Faceted crack initiation characteristics for high-cycle and very-high-cycle fatigue of a titanium alloy under different stress ratios

Ultrasonic fatigue tests were conducted at the stress ratios of −1, −0.5, −0.1, 0.1 and 0.5 for a Ti–6Al–4V alloy in high-cycle and very-high-cycle fatigue regimes. Experimental results showed that faceted crack initiation was the main failure mode for specimens at the stress ratios of −0.1, 0.1 and 0.5, and multi-site faceted crack initiation was observed at the stress ratios of 0.1 and 0.5. The measurements indicated that the number of facets increased with the increase of stress ratio. Based on the observations, the mechanism of faceted crack initiation was proposed, i.e., (i) cleavage of isolated primary α grains in cluster; (ii) gradual growth of originated cracks (facets), and the coalescence of adjacent facets; and (iii) coalesced facets forming a main crack in the cluster. Moreover, a model based on Poisson defect distribution is proposed to describe the effects of stress ratio on faceted crack initiation, which is in agreement with the experimental results.     

The effect of stress ratio on fatigue crack growth rate in the absence of closure

A fractographic study¹ was performed on Al-alloy fatigue fracture surfaces produced by programmed load sequences. The load sequences included steps of constant amplitude cycles at three different stress ratios, each step is preceded by a small number of high amplitude cycles designed to avoid the influence of crack closure and to serve as fractographic markers. The experiments were conducted on different specimen geometries to produce conditions associated with a long crack under fully elastic conditions and a short crack in a notched coupon seeing high local post yield stress conditions. Crack sizes covered in the study ranged from 0.02 to 12 mm, and growth rates ranged from 2×10⁻⁷ to 4×10⁻⁵ mm cycle⁻¹. Fractographic evidence from the study suggests that the crack growth rate can vary by up to a factor of five with applied stress ratio change from 0.64 to 0.73. In the case of the long crack, the effect is less noticeable or totally absent. In the case of naturally initiating notch root cracks, the effect is more pronounced at higher stress level and lower crack growth rate.     

An integrated methodology for separating closure and residual stress effects from fatigue crack growth rate data

To properly interpret the results of standard fatigue crack growth tests it is often necessary to incorporate corrective techniques to the ΔK applied data. This is especially true in the near‐threshold regime where long crack data need to be closure corrected to predict small crack behaviour. It is also an issue in the presence of residual stress. A methodology to separate the influence of sample size, geometry, crack length and residual stress from the standard crack growth test data to obtain a true material response is presented. Stress ratio and residual stress contributions from known combinations of assumed crack size, applied stress and residual stress are also addressed and incorporated in the fatigue crack growth behaviour.     

Residual stress effects on fatigue crack growth in a Ti‐6Al‐4V friction stir weld

Recent studies have illustrated a predominant role of the residual stress on the fatigue crack growth in friction stir welded joints. In this study, the role of the residual stress on the propagation of fatigue cracks orthogonal to the weld direction in a friction stir welded Ti‐6Al‐4V joint was investigated. A numerical prediction of the fatigue crack growth rate in the presence of the residual stresses was carried out using AFGROW software; reasonable correspondence between the predictions and the experimental results were observed when the effects of residual stress were included in the simulation.     

Effects of microstructure on fatigue crack propagation behavior in a bi-modal TC11 titanium alloy fabricated via laser additive manufacturing

In this study, the crack propagation behaviors in the equiaxed and equiaxed-columnar grain regions of a heat-treated laser additive manufacturing (LAM) TC11 alloy with a special bi-modal microstructure are investigated. The results indicate that the alloy presents a special bi-modal microstructure that comprises a fork-like primary α (α_p) phase surrounded by a secondary α colony (α_s) in the β phase matrix after the heat treatment is completed. The samples demonstrate a fast crack growth rate with larger da/dN values through the equiaxed grain sample versus across the equiaxed-columnar grain sample at low ΔK values (<13.8). The differences that are observed between the crack propagation behaviors (in the crack initiation stage) of the samples can be mostly attributed to the different size and morphology of the α_p lamellae and α_s colony within the grains in the equiaxed and columnar grain regions rather than the grain boundaries. The cracks prefer to grow along the α/β boundary with a smooth propagation route and a fast propagation rate in the equiaxed grain region, where the α_p and α clusters have a large size. However, in the columnar grain region, small and randomly distributed α_p lamellae generate a zigzag-shaped propagation path with a reduction in the da/dN value. Additionally, the change in the size of the α_p lamellae in the equiaxed grains (heat affected bands, HAB) is also observed to influence the propagation behavior of the crack during the crack initiation stage.     

