Effects of strain-rate and temperature on ductile damage and Fracture of metallic materials

Abstract

Ductile damage appears in metallic materials when they undergo sufficiently high plastic strain. It is caused by voids and produces degradation on the mechanical properties that eventually leads to the ductile fracture of the material. Like other properties, this behaviour is affected by the temperature and velocity of material deformation. This thesis investigates the effects that temperature and strain-rate have on ductile damage from an experimental and a modelling point of view, and explores innovative experimental methods to quantify damage and models to represent it. Experimental tests for the characterisation of plasticity, ductile damage and ductile fracture were conducted on stainless steel 304L and copper C110. The experiments were run at three temperatures ranging from −80◦C to 180◦C, and at three strain-rates from 10−3 s−1 to 104 s−1. For the accurate representation of plasticity near fracture, a neck monitoring method was implemented from which local stresses and strains can be calculated once localisation occurs. The method was also employed to analyse the effect of stress triaxiality on fracture using notched samples. Ductile damage accumulation was obtained from the degradation of the elastic modulus and validated by a micrographic analysis of the samples. The experiments at different temperatures were conducted following the same methodology inside an environmental chamber. To be able to apply the techniques to high-speed tests, an in-situ shadowgraph method was developed and a sub-pixel edge detection algorithm implemented, resulting in very high quality results, even at the highest strain-rate tested. An innovative experimental rig was also designed to perform high-speed interrupted tests from which damage accumulation was measured. Making use of the experimental results, plasticity, ductile damage and fracture models were calibrated and their effectiveness assessed. Novel ductile fracture and damage accumulation models were developed, which include the effects of strain-rate and temperature. The models also include non-linear damage accumulation, and are capable of representing accurately the effects observed in the experiments.