冲击和非冲击疲劳载荷下过载对裂纹扩展的影响

对12CrNi3A 钢在冲击和非冲击疲劳栽荷下的过载裂纹扩展延迟效应研究表明:过载对随后裂纹扩展的影响不仅取决于过载力作用下的裂尖塑变,还取决于随后基载力作用下的裂尖塑变.过载对随后裂纹扩展的影响存在两个互为矛盾的方面:一是裂尖塑变损伤促进裂纹扩展;另一是产生残余压应力场和闭合效应延缓裂纹扩展,两方面共同作用结果才能最终决定对随后裂纹扩展的影响.实验结果还表明,冲击疲劳载荷下的过载延迟效应与过载力的冲击速度有关,相同条件下,冲击疲劳载荷下的延迟期 Nd 高于非冲击疲劳载荷;各种因素对过载延迟的影响程度在两种载荷制度下不相同.     

7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳断口

利用电子显微镜和扫描电镜对7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳断口进行研究,分析了 7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳裂纹的发展过程。结果表明,搅拌摩擦焊接头腐蚀疲劳裂纹萌生 于焊核区和热力影响区的交界处并逐渐向热力影响区内部扩展,最终延伸并断裂于热影响区。腐蚀疲劳断 口存在多个裂纹源,裂纹源萌生于腐蚀坑处。腐蚀疲劳裂纹扩展分为低速扩展、稳态扩展和快速亚稳态扩展 3个阶段。在低速扩展阶段,产生了很多点蚀坑。在稳态扩展阶段,腐蚀疲劳具有脆性断裂特征。在快速亚 稳态扩展阶段,循环荷载成为裂纹扩展的主要驱动力而腐蚀作用的影响减弱。腐蚀疲劳裂纹扩展的瞬断区 呈现以脆性断裂为主的断裂特征。     

60Si_2Mn钢变幅疲劳的扫描电镜直接观察

应用自制的扫描电镜疲劳加载台,对60Si2Mn 弹簧铜过载或欠载时疲劳裂纹扩展的延迟与加速效应进行了扫描电镜的直接观察.结果表明,尖峰载荷会使闭合力K_(c1) 发生明显变化.欠载会使 K_(c1) 下降,其与裂纹顶端的反转压缩塑性区有关.     

Anodic dissolution of a crack tip at AA2024-T351 in 3.5wt% NaCl solution

The anodic dissolution process of a crack tip at 2024-T351 aluminium alloy (AA2024-T351) was determined by means of scanning Kelvin probe (SKP). Wedge-open loading (WOL) specimens were immersed in a 3.5wt% NaCl solution. After various durations of immersion, the Volta potential distributions around the crack were measured by SKP and the surface morphologies were observed by scanning electron microscopy (SEM). It is found that there is a nonuniform distribution of Volta potential around the crack. Before immersion, the Volta potential at crack tip is more negative than that in other regions. However, after immersion, a converse result occurs with the most positive Volta potential measured at the crack tip. SEM observations demonstrate that the noticeable positive shift of Volta potential results from the formation of corrosion products which deposit around the crack tip. Energy-dispersive spectrometry (EDS) analysis shows that the corrosion products are mainly Al oxide and Cu-rich particles. These observations implicate that the applied stress contributes to the preferential anodic dissolution of the crack tip and the redistribution of Cu.     

二氧化硫复合盐雾环境下2024-T351铝合金应力腐蚀开裂

通过SO2复合盐雾试验模拟工业污染海洋大气环境,结合有限元模拟分析、扫描电镜/能谱仪、光电子能谱分析等技术研究2024-T351铝合金在弹性应力区间的应力腐蚀开裂行为.结果表明:应力腐蚀开裂行为优先发生在2024-T351铝合金C型环的顶部应力集中区域;疏松的腐蚀产物层的形貌经历了由细棒状、团絮状到板块状的变化;试验6h就可以监测到裂纹,进行到480h的时候有贯穿裂纹形成,720h的时候试样完全断裂;裂纹为穿晶和沿晶混合机制,主裂纹以穿晶机制沿C型环法线扩展,二次裂纹沿晶界扩展.     

铸造奥氏体不锈钢的疲劳裂纹扩展

用紧凑拉伸试样研究了载荷比、单峰过载和两步高-低幅加载对Z3CN20-09M铸造奥氏体不锈钢疲劳裂纹扩展速率的影响。当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的增大而增大。单峰过载使裂纹扩展速率先有短暂的增加后长距离的减速扩展,出现裂纹扩展迟滞现象。两步高-低幅加载时,若两步的最大载荷不同,第二步裂纹扩展也会出现迟滞现象。用两参数模型和Wheeler模型能够预测恒幅载荷和变幅载荷下的疲劳裂纹扩展行为。     

激光功率对6061-T6铝合金激光填丝焊接头组织及性能的影响

采用激光填丝焊将厚度为2 mm和3 mm的6061-T6铝合金板材进行搭接叠焊,研究激光功率对接头成形质量的影响,分析了接头的显微组织和力学性能。结果表明,增大激光功率可以有效增加热输入量,焊道逐渐宽化,焊缝熔深增加。进一步分析发现,焊缝中心区和热影响区的析出相均为Mg₂Si。硬度试验结果表明,焊缝中心区由于细小等轴晶和析出相的双重作用,硬度远高于母材区与热影响区的。拉伸试验结果表明,接头的抗拉强度随激光功率的增大而升高,激光功率由2.0 kW升高至2.8 kW,抗拉强度升高约39%。     

7075-T6合金超声辅助搅拌摩擦焊接头搅拌区组织与力学性能

搅拌区的金属流动不充分容易导致轴肩影响区与搅拌针影响区之间的过渡区出现疏松缺陷并恶化接头的力学性能.在不同焊接参数条件下,对7075-T6铝合金分别进行常规搅拌摩擦焊接(FSW)和超声辅助搅拌摩擦焊接(UAFSW),研究了超声振动对接头搅拌区的金属流动行为、微观组织特征和力学性能的影响.结果表明:相同焊接参数下,UAFSW接头搅拌区的力学性能均优于FSW接头.焊接参数为1000r/min-110mm/min 的UAFSW接头搅拌区的抗拉强度和延伸率最高,分别达到515MPa和17.3%.在搅拌摩擦焊接过程中施加轴向超声振动可以显著降低搅拌区金属的屈服应力和流变应力,促进塑化金属沿板厚方向的流动.消除搅拌区中过渡区的疏松缺陷,并细化微观组织,是接头力学性能提高的主要原因.     

止裂孔维修对中心裂纹板疲劳寿命的影响分析

以铝合金2024-T3的中心裂纹板为研究对象,根据结构细节疲劳额定值(DFR)法计算了不同裂纹长度及不同止裂孔尺寸下结构的疲劳寿命;并通过试验得到半裂纹长度分别为10 mm、15 mm和20 mm,止裂孔半径为2.5 mm和3.5 mm下的结构试验寿命,对比结果验证了该模型及对应方法的可行性;随后对裂纹长度/板宽(2a/W)在0.2~0.6、止裂孔直径/板宽(D/W)0.03~0.20范围内的疲劳寿命进行了预估。根据分析结果得到了止裂孔效果最佳时D/W与2a/W的线性关系式,为工程维修提供技术参考。     

2024-T351铝合金在酸性NaCl溶液中SCC行为的电化学噪声检测

 采用电化学噪声技术对2024-T351铝合金在酸性NaCl溶液中的应力腐蚀开裂过程进行检测,并通过对电流噪声信号进行时域和频域分析,研究裂纹尖端腐蚀行为以及裂纹萌生和扩展过程的电化学噪声特征,并得出裂纹扩展的判断依据。结果表明,在浸泡初期,电流噪声时频谱中的振幅较小,能量主要集中于高频段,表明电极表面主要发生了膜破裂-再钝化等快反应过程,裂纹处于萌生阶段。随着浸泡时间的增加,电流噪声中出现特征峰。此时,低频段能量逐渐增加,表明裂纹进入扩展阶段。通过对电流噪声信号进行小波分解,可以区分应力腐蚀开裂过程中裂纹萌生和扩展的不同阶段。     

Temperature distribution study during the friction stir welding process of Al2024-T3 aluminum alloy

Heat flux characteristics are critical to good quality welding obtained in the important engineering alloy Al2024-T3 by the friction stir welding (FSW) process. In the present study, thermocouples in three different configurations were affixed on the welding samples to measure the temperatures: in the first configuration, four thermocouples were placed at equivalent positions along one side of the welding direction; the second configuration involved two equivalent thermocouple locations on either side of the welding path; while the third configuration had all the thermocouples on one side of the layout but with unequal gaps from the welding line. A three-dimensional, non-linear ANSYS computational model, based on an approach applied to Al2024-T3 for the first time, was used to simulate the welding temperature profiles obtained experimentally. The experimental thermal profiles on the whole were found to be in agreement with those calculated by the ANSYS model. The broad agreement between the two kinds of profiles validates the basis for derivation of the simulation model and provides an approach for the FSW simulation in Al2024-T3 and is potentially more useful than models derived previously.     

7075-T6铝合金单向超声振动车削表面质量及形貌特征

对7075-T6铝合金试件采用普通切削(CT)和振动切削(UVC)加工方法进行了加工实验,分析了2种切削加工方法在不同参数下对铝合金试件加工表面粗糙度的影响.试验研究结果表明:在相同的进给量和切削深度情况下,随着车削速度的变化,普通切削获得的加工表面粗糙度先减小后增大再减小,但随着转速的增大进入高速切削后,工件表面粗糙度值逐渐趋于稳定;随着进给量的增加超声振动硬车削与普通硬切削加工表面粗糙度都呈上升趋势,超声振动切削表面粗糙度较小;在切削速度、进给量相同的条件下,普通硬质切削7075铝合金加工表面粗糙度随着切削深度的增加而增加,而超声振动切削7075铝合金加工表面粗糙度随着切削深度的增加先减小后增加.通过对获得实验数据的分析以及对车削加工获得的加工表面显微形貌的观测,证实了超声振动加工在难加工材料加工中的优势.     

Stress-corrosion behavior and characteristics of the friction stir welding of an AA2198-T34 alloy

To better understand the stress-corrosion behavior of friction stir welding(FSW), the effects of the microstructure on the stress-corrosion behavior of the FSW in a 2198-T34 aluminum alloy were investigated. The experimental results show that the low-angle grain boundary(LABs) of the stir zone(SZ) of FSW is significantly less than that of heated affected zone(HAZ), thermo-mechanically affected zone(TMAZ), and parent materials(PM), but the grain boundary precipitates(GBPs) T1(Al_2CuLi) were less, which has a slight effect on the stress corrosion. The dislocation density in SZ was greater than that in other regions. The residual stress in SZ was +67 MPa, which is greater than that in the TMAZ. The residual stress in the HAZ and PM is -8 MPa and-32 MPa, respectively, and both compressive stresses. The corrosion potential in SZ is obviously less than that in other regions. However, micro-cracks were formed in the SZ at low strain rate, which indicates that the grain boundary characters and GBPs have no significant effect on the crack initiation in the stress-corrosion process of the AA2198-T34.Nevertheless, the residual tensile stress has significant effect on the crack initiation during the stress-corrosion process.     

压力容器疲劳裂纹扩展的超载迟滞效应及其估算

本文通过ST45钢轴向穿透裂纹模拟容器的内压疲劳裂纹扩展试验,探讨了在压力容器上推广、应用超载迟滞效应的问题。文中考虑常规压力容器水压试验的加载规定,着重研究了1.5倍小超载比情况下的迟滞特性,并讨论了超载点裂纹长度、基本循环内压大小及超载后停歇对迟滞效应的影响。最后选用 Wheeler、Will-enborg、广义Willcuborg、松冈(Matsuaka)、Maarse等五个较有名的数学模型计算试验容器的迟滞效应大小。研究表明:压力容器1.5倍的小超载,迟滞效应也是相当明显的,可使带有20mm左右裂纹的试验容器寿命提高约80％,Willenborg模型是估算压力容器上小超载迟滞效应较为理想的模型。     

铝合金CCT试样裂纹扩展三维尺寸效应金相分析

在三种厚度7075铝合金CCT试样随机谱裂纹扩展实验的基础上，对试样断口进行了精细的扫描电子显微镜分析，通过比较不同厚度试样在相同长度裂纹部位上同等放大倍数的金相照片，可以发现，试样的三维几何尺寸对裂纹扩展速率有着严重的影响。厚度越厚，则疲劳条带越稀，裂方扩展越快，最后对机理进行了分析。     

超载对X52管线钢疲劳裂纹扩展的影响

试验研究了裂纹扩展过程中超载载荷迟滞效应。在恒定的应力比下,在不同强度因子幅下通过计算过载影响区大小来确定影响区内的迟滞裂纹扩展速率,并与无超载情况下的裂纹扩展速率相比较,定性地说明X52管线钢裂纹扩展中的迟滞效应。     

7075-T6高强度铝合金温热条件下的拉深成形性能

采用温热拉深试验方法研究了在50~250 °C范围内，温度、压边力和成形速度对高强度铝合金7075-T6成形性能的影响.结果表明：在等温条件下，7075 T6的极限拉深比随着温度升高而先增后减；在差温条件下，极限拉深比随着温度升高而增大，当温度超过150 °C时，其极限拉深比较等温条件的大；随着温度升高，拉深成形所需要的力降低；在等温或差温条件下，虽其极限拉深比的差别较大，但其杯形件厚度的变化规律基本一致；当压边力为5 kN，速度为10 mm/min时，试样的成形性能最佳.     

压载对裂纹超载迟滞作用的影响

针对在随机载荷谱中拉超载对疲劳裂纹扩展的迟滞作用的研究较多，而对压载作用的研究很少的状况，着重研究了载荷谱中的压载对构件的疲劳寿命的影响。通过对裂纹尖端应力状态的研究，发现压载对超载迟滞现象有抵消作用；它使得有效应力强度因子值增大，并进一步给出了定量估计。     

Effect of cooling rate on plastic deformation of Zr-based bulk metallic glasses

The effect of cooling rate on plastic deformation of Zr-based bulk metallic glasses has been studied.The specimens with the diameters of 3 mm,4 mm and 6 mm cut from the same ladder-shaped Zr55Al10Ni5Cu...     

焊后热处理对6082-T6铝合金焊接接头组织和性能的影响

对6082-T6铝合金焊接接头进行固溶+时效和时效两种热处理，研究不同热处理制度对其组织和性能的影响。实验结果表明：未处理的6082-T6焊接接头抗拉强度为225 MPa，断裂位置位于热影响区，接头硬度最低值均在热影响区；经时效处理后的6082-T6焊接接头处强化相分布更加均匀，焊缝区组织无明显变化，熔合区和热影响区组织轻微细化，抗拉强度为264 MPa，断裂位置仍在热影响区，接头硬度最低值均在热影响区；经固溶+时效处理后的6082-T6焊接接头处重新析出细小的强化相，熔合区和热影响区组织有明显的细化，抗拉强度提高到302 MPa，断裂发生在焊缝区，硬度值明显高于未处理6082-T6焊接接头的，硬度最低值位于焊缝区。