Fractographic and microstructural analysis of fatigue crack growth in a Ti-6Al-4V fan disc forging

The constant amplitude fatigue crack growth behaviour of a conventionally (+β) solution treated and aged Ti-6Al-4V fan disc forging was examined by fractographic and microstructural analysis. The crack growth process was complex with many interrelated fracture features. A transition in the fatigue crack growth curve correlated with a change from structure-sensitive to continuum-mode crack growth, primarily in the transformed and aged β grains, and a decrease in fracture surface roughness. The transition was probably caused by the cyclic plastic zone size becoming equal to and exceeding the average platelet packet size. The significance of such transitions for prediction of fatigue crack growth and service failure analysis is discussed.     

A finite element analysis of fretting fatigue crack growth behavior in Ti-6Al-4V

There is a need for methodology(ies) to analyze the crack growth behavior under fretting fatigue condition since its experimental determination is a challenging task. A finite element sub-modeling method was used to estimate the crack propagation life in titanium alloy, Ti-6Al-4V specimens. Two contact geometries, cylinder-on-flat and flat-on-flat, were analyzed. The computed crack propagation lives were combined with the results of an experimental study where total fatigue lives were measured. The combined numerical-experimental approach provided the crack initiation lives. The crack propagation life increased with increasing applied cyclic bulk stress in similar manner for both contact geometries. Almost 90% of the fretting fatigue life was spent during the crack nucleation and initiation phases in the high cycle fatigue regime. A parametric study was also conducted to investigate the effects of contact load, coefficient of friction and tangential force on the crack growth behavior. The crack propagation life decreased with increase of these three parameters. This decrease was similar for the contact load and the tangential force in both contact geometries, however, the decrease in the case of coefficient of friction was relatively more in the cylindrical pad than in the flat pad.     

3D shear-mode fatigue crack growth in maraging steel and Ti-6Al-4V

Fatigue crack growth tests in mixed-mode II + III were performed on maraging steel and Ti-6Al-4V. The 3D evolutions of the crack fronts -measured by SEM after interrupted tests- were analyzed, taking into account the reduction in effective crack driving force by the interlocking and friction of the asperities of the crack surface. Under small-scale yielding conditions, the mixed-mode crack growth rates were found to correlate best with \({\sqrt{{\Delta {\rm K}}_{\rm II}^{{\rm eff}^{2}}+1.2\Delta {\rm K}_{\rm III}^{{\rm eff}^{2}}}}\) in maraging steel, while for Ti-6Al-4V, \({\sqrt{\Delta {\rm K}_{\rm II}^{{\rm eff}^{2}}+0.9\Delta {\rm K}_{\rm III}^{{\rm eff}^{2}}}}\) appeared suitable. For extended plasticity, a crack growth prediction method is proposed and validated for Ti-6Al-4V. This method is based on elastic-plastic F.E. computations and application, ahead of each node of the crack front, of a shear-dominated fatigue criterion.     

Fusion zone microstructure and fatigue crack growth behaviour in Ti-6Al-4V alloy weldments

Abstract

The fatigue crack growth resistance of α–β titanium alloys can be altered by microstructural modification. During welding, the fusion zone microstructure depends on cooling rate. In the present work, the alloy Ti-6Al-4V was welded over a range of heat inputs, using electron beam and gas tungsten arc welding. The weld microstructure varied from predominantly martensitic under rapid cooling conditions to a mixture of martensite and diffusional products on slower cooling. Post-weld heat treatment resulted in a basketweave α–β aggregate that coarsened with temperature and time. In all welded and heat treated conditions, the fusion zone exhibited a fatigue crack growth resistance superior to that of the base material, which was in part attributed to the lamellar microstructure of the fusion zone. Welding residual stresses also played a beneficial role in the as welded condition. Post-weld heat treatment eliminated the advantage resulting from the welding stresses but not that as a result of microstructure.     

Effect of mean stress on fretting fatigue of Ti-6Al-4V on Ti-6Al-4V

Fretting fatigue tests of Ti‐6Al‐4V on Ti‐6Al‐4V have been conducted to determine the influence of stress amplitude and mean stress on life. The stress ratio was varied from R=−1 to 0.8. Both flat and cylindrical contacts were studied using a bridge‐type fretting fatigue test apparatus operating either in the partial slip or mixed fretting regimes. The fretting fatigue lives were correlated to a Walker equivalent stress relation. The influence of mean stress on fretting fatigue crack initiation, characterized by the value of the Walker exponent, is smaller compared with plain fatigue. The fretting fatigue knockdown factor based on the Walker equivalent stress is 4. Formation of fretting cracks is primarily associated with the tangential force amplitude at the contact interface. A simple fretting fatigue crack initiation metric that is based on the strength of the singular stress field at the edge of contact is evaluated. The metric has the advantage in that it is neither dependent on the coefficient of friction nor the location of the stick/slip boundary, both of which are often difficult to define with certainty a priori.     

