Constant Maximum Stress Intensity Research Articles

Elasto-Plastic Fatigue Crack Growth Behavior of Extruded Mg Alloy with Deformation Anisotropy Due to Stress Ratio Fluctuation.

Fatigue crack growth (FCG) experiments were performed using a low-temperature extruded magnesium alloy AZ31 with texture. Under a constant maximum stress intensity factor (Kmax), the stress ratio R was changed from 0.1 to −1 during the fatigue crack growth process, and the FCG behavior before and after the R change was investigated. As a result, tensile twins were generated owing to the fatigue load on the compression side of R = −1, and the FCG velocity was accelerated. In addition, when the maximum compressive stress at R = −1 (|(σmin)R = −1|) exceeded the compressive yield strength of the material (σcy), the FCG velocity after R fluctuation greatly accelerated. On the other hand, under the condition |(σmin)R = −1| < σcy, the degree of acceleration of the FCG velocity due to R fluctuation was small. In either case, the degree of acceleration in the FCG increased as the Kmax value increased. The above FCG acceleration mechanism due to the R fluctuation was considered based on the observation of the deformation and twinning states of the fatigue crack tip, the fatigue crack closure behavior, and the cyclic stress–strain curve of the fatigue process. The FCG acceleration mechanism was as follows: First, the driving force of the FCG increased owing to the increase in crack opening displacement due to the generation of tensile twins. Second, the coalescence of the main crack and a plurality of microcracks were generated at the twin interface. The elasto-plastic FCG behavior after the stress ratio fluctuations is defined by the effective J-integral range ΔJeff.

Fatigue crack growth of a railway wheel

Abstract Typically, fatigue crack propagation in railway wheels is initiated at some subsurface defect and occurs under mixed mode (I–II) conditions. For a Spanish AVE train wheel, fatigue crack growth characterization of the steel in mode I, mixed mode I–II, and evaluation of crack path starting from an assumed flaw are presented and discussed. Mode I fatigue crack growth rate measurement were performed in compact tension C(T) specimens according to the ASTM E647 standard. Three different load ratios were used, and fatigue crack growth thresholds were determined according to two different procedures. Load shedding and constant maximum stress intensity factor with increasing load ratio R were used for evaluation of fatigue crack growth threshold. To model a crack growth scenario in a railway wheel, mixed mode I–II fatigue crack growth tests were performed using CTS specimens. Fatigue crack growth rates and propagation direction of a crack subjected to mixed mode loading were measured. A finite element analysis was performed in order to obtain the K I and K II values for the tested loading angles. The crack propagation direction for the tested mixed mode loading conditions was experimentally measured and numerically calculated, and the obtained results were then compared in order to validate the used numerical techniques. The modelled crack growth, up to final fracture in the wheel, is consistent with the expectation for the type of initial damage considered.

Near threshold fatigue crack propagation in railways’ steels: Comparison of two testing techniques

Mode I fatigue crack growth thresholds of steels for rail and wheel used in railways were determined according to two different procedures. Although fatigue crack propagation data in the Paris regime is usually straightforward to obtain, the same does not apply to the threshold regime, where crack growth rates are very small up to inexistent, and therefore tests are very time consuming. Furthermore, the very small fatigue crack growth rates involved in testing for threshold determination require very thorough and careful experimental procedures if extraneous effects – as for example interaction effects such as crack retardation or acceleration – are to be avoided. Two methods for evaluation of fatigue crack growth threshold value of the range of stress intensity factor were used: load shedding and constant maximum stress intensity factor with increasing load ratio R (R=minimum load/maximum load), taking into account the ASTM standard E647 prescriptions. The obtained results and lessons learned in conducting these tests are discussed, and the data generated is compared with data published.

Crack Closure Behavior under High-Load Ratio for AISI 4340 Steel

In this paper, fatigue crack growth rate (FCGR) analyses were conducted on compact specimens of an AISI 4340 alloy to study the behavior over a range in load ratios (0.1 ≤ R ≤ 0.95) and constant maximum stress intensity factor (Kmax) condition. Previous study had indicated that high R &gt; 0.7 and constant Kmax test conditions near threshold conditions were suspected to be free of crack-closure and that any differences were caused by Kmax effects, from threshold to near fracture conditions. Cracks in high-cycle fatigue (HCF) components spend a large portion of their fatigue life near threshold conditions. In order to characterize the evolution of damage and crack propagation during these conditions, fatigue crack growth rate (FCGR) data at threshold and near-threshold conditions are essential in predicting service life and in determining the proper inspection intervals. Fatigue crack growth model, namely the Forman model were examined, this model implicit the effect of R ratio and ease of curve fitting to measured data. The Forman model may be suggested for use in critical applications in studying fatigue crack growth for different load ratios.

