Dynamic fracture toughness of 4340 VAR steel under conditions of plane strain
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INTRODUCTION
A central issue in the dynamic fracture of engineering materials is the initiation of fracture from a pre-existing crack in a deformable body due to stress wave loading. Under these high rate of deformation conditions, the effects of material inertia and strain rate sensitivity are called into play and frac~re may evolve differently than, say, under quasistatic loading. Even in cases when the loading is quasistatic, the loading rates and strain rates may be high near the tip of a dynamically propagating crack. Improved understanding of crack initiation and crack propagation at these high loading rates, with due considerations of the effect of inertia and strain rate sensitivity of flow stress, is important to the development of methodologies for designing more failure resistant structures. Some of the early important contributions to the study of dynamic fracture initiation are due to Krafft,m who tested mild and high strength steel over a range of temperature and loading rates. Rapid loading was achieved through the use of a hydraulic testing machine. Eflis and Krafftt2] extended this work to tests on mild steel plates. These investigators were able to determine the critical values of stress intensity factor, Kia, for crack tip loading rates u p to K 1 ~ 103 MPa~/m/s. Using double cantilever beam
YOUNGSEOG LEE, Graduate Student, and VIKAS PRAKASH, Assistant Professor, are with the Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH 44106. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, trader the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
specimens and the optical method of caustics, Kalthoff e t studied rapid crack propagation and arrest in structural steels. A similar technique was used by Rosakis e t al.t4] to measure the stress intensity factor during rapid crack propagation in 4340 VAR steel having a hardness of 45 on the Rockwell C scale. Similar studies were conducted by Kobayashi and Dally[5] and Shukla e t aL, t61 who made photoelastic measurements of the crack tip fields in 4340 VAR steel by using a birefringent coating on the specimen surface. In an attempt to study dynamic fracture initiation in structural materials at even higher crack tip loading rates, several loading techniques have been developed. Impact loading of three-point ductile fracture bend specimens (Nakamura eta/. [71) allowed the measurement of dynamic fracture toughnessof structural metals at crack tip loading rates of 106 MPa~/m/s. Similar rates were achieved by Curran e t aL tS] and by Klepaczko and Solecki[gl by using a compact tension specimen loaded by a compression split Hopkinson pressure bar. Recently, Rosakis and Zhender~l~ and Krishnaswamy e t aL tx~l used a configuration consisting of a three-point bend specimen in a drop weigh
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