Crack Closure Stress Intensity Research Articles

Effects of deformation behavior on fatigue fracture surface morphology in a nickel-base superalloy

Fatigue crack propagation fracture surface morphologies in nickel-base superalloys vary substantially with changes in loading parameters such as temperature, Δ K, load ratio, frequency, and additionally microstructure. Quantitative fracture surface roughness can vary from sub-micron levels to a maximum value of approximately half the grain size. Atomic Force Microscope studies of surface slip traces in compression specimens revealed a clear relationship between slip homogeneity in compression testing and fracture surface roughness under similar fatigue loading conditions. It has been shown in this study that changes in Δ K, strain level, temperature, grain size, and load ratio can all affect slip heterogeneity, which in turn controls the fracture surface roughness. Finally, a model is developed that quantitatively predicts fracture surface roughness and roughness-induced crack closure stress intensity values from measurements of slip line spacing in a compression specimen.

Plasticity-induced crack closure under plane-strain conditions in the near-threshold regime

Abstract The change in cyclic plastic deformation at a fatigue crack tip as a function of crack extension and the variation in the crack closure are investigated. The cyclic plastic deformation is modelled by the motion of discrete dislocations. At constant applied crack driving force, the cyclic plastic deformation, that is the cyclic crack-tip-opening displacement, decreases with increasing crack extension. This is caused by an increase in stress intensity factor where the crack closes or, in other words, by a decrease in the local driving force. The crack closure stress intensity reaches a constant level after a crack extension of the order of the size of the plastic zone. Near the threshold of stress intensity this effect, namely plasticity-induced crack closure, is somewhat larger than in the Paris regime.

A multiparameter approach to the prediction of fatigue crack growth in metallic materials

A multiparameter approach is proposed for the characterization of fatigue crack growth in metallic materials. The model assesses the combined effects of identifiable multiple variables that can contribute to fatigue crack growth. Mathematical expressions are presented for the determination of fatigue crack growth rates, da/dN, as functions of multiple variables, including stress intensity factor range, ΔK, stress ratio, R, crack closure stress intensity factor, Kcl , the maximum stress intensity factor Kmax , nominal specimen thickness, t, frequency, Ω, and temperature, T. A generalized empirical methodology is proposed for the estimation of fatigue crack growth rates as a function of these variables. The validity of the methodology is then verified by making appropriate comparisons between predicted and measured fatigue crack growth data obtained from experiments on Ti–6Al–4V. The effects of stress ratio and specimen thickness on fatigue crack growth rates are then rationalized by crack closure considerations. The multiparameter model is also shown to provide a good fit to experimental data obtained for HY‐80 steel, Inconel 718 polycrystal and Inconel 718 single crystal. Finally, the implications of the results are discussed for the prediction of fatigue crack growth and fatigue life.

A NEW MULTIPARAMETER APPROACH TO THE PREDICTION OF FATIGUE CRACK GROWTH

A new multiparameter approach is proposed for the prediction of the combined effects of multiple variables on fatigue crack growth. The method, which is based on multiple linear regression analysis, involves the statistical formulation of mathematical expressions for the crack growth rate, da/dN, as a function of multiple variables, e.g. stress intensity factor range, ΔK, crack closure stress intensity factor, Kcl , and stress ratio, R. A general empirical approach is proposed for the estimation of the fatigue crack growth rate as a function of the above variables. The predictive capability of the empirical approach is then verified by comparing predicted and measured fatigue crack growth and crack growth rate data obtained from tests on a quenched and tempered Q1N (HY80) pressure vessel steel. Error ranges and reliability functions are presented within a probabilistic mechanics framework, and the implications of the results are discussed for the development of generalized fatigue life prediction methods.

Cyclic fatigue crack growth in silicon nitride: Influences of stress ratio and crack closure

The characteristics of cyclic fatigue crack growth was investigated in silicon nitride which hardly shows rising R-curve behavior, using single edge precraked beam specimens. Crack growth behavior is known to be explicable by a power law relationship including both the stress intensity range ΔK and the maximum stress intensity Kmax; i.e. da/dN = C (Kmax)p(ΔK)q. However, when Kmax is a constant, the crack growth rate da/dN is found to be not a unique power law function of ΔK. The curve in a double-logarithmic plot of da/dN and ΔK is composed of two regions. The slope q in the lower ΔK region is larger than that in the higher ΔK one. The difference of the q value between the two regions is primarily responsible for crack closure. Namely, the crack growth rate for a constant Kmax does correlate with the range of the effective stress intensity ΔKeff defined as ΔKeff = Kmax − Kcl, where Kcl is the crack closure stress intensity. It is, therefore, suggested that the crack growth behavior is expressed by a crack growth law containing the effect of crack closure, i.e. da/dN = C (Kmax)p(ΔKeff)q. On the basis of these results and other data available, cyclic fatigue mechanisms are discussed.

