Showing papers in "Experimental Mechanics in 2017"

A Review of Speckle Pattern Fabrication and Assessment for Digital Image Correlation

Abstract: As a carrier of deformation information, the speckle pattern, or more exactly the random intensity distributions, which could be naturally occurred or artificially fabricated onto test samples’ surface, plays an indispensable role in digital image correlation (DIC). It is now well recognized that the accuracy and precision in DIC measurements not only rely on correlation algorithms, but also depend highly on the quality of the speckle pattern. Considering the huge diversity in test materials, spatial scales and experimental conditions, speckle pattern fabrication could be a challenging issue facing DIC practitioners. To obtain good speckle patterns suitable for DIC measurements, some key issues of fabrication methods and quality assessment of speckle patterns must be well addressed. To this end, this review systematically presents the speckle pattern classification and fabrication techniques for various samples and scales, as well as some typical quality assessment metrics.

Recent Progress in Digital Image Correlation: Background and Developments since the 2013 W M Murray Lecture

Abstract: Since presentation of the 2013 Murray Lecture focusing on developments in digital image correlation (DIC), the methods have continued to expand internationally and their use has begun to grow in fields where there was less activity in the past. First, a brief history of digital image correlation methods is presented from the perspective of the first author, followed by a discussion of recent trends associated with the use of digital image correlation methods in academics, governmental laboratories and industrial settings. In the remainder of the article, new results are provided in three areas where DIC methods have seen rapid growth; application of StereoDIC or three-dimensional DIC (3D-DIC) to the study of wall structures in civil engineering; the use of Volumetric DIC or Digital Volume Correlation (DVC) to quantify the internal response of a specially-designed composite material and in the area of model validation for another application in civil engineering; transfer length measurements in pre-stressed concrete beams.

A Critical Comparison of Some Metrological Parameters Characterizing Local Digital Image Correlation and Grid Method

Abstract: The main metrological performance of two full-field measurement techniques, namely local digital image correlation (DIC) and grid method (GM), are compared in this paper. The fundamentals of these techniques are first briefly recalled. The formal link which exists between them is then given (the details of the calculation are in Appendix 1). Under mild assumptions, it is shown that GM theoretically gives the same result as DIC, since the formula providing the displacement with GM is the solution of the minimization of the cost function used in DIC in the particular case of a regular marking. In practice however, the way the solution is found being totally different from one technique to another, they feature different metrological performance. Some of the metrological characteristics of DIC and GM are studied in this paper. Since neither guideline nor precise standard is available to perform a fair comparison between them, a methodology must first be defined. It is proposed here to rely on three metrological parameters, namely the displacement resolution, the bias and the spatial resolution, to assess the metrological performance of each technique. These three parameters are thoroughly defined in the paper. Some of these quantities depend on external parameters such as the pattern of the surface of interest, so the same set of grid images is processed with both techniques. Only the contribution of the camera sensor noise to the displacement resolution is considered in this study. The displacement resolution, the bias and the spatial resolution are not independent but linked. These links are therefore studied in depth for DIC and GM and compared. In particular, it is shown that the product between the displacement resolution and the spatial resolution can be considered as a metric to perform this comparison. The extension to speckled patterns of the lessons drawn from grids is finally addressed in the last part of the paper. As a general conclusion, it can be said that for the value of the bias fixed in this study, the additional cost due to grid depositing offers GM to feature a better compromise than subset-based local DIC between displacement resolution and spatial resolution.

Active Slip System Identification in Polycrystalline Metals by Digital Image Correlation (DIC)

Abstract: In this paper, a novel approach was proposed to increase the confidence of active slip system identification in polycrystalline metals. The approach takes advantage of microscale deformation tracking via Digital Image Correlation (DIC) combined with scanning electron microscopy (SEM). The experimentally-obtained high-resolution deformation fields were mapped to an undeformed configuration, which gives slip traces suitable for comparison with undeformed crystal orientation data. A metric, named herein as the ‘relative displacement ratio’ (RDR), is calculated from the displacement fields near slip traces to characterize the localized deformation due to slip. In validation cases, the experimentally-measured RDRs matched well with RDRs theoretically-calculated from active slip systems. In test cases, active slip system identification by incorporating RDR as an additional constraint was demonstrated to be preferable to using Schmid factor alone as a constraint. The proposed approach supplements existing techniques for slip system identification with increased confidence.

