Small Fatigue Crack Growth Research Articles

Copper-bearing high-strength low-alloy steels: the influence of microstructure on the initiation and growth of small fatigue cracks : Davidson, D.L., Chan, K.S. and McClung, R.C. Metallurgical and Materials Transactions A (Sept. 1996) 27A, 2540–2556

In this work, the influences of laser shock peening (LSP) on the residual stress state, microstructures and mechanical properties of Ti-6Al-4V alloy fabricated by selective laser melting (SLM) were researched. Particularly, the microstructural evolution before and after LSP was clarified using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations. The results showed that the residual stress was adjusted from tensile to compressive state. Furthermore, high-density dislocations and a great number of parallel nano mechanical twins (nano-MTs) were promoted in the coarse lamellar α' structures, contributing to the grain refinement. The ultimate tensile strength and elongation of the SLM-manufactured (SLMed) Ti-6Al-4V alloy increased by approximately 14.3% and 18.3% after LSP treatment, respectively. Finally, the dominant mechanism of tensile property enhancement by LSP was revealed. High levels of compressive residual stress and grain refinement of the α' structures induced by LSP realized the excellent combination of strength and ductility.

Temperature dependence of the intrinsic small fatigue crack growth behaviour in nickel-base superalloys based on measurement of crack closure : Okazaki, M., Yamada, H. and Nohmi, S. Metall. Mater. Transact. A (Apr. 1996) 27A (4), 1021–1031

By integrating four lines of thoughts: symmetry breaking originally advanced by Anderson, bifurcation from nonlinear dynamical systems, Landau’s phenomenological theory of phase transition, and the mechanism of emergent rare events first studied by Kramers, we introduce a possible framework for understanding mesoscopic dynamics that links (i) fast microscopic (lower level) motions, (ii) movements within each basin-of-attraction at the mid-level, and (iii) higher-level rare transitions between neighboring basins, which have slow rates that decrease exponentially with the size of the system. In this mesoscopic framework, the fast dynamics is represented by a rapidly varying stochastic process and the mid-level by a nonlinear dynamics. Multiple attractors arise as emergent properties of the nonlinear systems. The interplay between the stochastic element and nonlinearity, the essence of Kramers’ theory, leads to successive jump-like transitions among different basins. We argue each transition is a dynamic symmetry breaking, with the potential of exhibiting Thom-Zeeman catastrophe as well as phase transition with the breakdown of ergodicity (e.g., cell differentiation). The slow-time dynamics of the nonlinear mesoscopic system is not deterministic, rather it is a discrete stochastic jump process. The existence of these discrete states and the Markov transitions among them are both emergent phenomena. This emergent stochastic jump dynamics then serves as the stochastic element for the nonlinear dynamics of a higher level aggregates on an even larger spatial and slower time scales (e.g., evolution). This description captures the hierarchical structure outlined by Anderson and illustrates two distinct types of limit of a mesoscopic dynamics: A long-time ensemble thermodynamics in terms of time t→∞ followed by the size of the system N→∞, and a short-time trajectory steady state with N→∞ followed by t→∞. With these limits, symmetry breaking and cusp catastrophe are two perspectives of the same mesoscopic system on different time scales.

Fatigue damage in two step loading of 316L steel. I. Evolution of persistent slip bands : Polák, J., Vasek, A. and Obrtlik, K. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (2–3), 147–155

The work reported in this paper is a summary of the results obtained from several projects run over a five year period. A number of thrusts were encompassed in the project scope, which was to examine the fatigue strength of extruded 6261-T6 I-beams with centrally located, welded cover plates under constant amplitude (CA) and variable (two-level repeated block) amplitude (VA) loading. The CA part of the work examined the effect of cover plate geometry, specimen size, thermal and vibratory stress relief, and differences in fabrication quality on fatigue performance. A number of cases were identified which gave rise to cross-overs in performance in moving from shorter to longer fatigue lives. The aim of the VA part of the work was to identify situations which might give rise to fatigue lives shorter than predicted by linear damage summation models, such as Miner's rule. Two such cases were identified: specimens which had been subjected to vibratory stress relief; and specimens in which weld `quality' (as assessed by profile, heat affected zone hardness, extent and microstructure) varied. Explanations for these observations are proposed and supported by interpretation of a two-level block of small fatigue crack growth.

