CRACK GROWTH, FRACTURE FEATURE AND RUPTURE LIFEDURING CYCLIC CREEP OF SUPERALLOY GH2132

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Torsional Fatigue Cracking and Fracture Behaviors of Cold-Drawn Copper:Effects of Microstructure and Axial Stress

The fatigue cracking and fracture behavior of cold-drawn copper subjected to cyclic torsional loading were investigated in this study.It was found that with increasing stress amplitude,the fracture mode of cold-drawn copper gradually changes from a shear fracture on transverse maximum shear stress plane to a mixed shear mode on both transverse and longitudinal shear planes and finally turns to the shear fracture on multiple longitudinal shear planes.Combining the cracking morphology and the relationship between torsional fatigue cracking and the grain boundaries,the fracture mechanism of cold-drawn copper under cyclic torsional loading was analyzed and proposed by considering the effects of the microstructure and axial stress caused by torsion.Because of the promotion of the grain boundary distribution on longitudinal crack propagation and the inhibition of axial stress on transverse crack grown,the tendency of crack propagation along the longitudinal direction increases with increasing stress levels.     

热障涂层系统的热梯度机械疲劳应力分析

基于IN738高温合金基体上涂覆的热障涂层系统(Thermal barrier coating system,TBCs),分析热循环和热梯度机械疲劳加载条件下涂层的应力分布及演变。通过有限元分析研究了热生长氧化层(Thermally growth oxidation,TGO)的应力分布,以预测不同载荷作用下TBCs的失效行为。结果可知,在热循环的基础上施加应变载荷会造成TGO应力性质及大小的改变。只施加温度载荷,在加热过程中TGO/粘结层(Bond coat,BC)界面波峰位置会承受轴向较大的拉伸应力,裂纹多会在此处萌生,且以层间开裂的方式失效。而在温度与机械载荷的共同作用下,冷却过程中会承受较大的拉伸应力,显著增大的轴向应力与径向应力共同作用,使垂直于TGO/BC界面的裂纹沿着界面方向扩展,从而造成陶瓷层(Top coat,TC)剥落。进一步对比分析了同相和反相加载时的应力分布,结果表明反相加载时一次循环周期内会产生拉伸平均应力,更易发生TBCs的失效。     

Discrete dislocation plasticity modeling of short cracks in single crystals

The mode-I crack growth behavior of geometrically similar edge-cracked single crystal specimens of varying size subject to both monotonic and cyclic axial loading is analyzed using discrete dislocation dynamics. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation incorporated through a set of constitutive rules. The fracture properties are specified through an irreversible cohesive relation. Under monotonic loading conditions, with the applied stress below the yield strength of the uncracked specimen, the initiation of crack growth is found to be governed by the mode-I stress intensity factor, calculated from the applied stress, with the value of K_init decreasing slightly with crack size due to the reduction in shielding associated with dislocations near a free surface. Under cyclic loading, the fatigue threshold is ΔK-governed for sufficiently long cracks. Below a critical crack size the value of ΔK_I at the fatigue threshold is found to decrease substantially with crack size and progressive cyclic crack growth occurs even when K_max is less than that required for the initiation of crack crack growth in an elastic solid. The reduction in the fatigue threshold with crack size is associated with a progressive increase in internal stress under cyclic loading. However, for sufficiently small cracks, the dislocation structure generated is sparse and the internal stresses and plastic dissipation associated with this structure alone are not sufficient to drive fatigue crack growth.     

