Cyclic deformation and fracture behavior of Al alloy 6061 under the action of positive mean stresses

Cyclic deformation and fatigue fracture behavior of Al alloy 6061 were studied over an extensive range of positive mean stress. In the regime of low cyclic stress amplitude, both secondary stage creep rate and fracture process are retarded by decreasing mean stress, but they are accelerated if the cyclic stress ratio is smaller than 0. 75. The superimposition of cyclic compressive stress is found to decrease the creep rate, but it has no effect on the relationship between fatigue life and mean stress. Characteristic patterns on the specimen fracture surfaces, representing the interaction between creep and fatigue and also the mechanism of the fatigue-creep deformation in different mean stress ranges, are discussed analytically. Finally, the concept of equivalent creep rate is proposed and a method to predict fatigue life with positive mean stresses is established and verified using the present experimental results. Leave from the State Key Laboratory for Leave from the State Key Laboratory for Leave from the State Key Laboratory for     

Investigating and Understanding the Cyclic Fatigue,Deformation, and Fracture Behavior of a Novel High Strength Alloy Steel: Influence of Orientation

In this paper, the results of a recent study aimed at understanding the influence of orientation on high cycle fatigue properties and final fracture behavior of alloy steel Pyrowear 53 is presented and discussed. This alloy steel has noticeably improved strength, ductility, and toughness properties compared to other competing high strength alloy steels having a near similar chemical composition and processing history. Test specimens of this alloy steel were precision machined and conformed to the specifications detailed in the ASTM standards for tension testing and stress‐controlled cyclic fatigue tests. Test specimens were prepared from both the longitudinal and transverse orientations of the as‐provided alloy steel bar stock. The machined test specimens were deformed in cyclic fatigue over a range of maximum stress and under conditions of fully reversed loading, i.e., at a load ratio of ?1, and the number of cycles‐to‐failure recorded. The specific influence of orientation on cyclic fatigue life of this alloy steel is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to characterize the intrinsic features on the fatigue fracture surfaces. The conjoint influence of microstructure, orientation, nature of loading, and maximum stress on cyclic fatigue life is discussed.     

Fatigue life prediction of sintered steels

Abstract

Low cycle fatigue damage of a sintered alloy was studied by comparing experimental results with those obtained from a model suggested in the literature, which considers the effect of non-zero mean strain on life prediction. Specimens of sintered Fe–C–Cu alloy with two different porosities were used. Experimental tests were carried out on a servohydraulic machine and the specimens were subjected to partially random loads with a maximum tensile stress equal to 80% of monotonic yield stress for each porosity. For a correct translation of the load sequence into damaging events, cycle counting was performed using the rain flow method and appropriate correlations between cyclic stress and cyclic deformation values were made. Both stress and strain controlled experiments were carried out in order to determine all the required model parameters. The fatigue life obtained experimentally was smaller than that predicted by the model and this difference increased with increasing porosity.     

稀土元素La对AZ91D镁合金低周疲劳行为的研究

研究了AZ91D＋0．3％La镁合金在总应变幅△εt／2为0.2％～1.2％范同内循环加载时的低周疲劳行为。结果表明,在较大应变幅下,AZ91D＋0．3％La镁合金的循环滞后回线上分别出现捌点、拉压不对称和锯齿现象;镁合金的低周疲劳寿命随应变幅值的提高而下降,并且符合Manson—Coffin关系式;镁合金的循环应力响应行为和循环应力应变行为均呈现明显的循环硬化现象;疲劳断口上出现了多疲劳源现象,且随应变幅的增加越来越凹凸不平。     

High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy

An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.     

