Fracture Beyond Fatigue

Abstract:  

“For decades, the steel design community has relied on well-established fatigue and fracture frameworks to ensure structural safety. These frameworks are grounded in stress range-based fatigue categories, fracture toughness requirements, initial flaw size assumptions, fabrication-related considerations, and anticipated service temperatures. They have served the profession well and continue to underpin modern design practice. In most cases, fracture in steel structures is understood as the result of fatigue crack initiation and propagation under repeated loading, or as originating from a pre-existing flaw, with material toughness acting as the final safeguard against unstable failure.

Yet, experience has shown that not all fractures follow this expected path. In some cases, a steel component may fracture without evidence of classical fatigue damage or an identifiable pre-existing flaw. This lecture explores two distinct cracking mechanisms that can occur outside traditional fatigue-based expectations and discusses approaches for achieving reliable structural performance in each case.

The first examines cracking associated with hot-dip galvanizing of welded steel components. Galvanizing introduces large thermal gradients and exposure to molten zinc, creating conditions under which liquid metal assisted cracking (LMAC) can occur in susceptible details. This form of fracture is not driven by cyclic loading, nor does it depend on inadequate fracture toughness in the conventional sense. Instead, cracking can develop from the interaction of stress, temperature, material condition, and exposure to liquid zinc during a transient manufacturing process. This portion of the lecture illustrates how design, detailing, fabrication, and galvanizing choices influence susceptibility, and how informed engineering judgment can be used to mitigate risk while preserving the durability benefits of galvanizing.

The second focuses on constraint-induced fracture (CIF), a mechanism in which local geometric constraint elevates stress triaxiality and suppresses plastic deformation, enabling brittle fracture to occur under nominally ductile conditions. Unlike cracking that can develop during galvanizing, CIF develops in-service and is largely governed by structural detailing and local constraint conditions. This lecture explores how small changes in detail geometry, such as constraint-relief gaps and attachment configurations, can fundamentally alter fracture behavior.

Together, these topics highlight fracture considerations that are relevant across a wide range of steel applications and project phases, from detailing and fabrication through long-term service. The lecture is aimed at broadening how engineers think about fracture in steel structures when behavior extends beyond traditional fatigue-based expectations.

Bio:

“Caroline Bennett is the Charles E. & Mary Jane Spahr Professor in the Department of Civil, Environmental, and Architectural Engineering (CEAE) at the University of Kansas (KU). She earned her Bachelor of Science and doctorate from the University of Cincinnati, and joined the KU faculty in 2006. She currently serves as Chair of the KU CEAE Department.

Dr. Bennett’s research has addressed some of the most pressing challenges in structural engineering, including fatigue and fracture performance of steel infrastructure, cracking in galvanized structures, constraint-induced fracture in steel bridges, and innovative structural retrofit strategies for a wide range of structures. She has led national projects with the Federal Highway Administration, the National Cooperative Highway Research Program, the U.S. Army Corps of Engineers’ Engineer Research and Development Center, and state Departments of Transportation, delivering solutions that extend the service life of structures and improve infrastructure performance.

Dr. Bennett serves on AISC’s Committee on Research and TC10 Materials and Fabrication Committee. She recently served on AISC’s Game Changer panel. She contributes to the American Iron and Steel Institute (AISI) Steel Bridge Task Force and Transportation Research Board committees, strengthening connections between research and practice. Bennett received AISC’s Special Achievement Award in 2024, AISC’s inaugural Early Career Faculty Award in 2016, and was named the Robert J. Dexter Memorial Lecturer in 2013 by AISI.