Fibre bridging during high temperature fatigue crack growth in Ti/SiC composites

Synchrotron X-ray diffraction has been used to map the crack tip stress field, load redistribution and the variation in interfacial shear stress along bridging fibres local to a matrix crack during fatigue crack growth in Ti-6Al-4V/unidirectional SCS-6 SiC monofilament composite at elevated temperature. Quasi-static fatigue cycles were applied in a diffractometer at the same stress amplitudes and temperatures (120 and 300 °C) as those used in prior off-line fatigue testing. The elastic fibre strains were measured ply by ply along the fibres and in the matrix in the region of the crack. In this manner the crack affected zone was mapped, and subsequently the interfacial shear stress levels deduced as a function of distance from the crack at these temperatures. The results are compared with previous studies of load redistribution at room temperature and the fibre sliding stresses compared with those needed to slide pristine fibres to evaluate degradation of the interfacial shear strength caused by interfacial wear during fatigue. The implications for the use of such composites at elevated temperatures, for example in aero engines, are discussed.     

CRACK GROWTH BEHAVIOR IN TITANIUM ALLOY TC11AT ROOM AND HIGH TEMPERATURES

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Influence of cubic boron nitride grinding on the fatigue strengths of carbon steels and a nickel-base superalloy

The influence of cubic boron nitride (CBN) grinding on fatigue strength was investigated on an annealed carbon steel, a quenched and tempered carbon steel at room temperature, and a nickel-base superalloy, Inconel 718, at room temperature and 500 °C. The results were discussed from several viewpoints, including surface roughness, residual stress, and work hardening or softening due to CBN grinding. The fatigue strength increased upon CBN grinding at room temperature, primarily because of the generation of compressive residual stress in the surface region. However, in the case of Inconel 718, this marked increase in the fatigue strength tended to disappear at the elevated temperature due to the release of compressive residual stress and the decrease of crack growth resistance at an elevated temperature.     

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。     

High temperature high cycle fatigue behavior of new aluminum alloy strengthened by (Co,Ni)3Al4 particles

High cycle fatigue (HCF) behavior of a new heat-resistant aluminum alloy at elevated temperature was investigated. This alloy consists of an α-Al matrix, a small amount of precipitated Mg₂Si, and distributed (Co, Ni)₃Al₄ strengthening particles. HCF tests were conducted with a stress ratio of (R)=0 and a frequency of (F)=30 Hz at 130 °C. The fatigue limit (maximum stress) of this alloy was 120 MPa at 10⁷ cycles. This is a value superior to that of conventional heat-resistant aluminum alloys such as the A319 alloy. Furthermore, regardless of the stress conditions, the new heat-resistant Al alloy has an outstanding fatigue life at high temperatures. The results of fractography observation showed that second phases, especially (Co, Ni)₃Al₄ particles, were effective to the resistance of fatigue crack initiation and propagation. On the other hand, Mg₂Si particles were more easily fractured by the fatigue crack. This study also clarifies the micromechanism of fatigue deformation behavior at elevated temperature related to its microstructure.     

TB6钛合金疲劳及裂纹扩展性能研究

本文对TB6钛合金锻件弦向和径向两种取样方向分别进行了室温和200℃下旋转弯曲高周疲劳、轴向低周疲劳和疲劳裂纹扩展性能试验研究。试验结果表明,弦向（C）和径向（R）两种取样方向对该合金锻件的旋转弯曲高周疲劳、轴向低周疲劳性能和疲劳裂纹扩展性能没有影响;温度升高可加速该合金锻件的疲劳裂纹萌生,但在裂纹扩展阶段,该合金高温下的韧性优势与屈服强度降低的劣势平衡的结果使其在室温～200℃温度范围内的疲劳性能基本不受温度的影响;在10—20mm的厚度范围内,厚度对该合金的疲劳裂纹扩展性能没有影响;在3．5％NaCl盐雾环境中。腐蚀介质对TB6钛合金的疲劳裂纹扩展速率在初始阶段有迟滞作用,但在应力强度因子范围大于14MPa m后有加速作用。     

INVESTIGATION OF THE LOW-CYCLE FATIGUE AND FATIGUE CRACK GROWTH BEHAVIORS OF P91 BASE METAL AND WELD JOINTS

Low cycle fatigue tests and crack growth propagations tests on P91 pipe base metal and its weld joints were conducted at three different temperatures: room temperature, 550℃ and 575℃. The strain-life was analyzed, and the changes in fatigue life behavior and fatigue growth rates with increasing temperature were discussed. The different properties of the base metal and its weld joint have been analyzed.     

