The growth of cracks in Ti–6Al–4V plate under combined high and low cycle fatigue

The fatigue crack growth rates in cross-rolled Ti–6Al–4V plate subjected to combined major and minor stress cycles have been measured at room temperature. The concept of crack closure was used to model the data for a test sequence using 1000 minor cycles per major cycle, and the model validated by either the accurate or safe prediction of the crack growth rates for a second series of tests involving 10,000 minor cycles per major cycle. Fatigue threshold values for the minor cycles derived from the growth rate data for combined major and minor cycle loadings were lower than those determined by the conventional load shedding method. In comparison with the behaviour of Ti–6Al–4V disc material which had been forged, the cross-rolled plate material exhibited: first, a clearly defined bilinear growth rate curve under a separate major cycle loading; second, similar or lower derived threshold values with separate minor cycle loadings; and third, reduced crack propagation lives for loadings combining major and minor cycles.     

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.     

The influence of minor cycles on low cycle fatigue crack propagation

The influence of relatively high frequency low amplitude vibrations superimposed on higher amplitude low frequency major cycles is studied, for titanium alloy (Ti-6Al-4V). It is concluded that the major cycles causing low cycle fatigue are of greatest significance during crack formation and for fatigue crack growth until the minor cycles are triggered by exceeding the appropriate fatigue threshold level. Beyond this point, for realistic numbers of minor cycles, as might occur in gas turbine and compressor blades and discs, the damage caused by high cycle fatigue dominates. For all practical purposes, once minor cycles activity is triggered, the component should be considered to have used up its useful life.     

Influence of sharp microstructural gradients on the fatigue crack growth resistance of α+β and near-α titanium alloys

The present study focuses on the effect of microstructural gradients on the fatigue crack growth resistance of Ti‐6Al‐4V and Ti‐6242 titanium alloys. Sharp microstructural gradients from fine‐grained bimodal to coarse‐grained lamellar microstructures were obtained by heat treating only a portion of fine‐grained plates in the β single‐phase field using a high‐frequency induction coil. For fatigue crack growth from a bimodal into a lamellar microstructure, it was found that the initial crack extension past the microstructural transition within the lamellar microstructure shows the same crack growth resistance as the reference bimodal microstructure. Similarly, for fatigue crack growth from a lamellar into a bimodal microstructure, the initial crack extension past the microstructural transition within the bimodal microstructure shows same crack growth resistance as the reference lamellar microstructure. Based on detailed crack front profile investigations using optical light and scanning electron microscopy as well as heat tinting procedures, these findings can be mainly attributed to the effect of the crack front geometry.     

On the application of the Kitagawa-Takahashi diagram to foreign-object damage and high-cycle fatigue

The role of foreign-object damage (FOD) and its effect on high-cycle fatigue (HGF) failures in a turbine engine Ti-6Al-4V alloy is examined in the context of the use of the Kitagawa-Takahashi diagram to describe the limiting conditions for such failures. Experimentally, FOD is simulated by firing 1 and 3.2 mm diameter steel spheres onto the flat specimen surface of tensile fatigue specimens at velocities of 200 and 300 m/s. Such damage was found to markedly reduce the fatigue strength of the alloy, primarily due to four factors: stress concentration, microcrack formation, impact-induced plasticity and tensile residual stresses associated with the impact damage. Two groups of fatigue failures could be identified. The first group initiated directly at the impact site, and can be readily described through the use of a fatigue-crack growth threshold concept. Specifically, a Kitagawa-Takahashi approach is presented where the limiting threshold conditions are defined by the stress-concentration corrected smooth-bar fatigue limit (at microstructurally small crack sizes) and a “worst-case” fatigue-crack growth threshold (at larger crack sizes). The second group of failures was caused by fatigue cracks that initiated at locations far from the impact site in regions of high tensile residual stresses, the magnitude of which was computed numerically and measured experimentally using synchrotron X-ray diffraction. Specifically, these failures could be rationalized due to the superposition of the residual stresses on the far-field applied mean stress, leading to a locally elevated load ratio (ratio of minimum to maximum loads). The effects of residual stress, stress concentration, and microstructurally small cracks are combined in a modified Kitagawa-Takahashi approach to provide a mechanistic basis for evaluating the detrimental effect of FOD on HCF failures in Ti-6Al-4V blade alloys.     

