Fatigue crack growth in plate specimens under Mode III loading

Fatigue crack growth in plate specimens of aluminium and an aluminium alloy has been investigated under Mode III loading conditions. It has been found that only when a fully plastic situation exists, does crack extension proceed by a valid Mode III mechanism. When plasticity is restricted to planes of maximum shear there is a strong component of Mode I cracking which results in delamination in the direction of macroscopic growth. Under constant load amplitude cycling the crack growth curve exhibits three stages: a reduction in rate is followed by a period of steady growth prior to acceleration in the final stages. Such a profile is independent of material condition or failure mechanism. Cycling between fixed displacement limits leads to a reducing growth and crack arrest which occurs at a length proportional to the square of the displacement.     

Fatigue crack growth simulation of aluminium alloy under spectrum loadings

Fatigue crack growth in structure components, which is subjected to variable amplitude loading, is a very complex subject. Studying of fatigue crack growth rate and fatigue life calculation under spectrum loading is vital in life prediction of engineering structures at higher reliability. The main aim of this paper is to address how to characterize the load sequence effects in fatigue crack propagation under variable amplitude loading. Thus, a fatigue life under various load spectra, which was predicted, based on the Austen, Forman and NASGRO models. The findings were then compared to the similar results using FASTRAN and AFGROW codes. These models are validated with the literature-based fatigue crack growth test data in 2024-T3 Aluminium alloys under various overload, underload, and spectrum loadings. With the consideration of the load cycle interactions, finally, the results show a good agreement in the behaviour with small differences in fatigue life compare to the test data.     

Fatigue crack growth threshold conditions at notches. Part II: generalization and application to experimental results

The theoretical foundation of a micromechanical model that accounts for the fatigue crack growth threshold conditions at notches was described in Part I of this study. Strictly speaking, the proposed formulation is restricted to the analysis of a component with an elliptical notch under antiplane stress. In this section of the study, the expressions derived in Part I are generalized for application to axial stress states and non-elliptical notch geometries. The procedure is validated by comparing the model's predictions with reported experimental results.     

Fatigue life and crack path predictions in generic 2D structural components

This paper proposes a reliable and cost-effective two-phase methodology to predict crack propagation life in generic two-dimensional (2D) structural components. First, the usually curved fatigue crack path and its stress-intensity factors are calculated at small crack increments in a specialized finite-element software, using automatic remeshing algorithms, special crack tip elements and appropriate crack increment criteria. Then, the computed stress-intensity factors are transferred to a powerful general-purpose fatigue-design program, which has been designed to predict both initiation and propagation fatigue lives by means of classical design methods. Particularly, its crack propagation module accepts any K_I expression and any crack growth rate model, considering sequence effects such as overload-induced crack retardation to deal with 1D and 2D crack propagation under variable amplitude loading. Non-trivial application examples compare the numerical simulation results with those measured in physical experiments.     

Fatigue crack propagation in the threshold regime after rapid load reduction

In this work, the influence of rapid load reduction on fatigue crack growth in the threshold regime of the aluminium alloy 2024-T3 has been studied. It can be shown that fatigue crack growth may be severely influenced by crack closure due to oxide formation and fracture surface roughness. After rapid load reduction, crack arrest could be observed at K_max values 10–100% above the constant amplitude threshold, depending on the environment. With measurements in different environments (humid air and vacuum), the oxide-induced crack closure effect could be recognized as being mainly responsible for an increase of the stress intensity threshold. Using high-frequency fatigue testing equipment, it was possible to show that after rapid load reduction in a vacuum, cracks may begin to grow again after crack arrest of more than 5 × 10⁷ cycles.     

Fatigue crack growth threshold at cryogenic temperatures: A review

The cryogenic temperature near-threshold fatigue crack growth behavior of several structural materials: aluminum alloys, copper, steels, nickel alloys and titanium alloys, was reviewed. It was observed that the resistance to near-threshold fatigue crack propagation in the alloy systems investigated generally improved with decreasing temperature. Environmental effects were not responsible for the influence of temperature on near-threshold crack growth rate behavior. Furthermore, crack closure alone could not account for this temperature effect. The dislocation dynamics model appears to offer a possible rationale to explain the improved near-threshold crack growth performance typically exhibited at cryogenic temperatures by the variety of materials examined herein. Crack closure, on the other hand, rationalized the influence of load ratio on low temperature near-threshold crack propagation behavior.     

