Controlled Fatigue Crack Growth Research Articles

In-situ experimental study on the fatigue crack propagation behavior of 7075-T6 and 2024-T3 aluminium alloys under variable amplitude loading and retardation model modification

Fatigue crack propagation behavior of Al7075-T6 and Al2024-T3 under four typical variable amplitude loading (VAL) patterns are investigated by using in-situ testing. The near-tip strain fields and crack opening level are measured combining in-situ optical microscopy testing and digital image correlation (DIC) technique. The crack tip deformation process is directly observed by in-situ scanning electron microscope (SEM). The results show that both the crack propagation behaviors and the crack-tip strain distributions are greatly influenced by the VAL patterns. The shape effect of the overload-induced plastic zone may prolong the crack growth retardation. The crack-front plasticity plays a dominant role in controlling fatigue crack growth. The crack-front plasticity supplies a link between crack closure and crack growth. The crack tip resharpening caused by the underload applied immediately after the overload may reduce the retardation extent. Finally, a modified retardation model is proposed and gives reasonable prediction results of the crack growth rate under VAL.

Tensile and fatigue crack growth behavior of commercially pure titanium produced by laser powder bed fusion additive manufacturing

The effects of build orientation and post heat treatments on the tensile and fatigue crack growth (FCG) behavior of commercially pure titanium (CP-Ti) manufactured by laser powder bed fusion (LPBF) using grade 2 powder were examined. Two orthogonal build orientations were used in conjunction with hot isostatic pressing (HIP) both above (950 °C) and below (730 °C) the β-transus temperature and property comparisons were also made to commercially available wrought material. The HIP treatments coarsened the α grain structure, reduced the tensile strength, and increased the fatigue crack growth threshold. The LPBF materials were generally stronger and more fatigue resistant than the wrought material due higher interstitial oxygen and nitrogen content. Additionally, higher tensile strength values were found for one build orientation with higher nitrogen content that was attributed to the different thermal histories during LPBF. However, the build orientation effect was not observed for the FCG behavior of the LPBF material and the FCG resistance at low growth rates were mainly controlled by the grain size. This was in sharp contrast to the wrought material which showed strong anisotropy in the microstructure sensitive fatigue crack growth regime due to strong crystallographic texture. At higher growth rates, FCG became microstructure insensitive when the cyclic plastic zone size became of similar order of magnitude to the grain size. • Fatigue crack growth rates measured for laser powder bed fusion CP-Ti. • Interstitial pickup during fabrication raised tensile strength and fatigue resistance. • Microstructure sensitive growth when cyclic plastic zone - grain size ratio is ≲1–4. • α'/α lath thickness and interstitial content controlled fatigue crack growth rates. • Build orientation had negligible effect on fatigue crack growth.

Fatigue crack deflection and branching behavior of low carbon steel under mechanically large grain condition

In situ SEM observation of fatigue crack growth behavior of low carbon steel under mechanically large grain condition, and EBSD analysis were carried out to investigate mechanisms of crack deflection and branching and effects of crystal orientation on them. It is found that the crack propagates along the activated slip bands in the present material and the crack deflection can be formed when a new activated slip band is created in a different direction from the growing crack. The crack branching can be formed by the following process: (1) the slip band along the crack cannot be activated further due to the restraint by such as grain boundary or strain hardening, (2) the crack cannot propagate further in the same direction, and (3) the secondary slip plane is activated to form a new slip band behind the crack tip. Not the stress intensity factors Kdef for deflected crack and ΔKeff for crack closure but rather the ΔKeff,tip is the intrinsic fracture mechanics parameter for controlling fatigue crack growth behavior of the material with crack deflection and branching.

