shear strength at low and high rates of deformation. SEM observations are performed to examine the effect of deformation rate on the failure modes in the two kinds of the DNS specimens. 1. INTRODUCTION Composite materials have been extensively used in a variety of industries, because of high specific stiffness and strength, long fatigue life and superior corrosion resistance. Laminated composites are, however, susceptible to failure in the matrix-rich interlaminar regions. Such interlaminar failure (or delamination) may lead to loss of stiffness and possible structural failure. Hence, it is important to understand the failure mechanisms leading to delamination. In practical laminated composite structures, the initiation and growth of a delamination crack are caused by the presence of interlaminar shear and normal stresses. So far, the four test techniques have often been used for characterizing the interlaminar shear resistance of composite materials against delamination: short-beam shear (SBS) test (ASTM D3) [1], four-point flexural test, Iosipescu shear test (ASTM D5379) and double-notch shear (DNS) test (ASTM D386) []. Nevertheless, there are no standard test methods for determining the interlaminar shear strength (ILSS) of composite materials under impact loading. Several attempts have been made to determine the effect of strain rate on the ILSS of laminated composites. Harding and Li [3] determined the high strain-rate ILSS of woven carbon/epoxy and glass/epoxy laminates by the tensile split Hopkinson bar (SHB) tests on double-lap shear specimens. Bouette et al. [] also attempted to measure the interlaminar shear properties of a unidirectional carbon/epoxy laminate at high strain rates from the tensile SHB tests on single-overlap shear specimens. Subsequently, Dong and Harding [5], and Hiley et al. [6] investigated the effect of strain rate on the ILSS of carbon/epoxy and carbon/peek laminates using the conventional split Hopkinson pressure bar (SHPB) tests on single-lap shear specimens. However, the high strain-rate in-plane shear strength (IPSS) of laminated composites has not been well understood as yet.

2 The purpose of the present work is to evaluate both the impact ILSS and IPSS parallel to the reinforcing fibers in a unidirectional carbon/epoxy (T7/51) laminated composite. DNS specimens are used in the static and impact compressive-shear tests, respectively. Finite element analyses are conducted to examine the shear stress and normal stress distributions on the expected failure plane between the two notches. The conventional split Hopkinson pressure bar (SHPB) [7] is applied to measure the dynamic compressive loaddeformation relations for the DNS specimens.. TEST PROCEDURE.1 Material and Specimen Geometry The material tested was a -ply unidirectional carbon/epoxy (T7/51) laminated composite with a thickness of 1.5 mm. The type of reinforcing fiber, matrix resin and fiber volume fraction are given in Table 1. The in-plane mechanical properties of the carbon/epoxy laminated composite were determined from tension tests on flat specimens (ASTM D339) in an Instron 55R testing machine and are listed in Table. The geometry and nominal dimensions of the DNS specimen used for ILSS and IPSS measurements are shown in Fig. 1. A notch distance was selected based on the ASTM D386 specification. The notches (or grooves) were carefully cut with a diamond wheel blade, and the notch tips were chosen to be of round profile with a fillet radius of.5 mm. Table 1 Type of reinforcing fiber and matrix resin used in unidirectional carbon/epoxy laminated composite Table In-plane tensile properties of unidirectional carbon/epoxy laminated composite Young's modulus Tensile strength Fracture strain Mass density L T.6 R.5 1. P P P P 3(z ) R (y ) 6.5 (DIMENSIONS IN MM) 1(x ) Fig. 1 Shape and nominal dimensions of double-notch shear (DNS) specimens under out-of-plane and in-plane pure shear loading (principal material axes are designated as 1, and 3). Finite Element Analysis The stress concentrations at the notches and edges of the DNS specimen are an important issue. Therefore, numerical stress analyses were performed using the MSC/NASTRAN package to study the stress distributions on three interlaminar planes between the two notches in the DNS specimen with lateral constraint. The unidirectional carbon/epoxy laminated composite was modeled as a transversely isotropic linear elastic material. The finite element model (see Fig. ) of the DNS specimen with slightly-overcut notches was used,

3 because some reports [8, 9] indicate that the slightly-overcut specimens produce more uniform shear stress distributions than the precisely-cut specimens. The computations were conducted under plane stress conditions with an external distributed load of p =.6kN/1.5mm, which was determined from the static interlaminar compressive-shear tests (see Fig. 6). The results of stress analysis are shown in Fig. 3. As expected, the shear stress distributions are not uniform on the interlaminar planes, but the normal stresses are negative (i.e., compressive), which suppress the initiation of delamination at stress concentrations around the two notches. The measured static ILSS of 6. MPa agrees well with the uniform shear stress level on the interlaminar plane away from both notches. (a) p 3 1 L1 (b) L L3 Fig. (a) Finite element model of DNS specimen with lateral constraint under out-of-plane pure shear loading; (b) details of FE mesh SHEAR STRESS 13 (MPa) 15 L1 L L3 1 ILSS = 6. (MPa) x/l x/l Fig. 3 Shear stress and normal (peeling) stress distributions along three lines on interlaminar planes within DNS specimen under out-of-plane pure shear loading NORMAL STRESS 33 (MPa) L1 L L

4 3. EXPERIMENTAL DETAILS 3.1 Split Hopkinson Pressure Bar Arrangement A schematic diagram of the standard SHPB arrangement is presented in Fig. (associated recording system not shown). Details of the test system have been given elsewhere [1]. The DNS specimen with the supporting jig to prevent buckling is held between the two 775-T6 Al Hopkinson bars (see Fig. 5). When the input bar is impacted with the striker bar launched through a gun barrel, a compressive strain pulse ( i )is generated in the input bar and travels towards the specimen. At the specimen/bar interface, because of the impedance mismatch, part of the pulse is reflected back into the input bar ( r ) and the remaining part is transmitted through the specimen into the output bar ( t ). The pulse in the specimen undergoes numerous internal reflections during the test. The incident, reflected and transmitted strain pulses are then recorded with two pairs of semi-conductor strain gauges mounted on the Hopkinson bars. PRESSURE CHAMBER 1 GUN BARREL STRIKER BAR ( ) INPUT BAR 75 GAUGE NO.1 15 OUTPUT BAR 75 GAUGE NO. SUPPORT BLOCK CHANNEL STEEL STOPPER RING V-BLOCK COMPRESSOR PHOTO-DIODES START STOP START STOP COUNTER INPUT BAR DNS SPECIMEN SUPPORT STAND OUTPUT BAR (DIMENSIONS IN MM) Fig. Schematic diagram of standard SHPB apparatus for impact compressive-shear testing d =16 mm DNS SPECIMEN INPUT BAR OUTPUT BAR SUPPORTING JIG Fig. 5 Edge view of DNS specimen with supporting jig held between two Hopkinson bars under out-of-plane pure shear loading 3.DataAnalysis By applying the elementary one-dimensional elastic wave propagation theory [11], we can determine the deformation (t), deformation rate (t) and compressive load P (t) for the DNS specimen as

5 t (t) = u 1 (t) u (t) = c o { i ( t ) t ( t )}dt (1) (t) = u 1 (t) u (t) = c o i (t) t (t) { } () P(t) = P (t) = AE t (t) (3) under the assumption of dynamic force equilibrium across the DNS specimen. Here c o denotes the longitudinal elastic wave velocity in the Hopkinson bars; i (t) and t (t) are the time-resolved incident and transmitted strain pulses; t is the time; A and E denote the cross-sectional area and Young s modulus (=71 GPa) of the Hopkinson bars. Eliminating time t through Eqs. (1) to (3) yields the compressive load-deformation and deformation rate-deformation relations for the DNS specimen.. RESULTS AND DISCUSSION.1 Static Compressive-Shear Tests The DNS specimens with the supporting jig were tested in compression with the Instron 55R testing machine at a crosshead speed of mm/min. Figure 6 shows the typical static compressive load-deformation relation. It is observed that the compressive load increases linearly until a sudden drop occurs. By assuming that failure occurs at the peak load, we can determine the ILSS or IPSS from the relation ILSS or IPSS = P max A () UD-CFRP h = 1.5mm =.6 kn P MAX. ( = 3. 1 FAILURE -5 m/s) f =.15 mm DEFORMATION (mm) Fig. 6 Compressive load-deformation relation from static test on DNS specimen under out-of-plane pure shear loading Table 3 Comparison of average static shear strengths as measured using two different test methods Test method No. of specimen Static shear strength (MPa) Out-of-plane In-plane DNS test SBS test