Sustained Mode I and Mode II fatigue crack growth in flat plates

This study was conducted to contribute to the understanding of fatigue crack growth under mixed mode loading. This was accomplished by developing and analyzing a flat plate specimen capable of maintaining crack growth on a plane oblique to the direction of the applied load. Several specimens were built and exposed to controlled fatigue loading in the laboratory. These specimens were then modeled using finite elements to determine the stress intensity factors (SIF). For the “Mode I/Mode II” specimens developed, the crack was forced to grow in a direction other than perpendicular to the load. The resulting crack front did not remain straight and flat, but stabilized into a curved or warped shape. Based on finite element analyses of these curved specimen cracks, it is concluded that the SIR were predominantly Mode I, with the Mode II and III SIR being negligible.     

Fatigue threshold and crack growth in an electron beam weld made of steel and bronze

In this research topic some experimental tests with single-edge notched beams were performed to determine the threshold load value for fatigue crack growth and to characterize fatigue crack behaviour of an electron beam weld made of steel and bronze. Subsequently, numerical analyses were done to estimate the threshold value ΔK_th and to simulate the fatigue crack growth. The calculated crack path was compared to those determined experimentally. The objective was to find out the necessary fracture properties for an analysis of an electron beam welded worm wheel and to asses the capability of usual fracture analysis software to simulate fatigue crack growth in welds.     

Mode II intersonic crack propagation in poroelastic media

A crack is steadily running in an elastic isotropic fluid-saturated porous solid at an intersonic constant speed c. The crack tip speeds of interest are bounded below by the slower between the slow longitudinal wave-speed and the shear wave-speed, and above by the fast longitudinal wave-speed. Biot’s theory of poroelasticity with inertia forces governs the motion of the mixture. The poroelastic moduli depend on the porosity, and the complete range of porosities n ∈ [0, 1] is investigated. Solids are obtained as the limit case n = 0, and the continuity of the energy release rate as the porosity vanishes is addressed. Three characteristic regions in the plane (n, c) are delineated, depending on the relative order of the body wave-speeds. Mode II loading conditions are considered, with a permeable crack surface. Cracks with and without process zones are envisaged. In each region, the analytical solution to a Riemann–Hilbert problem provides the stress, pore pressure and velocity fields near the tip of the crack. For subsonic propagation, the asymptotic crack tip fields are known to be continuous in the body [Loret and Radi (2001) J Mech Phys Solids 49(5):995–1020]. In contrast, for intersonic crack propagation without a process zone, the asymptotic stress and pore pressure might display a discontinuity across two or four symmetric rays emanating from the moving crack tip. Under Mode II loading condition, the singularity exponent for energetically admissible tip speeds turns out to be weaker than 1/2, except at a special point and along special curves of the (n, c)-plane. The introduction of a finite length process zone is required so that 1. the energy release rate at the crack tip is strictly positive and finite; 2. the relative sliding of the crack surfaces has the same direction as the applied loading. The presence of the process zone is shown to wipe out possible first order discontinuities.     

Mode II Fatigue Failure in Transparent Adhesive Joints

We study the fatigue cross section of an adhesive lap joint of glass and polycarbonate with regard for the procedure of application of the adhesive layer, its area, the levels of principal stresses, loading frequency, and the stress ratio. It is discovered that the fatigue life of the joint in laboratory air is independent of the stress ratio and loading frequency and the total number of cycles to failure is determined by the initial size of the defect. It is shown that the quality of adhesion is the principal factor guaranteeing the best fatigue life results.     

Four lectures on fatigue crack growth: III. Fatigue crack propagation,prediction and correlation

Two prediction techniques are introduced, (1) cycle-by-cycle prediction and (2) prediction by correlation. Attention is paid to the problem of describing variable-amplitude loading in terms of load cycles. Aspects of fatigue damage are reviewed with reference to interaction effects and weaknesses in cycle-by-cycle prediction methods. The discussion on prediction by correlation is restricted to constantamplitude loading. The validity of the similarity concept based on $\text{K-}\text{factors}$ is reconsidered. Application of simple specimen data to complex structures is shown. Finally a variety of crack growth equations is reviewed, including aspects of curve fitting, a comparison between formulas of Walker and Elber and asymptotic values in the $\text{da/dn}$ — Δ$\text{K}$ relation.