The statistical nature of fatigue

Some understanding of the statistical nature of fatigue is necessary for an appreciation of the scatter and variation of fatigue test results and predictions.

The scatter in fatigue test results comes from several sources. There are natural variations in material properties, component dimensions, customer service loads, and manufacturing tolerances. In comparison with the others, the variation in customer usage is typically largest. Probability distributions can show the variation for these variables.

This variation results in scatter in fatigue test results. For well-made components, a factor-of-two variation in fatigue life is not unusual in the mid- to high-cycle regime for tests conducted under the same nominal conditions.

Care must also be taken when interpreting fatigue life calculations because of this inherent scatter. Proper loads and representative material properties should be used.

In practice, design loads are often taken from a statistical analysis of actual measurements, extrapolated to represent a severe customer or severe usage such as the 99th percentile customer. Specialist Durability provides specialized tools for load data analysis and synthesis that can be used to extrapolate rainflow matrices of customer usage to a severe customer level.

The Likelihood of Weak Components Subjected to High Loads is Minimized in a Probabilistic Design Approach

In a probabilistic design approach, the goal is to minimize the likelihood of a very severe customer receiving an insufficiently strong component, as illustrated in the figure above.

The confidence levels for SN-curves are often taken at the 10%, 50%, and 90% probability of failure as in the figure below. A 10% failure probability means that for a given stress level there is a 10% probability that the specimen will fail before reaching the cycles to failure specified for the 10% probability to failure SN-curve. The 50% and 90% curves are less conservative than the 10% curve.

A typical measure of how much scatter is in the data is given by the ratio of the loads for 90% and for 10% probability of failure.

Note:

In Specialist Durability, this scatter value can be entered with the SN-curve and the strain-life material data. This scatter allows you to convert the material data from one probability of failure to another.

Typical values for the scatter are given in the next table.

Property Influencing Fatigue	Typical Scatter TS
Manufacturing geometry	1.02
Material: controlled	1.15
Material: welded	1.45
Applied loads	2.00

Use caution when performing fatigue calculations with material properties from handbooks or technical journal articles because the associated probability to failure is not often reported.

SN-Curve of a Material as Determined from Test Data for the 10%, 50%, and 90% Failure Probability Levels

Learn more

The two basic finite life approaches

Concepts in common

Damage-based data reduction methods

Non-local and surface effects

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

Browse Simcenter 3D help by product area

The statistical nature of fatigue, Simcenter 3D 2021.1 Series

© 2020 Siemens

window.mainLanguage="en_US"

window.delivId=""

window.projectId=""

MathJax.Hub.Config({ TeX: { extensions: ["autoload-all.js"] }, tex2jax: { displayMath: [ ] }, "SVG": { scale: 125 } });