Simulated flight fatigue of an alpha/beta titanium alloy

Simulated flight (FALSTAFF) fatigue tests have been carried out on precracked single edgenotch test-pieces of (Ti4Al4Mo2Sn0.5Si) IMI 550 titanium alloy. Predictions of simulated flight fatigue behaviour have been made from constant amplitude fatigue data, using a damage accumulation approach, with no allowance for load history. The predicted lives were conservative compared with the measured lives, and accurate within a factor of approximately two. Retardation of fatigue crack growth increased with increasing load amplitude. The microstructure produced by β-solution heat treatment at 1010°C, followed by ageing, was found to improve simulated flight fatigue lives by up to approximately 100% compared with standard solution treatment at 900°C, followed by ageing.     

The effect of grain size on small fatigue crack growth in pure titanium

The growth behaviour of small fatigue cracks has been studied in both fine- and coarse-grained versions of a pure titanium under axial loading at stress ratio, R, of −1. The growth behaviour and its statistical properties in a coarse-grained version of a different pure titanium have also been investigated under rotating bending (R = −1), and the results obtained were compared with those of a fine-grained version of this titanium in a previous report. Under both loading conditions, small cracks grew faster than large cracks. As the growth data were plotted in terms of the effective stress intensity factor range ΔK_eff (after allowing for crack closure, the growth rates could be well correlated with large-crack data in a large-crack regime. In a small-crack regime, however, small cracks still grew faster than large cracks. Small cracks in coarse-grained material showed higher growth rates than those in fine-grained material owing to a much smaller effect of microstructure such as grain boundaries and crack deflection. Stage I facets were observed in all the specimens tested, and their depths were less than the maximum grain size estimated by the statistics of the extreme values, but the distribution of stage I facet depths approximately corresponded to the maximum value distributions of grain size of the materials. The growth rates of small cracks followed log-normal distributions independent of grain size. The coefficients of variation, η, of growth rate in coarse-grained material were smaller than those in fine-grained material. The η values were significantly large at a/d 3 (a = crack depth, D = grain size), indicating that the relative size of microstructurally small cracks was not dependent on grain size.     

Fatigue crack growth testing at negative stress ratios: discussion on the comparability of testing results

It is an accepted fact in fatigue community that compressive loads contribute to fatigue crack growth. Evidences range from fatigue crack growth under fully compressive loads to effects of compressive underloads to negative stress ratio loading. Because the crack closes under compression and the crack flanks transmit compressive stresses, the loading situation is completely different to those of tensile loading. The present paper addresses the comparability of crack growth testing procedures at negative stress ratios. It reveals that compressive loading at the crack tip differs in different specimens for an equal maximum stress intensity factor K_max and negative stress ratio R. Furthermore, the crack length can significantly influence the loading conditions at the crack tip for tension–compression loading. Depending on the specimen type and crack length, a negative force ratio may lead to a change of algebraic sign of the stresses at the crack tip or not. As a consequence, the comparability of available literature results for R ≤ 0 tests is not ensured. Proposals to improve the comparability of tension–compression crack growth testing will be given.     

High‐cycle and very‐high‐cycle fatigue behaviour of a titanium alloy with equiaxed microstructure under different mean stresses

The fatigue behaviour of a titanium alloy Ti‐6Al‐4V with equiaxed microstructure (EM) under different values of tensile mean stress or stress ratio (R) was investigated from high‐cycle fatigue (HCF) to very‐high‐cycle fatigue (VHCF) regimes via ultrasonic axial cycling. The effect of mean stress or R on the fatigue strength of HCF and VHCF was addressed by Goodman, Gerber, and Authors' formula. Three types of crack initiation, namely, surface‐with‐RA (rough area), surface‐without‐RA, and interior‐with‐RA, were classified. The maximum value of stress intensity factor (SIF) at RA boundary for R < 0 keeps constant regardless of R in HCF and VHCF regimes. The SIF range at RA boundary for R > 0 also keeps constant regardless of R in VHCF regime, but this value decreases linearly with the increase of R for surface RA cases. The microstructure observation at RA regions gives a new result of nanograin formation only in the cases of negative stress ratios for the titanium alloy with EM, which is explained by the mechanism of numerous cyclic pressing.     

