Einfluss eines Gefügegradienten auf die rauhigkeitsinduzierte Rissschließung bei TIMETAL 1100

Influence of a Grain Size Gradient on Roughness Induced Crack Closure of TIMETAL 1100 Crack closure effects, which affect the crack propagation behaviour at low R‐values, become crack length dependent in gradient materials. It is not known, if the crack closure at a given ΔK is locally identical to that in a homogeneous microstructure or if closure effects are crack direction dependent. To get some insights to these questions, coarse lamellar microstructures (showing only roughness induced crack closure) with a gradient in the lamellae package sizes where produced on TIMETAL 1100 by recrystallisation. Their crack closure behaviour was compared to that of two different homogenous microstructures. The results clearly show, that crack closure effects show identical dependencies on the stress intensity and the local fracture surface roughness for all microstructures. The roughness, in turn, is uniquely determined by the sizes of the lamellae. Thus no crack direction effects where observed. It is shown that a model for roughness induced crack closure, which was developed for homogeneous microstructures, can be used to calculate the crack closure stress intensity for any gradient and loading condition.     

A STUDY ON THE CHANGE OF FATIGUE FRACTURE MODE IN TWO TITANIUM ALLOYS

The appearance of the fatigue fracture surface and crack growth curve have been examined for a Ti–2.5Cu alloy with different microstructures (two equiaxed and two lamellar microstructures), and for TIMETAL 1100 with a lamellar microstructure. With increasing Δ K , a slope change in the crack growth curve correlates with a transition in the fracture surface appearance (induced by a fracture mode transition); this being found in each microstructure. The microstructure size that controls the fatigue fracture is found to be the grain size for equiaxed microstructures and the lamella width for lamellar microstructures. The transitional behaviour can be interpreted in terms of a monotonic plastic zone size model in microstructures having a coarse microstructure size and in terms of a cyclic plastic zone size model for microstructures having a fine microstructure size.     

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.     

Fatigue crack closure: a review of the physical phenomena

Plasticity‐induced, roughness‐induced and oxide‐induced crack closures are reviewed. Special attention is devoted to the physical origin, the consequences for the experimental determination and the prediction of the effective crack driving force for fatigue crack propagation. Plasticity‐induced crack closure under plane stress and plane strain conditions require, in principle, a different explanation; however, both types are predictable. This is even the case in the transition region from the plane strain to the plane stress state and all types of loading conditions including constant and variable amplitude loading, the short crack case or the transition from small‐scale to large‐scale yielding. In contrast, the prediction of roughness‐induced and oxide‐induced closures is not as straightforward.     

Fatigue crack closure and crack growth behaviour in a titanium alloy with different microstructures

Fatigue crack closure and crack growth behaviour in Ti–2.5 wt % Cu alloy with two equiaxed and two lamellar microstructures have been investigated by constant-load amplitudetests. Plasticity-induced crack closure and roughness-induced crack closure have been characterized separately by experimental methods. A change in closure mechanism from plasticity-induced crack closure at high K values (region of high stress intensity ranges)to roughness-induced crack closure at low K values occurs in a solution-annealed equiaxed microstructure, while plasticity-induced crack closure is the operative closure mechanism in an over-aged equiaxed microstructure over the whole range of K and roughness-induced crack closure occurs in two lamellar microstructures. The crack closing stress intensity factor for plasticity-induced crack closure increases continuously with increasing maximum stress intensity. The crack closing stress intensity factor for roughness-induced crack closure increases with increasing maximum stress intensity at low K, and remains constant at high K. Crack closure and crack path deflection have a significant influence on the crack growth rates. © 1998 Kluwer Academic Publishers     

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.     

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.     

The effect of microstructure and environment on fatigue crack growth in 7049 aluminium alloy at negative stress ratios

The influence of environment and microstructure on fatigue crack growth has been investigated on a high strength 7049 aluminium alloy. This aluminium alloy was artificially aged to underaged (UA) and overaged (OA) microstructures. The heat treatment procedure was performed in order to obtain an UA and OA microstructure having the same yield strength properties, but differing in the mode of slip deformation: the UA alloy deforms by planar slip and that of the OA alloy by wavy slip. The crack growth measurements were performed in MT specimens at constant load ratios for R=0, −1, −2, −3 near-threshold and Paris regime in ambient air and vacuum conditions. Crack closure loads were measured in order to determine the P_open for each R ratio. Micromechanisms of near-threshold crack growth are briefly discussed for several concurrent processes involving environmentally assisted cracking with intrinsic microstructural effects. The results showed that the presence of humid air leads to a larger reduction in ΔK_th for both the ageing conditions, but the UA specimens were superior probably because of crack branching. The role of environmental effect and microstructures near-threshold regime seems to be more significant than any mechanical contributions to the crack closure, such as plasticity, roughness, oxide, etc.     

