激光-电弧复合焊接7075-T6铝合金疲劳断裂特性

研究激光一电弧复合焊接2mm板7075-T6铝合金在不同应力比尺和应力幅σa下的疲劳裂纹扩展行为．结果表明,优选的复合焊工艺参数激光功率3kW、电流110A和焊接速度3m／min条件下接头和母材的疲劳裂纹扩展速率曲线存在交叉现象,即当应力强度因子幅△K小于15．6MPa·m1/2。时,接头的疲劳裂纹扩展速率小于母材,反之则接头的疲劳裂纹扩展速率大于母材．而对应同一△K值,高应力比下的疲劳裂纹扩展速率快于低应力比条件下的扩展速率．应力幅或平均应力是影响疲劳裂纹扩展特性的主要因素．     

16MnR钢在不同条件下的疲劳裂纹扩展规律

采用压力容器用16MnR热轧钢不同缺口尺寸、不同厚度的紧凑拉伸 (CT)试样, 进行了不同温度、不同应力比条件的一系列疲劳裂纹扩展实验, 得到了相应实验条件下的疲劳裂纹扩展速率. 讨论了高温环境、缺口半径、应力比及试样厚度对疲劳裂纹扩展行为的影响规律. 结果表明: 16MnR钢在环境温度为150和300 ℃时的疲劳裂纹扩展速率比25和425 ℃时低, 300℃时疲劳裂纹扩展速率最低, 300℃以上时随温度的升高裂纹扩展速率增大; 缺口半径的大小对初期疲劳裂纹扩展有较大的影响; 应力比对16MnR钢的疲劳裂纹扩展行为没有影响; 疲劳裂纹扩展速率随试样厚度的增大而增大.     

8090Al—Li合金的疲劳裂纹扩展

研究了8090型Al—Li合?疲劳裂纹扩展行为以及组织结构和环境的影响,结果表明,在空气中,自然时效和欠时效?现出最好的疲劳裂纹扩展阻力,其次是峰时效状态,而过时效状态的疲劳裂纹扩展阻力最低,在3.5%NaCl水溶液中,时效状态对疲劳裂纹扩展行为有着同空气中相同的影响规律,但在同样的时效条件下,在3.5%NaCl水溶液中的疲劳裂纹扩展阻力要比空气中的低,在相同的应力强度因子范围△K作用下,缺口短裂纹和物理短裂纹均表现出比长裂纹高的扩展速率,用位错的平面滑移性和循环滑移可逆性解释了时效的影响,用裂纹的闭合效应和裂纹尖端塑性区尺寸说明了长短裂纹扩展行为的差别。     

载荷比对含氢Ti-4Al-2V钛合金疲劳裂纹扩展速率的影响

研究了含氢量为110μgg/和280μg/g的Ti-4Al-2V钛合金在不同载荷比下的疲劳裂纹扩展行为。载荷比降低，da/dN曲线向高△K方向移动，并逐渐不出现疲劳裂纹的近门槛扩展阶段；无论载荷比和含氢量的高低，稳态裂纹扩展区的裂纹扩展行为符合Paris幂律关系，即da/dN=C(△K）^m；含氢量为110μg/g时，载荷比对稳态裂纹扩展区的da/dN没有影响；含氢280μg/g时，载荷比降低，稳态裂纹扩展区的da/dN也降低；载荷比影响裂纹由近门槛扩展区进入稳态裂纹扩展区的应力强度因子范围△Kp和开始失稳扩展的应力强度因子范围△K1H。载荷比越小，△Kp和△K1H越大。     

38CrMoAlA钢表面氮化多冲疲劳性能研究

研究了38CrMoAlA钢表面氮化多冲疲劳裂纹萌生寿命、裂纹断裂寿命及裂纹扩展速率。结果表明:在低冲击能量状态下氮化对裂纹萌生影响不显著;而高冲击能量状态下氮化显著提高裂纹的萌生寿命和断裂寿命。冲击能量和氮化对裂纹扩展速率影响不显著,在相同的冲击能量和应力强度因子范围ΔK下,氮化试样的裂纹扩展速率低于未氮化试样的裂纹扩展速率。     