Color Stereo-Digital Image Correlation Method Using a Single 3CCD Color Camera

Abstract: This paper presents a novel color stereo-digital image correlation (stereo-DIC) method using a single 3CCD color camera for full-field shape, motion, and deformation measurements without any sacrifice of the camera sensor spatial resolution. With the aid of a specially designed color separation device using a beam splitter and two optical bandpass filters, images of blue and red colors are simultaneously recorded by the 3CCD camera from two different optical paths. The blue and red channel sub-images extracted from the recorded color images can be analyzed using the regular stereo-DIC algorithm to obtain the full-field three dimensional (3D) information of a test object surface. The effectiveness and accuracy of the proposed technique are demonstrated by a series of real shape, in-plane and out-of-plane translation, and 3D deformation tests.

Experimental Investigation of Fracture Process Zone in Rocks Damaged Under Cyclic Loadings

Abstract: Compared with other materials, most rocks generally fail in a brittle fashion rather than exhibiting yielding or purely plastic deformation. However, the initiation and coalescence of micro-cracks in the nonlinear region, known as the ‘fracture process zone’ (FPZ), are the primary reason for fracture propagation in rocks. Different elasticity-related models proposed for determining the features of the FPZ have not achieved an adequate understanding of its various fracture patterns. Based on previous experiments and numerical models, micro-crack density has been shown to be a function of loading history and to vary depending on whether the loading is monotonic or cyclic. The aim of the study reported here was to examine the different patterns of the FPZ under various types of cyclic loading and to quantitatively define damage and fracture patterns through the grains or rock matrix. Considerable laboratory testing was conducted, and fractured samples were investigated by computerised tomography scanning, supported by thin-section analysis. In the study, two different types of cyclic loading were tested: stepped and continuous. A diametral compressive loading was applied at predetermined amplitude and frequency with the continuous cyclic loading. The applied cyclic diametral compressive load was returned to the original level after each step, and at the next step, the amplitude started from zero, with stepped cyclic loading (SCL). An average 30 % strength reduction was found due to the SCL and emergence of high micro-fracture density in the FPZ. We presume that hard rock breakage techniques will be improved, especially for rock-cutting technologies, such as drag bits and oscillating disc cutting, by understanding the effects of cyclic loading on rock strength.

Operational Modal Analysis of a Helicopter Rotor Blade Using Digital Image Correlation

Abstract: A novel procedure to perform operational modal analysis on a reduced-scale, 2 m diameter helicopter rotor blade is described. Images of the rotor blade rotating at 900 RPM are captured by a pair of high-speed digital cameras at a sampling rate of 1000 frames per second. From these images, the out-of-plane bending deformation of the rotor blade is measured using Digital Image Correlation, with a spatial resolution of 7.2 mm and an accuracy of 60 μm, or 0.006 % of the rotor radius. Modal parameters including natural frequencies and mode shapes are determined from the bending deformation through application of the Ibrahim Time Domain method. The first three out-of-plane bending modes were identified at each rotational speed and compared to an analytical finite element model of the rotor blade. The experimental and analytical natural frequencies agreed to within 0.2 % in the best case and 10.0 % in the worst case. The experimental mode shapes were also found to closely match the analytical predictions. The results of this study demonstrate the ability of this procedure to accurately determine the modal parameters of rotating helicopter rotor blades.

J-Integral Calculation by Finite Element Processing of Measured Full-Field Surface Displacements

Abstract: A novel method has been developed based on the conjoint use of digital image correlation to measure full field displacements and finite element simulations to extract the strain energy release rate of surface cracks. In this approach, a finite element model with imported full-field displacements measured by DIC is solved and the J-integral is calculated, without knowledge of the specimen geometry and applied loads. This can be done even in a specimen that develops crack tip plasticity, if the elastic and yield behaviour of the material are known. The application of the method is demonstrated in an analysis of a fatigue crack, introduced to an aluminium alloy compact tension specimen (Al 2024, T351 heat condition).