GH2036高温合金平板裂纹闭合效应及裂纹扩展模型

为探明GH2036高温合金的低循环疲劳裂纹扩展机理,对GH2036高温合金平板在550℃、不同应力比下的低循环疲劳裂纹扩展特性进行了试验研究,采用数字图像相关(DIC)方法确定了GH2036高温合金的张开应力强度因子。结果表明,温度550℃、应力比大于0.7时GH2036高温合金无裂纹闭合现象,在此基础上建立了以残余裂尖张开位移、应力比为参量的GH2036高温合金裂纹闭合模型。而后,断口的SEM分析表明:随着应力比的增加,裂纹扩展区由穿晶断裂向沿晶断裂转化。最后,基于GH2036高温合金的裂纹闭合模型,建立了GH2036高温合金平板的低循环疲劳裂纹扩展寿命预测方法,与试验数据吻合良好,验证了方法的准确性。     

Mechanisms governing deformation and damage during elevated-temperature fatigue of an aluminum-magnesium-silicon alloy

The cyclic deformation and fracture characteristics of aluminum alloy 6061 are presented and discussed. The specimens were cyclically deformed using fully reversed tension-compression loading under total strain-amplitude control, over a range of temperatures. The alloy showed evidence of softening to failure at all test temperatures. The degree of softening during fully reversed deformation increased with test temperature. The presence of shearable matrix precipitates results in a microstructure that offers a local decrease in resistance to dislocation movement, causing a progressive loss of strengthening contributions to the matrix. At elevated temperatures, localized oxidation and embrittlement at the grain boundaries are exacerbated by the applied cyclic stress and play an important role in accelerating crack initiation and subsequent crack propagation. The fracture behavior of the alloy is discussed in light of competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic plastic strain amplitude and concomitant response stress, and the test temperature.     

工作介质中材料的应力腐蚀性能

为了研究某材料在工作介质中的应力腐蚀问题,开展了光滑试样和中心穿透预裂纹试样在温度为T_0、不同试验介质中、施加一定预应力水平的恒载荷试验,测试试样的力学性能,分析其金相组织,并测量了预裂纹试样在恒载试验过程中裂纹的扩展量.结果表明:在不同试验介质中,施加(90～95)% σ0.2载荷的光滑试样和施加(30～90)% K_c载荷的预制裂纹试样,经过约13000 h的试验后,没有试样发生滞后断裂,试验前后材料的抗拉强度、断裂韧性没有发生显著性变化;预制裂纹试样在恒载过程中裂纹向前扩展,扩展速率在10~(-5) mm/h的数量级;材料在工作介质中不存在应力腐蚀.     

Effect of dwell condition on fatigue behavior of a high-Nb TiAl alloy at 750 °C

The strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of a newly developed Ti-45Al-8Nb-0.2W-0.2B-0.02Y (at.%) alloy were carried out at 750 °C in air. The hysteresis loop, cyclic stress response and life modeling as well as failure mechanism of the alloy were investigated in detail. It was revealed that the tensile and compressive mean stresses would generate when the dwell condition was introduced at minimum and maximum strain, respectively. In addition, the dwell condition, especially for the compressive dwell condition, would significantly decrease the fatigue life. The typical continuum damage accumulation(CDA) and modified CDA life models proposed in the present study were employed to predict both LCF and CFI life of the alloy, which showed that the modified CDA life model had a higher accuracy than the typical CDA life one. Moreover, only single crack initiation source was observed at 92% (i.e. 11/12) of LCF fracture while multiple crack initiation sources at 84% (i.e. 31/37) of CFI fracture. Apparently different from LCF specimen showing more transgranular appearance, CFI specimen shows more intergranular appearance.     

DAMAGE MECHANISMS OF INCONEL 738LC UNDER CREEP-FATIGUE LOADING CONDITIONS

1.IntroductionFirststagebladesofhigh--temperaturelandbasegasturbinesaredamagedbyacombinationofcreepandfatigue.Theleadingedgeofthebladeinparticularissubjectedtoacompressivestrainholdconditionduringgasturbineoperation.Suchconditionwasknownasthesevereconditionsfornickel-basedsuperalloysandsignificantreductionofcreep-fatiguelifehasbeenreported['--'].Althoughsomelifepredictionmethodsforsuchconditionshavebeenproposed[3],theyarenotsufficienttopredictthecreep--fatiguelifeingeneralduetolackofknowledgec…     

