In this paper we describe an experimental study of the CTOD under impact loading in an attempt to clarify dynamic fracture behavior of mild steel. Impact three-point bend tests were carried out to investigate the effects of displacement rate on fracture toughness by using the drop test apparatus and air-gun experimental apparatus. Three-dimensional FEM calculations were performed and compared with the experimental results. Impact fracture toughness (critical CTOD) of SM-41B was evaluated by using the CTOD- Δ a curve (R-curve) under three different displacement rates. The effects of displacement rate on CTOD and critical CTOD were discussed based on experimental results. The relationships between fracture morphology and the displacement rate were investigated using the scanning electron microscope. It is found that the critical CTOD values decrease with increasing displacement rate, which is assumedly caused by intensive strain localization near the crack tip due to inertia force.
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