CF/Al复合材料横向拉伸渐进损伤与弹塑性力学行为

针对真空压力浸渗制备的单向碳纤维增强铝合金复合材料(CF/Al复合材料),采用细观力学数值模拟与实验结合的方法研究了其横向拉伸损伤演化和断裂力学行为,并分析了界面对复合材料横向拉伸力学性能的影响。结果表明,基于基体合金延性损伤和界面内聚力损伤本构所建立的细观单胞有限元模型,可以实现CF/Al复合材料横向拉伸弹塑性力学响应的计算和预测。复合材料横向拉伸时先后发生界面损伤、界面失效以及基体损伤累积与失效现象,界面损伤脱粘并诱发基体塑性损伤和失效是导致复合材料横向断裂的主要机理。增加界面强度有利于提高横向拉伸屈服强度和极限强度,界面刚度对极限强度影响不大,但增加界面刚度可有效提高复合材料横向拉伸弹性模量。     

循环载荷幅值对7075-T7451铝合金疲劳裂纹扩展行为的影响

在不同幅值循环载荷条件下对7075-T7451铝合金紧凑拉伸(CT)试样进行拉伸疲劳试验,对其疲劳裂纹扩展速率和应力强度因子幅值ΔK进行了研究,并用扫描电子显微镜观测试样的断口形貌。结果表明：随着循环载荷幅值的增大,试样的疲劳寿命缩短,裂纹的扩展速率增大;试样宏观断口形貌的裂纹稳态扩展区域减小,而瞬时断裂区域增大。稳态扩展区主要以疲劳条带扩展机制为主,且疲劳条带间距随循环载荷幅值的增大而增大;瞬断区的断口形貌以韧窝断裂为主,韧窝尺寸随循环载荷幅值的增大而减小。     

不同工艺对FGH95合金力学性能的影响

通过在零件上取小试样进行拉伸及拉-拉疲劳性能测试,分析研究热等静压+挤压+等温锻（EX）及直接热等静压（HIP）两种工艺在模拟服役条件下对FGH95合金力学性能的影响。结果表明,EX试样抗拉强度的稳定性高于HIP试样。从线性回归应力寿命（S-N）曲线中可以得到,在给定的循环载荷条件下,EX试样失效前承受的循环次数更多,分散性更小。断口断裂特征表明,EX工艺条件下合金以塑性断裂机制为主,而HIP工艺条件下以脆性断裂机制为主。     

热轧制镁基叠层复合材料的力学性能

采用热轧制备7075Al/MGY/7075Al叠层复合材料,材料的拉伸强度达300MPa。基于经典叠层板理论计算该材料的首层失效强度,其计算结果与试验结果吻合较好,表明经典叠层板理论可用于预测轧制制备的叠层复合材料首层失效强度。结果表明:随着铝合金厚度的增加,镁基叠层复合材料的拉伸强度、弯曲刚度、压缩强度和弯曲比强度增加,弯曲比刚度和拉伸比强度先增加后降低,且7075Al/MGY/7075Al、7075Al/AZ31/7075Al和3003Al/MGY/3003Al叠层复合材料的拉伸比刚度增加,但3003Al/AZ31/3003Al叠层复合材料的拉伸比刚度逐渐减小。     

缝合复合材料层合板疲劳寿命预测

复合材料层合板疲劳损伤机理和寿命预测是关系到现代航空结构损伤容限设计的关键问题。通过对复合材料层合板疲劳损伤特征研究,从应变等效性出发,结合经典刚度降法,建立层合板疲劳寿命预测两阶段宏观唯象模型,弥补了经典刚度降法和S-N曲线模型的不足。应用此模型对新型的缝合复合材料层合板进行了相关分析与研究,并将预测结果与试验结果进行了对比。结果表明,所建立的疲劳寿命预测模型结果与试验结果吻合良好,可为缝合复合材料的失效分析与工程应用奠定一定的理论基础。     

