Fatigue and Fracture of Engineering Materials and Structures

Structural components are generally subjected to a wide stress spectrum over their lifetime. Service loads are accentuated at the areas of stress concentration, mainly at the connection of components. When there is a critical level of multiple site damage at connections, cracks link up to form a large crack which abruptly reduces the residual strength of the damaged structural member. Therefore, it is important to estimate the fatigue life before the cracks link up due to critical multiple site damage. In this study, the extended finite element method was applied to predict lifetime under constant amplitude cyclic loadings of fatigue tests on several multiple site damage specimens made of Al 2024‐T3. Then the multiple crack growths under service stress spectra are calculated to investigate the effects of compressive stress, stress orders and the effect of sequence cyclic loadings on stress levels by using Forman and NASGROW equations.

Fatigue crack growth behaviour under mixed modes I and II was studied by applying in‐phase alternating tensile and torsional loading to a thin‐walled hollow cylindrical specimen with an initial crack.

In the linear region of a log‐log plot where da/dN=A(ΔK)^m, da/dN at first decreases with increasing ΔK₁₁₀ component and then approaches a minimum close to the value of ΔK₁₁₀/ΔK₁₀∼ 0.58; here ΔK₁₁₀/ΔK₁₀ is the ratio of the initial ΔK_II to the initial ΔK₁., When ΔK₁₁₀/ΔK₁₀ increases further, da/dN increases. Under shear mode, da/dN becomes higher than that under mode I. The ΔK₁, and ΔK₁₁ components during fatigue crack growth under mixed mode loading increase and decrease, respectively, with an increase in da/dN In the low crack growth rate region the fatigue crack growth rates accelerate with an increase of the initial ΔK₁₁ component, ΔK₁₁₀. Fatigue life increases with increase of ΔK₁₁₀/ΔK₁₀ under the test condition of equivalent stress range being kept constant and the pre‐crack length being the same.

Abstract— This paper reports a study on the factors influencing dislocation slip during cyclic deformation of 316L austenitic stainless steel. TEM investigations show that low temperature and interstitial nitrogen content favour planar slip and lead to higher erective stress values. Measurements of effective and internal stresses with the Handfield‐Dickson technique indicate that the contribution of nitrogen in the effective component is more important than that of temperature. It is deduced that nitrogen acts through a pinning effect, while low temperature exerts an effect on friction stress. The results also suggest that cyclic plasticity could modify the short range order leading to a redistribution of nitrogen.

A new mechanism modelling is proposed in this paper to explain the shot peening effect on fatigue life predictions of mechanical components. The proposed methodology is based on the crack growth analysis of shot peened specimens, which are affected by the interaction of surface roughness and residual stress produced during the shot peening process. An asymptotic stress intensity factor solution is used to include the surface roughness effect and a time‐varying residual stress function is used to change the crack tip stress ratio during the crack propagation. Parametric studies are performed to investigate the effects of surface roughness and the residual stress relaxation rate. Following this, a simplified effective residual stress model is proposed based on the developed mechanism modelling. A wide range of experimental data is used to validate the proposed mechanism modelling. Very good agreement is observed between experimental data and model predictions.

Abstract— The whole damage process in a finite sized specimen with interacting microcracks is simulated by a method combining the closed form crack solutions with boundary elements. Interactions among microcracks and boundary elements are taken into account with an explicit interaction matrix. A coalescence criterion is assumed to rule the intersection behaviour and propagation arrest. The fatal coalescence cluster resulting in the failure of the specimen, out of many intersections of propagating microcracks, is identified with a particular coalescence matrix. The numerical model proposed in this paper can be used to simulate the damage process in a brittle specimen of any shape, under arbitrary plane stress conditions.

Adhesively bonded lap shear joints have been investigated widely and several ideas have been proposed for improving joint strength by reducing bondline stress concentrations. These include application of adhesive fillets at the overlap ends and use of adhesive with graded properties in the overlap area. Another, less common, approach is to deform the substrates in the overlap area in order to obtain a more desirable bondline stress distribution.

Previous work carried out by the authors on a number of different substrate materials indicated that a reverse‐bent joint geometry is useful for increasing joint strength. Results from static stress analysis and experimental testing demonstrated that significant improvements could be achieved. This paper presents results of further work carried out to assess the fatigue performance of reverse‐bent joints. Substrates with different yield and plastic deformation characteristics were used and the effects of different overlap lengths on strength were examined. The results of this research show that the improvements obtained under static tests conditions translate to even higher benefits in fatigue. The paper also explains the failure mechanism of the joints under fatigue loading.

A novel wavy lap joint design was further studied. Our previous studies using cross‐ply composite adherends showed that the new design was indeed much stronger than the conventional flat joint. In order to fully demonstrate advantage of the new wavy lap joint over the conventional single lap joint, comparative fatigue tests were performed to determine the durability performance of the wavy joint. In this study, a comparative static strength test of the conventional flat joint and the wavy joint was first carried out using unidirectional composite adherends. Then fatigue tests at different load levels and load frequencies were conducted. The test results showed that the wavy lap joint had a much longer fatigue life than the conventional lap joint.

Fatigue damage modelling and life prediction of engineering components under variable amplitude loadings are critical for ensuring their operational reliability and structural integrity. In this paper, five typical nonlinear fatigue damage accumulation models are evaluated and compared by considering the influence of load sequence and interaction on fatigue life of P355NL1 steels. Moreover, a new nonlinear fatigue damage accumulation model is proposed to account for these two effects. Experimental datasets of pressure vessel steel P355NL1 and four other materials under two‐block loadings are used for model comparative study. Results indicate that the proposed model yields more accurate fatigue life predictions for the five materials than the other models.

High‐cycle fatigue (HCF) properties of two Al‐Si‐Cu‐Mg‐Ni alloys with different defect sizes named as alloys A (smaller ones) and B (bigger ones) were investigated at 350°C and 425°C, respectively. The results indicate that fatigue strengths of both alloys decrease as the temperature increases. Fatigue cracks originated from pores and oxide films at both temperatures. They propagated preferentially through cracked matrix at 350°C and debonded interface and grain boundary at 425°C. Alloy A exhibits higher fatigue life and fatigue strength than alloy B at 350°C due to its smaller pore sizes. However, it has slightly worse fatigue properties than alloy B at 425°C because the fatigue crack initiation is controlled by oxide film at this temperature and is not affected by its size. This indicates that there is a transition of predominant initiation site from pores to oxide films when the temperature increases. The fatigue strength estimated through defect size is consistent with the experimental results at 350°C, while unsuitable at 425°C.