6 where P max is the applied maximum load, and A is the area of failure plane. For comparison, the IPSS was also determined from in-plane tension tests on the DNS specimens with longer tabs and from three-point bend tests on the SBS specimens in the Instron 55R testing machine. The average ILSSs and IPSSs are summarized in Table 3. It is seen that the IPSSs from two different test methods almost coincide with each other, and, hence, the stress concentration effects associated with the notches of the DNS specimens are negligibly small. Note that the ILSS is nearly 3% lower than the IPSS at low deformation rates.. SHPB Tests A series of SHPB tests was performed at room temperature. Figure 7 shows typical oscilloscope records from the SHPB test on the DNS specimen under out-of-shear pure shear loading. The upper trace gives the incident and reflected strain pulses ( i and r ), and the lower trace gives the transmitted strain pulse ( t ). Figure 8 presents the resulting dynamic compressive load histories at the front and back faces of the DNS i r t Sweep rate: 1μs/div Vertical sensitivity: Upper trace : mv/div (63μ /div) Lower trace : mv/div (66μ /div) Fig. 7 Oscilloscope records from SHPB test on DNS specimen under out-of-plane pure shear loading (striker bar velocity: V=7.7 m/s) 1 8 UD-CFRP STRIKER BAR VELOCITY: V =7.7m/s FRONT LOAD BACK LOAD P 1 P h = 1.5mm 6 P P TIME t ( s) Fig. 8 Compressive load histories at front and back ends of DNS specimen under out-of-plane pure shear loading

7 specimen. It is thus verified that the dynamic force equilibrium holds across the DNS specimen. Figure 9 shows the resulting dynamic compressive load-deformation and deformation rate-deformation curves. The peak load marked by a X on the curve corresponds to a shear failure on the interlaminar plane between two notches. Note that the deformation rate increases abruptly at.1 mm just after failure initiation. The deformation rate does not remain constant during the test duration, and hence the deformation rate =.8 m/s given indicates the average deformation rate up to failure. In order to discuss the effect of deformation rate at failure on the ILSS and IPSS, the static and impact ILSSs and IPSSs are plotted in Fig. 1 against the deformation rate at failure defined as f = f /t f. Both of the ILSS and IPSS are seen to increase very slightly with deformation rate at failure up to f = 3. m/s. Similar rate independence of the ILSS for a unidirectional carbon/epoxy (T3/58) laminates is found by Bouette et al. []. 1 1 UD-CFRP STRIKER BAR VELOCITY: V =7.7m/s 8 6 P MAX h = 1.5mm =6.1kN FAILURE =.8m/s. f =.11mm f DEFORMATION (mm) Fig. 9 Compressive load-deformation and deformation rate- deformation relations from SHPB test on DNS specimen under out-of-plane pure shear loading 1 UD-CFRP 5 Ç É É Ç É HH H H J J J OUT-OF-PLANE h =1.5 (mm) IN-PLANE h = 3.85 (mm) INSTRON É Ç SHPB H DEFORMATION RATE AT FAILURE m/s f SHEAR STRAIN RATE 1/s Fig. 1 Effect of deformation rate at failure on out-of-plane and in-plane shear strengths