The Effect of Load Reduction Scheme on Crack Closure in the Near-Threshold Regime

Abstract Three-dimensional elastic-plastic finite element analyses were conducted to model fatigue crack growth in an M(T) specimen. Variable amplitude loading with a continual load reduction was used to simulate the load history associated with fatigue crack growth threshold measurement. Load reductions with both constant load ratio R and constant maximum stress intensity Kmax were used. Results indicated that load reduction with constant R generated a plastic wake such that remote crack opening occurred during loading, with the crack front opening prior to a region remote to the crack front. The last region to open was located at the point at which the load reduction originally began, and at the free surface. In contrast, for load reduction with constant Kmax, the crack front was the last to open. The results also indicated the crack opening process is three-dimensional in nature, with regions in the interior opening prior to regions near the free surface.

Modeling of fatigue crack growth threshold development for a microstructurally small crack

A method for predicting the fatigue crack growth threshold using finite element analysis is investigated. The proposed method consists of monitoring the plastic strain hysteresis energy dissipation in the crack tip plastic zone, with the threshold being defined in terms of a critical value of this dissipated energy. Two-dimensional plane-strain elastic–plastic finite element analyses are conducted to model fatigue crack growth in a middle-crack tension M(T) specimen. A single-crystal constitutive relationship is employed to simulate the anisotropic plastic deformation near the tip of a microstructurally small crack without grain boundary interactions. Variable amplitude loading with a continual load reduction is used to generate the load history associated with fatigue crack growth threshold measurement. Load reductions with both constant load ratio R and constant maximum stress intensity K max are simulated. In comparison with a fixed K max load reduction, a fixed R load reduction is predicted to generate a 35% to 110% larger fatigue crack growth threshold value.

Fatigue crack propagation and closure behavior of modified 1070 steel: Experimental results

A combined experimental and finite element study of fatigue crack propagation and crack closure behavior, in a modified 1070 steel, has been conducted. In this paper, the experimental aspects of this study are presented and discussed. A comparison of crack closure measurement techniques using crack mouth opening displacement, back-face strain gage and a new surface strain gage method was performed. For two thicknesses of compact tension specimens, a series of constant maximum stress intensity and constant load ratio, constant load and constant load ratio, constant maximum stress intensity and increasing load ratio, single tensile overload, and conventional fatigue crack propagation tests were conducted. Implications of the influence of specimen thickness, crack length and test conditions on closure and crack growth behavior are detailed. Electron microscopy observations of the fatigue fracture surfaces were performed to assess the relative importance of crack path meandering, oxide-induced ind surface roughness-induced crack closure mechanisms. In the domain of stable crack growth, closure is dictated by the mechanism of plasticity-induced crack closure; while near-threshold behavior was found to be dominated by the conjoint and mutually interactive mechanisms of oxide-induced and surface roughness-induced crack closure. Test results reveal a significant influence of specimen thickness on closure and overall growth rate behavior. The salient advantages of the new surface strain gage method are elucidated and the conformance of experimentally measured and calculated growth rates is highlighted.

Initiation and growth of small fatigue cracks in a Ni-base superalloy

This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (Kmax). Small surface cracks initiated predominantly at (Nb,Ti)xCy, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔKth, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Δσth) and the threshold cyclic stress intensity (ΔKσth) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime.

Cyclic fatigue crack growth of silicon nitride under a constant maximum stress intensity

Cyclic fatigue crack growth of silicon nitride under a constant maximum stress intensity

A comparison of different methods to determine the threshold of fatigue crack propagation

Three different methods for determining the threshold value for fatigue crack growth — the load-shedding technique, the stepwise increase of load amplitude on specimens precracked in cyclic compression, and decrease of stress intensity range at a constant maximum stress intensity — were applied to a high-strength aluminium alloy. The load-shedding technique tended to lead to higher values of the threshold, especially at low R-ratios. The threshold determined with decreasing stress intensity range at a constant maximum stress intensity was larger than the effective threshold determined with stepwise increasing of load amplitude on specimens precracked in cyclic compression.

