The **Ramberg-Osgood equation** describes the nonlinear relationship between the stress and the strain of a material around the yield point of the material. The equation is often used in conjunction with **Neuber’s rule**, to estimate plastic stress and strain based on linear stress calculation results.

# Blog

## Certainty of Survival

The standard error SE of log_{10}(N) is used to adjust the fatigue life or the damage predicted to any given **probability of survival**. Fatigue life data always include some scatter, and at any given level of stress, the distribution of fatigue lives is assumed to be a log-normal distribution. Or stated differently, a Normal or Gauss distribution of the logarithm of the fatigue life.

## Surface Roughness Factor KR

Fatigue tests of specimens are usually done on components with mirror polished surfaces. However, a component with a larger surface roughness will have a reduced fatigue strength. To account for the effect of this surface roughness to the fatigue strength of a material, a **roughness factor K _{R}** is used. This surface roughness factor is used to adjust the material S-N curve. This article describes the FKM method for obtaining the surface roughness factor and how to apply this factor to the S-N curve.

## What is Fatigue?

Failure due to **fatigue **is an important cause of structural failure. But what is metal fatigue exactly? And which factors influence the fatigue strength of components?