8 .3 Microscopic Examinations In an effort to identify the type of failure mode in the DNS specimens, we examined the failure surfaces with a scanning electron microscope (Hitachi X-65). Figures 11 and 1 indicate broken halves of the DNS specimens, where open circles denote SEM observation areas. Figure 13 shows the scanning electron micrographs of the static and impact failure surfaces for the DNS specimens. Microscopic examinations reveal that the failure surfaces are quite similar for both low and high rates of deformation, and resin hackles, typical of mode II shear failure, are visible on both surfaces. Little effect of the deformation rate on the failure mode can be detected. ILSS =6.MPa ILSS =63.MPa INP =77.9MPa INP =79.MPa 1 1 (a) DNS specimens under out-of-plane pure shear loading 1 1 (b) DNS specimens under in-plane pure shear loading Fig. 11 Appearance of failed DNS specimens under out-of-plane and in-plane pure shear loading ILSS =6.MPa OUT-OF-PLANE ILSS =63.MPa INP =77.9MPa IN-PLANE 3 INP =79.MPa 1 1 (a) DNS specimens under out-of-plane pure shear loading (b) DNS specimens under in-plane pure shear loading Fig. 1 Macrographs of static and impact DNS specimens failed (principal material axes are designated as 1, and 3) LOADING DIRECTION ILSS =6.MPa ILSS =63.MPa INP =77.9MPaLOADING DIRECTION INP =79.MPa LOADING DIRECTION LOADING DIRECTION (a) 1- plane of DNS specimen (b) 1-3 plane of DNS specimen Fig. 13 Scanning electron micrographs of static and impact failure surfaces for DNS specimens

9 5. CONCLUSIONS The impact interlaminar (out-of-plane) and in-plane shear strengths of the unidirectional carbon/epoxy (T7/51) laminated composite have successfully been characterized using the conventional SHPB. The influences of deformation rate at failure f on the two shear strengths and failure mode were discussed from a macroscopic and a microscopic point of view. From the results of the present investigation, we obtain the following conclusions: (1) A testing technique is developed for determining the interlaminar and in-plane shear strengths of laminated composites at strain rates of nearly 1 3 /s using the DNS specimen. () The ILSS is by 1% - 9% lower than the IPSS at low and high rates of deformation. (3) There is little effect of the deformation rate at failure f on the ILSS and IPSS up to nearly 3. m/s (corresponding to 6/s). References [1] ASTM D3-8: Standard test method for apparent interlaminar shear strength of parallel fiber composites by short-beam method, ASTM Standards and Literature References for Composite Materials, nd Ed., ASTM, Philadelphia, p. 15 (199). [] ASTM D386-79: Standard test method for in-plane shear strength of reinforced plastics, ASTM Standards and Literature References for Composite Materials, nd Ed., ASTM, Philadelphia, p. 396 (199). [3] Harding, J. and Li, Y.L.: Determination of interlaminar shear strength for glass/epoxy and carbon/epoxy laminates at impact rates of strain, Composites Science and Technology, Vol. 5, (199). [] Bouette, B., Cazeneuve, C. and Oytana, C.: Effect of strain rate on interlaminar shear properties of carbon/epoxy composites, Composites Science and Technology, Vol. 5, (199). [5] Dong, L. and Harding, J.: A single-lap shear specimen for determining the effect of strain rate on the interlaminar shear strength of carbon fibre-reinforced laminates, Composites, Vol. 5, No., (199). [6] Hiley, M.J., Dong, L. and Harding, J.: Effect of strain rate on the fracture process in interlaminar shear specimens of carbon fibre-reinforced laminates, Composites, Part A, Vol. 8A, (1997). [7] Kolsky, H.: An investigation of the mechanical properties of materials at very high rates of loading, Proceedings of the Physical Society, London, Vol. B6, (199). [8] Chiao, C.C., Moore, R.L. and Chiao, T.T.: Measurement of shear properties of fibre composites, part 1. evaluation of test methods, Composites, Vol. 8, (1977). [9] Zhang, C., Hoa, S.V. and Ganesan, R.: Experimental characterization of interlaminar shear strengths of graphite/epoxy laminated composites, Journal of Composite Materials, Vol. 36, No. 13, (). [1] Yokoyama, T.: High strain-rate compressive characteristics of carbon/epoxy laminated composites in through-thickness direction, Applied Mechanics and Materials, Vol. 1/, (). [11] Graff, K.F.: Wave Motion in Elastic Solids, Clarendon Press, Oxford, (1975).