Crack closure and effect of load variation on small fatigue crack growth behaviour : Jono, M. and Sugeta, A. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (2–3), 165–174

A variable-amplitude block-loading history consisting of high, near-yield-stress, underloads or compression-tension overloads followed by constant-amplitude small cycles was used to examine underload and compression-tension overload induced reductions in crack closure and the subsequent build-up of crack-opening stress to its steady-state level in a 2024-T351 aluminium alloy. Special attention was given to the near-threshold region, where the crack growth rate was less than 10−9 m cycle−1 The crack-opening stress level and the crack growth rate were measured for four different R-ratios of the small cycles following underloads and compression-tension overloads using a 900 power short focal length optical microscope. The crack-opening stress levels were measured at frequent intervals after the underload and compression-tension overload applications until the crack-opening stress returned to a steady-state level. The crack-opening stress build-up was then described by an empirical formula in terms of the ratio of the difference between the instantaneous crack-opening stress of the small cycles (Sop) and the post-underload and compression-tension overload crack-opening stress levels (Sopol), and the difference between the steady-state crack-opening stress of the small cycles (Sopss) and the post-underload and compression-tension overload crack-opening levels, (Sop− Sopol)(Sopss−Sopol). Effective stresses calculated using this formula were used to predict crack growth rates. The formula's predictions showed good agreement with experimentally measured crack growth rates. For simplicity, both underloads and compression-tension overloads will be referred to as overloads in the remainder of this paper.

Dislocation substructures at fatigue crack tips of 304 stainless steel cycled in air or vacuum : McEvily, A.J., Gonzalez, J.L. and Hallen, J.M. Scripta Materialia (1996) 35, 761–765

The work reported in this paper is a summary of the results obtained from several projects run over a five year period. A number of thrusts were encompassed in the project scope, which was to examine the fatigue strength of extruded 6261-T6 I-beams with centrally located, welded cover plates under constant amplitude (CA) and variable (two-level repeated block) amplitude (VA) loading. The CA part of the work examined the effect of cover plate geometry, specimen size, thermal and vibratory stress relief, and differences in fabrication quality on fatigue performance. A number of cases were identified which gave rise to cross-overs in performance in moving from shorter to longer fatigue lives. The aim of the VA part of the work was to identify situations which might give rise to fatigue lives shorter than predicted by linear damage summation models, such as Miner's rule. Two such cases were identified: specimens which had been subjected to vibratory stress relief; and specimens in which weld `quality' (as assessed by profile, heat affected zone hardness, extent and microstructure) varied. Explanations for these observations are proposed and supported by interpretation of a two-level block of small fatigue crack growth.

Crystallographic preferred orientation induced by cyclic rolling contact loading : Voskamp, A.P. and Mittemeijer, E.J. Metall. Mater. Transact. A (1996) 27A (11), 3445–3465

The work reported in this paper is a summary of the results obtained from several projects run over a five year period. A number of thrusts were encompassed in the project scope, which was to examine the fatigue strength of extruded 6261-T6 I-beams with centrally located, welded cover plates under constant amplitude (CA) and variable (two-level repeated block) amplitude (VA) loading. The CA part of the work examined the effect of cover plate geometry, specimen size, thermal and vibratory stress relief, and differences in fabrication quality on fatigue performance. A number of cases were identified which gave rise to cross-overs in performance in moving from shorter to longer fatigue lives. The aim of the VA part of the work was to identify situations which might give rise to fatigue lives shorter than predicted by linear damage summation models, such as Miner's rule. Two such cases were identified: specimens which had been subjected to vibratory stress relief; and specimens in which weld `quality' (as assessed by profile, heat affected zone hardness, extent and microstructure) varied. Explanations for these observations are proposed and supported by interpretation of a two-level block of small fatigue crack growth.

RELAXATION OF RESIDUAL STRESSES AT COLD‐WORKED FASTENER HOLES DUE TO FATIGUE LOADING

Abstract— Cold‐expansion of fastener holes is now commonly used within the aerospace industry to increase the fatigue endurance of airframes. Although a number of methods of cold expansion are possible, the split‐sleeve cold‐expansion process is the most widely accepted and is frequently used in the repair and manufacture stages of both military and civil aircraft. In the present work, the redistribution of residual hoop stresses due to the application of constant amplitude fatigue loading at 4% cold‐expanded holes has been studied. A modified Sachs method was adopted to evaluate the residual stress profiles and a replication technique was used to quantify crack growth. It was found that the decay of the residual hoop stress profile near the bore of the hole was due to the initiation and growth of small fatigue cracks. Cracks were found to initiate both near and below the fatigue limit, but subsequently arrested so stabilising the overall residual stress profile.

