Microscale modeling of fatigue crack growth at particle interfaces in viscoelastic asphalt materials

The viscoelastic damage evolution at the microscale particle interfaces of asphalt mortar or mixtures fundamentally determines the material's fatigue crack growth and failure at the macroscale. However, existing microscale damage models are often based on empirical assumptions that depend on interparticle stresses or forces, which are inaccurate or even incorrect for viscoelastic asphalt materials. In these materials, interfacial crack growth is governed by the viscoelastic energy release rate. To address the limitations of current models, a microscale viscoelastic fatigue damage model was developed using a pseudo J-integral-based Paris’ law and implemented in a discrete element model of asphalt mortar using the PFC2D program. The viscoelastic constitutive behavior was represented by a generalized Maxwell model, and the relaxation moduli were determined through a uniaxial compressive dynamic modulus test. The Paris’ law coefficients were calibrated by comparing model predictions with experimental results from indirect tensile fatigue tests of the material. The results show that the simulated fatigue life and crack area closely match laboratory test data, with an error margin within 15%. During the simulation of microscopic IDT fatigue damage, cracks hinder the horizontal transfer of forces within the cracked region, leading to stress concentrations in surrounding particles and a marked increase in their relative displacement. The connection of upper and lower cracks significantly reduces the specimen's load-bearing capacity. The variation in the number of contact breaks with fatigue load cycles is unaffected by the type of asphalt but is influenced by the applied stress level. These findings demonstrate that the pseudo J-integral-based Paris’ law, when applied at particle interfaces, can effectively model crack growth at the microscale and accurately predict the fatigue damage performance of viscoelastic asphalt materials at the macroscale.