DAMAGE MECHANICS APPROACH TO LIFE PREDICTION AND DAMAGE ANALYSIS OF HIGH TEMPERATURE COMPONENTS

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The present project is concerned with the development of an effective and rational approach to life prediction and damage analysis of high-temperature components on the basis of damage mechanics. The results of the project are summarized as follows: 1. As the most fundamental problems of damage mechanics, mechanical modelling of material damage was discussed first. It was shown that a general state of anisotropic damage can be described by a second rank symmetric tensor. 2. As an application of damage mechanics to the life prediction of high temperature components, creep crack growth under multiaxial state of stress was analyzed, and the effectiveness and utility of this method was ascertained. 3. Damage of materials may have influence on their deformation characteristics, rigidity and fracture toughness. However, as regards damage and fracture of creep, the influence of the change of elasticity modulus was insignificant. 4. The characteristic aspect of the damage mechanics approach is the simultaneous analysis of damage process and deformation process. Thus, as the second purpose of the present project, practical and effective constitutive equations of inelasticity were developed. 5. For this purpose, creep, plasticity and cyclic plasticity tests were performed for type 316 stainless steel and modified 9Cr-1Mo steel under high temperature multiaxial stress condition, and a new unified constitutive equation to describe these results was developed. 6. Constitutive equations of cyclic plasticity were also proposed to describe the effects of the history of temperature change and that of non-proportional loading path. Extension of these equations to damage materials was also discussed.