Electrical Resistance Decrease Due to Grain Coarsening Under Cyclic Deformation

Electrical resistance of polymer-supported thin metal films subjected to cyclic tensile loading typically is expected to grow with the cycle number due to fatigue-induced damage. Here it is demonstrated that electrical resistance can also decrease with the cycle number. Using electron backscatter diffraction analysis, it is shown that significant strain-induced, room-temperature grain coarsening is responsible for the resistance decrease. Grain coarsening has a vital effect on the electrical stability of metal films in a low-cycle fatigue regime.     

Three-dimensional fatigue crack growth behavior in an aluminum alloy investigated with in situ high-resolution synchrotron X-ray microtomography

The fatigue crack propagation process in an Al–Mg–Si alloy was investigated using in situ high-resolution synchrotron radiation X-ray microtomography. Tomography datasets were obtained at periodic intervals throughout the 120,000 fatigue cycles. Three-dimensional rendering of the through-thickness crack shape indicates that in a number of regions the adjacent sides of two branched cracks tend to overlap with fatigue cycling and form a crack overlapping region. Measured crack growth rates in each tomographic slice show that crack growth retardation generally occurs in these crack overlapping regions. The through-thickness variation in crack tip opening displacement was also measured and was used to account for the observed crack propagation behavior. Crack morphologies were observed at different load levels in a fatigue cycle. The crack closure level varied for two selected regions comprising different overlapping cracks. The correlation of the crack growth rate with both crack opening and closure levels was discussed and interpreted.     

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.     

一种等应力试样中的疲劳短裂纹行为

设计了一种三角形试样,当施加弯曲载荷时,该试样等腰三角形段内的表面拉应力为常数。采用振动试验机对所设计的试样进行疲劳试验,以研究一种焊接金属表面疲劳短裂纹萌生和发展特征。结果表明,疲劳短裂纹起源于铁素体晶内的滑移带。随疲劳周次增加,短裂纹发展以裂纹密度不断增加为主要特征。短裂纹的汇合造成裂纹发展且裂纹路径以穿晶为主。     

ZL101铝合金低周疲劳行为研究

研究了ZL101-T6铝合金的拉压低周疲劳行为,并重点分析了疲劳裂纹在材料中萌生与扩展的过程。通过扫描电镜和金相显微镜的观察分析表明:共晶硅相的断裂是材料低周疲劳裂纹萌生的主要方式,硅颗粒断裂的数量随疲劳循环周次的增加而增加,应变幅值的增加加快了硅颗粒的断裂速率,使疲劳裂纹的萌生速率加快;疲劳裂纹在循环初期主要通过断裂硅颗粒的互相连接进行扩展,随疲劳裂纹的长大,裂纹可穿过铝基体连接形成主裂纹并导致材料破坏,穿晶断裂为最终断裂的主要特征。     

SHORT FATIGUE CRACK BEHAVIOUR IN ISO-STRESS SPECIMENS

Fatigue tests were made on a vibration machine using specially designed specimens to investi-gate the initiation and propagation features of short fatigue cracks in a weld metal.The spec-imens were triangular in shape so that when loaded as a cantilever beam the surface tensilestress is constant.Test results show that short fatigue cracks mainly originate from slip bandswithin ferrite grains.The development of short fatigue cracks is marked by a gradual increasein crack density with fatigue cycles.Coalescence of short cracks leads to crack propagationand the crack path is predominantly transgranular.     

On the Use of Infrared Thermography for Analysis of Fatigue Damage in Ti6Al4V-Welded Joints

The present work is aimed at comparatively studying fatigue damage evolution of a pulsed Nd:YAG laser beam-welded (LBW) joint and the base metal (BM) of Ti6Al4V alloy subjected to cyclic loading. To reveal crack nucleation and propagation during the fatigue process, in situ fatigue was generated using infrared measurement methods. The results indicate that the rate of damage accumulated in the LBW joint was higher than in the BM specimens during a fatigue test, which decreased the fatigue life of the LBW joint. This observation is attributable to the LBW joint fusion zone microstructure, which has a higher void nucleation and growth rate compared with the BM microstructure.     

Fatigue Crack Nucleation Studies on Sulfuric Acid Anodized 7075-T73 Aluminum

The influence of a sulfuric acid anodic coating process on the fatigue crack nucleation behavior of 7075-T73 aluminum alloy was investigated. Silicone surface replication in combination with carbon sputter coating and scanning electron microscopy (SEM) allowed for in situ monitoring of the number of cycles for crack nucleation. A single edge circular notch (SECN) coupon was designed for the present study to localize fatigue damage thus enhancing fatigue crack detection and capture the effects of multiaxial stress conditions indicative of a majority engineering applications. Linear elastic finite element modeling of the SECN coupon was performed to quantify the von Mises equivalent stress distribution and the stress concentration factor of the notched region. The experimental results indicate that the presence of localized pitting corrosion initiated during the anodic coating pretreatment process had an adverse effect on fatigue performance. Specifically, multiple crack nucleation sites were evident as opposed to a single crack origin for the untreated specimens. Post-cycling SEM surface examinations displayed networks of micro-cracks in the anodic coating emanating from the pits although these were not found to be fatigue crack origin sites during post SEM fractographic exams. Thus, the stress concentration effect of the corrosion pits was found to be predominant. The total cycles to failure on average was reduced by approximately 60% for the anodic coated versus untreated specimens. A strategy is also discussed on how to mitigate accelerated crack nucleation by controlled surface pretreatment and use of a chromated chemical conversion coating in lieu of an anodic coating for selective applications.     

