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Congratulation Doctor Nader Heshmati!
Nader Heshmati defended his PhD “Effect of microstructure on pre- and post-punching fatigue behavior of hot-rolled thick-plate advanced high-strength steel” on January 31, at KTH.
His research tackles a key challenge in heavy vehicle manufacturing: how to make truck chassis lighter and stronger using Advanced High-Strength Steels (AHSS). While these steels are great for safety and fuel efficiency, thick AHSS plates can be tricky to process—especially when it comes to hole punching, which can weaken the material.
Nader’s thesis dives into how punching affects fatigue resistance in different types of AHSS. His findings? The results show that punching alters the microstructure of the steel, creating defects, refining grains, and introducing residual stresses that impact fatigue resistance. The study found that at high applied stresses, the post-punching fatigue performance is primarily governed by the fatigue crack propagation behavior of the as-rolled microstructure. However, at low applied stresses, strain localization between microconstituents, along with punching-induced changes such as surface roughness, residual stress, and post-punching microstructure, have a more significant effect on fatigue behavior after punching.
Opponent: Kip Findley, professor at Colorado School of Mines.
Supervisors: Peter Hedström and Annika Borgenstam, KTH.