On the growth of small fatigue cracks in γ-based titanium aluminides

The cyclic response and the microstructure evolution of the near γ-TiAl alloy are investigated by a blend of contemporary experimental techniques centred around in-situ acoustic emission (AE) measurements reflecting the cyclically-induced structural changes in the real-time scale. TEM and SEM – ECCI/EBSD examinations provided an adequate qualitative description of microstructural features associated with populations of dislocations and mechanical twins evolving concurrently in the course of cyclic deformation. Since the TEM offers only local and post-mortem information, the in-situ neutron diffraction technique scanning a large part of the gauge length was employed to characterise the lattice strain distributions with cycling. The volume fraction of twins as a function of loading cycles was obtained after loading or unloading half-cycles. The processes controlling the cyclic strain hardening during each deformation cycle were assessed by (i) the statistical analysis of the shape of the hysteresis loop aiming at the characterisation of distribution internal stress barriers for deformation mechanisms involved and (ii) the spectral and statistical analysis of AE data providing information on the kinetics of these mechanisms. Each of the used experimental methods brings its own set of advantages and limitations in terms of characterisation and interpretation. Their unique combination ensures a host of benefits that promote a comprehensive understanding of primary deformation mechanisms - deformation twinning, dislocation slip and detwinning. It is shown that the cyclic mechanical behaviour of the TiAl alloy can be comprehensively explained by the interplay between these mechanisms co-operating during each loading cycle. This interplay governs the behaviour of underlying mechanisms in the early stages of the fatigue damage evolution and likely determines the overall fatigue response of near γ-TiAl alloy at room temperature.

Fatigue performance of 6261-T6 aluminium alloy – constant and variable amplitude loading of parent plate and welded specimens

The work reported in this paper is a summary of the results obtained from several projects run over a five year period. A number of thrusts were encompassed in the project scope, which was to examine the fatigue strength of extruded 6261-T6 I-beams with centrally located, welded cover plates under constant amplitude (CA) and variable (two-level repeated block) amplitude (VA) loading. The CA part of the work examined the effect of cover plate geometry, specimen size, thermal and vibratory stress relief, and differences in fabrication quality on fatigue performance. A number of cases were identified which gave rise to cross-overs in performance in moving from shorter to longer fatigue lives. The aim of the VA part of the work was to identify situations which might give rise to fatigue lives shorter than predicted by linear damage summation models, such as Miner's rule. Two such cases were identified: specimens which had been subjected to vibratory stress relief; and specimens in which weld `quality' (as assessed by profile, heat affected zone hardness, extent and microstructure) varied. Explanations for these observations are proposed and supported by interpretation of a two-level block of small fatigue crack growth.

In situ SEM observations of the contrasting effects of an overload on small fatigue crack growth at two different load ratios in 2024-T351 aluminium alloy

In situ SEM observations and measurements were made of near-tip COD and crack growth at small, thumb-nail cracks subjected to a single overload during fatigue at constant load amplitude in 2024-T351 aluminium alloy. A delayed retardation in crack growth was observed at R = 0.05, but not at R = −1. During retarded growth at R = 0.05, near-tip COD decreased below that measured before overload and the level of crack closure increased. These events were not evident at R = −1. An elastic/plastic finite element modelling of the growth of similar small cracks predicted enhanced residual compressive stresses ahead of the crack tip for R = 0, but not for R = −1. The experimental and modelling work are consistent in linking differences in the crack growth profiles at the two R ratios with differences in COD, crack closure and residual compressive stress ahead of the crack tip following overload.

CRACK INITIATION AND SMALL FATIGUE CRACK GROWTH BEHAVIOUR OF SQUEEZE‐CAST Al‐Si ALUMINIUM ALLOYS

Abstract— Fatigue strength, crack initiation and small crack growth behaviour in two kinds of squeeze‐cast aluminium alloys, AC8A‐T6 and AC4C‐T6 were investigated using smooth specimens subjected to rotatary‐bending fatigue at room temperature. Fatigue resistance of these alloys was almost the same as that of the wrought aluminium alloys because of their fine microstructure and of the decrease in defect size due to squeeze‐casting. Fatigue crack initiation sites were at the eutectic silicon particles on the surface of specimens or at internal microporosity in the specimens. Crack initiation life, defined as a crack length of 50 μm on the specimen surface, was successfully estimated from an evaluation of initiation sites using fracture mechanics and the statistics of extrema. Small fatigue crack growth in the two kinds of alloys obeys the relation proposed by Nisitani et al., namely that d(2c)/dN = C(σa/σB)n· (2c), where C is a constant and σB is the ultimate tensile strength. It is pointed out that an improvement in fatigue strength of cast aluminium alloys can be expected by refining the eutectic silicon rather than by an increase in static strength.

