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Mode I fracture toughness was determined for several unidirectional, continuous fiber reinforced polymer matrix composites and a monolithic homogeneous alloy using a modified chevron-notch short bar (CNSB) specimen. The modified short bar specimen consisted of two dissimilar materials and is, consequently, referred to as the hybrid short bar (HSB) specimen. The HSB specimen comprised the host material that contained the chevron notch ligament through which fracture occurred during testing. Bonded to both sides of the host material were metallic adherends. Similar geometric dimensions were maintained between the monolithic (i.e., standard) and the hybrid short bar specimens. Maintaining geometric consistency between the specimens enabled standard short bar fracture toughness analysis techniques to be used in the data reduction of all specimens. Using metal/laminate HSB specimens, Mode I delamination fracture toughness was determined for graphite epoxy and graphite thermoplastic polymer matrix composite laminates. Also, plane strain chevron notch fracture toughness, KIv, was determined for 6061-T651 Al using a metal/metal HSB specimen. The metal/metal HSB specimen consisted of Al 6061-T651 as the host material onto which Ni adherends were bonded via electroplating. Using a low-temperature electroplating process in fabricating the metal/metal HSB specimens provided two significant benefits. First, a strong metallurgical bond was achieved between the dissimilar materials of the metal/metal HSB specimen. Second, during the plating process, modification of the host material microstructure was insignificant. Maintaining consistent microstructures between the metal/metal HSB and the monolithic short bar (SB) specimens permitted direct comparison of experimental fracture toughness test results. After correcting the HSB fracture toughness data to account for effects due to compliance mismatch of dissimilar materials, good agreement was observed between the toughness results obtained using hybrid short bar and monolithic short bar specimens.
fracture toughness, delamination toughness, monolithic short bar, hybrid short bar, composite laminates
Associate Professor, Michigan State University, East Lansing, MI