Modified Mixed-Mode Bending Test Apparatus for Measuring Delamination Fracture Toughness of Laminated Composites

Abstract

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, and has very few attachments. The use of needle bearings and a lightweight loading lever makes the apparatus particularly suitable for fatigue tests. The design parameters of the apparatus were selected to minimize 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 energy release rates ( G) were derived and were found to be accurate. The test apparatus was used to measure mixed-mode fracture toughness of unidirectional IM7/5260 graphite/Bismaleimide composites. The delamination growth was stable for mode I loading and mixed-mode loading with G I/ G II = 4.55/1 and 1.16/1. Whereas, it was unstable for mode II and mixed-mode loading with G I/ G II = 1/3.4. The average mode I and mode 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) = 4.55/1, 1.16/1, and 1/3.4 was 2.18 lb/in (382 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 Ic m/ G IIc m) found to give a lower bound solution. The delamination growth resistance and the mixed-mode ratio were practically constant.