What is it about?
We create a number of example geometries to represent a test specimen. The surface is represented with a roughened profile, and a number of randomly positioned pores are included in the subsurface. A loading cycle is defined and the stresses arising from those loads are calculated for each geometry. The nominal stress level, being the level that would be seen in a similar geometry but with a perfectly smooth surface and no porosity, is set to be below the stress level for material failure, but the presence of roughness or porosity leads to localised regions that have higher stresses. These stresses cause localised plastic yielding, and on repeated loading, this yield develops. We compare the effects of surface roughness, different pore sizes and the proximity of pores, to understand how certain arrangements can be more detrimental than others.
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Why is it important?
The way in which multiple load cycles leads to component failure is known as "fatigue". While fatigue is a well-known problem, and there are well defined testing strategies to determine the fatigue life of an engineering component, these tests are expensive to perform, and the cost of fatigue testing, and the necessary time needed to carry out that testing, is a major obstacle to the efficient design of high duty components. We are doing this work to obtain a phenomenological understanding, which should - eventually - mean that cheaper and shorter measurement and test programmes can be developed.
Perspectives
In carrying out this work, and future work to look at other aspects that would need to be considered (such as pore shape, pores breaking the surface, variation in loading cycles or combination loads, etc), I have been adopting an approach set out in Sir Francis Bacon's "Novum Organum". This book, published in 1620, sets out principles for the scientific method, and Bacon anticipates the need to understand complex issues, and tease out the effects of multiple variables. Science problems with multiple complex interactions are becoming solvable in modern times, thanks to the growing capability of computers, and computational algorithms. The "fun fact" here is that the need for this type of thinking was anticipated so long ago.
Alison McMillan
Glyndwr University
Read the Original
This page is a summary of: Combined effect of both surface finish and sub‐surface porosity on component strength under repeated load conditions, Engineering Reports, August 2020, Wiley,
DOI: 10.1002/eng2.12248.
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