Fatigue mechanisms in poly (methyl methacrylate) at threshold: effects of molecular weight and mean stress

Fatigue tests were conducted on three linear poly (methyl methacrylate) (PMMA) resins having weight average molecular weights (Mw) of 82 000, 205 000 and 390000 and on a fourth, cross-linked sample (Mc=3337 g mol−1). Fatigue threshold test conditions included a constant load ratio (Rc=0.1) and a constant maximum stress intensity level (K max c =0.52 MPa m1/2). TheRc=0.1 test results demonstrated that fatigue resistance increased with increasingMw, and that the cross-linked sample possessed a higher fatigue threshold than the linear Iow-Mw material. However, the K max c test results revealed the opposite trend, with fatigue resistance decreasing with increasingMw and chemical crosslinking. The marked change in relative fatigue resistance of the PMMA resins investigated under high mean stress conditions is believed to be a consequence of the competition between two molecular deformation mechanisms: chain scission and chain slippage. The presumed shift in operative mechanism as a function of theR level is reflected in differences noted on the fracture surfaces of the PMMA resins studied. Discontinuous growth band formation, which is indicative of large amounts of chain slippage, is favoured by lowMw and lowR ratios, but disappears in association with high-Mw and highR-ratio test conditions.

The influence of air humidity on near-threshold fatigue crack growth of 2024-T3 aluminum alloy: Stanzl, S.F., Mayer, H.R., and Tschegg, E.K. Mater. Sci. Eng. A Oct. 1991 A147 (1) 45–54

A combined experimental and finite element study of fatigue crack propagation and crack closure behavior, in a modified 1070 steel, has been conducted. In this paper, the experimental aspects of this study are presented and discussed. A comparison of crack closure measurement techniques using crack mouth opening displacement, back-face strain gage and a new surface strain gage method was performed. For two thicknesses of compact tension specimens, a series of constant maximum stress intensity and constant load ratio, constant load and constant load ratio, constant maximum stress intensity and increasing load ratio, single tensile overload, and conventional fatigue crack propagation tests were conducted. Implications of the influence of specimen thickness, crack length and test conditions on closure and crack growth behavior are detailed. Electron microscopy observations of the fatigue fracture surfaces were performed to assess the relative importance of crack path meandering, oxide-induced ind surface roughness-induced crack closure mechanisms. In the domain of stable crack growth, closure is dictated by the mechanism of plasticity-induced crack closure; while near-threshold behavior was found to be dominated by the conjoint and mutually interactive mechanisms of oxide-induced and surface roughness-induced crack closure. Test results reveal a significant influence of specimen thickness on closure and overall growth rate behavior. The salient advantages of the new surface strain gage method are elucidated and the conformance of experimentally measured and calculated growth rates is highlighted.

Some observations on the cyclic unloading effects of a fatigue crack

An experimental investigation has been performed on a low-carbon structural steel to determine the order of events occurring both at the fatigue crack tip and in the crack wake owing to an applied cyclic unloading sequence. The two-step loading sequence consisted of unloading a fatigue crack, grown at positive load ratios of R = 0.6 and R = 0.2, to zero load. The fatigue crack was then cycled manually at R = 0 for a period of 10 cycles whilst maintaining a constant maximum stress intensity value. Observations were made of the crack tip geometry and the fatigue crack wake by monitoring the electrical potential drop across the crack as a function of load. The resultant hysteresis effect was explained in terms of an asperity crushing model. The effect of a series of underload cycles following a single-peak overload was also investigated, and the underload cycles were shown to induce surfac cracking on the ductile stretch zone formed by the overload. The surface cracking, across the thickness of the specimen, provided evidence that a hump of plastically formed material is generated upon application of the overload cycle and this may affect the subsequent fatigue crack growth characteristics.

THE EFFECTS OF STRESS RATIO ON THE GROWTH BEHAVIOUR OF SMALL FATIGUE CRACKS IN AN ALUMINUM ALLOY 7075‐T6 (WITH SPECIAL INTEREST IN STAGE I CRACK GROWTH)

Abstract— The growth behaviour of small fatigue cracks has been investigated on aluminum alloy 7075‐T6 at stress ratios R of 0, −1 and −2. The effects of stress ratio are discussed with special interest in the stage I region of small crack growth. Cracks which initiated at R=−1 and −2, grew by a stage I mechanism up to a certain depth followed by stage II crack growth. The stage I to stage II transition occurred under a constant maximum stress intensity factor which was approximately consistent with the threshold effective stress intensity range, λKeff,th, for large cracks. At R= 0, on the other hand, stage I crack growth was not observed because of crack initiation at inclusions. Small cracks grew more rapidly than large cracks subjected to the same nominal stress intensity ranges at all the stress ratios, and they grew below the threshold stress intensity range, λKth, for large cracks. Stage I cracks, in particular, showed much higher growth rates than large cracks and grew even below λKeff,th. It is suggested that stage II crack growth rates should be characterized in terms of an effective stress intensity range, while a micromechanics approach will be necessary to evaluate stage I crack growth rates.

