High strain-rate-induced cleavage fracture in mild carbon steel
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INTRODUCTION
THE
fracture toughness of metals containing brittle second-phase particles can be influenced by the initiation of microcracks in second-phase particles located in the zone of elevated stress ahead of a preexisting flaw. t~ Recently, Lin et al. 121 have shown that low-temperature cleavage resistance in a spheroidized mild steel is dependent on both the distribution of carbide particles and the applied stress state. They independently varied the grain diameter in conjunction with either a fine or coarse carbide distribution in a 1008 steel. Their results indicated that a fine carbide distribution enhanced the fracture toughness of specimens containing a blunt notch but decreased the fracture toughness of specimens containing a sharp crack. Fracture mechanics considerations were used to create a model which assigns an effective particle strength level as an inverse function of carbide size, and the results were explained on the basis of the number of carbides having a strength less than the peak stress in the plastic zone of the crack. The peak stress ahead of a sharp crack is quite high, but the volume of material which experiences the peak stress level is very small. Therefore, the probability of finding an activated particle in the plastic zone of a sharp crack in a material with a given volume fraction of particles and, thus, the probability of microcrack initiation is higher with a distribution of fine particles. Conversely, the low peak stress but large plastic zone volume attendant to a blunt notch favors microcrack initiation in a microstructure which contains a distribution of coarse particles. It should be noted that as proposed by Lin et al.,[21 excessive coarsening of the particles (spheroidization treatment over 30 days resulting in very large carbide particles) can decrease the fracture resistance in all cases. If a specimen were subjected to a large uniform tensile stress, the model of Lin et al. t21 would predict a higher fracture resistance with a distribution of coarse particles. Such a loading condition can be achieved in an instrumented spall test. In this test, the sample is impacted at a high velocity by a flyer plate. The resultant planar shock waves in the flyer plate and specimen meet at the midsection of the sample and produce a high-amplitude ten-
A N N A K. ZUREK, Staff Member, and PAUL S. FOLLANSBEE, Group Leader, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. JOHN H A C K , Assistant Professor, is with the Department of Mechanical Engineering, Yale University, New Haven, CT 06520. Manuscript submitted March 2, 1989. METALLURGICAL TRANSACTIONS A
sile wave. The subsequent high-rate tensile loading fractures the specimen at a stress level approaching a significant fraction of the theoretical fracture strength of the specimen. This fracture stress is referred to as the spall strength. The purpose of this investigation was to study the influence of high strain-rate loading and carbide distribution on the spall strength of 1008 steel. II.
EXPERIMENTAL PROCE
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