Anisotropic Mechanical Properties of SAC Solder Joints in Microelectronic Packaging and Prediction of Uniaxial Creep Using Nanoindentation Creep

Abstract: In this paper, the mechanical properties and creep behavior of lead-free solder joints has been characterized by nano-mechanical testing of single grain SAC305 solder joints extracted from plastic ball grid array (PBGA) assemblies. The anisotropic mechanical properties characterized include the elastic modulus, hardness, and yield stress. An approach is suggested to predict tensile creep strain rates for low stress levels using nanoindentation creep data measured at very high compressive stress levels. The uniaxial creep rate measured on similarly prepared bulk (large) specimens was found to be of the same order-of-magnitude as the creep rate observed in single-grain BGA joints, with chararacteristically (slightly) higher creep strains measured during nanoindentation. This suggests that the same creep mechanism operates in both size domains. Electron backscattered diffraction (EBSD) and nanoindentation testing further showed that the modulus, hardness, and creep properties of solder joints are highly dependent on the crystal orientation.

Evaluation of Errors Associated with Cutting-Induced Plasticity in Residual Stress Measurements Using the Contour Method

Abstract: Cutting-induced plasticity can lead to elevated uncertainties in residual stress measurements made by the contour method. In this study plasticity-induced stress errors are numerically evaluated for a benchmark edge-welded beam to understand the underlying mechanism. Welding and cutting are sequentially simulated by finite element models which have been validated by previous experimental results. It is found that a cutting direction normal to the symmetry plane of the residual stress distribution can lead to a substantially asymmetrical back-calculated stress distribution, owing to cutting-induced plasticity. In general, the stresses at sample edges are most susceptible to error, particularly when the sample is restrained during cutting. Inadequate clamping (far from the plane of cut) can lead to highly concentrated plastic deformation in local regions, and consequently the back-calculated stresses have exceptionally high values and gradients at these locations. Furthermore, the overall stress distribution is skewed towards the end-of-cut side. Adequate clamping (close to the plane of cut) minimises errors in back-calculated stress which becomes insensitive to the cutting direction. For minimal constraint (i.e. solely preventing rigid body motion), the plastic deformation is relatively smoothly distributed, and an optimal cutting direction (i.e. cutting from the base material towards the weld region in a direction that falls within the residual stress symmetry plane) is identified by evaluating the magnitude of stress errors. These findings suggest that cutting process information is important for the evaluation of potential plasticity-induced errors in contour method results, and that the cutting direction and clamping strategy can be optimised with an understanding of their effects on plasticity and hence the back-calculated stresses.

Fracture Characterization of Rolled Sheet Alloys in Shear Loading: Studies of Specimen Geometry, Anisotropy, and Rate Sensitivity

Abstract: Two different shear sample geometries were employed to investigate the failure behaviour of two automotive alloy rolled sheets; a highly anisotropic magnesium alloy (ZEK100) and a relatively isotropic dual phase steel (DP780) at room temperature. The performance of the butterfly type specimen (Mohr and Henn Exp Mech 47:805–820, 16; Dunand and Mohr Eng Fract Mech 78:2919-2934, 17) was evaluated at quasi-static conditions along with that of the shear geometry of Peirs et al Exp Mech 52:729-741, (27) using in situ digital image correlation (DIC) strain measurement techniques. It was shown that both test geometries resulted in similar strain-paths; however, the fracture strains obtained using the butterfly specimen were lower for both alloys. It is demonstrated that ZEK100 exhibits strong anisotropy in terms of failure strain. In addition, the strain rate sensitivity of fracture for ZEK100 was studied in shear tests with strain rates from quasi-static (0.01 s−1) to elevated strain rates of 10 and 100 s−1, for which a reduction in fracture strain was observed with increasing strain rate.

