High ΔK Levels Research Articles

Application of a Cohesive Zone Model for Simulating Fatigue Crack Growth from Moderate to High ΔK Levels of Inconel 718

A cyclic cohesive zone model is applied to characterize the fatigue crack growth behavior of a IN718 superalloy which is frequently used in aerospace components. In order to improve the limitation of fracture mechanics-based models, besides the predictions of the moderate fatigue crack growth rates at the Paris’ regime and the high fatigue crack growth rates at the high stress intensity factor ΔK levels, the present work is also aimed at simulating the material damage uniformly and examining the influence of the cohesive model parameters on fatigue crack growth systematically. The gradual loss of the stress-bearing ability of the material is considered through the degradation of a novel cohesive envelope. The experimental data of cracked specimens are used to validate the simulation result. Based on the reasonable estimation for the model parameters, the fatigue crack growth from moderate to high ΔK levels can be reproduced under the small-scale yielding condition, which is in fair agreement with the experimental results.

Effect of Microstructures on Fatigue Crack Growth Behavior of Friction Stir Processed NiAl Bronze Alloy

Friction stir processed (FSPed) NAB alloy exhibits inhomogeneous microstructures that can be divided into three subregions from the top surface to the bottom according to α phase morphologies: Widmanstatten α subregion, banded α colonies, and stream-like α colonies. In this study, a constant stress intensity range (ΔK) was used for each sample to study the effect of microstructures on the fatigue crack growth rate (FCGR) of FSPed NAB alloy. The results show that α phase in banded and stream-like α colonies experiences completely dynamic recrystallization and forms equiaxed α grains during FSP. The FCGR of FSPed NAB alloy continuously decreases from the top surface to the bottom. In the subregion with stream-like α colonies, the alloy containing a higher content of equiaxed α grains and fine κ iv phase, and less retained β (β′) phase exhibits the best FCG resistance. The equiaxed α grains deflect the main crack and increase crack tortuosity effect, which make a main contribution to FCG resistance of FSPed NAB alloy, while martensite β′ phase produced during FSP accelerates its fatigue crack growth. Compared to matrix alloy, FSPed NAB alloy exhibits better FCG resistance only at high ΔK levels. At low ΔK levels, the crack deflection effect caused by coarser κ phase in the matrix alloy obviously improves its FCG resistance. With the increasing ΔK, the aforementioned crack deflection effect gradually diminishes and fatigue crack prefers to propagate in a flat way, resulting in higher FCGR of matrix alloy.

The effects of colony microstructure on the fatigue crack growth behavior for Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb titanium alloy

The fatigue crack growth (FCG) behavior for materials applied in aerospace field is very important, for the majority of incidents happened were due to fatigue fracture. Six double annealing processes were conducted to get different colony features for Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb alloy. Then the effects of the colony microstructure on the FCG behavior of this alloy are studied. The results show that the FCG curves for Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb alloy under four processes are distinguished by a clearly defined transition point. The values of ΔKT (transitional stress intensity range) under these processes are almost the same, about 15MPam. The fatigue crack growth rates (FCGR) for six microstructures vary obviously at low ΔK-levels (ΔK≤ΔKT) and then tend to approach each other at high ΔK-levels (ΔK≥ΔKT). Above phenomena are all due to the difference of crack deflection effect caused by different colony microstructures. The α platelet thickness and colony size are two key parameters determining the crack deflection effect of colony microstructure. For microstructure with thin α platelets, the main crack cuts the α platelets easily and propagates in a flat way. In this situation, the colony size exerts no influence on crack propagation path. For microstructure with thick α platelets, the main crack tends to propagate along the colony boundaries or the α platelet spacings. In this situation, the colony size can greatly influence the crack propagation path. The bigger the colony size, the more the tortuous crack path.

Fatigue crack growth behaviour of a near α titanium alloy Timetal 834 at 450 °C and 600 °C

Fatigue crack growth behaviour of a near α titanium alloy Timetal 834 has been studied at maximum dynamic strain ageing temperature (450°C) and at the service temperature (600°C). Compact-tension type fracture mechanics specimens have been tested with stress ratio and frequency of 0.1 and 1Hz, respectively, under constant amplitude loading. The alloy showed higher crack growth rate at 450°C due to dynamic strain ageing which causes higher stress concentration at the crack tip as compared to 600°C. The pronounced effect of oxide and roughness induced crack closure revealed from Auger electron spectroscopy and surface profiler, respectively, have been attributed to reduce the stage I as well as stage II fatigue crack growth rate at 600°C. Fractographic and crack path observations confirmed that the crack growth mechanism remained highly faceted in nature at low ΔK levels which changes subsequently to striation mode of fracture at relatively higher ΔK levels.

A damage model for fatigue crack propagation from moderate to high ΔK levels

ABSTRACTDifferent approaches have been developed to address the difficult issue of crack propagation simulation under monotonic loading. However, numerical models that can also account for crack growth under cyclic loading are much less numerous. The aim of the paper is to develop a numerical model to predict the behaviour of cracked panels under monotonic as well as under cyclic loading. In particular, the model has to simulate fatigue crack propagation at high ΔK levels for a wide variety of metallic alloys including aluminium alloys, titanium alloys and steels. An advanced fatigue model, developed in the framework of Continuum damage mechanics (CDM), is discussed in this paper. The CDM approach, initially proposed by Lemaitre, considers the effects associated to a given damage configuration through the definition of a thermodynamic state variable. This proper thermodynamic framework provides numerical robustness. This study examines the possibility to use such models to simulate fatigue crack propagation. The analysis of fracture modes and of –ΔK curves at different load ratios suggests that two damage mechanisms are involved in fatigue crack propagation. To improve the damage description of the Lemaitre model, two distinct damage variables are introduced. The evolution of the first one, called ‘static damage’, follows the classical Lemaitre formulation. The second one, ‘cyclic damage’ is related to cyclic cumulated plastic strain.

Fatigue Crack Growth Rate and Stress-Intensity Factor Corrections for Out-of-Plane Crack Growth

Abstract Fatigue crack growth rate testing is performed using automated data collection systems that assume straight crack growth in the plane of symmetry and that use standard polynomial solutions to compute crack length and stress-intensity factors from compliance or potential drop measurements. Visual measurements used to correct the collected data typically include only the horizontal crack length, which underestimates the crack growth rates for cracks that propagate out-of-plane. The authors have devised an approach for correcting both the crack growth rates and stress-intensity factors based on two-dimensional mixed mode-I/II finite element analysis (FEA). The approach is used to correct out-of-plane data for 7050-T7451 and 2025-T6 aluminum alloys. Results indicate the correction process works well for high ΔK levels, but it fails to capture the mixed-mode effects at ΔK levels approaching threshold (da/dN ∼ 10−10 meter/cycle). Based on the results presented in this paper, the authors propose modifications to ASTM E 647: to be more restrictive on the limits for out-of-plane cracking (15°); to add a requirement for a minimum of two visual measurements (one at test start and one at test completion); and to include a note on crack twisting angles, with a limit of 10° being acceptable.

Effects of high temperature excursion on mechanical and fatigue properties of nuclear pressure vessel steel during fabrication

Presented is study concerning the effects of introducing a high temperature cycle, within the range of 1200 to 1500°C, during the fabrication heat treatment on the mechanical and fatigue properties of a reactor pressure vessel (RPV) steel. The tensile properties, viz., tensile strength and reduction in area both exhibited small discontinuous decreases over the temperature range of 1200 to 1500°C while the room temperature Charpy impact energy was dramatically reduced by 50%. At room temperatures in a deoxygenated, demineralised aqueous environment (PWR water) it was observed that a high temperature cycle caused a significant increase in fatigue crack growth behaviour. However at ΔK levels above 8 MPa √m the high temperature cycled steel (termed overheated) and the as-received steel (termed normal) exhibited similar fatigue crack growth rates. The increased fatigue crack growth rates were caused by the occurrence of significant amounts intergranular failure facets of the fatigue surfaces of the overheated steel. In PWR water at a temperature of 120°C it was shown that, at initial ΔK levels the overheated pressure vessel steel exhibited fatigue crack extension characteristics which were around ten and twenty times slower (depending upon the overheating temperature) than the normal pressure vessel steel data. Such differences were explained in terms of the fractographic features. It was argued that the high temperature cycle treatment caused the absences of EAC growth in PWR water at 120°C by causing the liquation and subsequent re-precipitation of numerous, very small, sulphide inclusions at prior austenite grain boundaries. Finally, at higher ΔK levels, a high temperature cycle promoted the onset of stage III fatigue, typified by the occurrence of intergranular failure, by reducing the fracture toughness of the pressure vessel steel.