变幅载荷下316N不锈钢的力学性能

为了分析一批承受50万周次拉伸变幅载荷Type 316N不锈钢螺栓材料的力学性能变化,对该批中未失效螺栓进行室温拉伸、冲击和疲劳寿命测试,对已断裂失效螺栓进行组织和位错观察。结果表明:在循环载荷作用下,材料内部产生高密度的位错,并形成驻留滑移带,从而使服役后的螺栓材料屈强比上升,塑性下降。对未失效螺栓进行疲劳寿命试验,发现该批螺栓服役寿命仅为恒幅载荷条件下设计寿命的1/2左右,这是由于变幅载荷与恒幅载荷下材料的力学行为存在较大差异,从而导致其与按恒幅载荷设计的疲劳寿命出现偏差。     

缺口对TC17钛合金拉伸性能和低周疲劳性能的影响

研究了不同缺口(缺口应力集中系数)对TC17钛合金拉伸性能和低周疲劳性能的影响。结果表明,缺口试样抗拉强度随着缺口应力集中系数的增大而提高,伸长率随着缺口应力集中系数的增大而减小;低周疲劳寿命随缺口应力集中系数的增加而降低,当应力集中系数大于1.92时,疲劳寿命急剧下降。断口分析表明,试样拉伸断口由正应力拉断区和剪切断裂区两个典型区域构成。正应力拉断区为穿晶和沿晶相混合的断裂,剪切断裂区为穿晶断裂,断口上均为韧窝形貌。低周疲劳断口由疲劳断裂区、正应力拉断区和剪切断裂区三个典型区域构成。疲劳断裂区比较平坦,有明显的疲劳条带和平行的二次裂纹;低周疲劳断口的正应力拉断区和剪切断裂区的形貌特征与拉伸断口类似。     

SiCf/TC17复合材料断裂机理

利用扫描电镜和光学显微镜研究了C涂层SiC_f/TC17复合材料室温拉伸、高温拉伸(400℃)和高温疲劳(400℃)的断裂机理。结果表明,室温拉伸和高温拉伸断裂机理相似,均为界面脱粘和纤维拔出。室温拉伸断口由高低起伏的大断面组成。高温拉伸断口由众多的小断面组成,纤维拔出高度和界面脱粘程度比室温更剧烈。复合材料、包套和复合材料与包套结合区的组织分别为双态组织、网篮组织和粗大的双态组织,其疲劳性能依次递减,因此高温疲劳断裂中主裂纹萌生于包套与复合材料结合区,呈脆性断裂。主裂纹首先向包套扩展,随后向复合材料扩展,引起界面脱粘和纤维拔出。     

喷丸成形弹坑尺寸对2324铝合金疲劳性能的影响

定性和定量分析喷丸成形弹坑尺寸对 2324 铝合金疲劳性能的影响,研究改善喷丸成形零件疲劳性能的工艺方法。 采用白光干涉仪测试不同喷丸成形工艺下的弹坑尺寸,测试分析其对应的轴向拉伸疲劳寿命。 结果表明:喷丸能量越大,则弹丸深度(H)越大、弹坑直径(R)也越大,喷丸成形后再进行喷丸强化处理,则弹坑直径增大 19%,弹坑深度减小 58%。 喷丸成形引入深弹坑使疲劳裂纹源过早的形成和发展,造成预拉伸处理的喷丸成形上限试样疲劳寿命降低 61%,成形上限+喷丸强化试样疲劳寿命比仅喷丸成形试样提高 140%。 H/ R 值与机械加工状态试样平均疲劳寿命(L0 ) / 喷丸状态试样平均疲劳寿命(L)值呈现较好地线性关系,说明 H/ R 值可较好的表征喷丸成形造成的表面应力集中程度。     

23CrNi3Mo钢钎具断裂失效分析

采用扫描电镜和光学显微镜观察了不同类型23CrNi3Mo钢钎具疲劳破坏缺陷试样断裂形貌及显微组织,分析讨论了缺陷试样失效的原因。结果表明,高疲劳寿命的阿特拉斯钎尾缺陷试样在疲劳源裂纹萌生、扩展过程中,塑性变形特征明显,表明其具有良好的抵抗疲劳裂纹扩展能力,而国内潜孔钻头试样脆性断裂特征明显。两种缺陷试样表面均为高硬度渗碳马氏体层,但阿特拉斯钎尾缺陷试样渗碳层与基体组织的下贝氏体过渡区域较宽,基体以贝氏体组织为主,硬度梯度过渡平缓,具有良好的强度与韧性匹配;而国内潜孔钻头试样的过渡区域较窄,基体是马氏体组织。可见合理的硬度梯度分布对钎具钢强韧性匹配和疲劳寿命影响很大。     