Rapid method for low cycle fatigue properties: thickness effect on the fatigue crack initiation life of welded joints

Welded assemblies are commonly used in the shipbuilding industry. Because of the combination of stress concentration and cyclic loading, welded joints could be a critical area for fatigue damage. Thus, knowing stress and strain histories at the critical points of the structure is necessary, particularly when a confined plasticity occurs, to determine the fatigue life of welded assemblies. To avoid time‐consuming nonlinear finite element analyses (FEA), simplified estimation methods of the elastic–plastic strain/stress can be used. In a previous work, an approach to estimate stress state at critical points was developed and employed in the case of double‐notched specimens. The present paper focuses on welded joints in order to validate this strategy with the aim to estimate the fatigue crack initiation life of T‐joints. To go further, a parametric approach has been adopted to take into account the local geometries of welded joints and to determine the constraint operator without any FEA. The results predicted by this approach are compared with experimental fatigue results.     

A cumulative damage model for fatigue life estimation of high‐strength steels in high‐cycle and very‐high‐cycle fatigue regimes

A cumulative fatigue damage model is presented to estimate fatigue life for high‐strength steels in high‐cycle and very‐high‐cycle fatigue regimes with fish‐eye mode failure, and a simple formula is obtained. The model takes into account the inclusion size, fine granular area (FGA) size, and tensile strength of materials. Then, the ‘equivalent crack growth rate’ of FGA is proposed. The model is used to estimate the fatigue life and equivalent crack growth rate for a bearing steel (GCr15) of present investigation and four high‐strength steels in the literature. The equivalent crack growth rate of FGA is calculated to be of the order of magnitude of 10^?14–10^?11 m/cycle. The estimated results accord well with the present experimental results and prior predictions and experimental results in the literature. Moreover, the effect of inclusion size on fatigue life is discussed. It is indicated that the inclusion size has an important influence on the fatigue life, and the effect is related to the relative size of inclusion for FGA. For the inclusion size close to the FGA size, the former has a substantial effect on the fatigue life. While for the relatively large value of FGA size to inclusion size, it has little effect on the fatigue life.     

Correlation of fatigue properties and microstructure in investment cast Ti-6Al-4V welds

The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime.     

Numerical simulation of the coupling between thermal dissipation and fish‐eye crack growth in very high cycle fatigue regime

In this paper, we study the temperature field associated with the propagation of a fatigue crack in a very high cycle fatigue regime during ultrasonic fatigue testing. We use a Paris–Hertzberg crack growth law to compute the evolution of the crack and a perfectly elastic–plastic constitutive law to compute the plastic dissipation per cycle at the tip of the crack. A thermomechanical finite element model is proposed to estimate the evolution of the temperature field during the crack propagation. Numerical results obtained agree fairly well with experimental results.     

Nanograin layer formation at crack initiation region for very‐high‐cycle fatigue of a Ti–6Al–4V alloy

The microstructural features and the fatigue propensities of interior crack initiation region for very‐high‐cycle fatigue (VHCF) of a Ti–6Al–4V alloy were investigated in this paper. Fatigue tests under different stress ratios of R = ?1, ?0.5, ?0.1, 0.1 and 0.5 were conducted by ultrasonic axial cycling. The observations by SEM showed that the crack initiation of VHCF presents a fish‐eye (FiE) morphology containing a rough area (RA), and the FiE and RA are regarded as the characteristic regions for crack initiation of VHCF. Further examinations by TEM revealed that a layer of nanograins exists in the RA for the case of R = ?1, while nanograins do not appear in the FiE outside RA for the case of R = ?1, and in the RA for the case of R = 0.5, which is explained by the Numerous Cyclic Pressing model. In addition, the estimations of the fatigue propensities for interior crack initiation stage of VHCF indicated that the fatigue life consumed by RA takes a dominant part of the total fatigue life and the related crack propagation rate is rather slow.     

Evaluation of the overload effect on fatigue crack growth with the help of synchrotron XRD strain mapping

The overload retardation effect on fatigue crack growth rate (FCGR) in titanium alloy Ti-6Al-4V is studied. Synchrotron X-ray diffraction strain mapping of near-crack tip regions of pre-cracked fatigued samples is used to determine the effective stress intensity factors experienced by the crack tip. The effective stress intensity factor values are computed by finding the best match between the experimental strain maps and linear elastic fracture mechanics (LEFM) predictions. The dependence of the effective stress intensity factor, K, on the applied load is plotted, and an interpretation of the overload retardation effect is proposed. The present approach permits to reconcile the traditional LEFM fatigue crack propagation prediction and the experimental measurement of strain fields.     