THE INFLUENCE OF AGE-HARDENING ON FATIGUE CRACK PROPAGATION BEHAVIOUR IN 7075 ALUMINIUM ALLOY IN VACUUM

Abstract— The influence of age-hardening on the middle and low crack growth rates of a 7075 Al alloy is studied in vacuum. A transition in fracture surfaces morphology and crack growth curves is observed with the T 651 and T 7351 treatments in the near-threshold regime. Measurements of crack closure show its dependance on surfaces roughness and explain the lack of dependance of ΔK_th with load ratio, except for the T 7351 alloy. An equation of crack growth rate to the fourth power of ΔK_eff is in good agreement only with the crack propagation curves obtained for microstructure with an homogeneous deformation mode.     

Improved test method and analytical modelling for fatigue crack growth in coarse‐grain titanium alloy with rough fatigue surfaces

Fatigue crack growth rate properties are typically determined by experimental methods in accordance with ASTM Standard E647. These traditional methods use standard notched specimens that are precracked under cyclic tensile loads before the main test. The data that are produced using this approach have been demonstrated elsewhere to be potentially adversely affected by the test method, particularly in the threshold region where load reduction (LR) methods are also required. Coarse‐grained materials that exhibit rough and tortuous fatigue surfaces have been observed to be strongly affected by the tensile precracking and LR, in part because the anomalies caused by crack closure and roughness‐induced closure become more important. The focus of the work reported in this paper was to further develop methods to determine more accurate fatigue crack growth rate properties from threshold through to fracture for coarse‐grained, β‐annealed, titanium alloy Ti‐6Al‐4V extra low interstitial thick plate material. A particular emphasis was put upon the threshold and near threshold region, which is of strong importance in the overall fatigue life of components. New approaches that differ from the ASTM Standard included compression precracking, LR starting from a lower load level and continuing the test beyond rates where crack growth would otherwise be considered below threshold. For the threshold regime, two LR methods were also investigated: the ASTM method and a method where the load is reduced with crack growth such that the crack mouth opening displacement is held constant, in an attempt to avoid remote closure. Constant amplitude fatigue crack growth rate data were produced from threshold to fracture for the titanium alloy at a variety of stress ratios. Spike overload tests were also conducted These data were then used to develop an improved analytical model to predict crack growth under spectrum loading and the predictions were found to correlate well with test results.     

Short fatigue cracks in intermetallic γ‐TiAl‐alloys

Fatigue crack growth at room temperature and its relation to the local microstructure is studied for four different γ‐TiAl‐alloys with microstructures ranging from coarse and fully lamellar to fine and partly lamellar. It is shown that the number of cycles to failure depends strongly on the efficiency of the first barrier to crack extension, as crack growth rates may increase rapidly once this barrier has been breached by a specific crack. The crack extension behaviour for two typical barriers (colony boundary and twin boundary) is studied using high‐resolution optical and scanning electron microscopy.     

Vorhersagemodell für die Wachstumsraten kurzer Risse auf der Basis von Kmax‐konstant‐Versuchen an M(T)‐Proben

Prediction model for the growth rates of short cracks based on K_max‐constant tests with M(T) specimens The fatigue crack growth behaviour of short corner cracks in the Aluminium alloys Al 6013‐T6 and Al 2524‐T351 was investigated. The aim was to determine the crack growth rates of small corner cracks at stress ratios of R = 0.1, R = 0.7 and R = 0.8 and to develop a method to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T)‐specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The pre‐cracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a Focussed Ion Beam. The results of the fatigue crack growth tests with short corner cracks were compared with long fatigue crack growth test data. The short cracks grew at ΔK‐values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK‐values and the same stress ratios. A model was developed on the basis of K_max‐constant tests with long cracks that gives a good and conservative prediction of the short crack growth rates.     

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.     