离子渗氮化合物层对H13钢热疲劳性能的影响

采用Uddeholm热疲劳试验方法，用机械磨去法和在等离子体气氛内加氩的方法除去化合物层，并与有化合物层的热疲劳试样进行对比，以研究化合物层对H13钢热疲劳性能的影响。研究结果表明，化合物层所具有的高硬度和高强度，能使其推迟热疲劳裂纹的萌生，并在一定程度上阻止热裂纹向基体内部扩展，但有化合物层试样的热裂纹在表面扩展较快，经3000周热循环后，观察到横断面的热裂纹宽度较大、数量较多。     

基于原位SEM的激光-MIG复合焊接7075-T6铝合金疲劳裂纹扩展行为

通过扫描电镜原位观察激光复合焊接头各区裂纹的扩展行为. 结果表明,焊缝各区组织的不同使得疲劳裂纹扩展行为发生明显改变,疲劳裂纹位于焊缝中心时,裂纹总体沿着垂直于载荷主轴的方向扩展;疲劳裂纹位于热影响区时,裂纹大致成"Z"字型路径进行扩展;疲劳裂纹在焊缝中心和热影响区扩展时都存在二次裂纹;疲劳裂纹在母材区扩展时,呈现出单一和典型的裂纹扩展模式. 此外,通过原位SEM观察获得不同循环周期下的裂纹扩展长度,进而推算得到7075-T6铝合金接头各区内疲劳裂纹扩展速率的Paris公式.     

（F＋M＋A）三相钢低周疲劳的扫描电镜动态研究SCIEI

用扫描电子显微镜疲劳加载台进行动态原位观察，研究了奥氏体（Ａ）在（Ｆ＋Ｍ＋Ａ）三相钢低周疲劳裂纹萌生和扩展过程中的作用．结果表明，在低周疲劳裂纹的萌生和早期扩展阶段，三相钢中的奥氏体会降低疲劳缺口敏感性，延长裂纹萌生期，使裂纹路径弯折并降低扩展速率．在高应变疲劳阶段，因奥氏体应变诱发相变生成马氏体，其边界是裂纹扩展的低能通道，从而加速裂纹的扩展．并提出了一个能量判据：当裂尖扩展功小于Ａ→Ｍ转变的能量时，奥氏体会提高材料的疲劳抗力，反之奥氏体会加速疲劳裂纹的萌生和扩展．     

GJW35钢结硬质合金热疲劳裂纹萌生初步探讨

GJW35钢结硬质合金是一种用于热作工模具的新材料。对该合金的热疲劳裂纹萌生全过程进行了观察,结果表明,在裂纹孕育期间,试样缺口边缘上出现凹坑,且随着循环次数的增加,凹坑数目增多,尺寸增大,热疲劳裂纹在凹坑底部萌生。在缺口前沿区内的微孔也随着循环次数的增加而增多、增大;最终,微孔连结形成裂纹。     

Effects of bending fatigue on the electrical resistance in metallic films on flexible substrates

The increase of electrical resistance during the strain-controlled bending fatigue of 2 μm-thick inkjet-printed or vacuum deposited metallic films (Cu, Ag) on flexible substrates (BT: Bismaleimide Triazine, PI: Polyimide) was investigated. Electrical resistance increased with an increase in the number of fatigue cycles. The rate of increase in the electrical resistance of inkjet-printed Cu films was lower than that of thermally evaporated films. This phenomenon is attributable to the porous microstructure of inkjet-printed Cu films. The porous structure contains a lot of free volume and a large area of free surface, which can be a sinking site for vacancies formed during the cyclic deformation. It was confirmed that a smaller grain size leads to a lower rate of increase in the electrical resistance, which was ascribed to the easy vacancy annihilation due to a short diffusion length of the vacancy to the grain boundary which is a vacancy sinking site. The rate of increase in the electrical resistance was also influenced by the grain boundary geometry. The lower rate of the evaporated Ag film on a BT substrate was attributed to the crack-like grain boundaries, which were expected to behave like pores.     

