Disturbed state constitutive modeling based on stress-strain and nondestructive behavior

The proposed disturbed state concept (DSC) is based on the idea that a deforming material element can be treated as a mixture of two constituent parts in the relative intact (RI) and fully adjusted (FA) states, referred to as reference states. During external loading, the material experiences internal changes in its microstructure due to a process of self-adjustment, and as a consequence, the initial RI state transforms continuously to the FA state. The self-adjustment process, which may involve relative motions of material particles that can lead to microcracking and damage or strengthening, can cause disturbances in the observed response with respect to the responses under the two reference states. Then, the observed response is expressed in terms of the responses for the RI and FA states that are determined from laboratory tests on material specimens. The DSC permits development of unified constitutive models that include, hierarchically, other previous continuum and damage models as special cases. Various aspects of the DSC are verified here with respect to laboratory behavior of two materials, a cemented sand and a ceramic composite. Some of the unique features of this study are that (1) the constitutive behavior and parameters can be obtained from the stress-strain-volume change behavior, and from the measurements of ultrasonic P-wave velocities, (2) correlations between mechanical and ultrasonic response can be established, (3) the concept can provide a description of the crack density, and (4) it can be simplified for predicting the remaining life of materials through definition of constitutive models and evaluation of design moduli affected by mechanical and environmental loading.