Dynamic fracture of high-strength metallic alloys: experiments and modelling

Abstract

Fracture toughness is a property which describes the ability of a material containing a crack to resist fracture. Such a characteristic is one of the most important properties for describing the failure criteria of materials and may be a function of loading rate and temperature. Therefore, in the case of materials that may be subjected to dynamic loads or extreme conditions, it is crucial to be aware of the evolution of their fracture behaviour with such variables.
The main objective of this PhD thesis was to design and develop a novel experimental technique that allowed measuring the dynamic fracture-initiation toughness in a systematic manner for a wide range of loading rates. For such a purpose, two different high-strength metallic alloys, the AA7017-T73 and Mars® 240 steel were studied. Both materials were initially characterised at different strain rates and variying temperatures through uniaxial tensile tests performed in a universal servo-hydraulic machine equipped with a temperature chamber and a Hopkinson pressure bar system. As expected, the aluminium alloy presented a very mild strain rate hardening. Conversely, the armour steel exhibited a significant strain rate dependency.
The load obtained from properly calibrating the strain measurements of a strain gauge bonded close to the crack tip together with the stationary crack hypothesis, made possible the calculation of the dynamic fracture-initiation toughness from direct experimental measurements of three-point bending experiments in a wide range of loading rates. The obtained results corroborated the experimental observations of the uniaxial tensile tests, which helped to validate the experimental technique.
A numerical study on the mechanical response of both materials under three-point bending loading was conducted by comparing representative experimental measurements with their virtual counterparts. The anisotropic elastic-viscoplastic models employed for such a task were initially calibrated with the results obtained from the uniaxial tensile experiments. The numerical results showed reasonable agreement with the experimental response. In order to complete the numerical study, the accuracy on the fracture-initiation prediction was analysed with two stress-based fracture-initiation criteria.