The microstructure and fatigue properties of three model AS7G03 cast aluminium alloys containing artificial pores have been studied. Synchrotron X-ray tomography has been used to characterise in three dimensions the pore population in the alloys. The development of fatigue cracks in relation with local crystallography has been studied by means of electron back scattered diffraction (EBSD). Both the average number of cycles to failure and the lifetime scatter depend on the pore content specially at high stress level. The mechanism leading to the initiation of a crack from a pore has been identified. The crack propagation at high stress level appears to be quite insensitive to microstructural barriers and can be reasonably well described by a Paris type law. At low stresses, however, short cracks are often observed to be stopped at grain boundaries and the fatigue life is no longer predicted by a simple propagation law.

Experimental study of porosity and its relation to fatigue mechanisms of model Al–Si7–Mg0.3 cast Al alloys

In this model the growth of a crack is analysed in terms of the successive blocking of the plastic zone by slip barriers (e.g. grain boundaries) and the subsequent initiation of the slip in the next grain. The discontinuous character of the slip process (slip jumps) plays a fundamental role in the model. The factor governing the transfer of slip across a grain boundary is considered to be the stress concentration ahead of the plastic zone which, for a constant applied stress τ, is found to be dependent only on a parameter n = a/c defining the position of the crack tip relative to the grain boundary. The discrete behaviour of the slip has a strong influence in the short-crack period and hence cannot be neglected in the analysis of the crack growth rate. This period is characterized by large variations in the parameter n. In the long-crack period the slip jumps do not influence the overall description of the growth and the parameter n is almost constant. By making the crack extension per cycle prop...

Short and long fatigue crack growth: A unified model

http://qmxmt.com/scans/dave/other/papers/xmt%20applications/xmt_metalfatigue.pdf

Study of the interaction of a short fatigue crack with grain boundaries in a cast Al alloy using X-ray microtomography

Modelling of small fatigue crack growth interacting with grain boundary

Mechanics issues related to the formation and growth of cracks ranging from subgrain dimension to up to the order of one mm are considered under high cycle fatigue (HCF) conditions for metallic materials. Further efforts to improve the accuracy of life estimation in the HCF regime must consider various factors that are not presently addressed by traditional linear elastic fracture mechanics (LEFM) approaches, nor by conventional HCF design tools such as the S-N curve, modified Goodman diagram and fatigue limit. A fundamental consideration is that a threshold level for ΔK for small/short cracks may be considerably lower than that for long cracks, leading to non-conservative life predictions using the traditional LEFM approach. Extension of damage tolerance concepts to lower length scales and small cracks relies critically on deeper understanding of (a) small crack behavior including interactions with microstructure, (b) heterogeneity and anisotropy of cyclic slip processes associated with the orientation distribution of grains, and (c) development of reliable small crack monitoring techniques. The basic technology is not yet sufficiently advanced in any of these areas to implement damage tolerant design for HCF. The lack of consistency of existing crack initiation and fracture mechanics approaches for HCF leads to significant reservations concerning application of existing technology to damage tolerant design of aircraft gas turbine engines, for example. The intent of this paper is to focus on various aspects of the propagation of small cracks which merit further research to enhance the accuracy of HCF life prediction. Predominant concern will rest with polycrystalline metals, and most of the issues pertain to wide classes of alloys.

Basic issues in the mechanics of high cycle metal fatigue

The growth characteristics of small fatigue cracks were investigated under rotary bending in a low alloy steel prepared with two prior austenite grain sizes of 15 μm (fine grain) and 91 μm (coarse grain). The influence of grain boundaries on crack growth rate and the crack aspect ratio was examined, and the critical crack length above which linear elastic fracture mechanics (LEFM) is applicable was evaluated for a growing small crack. When the surface crack length is shorter than three grain diameters (3d), crack growth rates decrease near the grain boundaries. Aspect ratios are also affected by the microstructure and thus vary widely. Cracks longer than 3d are not influenced by the microstructure, but they grow faster than would be expected based on LEFM until their lengths reach 3d+ 150 μm. This behaviour may be attributed to the difference in crack closure between small cracks and large cracks. If the contribution of crack closure to the growth of small cracks can be established experimentally or analytically, the critical crack length above which LEFM is applicable would be 3d. However, because it is difficult to evaluate crack closure, 3d+ 150 μm is considered to be the critical crack length for engineering applications.

Limitations of linear elastic fracture mechanics in respect of small fatigue cracks and microstructure

https://www.jstage.jst.go.jp/article/jsms1963/34/382/34_382_816/_pdf

各種環境下におけるオーステナイト系ステンレス鋼の疲労強度 : 疲労微小き裂の発生, 密度・分布および成長

Characteristics of fatigue crack propagation have been investigated in a low carbon steel and a high tensile strength steel to evaluate the effect of sheet thickness. Crack propagation data are generated over a wide range of growth rates, from 10-8 to 10-3 mm/cycle, for load ratios of 0.05 and 0.70 at room temperature in laboratory air. Particular emphasis is placed on behaviour at near-threshold growth rates. Near-threshold fatigue crack propagation behaviour is found to show a marked sensitivity to sheet thickness, and near-threshold growth rates decrease and threshold values increase with increasing sheet thickness. Roughness- and oxide-induced crack closure are proposed as a mechanism for the effect of sheet thickness on near-threshold fatigue crack propagation. It is also shown that the requirement for specimen thickness recommended by ASTM, W/20lBlW/4, is not valid for near-threshold fatigue crack propagation.

Zenji Ando

Papers

Limitations of linear elastic fracture mechanics in respect of small fatigue cracks and microstructure

各種環境下におけるオーステナイト系ステンレス鋼の疲労強度 : 疲労微小き裂の発生, 密度・分布および成長

The Effect of Sheet Thickness on Near-Threshold Fatigue Crack Propagation and Oxide and Roughness-Induced Crack Closure

The effect of sheet thickness on near-threshold fatigue crack propagation and oxide- and roughness-induced crack closure.

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments