S-N Curve Characterisation for Composite Materials and Prediction of Remaining Fatigue Life Using Damage Function

Ho Sung Kim,Saijie Huang

doi:10.3390/jcs5030076

Ho Sung Kim, Saijie Huang

Open Access

https://doi.org/10.3390/jcs5030076

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Abstract

S-N curve characterisation and prediction of remaining fatigue life are studied using polyethylene terephthalate glycol-modified (PETG). A new simple method for finding a data point at the lowest number of cycles for the Kim and Zhang S-N curve model is proposed to avoid the arbitrary choice of loading rate for tensile testing. It was demonstrated that the arbitrary choice of loading rate may likely lead to an erroneous characterisation for the prediction of the remaining fatigue life. The previously proposed theoretical method for predicting the remaining fatigue life of composite materials involving the damage function was verified at a stress ratio of 0.4 for the first time. Both high to low and low to high loadings were conducted for predicting the remaining fatigue lives and a good agreement between predictions and experimental results was found. Fatigue damage consisting of cracks and whitening is described.

Highlights

S-N curve characterisation is important for engineering materials and for the fail-safe design and fatigue life prediction of various components subjected to dynamic loading
The literature shows that a data point at the lowest fatigue life for an S-N curve has been arbitrarily determined for fatigue characterisation, and that an ultimate strength obtained from the static test at an arbitrary loading rate (e.g., 1 mm/min [6] or 5 mm/min [7]) was used as the peak stress at the lowest number of loading cycles
Eskandari and Kim [8] recently rationalised that the lowest number of loading cycles for S-N behaviour should be 0.5 in the case of a stress ratio (R) of zero for predicting the remaining fatigue life according to the fatigue damage theory

Summary

Introduction

S-N curve characterisation is important for engineering materials and for the fail-safe design and fatigue life prediction of various components subjected to dynamic loading. Eskandari and Kim [8] recently rationalised that the lowest number of loading cycles for S-N behaviour should be 0.5 in the case of a stress ratio (R) of zero for predicting the remaining fatigue life according to the fatigue damage theory. In the light of the deficiencies in the past methodology and verification for the theory of fatigue damage, the purpose of this paper was to: (a) develop a method for determining the initial peak stress within the 1st loading cycle using tensile test results; (b) verify the damage function proposed by Eskandari and Kim [8] for predicting the remaining fatigue life at a high stress ratio of 0.4 using validly determined initial peak stress; and (c) investigate the S–N fatigue behaviour of PETG. A Matlab script for determining α and β is given in the Appendix A

Prediction of Remaining Fatigue Life

Determination of the Exponent n

Development of Method for Data Point at the Lowest Number of Loading Cycles

Conclusions