Identification of damage mechanism and validation of a fracture model based on mesoscale approach in spalling of titanium alloy

Abstract

The subject of this paper is identification of the physical mechanisms of spalling at low impact velocities for Ti-6Al-4V alloy and determination of the macroscopic stress of spalling via meso-macro approach. Spalling is a specific mode of fracture which depends on the loading history. The aspects of the initial microstructure and its evolution during plastic deformation are very important. In order to identify the spalling physical mechanisms in titanium alloy, numerous pictures by the optical microscopy of the spall surfaces created by plate impact technique have been taken. The scenario of failure observed is in complete agreement with known physical micro-mechanisms: namely nucleation, propagation and coalescence by adiabatic shearing of micro-voids. The most interesting point in spall fracture of Ti-6Al-4V alloy is the nucleation of micro-voids. A significant amount of small micro-voids in the region of the expected spall plane has been observed. It appears that microstructural effects are important due to dual a-b phase microstructure, called Widmanstätten structure. The orientation of microstructure has a direct influence on nucleation mechanism by means of distribution of nucleation sites and decohesion between the softer particles (a-phase lamellae) and the harder lattice (b-phase). According to these observations, a fracture model has been developed. This model is based on the numerous post-mortem microscopic observations of spall specimens. The goal is to determine the macroscopic stress of spalling in function of loading time and damage level via a meso-macro approach.

The shock and spall response of three industrially important hexagonal close-packed metals: magnesium, titanium and zirconium

“…Microstructural analysis of recovered specimens by Tyler et al [78] showed that voids seemed to nucleate at the interface between the α and β phase. Boidin et al [79] also observed that voids nucleated at the interface between the α and β phase in Ti-6Al-4V with a lamella-based microstructure. The voids were observed to coalesce owing to highly localized plastic bridging between facets.…”

Constitutive behaviour of anisotropic materials under shock loading

“…Steinberg, 1991;Meyers, 1994;Johnson et al, 1994;Kanel et al, 1997;Millett et al, 2002;Lopatnikov et al, 2004;and Zaretsky et al, 2005;Gebbeken et al, 2006;Gu and Ravichandran, 2006;Bronkhorst et al, 2006), and numerous phenomenological as well as microscopic models have been developed (e.g. Wallace, 1981;Steinberg et al, 1980;Swegle and Grady, 1985;Meyers, 1994;Kanel et al, 1995;Nellis et al, 2003;Bourne and Gray, 2003;Krü ger et al, 2003;and Chijioke et al, 2005;Boidin et al, 2006;Petit and Dequiedt, 2006). However, in spite of a perfectly adequate general understanding, experimental methodology, and theory, material models do not agree in detail, especially for anisotropic materials.…”

Microstructure evolution of adiabatic shear band in AZ31 alloy under dynamic compression

“…Some researchers found that high microhardness of ASB in zirconium alloy was mainly due to grain refining [19]. However, other researchers considered that the higher hardness within ASB was because its microstructure was quenched by the surrounding material [52]. In the present experiments, the gradient structure of the shear region after large plastic deformation led to the difference in microhardness and a large number of fine equiaxed grains in the ASB caused the microhardness to increase.…”