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of HT80 steel at several stress ratios

Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.     

Effect of stress ratio and frequency on fatigue crack growth rate of 2618 aluminium alloy silicon carbide metal matrix composite

Effect of stress ratio and frequency on the fatigue crack propagation of 2618 aluminium alloy-silicon carbide composite were investigated at ambient temperature. With the first set of specimens, the fatigue crack growth rates were studied at three frequencies of 1 Hz, 5 Hz and 10 Hz at a stress ratio of 0.1 whereas the effects of stress ratios of 0.1, 0.25 and 0.50 were studied with the second set of specimens. The study showed that the fatigue crack propagation behaviour of this metal matrix composite was influenced to an appreciable extent by the stress ratio, but not by the fatigue frequencies used in this investigation.     

Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of ?1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 μm particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R=?1, the fatigue strength of the SiC_p/Al composite was the same as that of the unreinforced alloy, but at R= 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiC_p/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10⁷ cycles in the unreinforced alloy, but significantly unconservative estimation in the SiC_p/Al composite. At R= 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R=?1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiC_p/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.     

Local texture and fatigue crack initiation in a Ti-6Al-4V titanium alloy

ABSTRACT Fatigue crack initiation was studied in a bimodal TA6V titanium alloy. A ghost structure inherited from the forging process, the scale of which is roughly 100 times the apparent grain size, was found to govern the initiation process. In these macrograins, that we have labelled macrozones, most of the primary alpha grains (α_p) are found to display the same crystallographic orientation. Fatigue cracks are initiated on the basal plane or, if basal slip is difficult, on the prismatic plane. Thus in macrozones, where basal or prismatic slip is easy, numerous neighbouring tiny cracks appear over the whole macrozone, which have the size of the primary α_p grains. In these macrozones the contribution of crack coalescence to crack growth is consequently very significant. On the contrary, if basal and prismatic slips are both difficult in the macrozone, no crack can be found in the corresponding macrozone. The crack initiation process is thus highly heterogeneous at the scale of the macrozone. Furthermore, this microstructure is found to induce a large scatter in the fatigue life of notched samples.     

Local strain accumulation in fatigue crack propagation process of Ti‐6Al‐4V alloy

Fatigue crack growth behaviours of the titanium alloy Ti‐6Al‐4V, with two different microstructures, at different maximum stresses were identified by digital image correlation technique. Full‐field strains were monitored around fatigue cracks after consecutive cycles in fatigue crack growth experiments. Results indicated that the Ti‐6Al‐4V alloy with a bi‐modal microstructure had a better fatigue resistance than that with a primary‐α microstructure. Typical behaviours of small cracks and the evolution of multi‐scale fatigue cracks were clarified. The strain accumulations around the micro‐notch and fatigue crack increased with increasing number of load cycles. On the basis of von Mises strain mapping, it was found that crack growth rate could be characterized by crack‐tip plastic zone size.     

Environmental fatigue crack growth in titanium aluminides and hydrogen evolution behaviour

The influence of environment on fatigue crack growth behaviour was investigated both in nearly lamellar and in duplex titanium aluminides, and the hydrogen evolution kinetics was analysed by thermal desorption spectroscopy. The tensile strength of the duplex material decreases in the order of the extent of the water molecule content in the environment: the strength in vacuum is the highest, and decreases in the order of laboratory air and finally in water. In the case of the lamellar material, the fatigue crack growth rate in dry air is higher in the R–C crack plane orientation than that in the L–C crack plane orientation. The crack growth rate becomes higher when the crack grows as the lamellae tear. However, in the case of the duplex material, the crack growth rate in the R–C crack plane orientation is smaller in the low Δ K (Δ K _eff ) region. When cathodic charging is applied, the fatigue crack growth rate becomes higher than in dry air, particularly in the higher stress intensity factor range. The hydrogen evolution rate is increased by cathodic charging, with lower temperature peaks and higher ones. The peaks at lower temperatures are correlated with the decomposition of hydrides and de-training of hydrogen from microstructural imperfections such as microvoids. As-received materials also show an evolution peak at a higher temperature, and the evolution rate is almost independent of cathodic charging. In addition, the evolution rate at a high temperature (above 800 °C) is increased by cathodic charging. The hydrogen is considered to have an important role on fatigue crack growth acceleration.     

