Seminar by Dr. Emiel Amsterdam, Netherlands Aerospace Centre (NLR): "The effect of crack length and maximum stress on the fatigue crack growth of engineering alloys"

The Multi-Scale Mechanics (MSM) group (PI: Dr. Francesco Maresca) presents: The effect of crack length and maximum stress on the fatigue crack growth of engineering alloys Dr. Emiel Amsterdam, Netherlands Aerospace Centre (NLR)

Abstract:
Metal fatigue represents a lively interdisciplinary field of research, both materials science and materials engineering oriented, where the very first study dates back to the German mining advisor Wilhelm Albert in 1829. Besides the academic interest in the fundamentals, economic and safety considerations have been strong driving forces for making progress ever since the discovery of the phenomenon. Currently, the worldwide annual loss of engineering structures due to metal fatigue is estimated at billions of dollars and replacement of these structures consumes a large amount of labour, energy and raw materials.

The fatigue crack growth rate (FCGR) curve of metallic alloys is usually divided into three regions. Region II is often referred to as the Paris regime and is usually modelled with a power law relationship with a single exponent. Regions I and III are located at the beginning and end of the FCGR curve, respectively, and are frequently modelled with asymptotic relationships. Here, we hypothesize that fatigue crack growth is governed by power law behaviour at all crack lengths and all stress intensity factor ranges (ΔK). To accommodate for the changes in the FCGR slope at regions I - III mathematical pivot points are introduced in the Paris equation. Power law behavior with the presence of pivot points enables direct fitting of the crack length vs. cycles (a-N) curve to obtain the FCGR as a function of ΔK. This novel approach is applicable to small and long crack growth curves and results in accurate multilinear FCGR curves that are suitable for reconstruction of the measured a-N curves. The method also shows that the FCGR increases with maximum stress for a given ΔK and stress ratio when the maximum stress approaches the yield stress. This newly discovered maximum stress phenomenon becomes important in the case of fatigue testing, where the initial crack lengths are usually small and maximum stresses are high.

Bio
Dr. Emiel Amsterdam obtained his MSc in Physics at the University of Groningen (RUG). The subject of his master thesis was the exciton diffusion length and photoluminescence in conjugated polymers. In 2008 he obtained his PhD at the group of Materials Science in Groningen. The topic of his thesis was related to structural performance and failure analysis of aluminium foam. After his PhD he got a position at the Royal Netherlands Aerospace Centre (NLR) in Marknesse and worked on life assessment of high temperature parts in turbine engines. Over the last 14 years, he has conducted numerous failure analysis investigations on aircraft & engine parts and was guest editor for the journal Engineering Failure Analysis. As a failure investigator he was also part of the team investigating the downing of flight MH17 over eastern Ukraine. Currently, he is a senior scientist and his research topics are related to fatigue and fatigue crack growth in engineering alloys, composites and 3D printed metals.