Deformation analysis of lap-shear testing of solder joints

A combined numerical and experimental study of lap-shear testing of solder joints was carried out. The primary objective was to provide a quantitative evaluation of shear deformation experienced by the solder. The numerical analysis was based on finite element modeling of deformation in the substrate–solder assembly. The calculated shear response, utilizing both the commonly adopted far-field measurements and the actual shear strain in solder, was found to differ significantly. The geometric and material parameters during lap shear were explored to provide physical insight into the problem. The nature of deformation of the substrate was seen to greatly influence the shear behavior of the solder. The “transmission” of shear strain into the solder is very ineffective when the solder is in the elastic state, resulting in smaller shear strains in the solder than the nominal values. This effect is also true even when the solder is deforming plastically. Experimental measurements of solder shear using direct optical measurements confirmed the numerical findings. Subtraction of the average deformation of the Cu substrate provides a reasonable, albeit not complete, approximation of the solder strain.     

Strain analysis in machining using metallographic methods

Strain patterns in 4340 steel chips created during turning at speeds between 36 and 440 m/min (119 and 1442 sfpm) are examined metallographically. A new method, based on the grain elongation orientation in the chip, is developed for quantification of local strains created during plane strain deformation in the primary and secondary deformation zones. This method is applied to both continuous and to sawtooth type chips. Strains determined by this method compare well with strains computed from chip thickness measurements, but there appear to be systematic differences. Strain localization is evident in chips formed at the higher cutting speeds. This strain localization does not appear to extend to the rake face side of the chip. The shear angle calculated from the strain in the strain-localized regions compares favorably with the orientation of these regions.     

Assessment of Fatigue Resistance of Aluminide Layers on MAR 247 Nickel Super Alloy with Full-Field Optical Strain Measurements

This work presents the results of fatigue tests of MAR 247 alloy flat specimens with aluminides layers of 20 or 40 µm thickness obtained in CVD process. Fatigue test was conducted at amplitude equal to half of maximum load and ranging between 300 and 650 MPa (stress asymmetry ratio R = 0, frequency f = 20 Hz). Additionally, 4 of the tests, characterized by the highest amplitude, were accompanied with non-contact strain field measurements by means of electronic speckle pattern interferometry and digital image correlation. Results of these measurements allowed to localize the areas of deformation concentration identified as the damage points of the surface layer or advanced crack presence in core material. Identification and observation of the development of deformation in localization areas allowed to assess fatigue-related phenomena in both layer and substrate materials.     

Critical Deposition Condition of CoNiCrAlY Cold Spray Based on Particle Deformation Behavior

Previous research has demonstrated deposition of MCrAlY coating via the cold spray process; however, the deposition mechanism of cold spraying has not been clearly explained—only empirically described by impact velocity. The purpose of this study was to elucidate the critical deposit condition. Microscale experimental measurements of individual particle deposit dimensions were incorporated with numerical simulation to investigate particle deformation behavior. Dimensional parameters were determined from scanning electron microscopy analysis of focused ion beam-fabricated cross sections of deposited particles to describe the deposition threshold. From Johnson-Cook finite element method simulation results, there is a direct correlation between the dimensional parameters and the impact velocity. Therefore, the critical velocity can describe the deposition threshold. Moreover, the maximum equivalent plastic strain is also strongly dependent on the impact velocity. Thus, the threshold condition required for particle deposition can instead be represented by the equivalent plastic strain of the particle and substrate. For particle-substrate combinations of similar materials, the substrate is more difficult to deform. Thus, this study establishes that the dominant factor of particle deposition in the cold spray process is the maximum equivalent plastic strain of the substrate, which occurs during impact and deformation.     

Multilayer thin films/substrate system with variable film thickness subjected to non-uniform misfit strains

Current methodologies used to infer thin-film stress from curvature measurements are strictly restricted to stress and curvature states that are assumed to remain uniform over the entire film/substrate system. These methodologies have recently been extended to a single thin film of non-uniform thickness deposited on a substrate and subjected to the non-uniform misfit strain. Such methodologies are extended to multilayer thin films of non-uniform thickness deposited on a substrate in the present study. Each thin film may have its own non-uniform misfit strain and non-uniform thickness. We obtain the film stresses and system curvatures in terms of the misfit strains and thickness in thin films. We derive the film stresses and interface shear stresses in terms of system curvatures and film thicknesses. They all feature a “non-local” dependence on curvatures, which make full-field measurement a necessity for the experimental inference of such stresses.     

