Bifurcation analysis of the Hobson short fatigue crack growth law

A bifurcation analysis of the Hobson short fatigue crack growth law is presented. The analysis reveals that, although the growth law is non‐linear, it contains no bifurcation points.     

On the corrosion fatigue crack initiation model and expression of metallic notched elements

In the present study, attempts are made to extend the application of the mechanical model for the fatigue crack initiation (FCI) and the FCI life formula of metallic notched elements in laboratory air to those in the corrosive environment. The test results and analysis of the corrosion FCI (CFCI) life of aluminum alloys and Ti---6A1---4V show that the expression of the CFCI life obtained by modifying the FCI life formula in laboratory air can give a good fit to the test results of the CFCI life. The salt water (3.5% NaCl) environment has no effects on the CFCI resistant coefficient compared with the FCI resistant coefficient in laboratory air. However, 3.5% NaCl environment greatly decreases the CFCI threshold of aluminum alloy, but has little effect on the CFCI threshold of Ti---6A1---4V. The loading frequency ranging from 1 Hz to 10 Hz has no appreciable effect on the CFCI life, and thus, the CFCI threshold of aluminum alloys investigated. Hence, the expression for the CFCI life of metallic notched elements proposed in this study is a better one, which reveals a correlation between the CFCI life and the governing parameters, such as, the geometry of the notched elements, the nominal stress range, the stress ratio, the tensile properties and the CFCI threshold. However, this new expression of the CFCI life needs to be verified by more test results.     

Initiation and growth behaviour of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation microcomputed tomography

Small internal fatigue cracks initiated in Ti‐6Al‐4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR‐μCT) at SPring‐8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN‐ΔK relationship was measured and compared with that obtained in a high vacuum environment. SR‐μCT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10^?10 m/cycle, and extremely slow rates ranging from 10^?13 to 10^?11 m/cycle were measured in a lower ΔK regime below 5 MPa√m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum‐like environment inside the internal cracks.     

Numerical modelling of fatigue crack initiation and growth of martensitic steels

This paper presents a numerical simulation of micro‐crack initiation that is based on Tanaka‐Mura micro‐crack nucleation model. Three improvements were added to this model. First, multiple slip bands where micro‐cracks may occur are used in each grain. Second improvement deals with micro‐crack coalescence by extending existing micro‐cracks along grain boundaries and connecting them into a macro‐crack. The third improvement handles segmented micro‐crack generation, where a micro‐crack is not nucleated in one step like in Tanaka‐Mura model, but is instead generated in multiple steps. High cycle fatigue testing was also performed and showed reasonably good correlation of proposed model to experimental results. Because numerical model was directed at simulating fatigue properties of thermally cut steel, edge properties of test specimens were additionally inspected in terms of surface roughness and micro‐structural properties.     

Improved fatigue crack growth resistance by retrogression and re‐aging heat treatment in 7010 aluminum alloy

Aircraft grade 7010 aluminum alloy was heat treated to two different conditions: (1) standard peak aging (T6) and (2) retrogression and re‐aging (RRA). The microstructures of these alloys were characterized by using transmission electron microscope. Fatigue crack growth rate (FCGR) tests were conducted using standard compact tension specimens, following ASTM standards. Tests were conducted at various stress ratios, R ranging from 0.1 to 0.7. The RRA‐treated alloy was observed to contain coarsened η′ (MgZn₂) precipitates with higher inter‐particle spacing when compared with T6‐treated alloy. The grain boundary precipitates (GBPs) were also coarsened and discontinuous in RRA‐treated alloy as compared with continuous GBPs in T6 condition. The FCGR was lower and ΔK_th was higher in RRA‐treated alloy compared with T6‐treated alloy at all the stress ratios investigated. Improved fatigue crack growth resistance in RRA‐treated alloy was correlated to the modified microstructure and enhanced crack closure levels.     

On some criterial problems of fatigue crack initiation and growth in polycrystals

We address a relationship between mechanisms and direction of a fatigue crack growth at stages I and II. The criteria for the fatigue crack growth direction at those stages are substantiated based on a study of physical and mechanical features of the fatigue crack propagation in polycrystals. We discuss the conditions under which microcracks may initiate brittle fracture. __________ Translated from Problemy Prochnosti, No. 4, pp. 5–24, July–August, 2008.     

