Dynamic Fracture Toughness Test Using Hopkinson Bar
Various loading and measuring configurations have been developed in Hopkinson bar fracture toughness experimental techniques. It is well known that several fundamental issues, such as force equilibrium, pulse shaping, stress-wave propagation, etc., must b
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Dynamic Fracture Toughness Test Using Hopkinson Bar Wei-Yang Lu, Bo Song, and Kenneth Gwinn
Abstract Various loading and measuring configurations have been developed in Hopkinson bar fracture toughness experimental techniques. It is well known that several fundamental issues, such as force equilibrium, pulse shaping, stress-wave propagation, etc., must be evaluated in order to obtain a reliable measurement. In our previous work of characterizing Mode II dynamic fracture toughness of a woven composite, highly sensitive polyvinylidene fluoride (PVDF) force transducers were employed to check the forces on the front wedge and back spans in a SHPB ENF experiment. The results show that proper pulse shaping is necessary so the specimen can achieve stress equilibrium before the crack starts to propagate. This study addresses the issue that stress wave propagates through the non-uniform section, which is between the incident and transmission bars including the specimen, loading wedge, and supporting fixture. The transmitted signals are compared with PVDF measurements, and also with numerical simulations of stress waves propagate through supporting fixture and down to the transmission bar. Keywords Mode II fracture • Dynamic fracture toughness • PVDF • Woven glass composite • Hopkinson Bar
Various loading and measuring configurations have been developed in Hopkinson bar fracture toughness experimental techniques. It is well known that several fundamental issues, such as force equilibrium, pulse shaping, stress-wave propagation, etc., must be evaluated in order to obtain a reliable measurement. A typical experimental setup for Mode II dynamic fracture toughness is shown in Fig. 64.1. In our previous work of characterizing a woven composite [1, 2], highly sensitive polyvinylidene fluoride (PVDF) force transducers were employed to check the forces on the front wedge and back spans in a high rate end notch flexure (ENF) fracture experiment, shown in Fig. 64.2. The results show that proper pulse shaping is necessary to achieve stress equilibrium in the specimen before the crack starts to propagate. This study addresses the issue that stress wave propagates through the non-uniform section, Fig. 64.3, which is between the incident and transmission bars including the specimen, loading wedge, and supporting fixture. Except the composite specimen, all are made of steel. Note that force equilibrium of the specimen, F1 ¼ 2 F2 ¼ 2 F3, is not equivalent to the equilibrium of the test section, F4 ¼ F5, which is usually the requirement of Hopkinson bar force measurement under dynamic loading. PVDF transducers measure the forces F1, F2 and F3 at contact interfaces; Hopkinson bars evaluate F4 and F5 at a distance away from the contact points. In an experiment, where a pulse shaper was used, the force histories measured from PVDF transducers are shown in Fig. 64.4. It takes the transverse wave approximately 40 ms to propagate from the central wedge to the end rail supports. Both forces F1 and F2 + F3 increased slowly in the beginning but at
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