超载对X52管线钢疲劳裂纹扩展速率的影响

用紧凑拉伸试样研究了不同应力比和超载比对X52管线钢疲劳裂纹扩展速率的影响,并对疲劳断口形貌进行SEM分析.结果表明,拉伸超载有阻滞X52钢疲劳裂纹扩展的作用,使疲劳裂纹扩展由超载扩展,减速扩展和恢复扩展三个阶段组成.在恒定的应力比下,超载比越大,裂纹扩展的阻滞效应也越明显.超载前后形貌差别越大;在恒定超载比下,低应力比的裂纹扩展阻滞现象更明显.     

2524铝合金不同应力比下的疲劳裂纹扩展行为

研究了不同应力比(R=0.1,0.2,0.3,0.5)对2524-T3铝合金的疲劳裂纹扩展速率的影响,并结合扫描电镜(SEM)下裂纹的断口形貌特征进行了分析。结果表明:应力比R对疲劳裂纹扩展寿命和速率存在明显影响,在恒定最大载荷条件下,应力比越小则裂纹扩展寿命越短;当应力强度因子范围△K相同时,应力比越大,裂纹扩展速率da/dN越快。利用Paris公式对数据进行回归分析,da/dN-△K曲线呈收敛趋势,进一步采用Elber模型进行拟合,从裂纹闭合的角度解释了应力比R对裂纹扩展速率的影响,结果表明该模型能够较好地描述2524-T3铝合金在不同应力比下的疲劳特性。疲劳断口呈现出早期裂纹扩展、稳定扩展和快速扩展3个阶段,不同应力比下,疲劳辉纹间距和二次裂纹数量存在明显差异。     

应力比对TC4-DT钛合金疲劳裂纹扩展速率的影响

对TC4-DT钛合金在不同应力比下的疲劳裂纹扩展行为进行了研究,绘制出疲劳裂纹扩展速率和应力强度因子幅值ΔK之间的关系曲线,用SEM对断口形貌进行了观测。实验结果表明,随着应力比增加,裂纹扩展速率增加;应力比降低,da/dN曲线向高ΔK方向移动。预裂区主要是以微区解理断裂机制为主,稳态扩展区主要是以疲劳条带扩展机制为主,同时也存在微区解理断裂机制,快速扩展区的断口形貌呈韧窝型静载断裂特征。     

铸造Ti-6Al-4V合金的疲劳性能研究

通过铸造Ti-6Al-4V合金疲劳性能试验,绘制出了铸造Ti-6Al-4V合金在不同应力比下裂纹扩展速率da／dN-△K曲线。根据logda／dN和log△K的拟合曲线方程,计算出Paris公式下材料常数c、n值。经SEM对铸造Ti-6Al-4V疲劳断口进行分析,发现随着应力比R的增大,疲劳条带越宽,裂纹扩展速率越大。     

喷射沉积Al-Si/SiCp制动盘材料的热疲劳微裂纹扩展行为

采用V形缺口试样,研究喷射沉积Al-Si/SiCp复合材料制动盘在25(450 ℃热循环下的热疲劳行为.通过金相显微镜和扫描电镜观察了复合材料的组织和热疲劳裂纹形貌,研究热疲劳裂纹形成与扩展机制.结果表明:热疲劳主裂纹主要从V形缺口处萌生;在同样的热循环次数下,热处理前的试样要比热处理后的试样先出现裂纹,且裂纹扩展的速率较快;裂纹绕过Si颗粒向前扩展以及裂纹穿过Si颗粒向前扩展是裂纹与Si颗粒相互作用的主要机制;SiC颗粒与热疲劳裂纹有明显的交互作用.因此,改善Si相的形态和分布以及加强Al/SiC颗粒间的界面结合有利于提高热疲劳裂纹扩展的抗力.     

