Velocity Effect on Fast Fracture-Fracture Surface Formation Process

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Mechanical meaning of fracture velocity (V) on dynamic fracture was assessed in this research. Systematic experimental study was performed to understand the relationship between dynamic fracture toughness (K), fracture surface roughness (R) and V by dividing a fracture process into two parts, namely, acceleration and deceleration regions. Handmade dual focus high-speed camera (RIAM-DFC2) was used to measure K and V accurately, and a laser roughness mater was used to measure R.The following conclusions were obtained :(1) For tensile SEN specimens of an extruded PMMA of low molecular weight (10^4 order), annealing at 90℃, 24hrs increased V, K and R of the material. However, CT+SEN specimens didn't show such annealing effect. Thus, specimen geometry dependence of the annealing effect was recognized.(2) CT+SEN type specimens of an epoxy resin and a low molecular weight PMMA exhibited that the peak of V appears prior to that of K, thus there exists time difference between the peaks of V and K, and the difference increases as the distance between crack and loading point increases. It was also shown that the peak of unloading rate lies between the two peaks of V and K.(3) Dynamic fracture experiment of a low molecular weight PMMA using a SEN specimen with circular holes enabled us to study the increase, decrease and reincrease processes of V and K.This experiment showed that in the fracture process, K is not a single-valued function of V and depends upon the acceleration (A).(4) It was clarified that if K and V are the same at different acceleration and reacceleration points, A values are almost identical. This implies that the relationship between K and V can be expressed explicitly by introducing A.(5) It was clarified that in dynamic fracture of low molecular weight PMMA, about a half of external work is converted into fracture energy.