A Novel Physics-Based Predictive Model for Small Fatigue Crack Growth with Microstructural Sensitivity in Cast Aluminum Alloys

A novel predictive model for microstructurally small fatigue crack growth rates was developed using a three-part methodology. First, a deterministic model was created to predict microstructurally small fatigue crack growth behavior from long fatigue crack growth data using considerations of crack tip plasticity. Subsequently, microstructural barrier characteristic spacing/strength were modeled to introduce characteristic acceleration and deceleration mechanisms of grain- and secondary phase-controlled cracks. Finally, the deterministic model was coupled with a Monte Carlo technique, and used to make predictions of lifetime distributions and S–N curves with material and component specificity. Simple, metallographically measured parameters are used to make predictions, and the model provides insight into their respective roles in controlling fatigue crack growth lifetimes, and enables design practice optimization. The model predicts that, for cast Al-Si-Mg alloy A356, increasing matrix strength, grain size, and secondary dendrite arm spacing enhances overall fatigue crack growth resistance. Comparisons are made to experimental data, and show that successful predictions are made.

Al7075-T651의 마찰교반용접된 접합부의 피로균열전파율의 통계적 분포

This paper deals with the effects of driving force and material properties on statistical distribution of fatigue crack growth rate (FCGR) for the friction stir welded joints of Al 7075-T651 aluminum plate. In this work, the statistical probability distribution of fatigue crack growth rate was analyzed by using our previous constant stress intensity factor range controlled fatigue crack growth test data. As far as this study are concerned, the statistical probability distribution of fatigue crack growth rate for the friction stir welded (FSWed) joints was found to evaluate the variability of fatigue crack growth rate for base metal (BM), heat affected zone (HAZ) and weld metal (WM) specimens. The probability distribution of fatigue crack growth rate for FSWed joints was found to follow well log-normal distribution. The shape parameter of BM and HAZ was decreased with increasing the driving force, however, the shape parameter of WM was decreased and increased with increasing the driving force. The scale parameter of BM, HAZ and WM was increased with the driving force.

Control Effect on Fatigue Crack Propagation of TiNi Fiber Reinforced PMMA Composites

In this paper, the TiNi fiber reinforced/PMMA (Poly methyl methacrylate) composite is developed, and its effectiveness of controlling fatigue crack growth is studied. The TiNi fiber reinforced/PMMA composite’s mechanical property enhancement and deformation resistance are also studied. The fatigue behavior and crack propagation are observed with a SEM servo-pulser (fatigue testing instrument with scanning electron microscope) while increasing temperature. As the results, it is confirmed that the fatigue life and resistance are improved. How the shape memory effect and expansion behavior of the matrix caused by temperature increasing affect the fatigue crack propagation control is examined. It is verified that the control of fatigue crack growth is attributed to the compressive stress field in the matrix due to shrinkage of the TiNi fibers above austenitic finishing temperature (Af).

Crack retardation equations for the propagation of branched fatigue cracks

The stress intensity factors (SIF) associated with branched fatigue cracks can be considerably smaller than that of a straight crack with the same projected length, causing crack growth retardation or even arrest. This crack branching mechanism can quantitatively explain retardation effects even when plasticity induced crack closure cannot be applied, e.g. in high R-ratio or in some plane strain controlled fatigue crack growth problems. Analytical solutions have been obtained for the SIF of branched cracks, however, numerical methods such as Finite Elements (FE) or Boundary Elements (BE) are the only means to predict the subsequent curved propagation behavior. In this work, a FE program is developed to calculate the path and associated SIF of branched cracks, validated through experiments on 4340 steel ESE(T) specimens. From these results, semi-empirical crack retardation equations are proposed to model the retardation factor along the crack path. The model also considers the possible interaction between crack branching and other retardation mechanisms.

Stress intensity factor equations for branched crack growth

Overload-induced fatigue crack branching is a well-known crack growth retardation or arrest mechanism, which can quantitatively explain such effects even when arguments based on plasticity induced crack closure cannot be applied, e.g. in high R-ratio or in plane strain controlled fatigue crack growth. However, the few results available for branched cracks cannot be used to predict the subsequent crack growth nor account for the delays observed in practice. In this work, specialized finite element (FE) and fatigue life assessment software are used to solve this problem. The crack path and associated stress intensity factors (SIF) of kinked and bifurcated cracks are numerically obtained by the FE program for several angles and branch lengths, and the companion life assessment program is used to estimate the number of delay cycles associated with them. From these results, crack retardation equations are proposed to model the number of delay cycles and the retardation factor along the crack path, allowing for a better understanding of the influence of crack deflection in the propagation life of structural components.

