Determining the endurance limit of AISI 4340 steels in terms of different statistical approaches

Rıza Gürbüz,Salim Çalışkan

doi:10.3221/igf-esis.58.25

Rıza Gürbüz, Salim Çalışkan

Open Access

https://doi.org/10.3221/igf-esis.58.25

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Abstract

In engineering applications, fatigue phenomenon is a key issue and needs to be analyzed in the beginning of design phase in case of any component exposed to alternating loading on operation otherwise catastrophic fatigue failure may cause. Component can be designed with safe-life, fail-safe, and damage tolerant approach based on whether redundant load path and damage sensitive. Before starting analyzing the structure, material allowable data needs to be presented in a reliable way to predict fatigue life of components. SN curves with presented confidence levels are the robust approach to make a prediction on safe life of a structure in terms of fatigue. In this point, there are so many approaches to determine fatigue limit of materials and issue shall be handled by statistical manner. In literature, different staircase and curve fitting methods were presented to estimate endurance limit of materials and some reliability manuscript published. In this paper, fatigue limit of AISI 4340 steel will be investigated through most convinced staircase and curve fitting approaches and their reliability will be queried.

Highlights

S N curves have been firstly used by German engineer, Wöhler and stress-life was formulated in linear equation; hereby, logarithm of life is correlated with oscillating stress [1]
Even though fatigue strength exhibits normal distribution extensively, coefficient of variance in fatigue limit reflects lognormal distribution and scatter is greater than initiation dominant life region compared to propagation one that is under high stress loading
Fatigue strength of a material can be determined by evaluation of staircase or curve fitting methods statistically

Summary

Introduction

S N curves have been firstly used by German engineer, Wöhler and stress-life was formulated in linear equation; hereby, logarithm of life is correlated with oscillating stress [1]. Schijve et al investigated fatigue limits statistical point of view and resulted in Weibull is better suited than log-normal distribution Another approach is proposed by Stüssi and non-linear equation is formulated with stress and life relation taking ultimate and infinite endurance stress into formulation. Model uses log-normal distribution and it can be solved by linear regression based on Maximum likelihood approach It gives information about how many specimens are needed to get reliable data. Increment range shall be between 0.5σ and 2σ for good estimation if standard deviation is known; otherwise, small interval results in spending time unless starting point close to mean value [16] This approach is modified by Dixon and proposed that increment step can be estimated as σ and maximum 50% error of standard deviation can be tolerated. Primary condition of staircase method is to estimate by normal distribution; safety factor needs to be evaluated through log-normal distribution to prevent oversizing

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