Impact of temperature excursion on stress corrosion cracking of stainless steels in chloride solution

The impact of a temperature excursion on the subsequent stress corrosion crack growth at the normal operating temperature has been investigated for 321 stainless steel (UNS32100) and 316L stainless steel (UNS31603) using precracked compact tension specimens. Although the data are preliminary the indication is that once crack growth has initiated in 321 SS at the elevated temperature, 130 °C in this study, the crack growth may be sustained at the lower temperature (40 °C), at least over the exposure time of about 700 h. However, the growth rate of 316L SS at the lower temperature was significantly lower than for 321 SS and tended to zero after 2000 h. For the 316 SS a temperature transient should not impact on structural integrity, provided it is short in duration.     

Fatigue crack growth and fracture toughness properties of 304 stainless steel pipe for LNG transmission

The fatigue crack growth rate and fracture toughness tests of type 304 stainless steel were studied over a temperature range of −162°C to room temperature. Girth weld metal specimens were fabricated using a combination of gas-tungsten-arc-welding and shielded-metal-arc-welding. The seam weld metal was made with submerged arc welding. Fatigue crack growth rate tests were conducted using compact tension specimens in accordance with ASTM E647. Fracture toughness was evaluated through CTOD tests with three point bend specimens. The CTOD values were affected by crack orientation with respect to the rolling direction, but orientation had no influence on fatigue crack growth rates. The fatigue crack growth rates and the CTOD values decreased with decreasing test temperature.     

Rolling contact fatigue of post-treated WC–NiCrBSi thermal spray coatings

The aim of this experimental study was to comprehend the relative rolling contact fatigue (RCF) performance and failure modes of functional graded WC–NiCrBSi thermal spray coatings in the as-sprayed and post processed state, by means of Hot Isostatic Pressing (HIPing) and vacuum heating. Functional graded WC–NiCrBSi coatings were deposited by a JP5000 system. HIPing was carried out at two different furnace temperatures of 850 and 1200 °C, while vacuum heating was performed at the elevated temperature of 1200 °C. The rate of heating and cooling was kept constant at 4 °C/min. Rolling contact fatigue tests were conducted using a modified four ball machine under various tribological conditions of contact stress and configuration, in full film elasto hydrodynamic lubrication. Results are discussed in terms of the relative RCF performance of the as-sprayed and post-treated coatings, and also surface and sub-surface examination of rolling elements using scanning electron microscope (SEM), light microscope and surface interferometry.

Test results reveal that performance of the coating was dependant on the microstructural changes due to post-treatment. Coatings heat-treated at 1200 °C displayed superior performance in RCF testing over the as-sprayed coatings at all stress levels (2, 2.3, 2.7 GPa) with emphasis on RCF performance at lower stress load of 2 GPa, where no failure occurred. Improvement in RCF performance was attributed to the diffusion between the carbides and matrix resulting in improved strength. At higher levels of contact stress, failure was surface initiated, and was attributed to initiation and propagation of micro-cracks at the edge of rolling contact region which led to coating delamination.     

Influence of Elevated Temperature and Stress Ratio on the Fatigue Response of AM60B Magnesium Alloy

The fatigue response of a high pressure die-cast AM60B Mg alloy is studied at room and elevated temperatures. The fatigue tests are conducted with stress ratio of R?=?0.1 and frequency of 30?Hz. The main objective is to determine whether elevated temperature would affect the fatigue response of the alloy. In addition, fatigue crack growth characteristics of the alloy is investigated at room temperature. The purpose of this test is to ascertain the capability and accuracy of a finite element approach coupled with the Walker model in assessing the life cycle of the alloy, in consideration of the influence of stress ratio.     