Distortion Correction Protocol for Digital Image Correlation after Scanning Electron Microscopy: Emphasis on Long Duration and Ex-Situ Experiments

Abstract: Recently, scanning electron microscopy (SEM) has been used for digital image correlation (DIC), as micrographs can be acquired with high magnification, providing improved resolution to quantify strain heterogeneities. However, it has been shown by researchers that SEM images can be problematic due to inherent electromagnetic distortions that are not present in optical images. Drift, spatial distortions, and magnification uncertainties are the main issues that can seriously affect the accuracy of localized strain measurements. The present work focuses on long duration experiments, for which images are taken days or weeks apart. We have proposed a systematic procedure to reduce drift, correct spatial distortion, and account for magnification variations between pairs of acquired images. Additionally, SEM parameters are discussed and chosen to increase the signal-to-noise ratio and improve the accuracy of the DIC measurements. The spatial distortion correction increases the repeatability of the correlated values and the precision of the measurements. The implementation for this type of correction is done by applying the measured distortion gradient of a certified grid onto the DIC strain field. The magnification adjustment increases the reliability of the strain maps, ensuring the measurements are in agreement with the actual strain induced during the experiment. We have presented a systematic protocol for ex-situ DIC experiments within the SEM and some basic cross-check procedures that can be performed to evaluate the reliability of the reference grid and the precision of the final strain map.

Stereo-DIC Uncertainty Quantification based on Simulated Images

Abstract: Stereo digital image correlation (stereo-DIC) is in wide-spread use for full-field shape, motion and deformation-measurements. However there are very few papers investigating the influence of the setup on the measurement uncertainty. This is mainly due to the highly non-linear measurement chain involving both optical and numerical aspects, making it difficult to investigate how error sources are propagated through the stereo-DIC chain. Indeed, it is impossible to separate all the error sources that are present during a physical measurement. This paper tries to investigate a selection of error sources that are present during experiments. This is based on a simulator introduced in a previous article (Balcaen et al., Exp Mech, 1–16 2017) and briefly reviewed here. Based on these simulations we suggest some “best-practices” guidelines of optimal stereo-DIC setups.

Stereo-DIC Calibration and Speckle Image Generator Based on FE Formulations

Abstract: Stereo digital image correlation (stereo-DIC) is being accepted by the industry as a valid full-field measurement technique for measuring shape, motion and deformation, and it is therefore of utmost importance to provide uncertainties on the obtained measurements. However, the influences on a stereo-DIC measurement are not fully understood; indeed, stereo-DIC is a complex optical-numerical process and it is not always clear how errors are propagating throughout the measurement chain. In order to investigate the magnitude of the different error-sources a simulator for stereo-DIC is proposed. This simulator is able to generate realistic synthetic images as if they were made during a real set-up, so the error sources can be investigated separately and an optimal set-up can be chosen before any physical test is performed. We present in this paper the mathematical approach to the DIC simulator including details on how to convert FE displacement field results to stereo-DIC images. The simulator includes the ability to control the lighting and to create synthetic calibration images. The synthetic images are compared to simulations for a bulge test as a validation of the simulator. Synthetic calibration images are compared to experimental calibration studies to verify those. Finally a brief look at how the simulator could be used for looking at calibration quality is conducted.

Equal Noise Resistance of Two Mainstream Iterative Sub-pixel Registration Algorithms in Digital Image Correlation

Abstract: The inverse compositional Gauss-Newton (IC-GN) algorithm and the forward additive Newton-Raphson (FA-NR) algorithm are two mainstream iterative sub-pixel registration algorithms in digital image correlation. This study compares the accuracy and convergence ability of the two algorithms by theoretical analysis and numerical experiments in the speckle images that have been contaminated with artificial Gaussian noise. Based on the derived error model, the systematic errors of the two algorithms are dominated by interpolation-induced error and are insensitive to noise. The random errors are proportional to the noise level. The noise also reduces the convergence radius and rate in the two algorithms. The two algorithms demonstrate equal noise resistance due to their mathematical equivalence. These conclusions are well supported by the experimental study. The recently reported vulnerability of the FA-NR algorithm to noise is not associated with the inherent flaw of the algorithm but with its implementation. If an inappropriate method is employed to estimate the gradients at sub-pixel locations in the FA-NR algorithm, abnormally large errors may be induced. This problem can be eliminated using the method that is proposed in this study, which has an insignificant extra-computation cost.