循环条件对金属陶瓷热冲击疲劳的影响

研究了循环条件对40％Ni－Ti（C,N）金属陶瓷热冲击疲劳的影响。结果表明,循环条件对金属陶瓷热冲击疲劳有很大影响。随循环温度和冷却速率的增加,裂纹形核的孕育期缩短,裂纹扩展速率增加。扫描电镜观察发现,断口形貌中存在疲劳条纹。     

Fatigue crack growth under a constant amplitude loading of Al-6061-T6 welds obtained by modified indirect electric arc technique

Abstract

The characteristics of the fatigue crack growth in the base metal, weld metal and heat affected zone (HAZ) were quantified by testing compact type specimens of 6061-T6 welds obtained by the modified indirect electric arc technique. The fatigue crack growth depends on the microstructure imposed by the welding thermal cycle and it was observed that in the HAZ the crack growth rate is lower than that in the weld metal, but higher than that in the base metal. Microhardness maps revealed that this behaviour is due to the formation of a larger plastic zone around of the crack tip produced by loss of hardening. A comparison of fatigue crack growth of weld metal and HAZ for modified indirect electric arc and friction stir welding shows that the weld metal produced by friction stir welding exhibits better resistance to crack propagation, but both processes behaved similarly in the HAZ.     

石墨形态及合金元素对铸铁抗热疲劳性能的影响

对石墨形态、合金元素等对铸铁抗热疲劳性能的影响进行了研究。加入提高抗氧化性的合金元素可显著提高铸铁的抗热疲劳性能。从首次产生裂纹的循环次数、裂纹扩展速度及变形量等综合考虑,蠕铁的抗热疲劳性能最好。     

Damage of the unalloyed-plutonium structure as a result of phase naklep and self-irradiation: An electrical-resistance study

Damage of the structure of the α phase of plutonium as a result of repeated α ? β phase recrystallization and prolonged self-irradiation was estimated based on the changes in the electrical resistance measured at room temperature before and after phase recrystallization. Samples with different time of storage were used. An analysis of the results showed that the electrical resistance of samples grows linearly depending on the number of cycles of phase recrystallization. It has been established that the slope of the dependences obtained is determined by the maximum temperature of the sample in the cycle. At a maximum temperature in the cycle equal to 170°C, an increase in the relative electrical resistance of the samples was ～2.5%/cycle; when heating to 125°C, ～0.3%/cycle. The variation of the electrical resistance of the samples stored for ～20 years depending on the number of cycles of phase recrystallization exhibited a single feature, namely, a decrease in resistance in the first cycle of heating to 170°C rather than an increase observed in the experiments with the samples stored for ～2.5 year. This decrease is connected with the influence of the possible beginning of migration of radiogenic helium. In the subsequent cycles, the change in the electrical resistance of samples upon prolonged storage was analogous to that of the samples with a short time of storage.     

纳米晶Ni疲劳行为的实验研究

系统研究了纳米晶Ni与粗晶Ni的疲劳行为. 通过疲劳实验获得了这2种材料的疲劳应力--寿命曲线, 并采用AFM对纳米晶Ni样品表面进行观察以研究其裂纹萌生的微观机制, 利用纳米压痕仪对疲劳实验前后样品的力学性能和显微组织变化进行了研究. 结果表明, 纳米晶Ni具有比粗晶Ni更高的疲劳极限. AFM观察表明,纳米晶疲劳后样品表面出现平均尺寸为73 nm的胞状起伏, 疲劳后样品的晶粒尺寸未发生明显改变. 压痕硬度结果表明, 疲劳过程材料的力学性能也未发生明显变化.     

拉保持对316L奥氏体不锈钢焊接接头低周疲劳循环应力响应及损伤机制的影响

在873 K温度下对316L奥氏体不锈钢母材和焊缝分别进行连续和拉保持低周疲劳试验,使用光学显微镜(OM)和透射电镜(TEM)对试样位错微观结构和裂纹扩展形貌进行观察,分析拉保持对母材和焊缝低周疲劳循环应力响应及损伤机制的影响.结果表明,母材在连续和拉保持低周疲劳试验初期都发生循环硬化,焊缝在连续低周疲劳试验中发生连续循环软化,而在拉保持试验后期则出现明显循环稳定阶段;母材和焊缝试样拉保持低周疲劳寿命低于其连续低周疲劳寿命;母材和焊缝在连续低周疲劳试验中裂纹主要以穿晶方式扩展,而在拉保持试验中焊缝是以沿晶或混合方式扩展.     

轧辊堆焊金属的耐热疲劳性能

通过设计的热疲劳试验方法,对三种自制药芯焊丝和一种国产堆焊轧辊用实心焊丝的耐热疲劳性能及其影响因素进行了深入研究,同时对堆焊金属中热疲劳裂纹形成以及扩展机理做了进一步探讨,研究表明,堆焊金属中热疲劳裂纹的形成以及扩展是循环应力和氧化气氛共同作用的结果,堆焊金属的抗氧化性越高,抗启裂能力越强,在此基础上,得出了热疲劳裂纹形成以及扩展的机理模型图,另外,研究结果还表明,堆焊金属的组织以及组织中存在的夹杂物对耐热疲劳性能也有很大影响,均一的组织有利于提高其耐热疲劳性能,而夹杂物则促使热疲劳裂纹形成、扩展、分叉。