Investigative Method for Fatigue Crack Propagation Based on a Small Time Scale

Abstract

In-situ scanning electron microscopy (SEM) testing based on a small time scale is proposed to integrally investigate the fatigue crack growth behavior and mechanisms, which is different from the widely-used, cycle-based approach due to its small time scale and comprehensive analysis of the effects of microstructure, crack closure and applied loading on crack growth. In the proposed methodology, the behavior of fatigue crack growth at any time within a loading cycle is observed by SEM to investigate the influence of microstructure on crack growth. Images with high resolution are taken to measure the crack tip opening displacement (CTOD), and the correlation between CTOD and the stress intensity factor (SIF) K is studied. A model based on experimental data is used to predict the CTOD variation. The unstable crack growth of aluminum alloy 7050-T7451 is investigated using the proposed method. Results show that this method has great potential in fatigue crack growth mechanism research compared with the traditional cycle-based approach.

In situ SEM fatigue testing technology for metallic materials: a review

“…• Small time scale method • DIC • Effect of boundaries and texture on fatigue crack growth behavior [58][59][60][61] • Overloading in fatigue testing…”

“…51,[65][66][67] Both hydrogen charging and corrosion fields cause an increase in the fatigue crack growth rate of the material, but due to the limitation of conventional SEM imaging conditions, such experiments need to be conducted under ESEM, and the related work has not been carried out sufficiently. Moreover, in situ SEM fatigue technology has been developed for a variety of applied load variations, such as stress ratio, 80 creep-fatigue, 95 fretting fatigue 38,41 and small time scale methods, 59,60 which make it uniquely suited for exploring the fatigue failure behavior of materials under different loading scenarios. Among these, the small time scale method is an excellent way to investigate the fatigue crack closure behavior of materials, which will be mentioned in Section 4.…”

“…Moreover, the small time scale method is useful for studying the unstable crack propagation process. By measuring the crack tip opening displacement (CTOD) under different stress conditions within one fatigue cycle, Wang et al 60 established a CTOD–stress intensity factor range (Δ K ) model, which shows a continuous relationship between the fatigue crack increment and applied loading, and the crack length at any arbitrary time can be calculated through the integration of the instantaneous crack growth rate.…”

Effect of Precipitates on the Mechanical Performance of 7005 Aluminum Alloy Plates

“…However, there are two prevalent views on the effect of precipitate particles on the fatigue properties of aluminum alloy plates [ 8 ]. A few reports have suggested that small GP zones, which are completely coherent with the matrix, contribute to improving the fatigue performance of aluminum alloy plates.…”

Uncertainty Quantification in Small-Timescale Model-Based Fatigue Crack Growth Analysis Using a Stochastic Collocation Method

“…Extensive experiments under both constant and variable amplitudes have been implemented to validate the accuracy of the novel small-timescale model for FCG analysis [30][31][32][33][34]. Previous investigations have indicated that the presented model greatly promotes the fatigue prognosis for structural systems.…”