An analysis of reported fatigue crack growth rate data with special reference to Ti-6A1-4V

A review of the published literature on fatigue crack growth suggests that power-law growth in Ti-6A1-4V is sensitive to microstnictural changes which result in variations in the fatigue mechanism. Microstnictures which promote secondary cracking along α/β interfaces display slow growth rates while microstructures which promote dimpled rupture display fast growth rates. Examples of similar effects are found in other alloy systems.Typically, the power-law growth are found in other alloy systems. It is also suggested that the power-law regime begins at $\text{ΔK ～- 13 MNm}{}^{\mathrm{<mtext>?</mtext><mtext>case3</mtext><mtext>2</mtext>}}$, coinciding with the lower limit of striation formation on the fracture surface. The upper limit occurs at about $\text{K}{}_{\max }\text{= 1/2K}{}_{\mathrm{c}}$. At higher growth rates, the Forman equation appears to be adequate.The normalized stress intensity factor, $\text{ΔK/E}$, required to produce a given growth rate in Ti-6A1-4V is on the order of that for other Ti-base alloys, ferritic steels, martensitic steels and aluminum alloys. Austenitic steels, which deform by planar slip are much more resistant to crack growth over much of the stress intensity range normally encountered.     

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.     

Modeling the fatigue crack growth behavior of Ti-6Al-4V by considering grain size and stress ratio

Ti-6Al-4V is a commonly used titanium base alloy in aerospace applications. The increasing demand for damage-tolerant designs of such components necessitates a detailed knowledge of its crack growth behavior. The aim of this research was the characterization and phenomenological modeling of long crack growth behavior with respect to microstructure and stress ratio. Therefore, the long crack propagation was characterized for eight different heat treatment conditions and four stress ratios. For comparison, physically short crack growth tests were also performed. The long crack growth threshold was found to be dominated by roughness-induced crack closure, and the fracture surface roughness is controlled by the primary α-grain size. The reason for this correlation is a near-threshold crack propagation mode, which is dominated by the transcrystalline fracture of α-grains. This correlation was used to model the crack growth threshold with respect to microstructure. A linear relation was determined between the stress ratio and the threshold value, which was also found in this approach. Further presented models cover the crack growth behavior in the near-threshold (Stage I) and mid-growth rate regions (Stage II).     

Acoustic emission during fatigue of Ti-6Al-4V: Incipient fatigue crack detection limits and generalized data analysis methodology

The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6Al-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 10 m. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.     

The high-cycle fatigue behavior of Ti-6Al-4V alloy

The cyclic stress-strain behavior and the S/N behavior of a “pseudo-elastic” alloy, Ti-6Al-4V of coarsened microstructure and of a “plastic” material, copper, are compared. In accord with views recently put forth by Freudenthal, important differences are noted between these two types of materials. These differences include the mode of crack initiation, the intensity of the Bauschinger effect, and the level below macroscopic yield at which long-life fatigue data lie.     

Plain fatigue and fretting fatigue behaviour of plasma nitrided Ti-6Al-4V

Fatigue tests with and without fretting against unnitrided fretting pads were conducted on unnitrided and plasma nitrided Ti-6Al-4V samples. Plasma nitrided samples exhibited higher surface hardness, higher surface compressive residual stress, lower surface roughness and reduced friction force compared with the unnitrided specimens. Plasma nitriding enhanced the lives of Ti-6Al-4V specimens under both plain fatigue and fretting fatigue loadings. This was explained in terms of the differences in surface hardness, surface residual stress, surface roughness and friction force between the unnitrided and nitrided samples.     

Tensile and fatigue strength of hydrogen-treated Ti-6Al-4V alloy

Tensile, fatigue and fractographic data on Ti-6Al-4V microstructures attained through a series of post--annealing treatments which used hydrogen as a temporary alloying element are presented. Hydrogen-alloying treatments break up the continuous grain boundary and colony structure, and produce a homogeneous microstructure consisting of refined -grains in a matrix of discontinuous . These changes in microstructural morphology result in significant increases of the yield strength (974 to 1119 MPa), ultimate strength (1025 to 1152 MPa) and high cycle fatigue strength (643 to 669 MPa) compared to respective values for lamellar microstructures (902, 994, 497 MPa). The strengths are also significantly greater than the strengths of equiaxed microstructures (914, 1000, 590 MPa). The strengths of hydrogen-alloy treated samples are therefore superior to strengths attainable via other thermal cycling techniques.The fatigue fracture surfaces of the hydrogen-alloy treated samples were topographically similar to equiaxed samples. Fatigue crack initiation was characterized by faceted regions. As crack length and K increased, the crack surface changed to a rounded, ductile topology, with microcracks and locally striated regions. Fracture primarily followed the - interfaces. This is rationalized by the fact that hydrogen-alloyed microstructures are very fine Widmanstatten microstructures having reduced aspect ratios, and these microstructures fail along - interfaces.     