Environmental cracking of AISI 4145 steel in drilling muds

The environmental cracking of a quenched and tempered AISI 4145 steel in a water-based drilling mud has been investigated under slow strain-rate tensile and cyclic-loading conditions. The variables investigated were pH, potential, temperature and the addition of copper- and zinc-containing lubricants. These tests were supported by detailed electron fractography. Under tensile loading conditions, progressive loss of ductility occurs with decreasing potential and pH, and with increasing temperature. The addition of 25 wt% copper- and zinc-containing lubricants generally reduces this embrittlement. The corrosion fatigue crack propagation rate increases with decreasing potential and pH. At low ΔK values, temperature does not affect the corrosion fatigue crack propagation rate, but it is accelerated by increasing temperature at higher ΔK levels. In the presence of copper-containing lubricant, the crack growth rate is always lower than that in copper-free mud. The addition of zinc-containing lubricant to the mud accelerates the crack growth rate at low ΔK values, but does not affect it at higher ΔK values. It is concluded that the decrease in ductility and the increase in the fatigue crack growth rates in drilling muds, relative to air, are due to the appearance of brittle forms of fracture (intergranular, transgranular and quasi-cleavage fracture modes) which are the result of hydrogen-induced cracking.

Fatigue crack growth behavior of D6AC space shuttle steel

Fatigue crack propagation rate properties of a D6AC steel were developed, covering a wide range of crack growth rates from approximately 10 −3 m/cycle (4 × 10 −2 in./cycle) to 10 −10m/cycle (4 × 10 −9 in./cycle). The low crack propagation rates were obtained using a decreasing ΔK test method, while the fast crack growth rates were generated by the J- controlled test technique. At high ΔK levels [> 122 MPa √ m (111 ksi√ in.)], dimpled fracture was the dominant fracture mode, while at ΔK levels between 122 MPa √m (111ksi √in.) and 176.6 MPa√m (16 ksi√in.), the transgranular fracture mode was found. Below ΔK levels of 17.6 MPa√m (16 ksi√in.), some intergranular fracture mixed with transgranular fracture was observed. At near-threshold levels, oxide deposits were present on fracture surfaces. Oxide thickness was measured, and was found to be much less than crack tip opening displacement. Thus, oxide-induced crack closure was negligible during near-threshold crack growth testing.

Predicting fatigue crack growth rates and thresholds at low temperatures

In this paper an attempt was made to offer a new method for predicting fatigue crack growth (FCG) rates and thresholds of metals in low temperature structural applications. The experimental results available have indicated that a decrease in temperature can influence the FCG mechanism, as well as the FCG rates, and the fatigue ductile-brittle transition (FDBT) may occur in some metals. Accordingly, metals can be divided into two groups, i.e. alloys without an FDBT and those with an FDBT. In the former, the fracture mode of FCG is ductile transgranular for temperatures down to 4 K and the crack growth rates in the intermediate region (d a/d N = 10 −8−10 −6 m cycle −1) depend mainly upon Young's modulus. In the latter, the FCG behaviour above the FDBT temperature is similar to that of the alloys without an FDBT. When the temperature is below the FDBT temperature, the FCG rates (especially at higher ΔK levels) will be enhanced by the brittle transition of the FCG mechanism and both the strength and the ductility of the alloys will have significant effects on the FCG rates. The expression previously proposed by one of the present authors and a coworker for FCG rates not only explains the low temperature FCG behaviour of the metals mentioned above but also predicts the low temperature FCG rates in both the near-threshold region and the intermediate region. Finally, a new method to predict ΔK th at low temperatures is tentatively proposed based on the stratic fracture model.

Fatigue in metallic alloys containing non-metallic particles

This paper examines the effects of non-metallic particles on fatigue performance and, in particular, their influence on fatigue crack propagation at high ΔK (K max) levels. The nature and properties of a number of common non-metallic particles found in Fe- and Al- based alloys are described, and consideration is given to the consequences of mismatch of physical and chemical properties between particle and matrix. Effects of particles on fatigue in conventional alloys are illustrated and compared with the behaviour of Al/SiC p MMC. The problems associated with developing particulate reinforced MMC with adequate fatigue crack growth resistance and toughness for structural applications are discussed.