2D70铝合金不同环境低周疲劳性能研究

对2D70铝合金在试验室空气、湿空气、盐雾及盐雾＋SO2四种环境下进行低周疲劳试验,将得到的四种环境下的循环δ-ε曲线及应变一寿命曲线分别进行对比,得出结论：相同应变下,2D70铝合金随腐蚀环境增强,循环应力降低;相同应变下,2D70铝合金在盐雾及盐雾＋SO2两种环境下低周疲劳寿命相近,并且随着腐蚀环境的增强,低周疲劳寿命降低。     

The importance of surface layer on fatigue behavior of a Ti- 6AI- 4V alloy

The effects of precycling and surface removal on the fatigue life and fatigue limit of a Ti-6A1-4V alloy were investigated. It was shown that both the fatigue life and fatigue limit were strongly dependent on the severity of precycling. The fatigue limit lost its significance if the alloy was subjected to a precycling treatment with a high stress amplitude. Cycling with stress amplitude below the fatigue limit after precycling showed a dependence of the logarithmic number of cycles to failure on the fraction of prefatigue damage. The interdependence of fatigue life and fatigue limit to precycling history was attributed to microcrack formation, principally restricted to a surface layer of less than 100 μm. Depending on the severity of precycling and on the magnitude of the applied cyclic stress, the fatigue damage could be either partially or totally eliminated by surface removal. The α/β interphase region of the surface layer appeared to offer preferred sites for dislocation pile-ups and crack initiation.     

The effect of mean stress and environment on corrosion fatigue behavior of 7075-T6 aluminum

Axial fatigue tests were performed on a 7075-T6 aluminum alloy in tension-compression and under superimposed positive mean stresses in dry air and in aqueous 0.5N NaCl solution. Both corrosive environments and positive mean stresses resulted in lower fatigue lives but no interaction between these variables was observed. Crack initiation in air occurred at electropolish pits at inclusion/alloy interfaces and propagated primarily in a Stage I (crystallographic) mode. Crack initiation in NaCl solutions occurred at heavily corroded regions surrounding non-metallic inclusions and propagated in a cleavage mode normal to the direction of applied stress. The relative number of cycles to crack initiation is shown to be a function of the magnitude of cyclic stress but not of mean stress. Similarly, the percentage of reduction in fatigue life due to corrosive environments is approximately constant at all mean stress levels. These data indicate that fatigue crack initiation is primarily related to mobile dislocations associated with cyclic deformation. Crack propagation on the other hand appears to be controlled by the maximum applied stress. A model for environment assisted cracking is presented which suggests that hydrogen induced cleavage is responsible for the degradation in fatigue properties of this alloy. Formerly Research Assistant, Materials Division, Rensselaer Polytechnic Institute, Troy, N. Y. 12181.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Facture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100 ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ _t /2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ _t /2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress amplitude was observed at Δɛ _t /2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with an inflection point at about Δɛ _p /2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111} slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was discussed.     

Experimental and Numerical Study of the Fatigue Strength of Double Lap Bolted Joints and the Effect of Torque Tightening on the Fatigue Life of Jointed Plates

In this research, the effects of clamping force on the fatigue life of 2024-T3 aluminum alloy double lap bolted joints have been studied experimentally and numerically. To do so, three sets of the specimens were prepared and each subjected to torque of 1, 2.5 and 5 N m and then fatigue tests were carried out under different cyclic longitudinal load levels. In the numerical method, finite element ANSYS code was used to obtain stress distribution in connection plates due to clamping force and longitudinal applied loads. Numerical simulation and experimental results revealed that the fatigue life of double lap bolted joints were improved by increasing the clamping force due to compressive stresses which appeared around the hole.     