アルミニウム合金７０７５‐Ｔ６の微小疲労き裂成長挙動の応力比依存性 主として第Ｉ段階き裂成長過程に注目して

The growth behaviour of small fatigue cracks was investigated on an aluminum alloy 7075-T6 at the stress ratios R of 0, -1 and -2. The effect of stress ratio was discussed on the basis of the detailed observation with special interest in the stage I region of small crack growth.Cracks initiated at R=-1 and -Z, grew to a certain depth by the stage I growth mechanism, and the stage II crack growth followed. The stage I to stage II transition occurred under a constant maximum stress intensity factor which was approximately consistent with the effective threshold stress intensity range, ΔKeff, th, for large cracks. At R=0, on the other hand, the stage I crack growth was not observed because of crack initiation at inclusions.At all the stress ratios, small cracks grew more rapidly than large cracks in the same nominal stress intensity range, and grew below the threshold stress intensity range, ΔKth, for large cracks. Particularly, the stage I cracks showed much higher growth rate and grew below ΔKeff, th for large cracks. It is suggested that the stage II crack growth rate can be characterized in terms of the effective stress intensity range, while a micromechanics approach is successful to evaluate the stage I crack growth rate.

A SIMPLIFIED LABORATORY APPROACH FOR THE PREDICTION OF SHORT CRACK BEHAVIOR IN ENGINEERING STRUCTURES

Abstract— Conventionally determined fatigue threshold information (ASTM E647) can lead to non‐conservative estimates of fatigue lifetimes when these data are utilized in damage tolerant design assessments. The non‐conservative nature of such data can be attributed primarily to the development of excessively large amounts of crack closure at low R‐ratios, particularly at near threshold stress intensity factor levels. These high closure levels attenuate the effective stress intensity condition prevailing at the crack tip and confound attempts to predict the behavior of short cracks that exhibit limited crack closure. A modified test procedure, involving constant maximum stress intensity factor (Kcmax) test conditions, is described which identifies fatigue crack propagation (FCP) threshold behavior in the absence of detectable amounts of crack closure. These data have been generated with conventional long crack specimens for several aluminum, iron, and nickel‐based alloys and which are shown to closely simulate the FCP response of short cracks in these engineering materials. As such, the modified threshold test procedure, incorporating constant Kmax loading conditions, represents a valuable tool in the prediction of the cyclic lifetime of engineering components. The stress‐cyclic lifetime (S‐N) curve for aluminum butt‐welded beams was computed based on Kcmax data and found to be in excellent agreement with actual test results.

水素供給下における高強度鋼の疲労き裂進展下限界特性について

Constant maximum stress intensity (Kmax) fatigue tests were carried out to obtain the near-threshold characteristics in fatigue crack growth of high strength steel. Specimens were loaded in in-plane bending at various Kmax under hydrogen supply and in room air. Fractographic observations were made on the fracture surfaces at the near-threshold regions.The results obtained are as follows;(1) The threshold stress intensity factor range (ΔKth) obtained under hydrogen supply was equivalent substantially to that in room air with an exception of the region of Kmax>KISCC under hydrogen supply.(2) ΔKth decreased linearly with an increase in stress ratio (R). In this regard, the effective value of the threshold stress intensity factor range (ΔKeffth) was constant, ΔKeffth≈3MPa√m, in the region of R 0.7, ΔKth decreased with increasing R. From these results, it is noted that the design based on ΔKth obtained at lower R is dangerous even in room air.(3) On the fractographic observations at the near-threshold region, the striation-like pattern occupied the major portion of the fracture surfaces in the region of R 0.7, the proportion of intergranular facet increased with R.(4) It is confirmed that the constant-Kmax fatigue test is suitable for the determination of ΔKth.

On the influence of environment on the load ratio dependence of fatigue thresholds in pressure vessel steel

A study has been made of the influence of load ratio R on fatigue crack propagation behavior and specifically on the value of the fatigue crack growth threshold, Δ K 0 , in a bainitic 2.25 Cr-1Mo pressure vessel steel tested at 50 Hz in aqueous, and moist and dry gaseous environments. Data are obtained for crack growth in a distilled water environment and are compared to previously published results in air and hydrogen. It is found that in distilled water the dependence of thresholds Δ K 0 values on R is far less marked than in moist air and dry hydrogen atmospheres where Δ K 0 values decrease sharply with increasing R. Furthermore, whereas in air and hydrogen, the threshold condition is characterized by a constant maximum stress intensity at low load ratios, and a constant alternating stress intensity at high load ratios, no such behavior is observed in water. Based on extensive measurements of crack face oxidation products using scanning Auger speetroscopy and on previous crack closure measurements using ultrasonics techniques, the role of load ratio in influencing near-threshold fatigue behavior is ascribed to mechanisms of crack closure specifically plasticity-induced closure and closure arising from crack face oxide debris. The implications of such plasticity-induced and oxide-induced closure to the load ratio-dependence of near-threshold fatigue behavior in various environments are discussed in detail.