Published fatigue crack closure mechanisms are reviewed in the context of the following five ideas that have been developed over the past twenty years to explain the closure effects on near-threshold crack growth behaviour: (1) oxide, (2) asperity, (3) plasticity, (4) phase transformation and (5) viscous fluids. The first three have been considered as more important than the last two. Our analysis indicates that (a) there can be no contribution from plasticity to crack closure, (b) the crack closure contribution can be significant only if it is closed fully, which is reflected as an infinite slope in the load-displacement curve, (c) formation of oxide asperities from a fretting action is a random process and not a deterministic one, and therefore cannot explain the deterministic behaviour of the effect of load ratio on the threshold, and (d) the closure contribution from the asperities resulting from oxides or corrosion products,or surface roughness, it is less than 20% of what has been deduced based on the change in the slope of the load-displacement curves. Thus the analyses show that crack closure can exist, but its magnitude is either small or negligible. The critical evaluation of the literature data on (1) the threshold stress intensity variation with load ratio on many materials, and (2) and examination of the experimentally observed load-displacement curves confirm the above conclusions. Hence to rationalize the observed variations in the fatigue crack threshold with load ratio, we have proposed a new model. It postulates a requirement for two critical stress intensity parameters, namely ΔK th ∗ and K max ∗ , that must be satisfied simultaneously as the crack tip driving forces for fatigue crack growth. This requirement is fundamental to fatigue, since an unambiguous description of cyclic loads requires two independent load parameters. Several experimental results from the literature are presented in support of this postulation. Using these two critical parameters, the entire functional relationship between Kmax, ΔKth and R is explained without invoking an entrinsic factor, namely crack closure. In addition, for a given material and its crack tip environment, a unique relationship between ΔKth and Kmax exists that is independent of test methods used in determining thresholds. Finally, because of this two parameter requirement, all fatigue crack growth data need to be represented in terms of three-dimensional plots involving da/dN, ΔK and Kmax. For a two-dimensional representation, the data need to be transformed correctly, defining the net driving force involving both ΔK and Kmax parameters. The concepts presented are independent of whether crack closure exists or not, or even whether cracks exist or not.

A review of crack closure, fatigue crack threshold and related phenomena

A state-of-the-art review on passivation and biofouling of Ti and its alloys in marine environments

Effects of surface treatments on high cycle corrosion fatigue of metallic implant materials

Cyclic stress-strain response and dislocation substructure evolution of a ferrite-austenite stainless steel

A simple test procedure involving stress ratio changes at the fatigue threshold is proposed to reveal the role of the lower portion of the loading cycle below Kop in the fatigue crack growth behaviour. It is observed for both Al 2024-T3 and Al 7475-T761 alloys that the initially non-propagating fatigue crack at the fatigue threshold resumes growth upon diminishing Kmin while keeping Kth,max constant. These experimental findings can be interpreted by means of a modified crack closure concept in which the contribution of the lower portion of the loading cycle below Kop (including also a part of compressive loading if R < 0) to the variation in the stress state experienced by the fatigue crack tip is taken into account.

Contribution of the cyclic loading portion below the opening load to fatigue crack growth

In this study, the effects of stress ratio, microstructure and fracture surface roughness on the fatigue properties of a two-phase cast stainless steel were investigated. This behaviour was examined by means of the fracture mechanics approach and fractography. The fatigue crack growth rate decreased with decreasing stress ratio. The stress ratio markedly influenced the fatigue crack growth rate as ΔK approached the ΔKth value. The roughness of the fracture surface was greater in the as-cast material than in the heat-treated material. Analysis of the crack growth data using ΔKeff showed that the effect of R ratio could be explained but that the effect of microstructure on crack growth rate could not.