A critical assessment of the mechanistic aspects in HAYNES 188 during low-cycle fatigue in the range 25 °C to 1000 °C

The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of temperatures between 25 °C and 1000 °C employing a triangular waveform and a constant strain amplitude of ±0.4 pct. Correlations between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the crack initiation and propagation modes, deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic strain aging (DSA) was found to occur over a wide temperature range between 300 °C and 750 °C. In the DSA domain, the alloy exhibited marked cyclic hardening with a pronounced maximum at 650 °C. Dynamic strain aging has been documented through the occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in the DSA regime. These macroscopic features in the DSA domain were accompanied by the substructure comprised of coplanar distribution of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end of the DSA domain, dislocation pinning by M₂₃C₆ precipitates occurred predominantly. The deformation behavior below and above the DSA domain has also been investigated in detail. The temperature dependence of LCF life showed a maximum at ≈300 °C. The drastic reduction in life between 300 °C and 850 °C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower life at temperatures less than 200 °C has been attributed to the combined influence of low ductility and larger cyclic response stress.     

Effect of Proximity and Dimension of Two Artificial Pitting Holes on the Fatigue Endurance of Aluminum Alloy AISI 6061-T6 Under Rotating Bending Fatigue Tests

This work deals with the study of the two artificial pitting holes effects, caused by their dimensions and proximity, on the fatigue endurance of aluminum alloy AISI 6061-T6 under rotating bending fatigue tests. Stress concentration induced by artificial pitting holes is analyzed and correlated with the experimental fatigue life. It is found that the stress concentration increases exponentially when the two pitting holes approach, and this induces an important reduction in the fatigue life. Concerning the diameter variation of one pitting in regard to the second, no important influence was observed on fatigue life for a given separation between them; this implies that the separation between the two artificial pitting holes and the associated stress concentration is the principal parameter on the fatigue life under these conditions. Finally, results are discussed and conclusions are presented involving the fatigue life, proximity, and dimension of pitting holes, stress concentration factor, and fracture surfaces where the failure origin is identified.     

Influence of subsolvus thermomechanical processing on the low-cycle fatigue properties of haynes 230 alloy

Strain-controlled low-cycle fatigue tests have been conducted in air at elevated temperature to determine the influence of subsolvus thermomechanical processing on the low-cycle fatigue (LCF) behavior of HAYNES 230 alloy. A series of tests at various strain ranges was conducted on material experimentally processed at 1121 °C, which is below the M₂₃C₆ carbide solvus temperature, and on material fully solution annealed at 1232 °C. A comparative strain-life analysis was performed on the LCF results, and the cyclic hardening/softening characteristics were examined. At 760 °C and 871 °C, the fatigue life of the experimental 230/1121 material was improved relative to the standard 230/1232 material up to a factor of 3. The fatigue life advantage of the experimental material was related primarily to a lower plastic (inelastic) strain amplitude response for a given imposed total strain range. It appears the increase in monotonic flow stress exhibited by the finer grain size experimental material has been translated into an increase in cyclic flow stress at the 760 °C and 871 °C test temperatures. Both materials exhibited entirely transgranular fatigue crack initiation and propagation modes at these temperatures. The LCF performance of the experimental material in tests performed at 982 °C was improved relative to the standard material up to a factor as high as 2. The life advantage of the 230/1121 material occurred despite having a larger plastic strain amplitude than the standard 230/1232 material for a given total strain range. Though not fully understood at present, it is suspected that this behavior is related to the deleterious influence of grain boundaries in the fatigue crack initiations of the standard processed material relative to the experimental material, and ultimately to differences in carbide morphology as a result of thermomechanical processing. Formerly Graduate Student, University of California, San Diego, La Jolla, is Senior Material Engineer, Solar Turbines Inc., San Diego, CA 92186-5376.     

Corrosion fatigue of a precipitation-hardened Al-Li-Zr alloy in a 0.5 M sodium chloride solution

Corrosion fatigue (CF) experiments have been performed on a high-purity Al-2.5Li-0.12Zr alloy in a deaerated 0.5 M sodium chloride solution as a function of aging time. The results of these tests were compared to the results of fatigue tests performed in dry air to investigate the effect of aging on the CF susceptibility of the alloy. It was found that the high cycle fatigue strength of the alloy was dramatically reduced by the aqueous environment. Examinations of the relative fatigue strength (Δσ_NaCl/Δσ_air) indicated that the underaged (UA) alloys were more susceptible to CF than the overaged (OA) alloys over the stress ranges studied, but the difference of the susceptibility between the UA and the OA alloys was reduced by decreasing the applied cyclic stress. The evidence suggests that, for the UA Al-Li-Zr alloys, the CF resistance is determined by both slip-enhanced dissolution and hydrogen embrittlement at high stress ranges, while at low stress ranges, the CF life is predominantly controlled by pitting-induced crack initiation regardless of the aging condition of the alloy.     

Q235钢结构材料的超低周疲劳性能

对钢结构材料 Q235钢的超低周疲劳性能进行了研究。采用横向应变控制方法,保持频率1 Hz 恒定,在岛津电液伺服疲劳试验机上开展了试验钢的超低周疲劳试验。获得了循环应力响应特征曲线等实验数据,并在此基础上分别建立了试验钢基于塑性应变幅及应变速率的超低周疲劳寿命预测公式,且2种公式均能较好地对其寿命进行预测。通过电镜扫描(SEM),分析了试验材料超低周疲劳下的微观断裂机理。研究结果表明,试验材料在超低周疲劳与低周疲劳下的疲劳性能,如循环响应特征、寿命预测公式以及微观断裂机理等方面均存在一定的差异。     

High-temperature low-cycle fatigue of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr

The low-cycle fatigue (LCF) behavior of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr in fully lamellar (FL) and nearly lamellar (NL) microstructural conditions is studied at 650 °C and 800 °C, with and without hold times. At 650 °C and 800 °C, the alloy in either condition exhibits cyclic stability at all strain levels studied, excepting the NL structure which shows slight cyclic hardening at higher strain levels at 650 °C. Fracture in the FL condition occurs by a mixed mode comprising delamination, translamellar fracture, and stepwise fracture. On the other hand, fracture occurs mostly by translamellar mode in the NL condition. At both test temperatures, the alloy in the FL condition obeys the well-known Manson-Coffin behavior. The fatigue resistance of the alloy at 650 °C in the FL condition is very much comparable to, while in the NL condition it is superior to, that of Ti-24Al-llNb alloy. At 650 °C, a 100-second peak tensile strain hold doubles the fatigue life of the alloy in the FL condition, while a 100-second hold at compressive peak strain or at both tensile and compressive peak strain degrades fatigue life. The observed hold time effects can primarily be attributed to mean stress. Irrespective of the nature of the test, the hysteretic energy (total as well as tensile) per cycle remains nearly constant during the majority of its life. The total and tensile hysteretic energy to fracture, at both test temperatures, increase with cycles to failure, and the variation follows a power-law relationship. Formerly NRC Senior Resident Associate, Wright Laboratory.     

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy

Tension-compression fatigue tests were conducted on forged (F-950) and rolled (R-950) Ti-6Al-4V alloys. The role of basal texture, produced by forging, was compared with prismatic texture, produced by rolling, on microstructurally short fatigue crack growth initiation and growth resistance. The fatigue life of R-950 alloy proved to be higher than that of F-950 alloy. Major differences in fatigue crack growth rate were detected in the short fatigue crack region at crack lengths of below 100 µm (F-950 alloy showing low resistance). To quantify the effect of texture alone, the opening/closure behaviour of a microstructurally small crack was analysed from the data obtained by combining an automatic in situ observation system with a digital image correlation technique. This analysis showed the crack opening stress of both alloys to be close to identical in the short fatigue crack growth region, nullifying the effect of crack closure. To study the effect of texture, cross-sectional electron backscatter diffraction analysis was performed at crack initiation sites. The results revealed that in F-950 alloy, short fatigue crack growth proceeded along basal planes that had similarly-oriented grains, whereas in R-950 alloy, short fatigue crack growth did not follow prismatic planes: the orientation varied.