AN ANALYSIS OF THE MECHANISM OF MICRODAMAGE HEALING BASED ON THE APPLICATION OF MICROMECHANICS FIRST PRINCIPLES OF FRACTURE AND HEALING

The fatigue process is viewed as the result of the competing processes of crack growth and crack healing. The crack growth or damage process is one of microcrack development and growth and ultimately coalescence of microcracks and propagation in the form of macrocracks. During the entire process, and especially during the period of microcrack growth, microdamage healing strongly influences the process. The significance of the process of healing on fatigue damage is proven through laboratory testing during which a very significant recovery of dissipated pseudo energy is demonstrated following rest periods. The significance of the healing process is also proven from field data demonstrating recovery of stiffness measured by surface wave techniques. The mechanism of healing is discussed based on first principles of fracture and healing. These principles show that surface energy of the mixture constituents and mixture compliance must affect both fracture speed and healing speed. The impact of surface energy on the healing process was verified by comparing bitumen surface energies measured for various bitumens to the rate of healing of mixtures containing those bitumens. A micromechanics fracture and healing model linked to a finite element analysis of direct tensile, controlled-strain fatigue tests further verifies the link between surface energy and fracture and healing.