Digital image correlation analysis of the deformation behavior of Pb-free solders at intermediate strain rates

Digital image correlation (DIC) is a powerful tool for quantifying local stresses and strains. The demand for environmentally benign Pb-free solders and the push toward smaller portable electronics will make it more likely for solder interconnects to en-counter mechanical shock through dropping or mishandling. Thus, quantifying the strain rate behavior of Pb-free solders from the quasi-static to the shock regime is essential for developing reliable numerical models of the mechanical shock behavior. In this paper we report on the use of DIC to measure the local strain and strain rate occurring in the neck of Sn-3.5Ag-0.7Cu specimens, at the onset of necking. Tensile tests were conducted in the range 10⁻³s⁻¹–30 s⁻¹. A parametric study was conducted to identify the optimum DIC parameters for the experimental setup. The effect of microstructure and applied strain rate on the local values of strain and strain rate is discussed     

Strain and distortion monitoring during arc welding by 3D digital image correlation

A novel three-dimensional (3D) digital image correlation (DIC) approach using a stereovision system was developed to measure the evolution of strain and distortion near the fusion zone during the gas tungsten arc welding process. Unlike the previously reported two-dimensional (2D) DIC approach using a single camera, the 3D DIC method was immune to the out-of-plane displacement and was capable of measuring 3D deformation. Both 2D and 3D DIC approaches in welding applications were based on the utilisation of the novel high-temperature speckle and the pulsed laser illumination plus bandpass filters. However, the speckle pattern was partially specular reflective causing issues in subset matching in the 3D approach. A new algorithm and experimental procedure was incorporated to solve this problem.     

Characterization of Fatigue Fractures in Closed-Cell Aluminum Foam Using x-ray Micro-Computed Tomography

A post-mortem study of Alporas closed-cell aluminum foam specimens previously failed under strain-controlled fully reversed tension-compression fatigue was conducted using x-ray micro-computed tomography (μCT). Volumetric renders of the 3D structure of the material were produced. Fractures were identified and marked throughout voxel-based images of the specimens. This produced a 3D plot of fracture locations. At high strain amplitudes (0.175-0.5%), fractures formed an interconnected planar zone oriented approximately perpendicular to the loading axis; typically, the angle of the plane differed from that of a tension failure. Conversely, at low strain amplitudes (0.05-0.1%), short fractures have been formed diffusely within the specimen. In both cases, observed fractures were tortuous. Our previous work with surface strain mapping via digital image correlation (DIC) suggested that for all strain amplitudes, a crack, evidenced by a zone of high extensile strain, was formed and propagated through the material. This result was confirmed at high strain amplitudes, but not at low strain amplitudes. The discrepancy is attributed to three potential causes. Using DIC, short cracks cannot be accurately resolved with relatively coarse light intensity patterns. DIC images indicate fractures under load, while μCT imaging was conducted under zero load. Finally, the localized extension seen in DIC images could be attributed to strain with no resultant fractures.     

Experimental characterization of the intragranular strain field in columnar ice during transient creep

A digital image correlation (DIC) technique has been adapted to polycrystalline ice specimens in order to characterize the development of strain heterogeneities at an intragranular scale during transient creep deformation (compression tests). Specimens exhibit a columnar microstructure so that plastic deformation is essentially two-dimensional, with few in-depth gradients, and therefore surface DIC analyses are representative of the whole specimen volume. Local misorientations at the intragranular scale were also extracted from microstructure analyses carried out with an automatic texture analyzer before and after deformation. Highly localized strain patterns are evidenced by the DIC technique. Local equivalent strain can reach values as much as an order of magnitude larger than the macroscopic average. The structure of the strain pattern does not evolve with strain in the transient creep regime. Almost no correlation between the measured local strain and the Schmid factor of the slip plane of the underlying grain is observed, highlighting the importance of the mechanical interactions between neighboring grains resulting from the very large viscoplastic anisotropy of ice crystals. Finally, the experimental microstructure was introduced in a full-field fast Fourier transform polycrystal model; simulated strain fields are a good match with experimental ones.     