Fatigue behavior of a Zr-based bulk metallic glass under uniaxial tension–tension and three-point bending loading mode

The fatigue behavior of as-cast Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass was systematically investigated under uniaxial tension–tension and three-point bending loading modes. To obtain the fatigue stress-life (S–N) diagram, stress-controlled experiments were conducted using a computer-controlled material test system electrohydraulic testing machine at 25 Hz with a 0.1 R ratio in air at room temperature. The fatigue endurance limit (630 MPa) in stress range terms under cyclic tensile load was almost three times higher than that under the three-point bending condition (225 MPa). Both fatigue resistances were similar at higher stress level. The fatigue fracture morphologies associated with the S–N curve indicated that the defects have little to no influence on the crack initiation and the fatigue life in low cycle fatigue range. However, at lower stress level, the most detrimental factor was the number of defects that resulted in fatigue endurance limit discrepancy.     

Effect of Stress Ratio on the Fatigue Crack Propagation Behavior of the Nickel-based GH4169 Alloy

The fatigue crack growth behavior of the newly developed GH4169 nickel-based alloy at a maximum stress of700 MPa and different stress ratios was investigated in the present work employing the specimens with a single micronotch at a frequency of 129 Hz at room temperature. The results demonstrate a typical three-stage process of fatigue crack propagation processing from the microstructurally small crack(MSC) stage to the physically small crack(PSC) stage, and finally to the long crack stage. The crack growth rate in the MSC stage is relatively high, while the crack growth rate in the PSC stage is relatively low. A linear function of crack-tip reversible plastic zone size was proposed to predict the crack growth rate, indicating an adequate prediction solution.     

High-temperature cyclic fatigue-crack growth behavior in an in situ toughened silicon carbide

The growth of fatigue cracks at elevated temperatures (25–1300°C) is examined under cyclic loading in an in situ toughened, monolithic silicon carbide with Al–B–C additions (termed ABC–SiC), with specific emphasis on the roles of temperature, load ratio, cyclic frequency, and loading mode (static vs cyclic). Extensive crack-growth data are presented, based on measurements from an electrical potential-drop crack-monitoring technique, adapted for use on ceramics at high temperatures. It was found that at equivalent stress-intensity levels, crack velocities under cyclic loads were significantly faster than those under static loads. Fatigue thresholds were found to decrease with increasing temperature up to 1200°C; behavior at 1300°C, however, was similar to that at 1200°C. Moreover, no effect of frequency was detected (between 3 and 1000 Hz), nor evidence of creep cavitation or crack bridging by viscous ligaments or grain-boundary glassy phases in the crack wake. Indeed, fractography and crack-path sectioning revealed a fracture mode at 1200–1300°C that was essentially identical to that at room temperature, i.e. predominantly intergranular cracking with evidence of grain bridging in the crack wake. Such excellent crack-growth resistance is attributed to a process of grain-boundary microstructural evolution at elevated temperatures, specifically involving crystallization of the amorphous grain-boundary films/phases.     

High cycle fatigue and fracture behavior of 2124-T851 aluminum alloy

The high cycle fatigue properties and fracture behavior of 2124-T851 aluminum alloy were investigated roundly, including the fatigue crack growth rate, fracture toughness and fatigue S--N curve. Furthermore, the fatigue crack growth rate was analyzed by fitting the curves. And the microstructure of the alloy was studied using by optical microscopy, transmission electron microscopy and X-ray diffractometry, scanning electron microscop1/2 The results show that the fatigue strength and the fracture toughness of 2124-T851 thick plate are 243 MPa and 29.6 MPa.m at room temperature and R=0. 1, respectively. At high cycle fatigue condition, the characteristics of fatigue facture were observed obviously. And the higher the stress amplitude, the wider the space between the fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area and the greater the ratio between the intermittent fracture area and the whole fracture area.     

The effect of sensitization on the hydrogen-enhanced fatigue crack growth of two austenitic stainless steels

Fatigue crack growth tests were performed to evaluate the susceptibility to hydrogen-enhanced crack growth of AISI 304 and 316 stainless steels (SSs). Sensitization treatment at 650 °C 100 h played little effect on the fatigue crack growth behavior in air, regardless of testing specimens. However, hydrogen accelerated the fatigue crack growth of various specimens to different degrees; sensitized specimens were more susceptible as compared with the un-sensitized ones.

Fatigue fracture appearance of various specimens tested in air exhibited mainly transgranular fatigue fracture together with rarely intergranular fracture and twin boundary separation. Meanwhile, intergranular fracture was found for sensitized specimens tested in hydrogen. Extensive quasi-cleavage fracture related to the strain-induced martensite accounted for the hydrogen-accelerated fatigue crack growth of unstable austenitic SSs. On the other hand, the lower susceptibility of 316H specimens could be attributed to the partial austenite transformation, as evidenced by a mixture of transgranular fracture feature and quasi-cleavage.     