Environmentally Affected Small Cyclic Fatigue Crack Growth Behavior of Silicon Nitride in Atmospheric Air and Toluene.

To investigate the effect of atmospheric humidity on cyclic fatigue crack growth behavior, fourpoint bending fatigue tests were performed on silicon nitride in laboratory air and toluene. Crack aspact ratios for surface cracks in laboratory air were clarified to be lower than in toluene, The relationship between crack growth rate and maximum stress intensity factor for small surface cracks was obtained using the above crack aspect ratios. This result indicated that crack growth rate was accelerated by atmospheric humidity in the low stress intensity factor range, but not in the high range. This acceleration was clarified to be causes by the mutual action of cyclic stressing and atmospheric humidity. Microcrack initiation ahead of the main crack, the cutting process of ligament between the micro and the main crack, and cyclic loosening of crack bridging at wake of the crack tip were ciarified to be dominant factors which influence the crack growth behavior in both environments.

Degradation of components and structures by fatigue

This note examines briefly fatigue as a degradation mechanism and how engineers compensate for the fatigue phenomenon. Design is generally based on the stress-lifetime concept, but this concept is not very useful for lifetime assessment of operating components; thus, fatigue crack initiation and the growth of fatigue cracks become important. The numerous factors related to fatigue crack initiation and growth are briefly mentioned, and the importance of including the growth of small fatigue cracks is emphasized through the use of an illustration.

Cu-bearing high-strength low-alloy steels: The influence of microstructure on the initiation and growth of small fatigue cracks

The fatigue characteristics of a Cu-bearing high-strength low-alloy (HSLA) steel were investigated in air, relative humidity ≈50 pct, as a function of microstructure, which was altered by heat treatments and welding. Small fatigue cracks (≈30-Μm long) were naturally initiated from smooth specimens and grown past the transition length (≈200 Μm), where they exhibited the characteristics of large fatigue cracks. The number of cycles to crack initiation depended on stress magnitude but not on microstructure, although the site of initiation was microstructurally dependent. Small cracks in all microstructures grew at δK values below the large crack threshold. The as-received (polygonal ferrite) microstructure and one of the lath microstructures that resulted from heat treatment exhibited the same growth rate correlation as large cracks in the linear (Paris) region, and could be considered as an extension of the large crack growth region down to the point of initiation. Small cracks grew at rates faster than expected through one of the heat-treated and the weld microstructures; therefore, the number of cycles required for growth from initiation to the transition to large crack growth decreased about threefold, which is a potentially important factor in predicting lifetimes of structures made from this steel.

Small, short and long fatigue crack growth in an advanced silicon nitride ceramic material

In metallic materials, a number of workers have reported that the growth rates of small fatigue cracks cannot be correlated with the stress intensity factor range, {Delta}K. Small cracks normally exhibit faster growth rates than long cracks and often show growth rate minima. This anomalous behavior has been attributed to the failure of the linear elastic fracture mechanics parameter {Delta}K to characterize small, or short fatigue crack growth. Ceramic materials combine a lack of dislocation deformation and a very small grain size and thus the reasons for any observed anomalous small or short crack growth effect are less clear. Previous work on small or short fatigue crack growth in ceramics is limited, and work on silicon nitride which is one of the most promising structural ceramics is particularly sparse. As the majority of the fatigue lifetime of any silicon nitride component will be controlled by the propagation of a preexisting small flaw to a critical size, the presence of any short or small crack effect in this material is of engineering importance. Thus, the objective of the work presented here is to investigate the small, short and long crack growth in an advanced silicon nitride material.