Identification of Post-necking Tensile Stress–Strain Behavior of Steel Sheet: An Experimental Investigation Using Digital Image Correlation Technique

The stress–strain behavior of sheet metal is commonly evaluated by tensile test. However, the true stress–strain curve is restricted up to uniform elongation of the material. Usually, after the uniform elongation of the material the true stress–strain is obtained by extrapolation. The present work demonstrates a procedure to find out the true tensile stress–strain curve of the steel sheet after necking using digital image correlation (DIC) technique. Hill’s normal anisotropic yield criteria and local strains measured by DIC technique are used to correct the local stress and strain states at the diffuse necked area. The proposed procedure is shown to successfully determine the true tensile stress–strain curve of ferritic and dual-phase steel sheets after necking/uniform elongation.     

基于DIC与Irwin模型的工业纯钛疲劳裂纹尖端循环应变场表征

为了实现工业纯钛TA2疲劳裂纹尖端循环应变场的表征,采用数字图像相关(digital image correlation, DIC)方法并结合Irwin模型,研究了TA2紧凑拉伸试样在多级疲劳载荷下的裂纹扩展规律、循环应变场的实验划分方法以及循环应变环的演化规律。首先通过多级载荷试验获得了TA2紧凑拉伸试样的疲劳裂纹扩展规律,在此基础上结合DIC和Irwin模型,建立了疲劳裂纹尖端循环应变场的实验划分方法,实现了循环塑性区、单调塑性区和弹性区的划分。另一方面,采用DIC获得不同区域的滞回应变环,讨论了不同区域应变环的差异,从而论证了划分方法的有效性,并且揭示了裂纹扩展过程中裂纹尖端应变场从弹性区、单调塑性区到循环塑性区的演化规律。研究工作实现了TA2疲劳裂纹扩展过程中裂纹尖端循环应变场的表征,能够满足疲劳裂纹扩展研究的需要。     

Application of digital image correlation for the study of strain concentrations due to change in geometry in weld beads: parent interface of structural steel welds

The paper aims to characterise the strain concentration of welded junctions subjected to elastic loads using digital image correlation (DIC). When measuring such small strains and heterogeneous strains fields with DIC, a compromise between noise and accuracy arises where a suitable choice of the image processing settings becomes crucial to obtain accurate results. An extensive study of the influence of the different processing parameters was carried out to ensure the best trade-off between accuracy and noise. A practical application was later performed with the found optimal settings; welded specimens were subjected to uniaxial and biaxial loads to assess possible influence of the strain state on the strain concentration. Results show a great variability of the strain concentration along the weld bead base, evidencing a significant influence of local geometrical characteristics. On the other hand, the strain state influence was found to be moderate, or almost inexistent for lower strain concentrations.     

筒形件强力旋压的刚塑性有限元分析

筒形件强力旋压是当代的一种重要生产工艺过程。本文建立了筒形件强力旋压的平面变形力学模型，通过刚塑性有限元分析，获得了正旋工艺和反旋工艺的塑性流动速度场，以及应变和应变速率的分布。塑性流动模型与网格实验结果在趋势上得到较好的吻合。     

An Insight into Lüders Deformation Using Advanced Imaging Techniques

An investigation to explore the feasibility of simultaneous application of infrared thermography (IRT) and digital image correlation (DIC) for analysis of Lüders deformation is carried out. Physical models and proposed concepts explaining the dynamics of deformation localization associated with Lüders band phenomenon addressing band-formation mechanism, inhomogeneity in stress-strain distribution across the band front, and strain localization following band front propagation are successfully correlated with the thermal and strain evolutions obtained using IRT and DIC. The studies revealed the potential of using these techniques simultaneously in providing an enhanced understanding of micro mechanisms involved in Lüders deformation based on associated macroscopic thermal and strain evolutions in a noncontact, nondestructive manner.     