EVALUATION OF CRACK GROWTH RETARDATION IN BRANCHED FATIGUE CRACKS

Overload-induced fatigue crack branching (or bifurcation) is a phenomenon capable of inducing significant growth retardation or even crack arrest. Such behavior can quantitatively explain retardation effects even when plasticity induced crack closure cannot be applied, e. g. in high R-ratio or in plane-strain controlled fatigue crack growth. However, the few analytical models available for branched cracks cannot be used to predict the subsequent crack growth nor account for the delays observed in practice. In this work, specialized Finite Element (FE) and life assessment software are used to predict the reduction in the propagation rates in bifurcated cracks. The crack path and the associated stress intensity factors (SIF) of asymmetrically bifurcated cracks are numerically obtained for several bifurcation angles. The companion life assessment program is used to estimate the number of delay cycles associated with crack bifurcation. The proposed approach is validated experimentally, allowing for a better understanding of the influence of crack deflection in the propagation life of structural components. Key-words: fatigue, crack bifurcation, growth retardation.

Multiple mechanisms controlling fatigue crack growth

ABSTRACTRecognizing that fatigue is a two‐parameter problem requiring two load parameters to define cyclic loads unambiguously, a unified approach has been developed to account for crack growth behaviour in terms of ΔK and Kmax. Since both driving forces govern the crack growth rate, any analysis based on either ΔK or Kmax will provide only partial information about the fatigue behavior of materials. It is shown that ΔK–Kmax plots and the associated crack growth trajectory maps reflect the basic mechanisms that contribute to crack growth in a material. These plots also provide a convenient basis to recognize the changes in the micromechanisms that can occur as a function of load ratio or crack growth rate, or both. Taking examples from the literature, crack growth trajectory maps are provided showing such changes in the governing mechanisms of crack growth. It is shown that the ΔK–Kmax approach is not an alternative to crack closure models, but it reflects the intrinsic material behaviour that must be understood before reliable crack prediction models can be developed.

On the relationship and the short fatigue crack threshold

ABSTRACTMurakami and Endo have used the parameter to successfully predict the endurance limits and the threshold levels for components, which contain small defects or cracks. The present paper uses a modified linear–elastic fracture mechanics (LEFM) approach to examine the mechanistic basis for these correlations. The modifications include consideration of the endurance limit rather than the threshold level as a factor controlling fatigue crack growth in the very short crack growth range, consideration of elastic–plastic behaviour, and consideration of the role of crack closure in the wake of a newly formed crack. Predictions based upon the modified LEFM behaviour are found to be consistent with the earlier predictions of Murakami and Endo.

The effects of environment and loading waveform on fatigue crack growth in Inconel 718

The influence of environment and fatigue waveform shape on crack growth behaviour has been evaluated in a workhorse turbine disc material, Inconel 718, at 600°C through an experimental programme that investigated crack growth from a notch geometry. Crack growth data are presented for a variety of stress and air pressure conditions, with specific combinations typifying model component test conditions of particular interest. The effect of waveform characteristics covering a range of loading/unloading rates and different dwell periods at peak stress was evaluated. The rates of propagation and the resultant crack profiles were found to be significantly affected by the environmental conditions through control of the dominant fracture mechanisms of transgranular and intergranular cracking. Evidence for environment controlled fatigue crack growth is presented.