Effect of dwell on the growth of fatigue cracks in Ti-6Al-4V alloy bar

The growth of cracks in Ti-6Al-4V alloy was studied in fatigue tests in which the maximum load each cycle was held constant for a chosen dwell-time, and the results were compared with those obtained using sinusoidal stressing.

A 57 mm thick forged and rolled bar of Ti-6Al-4V alloy was used. It had been annealed at 700°C for 2 hr and the microstructure was mainly hexagonal -phase with a strong preferred orientation (TD texture). Test pieces were cut from the bar to select crystal orientations of fracture planes and directions. When the stressing direction was parallel to the basal plane, the crack growth rate of dwell-cycling at room temperature was similar to that for sinusoidal stressing at 0.3 and 25 Hz. But when the stress was normal to the basal plane the rate of growth was increased considerably, depending on the dwell-time and the stress intensity factor range (ΔK).

The dependence of the dwell-effect on microstructure and temperature was investigated and the preferred plane of dwell-fracture determined. The failure mechanism is discussed, together with the implications of the results for aircraft components subjected to in-service cyclic loads such that each flight is broadly simulated by a stress pattern having a dwell time at maximum load.     

Surface nanocrystallization of Ti-6Al-4V alloy: microstructural and mechanical characterization

In this study, microstructural and mechanical properties of Ti-6Al-4V alloy, before and after the SMA treatment (SMAT) as well as the duplex SMAT/Nitriding process at different treatment conditions, were investigated in order to deepen the knowledge of these properties for biomedical devices. For that purpose, tribological (wear resistance, coefficient of friction) and mechanical (Vickers microhardness) tests were performed. To carry out the microstructural and surface topographical characterization of the samples, the scanning electron microscopy (SEM) and the 3D-SEM reconstruction from stereoscopic images have been used. By means of profiles deduced from the 3D images, the surface roughness has been calculated. The obtained results allowed to find an interesting SMAT condition which, followed by nitriding at low temperature, can greatly improve tribological and mechanical properties of Ti-6Al-4V alloy. It was also shown from SEM characterization and the original method of 3D-SEM reconstruction, that SMAT can reduce the machined grooves and consequently the roughness of the samples decreases. Moreover, we demonstrated, for the first time, that instead of usual etching method, the ionic polishing allowed to reveal the grains, the grain boundaries and the twins as well as the surface nanocrystalline layer generated by SMAT. Thus, the thickness of the SMATed layer decreases with the nitriding temperature, whereas the surface grain size increases.     

Fatigue crack growth retardation after single-cycle peak overload in Ti-6Al-4V titanium alloy

Fatigue crack growth after single-cycle peak overload was investigated in Ti-6Al-4V sheet. Strain hardening was determined not to be the major controlling mechanism retarding crack growth after peak over-load, but instead, strain hardening slightly accelerated crack growth for the case when strain hardening was induced prior to crack initiation. Crack growth after peak overload was characterized by: (1) no effect after 20 per cent overload: (2) crack arrest immediately following 70 and 100 per cent overloads; (3) subsequent retarded crack growth rates after 70 and 100 per cent overloads; and (4) retardation but no arrest following 50 per cent overload. The Wheeler model of crack growth retardation was investigated. The physical appearance of post-test fracture surfaces were as hypothesized by the Elber concept of crack closure after overload. The The recovery of an overloaded crack was linear with respect to the constant load amplitude cyclic stress intensity.     

Effects of corrosive environments on near-threshold fatigue crack growth behavior of T1-6A1-4V

The near-threshold fatigue crack growth behavior of Ti-6A1-4V alloy has been investigated in low O₂ steam (< 1 ppm), high O₂ steam (40ppm), and boiling water with various concentrations of Nad and/or Na₂₂SO₄. At load ratio (R) of 0.5, high O₂ steam increased the crack propagation rates in the threshold region, relative to low O₂ steam. However, at R = 0.8, the near-threshold crack growth rates in low and high O₂ steam were comparable. Values of threshold stress intensity range, ΔK_th, slightly increased with an increase in the concentration of NaCl in the solution. Varying solution pH from 5.0 to 10.0 in a 0.1 g NaCl plus 0.1 g Na₂SO₄ per 100ml H₂O solution had no effect on the rates of near-threshold crack propagation. Increasing the hydrazine level from 30 to 10⁷ ppb in the same salt solution also did not change the resistance to crack growth. Comparing the present results with the previous data on 403 stainless steel, the near-threshold crack propagation rate performance in Ti-6Al-4V alloy is superior to that in 403 steel in both the steam and salt solution environments.