A Comparison of Macroscopic Fracture Surface and Crack Growth Rate of Ti-6Al-4V

A comparison between crack growth rate (da/dN) vs. effective stress intensity range factor (ΔKeff) curve behavior and microscopic and macroscopic fracture surface of commercial Ti-6Al-4V alloy are presented. Three different regimes are correlated with characteristics measured on the fracture surfaces. Three regions can be observed in which part I is rough and darker than others parts known as pre-transition, part II is smooth and light known as transition region and part III is a little darker than part II known as post-transition region. In the present investigation the correlation of fatigue crack growth rate for Ti-6Al-4V and microstructure of fracture surface has been presented.

Fatigue Crack Growth Rate Characteristics of Laser Shock Peened Ti-6Al-4V

The fatigue crack growth rate (FCGR) characteristics of Laser Shock Peened (LSP) titanium 6Al-4V were examined and compared to those of unprocessed material. The FCGR resistance of LSP processed material tested at low stress ratios (R) is shown to be significantly greater than for unprocessed, baseline material. The increased damage tolerance can be attributed to the large residual compressive stresses generated by the LSP process. Differences in the growth rate behavior due to LSP can be accounted for by using the closure corrected effective stress intensity range, ΔKeff, which takes into account only the portion of loading above the crack opening load. The rationale of using ΔKeff is also demonstrated through fractographic investigations.

The Overload Retardation Effect in Fatigue Crack Propagation of Worm-Like Graphite Cast Iron

In this paper, single and multiple overloading tests under cyclic controlled stress were carried out on specimens of Worm-like Graphite Cast Iron (WGCI), a brittle material. The results showed that the overloading had a strong retardation effect on this material when the overload ratio r (ratio between the overload range and the baseline load range) is larger than 1.2. For example, the number of cycles of retardation due to overloading, N’, may be as high as ten times the cycles, N, required for the same crack length under baseline loading. In addition, for the testing conditions examined, there is no significant difference in the retardation effect under single or multiple overloading for this material. An analytical model based on the concept of crack closure and the effective stress intensity range was developed for evaluating the effect of the retardation. The development led to a correlation between the number of applied cycles and the crack size after overloading and permits to calculate the retardation cycles N’. Finally, the mechanisms attributed to the retardation effect for WGCI are discussed.

Prediction of Fatigue Threshold of Notched Components

A consistent method is proposed to predict significant properties concerning the notch fatigue threshold from three experimental data: the fatigue limit of smooth specimens, the threshold stress intensity range for a long crack, and the effective stress intensity range at the threshold for a long crack. Based on the model of the intrinsic crack combined with crack closure, the fatigue limits for crack initiation and fracture from a notch, the length of a nonpropagating crack, and the notch tip radius at the branch point are all derived in a consistent way. The predicted results are compared with the experimental data of a low-carbon steel, and good agreement is obtained.