The Portevin–Le Chatelier (PLC) effect and shear band formation in an AA5754 alloy

A non-contact strain measurement technique, based on digital image correlation (DIC) analysis, has been utilized in order to observe Portevin–Le Chatelier (PLC) band behaviour during tensile deformation of AA5754 sheet and subsequently to measure the level of incremental plastic strain carried within the bands. In addition, PLC banding was studied as a function of prior deformation under positive strain rate sensitivity conditions (i.e., at low temperature). Small prestrains (of the order of the Lüders strain) do not affect subsequent room-temperature deformation. However, prestrains of 0.1 or higher change the nature of the PLC bands that form at room temperature. Finally, careful experiments were performed involving low-temperature prestrains sufficient to cause immediate necking when deformation was continued at room temperature. These suggest that shear localization is independent of existing PLC bands.     

Evaluation of the ductile-to-brittle transition temperature of a silicon steel under various strain rate conditions with a servo-hydraulic high speed testing machine

This paper is concerned with the construction of an empirical model of the Ductile-to-Brittle Transition Temperature (DBTT) for 3.4% silicon steel based on tensile test results at strain rates ranging from 0.001 s^?1 to 100 s^?1. Dynamic tensile tests are conducted using an in-house servo hydraulic tensile test machine at strain rates of 1 s^?1, 10 s^?1, and 100 s^?1 and quasi-static tensile tests are conducted using Instron 4206 at strain rates of 0.001 s^?1 and 0.01 s^?1 with an environmental chamber. Fracture elongations are measured by a DIC method during all tests using the high-speed camera for accurate measurement. The DBTT of 3.4% silicon steel is presented in terms of fracture strain with the variation of the temperature and the strain rate. It is demonstrated from the test results that the DBTT increases as the strain rate increases. An empirical model of the DBTT is constructed in terms of strain rate, temperature and fracture elongation. The parameters of the empirical model are calculated from experimental results obtained at various temperatures and strain rates.     

An alternative to the Shoji crack tip strain rate equation

Fundamentally derived crack tip strain rate (CTSR) equations are being sought for use in modelling stress corrosion crack growth. The CTSR equation derived by Shoji and coworkers is gaining prominence in modelling the effects of cold work on stress corrosion cracking of nuclear reactor materials due to an ability to model the effects of yield stress and strain hardening on CTSR. However, the Shoji equation is compromised by assumptions that are inconsistent with the Gao-Hwang (GH) crack tip strain equation that was used in derivation of Shoji’s equation. Moreover, the GH equation appears to have been incorrectly derived. As a result, the Shoji equation cannot be considered fundamentally supported. An alternative to the Shoji equation is developed here by extending the Rice, Drugan and Sham CTSR equation to include strain hardening.     

Stress-induced martensitic phase transformation in thin sheets of Nitinol

Using the in situ optical technique known as digital image correlation (DIC), full-field quantitative strain maps of localization have been experimentally obtained for the first time in thin sheets of Nitinol. The use of DIC provides new information connecting previous observations on the micro- and macro-scales. It shows that the transformation initiates before the formation of localized bands, and the strain inside the bands does not saturate when they nucleate. The effect of rolling texture on the macroscopic stress–strain behavior was observed and it is shown that the resolved stress criterion or Clausius–Clapeyron relation does not hold for polycrystalline Nitinol. Finally, the effect of geometric defects on localization behavior was observed.     

Mechanical behaviour of SSRT specimens optimized for IGSCC concerns

Simulating testing conditions leading to evaluate the intergranular stress corrosion cracking sensitivity of structural alloys is crucial to estimate the lifetime of in-service components. Former studies have pointed out that a simple modification of the design of slow strain rate tensile specimens was particularly convenient for evaluating the susceptibility to intergranular stress corrosion cracking of nickel-base alloys. The aim of the present work is to characterize and model the mechanical behaviour of such specimens. Validation of proposed modelling relies mainly on tensile tests carried on specimens equipped with strain gages. One of the striking results is that, for a given displacement rate of the heads of the specimen, a much slower strain rate can be obtained locally in comparison with the strain rate of an equivalent smooth specimen.     