Effect of temperature on fatigue crack growth in P92 steel

Fracture at high temperature has become a critical problem for such high temperature components as those used in power plants or oil refinery plants, because both high operating temperature and pressure are required for better thermal efficiency. Therefore, it is very important to approach such problems from the viewpoint of high temperature material properties. Since fatigue and creep are closely related to such components failures, the fracture behavior in high temperature components must be evaluated through fatigue and creep crack growth tests, and based on these results, better operating conditions can be determined. In this study, recently developed P92 (9Cr-2W) alloy steel, which is a high strength material for high temperature use, is investigated and its fatigue crack growth has been characterized by Paris law. A series of high temperature fatigue tests were carried out at 400, 500, 550, 600, 625, 650, and 700°C to verify the temperature effect. The results indicated that the Paris exponent remained at approximately the same value up to a certain temperature. From 600 to 700°C, creep rupture tests were conducted in order to investigate the creep behavior with temperature. Further analysis has also been carried out to investigate the effect of temperature on fracture mode shift, dimple formation, and its role in crack growth rate and deformability at high temperature.     

Fatigue Behavior and Damage Evolution of SiC Fiberreinforced Ti-6Al-4V Alloy Matrix Composites

研究SiC纤维增强钛基复合材料(SiCf/Ti-6Al-4V)室温疲劳行为和损伤演化机制。疲劳试验条件:载荷控制、应力比0.1和加载频率10 Hz。采用疲劳断裂试验建立最大加载应力为600~1200 MPa内SiCf/Ti-6Al-4V的S-N曲线。采用疲劳中止试验以及SEM显微分析研究应力水平对SiCf/Ti-6Al-4V疲劳损伤演化的影响。结果表明,SiCf/Ti-6Al-4V疲劳损伤萌生模式与演化过程与应力水平密切相关。在高应力水平(Smax=1000 MPa),纤维开裂是主要损伤萌生模式。一旦2或3根纤维断裂后,纤维裂纹和基体裂纹开始联接并形成宏观扩展裂纹。在中等应力水平(Smax=800 MPa),基体裂纹萌生与扩展是主要损伤模式。多条基体裂纹萌生于试样外表面棱边和离外表面附近试样内部开裂的纤维基体界面处。基体裂纹均沿垂直于加载方向扩展,且大部分纤维未断裂并纤维桥接基体裂纹。在低应力水平(Smax=600 MPa),仅在C涂层和界面反应层之间和C涂层内部观察到局部界面脱粘现象。     

Fatigue fracture of high-strength Al-Zn-Mg-Cu alloy

X-ray diffractometry(XRD), optical microscopy(OM), scanning electron microscopy(SEM) were used to study the fatigue fracture of the T7451 Al-Zn-Mg-Cu alloy (470 °C, 60 min+115 °C, 8 h+165 °C, 16 h). The study reveals mainly the microscopic structure of the alloy in the process of crack formation and crack growth. The fatigue fracture is characterized by three zones: fatigue crack source zone, fatigue crack propagation zone and fatigue fracture zone. The fatigue damage preferably incubates at the fractured inclusion particles at or near (about 25 μm) the specimen free surfaces, and these brittle Fe-rich intermetallic inclusion particles are (7–10) μm×(11–14) μm in size. Some features such as “feather-like”, “river and range” and boundary extrusions can be observed in the fatigue propagation zone, and in the fatigue fracture zone the surface is rough and uneven.     

Effect of pre-strain on fatigue crack growth of 2E 12 aluminum alloy

The influences of pre-strain on the mechanical property and fatigue crack growth of 2E12 aluminum alloy were evaluated by SEM, TEM, mechanical property and fatigue tests. The axial fatigue tests were conducted under a constant amplitude sinusoidal wave loading at stress ratio of 0.1 in laboratory air and salt fog at room temperature. The results show that the yield stress of pre strain material is higher than that of the material without undergoing pre-strain, but pre-strain can not make the increase of the growth rate of fatigue crack. Fatigue crack growth rates of the alloy in salt fog are higher than those in air. The increased fatigue crack growth of the alloy in a given environment and more brittle striations can be observed in salt fog.