An inspection of elevated temperature fatigue crack extension data in low alloy steels

The present paper describes a reasonable review of various high temperature fatigue crack growth data obtained from a range of low alloy steels under both continuous cycle conditions and those where a dwell time at maximum load had been imposed. The continuous cycle data all resided below the 1/2CTOD line and were found to be primarily frequency and Δ K 2 dependant with all crack extensions occurring in a transgranular fashion. The dwell time fatigue crack growth data could be reasonably portrayed in terms of maximum stress intensity K max, prevalent at the dwell portion of the fatigue cycle. Furthermore, it was established that the dwell time data fell roughly into two groups, viz., those for dwell times of 30 min or less and data with dwell times greater than this value. A comparison of similar dwell time results from various literature sources indicated reasonable commonality and it was observed that in the vast majority of cases: (1) fatigue crack growth rates resided above the 1/2CTOD line; (2) exhibited an approximately Δ K 10 relationship; and (3) fatigue crack growth was wholly intergranular in nature. Finally the existence of some critical stress intensity condition and/or cavity damage condition was discussed to account for the observed abrupt switch from continuum controlled fatigue crack growth with a Δ K 2 dependence to super-continuum growth characteristics which obeyed a Δ K 10 dependence.

Fatigue crack growth retardation assessment of 2024-T3 and 6061-T6 aluminium specimens

A fatigue crack growth retardation model is developed. It considers a strip plastic zone with material hardening effect which is taken as one of the basic mechanisms controlling fatigue crack growth. Crack growth is treated incrementally and corresponds to the failure of material elements ahead of an existing crack after a certain critical number of low cycle fatigue. Computed curves are correlated to test data obtained from the 2024-T3 and 6061-T6 aluminium specimens. Deviations from test data increase with increasing crack length.

Influence of mean stress on the fatigue crack growth characreristics of [formula omitted] CrlMo steel tube at elevated temperature

The fatigue crack growth characteristics of 2- 1 4 CrlMo steel have been investigated at 861 K over the R-ratio range 0.1–0.7 utilising a dwell time of 10 min. at maximum load. All tests were conducted under load control in a laboratory air environment. It was established that the R-ratio significantly affected the fatigue crack extension behaviour inasmuch that with increasing R-ratio, the critical ΔK level for the onset of creep fatigue interactive growth, ΔK IG, decreased from 20 to 7 MPa√m and the threshold stress intensity, ΔK th, decreased from 9 to about 3 MPa√m. At intermediate ΔK levels, i.e. between ΔK th and ΔK IG, the fatigue crack extension rates, for all R-ratio values, resided on or slightly below the CTOD line, which represents the upper bound for contrnuum controlled fatigue crack growth. Creep fatigue interactive growth was typified by crack extension rates that reside above the CTOD line with a ΔK IG dependence; the attainment of some critical creep condition or crack linkage condition which causes the abrupt change in crack extension behaviour at ΔK IG; and crack extension occurs almost exclusively in an intergranular manner. The R-ratio and ΔK IG followed a linear relation. A literature review concerning the effect of temperature on the threshold fatigue crack growth characteristics of low alloy ferritic steels demonstrated powerful effects of temperature; the magnitude of these effects, however, were dependent upon the testing temperature regime and R-ratio level. The effect of R-ratio on ΔK th was greatest at temperatures >400°C, significant at ambient temperatures and least in the temperature range 90°C to <300°C. The relationship between temperature and ΔK th, at a given R-ratio, exhibited a through and a minimum ΔK th value was observed in the temperature range 200–250°C. The magnitude of the temperature effects on ΔK th decreased with increasing R-ratio. Such effects of temperature and R-ratio on ΔK th was reasonably explained in terms of crack closure effects. Finally, the present elevated temperature fatigue crack growth data exhibited massive crack extension enhancement values when compared to ambient near-threshold fatigue crack growth data for 2- 1 4 CrlMo steel. Such large enhancement values were the combined effects of temperature (environment) and frequency.

Study of crack closure under single and intermediate tensile overload cycles for 6061-T6 aluminium alloy: Kumar, R. and Garg, S.B.L. Int. J. Pressure Vessels Piping 1989 36, (2), 111–123

A fatigue crack growth retardation model is developed. It considers a strip plastic zone with material hardening effect which is taken as one of the basic mechanisms controlling fatigue crack growth. Crack growth is treated incrementally and corresponds to the failure of material elements ahead of an existing crack after a certain critical number of low cycle fatigue. Computed curves are correlated to test data obtained from the 2024-T3 and 6061-T6 aluminium specimens. Deviations from test data increase with increasing crack length.

