MODIFIED MIXED-MODE BENDING TEST APPARATUS FOR MEASURING DELAMINATIONFRACTURE-TOUGHNESS OF LAMINATED COMPOSITES

A modified mixed-mode bending test apparatus was developed to measure delamination fracture toughness and fatigue delamination growth rates of laminated composites. The test apparatus is compact, lightweight, a nd has very few attachments. The use of needle bearings and a lightwei ght loading lever makes the apparatus particularly suitable for fatigu e tests. The design parameters of the apparatus were selected to minim ize the geometric nonlinear effects. The total weight of the aluminum lever and the saddle is about 1.02 lb (4.5 N). The lever is graduated to directly read the load position and calculate G(I)/G(II) ratio. For this apparatus, simple linear equations for mode I and II strain ener gy release rates (G) were derived and were found to be accurate. The t est apparatus was used to measure mixed-mode fracture toughness of uni directional IM7/5260 graphite/Bismaleimide composites. The delaminatio n growth was stable for mode I loading and mixed-mode loading with G(I )/G(II) = 4.55/l and 1.16/l. Whereas, it was unstable for mode II and mixed-mode loading with G(I)/G(II) = 1/3.4. The average mode I and mod e II fracture toughnesses were 1.53 lb/in (268 N/m) and 13.53 lb/in (2 ,370 N/m), respectively. The average fracture toughness for mixed-mode loading with (G(I)/G(II)) 4.55/l, 1.16/l, and 1/3.4 was 2.18 lb/in (3 82 N/m), 3.15 lb/in (552 N/m), and 7.44 lb/in (552 N/m), respectively, The linear mixed-mode delamination fracture criteria (G(lc)(m)/G(llc) (m)) found to give a lower bound solution. The delamination growth res istance and the mixed-mode ratio were practically constant.