Stage Of Fatigue Crack Growth Research Articles

Static Strength and Fatigue Performance of Aluminium-Adhesive T-Peel Joints

Numerical predictions of the tensile stresses within adhesively-bonded aluminium T-peel joints have established where critical fatigue defects are likely to initiate and propagate to fracture. These predictions of maximum stress correlate directly with static strength measurements of such joints. The results presented within this article elucidate the region of cohesive crack initiation, its subsequent direction of propagation and the relative duration of the different stages of fatigue crack growth. Separate stages of embedded, surface and through-width fatigue growth of cohesive defects with a T-peel joint are identified and compared. In this manner the fatigue life of typical bonded aluminium-epoxy T-peel joints has been established from the first stages of crack initiation to final joint fracture. It has been shown that joint static and fatigue strengths are maximized by having a maximum amount of adhesive within the fillet region of the joint.

Fatigue Growth of Cohesive Defects in T-Peel Joints

Abstract This paper uses 2D and 3D finite element models to predict the stresses within bonded and weld-bonded T-peel joints. Epoxy adhesive is modelled as a homogeneous layer providing a perfect bond between aluminium adherends. Knowledge of the critical tensile stresses enables the likely region of fatigue crack initiation to be predicted. The long term reliability and durability of a joint depend directly on its fatigue strength. This research elucidates the region of cohesive crack initiation, the subsequent direction of crack propagation and the relative duration of the different stages of fatigue crack growth. The various stages of embedded, surface and through-width fatigue growth of cohesive defects within a T-peel joint are compared. This establishes fatigue life from crack initiation to final joint fracture for typical bonded and weld-bonded T-peel joints.

Development of cohesive fatigue cracks in T-peel joints

This paper uses two- and three-dimensional finite element models to predict the stresses within bonded and weld-bonded T-peel joints. Epoxy adhesive is modelled as a homogeneous layer providing a perfect bond between aluminium adherends. Knowledge of the critical tensile stresses enables the likely region of fatigue crack initiation to be predicted. The long-term reliability and durability of a joint depend directly on its fatigue life. This research elucidates the region of cohesive crack initiation, the subsequent direction of crack propagation and the relative duration of the different stages of fatigue crack growth. The various stages of embedded, surface and through-width fatigue growth of cohesive defects within a T-peel joint are compared. This establishes fatigue life from crack initiation to final joint fracture for typical bonded T-peel joints.

The influence of hammer peening on fatigue in high-strength steel

An investigation of the influence of hammer peening on fatigue crack initiation and propagation in high-strength steel has been performed using edge notch specimens. The crack initiation time was found to decrease after peening; however, the fatigue crack growth stage remained unchanged or decreased depending upon the peening parameters. The results have been used to develop an analytical model to predict the fatigue crack growth rate in peened materials, based upon the compressive residual stress effect.

Fatigue crack propagation in high strength low alloy steels

This paper considers the crack propagation behavior of a bainitic HSLA-steel with a minimum yield strength of 600 MPa. As in the welded structures, crack-like defects are present and they can affect the resistance to failure by crack growth. Coarsening of the grain corresponded to a higher threshold level δK 0 during the early stage of fatigue crack growth. Sensitivity to the mean stress level increases with increasing crack growth rates.

Laws of surface crack growth in alloy AMtsS during low-cycle loading

1. Alloy AMtsS specimens with a crack fracture under low-cycle loading after three stages of fatigue crack growth: elastic crack growth (rp 10, δ>0.1 mm). 2. The fracture of flat-rolled structural elements made of alloy AMtsS with surface cracks is quasistatic under conditions of axial low-cycle loading. 3. The nominal net stress equal to the yield strength is a criterion of the transition from elastic-plastic to quasistatic crack growth in alloy AMtsS under low-cycle loading. 4. Crack opening equal to the critical crack opening in short-term static strength tests,\(\delta _{c_s } = \delta _{c_c } \), is a criterion of unstable growth of surface cracks in alloy AMtsS under low-cycle loading.

Micromechanisms of fatigue crack growth exposed to load transient effects

The present paper concentrates on microscale influences in the fatigue crack growth stage. Research into the area of fatigue crack growth exposed to load transient effects not only aids fatigue-life predictions but also adds to a fundamental understanding of fatigue-damage mechanisms. The present results refer to tests carried out on Al alloys and stainless steel.

The early stage of fatigue crack growth in martensitic steel

In low ductility and high strength steels, the early stage fatigue behavior associated with non-metallic inclusions is a highly localized phenomenon near the inclusions. However, the nature of the fatigue crack initiation process is not clear. In this paper, a special emphasis is placed on the possible differences in the mechanism of initiation and the early growth of fatigue cracks between a martensitic steel and ordinary ductile materials.

Minimizing Fatigue and Fracture in Steel Bridges

Steel bridges are subjected to live loads which produce variable stress ranges in bridge components. At welded bridge details, the existence of initial defects and residual stresses eliminate the initiation stage of fatigue crack growth, and stress range is found to be the controlling factor for crack propagation. Laboratory tests have resulted in stress range-fatigue life relationships for various bridge details. These data correlate well with fracture mechanics theory and with field data. Limits on live load stresses have been adopted for steel bridges. Coupled with material fracture toughness requirements, the stress range limits minimize the probability of fatigue and fracture in steel bridges.

Fatigue Life Estimates for Bluntly Notched Members

Methods for estimating the total fatigue lives of bluntly notched members are presented. Analytical estimates are compared to experimental data generated in the Society of Automotive Engineers Cumulative Fatigue Damage Test Program. The crack initiation lives were predicted using strain life concepts and fracture mechanics methods were employed to predict the fatigue crack growth stages. The methods used highlighted the proportions of the total fatigue life spent in crack initiation and propagation. These were found to depend on load amplitude and level of mean load. For the bluntly notched member investigated the initiation and early stages of crack formation formed the major part of the total life, when the loading was predominantly tensile. Conversely for a loading history where most of the cycles were compressive the majority of the fatigue life was expended in crack growth away from the notch.

Kinetics of fatigue crack growth in ferrosilicon in a vacuum and in air

1. Two stages of fatigue crack growth were observed in ferrosilicon, alloys in air and in a vacuum. The first is one of slow crack growth characterized by a low value of acceleration, while in the second stage acceleration abruptly increases. 2. The rate of crack growth is less in a vacuum than in air: 10 times less in the first stage, and 10–4.0 times less in the second stage in proportion to crack growth. 3. Fatigue crack growth in both stages is accompanied by the formation on the fracture surface of brittle cleavage faces and fatigue striae, as well as sections of tough fracture. 4. With loading both in air and in a vacuum, the spacing of striae over the entire length of the crack is practically constant. 5. The macroscopic rate of crack growth in the stage of slow growth is less than the rate measured by the spacing of striae, being less by a factor of 3 in air and by a factor of 30 in the vacuum. This can be explained by stoppages of the fatigue crack and, in particular, by the fact that a striation is not formed after each cycle. 6. The decrease in the rate of fatigue crack growth in a vacuum compared to the rate in air is apparently due to the facilitating of stress relaxation at the crack tip.