Cradle-to-Grave Life Cycle Prediction of Automotive Materials and Systems in Service: Impact of Ageing on Performance

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Material ageing is commonly understood as changes of material properties with time. This physical or chemical alteration has a detrimental effect on the material properties and leads to gradual loss of the design function and unacceptable loss of efficiency or ultimate failure (in some cases ageing may be observed as an improvement, for instance, engine performance). Ageing of a material system under normal service conditions is a difficult case to treat. Many factors can significantly affect its durability, such as temperature, irradiation, moisture, chemicals, mechanical creep and fatigue loading. Synergism in the global ageing often occurs when the simultaneous action of several stresses results in an ageing effect that differs from that which would be observed if the individual stresses were applied sequentially. In an effort to understand the ageing process, the first task is to identify the age parameter that represents, on a macroscopic scale, the micro and sub microscopic features, underlying processes such as nucleation and growth of micro defects, and/or physico-chemical transformations. The second task consists of formulation of a constitutive equation that can mathematically represent ageing. The third task obviously is the experimental examination of the condition that leads to catastrophes such as small perturbation in controlling parameters leading to large variations of the age parameter. A material tensor "g" can be introduced as an age parameter similar to stresses and temperature. A variational principle approach can then be followed to formulate a constitutive equation for ageing. Interestingly, an evolution of "g" in four dimensional material space-time continuum would lead to an inelastic behaviour, which manifests as time dependent material properties recorded by an external observer. For one of the simplest linearized case this approach leads to a semi-empirical creep behaviour model. Currently, most of the studies are based on accelerated ageing test at coupon level and require extrapolation to normal service conditions of the part/component, resulting in many uncertainties, leading to frequent diagnostic tests in the field (visual inspection, chemical measurements, physical measurements), impacting dramatically on cost. The aim of the proposed work is to develop an 'age-aware' comprehensive simulation tool for the prediction and assessment of critically important automotive components and systems during manufacturing, in-service and the end of life.