Superveniencia como irreducibilidad y dependencia

Under variable amplitude loading, the one phenomenon that is often used to explain the retardation o f the fatigue crack propagation life is crack closure. Numerous studies have been undertaken on fatigue crack growth retardation phenomena, but they often do not agree. The discussion is usually on the effect o f plasticity induced crack closure. This is where plastic deformation occurs in the crack wake, is sufficient to force the two faces of the crack to make contact before the applied load is fully released. Many theoretical arguments and finite element analyses have been produced, but insufficient experimental results have proved to be inconclusive. This could be due to the difficulties in defining the actual crack closure itself or the measurement o f closure phenomenon.

The understanding in the retardation mechanisms has yet to be clarified fully in terms of the effectiveness o f closure. Most o f the debates about crack closure arise from the work published by Elber [1.34] in 1970, with the concept o f crack closure due to residual stress or plasticity induced crack closure. This is a very important concept, as it indicates that the fatigue crack growth is not determined by AK, but rather by the effectiveness of AK. He based his work on the influence of prior plastic deformation on crack closure. His conclusion on the influence o f closure on fatigue crack growth remained controversial, but it does highlight that it is one of the mechanisms responsible for the closure. The closure phenomenon may not be the most crucial mechanism in the crack propagation process, however, one must be aware of its existence. There are four main closure mechanisms that are normally considered.

1.4.2.1 Plasticity Induced Crack Closure

The plasticity induced crack closure is more closely related to the work done by Elber. There are two main concepts for the plastic residual stress induced closure. First idea is that the plastic deformation occurred at the crack tip (Figure 1.7) due to overloading.

Chapter 1 Introduction and Background Work on Fatigue Testing

causing the crack tip to close. The second idea is that the plastic deformation left behind by the crack propagation forcing the crack surface together (Figure 1.8).

Elber [1.34] showed through observation on a thin sheet o f aluminium alloy that the fatigue crack can close during tensile loading. He suggested that the deformed wake of residual stress left behind by the crack growth is the major contributor in causing the closure o f the crack surface and slowing down the driving force for the crack advance. The mechanism is schematically displayed in Figure 1.8 showing the compressive residual stress left behind as the crack advances. However other work in the 70s and 80s have shown that Fiber’s closure mechanism is not the sole cause o f closure. Shih and Wei [1.35] disputed the work carried out by Elber, as the amount o f data was limited and concluded that crack closure is more dependent on the load ratio, R and Kmax-

1.4.2.2 Oxide Induced Crack Closure

This mechanism of crack closure was introduced when trying to explain the strange behaviour of fatigue crack growth near the threshold region, under the influence o f a corrosive environment in steels and aluminium. Paris [1.36] was the first to make reference to the oxide induced crack closure as represented in Figure 1.9. When a crack propagates, the new metal comes into contact with the environment, and a thin oxide layer is formed. The layer of metal oxide formed behind the crack tip can wedge the crack face, causing the closure effect. It has been proposed that as the crack propagates, the fretting mechanism causes the oxide layer to come loose and a new oxide surface is formed. This can be built up to twenty times the thickness o f a fireshly formed layer. However, this mechanism is only significant at a lower load ratio, R, while at a higher value of R, the crack will propagate faster, thus the fretting mechanism o f the oxide will not be as effective. To include the oxide induce crack closure into fatigue crack propagation is very difficult, because the oxide layer left behind usually has inconsistent thickness and cannot be quantified properly.

Chapter 1 Introduction and Background Work on Fatigue Testing

1.4.2.3 Roughness Induced Crack closure

Roughness induced closure (Figure 1.9) is like oxide induced crack closure as both mechanisms are usually used explaining the irregular microstructure crack growth behaviour near the threshold value. Originally, it was believed that fine grain and wavy slip formation give rise to better fatigue resistance. Later research [1.33] near the threshold stress range found that in most alloys, the mechanism is probably due to coarser grains and enhanced planar slip deformation mode. This mechanism is only effective with a low load ratio and a small crack tip opening displacement. In most offshore applications, the R value is likely to be high thus the effect o f this mechanism is expected to be minimal.

1.4.2.4 Problems in Quantifying Crack Closure

There are many complex issues in fatigue crack growth, in order to quantify the effect of crack closure in fatigue behaviour for both experimental and theoretical studies. This is because crack closure is very specific to certain conditions and the magnitude of the closure is very dependent on the material microstructure, environment and stress state. Small variations within the microstructure can affect the crack propagation rate, especially near threshold. Also, environmental conditions can exert small changes in the surface condition and affect the formation of oxide. Specimen size and geometry, crack size, stress state and load history also have great influence on the crack closure. The effect of crack closures can be included into the calculation, if they can be quantified. There are many methods for measuring crack closure; these include strain gauges, clip gauges, but the main difficulties are in distinguishing which mechanisms are responsible for the crack closure within. Thus, these methods do not provide adequate insight into the influence of closure mechanism on fatigue crack propagation. Net result is that whatever the mechanism or reason; the best method is to employ the effective stress range, AKg/f.

Chapter 1 Introduction and Background Work on Fatigue Testing

For most fatigue tested offshore welded joints, the load interaction effect is unlikely to be great. This could be down to several reasons. The specimens are large thus the closure effect should be negligible. The single high overload or peak stress are very unlikely to occur amongst smaller peaks as it is untypical for offshore service loading. Lastly, the steels employed for offshore structures normally have small plastic zones ahead of the crack tip, thus limiting the effect o f plasticity crack closure. In conclusion, the effect of crack closure is unlikely to greatly influence the crack growth rate in welded offshore steels, but its effect should be noted.