2050铝合金的疲劳裂纹扩展行为研究

研究取样方向、应力比以及微观组织对2050铝合金疲劳裂纹扩展速率的影响。结果表明:取样方向和应力比对疲劳裂纹扩展速率在近门槛区和快速扩展区的影响显著,而在稳态扩展区的影响则相对较小;L-T取样方向的速率最低,T-L取样方向的速率次之,S-L取样方向的速率最高,这主要与合金的晶粒取向、织构取向和第二相粒子取向有关;应力比为0.5的裂纹扩展速率高于应力比为0.1的速率,这在近门槛区主要与裂纹闭合效应有关,在快速扩展区主要受最大应力场强度因子Kmax的影响。     

2397合金高周疲劳性能及裂纹萌生扩展行为

研究了2397-T87铝锂合金的高周疲劳性能及裂纹萌生扩展行为。结果表明:在应力比R=0.1时,2397-T87铝锂合金L方向、LT方向和ST方向光滑试样(K_t=1.0)的疲劳寿命极限分别约为192,243和151 MPa;缺口试样(K_t=3.0)的疲劳寿命极限分别约为72,78和70 MPa。其疲劳裂纹主要萌生于试样表面,以及氧化物、夹杂等脱落形成的空洞,Al(CuFeMn)第二相杂质粒子。驻留滑移带(PSB)和晶粒取向对其疲劳裂纹早期扩展有重要影响。     

Damage mechanism of a nodular cast iron under the very high cycle fatigue regime

Fatigue behaviour in the very high cycle regime (VHCF) of 10¹⁰ cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = −1.

The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An advanced, high-speed, and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels caused by internal damping due to a very high frequency cycling. The temperature field on the surface specimen was determined by using a non-destructive measurement technique called infrared pyrometer. An infrared camera made up of a matrix of 320 × 240 detectors was used.

The S-N curves obtained show that fatigue failure occurred beyond 10⁹ cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Under the current test conditions, the high cycle fatigue (VHCF) behaviour of the cast iron exhibited a typical fatigue crack growth process, that is, fatigue initiation takes place always at the surface graphite or subsurface void; the distinctive stable fatigue crack growth zone can be found around the fatigue crack initiation site, the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition.

Under lower stress amplitude and high cycle condition, surface graphite fatigue initiation is predominantly depended on cyclic stress amplitude; subsurface void fatigue initiation is determined by maximum cyclic stress.

In the process of small crack propagation, the temperature in local plastic zone increase very sharply. The temperature field of ultrasonic fatigue specimen can be changed with the cooling condition; internal heating can accelerate surface fatigue crack initiation and propagation.

Fatigue properties in VHCF regime were studied for cast iron (GS51) at 20 kHz frequency and for the first time, crack initiation and propagation stages were analyzed using a high-sensitivity infrared camera. The new Paris's model for fish eye formation in the gigacycle fatigue were also confirmed by this study.     

激光冲击TC17钛合金疲劳裂纹扩展试验

为研究激光冲击强化对钛合金试件疲劳性能的影响,在标准试件的裂纹扩展路径上设计了全强化和间隔强化两种不同的强化方案,研究激光冲击强化对试件疲劳寿命和裂纹稳定扩展时速率的影响规律,利用有限元数值模拟和X射线残余应力测试获得了试件的残余应力场分布状态,并对比分析了试件的断口形貌和微观组织特征。结果表明:相比于未强化试件,激光冲击强化后试件的平均疲劳寿命分别提高了2.14倍和1.90倍,两种不同的冲击强化方法分别降低钛合金试件的裂纹扩展速率24%和15%。间隔强化后试件表面产生-512 MPa的最大残余压应力,裂纹扩展的C′值为-7.3,m值为2.6,而强化间隔区引入最大值为82.4 MPa的残余拉应力,裂纹扩展速率急剧升高,C′值减小至-13.6,m值为8.0。当裂纹扩展到强化区时,扩展速率再次降低,激光冲击强化对TC17钛合金疲劳裂纹扩展有显著的抑制作用。     

