Mirco D. Chapetti

Fatigue propagation threshold of short cracks under constant amplitude loading

Abstract In this study a threshold for fatigue crack propagation as a function of crack length is defined from a depth given by the position d of the strongest microstructural barrier to crack propagation, which defines the plain fatigue limit. The material threshold is estimated from the plain fatigue limit Δ σ eR , the position d of the strongest microstructural barrier and the threshold for long cracks, Δ K thR . The threshold for eight different materials for which experimental results can be obtained from the literature was estimated. Good agreement was observed in all cases. Some quantitative analyses of the fatigue propagation behavior of short cracks are carried out and discussed.

Prediction of the fatigue limit of blunt-notched components

Abstract A prediction of the fatigue limit of blunt-notched components of a low carbon steel was made on the basis that the fatigue limit of polycrystalline metals represents the critical conditions for the propagation of nucleated cracks. An expression for the material resistance to crack propagation as a function of the crack length is obtained for the first part of the short crack regime, which defines the blunt notch sensitivity to fatigue. The material resistance curve is modeled from a depth d, given by the position of the strongest microstructural barrier to microstructurally short crack propagation, which defines the plain fatigue limit. A microstructural threshold, ΔKd, is suggested as an intrinsic material resistance to microstructurally short crack propagation, defined by the plain fatigue limit Δσe0 and the position of the strongest microstructural barrier d. The modeled notch sensitivity fits reasonably well the experimental results for a low carbon steel.

Effect of Residual Stresses and Inclusion Size on Fatigue Resistance of Parabolic Steel Springs

Abstract The effect of inclusion size and residual stresses of spring steel grade 51CrV4 on fatigue lifetime in high cycle was investigated by experimental results obtained by fatigue testing. Since spring with obvious surface defects or inclusions at the surface are failed during pre-stress, the residual stress profile under surface becomes relevant for fatigue crack initiation and failure. Investigation shows that fatigue threshold for high cycle fatigue depends, besides residual stresses, on inclusion size and material hardness. In order to determine allowed size of inclusions in spring steel and the influence of residual stresses, the Murakamis and Chapettis models and concepts have been used. The stress loading limit regarding inclusion size and applied stress has been determined for loading ratio R = −1 and 0.1 on specimens with and without residual stresses.

The Crack Initiation and Propagation in threshold regime and S-N curves of High Strength Spring Steels

An integrated fracture mechanics approach is proposed to account for the estimation of the fatigue resistance of component. Applications, estimations and results showed very good agreements with experimental results. The model is simple to apply, accounts for the main geometrical, mechanical and material parameters that define the fatigue resistance, and allows accurate predictions. It offers a change in design philosophy: It could be used for design, while simultaneously dealing with crack propagation thresholds. Furthermore, it allows quantification of the material defect sensitivity. In the case of the set of fatigue tests carried out by rotational bending of specimens without residual stresses, the estimated results showed good agreement and that an initial crack length of 0.5 mm can conservatively explain experimental data. In the case of fatigue tests carried out on the springs at their final condition with bending at R = 0.1 our data shows the influence of compressive residual stresses on fatigue strength. Results also showed that the procedures allow us to analyze the different combinations of initial crack length and residual stress levels, and how much the fatigue resistance can change by changing that configuration. For this set of tests, the fatigue resistance estimated for an initial crack length equal to 0.35 mm, can explain all testing data observed for the springs.

Analysis of breathing cracks using vibrations

The influence of half-ellipse-shaped fatigue cracks and machined through-thickness cuts on the impact response of cantilever beam samples were analysed using the Finite Element Method and compared to experimental data for different crack depths. The effect of sample width was analysed for geometries with fatigue cracks (aspect ratio a/c = 0.45) and through-thickness cuts (rectangular cracks). The differences between both types of damage were analysed numerically and experimentally. The effect of sample width was studied and it was found that the behaviour of the cantilever with a fatigue-crack approaches that of the saw cut sample when the fatigue crack reaches the edges of the test piece. Acceleration data were analysed using the Fast Fourier Transform algorithm and Power Density Spectra (PSD) were obtained. For fatigue-cracked samples, harmonics of the frequencies of natural modes of oscillation were found. Analytical modelling was done using a mass-spring-damper system for studying the bilinear behaviour of the stiffness element. Harmonics found were assigned to the breathing behaviour of the crack. The detection threshold using frequency shifts inspection was twice as high as using spectral analysis. This would allow crack detection at half of the fatigue life of the specimen.