Role of silicon carbide particles in fatigue crack growth in SiC-particulate-reinforced aluminum alloy composites

A study has been made of the mechanistic role of silicon carbide (SiC) particles during fatigue crack propagation in powder metallurgy AlZnMgCu metal matrix composites reinforced with 20 vol.% SiC particulates (SiC p), with varying sizes of reinforcement phase. Crack growth and accompanying crack tip shielding (principally by crack deflection, closure and bridging) are examined in peak-aged alloys over a wide spectrum of growth rates from 10 −12 to 10 −4 m cycle −1 and are compared with corresponding behavior in the unreinforced matrix alloy. Crack growth resistance in the composites is found to be both superior and inferior to that of the unreinforced alloy, depending on how the SiC and SiC-matrix interface fracture. At low stress intensity ranges ΔK, the predominant fracture of carbides close to the crack tip results in low levels of crack closure and rapid growth kinetics for fine SiC p distributions, whereas for coarse SiC p distributions the rougher fracture surface promotes crack closure from asperity wedging and improved crack growth resistance. With increasing ΔK, the fracture of large carbides farther ahead of the crack tip leads to the development of non-uniform crack fronts, thereby promoting crack bridging via uncracked ligaments and a small improvement in crack growth resistance. At high ΔK levels approaching K Ic, growth rates are faster in the composites owing to their low toughness. On the basis of these results, the overall fatigue crack growth performance of the SiC pAl composites compared with that of the traditional monolithic aluminum alloys is briefly discussed.

Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.

Effect of Heat-Treatment Upon the Fatigue-Crack Growth Behavior of Alloy 718 Weldments—Part II: Microscopic Behavior

The microstructural features that influenced the fatigue-crack growth behavior of as-welded, conventional heat-treated, and modified heat-treated Alloy 718 GTA weldments were studied. Electron fractographic examination revealed that operative fatigue mechanisms were dependent on microstructure, temperature and stress intensity factor. All specimens exhibited three basic fracture surface morphologies at temperatures up to 538°C: crystallographic faceting at low stress intensity range (ΔK) levels, striation formation at intermediate values, and dimples coupled with striations in the highest ΔK regime. At 649°C, extensive amounts of intergranular cracking were observed. Laves and δ particles in the conventional heat-treated material nucleated microvoids ahead of the advancing crack front and caused an overall acceleration in crack growth rates at intermediate and high ΔK levels. The modified heat treatment removed many of these particles from the weld zone, thereby improving its fatigue resistance. The dramatically improved fatigue properties exhibited by the as-welded material were attributed to compressive residual stresses introduced by the welding process.

ON THE OVERLOAD INDUCED FATIGUE CRACK PROPAGATION BEHAVIOR IN ALUMINUM AND STEEL ALLOYS

The overload induced fatigue crack propagation behavior of several aluminum and steel alloys was examined as a function of the baseline stress intensity factor range (δKb). In order to gain a clearer understanding of the parameters which influence the cyclic delay phenomenon, under both plane strain and plane stress conditions, tests were conducted at δKb values ranging from the near threshold regime to high δK levels approaching fast fracture. Large amounts of overload induced cyclic delay (˜100,000 cycles) were observed at both high and low δK levels (provided the plastic zone size/thickness ratio and plastic zone size/grain size ratio approached unity, respectively) with significantly less delay occurring at intermediate δK values. All alloys examined exhibited this type of delay behavior which can be described by a “U‐shaped” plot. The delay phenomenon at high δKb levels under plane stress conditions was attributed to increased crack closure associated with large tensile displacements in the wake of the advancing crack. At low δKb levels increasing cyclic delay was attributed to an increased effective overload ratio as δK approached δKth.

Corrosion Fatigue Crack Propagation Behavior of a C-Mn-Cb Steel

The fatigue crack propagation properties of a C-Mn-Cb plate steel (SA633 Grade C) in a 3.5 percent NaCl solution have been evaluated for loading frequencies of 10, 1.0, and 0.1 Hertz. To reveal the influence of test specimen orientation and steel cleanliness, both a conventional sulfur level and a low sulfur-calcium treated plate were examined in the three major testing orientations. In addition to other basic testing of the plates, the elastic-plastic fracture toughness properties were also established. The fatigue crack growth rates at 0.1 Hz of both steels were increased by factors of 2–5 over air data, depending on the ΔK level and specimen orientation; some increase was also noted at 1.0 Hz. The acceleration due to the salt water environment was a result of a hydrogen embrittlement mechanism which resulted in bursts of faceted, cleavage-like, transgranular fracture of ferrite grains in this ferrite-pearlite steel. At higher ΔK levels, the calcium treated steel showed slower growth rates than the conventional sulfur level steel for all testing conditions. It was found that higher oxygen contents of a salt water solution could lead to corrosion product wedging at low ΔK levels, which could retard crack growth.