The effects of stress ratio on fatigue threshold

Arising from the fact that there is a reverse compression stress around a fatigue crack, it is supposed that there is an equivalent compressive high-strain area. Based upon the eigenequation of crack growth, the critical value, K th c, of the compressive stress intensity factor is derived. The analytical expression for the change in fatigue threshold with stress ratio is derived from the similarity between the crack closure stress intensity factor and K th c. The fatigue threshold values for some materials are calculated using the results of this paper; it is found that the theoretical values are in good agreement with the experimental results.

A CRACK CLOSURE STUDY TO PREDICT THE THRESHOLD BEHAVIOUR OF SMALL CRACKS

Abstract— The small crack problem is addressed within the applicability of Linear Elastic Fracture Mechanics as the result of crack closure phenomenon. The variation of crack closure stress intensity factory Kop as a function of crack length, a, was determined in two materials, namely a A508 steel and a 2024A1 alloy. These results were obtained on two‐dimensional small cracks (a≫ 0.1 mm) which were machined from long fatigue cracks. These measurements of Kop in addition to data published in the literature on a nodular cast iron and a 9Cr–1Mo steel yield to a unique characteristic function: Kop/Ko= 1 –exp(–ka) in which k is the only parameter to characterize the small crack effect. A prediction of the threshold behaviour of small and long cracks on A508 steel is made using the results of crack closure measurements.

CRACK CLOSURE AND FATIGUE CRACK GROWTH RATE IN THREE POINT BEND SPECIMENS OF DIFFERENT WIDTHS

Abstract— The crack closure stress intensity factor values and fatigue crack growth rates were determined in Three Point Single Edge Bend, SE(B), specimens prepared from rails manufactured using two different grades of rail steels. The width, and correspondingly the span, of the SE(B) specimens were varied eight fold; the thickness of all the specimens being the same. It is observed that the crack closure stress intensity factor values decrease with an increase in the width of SE(B) specimens. At a given value of ΔKeff, the fatigue crack growth rate (FCGR) is independent of the width. However, at a given value of ΔKeff, the FCGR is observed to decrease with increasing width. In view of the above results, the scope of application of the FCGR laws based on an effective stress intensity factor to the life prediction of components, requires careful examination.

The influence of grain size and fracture surface geometry on the near-threshold fatigue crack growth in ferritic steels

Fatigue crack growth in the threshold region is very much affected by the microstructure. A large portion of this microstructural influence is due to changes in the crack closure stress intensity K cl . The present paper is concerned with the influence of the grain size on the near-threshold fatigue crack growth in ferritic steels, with special emphasis on the influence of the microstructurally induced fracture surface roughness on the crack closure stress intensity. The experiments were performed on ferritic steels with grain sizes varying between 8 and 82 μm. An increase in the nominal and effective fatigue crack growth threshold values with increasing grain size was found. Quantitative fractographic measurements showed that the fracture surface roughness rises with the grain size independently of the stress intensity level. The results also indicate that K cl is proportional to H ̄ 1 3 , where H ̄ is the fracture surface roughness as expressed by the mean standard deviation of height. By rationalizing the experimental data, ΔK th has been quantified in terms of grain size effects and the fracture surface roughness.

A model for fatigue crack closure

The phenomenon of fatigue crack closure has attracted continued interest over recent years. This paper concerns itself with one aspect of the phenomenon namely the effects of a single asperity on the crack face close to the crack tip and under dominantly plane strain Mode 1 loading conditions. The model as developed is two dimensional in character but the nature of the closure processes allows it to be applied to real materials. The model illustrates the influence of asperity height, the distance from the crack tip of the asperity and rigidty of the asperity on the crack closure stress intensity. Application of the model to the case of microstructural asperities in nickel alloys and to oxide asperities in steels has met with reasonable success.

Variation of crack opening-load diagram with fatigue crack growth rate

This study is concerned with the fatigue threshold of the aluminium alloy AU2GN. By analysis of load-displacement curves, the authors have determined the crack closure load (Pop) and the elastic response of crack opening (Pel). Thus, it has been shown that at the fatigue threshold level, the crack is open elastically. The R ratio has little influence on crack closure but K max affects its behavior. K op disappears when K min is higher than the crack closure stress intensity factor.