TC4钛合金的超高周疲劳行为及其竞争失效模型构建

针对现有模型对TC4竞争失效预测的不准确性,建立了基于最大应力强度因子的竞争失效模型。在室温以及两种应力比下,针对TC4钛合金进行超高周疲劳试验,通过试验与最弱键竞争失效理论相结合的方法进行评估,研究其超高周疲劳性能。通过对试样断口形貌的观察,可将其失效模式分为如下两类:表面失效以及内部失效。对试样表面缺陷以及内部解理刻面尺寸进行测量,并评估其最大应力强度因子值。进一步通过正态分布得到最大应力强度因子的累计分布函数,基于两参数泊松分布建立了与最大应力强度因子有关的竞争失效模型。通过模型计算结果,可以得出在任一最大应力强度因子下试样发生各种失效模式的概率,且经分析对比,本文中TC4两种疲劳失效模式的失效概率评估结果与试验数据吻合较好,为分析TC4钛合金超高周疲劳状态下的疲劳失效模式提出了新的评估方法。     

Effect of post-weld heat treatment on fracture toughness and fatigue crack growth behaviour of electron beam welds of a titanium (α+β) alloy

The effects of a post-weld heat treatment on the fracture toughness and fatigue crack growth behaviour of electron beam welds of an α + β titanium alloy, Ti–6.5Al–1.9Zr–0.25Si have been studied. Welds in the stress-relieved condition exhibited poor fracture toughness due to poor energy absorbing capacity of the thin α and α' phases. Post-weld heat treatment which resulted in the decomposition of α' to α + β and the coarsening of intragranular and intergranular α resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and intergranular α phase. Fatigue crack growth resistance of welds was superior to the base metal in the α + β heat-treated condition. The superior crack growth resistance of the welds is due to the acicular α microstructure which results in a tortuous crack path and possible crack closure arising from crack path tortuosity.     

Multiscale fatigue crack growth modelling for welded stiffened panels

The influence of welding residual stresses in stiffened panels on effective stress intensity factor (SIF) values and fatigue crack growth rate is studied in this paper. Interpretation of relevant effects on different length scales such as dislocation appearance and microstructural crack nucleation and propagation is taken into account using molecular dynamics simulations as well as a Tanaka–Mura approach for the analysis of the problem. Mode I SIFs, K_I, were calculated by the finite element method using shell elements and the crack tip displacement extrapolation technique. The total SIF value, K_tot, is derived by a part due to the applied load, K_appl, and by a part due to welding residual stresses, K_res. Fatigue crack propagation simulations based on power law models showed that high tensile residual stresses in the vicinity of a stiffener significantly increase the crack growth rate, which is in good agreement with experimental results.     

Fatigue crack growth property of laser beam welded 6156 aluminium alloy

Fatigue crack growth behaviours in different welding zones of laser beam welded specimens were investigated using central crack tension specimens for 6156 aluminium alloy under constant amplitude loading at nominal applied stress ratio R = 0.5, 0.06, ?1. The experimental results showed that base metal (BM) exhibited superior fatigue crack resistance compared to weld metal (WM) and heat‐affected zone (HAZ). Crack growth resistance of WM was the lowest. The exponent m values for BM and HAZ at different stress ratios are close and around 2.6, while m for WM at different stress ratio is around 4.7. The discrepancy between crack growth rates for WM and BM is more evident with increasing stress ratio, while it is a little change for HAZ and BM. Change of the microstructure in WM deteriorates the resistance of fatigue crack growth compared to BM. It was mainly due to grain boundary liquation and dissolving of second‐phase particles in the weld region. It was also found that the variety of fatigue crack resistance for different welding zones is in conformity with the change of hardness. BM with the highest hardness exhibited the maximum resistance for fatigue crack, and WM with the lowest hardness exhibited the minimum fatigue crack resistance.