In situ investigation of small fatigue crack growth in poly-crystal and single-crystal aluminium alloys

Abstract   In situ scanning electron microscope observations of short crack growth in both a poly-crystal and a single-crystal alloy revealed that fatigue cracks may grow in a shear decohesion mode over a length that is several times the grain size, far beyond the conventional stage I regime. In the poly-crystal aluminium alloy 2024-T351, fatigue cracks were found to continue to grow along one shear band even after two mutually perpendicular shear bands had formed at the crack tip. For the single-crystal alloy specimen with the loading axis being nearly perpendicular to its main shear plane, mode I fatigue cracks were found to grow along the shear band. These two types of fatigue crack growth pose a significant challenge to the existing fatigue crack growth correlating parameters that are based on crack-tip opening displacement. In particular, it has been found that the cyclic crack-tip opening displacement, which accounts for both large-scale yielding and the lack of plasticity-induced crack closure, is unable to unify the growth rates of short and long cracks in aluminium 2024-T351, suggesting a possible dependence of crack growth threshold on crack length.     

Short crack growth and fatigue life in austenitic-ferritic duplex stainless steel

The kinetics of short crack growth has been studied in austenitic‐ferritic 2205 duplex stainless steel. Smooth cylindrical specimens and specimens with shallow notch were subjected to constant plastic strain amplitude loading. The crack growth was studied in notched specimens. The notch area has been mechanically and electrolytically polished to facilitate the observation of crack initiation and growth. The initiated cracks were observed in an SEM (scanning electron microscope). The crack growth was studied using long distance QUESTAR optical microscope equipped with high‐resolution camera. In constant plastic strain amplitude loading the microcracks were initiated and their growth kinetics has been studied. The characteristic features of the crack growth at different plastic strain amplitudes were recorded. Two approaches to analyse the crack growth rates were adopted. The comparison of the prediction of the fatigue life using the plastic‐strain‐dependent crack growth rate was compared with Manson–Coffin law and the relation between parameters of this law and parameters of the short crack growth law were established.     

The fatigue crack growth behaviour of 2524‐T3 aluminium alloy in an Al2O3 particle environment

The effect of the Al₂O₃ dust environment on the crack propagation behaviour of 2524‐T3 Al alloy was investigated. The results show that the Al₂O₃ dust environment reduces the fatigue crack growth rate (FCGR) of alloy especially at low ΔK. Many Al₂O₃ particles are deposited and stuck in the crack during fatigue loading which promotes crack closure, while this effect is gradually weakened with the increase of ΔK. The deposited Al₂O₃ particles induce the disorderly arranged slip bands (SBs) ahead of the crack tip which deflects the crack path making it more tortuous in the Al₂O₃ dust.     

Investigation of small fatigue crack initiation and growth behaviour of nickel base superalloy GH4169

Surface replication method was utilized to monitor the small fatigue crack initiation and growth process of single‐edge‐notch tension specimens fabricated by nickel base superalloy GH4169. Three different stress levels were selected. Results showed that small fatigue cracks of nickel base superalloy GH4169 initiated from grain boundaries or surface inclusions. The small fatigue crack initiation and growth stages took up about 80–90% of the total fatigue life. Multiple major cracks were observed in the notch root, and specimen with more major cracks seemed to have smaller fatigue life under the same test conditions. At the early growth stage, small crack behaviour might be strongly influenced by microstructures; thus, the crack growth rates had high fluctuations. However, the stress level effect on the small fatigue crack growth rates was not distinguishable for the three different stress levels. And no clear differences were found among the crack initiation lives by using replication technique.     

Very-high-cycle fatigue crack initiation and propagation behaviours of magnesium alloy ZK60

The aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.     

Prediction of short fatigue crack growth of Ti‐6Al‐4V

It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.     

A modification of UniGrow 2‐parameter driving force model for short fatigue crack growth

In this paper, a modification of the UniGrow model is proposed to predict total fatigue life with the presence of a short fatigue crack by incorporating short crack propagation into the UniGrow crack growth model. The UniGrow model is modified by 2 different methods, namely the “short crack stress intensity correction method” and the “short crack data‐fitting method” to estimate the total fatigue life including both short and long fatigue crack propagations. Predicted fatigue lives obtained from these 2 methods were compared with experimental data sets of 2024‐T3, 7075‐T56 aluminium alloys, and Ti‐6Al‐4V titanium alloy. Two proposed methods have shown good fatigue life predictions at relatively high maximum stresses; however, they provide conservative fatigue life predictions at lower stresses corresponding high cycle fatigue lives where short crack behaviour dominates total fatigue life at lower stress levels.     