Effects of constitutive parameters on adiabatic shear localization for ductile metal based on JOHNSON-COOK and gradient plasticity models

1Introduction Adiabatic shear band(ASB)is a very narrow zone with a high concentration of shear strain.It is believed that ASB is formed by a process of thermo-mechanical instability.ASB can be observed in the process of dynamic deformation of various fer…     

A mechanism governing oxidation-assisted low-cycle fatigue of superalloys

A model capable of characterizing oxidation-assisted low-cycle fatigue is described. It involves the following steps. After a few strain cycles, because of creep, a tensile stress develops during the de-straining phase of the cycle. This stress opens cracks present in the material and exposes the surfaces to the atmosphere, causing thermally grown oxide (TGO) growth. Dilatation takes place upon converting the alloy to oxide, with an associated strain rate that induces a compressive growth stress. Thereafter, during the re-straining phase of the cycle, transverse extension of the substrate induces in-plane tension in the TGO, which “pushes” the TGO into the substrate along the crack front. Finite element simulations of this process have been presented that predict crack growth per cycle, da/dN, comparable with experimental measurements. Trends in da/dN with the TGO dilatation rate and the creep strength of the superalloy have been elucidated.     

Residual Strain and Fracture Response of Al2O3 Coatings Deposited via APS and HVOF Techniques

The aim of this investigation was to nondestructively evaluate the residual stress profile in two commercially available alumina/substrate coating systems and relate residual stress changes with the fracture response. Neutron diffraction, due to its high penetration depth, was used to measure residual strain in conventional air plasma-sprayed (APS) and finer powder high velocity oxy-fuel (HVOF (θ-gun))-sprayed Al₂O₃ coating/substrate systems. The purpose of this comparison was to ascertain if finer powder Al₂O₃ coatings deposited via θ-gun can provide improved residual stress and fracture response in comparison to conventional APS coatings. To obtain a through thickness residual strain profile with high resolution, a partially submerged beam was used for measurements near the coating surface, and a beam submerged in the coating and substrate materials near the coating-substrate interface. By using the fast vertical scanning method, with careful leveling of the specimen using theodolites, the coating surface and the coating/substrate interface were located with an accuracy of about 50 μm. The results show that the through thickness residual strain in the APS coating was mainly tensile, whereas the HVOF coating had both compressive and tensile residual strains. Further analysis interlinking Vickers indentation fracture behavior using acoustic emission (AE) was conducted. The microstructural differences along with the nature and magnitude of the residual strain fields had a direct effect on the fracture response of the two coatings during the indentation process.     

A sintering model for plasma-sprayed zirconia thermal barrier coatings. Part II: Coatings bonded to a rigid substrate

The sintering model described in Part I, which relates to free-standing plasma-sprayed thermal barrier coatings, is extended here to the case of a coating attached to a rigid substrate. Through-thickness shrinkage measurements have been carried out for coatings attached to zirconia substrates, and these experimental data are compared with model predictions. The model is then used to explore the influence of the substrate material (zirconia vs. a nickel superalloy), and of the in-plane coating stiffness. Both differential thermal expansion stresses and tensile stresses arising from the constraint imposed on in-plane shrinkage can be relaxed via two diffusional mechanisms: Coble creep and microcrack opening. This relaxation allows progression towards densification, although the process is somewhat inhibited, compared with the case of a free-standing coating. Comparison of the stored elastic strain energy with the critical strain energy release rate for interfacial cracking allows estimates to be made of whether debonding is energetically favoured.¹     

The effect of employed loading mode on the mechanical cyclic stabilization of NiTi shape memory alloys

One of the effective parameters on the characteristic response of the nickel-titanium (NiTi) shape memory alloy (SMA) is the employed loading mode during the uniaxial cycling. In this study, the uniaxial tensile tests are conducted under an extended number of mechanical cycles in order to investigate the effect of force control on the pseudoelastic response of the NiTi SMA. The results which are obtained under the force control are compared with the mixed (force/displacement) control counterpart. An evolutionary stress-strain pattern is observed during the 100 cycles of mechanical loading. Additionally, it is noticed that the residual strain and mechanical hysteresis area also show an evolutionary pattern. The observed evolutionary patterns are delineated in three regimes: (i) early evolution; (ii) approach to a nearly stable stage (transient); and (iii) stabilization stage. Furthermore, the in-situ digital image correlation (DIC) is applied to receive the meso-/micro-scale full-field strain measurements. Severe strain localization is observed under force-controlled tensile test during meso-,/micro-scale DIC analysis.     

