By: Henry Karpovas Lisak Have you ever seen footage of the Tacoma Narrows Bridge collapse or read about the tragic failures of the de Havilland Comet, the world’s first commercial jet airliner? These are not just dramatic moments in engineering history – they are powerful reminders of how unexpected material failures can bring down even the most advanced structures. As a sophomore mechanical engineering student at the University of Minnesota, I’ve been exploring these concepts in MATS 2001: Introduction to the Science of Engineering Materials, taught by Professor Chris Haas. This course has been fundamental in helping me understand the relationship between material structure, mechanical properties, and real-world performance – and how failure often results from overlooked or misunderstood mechanisms. Fatigue: The Silent Accumulator Fatigue is one of the most common and dangerous forms of material failure. It occurs when a material is subjected to repeated loading and unloading, even when the loads are below the material’s yield strength. Over time, microcracks form and grow with each cycle until the material fails suddenly. A textbook example of this is the de Havilland Comet. In the 1950s, this jet experienced multiple in-flight breakups due to fatigue cracks that developed around the sharp corners of its square windows. The fuselage underwent cyclic pressurization with each flight, and the stress concentrations at the window corners accelerated crack growth, ultimately causing catastrophic failure. This tragedy led to major advancements in fatigue testing and design, including the adoption of rounded windows in all future aircraft. Creep: Deformation Over Time Creep is another critical mode of failure, especially in high-temperature environments. It refers to time-dependent deformation that occurs when a material is under constant stress at elevated temperatures. Early jet engines, for example, suffered from creep damage in their turbine blades, which operate under extreme heat and rotational stress. Over time, these blades would elongate and lose their shape, reducing engine performance and potentially leading to mechanical failure. Addressing this required the development of high-performance materials, such as nickel-based superalloys and single-crystal blades, which resist creep and maintain integrity over long periods. Torsion: Twisting into Instability The Tacoma Narrows Bridge collapse in 1940 is one of the most iconic failures in engineering history, often cited in materials and structures courses. Although not a case of material strength failure, it demonstrated the danger of torsional instability. Under steady winds, the bridge began to oscillate in a torsional mode due to aeroelastic flutter. The bridge lacked the necessary torsional stiffness, and the twisting oscillations amplified until the structure tore itself apart. This failure underscored the importance of considering dynamic forces – not just static loads – in structural design. Shear: Sliding at the Breaking Point Shear failure happens when a material or component fails along a plane due to forces acting parallel to each other. A real-world example is the collapse of the World Trade Center towers on September 11, 2001. While many factors contributed to the collapse, one mechanism involved the failure of shear connections between floor trusses and vertical columns. As intense fires weakened the steel, these connections were unable to resist the shear forces imposed by sagging floors. Once enough shear connections failed, the building’s structure could no longer support the upper floors, leading to a progressive and total collapse. Conclusion: Learning from Material Failures These historical examples show that materials can fail in many ways – through fatigue, creep, torsional instability, or shear – and understanding these mechanisms is essential to prevent future disasters. Courses like MATS 2001 have helped me develop a foundation in analyzing how materials respond to stress, temperature, and time. By learning from past failures, engineers can design safer, more resilient systems that stand the test of time.

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