PSI - Issue 57

Pierrick Lepitre et al. / Procedia Structural Integrity 57 (2024) 395–403 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Determining a full S – N curve is a time-consuming and an expensive task. Since numerous parameters influence fatigue properties, generating fatigue curves for every configuration is impractical. To speed up the fatigue life mapping process, instead of using an empirical approach and only considering the number of cycles at rupture, fatigue mechanisms can be investigated. To achieve such tasks, the self-heating under cyclic loadings method, proposed by Doudard et al. (2005) and Munier et al. (2014), studies the dissipation mechanisms and links them to the high cycle fatigue mechanism. The 300M steel, an ultra-high tensile strength steel (R p,0.2 ≈ 1 600 MPa, AMS6257-F (2016)), is used in aeronautics in landing gears to achieve strength, weight reduction and lifespan requirements. To improve fatigue properties, surfaces are conventionally shot peened. Few studies have been made on the fatigue properties and fatigue mechanisms of this configuration (Pei-ge and al. (1996), Pistochini and Hill (2011), Bag et al. (2019, 2020)). We propose to apply the self-heating method on polished 300M steel coupons; effects of shot peening on the self-heating curve and their links with the fatigue properties will be considered in future work. In this paper, self-heating testing and post-processing protocols are implemented for polished 300M steel on fatigue coupons to validate this approach for such a steel grade. The self-heating method has been validated for various steel grades by Munier et al. (2014) but to the authors ’ knowledge, this is the first time that the approach is applied for an ultra-high tensile strength steel. This paper is divided in four parts. Firstly, the experimental setups and the post processing protocol are described. Secondly, the results are exposed. To be validated, testing and post processing protocols are applied on six test coupons for a load ratio R = -1. Then the self-heating model is presented and applied to experimental results. These results are compared with conventional high cycle fatigue tests data, showing an excellent correlation. Finally, experimentalresults at another load ratio are correlated with the same level of success.

Nomenclature C p

Specific heat

Δ

Intrinsic dissipation

C the

Thermo elastic coupling term

S 0

Normalizing stress of the self-heating model Process intensity of the first regime of the self-heating model Process intensity of the second regime of the self-heating model Mean test coupon temperature rise Mean stabilized test coupon temperature rise Estimation of the fatigue limit by the self-heating model Density

E d Dissipated energy density per cycle P f, 1/99% Fatigue limit range for 1 & 99 % probability of failure K t Stress concentration factor R Stress load ratio ( ⁄ ) R p,0.2 Conventional yield strength at 0.2 % plastic deformation T bj Bottom jaw temperature T ep Test coupon temperature T tj Top jaw temperature f r Loading frequency m+2 Slope of the second regime of the self-heating model p Slope of the first regime of the self-heating model t Time Γ Gamma function

α

β

θ

θ stab

ρ

σ AE

σ D σ a

Mean conventional fatigue limit

Loading amplitude Maximum stress Minimum stress

σ max σ min

σ x τ eq

Mean stress

Equivalent time

RSD

Relative standard deviation