Modelling of metal flow during hot forging with regard to microstructural aspects

A mathematical model has been proposed for evaluation of velocity and temperature fields in hot forging operations for axisymmetric parts. The model can be applied to determine forging load, strain rate, strain and temperature distributions. In addition, the effects of temperature and strain rate on metal flow have been considered through simultaneous modelling of dynamic phase transformation within the deforming metal. Hot forging experiments have been carried out under different working conditions and the results have been compared with the predictions. A good agreement between the simulated and experimental results was found.     

DC05同材同厚拼焊板焊缝组织与力学性能

焊缝对同材同厚拼焊板成形的影响远大于母材强度或厚度差异的拼焊板.焊缝的力学性能是评价同材同厚拼焊板成形性及有限元仿真的关键.文中在"混合法"的基础上,通过微观组织观测与微硬度测试,结合数字影像相关技术,获得了拼焊板单向拉伸实时的全场应变场,解析计算出了焊缝的真实应力-应变关系曲线.将此曲线与小尺寸试件单向拉伸试验曲线对比分析后发现,曲线在前期稳定变形阶段是吻合的,在屈服阶段和颈缩阶段的差异是由于标准试件中母材对焊缝变形的限制作用,阻止焊缝过早颈缩.结果表明,焊缝的应变路径(应变比为-0.63)已经偏离单向拉伸状态(应变比为-0.50),纯焊缝试件则无此限制作用.计算所得曲线才是包含母材焊缝的真实应力应变曲线.     

Asymmetric microstructure and fracture behaviour of friction stir welded joints of 2024 aluminium alloy under cyclical load

In this study, the effect of asymmetric microstructure on fracture behaviour of friction stir welded joints of 2024-O aluminium alloy under cyclical load was investigated via optical microscopes, electron backscattering diffraction, microhardness and infrared thermography, and digital imaging correlation (DIC). The results demonstrate that the thermo-mechanical affected zone (TMAZ) at retreating side possesses a relatively higher fraction of low-angle grain boundaries and coincidence site lattice boundaries in comparison to that at the advancing side (AS), which results in the strain localisation and fracture path along the region between nugget zone and TMAZ at the AS under cyclical load. The profile of microhardness presents the asymmetric characteristic as well. The temperature evolution of the joint obtained by infrared camera is in good agreement with strain distribution measured by DIC. The thermography proves to be generally sufficient to predict the fracture path before failure under cyclical load.     

Measuring forming limit strains with digital image correlation analysis

Digital image correlation (DIC) analysis methods can provide very accurate strain measurement that is useful for identifying the onset of localized necking and thus determining the forming limit strains of sheet metals. Several different DIC procedures to establish a forming limit diagram (FLD) are compared including both position dependent and time dependent measurement methods where the latter is found to be more accurate for an Al–Mg–Si alloy. Since FLD standards are still being developed to include DIC methods, a new DIC-based time dependent measurement method for the forming limit strains is proposed in this research: (1) Surface topography is extracted from the DIC data and changes in surface height difference are obtained, of the test specimen; (2) The onset of localized necking is identified by the limit of stable flow of the surface height difference or the first derivative of the surface height difference near the development of a local neck when using the Marciniak test procedure. Compared to existing methods, this new procedure more accurately defines the onset of necking and the boundary of safe deformation of an FLD.     

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.     

基于数字图像相关技术的SLM-IN718细观表面裂纹扩展机制

开展了室温下不同应力比(R = 0.1及R = -1)的原位疲劳试验,并结合数字图像相关技术(DIC)分析了选区激光熔化IN718镍基高温合金(SLM-IN718)的细观超高周表面裂纹扩展机制。结果表明：首先,DIC分析表明SLM-IN718受载时裂纹尖端出现了类似于蝴蝶形的塑性应变区,这与采用Von Mises屈服准则相一致;其次,分析裂尖前方应变场特征、位移场特征,确定SLM-IN718存在裂纹闭合效应,并对其裂纹闭合效应进行评估,对SLM-IN718而言,R = 0.1与R = 0条件下载荷分别达到最大载荷的53%与29%时裂纹张开;此外,建立了虑及裂纹闭合效应的裂尖塑性区尺寸评估模型,计算值与实测值一致性较好;最后,结合DIC分析结果探讨了SLM-IN718低应力条件下的表面裂纹扩展机制。