Investigation on High Temperature Behavior of Laminate and Nanostructured Composite Materials.

摘要

A novel high-strain-rate and high temperature experimental set-up was developed to investigate failure of advanced materials. Experiments were performed on preheated Ti-6Al-4V specimens, at temperatures in the range 25-550 deg C, to determine the role of thermal activation on dynamic stress induced inelasticity and damage. Interferometric techniques were employed to record the free surface velocity of the target plates. The experimental results show that thermal activation overcomes the role of rate dependence in the material constitutive behavior. The Hugoniot Elastic Limit (HEL) and spall strength of Ti-6Al-4V significantly decrease with temperature despite of the high strain rates, about 10(exp 5)/s, used in the tests. Microscopy studies, performed on recovered samples, show that temperature substantially reduces the strain inhomogeneity leading to microvoid formation and that a change in void nucleation site occurs. A completely reversible shock-induced phase transformation alpha right arrow omega be present in the tested Ti-6Al-4V. Evidence of this phase transformation is observed in the velocity histories upon unloading of the first compressive pulse. The phase transformation is controlled by a combination of thermal and Stress driven mechanisms. Two oiler activities were completed under this grant, the development of software for finite element analysis of failure in brittle and layered materials, and the development of dynamic fracture experiments with full field measurements by means of speckle techniques. The new software accounts for crack initiation and propagation, finite deformation and surface roughness effects.