石墨烯层间摩擦的面内局部应变调控

目的 揭示面内应变对石墨烯层间摩擦行为的影响.方法 利用分子动力学模拟方法,基于支撑的石墨烯基底与单层六边形石墨烯滑片接触模型,重点考察滑片在加载面内局部拉伸应变的基底表面的滑动摩擦过程.着重分析滑片所受侧向力(即瞬时摩擦力)随滑动距离的变化规律,计算滑片分别与基底的加载应变区和非应变区的层间作用,通过计算层间作用势能、接触原子数、能量耗散等,直观揭示应变对摩擦的影响机制.结果 当滑片在基底的局部应变加载区域滑动接触时,由于滑片与基底界面公度性和层间势能降低,接触原子数减少,导致滑片所受侧向力的振幅比其在基底的非应变区滑动时明显降低.具体来讲,当应变由0增加至10％,处于应变区的滑片所受侧向力由1.33 nN减小至0.86 nN,降幅达35％.通过计算加载均匀应变的基底与滑片层间的摩擦力,表明摩擦力随应变的增加而逐渐减小,且载荷越高、滑片尺寸越大时,摩擦力降幅越明显.结论 局部应变可以有效调控摩擦的分布规律,降低石墨烯层间摩擦力大小.     

Dislocation accumulation and strengthening in Cu thin films

An analysis that addresses the strain-hardening behavior of thin metallic films on substrates is presented. Stress measurements were made on 0.5 μm thick Cu films on Si substrates during thermal cycling, during stress relaxation at room temperature (RT), and after quenching in liquid nitrogen. Significant strengthening was observed in the thermal cycle during cooling. The stress relaxation at RT shows a decrease of the stress from 360 MPa to 290 MPa within 15 months. A theoretical approach to the strengthening phenomenon is made on the basis of the Peach–Koehler dislocation-interaction forces. It shows that adding threading dislocations into a parallel array of dislocations at the film–substrate interface can contribute significantly to the strain hardening of thin films. The calculated strain hardening accounts for a large portion of the observed strengthening.     

On the elastic and creep stress analysis modeling in the oxide scale/metal substrate system due to oxidation growth strain

Considering the case of asymmetric oxidation, new elastic and creep analysis models are developed to elucidate the stress evolutions in an oxide scale/metal substrate system during isothermal oxidation, due to the oxidation growth strain in oxide scale. The theoretical works allow for the experimental inference of growth strain and stresses from the curvature measurements during oxidation. Moreover, they provide ways to explore and identify the main mechanisms for oxidation. Two sets of published experimental data are employed and analyzed by the elastic and creep analytical approaches. A novel simple determination method of the growth parameter is proposed and validated.     

Cr15Mn9Cu2Ni1N不锈钢连铸坯的高温拉伸变形特性

对于Cr15Mn9Cu2Ni1N不锈钢连铸坯,热变形过程中变形局部化的发生会影响其表面质量。从连铸坯的表层及芯部制取小型试样,利用热/力模拟试验机,进行温度950℃～1150℃范围内的拉伸试验。结果发现,随变形温度升高,该钢强度降低而延伸率提高;试样在发生颈缩,即变形局部化之前,要经历均匀变形和扩散颈缩变形,两种变形均使试样变形区获得均匀的宏观变形形貌;而高温拉伸的延伸率主要由扩散颈缩阶段的变形量决定。分析表明,均匀变形阶段主要靠应变强化抑制变形局部化的发生,而扩散颈缩变形阶段应变速率强化起主导作用。随变形温度升高,尤其在温度高于1100℃时,该钢的应变速率强化效应增强,可推迟最终变形局部化的发生,从而获得较大的延伸率。     

Size dependent nanoindentation of a soft film on a hard substrate

Nanoindentation experiments on Al/glass systems show that, as the indentation depth increases, the hardness decreases during a shallow indentation, and increases when the indenter tip approaches the film–substrate interface. We associate the rise in hardness during two stages with the strong strain gradient effects, the first stage is related with the small scale effects and the second stage with the strain gradient between the indenter and the hard substrate. Using the strain gradient theory proposed by Chen and Wang and the classical plasticity theory, the observed nanoindentation behavior is modeled and analyzed by means of the finite element method, and it is found that the classical plasticity cannot explain the experiment results but the strain gradient theory can describe the experiment data at both shallow and deep indentation depths very well. The results prove that both the strain gradient effects and substrate effects exist in the nanoindentation of the film–substrate system.     

