Doctoral Defense - Poornachandra Vaddy

About the Event

The Department of Civil & Environmental Engineering

Michigan State University

Ph.D. Dissertation Defense

July 21, 2025 at 11:00 AM EST

3540 Engineering Building (Seminar Room) and Zoom

Contact Department or Advisor for Zoom Information

ABSTRACT

DEVELOPMENT OF A NEW FATIGUE FAILURE CRITERION USING OPTICAL FLOW-BASED MULTI-GAUGE LENGTH APPROACH

By: Poornachandra Vaddy

Advisor: Muhammed Emin Kutay

Fatigue cracking remains one of the most critical and difficult-to-predict failure modes in asphalt concrete (AC) pavements. Conventional fatigue failure criteria, such as those based on stiffness degradation or stress × cycle product, often rely on bulk, fixed-gauge measurements that assume uniform damage accumulation throughout the specimen. However, asphalt concrete is a heterogeneous material where failure typically initiates and propagates in localized regions. This dissertation presents the development of a new fatigue failure criterion based on the onset of localized strain relaxation, captured through a high-resolution, non-contact, optical flow-based strain measurement system.

The experimental setup included a customized camera-based measurement system integrated with the Asphalt Mixture Performance Tester (AMPT). The system allowed localized strain tracking by dividing the specimen’s gauge length into multiple segments. Fatigue tests were conducted under cyclic loading, while companion uniaxial monotonic tests were used to study the initiation and propagation of cracks under controlled displacement conditions. Results from monotonic loading revealed that macrocrack formation coincided consistently with the inflection point of the load–time curve, confirmed across three different AC mixtures. Under cyclic loading, a novel failure definition was proposed as the first instance of localized strain relaxation, offering earlier and spatially resolved detection of damage compared to traditional global methods. The new failure criterion was benchmarked against the conventional stress × cycle approach. Across all tested specimens, the relaxation-based method identified failure significantly earlier—often two to five times sooner than the traditional approach. This was particularly pronounced in softer mixtures, where the relaxation behavior and strain divergence became more evident. In addition, block-wise strain measurements revealed consistent strain localization patterns, validating the need for spatial resolution in fatigue damage assessment.

While the findings demonstrate the robustness of the new failure definition, certain limitations were identified. These include limited camera field of view, and difficulty pinpointing inflection points in soft mixtures with gradual load response. Recommendations include expanding the imaging system to all sides of the specimen, improving synchronization, and testing additional mixtures, including highly modified binders such as HiMA. The developed criterion also presents opportunities for integration into structural models based on the Viscoelastic Continuum Damage (VECD) theory and/or Finite Element Method (FEM).

Overall, this study advances the understanding of fatigue failure in AC by redefining it as a spatially localized and temporally trackable event. The proposed framework enhances mechanistic fatigue modeling and supports the development of more resilient, performance- based pavement design strategies.

Persons with disabilities have the right to request and receive reasonable accommodation. Please call the Department of Civil and Environmental Engineering at 517-355-5107 at least one day prior to the seminar; requests received after this date will be met when possible.