Kinetics of corrosion fatigue crack growth in high strength low-alloy steels in 3.5%NaCl aqueous solution.

Corrosion fatique crack growth rates as a function of the stress intensity factor range were investigated in the 3.5% NaCl aqueous solution of various temperatures between 278K and 333K under a constant applied electrochemical potential of -1050mV vs. Ag/AgCl reference electrode. Generally speaking, crack growth is enhanced as solution temperature rises. The corrosion fatique crack growth rate-stress intensity factor range curve can be divided into three distinct stages independent of testing temperature; at low ΔK levels or Range I, corrosion fatigue crack growth rate is strongly dependent on ΔK and slightly on testing temperature. In the range of moderate ΔK or Range II, the extent of which depends on testing temperature, the ΔK dependence of the crack growth rate curves disappears and crack growth rate reaches a limiting value. The apparent activation energy for the rate limit region of Range II was found to be approximately 40.0kJ/mole, which can be compared well with that for apparent hydrogen diffusion in steel. At higher ΔK levels or in Range III, corrosion fatigue crack growth rates coincide with those obtained in ambient laboratory air, indicating that the corrosion fatigue crack growth in this range is not affected essentially by the aqueous environment.

On the cyclic behavior of cast and extruded aluminum alloys. Part B: Fractography

In a prior study [1], the fatigue crack propagation (FCP) response of a cast and an extruded aluminum alloy was examined as a function of mean stress and specimen orientation while crack closure data were collected. In this work, extensive electron fractographic studies were conducted on the previously generated fatigue fracture surfaces using both scanning and transmission electron microscopy. The threshold micromorphology revealed crisp, cleavage-like facets. Striation spacing measurements at intermediate and high ΔK levels were obtained to determine microscopic growth rates; these measurements were seen to vary with R ratio and were best correlated with ΔK EFF rather than ΔK APP . Slope changes in the da/da- ΔK plots were identified and attempts made to establish correlations between the associated plastic zone sizes and microstructural dimensions. Of particular note, a stage IIa to IIb transition in the extruded material was found to correspond to a micromechanism change from faceted growth to striated growth when the reversed plastic zone size was similar to the subgrain dimension.

Fatigue crack growth behavior of Ti-6Al-6V-2Sn in methanol and methanol-water solutions

Differences in the corrosion fatigue crack growth behavior of anα–β titanium alloy in chloride-containing aqueous and methanol environments are reported, and discussed in relation to differences in repassivation behavior for the two types of environments. Experiments have been conducted with various solution mixtures of water (a passive film-forming environment) and methanol (a nonfilm-forming environment) to define the role of repassivation in controlling fracture modes and crack growth rates at different frequencies. The critical event in determining whether the repassivation process can suppress environmental fatigue fracture is the interaction between the rate of exposure of fresh metal surfaces at the crack tip and the rate at which they can be repassivated. The out-come of this mechano-chemical interaction is shown to be dependent on the frequency and stress intensity(ΔK) level as well as the chemistry of the environment. As a result, differences in repassivation behavior for methanol-water solutions can be correlated with major differences in fatigue crack growth rates and fracture modes at low ΔK levels, whereas repassivation differences have little effect at high ΔK levels. Based on these low-ΔK corrosion fatigue characteristics, methanol solutions are concluded to be far more detrimental to titanium alloys than aqueous solutions.

The effect of intermediate thermomechanical treatments on the fatigue properties of a 7050 aluminum alloy

A study was undertaken to determine the effect of microstructures produced by different ingot processing techniques on the fatigue properties of a 7050 aluminum alloy. The different microstructures investigated were produced by hot-rolling to simulate commercial processing (CP) methods or intermediate thermomechanical treatments (ITMT). Characterization of the microstructures revealed that the CP 7050 material was partially recrystallized (<50 pct) due to the use of hot-rolling as the final deformation step. The ITMT materials were examined in the as-recrystallized (AR) condition or in AR + hot rolled condition (AR + HR). Results of the investigation showed thattotal fatigue life, both low and high cycle, were not greatly affected by the grain structures of the experimental materials. However, metallographic studies indicated that crack initiation is probably more difficult in the fine-grained AR material. The results of fatigue crack growth tests showed that higher crack growth rates observed at low ΔK values for ITMT {dy7050} were most likely due to the detrimental effects of undissolved Al2CuMg particles. These particles, which also contribute to low fracture toughness and higher crack growth rates at high ΔK levels, are formed during a furnace-cooling step in the ITMT processing schedule.