FATIGUE CRACK PROPAGATION IN Al-Li ALLOY 8090

Fatigue crack growth rates of Al-Li alloy 8090 in air were found to be strongly dependent up-on the aging conditions.The naturally aged and underaged specimens showed the highest re-sistance to the fatigue crack propagation.The fatigue crack growth resistance of the overagedspecimen is the lowest and that of the peakaged specimen in between.As compared to air,3.5% NaCl solution does not change the effect of aging conditions on the fatigue crackgrowth,but causes a decrease of the resistance to the fatigue crack growth under the same ag-ing condition.Both short cracks from notch and physically short cracks showed much higherrates of fatigue crack propagation in comparison with long cracks under the same aging condi-tion and stress intensity level.The growth behavior of the short crack depends on its type.Thegrowth rate of short crack from notch decreases first to a minimum and then increases with in-creasing △K.However,the physically short crack grows at a progressively increasing rate.The effects of aging conditions are explained in terms of the slip planarity of dislocations andthe cyclic slip reversibility.The observed short crack behavior is considered to be dependent onthe crack closure and the local plasticity near the crack tip.     

焊接残余应力对7N01铝合金疲劳裂纹扩展影响

焊接残余应力作为平均应力影响裂纹扩展. 将残余应力与外载平均应力分离,通过构建典型焊接残余应力场,借助扩展有限元计算焊接残余应力场的应力强度因子. 开展了紧凑拉伸（CT）试样的疲劳扩展试验,基于Walker公式将裂纹尖端平均应力强度因子K_m（静态量）和应力强度因子幅值ΔK（动态量）分离,获得疲劳裂纹扩展速率da/dN与K_m及ΔK的非线性关系. 结果表明,不同外载荷下,应力比与裂纹长度为非线性关系;残余应力对裂纹扩展存在尺度效应:CT试样裂纹长度小于2 mm时,残余应力场明显影响疲劳裂纹扩展速率;当裂纹长度大于2 mm,外载荷为主导因素.     

CRACK GROWTH BEHAVIOR IN TITANIUM ALLOY TC11AT ROOM AND HIGH TEMPERATURES

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喷丸残余应力对裂纹闭合效应影响的数值仿真

基于裂纹闭合效应,利用ABAQUS软件建立用于预测残余应力场中疲劳裂纹扩展特性的弹塑性有限元模型。考虑塑性和残余应力场对裂纹闭合的作用,分析残余应力、应力比和裂尖单元尺寸对裂纹闭合效应的影响。研究结果表明:未喷丸试样的裂纹闭合类型为塑性诱导裂纹闭合,喷丸残余应力场中的裂纹闭合为塑性和残余压应力共同作用,且裂纹张开力的大小与残余应力的分布相对应;正应力比越大,裂纹闭合效应越不明显,疲劳裂纹扩展速率越快;裂尖单元尺寸小于塑性区范围时可以真实反映裂尖的闭合状态;喷丸残余压应力通过提高裂纹闭合力,增强裂纹闭合效应,抑制疲劳裂纹扩展。     

Computer-aided experiment on arc welding

Fatigue properties of cast aluminium welded joints by friction stir welding (FSW) and MIG welding were investigated, comparing with that of the base plate. Fatigue crack propagation tests for the da/dN ? ΔK relation and bending fatigue tests for the S–N relation were carried out. Fatigue cracks in both FSW and MIG specimens were accelerated, when the fatigue crack tip reached the stir zone or the weld metal. This behaviour was discussed based on the crack closure induced by the crack surface roughness and the residual stress. In the S–N properties, the influence of specimen surface finishing on fatigue life was also examined. Fatigue lives of the FSW and MIG specimens in the ‘as weld’ condition were in the range of the largely scattered base plate fatigue lives, in spite of the different fatigue crack initiation sites in each specimen such as the porosity in the base plate, the tool mark bottom in the FSW and the weld toe in the MIG. The FSW specimens with the polished surface showed the particular improvement in fatigue strength for finite fatigue life.