The effect of bi-modal and lamellar microstructures of Ti-6Al-4V on the behaviour of fatigue cracks emanating from edge-notches

This paper is aimed at evaluating the influence of bi‐modal and lamellar microstructures on the behaviour of small cracks emanating from notches in α+β titanium Ti‐6Al‐4V alloy. Pulsating four point bending tests were performed at a nominal stress ratio of 0.1 and a frequency of 15 Hz on double‐edge‐notched specimens. The conditions of initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 88 and 58% of the 0.2% material yield stress. Crack closure effects were measured by an extensometric technique and discussed. Variations in crack aspect ratio were determined and considered in the ΔK calculation. Corresponding results were discussed by considering the effect of the yielded region at the notch tip calculated by elastic–plastic finite element modelling of the fatigue tests. The importance of the bi‐modal and lamellar microstructures on the material damage was highlighted and correlated to the observed oscillations in the crack growth rate. The crack growth rate data obtained were compared with those measured using standard C(T) specimens (long crack).     

Short and long crack data for fatigue of 2024‐T3 Al sheets: binariness of scale segmentation in space and time

Pedagogically speaking, crack initiation–growth–termination (IGT) belongs to the process of fracture, the modelling of which entails multiscaling in space and time. This applies to loadings that are increased monotonically or repeated cyclically. Short and long crack data are required to describe IGT for scale ranges from nano to macro, segmented by the SI system of measurement. Unless the data at the nano scale can be connected with the macro, IGT remains disintegrated. The diversity of non‐homogeneity of the physical properties at the different scale ranges results in non‐equilibrium. These effects dubbed as non‐equilibrium and non‐homogeneous are hidden in the test specimens and must be realized. They can be locked into the reference state of measurement at the mi‐ma scale range by application of the transitional functions and transferred to the nano‐micro and macro‐large scale ranges. The aim of this work is to convert the ordinary crack length data to those referred to as short cracks that are not directly measurable. All test data are material, loading and geometry (MLG) specific. The results obtained for the 2024‐T3 aluminium sheets hold only for the MLG tested. The differences are more pronounced for the short cracks. These effects can be revealed by comparing the incremental crack driving force (CDF) for the ma‐mi range the ma‐large range and the na‐mi range The CDF is equivalent to the incremental volume energy density factor (VEDF). The incremental mi‐ma CDF is found to be 10–10⁵ kg mm^?1 for cracks 3–55 mm long travelling at an average velocity of 10^?5 mm s^?1. The crack velocity rises to 10^?3 mm s^?1 when the incremental CDF is increased to 10⁵–10⁶ kg mm^?1, while the crack lengths are 49–260 mm. The crack velocity for the na‐mi range of 0.040–0.043 mm slowed down to 10^?8 mm s^?1, and the incremental CDF reduces further to 10^?8–10^?2 kg mm^?1. Note that changed several orders of magnitude while the crack advanced from 0.040 to 0.044 mm. Such behaviour is indicative of the highly unstable nature of nanocracks. All results are based on using the transitionalized crack length (TCL). The TCL fatigue crack growth increment Δa is postulated to depend on the incremental CDF ΔS or ΔVEDF. The form invariance of , and is invoked by scale segmentation to reveal the multiscale nature of IGT that is inherent to fatigue crack growth. While the choice of directionality from micro to macro is not the same as that from macro to micro, this difference will not be addressed in this work.     

Role of microstructures on the growth of long fatigue cracks

An analysis is presented to understand the role of microstructures on the two crack growth driving force parameters, and , without invoking the extrinsic crack closure concepts. Microstructural variables considered are: grain size, precipitates and stacking fault energy. It is shown that is strongly affected by the scale of the microstructure, such as grain size or precipitate spacing. For each case, the mode of slip deformation and environment affects the fatigue resistance as represented by . However, the microstructures seem to have a smaller effect on . Also, the enhanced planarity of slip from the reduction in stacking fault energy has a pronounced effect on when compared with the materials deforming under homogeneous slip.