| Theodore von Kármán Professor of Aeronautics and Mechanical Engineering |
Research Interests Experimental and Analytical Studies of Dynamic Crack Initiation in Highly Ductile Solids Collaborator: G. Ravichandran, Caltech
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A drop weight tower and a high speed gas gun are being used to provide a wide spectrum of loading rates for the study of dynamic crack initiation process in highly ductile metals. The effect of rate sensitivity on the value of the dynamic J integral (a parameter characterizing the near tip plastic strains at initiation) is investigated at a number of loading rates by means of high speed photography and the optical methods of reflected caustics, Shadow Moire, and Coherent Gradient Sensoring. The experimental results are compared to three-dimensional, elastic-plastic, numerical simulations of the impact event. Materials tested include HY100, HY130, HSSLA, 4340, 304 stainless Steels and Aluminum alloys. High-speed infrared temperature sensors are also being used to measure temperature generated at the crack tip plastic zone during dynamic loading prior to crack initiation. The temperature signature is used to measure the time history of J up to initiation and to thus establish the dependence of fracture toughness on the loading rate. Analytical models of unstable void growth (cavitation) are used to investigate the fracture initiation process in such highly ductile metals. In such solids cracks typically initiate and propagate in a "tunneling" mode through the specimen thickness. Initiation is then followed by the formulation of extensive shear lips on the specimen surface. The models view crack growth as a consequence of the nucleation growth and coalescence of microvoids in front of the main crack and provide estimates of the levels of fracture toughness as functions of material parameters, inclusion size and distribution, etc.
last
update:
05/31/2016