A Comparative Analysis of Step Loading and Staircase Testing for Fatigue
Strength Estimation of an Engine Component
Abstract
Staircase testing is a standard method for evaluating the fatigue
strength of components. However, staircase testing assumes a normal
distribution, while components can display bimodal behavior due to flaws
in material, or issues during the manufacturing process. Three unique
step loading data sets on different production crankshafts provide
evidence that step loading reliably identifies material or manufacturing
issues, that lower a component’s fatigue strength. Staircase testing has
an 87% or greater chance of overestimating the component’s fatigue
strength, which in turn overestimates the component’s expected
reliability. For example, a component with a 99.9% reliability, based
on staircase testing would only have a 74% reliability based on step
loading. If a component contains an undetectable manufacturing defect,
staircase testing has a 99% chance of overestimating the component’s
fatigue strength. Step loading reliably improves the estimation of a
component’s fatigue strength distribution while providing insights into
a component’s defect tolerance.