| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 90 === The dynamic shear deformation behavior and fracture characteristics of 304L stainless steel GTAW and SMAW joints are studied by torsional split-Hopkinson bar at room temperature under five strain rates (800s-1, 1200s-1, 1700s-1, 2200s-1 and 2800s-1). Experimental results indicate that strain rate significantly influences the mechanical properties of 304L stainless steel GTAW and SMAW joints. Flow stress, work hardening rate, strain rate sensitivity and deformation heat increase with strain rate, but activation volume decreases. Flow stress, work hardening rate, strain rate sensitivity and deformation heat for GTAW joints are larger than for SMAW joints. SEM fractographs show many dimples, revealing ductile fracture. We divide the fracture surface appearance into two characteristic zones, the densely dimpled and the smooth surfaced zones, and find significant differences between the two zones with increasing strain rate. Besides, comparison of GTAW and SMAW joints shows differing amounts of weld inclusions, which influences fracture appearance and fracture strain. For both joint types, fracture occurs only in the weld zone. Besides, δ ferrite sub-structures in the fusion zone are clearly twisted into band-like features from large localized shear deformation; it is a clear evidence of adiabatic shear fracture. The results indicate that microvoid nucleation and growth play a significant role in shear band formation, and that weld inclusions speed up adiabatic shear fracture formation by void nucleation and growth. The Kobayashi & Dodd constitutive equation accurately describes the high-strain-rate shear plastic behavior for 304L stainless steel GTAW and SMAW joints.
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