An assessment of the role of near-threshold crack growth in high-cycle-fatigue life prediction of aerospace titanium alloys under turbine engine spectra

Increasingly accurate life prediction models are required to utilize the full capability of current and future advanced materials in gas turbine engines. Of particular recent interest are predictions of the lifetimes of engine airfoil materials that experience significant intervals of high-frequency, high-cycle fatigue (HCF). Conventional life management practices for HCF in the turbine engine industry have been based principally on a total-life approach. There is a growing need to develop damage tolerance methods capable of predicting the evolution and growth of HCF damage in the presence of foreign object damage (FOD), low cycle fatigue (LCF), and surface fretting fatigue. To help identify key aspects of the HCF life prediction problem for turbine engine components, a review is pressented of the extensive results of an Air Force research contract with Pratt & Whitney on the high strength titanium alloy Ti-8Al-1Mo-1V. Data from this representative turbine-airfoil material are used to examine the applicability of linear elastic fracture mechanics methods for prediction of service lifetimes under load spectra that include high cycle fatigue. The roles of fatigue crack initiation and growth are examined for materials that are nominally-defect-free, as well for materials that have experienced significant prior structural damage. An assessment is presented of the potential utility of the conventional threshold stress intensity factor range, ΔK th, defined by testing specimens containing large cracks. Although the general utility of a large-crack-ΔK th approach is questionable due to the potentially rapid growth of small fatigue cracks, the low allowable stresses involved in turbine engine high cycle fatigue appear to limit and simplify the small-crack problem. An examination is also presented of the potential effects of high-cycle fatigue and low-cycle fatigue (HCF/LCF) interactions.

Temperature dependence of the intrinsic small fatigue crack growth behavior in ni-base superalloys based on measurement of crack closure

The effect of temperature on the small fatigue crack growth behavior of a single crystal and directionally solidified Ni-base superalloys was investigated at temperatures between 873 to 1123 K by measuring the crack closure. The results were also compared with those of the physically long crack. It was found that the propagation resistance and the fatigue threshold of the long cracks increased with temperature in all the materials. The long crack growth rates at three temperatures were approximately represented by an unique curve, after taking account of crack closure level and elastic modulus. In contrast, the small crack growth resistance decreased with temperature even when the crack closure phenomenon was taken into consideration. Furthermore, the small fatigue cracks exhibited considerably higher growth rates than the long cracks at a given effective stress intensity factor range and also grew under effective stress intensity factor ranges below the long crack threshold. The factors responsible for the lack of similitude in propagation rates between small and long cracks were also discussed, based on these observations and the chemical analysis near the crack tip using the electron probe microanalyzer.

Micromechanical model of stage I to stage II crack growth transition for aluminium alloys

Stage I to Stage II crack growth transition is a kinetic process that is particularly studied in this work. Crack growth transition is modelled to occur when small fatigue crack growth is deterred by an effective barrier that reduces the crack growth rate to a minimum. Effective blockage of small crack growth due to barriers is characterised by pile-up of dislocations. Crack tip pile-up sliding displacement is formulated by adopting a continuous configuration of dislocation pile-up. Transition condition is quantitatively determined when a crack tip sliding displacement of Stage I cracks just meets a crack tip opening displacement of Stage II cracks. As a result, a dislocation-based micromechanical model is systematically developed that enables kinetic predictions of crack growth transition and growth rates of small fatigue cracks. Moreover, the concept of microstructurally-affected-zone is further interpreted in terms of local microstructure and load levels, leading to good explanations for scatters of small crack growth. All model-based growth rate predictions show a good coincidence with experimental results.

Analysis of short crack growth from microscopic fatigue properties

Successful simulation of kinetics of small fatigue crack growth entails three aspects: Stage I, Stage II growth rate prediction and transition prediction. In this paper attention is focused on growth rate predictions. By using microstructurally-affected-zone and process zone concepts, microscopic fatigue behaviour of small fatigue crack propagation is logically linked with macroscopic fatigue behaviour, showing an intrinsic relation between small fatigue crack growth and macroscopic low-cycle fatigue properties during crack growth. Furthermore, variation of relatively big plastic zone size associated with a growing small fatigue crack is kinetically simulated. As a result a quantitative prediction model of growth rates for Stage I and Stage II growth has been developed whose explicit advantage is that the growth rate of small fatigue crack can now be predicted in terms of bulk fatigue properties in conjunction with local microstructural characteristics.

Initiation and growth of small fatigue cracks in a Ni-base superalloy

This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (Kmax). Small surface cracks initiated predominantly at (Nb,Ti)xCy, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔKth, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Δσth) and the threshold cyclic stress intensity (ΔKσth) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime.