Energy-based mechanistic approach for damage characterization of pre-flawed visco-elasto-plastic materials

Abstract Damage characterization plays a significant role in producing durable and high performance structural materials. However, it is somewhat complicated because of the particular characteristics of many materials, such as pre-existing flaws, time-dependent behaviors, and coexistence of cracking and permanent deformation. This kind of materials is pre-flawed visco-elasto-plastic material. In order to characterize damage in such materials, this paper proposes an energy-based mechanistic (EBM) approach that provides a complete solution to these problems. As typical pre-flawed visco-elasto-plastic materials, asphalt mixtures are selected to demonstrate the principles and applications of the EBM approach. When an asphalt mixture is not damaged, the pre-existing flaws are air voids, characterized by the average air voids size and number of air voids calculated by the EBM approach. The calculated values are more accurate than those measured by the X-ray Computed Tomography system. Due to the increased accuracy, it is discovered that the air voids becomes smaller when the mixture is aged, which serves as an evidence of the change of the internal structure of the material due to aging. When an asphalt mixture is damaged, the damage includes cracking and permanent deformation. The cracking damage is a multitude of randomly distributed cracks. A new concept, distributed continuum fracture (DCF), is introduced to model the distributed cracks in the EBM approach. Development of cracking damage is quantified by the evolution of damage density, average crack size and number of cracks. The damage densities of eight different mixtures are proven to correctly reflect the effect of mixture composition and aging. New features of number of cracks discovered lead to new definitions of cracking history in pre-flawed materials. The energy for permanent deformation is separated from that expended for cracking in the same asphalt mixture. Such a separation acknowledges the fact that cracking and permanent deformation always occur simultaneously. The separated energy for cracking is used to define a cracking energy dissipation rate, a direct indicator of cracking susceptibility of asphalt mixtures. In a word, the EBM approach is able to characterize damage in asphalt mixtures under various conditions using one type of test on one specimen. It requires simple inputs: stress, strain, and time, and all the calculations are performed automatically by the Excel. Using this approach to analyze the test data is more efficient than some alternative methods because of less testing effort and more informative results with improved accuracy.