Fatigue crack growth and crack closure in an AlMgSi alloy

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1-T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameter U was calculated and related with Δ K and the stress ratio, R . The threshold of the stress intensity factor range, Δ K _th , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, Δ K baseline level and R . The crack closure parameter U trends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1-T6 alloy for plane stress conditions.     

Verfahren zur vollständigen Ermittlung der R‐Abhängigkeit des Rissausbreitungsverhaltens mit nur einer Probe

Procedure for the determination of the complete R‐dependency of the crack growth behaviour with only one specimen A new concept for fatigue crack propagation tests has been developed. Using a single specimen, it is possible to determine fatigue crack growth curves (da/dN ‐ ΔK) for every stress ratio between R = 0.9 and R = ‐1. Additionally, the new concept also provides threshold values for fatigue crack growth for different values of R and K_max. In combination with a continuous crack length measurement tool (such as the DC potential drop method) this testing procedure can be performed with minimal effort of personnel and time. The test procedure consists of a sequence of K_max‐constant tests with decreasing crack growth rates. As the applied K_max is increasing stepwise there should be no load history effects. According to the procedures described in the ASTM Standard E 647, the results using this new testing procedure fit very well to the da/dN ‐ ΔK curves generated with different specimens. The tests also fulfil all the requirements of ASTM Standard E 647.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

A threshold fatigue crack closure model: Part II – experimental verification

Predictions from an analytical model that considers contributions and interactions between plasticity, roughness, and oxide induced crack closure are presented and compared with experimental data. The analytical model is shown to correctly predict the combined influences of crack roughness, oxide debris, and plasticity in the near‐threshold regime. Furthermore, analytical results indicate closure mechanisms interact in a non‐linear manner such that the total amount of closure is not the sum of closure contributions for each mechanism.     

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).     

Effects of microstructure on mixed-mode, high-cycle fatigue crack-growth thresholds in Ti-6Al-4V alloy

Effect of microstructure on mixed‐mode (mode I + II), high‐cycle fatigue thresholds in a Ti‐6Al‐4V alloy is reported over a range of crack sizes from tens of micrometers to in excess of several millimeters. Specifically, two microstructural conditions were examined—a fine‐grained equiaxed bimodal structure (grain size ～20 µm) and a coarser lamellar structure (colony size ～500 µm). Studies were conducted over a range of mode‐mixities, from pure mode I (ΔK_II/ΔK_I = 0) to nearly pure mode II (ΔK_II/ΔK_I ～ 7.1), at load ratios (minimum load/maximum load) between 0.1 and 0.8, with thresholds characterized in terms of the strain‐energy release rate (ΔG) incorporating both tensile and shear‐loading components. In the presence of through‐thickness cracks—large (> 4 mm) compared to microstructural dimensions—significant effects of mode‐mixity and load ratio were observed for both microstructures, with the lamellar alloy generally displaying the better resistance. However, these effects were substantially reduced if allowance was made for crack‐tip shielding. Additionally, when thresholds were measured in the presence of cracks comparable to microstructural dimensions, specifically short (～200 µm) through‐thickness cracks and microstructurally small (< 50 µm) surface cracks, where the influence of crack‐tip shielding would be minimal, such effects were similarly markedly reduced. Moreover, small‐crack ΔG_TH thresholds were some 50–90 times smaller than corresponding large crack values. Such effects are discussed in terms of the dominant role of mode I behaviour and the effects of microstructure (in relation to crack size) in promoting crack‐tip shielding that arises from significant changes in the crack path in the two structures.     

Transient behaviour in the numerical analysis of plasticity induced crack closure

A transient behaviour is observed in the numerical analysis of plasticity induced crack closure at the beginning of crack propagation, as the residual plastic field is being formed. The extent of crack propagation prior to plasticity induced crack closure measurement has a major influence on the accuracy of numerical prediction and on computation time. The objective here is to quantify and understand the minimum propagation, Δa_stb, required to obtain stabilized crack opening values. For plane stress state, Δa_stb was found to increase with ΔK. Under plane strain conditions, a peak of closure exists at the beginning of crack propagation for relatively low ΔK values, which promotes relatively large transient periods. Two driving forces explain the stabilization behaviour, the formation of residual plastic wake and the stabilization of plastic strain, but the second seemed to control the phenomenon. Finally, two strategies are proposed to accelerate convergence. The first, consisting of a progressive increase of maximum load, is relevant in plane strain and 3D studies, in order to eliminate the initial peak. The second strategy consists of an extrapolation model and is very effective for plane stress conditions.