Development of a New Fatigue Failure Criterion using Optical Flow-Based Multi–Gauge Length Approach

Fatigue failure in asphalt concrete (AC) pavements is typically assessed using global degradation metrics such as stiffness reduction or stress-cycle (S × N) relationships. However, these conventional approaches overlook the localized and heterogeneous nature of damage evolution in AC mixtures. This study introduces a novel fatigue failure criterion based on the onset of localized strain relaxation, identified using a high-resolution, non-contact optical flow-based strain measurement system developed for the Asphalt Mixture Performance Tester (AMPT). The non-contact strain measurement system, referred to as MIMAQ (Michigan IMage AcQuisition), enables microstrain-level tracking across multiple gauge segments without the need for speckle patterns or external synchronization. Fatigue and uniaxial monotonic tensile tests were conducted on AC specimens using a multi–gauge length approach, allowing for spatial resolution of strain evolution. Results from uniaxial cracking tests confirmed that macrocrack initiation consistently aligned with the load–time inflection point, while fatigue tests demonstrated that strain relaxation in localized regions preceded conventional failure indicators by a factor of two to five. A complementary metric, macrocrack propagation time (MPT), was introduced to quantify the temporal spread of failure across the specimen. The proposed relaxation-based failure criterion offers earlier, and more mechanistically interpretable detection of fatigue failure compared with traditional methods. It also enables performance assessment at lower target strain levels, reducing test durations. The findings support the potential for incorporating spatially resolved strain data into fatigue modeling frameworks such as viscoelastic continuum damage (VECD) or finite element models, contributing to the development of more resilient and performance-driven pavement designs.