Observation of Crack Growth Behavior of Various Cracks in 4.762mm Diameter Silicon Nitride Balls under Cyclic Compressive Load

In order to investigate the effect of pre-crack lengths on strength of silicon nitride balls under cyclic pressure loads, growth behavior of 600~700μm pre-cracks were compared to those of 200μm~300μm and 400~500μm pre-cracks. Furthermore, the change in initial threshold limit of the maximum stress intensity factor was discussed. It was found that the increasing ratio of stress intensity factor during N=0 and N=1000 distinguished the failure and non-failure, and pre-crack length had strong effect on the threshold limits of the increasing ratio.

Consideration on Fatigue Crack Growth Thresholds Under Negative Stress Ratio

Fatigue crack growth thresholds ΔKth are provided by several fitness-for-service (FFS) codes. When evaluating cracked components subjected to cyclic loading, maximum stress intensity factor Kmax and/or minimum stress intensity factor Kmin are required. However, the definitions of the thresholds ΔKth under negative stress ratio R are not clearly written, except for BS (British Standards) 7910. In addition, the ΔKth are given by constant values under negative R. Fatigue crack growth rates under negative stress ratio is recommended to use maximum stress intensity factor Kmax by ASTM (American Society of Testing and Materials) E 647, because of the Kmax being close to crack driving force. Therefore, it deems that the ΔKth under negative R seems to be Kmax. This paper shows that the Kmax converted by the ΔKth are not constant values under negative R based on the survey of experimental data. The Kmax decreases with decreasing the stress ratio R. Therefore, the ΔKth for the FFS codes are less conservative. As experimental data under negative stress ratio R were taken by Kmax – Kmin, the definition of the threshold ΔKth is benefit to use Kmax – Kmin, instead of Kmax.

Calculation of the Stress Intensity Factors for Elliptical Cracks Embedded in a Weld under Tensile Loading I：Single Crack

In this paper, an embedded elliptical crack in a weld of pressure vessels under tension was taken into consideration, and stress intensity factors at the crack tip were calculated numerically with the emphasis on the influences of the weld surface. It is found that when the embedded depth is 4 times larger than the minor semi-axis of the ellipse, the weld surface effects on the crack can be neglected and the numerical solutions for the stress intensity factors well agree with the analytical ones. This result can be used to distinguish a shallow embedded crack from a deep embedded crack. It is also found that the point with maximum stress intensity factor is always located at the end of minor axis of the ellipse no mater the shape of the ellipse is and how deep it was embedded.

The Evaluation of the Effects of the Maximum Stress Intensity Factor for Fatigue Crack Opening Behavior by Finite Element Analysis

The cyclic crack tip opening displacement is well related to fatigue crack opening behavior. In this paper, we investigate the effect of the maximum stress intensity factor, Kmax, when predicting fatigue crack opening behavior using the cyclic crack tip opening displacement obtained from FEA. The commercial finite element code, ANSYS, for fatigue crack closure analysis in this study is used. We derive the prediction formula of crack opening behavior when using the cyclic crack tip displacement obtained from the FEA. The numerical prediction shows the good results regardless of stress ratios. It is confirmed that the crack opening behavior depends upon the maximum stress intensity factor Kmax.

Fatigue Strength Evaluation of Spot-Welded Multi-Lap Joint by the Maximum Stress Intensity Factor, Kθmax

Fatigue strength and reliability of structural components in railroad commercial vehicles are usually decided by spot weld. Thus, it is very important to develop a fatigue design method for spot welded multi-lap joints in railroad commercial vehicles. The objective of this paper is to provide an integrated fatigue design criterion for spot-welded multi-lap joints used in the body of railroad commercial vehicles. Based on finite element analysis, the fatigue life data for spot-welded multi-lap joints is correlated in terms of the maximum stress intensity factor, K θ max , that is the fracture mechanical parameter. From the results, using Δ K θ max - N f relationship, a proper fatigue life design rule independent on geometric variables and material properties for components having spot welded multi-lap joints can be established.