Predictive modeling of hydrogen assisted cracking – a Micromechanics conquest

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• Dr Emilio Martínez-Pañeda, CUED
• Friday 23 February 2018, 14:00-15:00
• Oatley Seminar Room, Department of Engineering.

If you have a question about this talk, please contact Hilde Hambro.

The detrimental effect of hydrogen firmly challenges the use of high-performance materials in energy infrastructure – the ductility and toughness of structural alloys are dramatically reduced in corrosive environments. With current engineering approaches being mainly empirical and highly conservative, there is a strong need to understand the mechanisms of such hydrogen-induced degradation and to develop models able to predict the initiation and subsequent propagation of cracks as a function of material, environmental and loading variables.

However, hydrogen assisted cracking is a very complex mechanical-chemical problem that depends sensitively on mechanisms that pertain to the micro and atomic scales. The speaker and his collaborators have been actively engaged in the development of enriched continuum-like models that aim to incorporate the mechanisms governing hydrogen-assisted cracking. To this end, efforts have been devoted to investigating crack tip fields by means of strain gradient plasticity (SGP) models, as classical continuum theories are unable to adequately characterize behaviour at the small scales involved in crack tip deformation. Grounded on the physical notion of geometrically necessary dislocations (GNDs), SGP formulations have proven to quantitatively capture the hardening effects associated with large gradients in plastic strain. Finite element results reveal that GNDs close to the crack tip promote local strain hardening and lead to a much higher stress level as compared with conventional plasticity.

Gradient-enhanced predictions proved to be particularly relevant in hydrogen embrittlement models due to the essential role that the hydrostatic stress has on both interface decohesion and hydrogen diffusion. We show that, when gradient effects are accounted for, a small reduction in the cohesive strength due to hydrogen entails a sharp transition from microvoid damage to brittle failure. Encouraging agreement with experimental data has been obtained by incorporating the influence of dislocation hardening in the modelling of hydrogen transport and environmentally assisted cracking. The promising results achieved have attracted the interest of industrial partners and technical standards organizations, ending with a scientific/engineering handshake a journey that began from fundamental micromechanics.

This talk is part of the Engineering - Mechanics and Materials Seminar Series series.

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