THE INFLUENCE OF TEST VARIABLES ON THE FATIGUE CRACK GROWTH THRESHOLD

Abstract— A microcomputer controlled fatigue crack growth and threshold testing system has been used to investigate the influence of test variables on the measured values of ΔKth, the threshold for fatigue crack growth, using a C‐Mn steel. The work has examined: (1) the influence of crack length and test management; (2) the basic material scatter from repeated testing; (3) the effect of unloading rate C where C= (1/Δ/K)(d ΔK/d a); (4) the effect of step unloading; (5) the influence of minimum stress intensity factor, Kmin. Comparisons have been made between the results of this computer controlled work and those published previously but made using a manual load shedding technique. The results of ΔKth and fatigue crack growth rates are in general agreement with previous data and confirm the Kmin dependence of ΔKth and da/dn. The value of ΔKth is shown to be generally independent of the other test variables for a wide range of conditions and is reproducible with a low degree of scatter.

Displacement controlled fatigue crack growth in inelastic notch fields: Implications for short cracks

Much has been written documenting the crack growth from notches which suggests that LEFM does not consolidate such data in a manner consistent with those of compact tension and other specimens with long cracks. Such crack growth data are reviewed and causes for the observed “short crack” effect are advanced. Situations where the crack is contained within the inelastic field of a notch are explored at length. A postulate is advanced to characterize crack growth for this case in terms of a pseudoplastic stress intensity factor defined in analogy to LEFM as well as a simple approach to deal with local crack closure. The postulate is shown to be consistent with experimental observations. Predictions of the growth of cracks in the inelastic field are made and shown to confirm observed growth rates and trends.

The effects of liquid metal embrittlement on high temperature fatigue of 2.25 Cr-1 Mo steel in liquid lithium

The fatigue crack propagation characteristics of a 2.25 Cr-1 Mo steel were investigated in argon and lithium with either < 100 ppm or approximately 2000 ppm dissolved nitrogen. Data, which extend those presented previously, were obtained for temperatures of 673 to 873 K and loading frequencies of 0.05 to 20 Hz. At a constant imposed stress intensity factor, the growth rate in argon decreased with an increase in frequency for all test temperatures. In contrast, the growth rate was enhanced by the presence of lithium and shown to increase significantly with an increase in frequency at the lower test temperatures. The lithium enhanced growth rates resulted from strain rate induced liquid metal embrittlement (LME). The loading frequencytemperature conditions necessary for LME were predicted from a model based on the achievement of a critical strain rate dependent flow stress at the tip of a propagating fatigue crack. The results correlated with fractographic observations which showed a transition from deformation controlled fatigue crack growth at low frequencies to LME controlled cleavage formation at high frequencies.

The effects of load ratio on environmentally assisted fatigue crack growth

A modification to the model of Weir et al. for surface reaction and transport controlled fatigue crack growth has been developed to explicitly account for the effect of load ratio on environmentally assisted fatigue crack growth. Load ratio was found to affect principally gas transport to the crack tip, and therefore affected only transport controlled crack growth response. Experimental verification of the modified model was made by studying the room temperature fatigue crack growth responses at different load ratios for a 2219-T851 aluminum alloy exposed to water vapor. The results show that the effects of load ratio can be attributed to two different sources—one relating to its effect on local deformation at the crack tip and is reflected through the mechanical component, (d a/d N) 0 and the other on its role in modifying environmental effect and is manifested through the corrosion fatigue component, (da/dN) cf Furthermore, the results show that the saturation value of corrosion fatigue component, (da/dN) cf,s , is essentially independent of R, and that the exposure needed to produce “saturation response” ( P 0/2f) s , as a function of load ratio can be predicted from the modified model. The modified model, therefore, allows one to predict the corrosion fatigue crack growth response for any load ratio on the basis of measurements made at a single load ratio, provided that the values of ( da/dN), are known.