碳纤维增强复合材料双面埋头压铆模拟分析

铆接技术作为一种不可拆卸的连接方式,在飞机装配过程中被广泛使用，同时碳纤维增强复合材料由于自身优异的力学性能也成为了航空工业中热门。在碳纤维增强复合材料铆接的过程中,铆钉的变形会对铆接的碳纤维增强复合材料造成损伤,从而影响铆接的质量以及其使用寿命。针对这一问题，使用有限元仿真软件ABAQUS创建复合材料双面埋头压铆三维模型，通过ABAQUS子程序接口编写VUMAT用户材料自定义程序，分别取Hashin准则和Puck准则的优势作为三维失效准则，选用合理的刚度退化方式对压铆后的复合材料的渐进损伤进行分析。通过ABAQUS对复合材料压铆过程模拟，得到铆钉在压铆过程中的成形规律和复合材料层合板的主要损伤失效形式为基体的压缩和拉伸、纤维压缩以及其损伤失效的扩展方向。     

基于动态材料模型的材料热加工工艺优化方法

基于动态材料模型理论的加工图技术被广泛用于设计和优化材料热加工工艺中,以实现微观组织和性能的控制。综述动态材料模型的相关理论,介绍基于动态材料模型的稳定变形区和失稳变形区的各种判据及其物理含义,比较和分析各种判据的应用情况及其有效性,对判据在某些情况下预测失败的原因进行分析,指出利用各种判据优化材料热加工工艺时的选用原则。     

Fatigue crack initiation and multiplication of unnotched titanium matrix composites

Fatigue crack initiation and multiplication of the unnotched SCS-6 silicon carbide fiber-reinforced titanium matrix composites with different matrix and interfacial properties have been investigated experimentally and analytically. Ti–15V–3Al, Ti–6Al–4V, and Ti–22Al–23Nb were chosen as matrix materials. The initiation and propagation of each individual matrix crack as a function of fatigue cycles and applied stress levels were carefully monitored. The statistical distribution of crack growth rates in each composite has been constructed and analyzed. The evolution of normalized matrix crack density and stiffness reduction of these composites under fatigue loading also has been characterized. A modified shear-lag model, coupled with the strain-life equation and a fiber bridging model were used to predict the fatigue crack initiation life, matrix crack growth rate, normalized matrix crack density, and residual stiffness of the composites. The predicted fatigue properties correlated well with experimental results.     

Ultrasonic measurements of stiffness in thermal-mechanically fatigued IM7/5260 composites

In recent years, ultrasonic methods have been developed that can measure the mechanical stiffness of composites. The Lamb wave velocity is directly related to the material parameters, so an effective method exists to ascertain the stiffness of composites by measuring the velocity of these waves. In this study, a Lamb wave measurement system was used to measure the bending and out-of-plane stiffness coefficients of thermoset composite laminates undergoing thermal-mechanical loading. A series of 16 ply and 32 ply composite laminates were subjected to thermal-mechanical fatigue (TMF) in load frames equipped with special environmental chambers. The composite system studied was a graphite fiber-reinforced bismaleimide thermoset, IM7/5260. The samples were subjected to both high and low temperature profiles as well as high-strain and low-strain profiles. The bending and out-of-plane stiffnesses for composite samples that have undergone over 6000 cycles of combined thermal and mechanical fatigue are reported. The Lamb wave generated elastic stiffness results have shown decreases of up to 64% at 4706 cycles for samples subjected to TMF at high temperatures and less than a 10% decrease at over 6000 cycles for samples subjected to TMF at low temperatures.     

Interface effects on crack deflection and bridging during fatigue crack growth of titanium matrix composites

The effect of the interface on the crack deflection and crack bridging behavior of continuous fiber-reinforced titanium matrix composites has been investigated using three interfaces with significantly different mechanical characteristics. Each of these composites exhibited stress ranges in which fiber bridging was present and stress ranges in which stable fiber bridging was not present. The fatigue crack growth rate for all composites, even for the ones that did not exhibit fiber bridging was significantly below that of the matrix. This phenomenon, believed to be an effect of elastic crack shielding, was most significant for composites with the strongest interfacial bond. Interface failure ahead of the crack tip and its influence on the local stress intensity factor is believed to be responsible for the decrease in the shielding effect of low strength interfaces. Interface debonding was observed in all three composites, and damage to the interface ahead of the crack tip was seen in two of the three composites. A stress-based criterion for predicting debonding appeared to effectively explain the crack deflection behavior for the three composites. Evidence of crack deflection even for the strongest interface suggests that there is scope to increase the interface bond strength in SiC/Ti-alloy system for improved transverse properties without compromising the fatigue life.     

短纤维增强复合材料（MMC）的蠕变损伤失效的微观数值模拟

研究了短纤维增强复合材料（MMC）的蠕变损伤和失效的细观机理.采用微胞模型结合有限元软件ABAQUS进行.基体的损伤失效由K-R模型描述,纤维断裂由应力准则控制,它们已分别作为用户子程序编入到ABAQUS中.有限元分析表明,在纤维断裂前损伤主要位于纤维头顶角附近,断裂后集中在纤维断裂面附近.纤维断裂对基体蠕变损伤影响有限,但是断面附近产生很高的应力集中,该应力集中将有助于裂纹在基体中的发展.     

Damage Initiation and Growth in Fiber-Reinforced MMCs

Damage in continuous-fiber reinforced metal-matrix composites can be quite complex because of the number of constituents (e.g., fiber, matrix or fiber/matrix interface) that can fail. Multidirectional lay-ups have an even greater number of possible damage orientations. Based on the simplifying assumption of equivalent constituent strain states in the absence of damage, a strain-based failure criterion may be applied to determine when and where initial damage will occur. Depending on the relative fatigue behavior of the fiber and matrix, as well as the interface properties, the failure modes can be grouped into four categories.     

Fatigue testing welded joints for P/M Al-matrix composites

To meet their need for high stiffness, many bicycles currently rely on aluminum-matrix composites. The highest strength and highest stiffness form of these material, 6092/B4C/15p, is produced by powder metallurgy techniques. The bicycles are tungsten inert-gas welded and fully heat treated after welding. A test technique has been developed to assess the static and fatigue properties of welded joints. This test technique has revealed the increased stiffness and strength of a 6092/B4C/15p-T6P composite tube joint as compared with a 6061-T6 aluminum one. This test technique is currently being used to identify fatigue properties for both tube joints.     

微连接接头在热疲劳过程中的破坏规律

同时采用电阻测量方法和疲劳裂纹观测方法分析表面贴装结构焊点的热疲劳破坏过程.通过研究焊点在热疲劳过程中的电阻值变化和热疲劳裂纹的扩展过程,来比较传统锡铅钎料焊点和无铅钎料SAC305焊点的热疲劳破坏规律.并结合有限元分析,研究焊点在热疲劳过程中的电阻值变化与裂纹扩展之间的关系,从而获得一种工程上较为实用的焊点热疲劳失效的电阻值经验判据.结果表明,SAC305无铅钎料焊点相对于传统锡铅钎料焊点而言,具有较为优异的抗热疲劳性能.根据试验研究和有限元分析结果建立了焊点在热疲劳过程中发生失效的电阻值经验判据.

Abstract：

The failure process of soldered joint in SMT was investigated by electrical resistance measurement method and crack observation method. The characteristics of electrical resistance value variation of lead-tin and lead-free soldered (SA(305) joints during the thermal fatigue test were obtained. And at the same time the crack propagation in soldered joint was observed. According to these measurements, the failure roles of lead-tin and lead-free soldered joint were compared. The relationship between electrical resistance value variation and crack propagation of soldered joint during thermal fatigue test was studied by FEM, and an empirical criterion to estimate the failure of the soldered joint in the thermal fatigue test was obtained based on electrical resistance value variation. The experimental results show that the lead-free soldered joint has a higher resistibility in therrnal fatigue than the traditional lead-tin soldered joint. The criterion based on electrical resistance value variation was founded from the experimental and simulation results.