蠕变疲劳交互作用下裂纹萌生的有限元模拟

通过结合初始应力应变场与连续损伤力学理论以及单元失效和裂纹萌生准则,构建了蠕变-疲劳交互作用下裂纹萌生的预测模型,将模型编写为UMAT耦合到ABAQUS有限元分析软件中,实现了初始无缺陷结构蠕变-疲劳交互作用下裂纹萌生的有限元模拟,并分析了影响裂纹萌生寿命的因素.通过与线性累计损伤理论对比发现,裂纹萌生位置蠕变损伤和疲劳损伤具有相互促进作用,蠕变疲劳的交互作用使裂纹萌生寿命减小;蠕变和疲劳载荷加载顺序对损伤的累积具有很大影响,模拟发现承受蠕变载荷的结构在承受后续循环载荷下总损伤值更大,裂纹萌生寿命更短.     

STRESS RELAXATION BEHAVIOUR OF TITANIUM

Studies of stress relaxation were carried out on commercially pure titanium.It was found thatthe stress relaxation behaviour relates closely to the level of deformation.The amount of re-laxed stress σ_R is independent of the amplitude of stress dip in a certain range ofdeformation.However,it depend on the stress level at which stress relaxation begins in a cer-tain relaxation time t_R.Both back(critical)stress σ_c and flow stress σ_o vary with the strainin a similar manner.The effective stress on dislocations σ~* increases with increasing strainunder low strains,but keeps constant under higher strains.     

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.     

Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy

The fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K_max, and the stress intensity factor range, ΔK. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative effects of ΔK and K_max on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The influence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.     

GH720Li镍基高温合金蠕变-疲劳试验研究

对先进航空发动机涡轮盘的典型材料镍基高温合金GH720Li的蠕变-疲劳特性进行了试验研究。通过对真实涡轮盘上取样的圆棒试验件,开展了650℃不同保持时间下的GH720Li合金蠕变-疲劳试验。结果表明,650℃下保持时间对GH720Li合金的蠕变-疲劳特性影响较大,即,在较长的保持时间(30 min)下,蠕变损伤占主导地位,降低了材料的蠕变-疲劳寿命。SEM断口分析表明:随着保持时间的增加,裂纹形成区由穿晶和沿晶混合断裂向沿晶断裂转化;裂纹扩展区由穿晶断裂向穿晶、沿晶的混合模式转化。最后,基于GH720Li合金的蠕变-疲劳试验数据,分别选取载荷谱转换法和机械功密度法进行GH720Li合金的蠕变-疲劳寿命预测,并研究了2种模型的工程适用性。     

Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 ℃

Low-cycle fatigue (LCF) behaviors of a second-generation nickel-based single-crystal superalloys with [001] orientation at 760 °C have been investigated. Different strain amplitudes were introduced to investigate the creep-fatigue effects. The LCF life of none tensile holding (NTH) was higher than that of the 60-s tensile hold (TH) at any strain amplitude. As the strain amplitude was 0.7%, the stacking and cross-slip dislocations appeared together at the γ/γ’ coherent microstructure in both TH and NTH specimens. At the strain amplitude of 0.9%, plenty of the cross-slip dislocations appeared in γ channel and other dislocations were stacking at γ/γ’ interfaces. However, the SFs still appeared in γ’ phase with 60-s TH which caused cyclic softening. As the strain amplitude increased up to 1.2%, the dislocations are piling up at the γ/γ’ interfaces and cutting through the γ’ phase in both TH and NTH tests, which caused cyclic hardening. The influences of strain amplitude and holding time were complicated. Different stress response behaviors occurred in different loading conditions. The surface characteristic and fracture mechanism were observed by scanning electron microscopy. This result is helpful for building the relationship of various blade fatigue failure modes, cyclic stress response and microstructure deformation under different strain amplitudes.     

