A PSEUDO-ELASTIC MATERIAL DAMAGE MODEL INVOLVING LOCAL STIFFNESS CHANGE AND CRACK GROWTH.

摘要

Nonlinear structural response is the result of material yielding and/or fracture. Traditional methods of structural analysis address these two aspects of material behavior by separate failure criteria. In fact, yielding and fracture are intimately related by the irreversible microstructural processes that are responsible for both forms of material damage.;The fundamental hypotheses of the criterion are applied to a pseudo-elastic material. From the appropriate constitutive relation, increasing levels of strain energy density are associated with decreasing local stiffness of a material element. The range of material stiffness values delineate the regime of local nonlinear responses, from yield to fracture, available to describe the state of material damage in a structure. This behavior is incorporated into finite element procedures which predict the extent of material yielding and fracture for each load increment.;The nonlinear behavior and failure loads of two identical center cracked panel specimens are predicted for different material toughness values. Quantitative evaluation of yielding and fracture influence on total panel response is made by numerically suppressing, in turn, each of the mechanisms. The increased role of yielding for the higher toughness material is demonstrated.;The failure criterion is then used to address the problem of scaling structural component behavior. Structural behavior is known to be influenced by material selection, geometry and size, and loading rate because energy dissipation rates are sensitive to these factors. Twenty-seven axisymmetric tensile specimens are numerically analyzed to parametrically examine three materials, three loading rates and three specimen sizes. Simple design procedures are obtained using the linear nature of the strain energy density factor (which characterizes the crack tip region) versus crack growth relation. Changes in specimen size is seen to produce translation of these lines parallel to one another, while material and loading rate influence the slopes and intersection points.;This dissertation presents a failure criterion which addresses both yielding and fracture based on the absorbed strain energy density of the material. Higher values of strain energy density absorbed prior to fracture are associated with a higher material toughness.