Influence of the loading path on fatigue crack growth under mixed-mode loading

Fatigue crack growth tests were performed under various mixed-mode loading paths, on maraging steel. The effective loading paths were computed by finite element simulations, in which asperity-induced crack closure and friction were modelled. Application of fatigue criteria for tension or shear-dominated failure after elastic–plastic computations of stresses and strains, ahead of the crack tip, yielded predictions of the crack paths, assuming that the crack would propagate in the direction which maximises its growth rate. This approach appears successful in most cases considered herein.     

Analysis of the crack growth propagation process under mixed-mode loading

In the present paper, a computational model for crack growth analysis under Mode I/II conditions is formulated. The focus is on two issues – crack path simulation and fatigue life estimation. The finite element method is used together with the maximum principal stress criterion and the crack growth rate equation based on the equivalent stress intensity factor. To determine the mixed-mode stress intensity factors, quarter-point (Q-P) singular finite elements are employed. For verification purposes, a plate with crack emanating from the edge of a hole is examined. The crack path of the plate made of 2024 T3 Al Alloy is investigated experimentally and simulated by using the finite element method with the maximum tangential stress criterion. Then, the validation of the procedure is illustrated by applying the numerical evaluation of the curvilinear crack propagation in the polymethyl methacrylate (PMMA) beam and the Arcan specimen made of Al Alloy for which experimental results are available in the literature. In order to estimate fatigue life up to failure of the plate with crack emanating from the edge of a hole, the polynomial expression is evaluated for the equivalent stress intensity factor using values of stress intensity factors obtained from the finite element analysis. Additionally, the fatigue life up to failure of the Arcan specimen is analyzed for different loading angles and compared with experimental data. Excellent correlations between the computed and experimental results are obtained.     

The stress-level effect on fatigue-crack growth under constant-amplitude loading

The fatigue-crack-closure concept has been successfully used with stress-intensity factors to predict the growth of cracks under a wide variety of load histories and in complex crack configurations. Both test and crack-closure analyses have shown that the stress-intensity-factor-range-against-rate curves are affected by the stress ratio (R), the applied stress or load level (S_max or P_max), and the crack-front constraint (plane-stress or plane-strain behavior). However, most life-prediction codes use only linear-elastic fracture mechanics (LEFM) concepts, which neglect stress-level effects, to make life predictions. Thus, under some loading conditions, such as negative R ratios or high-applied stress levels, non-conservative life predictions are made using only LEFM procedures.Fatigue-crack-growth tests have been conducted on middle-crack tension M(T) specimens made of 2024-T3 thin-sheet (B = 2.3 mm) aluminum alloy over a wide range in applied stress levels (0.1–0.5 times the flow stress of the material) and for two stress ratios (R = 0.05 and −1). The FASTRAN life-prediction code, using either the crack-closure model or LEFM procedures, and the AFGROW code, which uses only LEFM procedures, were used to make crack-growth predictions from an initial crack size to failure in the M(T) specimens. The results from AFGROW and FASTRAN, using LEFM procedures, agreed very well with each other. The crack-closure model predicted all results with ±20%, whereas, the codes using LEFM procedures (neglecting stress-level effects) resulted in non-conservative life predictions as large as a factor-of-3 from the test results.     

The effect of environments on fatigue-crack propagation in an ultra-high-strength steel

The influence of various gaseous environments on the rate of fatigue crack growth at room temperature in an 0.45 percent carbon low-alloy ultra-high-strength steel tempered at 400°F and at 800°F has been investigated. The sensitivity of the rate of fatigue crack growth to moisture in this steel is found to be dependent on the fracture toughness of the material.

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Zusammenfassung Der Einfluss verschiedener Gasatmosphären auf die Zunahme von Ermüdungsrissen Raumtemperatur wurde untersucht an ultrahochfesten Kohlenstoffstahlen von niedriger Legieiung, welche bei 205 C. (400 F.) and 427 C. (800 F.) getempert wurden. Es wurde die Abängigkeit zwischen der Bruchstärke dieses Stables and der Einwirkung auf die Fortpflanzung von Ermudungsrissen durch Feuchtigkeit gefunden.

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Résumé L'influence de divers environnements gazeux sur le taux de croissance des fissures de fatigue à température ambiante d' un alliage à 0,451o de carbone d' un acier à très haute résistance trempé à 400 F. et à800 F.àétéétudiée.La sensibilité du taux de croissance des fissures de fatigue à l' humidité dans cet acier a été trouvée dépendre de la resistance à la cassure de ce matériau.

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This research was carried out while the authors were affiliated with the Applied Research Laboratory, United States Steel Corp. , Monroeville, Pennsylvania.     

Mechanisms of fatigue-crack propagation in ductile and brittle solids

The mechanisms of fatigue-crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatigue-crack growth as a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip (e.g., alternating crack-tip blunting and resharpening), which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip (e.g., crack closure and bridging), which impede it. The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., ΔK andK _max) provide a useful distinction of the process of fatigue-crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as load ratio and crack size. Finally, the differing susceptibility of ductile and brittle materials to cyclic degradation has broad implications for their potential structural application; this is briefly discussed with reference to lifetime prediction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Approximation of curved cracks under mixed-mode loading

The calculation of stress intensity factors or mechanical energy release rate for non-straight cracks can be complicated. Approximation to equivalent crack shapes can simplify calculations considerably, but this requires an understanding of the influence of key shape parameters on crack-tip stresses. A simple analytical model has been developed, based on the concept of a relaxed volume, to predict mechanical energy release rate and deflection angle for a range of crack shapes under mixed-mode loading. Results from this model compared well with those obtained from finite element (FE) simulations, and with predictions from previous analytical models. It was found that the crack length and orientation of the crack-tip with respect to loading direction are the key influences on fracture parameters, whilst curvature near the crack-tip can also affect results.     

