Effect of R load ratio on fatigue crack growth resistance of steels used in automotive applications: experimental results and use of performance prediction models

This research consisted in comparing the fatigue crack growth (FCG) performance of four HSLA/ AHSS steels used automotive applications and with different microstructures, application some prediction models for da/dN versus ΔK traditional sigmoidal curve as a function R load ratio. FCG tests were carried out on C(T) test specimens R-ratios varying between 0.03 0.7. Using original empirical methodology proposed by Paris Erdogan to describe da/dN-ΔK relationship, results showed significant differences microstructure, deleterious effect R-ratio increase rate. In order check existing methodologies based physical considerations predicting behavior materials mainly threshold ΔKth region, well-known closure model Elber, an approach using two parameters driving force Vasudevan co-authors combination these recently Zhu compared. The manifestation its qualitatively expected dependence verified studied steels, but Elber was not able provide master that accurately summarized steels. combined use critical thresholds, ΔKth* Kmax*, according also did accurate evaluating R-ratio. Regardless dispersions, there is connection two-parameter closure, hence could be promising alternative. However, this dispersions unable create adequately predict growth. Thus, it can concluded topic still open, requiring more in-depth phenomenological knowledge FCG.