Characterization of fatigue crack initiation and growth in hybrid aluminium–graphite fibre composite laminates using image analysis

Hybrid aluminum–graphite-epoxy fibre–metal laminates have been tested in open hole tension–tension fatigue at several stress levels. Crack initiation and growth in the outer aluminum layers was monitored by periodic digital imaging of the specimen surface. Image analysis was used to determine the crack length throughout the experiment. Crack growth results were consistent between specimens for each test configuration. A relationship between the crack growth results and stress level was developed using linear elastic fracture mechanics; this successfully characterized the data. The effect of shot peening and alternate hole finishing techniques were considered for fatigue behaviour improvement.     

Small fatigue crack growth behavior of titanium alloy TC4 at different stress ratios

In this paper, the small fatigue crack behavior of titanium alloy TC4 at different stress ratios was investigated. Single‐edge‐notch tension specimens were fatigued axially under a nominal maximum stress of 370 MPa at room temperature. Results indicate that fatigue cracks in TC4 initiate from the interface between α and β phases or within α phase. More than 90% of the total fatigue life is consumed in the small crack initiation and growth stages. The crack growth process of TC4 can be divided into three typical stages, ie, microstructurally small crack stage, physically small crack stage, and long crack stage. Although the stress ratio has a significant effect on the total fatigue life and crack initiation life at constant σ_max, its effect on crack growth rate is indistinguishable at R = ?0.1, 0.1, and 0.3 when crack growth rate is plotted as a function of ?K.     

Fatigue behaviors of laser hybrid welded AA7020 due to defects via synchrotron X‐ray microtomography

Defect or shrinkage is known to have a detrimental effect on the fatigue resistance of casting lightweight alloys and additively manufactured or 3D printed materials. However, very few works focus on the damage mechanism of fusion welded Al alloys due to gas pores or metallurgical defects. This paper performs an investigation on the effect of porosity on the damage evolution of laser hybrid welded 7020‐T651 alloys. The critical pore size comparable with average weld grain was assumed in terms of the population and dimension of micropores. To characterize the coupling effect between gas pores and cracks, an in situ fatigue testing rig was developed to well work at the synchrotron radiation tomography system. Combining synchrotron X‐ray microtomography and fatigue resistance testing, the pore size and location were correlated with the crack initiation and crack growth path but relatively less on the long crack propagation rate. Furthermore, the interaction between the porosity and stress concentration was elucidated by using finite element simulations, which shows that the gas pore appears to be a preferred cracking site especially near the surface.     

A cumulative model of fatigue crack growth and the crack closure effect

A model of fatigue crack growth based on an analysis of elastic/plastic stress and strain at the crack tip is presented. It is shown that the fatigue crack growth rate can be calculated using the local stress/strain at the crack tip by assuming that a small highly strained area $\text{x}{}^1$, existing at the crack tip, is responsible for the fatigue crack growth, and that the fatigue crack growth may be regarded as the cumulation of successive crack re-initiations over a distance $\text{x}{}^1$. It is shown that crack closure can be modelled using the effective contact zone g behind the crack tip. The model allows the fatigue crack growth rate over the near threshold and linear ranges of the general da/dN versus ΔK curve to be calculated. The fatigue crack growth retardation due to overload and fatigue crack arrest can also be analysed in terms of g and $\text{x}{}^1$.Calculated fatigue crack growth rates are compared with experimental ones for low and high strength steel.     

A new testing method to obtain mode II fatigue crack growth characteristics of hard materials

Flaking type failure in rolling‐contact processes is usually attributed to fatigue‐induced subsurface shearing stress caused by the contact loading. Assuming such crack growth is due to mode II loading and that mode I growth is suppressed due to the compressive stress field arising from the contact stress, we developed a new testing apparatus for mode II fatigue crack growth. Although the apparatus is, as a former apparatus was, based on the principle that the static K_I mode and the compressive stress parallel to the pre‐crack are superimposed on the mode II loading system, we employ direct loading in the new apparatus. Instead of the simple four‐point‐shear‐loading system used in the former apparatus, a new device for the application of a compressive stress parallel to the pre‐crack has been developed. Due to these alterations, mode II cyclic loading tests for hard steels have become possible for arbitrary stress ratios, including fully reversed loading (R=?1); which is the case of rolling‐contact fatigue. The test results obtained using the newly developed apparatus on specimens made from bearing steel SUJ2 and also a 0.75% carbon steel, are shown.