Contribution to the study of fatigue-crack propagation near threshold conditions

International Journal of Fracture -     

The effect of D2O on fatigue-crack propagation in a high-strength aluminum alloy

The rates of fatigue-crack propagation for a high-strength aluminum alloy (7075-T651) in an environment of D₂O (99.98% purity) at room temperature were determined and compared with data obtained in high-purity argon and distilled water. The results showed that D₂O caused a ten-fold increase in the rate of fatigue-crack propagation (up to 10^–4 inch per cycle), which is equal to the increase caused by distilled water. These results lend further support to the previous observation that the rate controlling process for fatigue-crack propagation in this alloy (at rates below 10^–4 inch per cycle) is the mechanical process of creating new crack surfaces, instead of either the transport of aggressive environment to the crack-tip or diffusion of hydrogen ions into the material ahead of the crack tip.     

Short fatigue crack growth behavior under mixed-mode loading

Mixed-mode loading represents the true loading condition in many practical situations. In addition, most of the fatigue life of many components is often spent in the short crack growth stage. The study of short crack growth behavior under mixed-mode loading has, therefore, much practical significance. This work investigated short crack growth behavior under mixed-mode loading using a common medium carbon steel. The effects of load mixity, crack closure, and load ratio on short crack growth behavior were evaluated by conducting experiments using four-point bending specimens with several initial K _II /K _I mixed-mode ratios and two load ratios. Cracks were observed to grow along the paths with very small K _II /K _I ratios (i.e. mode I). The maximum tangential stress criterion was used to predict the crack growth paths and the predictions were found to be close to the experimental observations. Several parameters including equivalent stress intensity factor range and effective stress intensity factor range were used to correlate short crack growth rates under mixed-mode loading. Threshold values for short cracks were found to be lower than those for long cracks for all the mixed-mode loading conditions. Crack closure was observed for the entire crack length regime with all load mixity conditions at R ≈ 0.05 and for short crack regime under high load mixity condition at R = 0.5. Several models were used to describe mean stress effects and to correlate crack growth rate data.     

Effect of frequency and environment on fatigue-crack propagation of SA533B-1 steel

The effects of a decrease in the frequency of cyclic loading on the fatigue-crack propagation characteristics of SA533B-1 steel in various environments were investigated. Frequency levels of 10 Hz, 1.0 Hz and 0.1 Hz were employed in laboratory air, distilled water and a 3.5% NaCl solution. As the loading frequency was decreased, statistically significant increases in the fatigue-crack growth rates (da/dN) for the distilled water and salt water environments, as compared to those measured in laboratory air, were observed. These increases in growth rates were limited to certain ranges of stress intensity range (ΔK) values depending upon the frequency level being tested. A hydrogen embrittlement mechanism is proposed to explain the increase in growth rates based upon a fractographic analysis.     

Micro-mechanical finite element analysis of Z-pins under mixed-mode loading

This paper presents a three-dimensional micro-mechanical finite element (FE) modelling strategy for predicting the mixed-mode response of single Z-pins inserted in a composite laminate. The modelling approach is based upon a versatile ply-level mesh, which takes into account the significant micro-mechanical features of Z-pinned laminates. The effect of post-cure cool down is also considered in the approach. The Z-pin/laminate interface is modelled by cohesive elements and frictional contact. The progressive failure of the Z-pin is simulated considering shear-driven internal splitting, accounted for using cohesive elements, and tensile fibre failure, modelled using the Weibull’s criterion. The simulation strategy is calibrated and validated via experimental tests performed on single carbon/BMI Z-pins inserted in quasi-isotropic laminate. The effects of the bonding and friction at the Z-pin/laminate interface and the internal Z-pin splitting are discussed. The primary aim is to develop a robust numerical tool and guidelines for designing Z-pins with optimal bridging behaviour.     

Simulation of corrosion fatigue crack growth under mixed-mode loading

The kinking of a corrosion crack due to mixed-mode fatigue loading is studied using an adaptive finite element procedure. The rate of material dissolution is assumed to be proportional to the stretching of the corroding surface. The dissolution of material is governed by a corrosion law, where no criterion is needed for neither crack growth nor growth direction. The problem is treated as a general moving boundary problem. The kink angles are found to be in very good agreement with results for sharp cracks using criteria reported in the literature.     

A weibull analysis of fatigue-crack propagation data from a nuclear pressure vessel steel

Techniques of curve fitting, based upon an equation in the form of the four parameter Weibull Survivorship Function, are presented for the reduction of fatigue-crack propagation rate data obtained from a nuclear pressure vessel steel. The importance of data in the near threshold and the high crack propagation rate regions of the $\text{D}\text{A/}\text{D}\text{N}$vs ΔK, curve, as it relates to the curve fit,i s demonstrated with respect to a change in the four parameters of the Weibull function.