Nonlocal approach in evaluating strain localization behaviors of voided ductile materials

Finite element analysis of the strain localization behaviors of a voided ductile material has been performed using a non-local plasticity, in which the yield strength depends on both an equivalent plastic strain measure (hardening parameter) and Laplacian equivalent. The introduction of gradient terms to the yield function was found to play an important role in simulating the strain localization behavior of the voided ductile material. The effect of the mesh size and characteristic length on the strain localization were also investigated. An FEM simulation based on the proposed non-local plasticity revealed that the load-strain curves of the voided ductile material subjected to plane strain tension converges to one curve, regardless of the mesh size. In addition, the results using non-local plasticity also showed that the dependence of the deformation behavior of the material on the mesh size was much less sensitive than with classical local plasticity and could be successfully eliminated through the introduction of a large value for the characteristic length.     

钢中剪切变形局部化的形成与发展

对低碳２０号钢经正火，淬火烽回火处理后在高速扭转过程中变形局部化的发生，发展演化过程以及最后形成的剪切带的微结构特征进行了观察。结果表明，变形局部化敏感于材料的强度、强度越高，越容易出现变形局部化．局部化的产生与发展涉及到一系列相继发生的晶体学和非晶体学的变形现象：析出相－基本间界面发射位错琪成有网络，位错胞形成并沼剪切带方向拉长。     

The influence of serrated flow on necking in tensile specimens

Three-dimensional finite element simulations are used to investigate the role of serrated flow on the strain at the onset of necking in a cylindrical uniaxial tension specimen. The material is idealized using a modified form of the McCormick constitutive equation, which has an additional material parameter that allows the rate of transient aging to be varied without affecting its steady-state response. Stability calculations and direct simulations show that, if the transient response is sufficiently slow, serrated flow can be suppressed, even though the material has negative steady-state strain rate sensitivity. This result is then used to determine the effect of suppressing serrated flow on the strain to localization. We find that negative steady-state sensitivity significantly reduces the strain required to initiate necking failure in a tensile specimen. However, the strain to failure is largely unaffected by the transient response of the material, and suppressing the serrated flow in particular has a negligible effect on the localization strain. We conclude that, while both serrated flow and reduced ductility are observed in materials with negative rate sensitivity, the reduction in ductility is not a direct consequence of serrated flow.     

Deformation behavior ofAZ31 magnesium alloy at different strain rates and temperatures

The deformation behavior of AZ31 was examined by compression and tension testes over a wide strain rate and temperature range, strain rate from 10^-3 to 10^3 s^-1, temperature from 300 to 623 K. Analysis of flow behavior and microstructural observations indicate that in tension tests dislocation glide is the most important deformation mechanism in the test strain rate and temperature range, while in compression tests twinning deformation mechanism is important at lower temperature when the strain rate ranges from 10^-3 to 10 s^-1. At 10^3 s^-1 strain rate, dislocation glide and twinning are present at the same time. At the strain rate of 2 964 s^-1, adiabatic shear band can be found easily, even at the strain rate of 1 537 s^-1 adiabatic shear localization zone can be found. In adiabatic shear localization zone, there are fine recrystallization grains. But in adiabatic shear band, the grains cannot be identified by optical microscopy.     

冲击载荷作用下钨合金材料绝热剪切带形成机理

对钨合金材料在受冲击载荷作用下绝热剪切带形成机理进行了研究,重点考察了受力条件的影响,用分离式Ｈｏｐｋｉｎｓｏｎ压杆装置对阶梯圆柱形、哑铃形和直圆柱形钨合金试件进行了冲击加载,利用光学显微镜对冲击后的试件进行了显微组织观察。用ＡＢＡＱＵＳ大型数值分析软件对试件进行有限元分析,材料模型采用热粘塑性形式的Ｊｏｈｓｎｏｎ－Ｃｏｏｋ模型,实验和数值计算结果表明：受力条件对钨合金材料的变形、损伤（微型纹和绝