Modelling the initiation of cleavage fracture of ferritic steels

The initiation of the cleavage fracture of ferritic steels at cracked grain boundary carbides situated in a plastic zone is modelled, using a 2D discrete dislocation simulation of the plastic zones associated with a microcrack under triaxial loading. Fracture is assumed to occur when the local stress intensity factor for the microcrack equals K_IC. For microcracks in the size range 0.1–10 μm, the applied stress at fracture, σ_F, is found to be independent of the yield stress (in the range 200–1600 MPa) and therefore of temperature. The dependence of σ_F on the crack size deviates from a Griffith type relation. These predictions are consistent with experimental data on steels in the literature. The values of σ_F predicted are in good agreement with the experimental values. The results suggest that only a small fraction of carbides are in configurations leading to fracture.     

Modeling crack growth from pores under compressive loading with application to metallic glasses

The mechanism of the fatigue-crack growth is essential to understand the fatigue and fracture behavior of bulk metallic glasses (BMGs) and is thus critical to predict the service lifetime of BMGs as potential engineering structural materials. Experiments indicate that fracture under compressive loading exhibits distinct behaviors different from that under tensile loading. A typical compression failure may initiate from micro porosity where cracks propagate in a direction generally parallel to the loading axis. Micromechanical stress analysis shows that pores cause axial tensile microcracks emanating from the pore. A simplified computational model based on the linear elastic fracture mechanics (LEFM) is proposed to investigate crack initiation and subsequent propagation under compressive load, where the effect of crack closure on mode-I fracture is considered. The stable crack length is characterized by a dimensionless fracture-mechanics quantity required to attain the associated crack length. The behavior of crack growth is examined based on the stress-intensity-factor (SIF) calculation, and its dependence on the loading and lateral confinement conditions is discussed.     

A dislocation model for the two critical stress intensities required for threshold fatigue crack propagation

We have examined the variations, with decreasing load ratio, of threshold peak and cyclic stress intensities required for fatigue crack growth in stage I (mainly mode II loading) using a simple model simulating dislocation motion near a crack tip. In this model the crack grows by dislocations running into the crack during loading and unloading phases. Initially we have studied the behaviour of a crack with a dislocation source relatively far away from the crack tip. Crack propagation rates showed a Paris regime at high ΔK, and an abrupt threshold value ΔK_th below which no crack growth occurred. The variation with load ratio of the peak (K_th) and cyclic (ΔK_th) stress intensities at the fatigue threshold showed that two different processes controlled the behaviour. At high load ratios dislocations are generated readily during loading and the threshold is controlled by the need for sufficient unloading to allow dislocations to run back into the crack, so that the criterion ΔK ≥ ΔK∗ results. At negative load ratios it is the generation of dislocations during the loading phase that controls the threshold condition, since once generated, the large unloading and reversed loading easily forces dislocations back to the crack. Under these conditions the threshold criterion becomes K_max ≥ K∗.     

MECHANISM OF HYDROGEN FACILICATED CLEAVAGE IN TITANUM ALUMINIDE

Both hydrogen induced cracking and overload crack initiated at same characteristic distance,r~*,within the plastic zone along the slip line when the plastic zone developed to a critical ex-tent.For the overload crack.K_(IC)=αr~*~(1/2)[σ_F~((n+1)/2n)/σ_(ys)~((1-n)/2n)],σ_F=σ_0+g[2μbσ_(th)/π~2L(1-v)]~(1/2)For the hydrogen induced cracking:K_(IH)=αr~*~(1/2)[σ_F(H)~((n+1)/2n)/σ_(ys)(H)~((1-n)/2n)],σ_F(H)={σ_0(H)+g[2μbσ_(th)(H)/π~2L(1-v)]~(1/2)}/kHydrogen pomoting the dislocation multiplication and motion would result in σ_0(H)<σ_0,k>1,Therefore,hydrogen promoting the cleavage fracture in titanum aluminide can be dueto that hydrogen facilitates the local plastic deformation,which results in σ_F(H)<σ_F and thenK_(IH)相似文献