The effects of microstructure and fracture surface roughness on near threshold fatigue crack propagation characteristics of a two-phase cast stainless steel

— Fatigue crack propagation was investigated in polycarbonate and glass fibre reinforced polycarbonate and the effect of stress ratio and glass fibre content determined. The addition of glass fibre increases the tensile strength, but does not always contribute to an increase in fatigue crack propagation resistance. For polycarbonate the effect of stress ratio can be partly explained by using crack closure concepts as other researchers have suggested, but for glass fibre reinforced polycarbonate this was not possible. Fractography revealed a void growth process, which occurred by decohesion at the interface of the glass fibres and the base material, which was dependent on the maximum stress intensity factor. The process of linking the voids and the main crack growth behavior depended on the stress intensity factor range, ΔK. A proposed crack propagation model can explain the effect of stress ratio on crack propagation in fibre reinforced polycarbonate.

Fatigue crack propagation and crack closure behavior in polycarbonate and fibre reinforced polycarbonate

The effects of stress ratio and microstructure on fatigue crack growth rate in air and natural seawater were investigated for pure titanium and its weld metal. The corrosion fatigue characterization of pure titanium was also studied under a cathodic potential in natural seawater. In air, the fatigue crack growth rates of pure titanium and its weld metal increased with increasing stress ratio. In natural seawater, the effect of stress ratio was similar to that in air. However, the crack growth rates were greater for pure titanium than for the weld metal. These results indicate that the corrosion action is sensitive to the microstructure in front of the crack tip. When the crack growth rate for the weld metal was plotted using the effective stress intensity factor range, the crack growth rate in natural seawater was coincident with that in air, regardless of stress ratio. For the base metal, there is a significant difference in the crack growth rate between natural seawater and air.

The effects of a marine environment on the corrosion fatigue crack propagation rate of pure titanium and its weld metal

— Impact fatigue tests were carried out using a rotating-disk type impact fatigue testing machine The influence of prior austenite grain size, ductile-brittle transition temperature and test temperature on impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varied from 8–3 to 25-4 μm The results in impact fatigue tests were compared to those under non-impact conditions The crack growth rates associated with striation formation were insensitive to the change in prior austenite grain size, ductile-brittle transition temperature and test temperature regardless of impact and non-impact fatigue When the material was in the brittle condition, impact fatigue gave rise to a transition from striation formation to intergranular and cleavage cracking Such a transition will result in the acceleration of crack growth rate The Paris Law exponent values in impact fatigue were reasonably expressed by the ratio of test temperature to ductile-brittle transition temperature

THE INFLUENCE OF TEST TEMPERATURE ON IMPACT FATIGUE CRACK GROWTH BEHAVIOR IN QUENCHED AND TEMPERED Cr‐Mo ALLOY STEELS WITH DIFFERENT PRIOR AUSTENITE GRAIN SIZES

The fatigue crack propagation behaviour of polycarbonate and glass fibre reinforced polycarbonate was studied in hot distilled water. The effects of temperature, distilled water and glass fibre content on fatigue crack growth rate were determined. In distilled water at 333 K, the fatigue crack growth rate decreased with increasing glass fibre content. A melting was observed of the adhesive bond between the glass fibres and the matrix. It was evident that the fracture morphology closely relates to the fatigue crack growth rate which depends on the ΔK parameter rather than the Kmax parameter when the materials are tested in hot distilled water.

Ri-ichi Murakami

Papers

The effects of microstructure and fracture surface roughness on near threshold fatigue crack propagation characteristics of a two-phase cast stainless steel

Fatigue crack propagation and crack closure behavior in polycarbonate and fibre reinforced polycarbonate

The effects of a marine environment on the corrosion fatigue crack propagation rate of pure titanium and its weld metal

THE INFLUENCE OF TEST TEMPERATURE ON IMPACT FATIGUE CRACK GROWTH BEHAVIOR IN QUENCHED AND TEMPERED Cr‐Mo ALLOY STEELS WITH DIFFERENT PRIOR AUSTENITE GRAIN SIZES

Fatigue crack propagation characteristics of glass fibre reinforced polycarbonate in hot distilled water