Measurement of Strain Localization Resulting from Monotonic and Cyclic Loading at 650 ∘ C in Nickel Base Superalloys

Abstract: A methodology is presented for the use of the oxide scale that develops in polycrystalline Ni-base superalloys at service temperature, as a speckle pattern for μm-scale resolution strain measurements. Quantitative assessment of the heterogeneous strain field at the grain scale is performed by high-resolution SEM digital image correlation under monotonic and cyclic loading in polycrystalline Ni-base superalloys up to 650 ∘C. In the Rene 88DT superalloy, strain localization is observed near twin boundaries during low cycle fatigue (LCF) at intermediate temperatures, correlating with activation of {111} 〈110〉 and {111} 〈112〉 slip systems. A strong correlation between the microstructural configuration that promotes strain localization during monotonic loading and crack initiation at 650 ∘C in low cycle fatigue was observed.

Development of High Temperature Nanoindentation Methodology and its Application in the Nanoindentation of Polycrystalline Tungsten in Vacuum to 950 °C

Abstract: The capability for high temperature nanoindentation measurements to 950 °C in high vacuum has been demonstrated on polycrystalline tungsten, a material of great importance for nuclear fusion and spallation applications and as a potential high temperature nanomechanics reference sample. It was possible to produce measurements with minimal thermal drift (typically ~0.05 nm/s at 750–950 °C) and no visible oxidative damage. The temperature dependence of the hardness, elastic modulus, plasticity index, creep, creep strain, and creep recovery were investigated over the temperature range, testing at 25, 750, 800, 850, 900 and 950 °C. The nanoindentation hardness measurements were found to be consistent with previous determinations by hot microhardness. Above 800 °C the hardness changes relatively little but more pronounced time-dependent deformation and plasticity were observed from 850 °C. Plasticity index, indentation creep and creep recovery all increase with temperature. The importance of increased time-dependent deformation and pile-up on the accuracy of the elastic modulus measurements are discussed. Elastic modulus measurements determined from elastic analysis of the unloading curves at 750–800 °C are close to literature bulk values (to within ~11 %). The high temperature modulus measurements deviate more from bulk values determined taking account of the high temperature properties of the indenter material at the point (850 °C) at which more significant time-dependent deformation is observed. This is thought to be due to the dual influence of increased time-dependency and pile-up that are not being accounted for in the elastic unloading analysis. Accounting for this time-dependency by applying a viscoelastic compliance correction developed by G. Feng and A.H.W. Ngan (J. Mater. Res. (2002) 17:660–668) greatly reduces the values of the elastic modulus, so they are agree to within 6 % of literature values at 950 °C.

Finite Element Stereo Digital Image Correlation: Framework and Mechanical Regularization

Abstract: The use of Finite Element meshes in Digital Image Correlation (FE-DIC) is now widespread in experimental mechanics. Up to now FE have been much less used in Stereo-DIC. The first goal of this paper is to explain in details how to use FE in Stereo-DIC using a formulation in the physical coordinate system. More precisely, it is shown how to perform the calibration of possibly nonlinear models, shape and displacement measurement based on a FE mesh. In addition it is shown that with such a framework it is possible to regularise the measurement with a FE model based on the same mesh. For instance, using this technique, it is shown that it is possible to measure the rotation field of a bending plate in addition to its displacement.

Effect of Particle Shape on Monotonic Liquefaction: Natural and Crushed Sand

Abstract: This study investigates the effect of particle shape on monotonic liquefaction of the soil by performing a series of static triaxial compression tests. The tests conducted on three types of natural sand (NS), crushed sand (CS), and mixed sand (MS) (i.e., 50% natural sand +50% crushed sand by dry weight of the soil) with different particle shape descriptors consist of roundness (R), sphericity (S), and regularity (ρ). The shearing responses showed that the CS and MS specimens showed a dilative response whereas the natural sand had a strain-softening contractive behaviour. Also, the interpreted results based on a framework of the critical state of the soil mechanic (CSSM) showed that in e-p′ plane, the specimen with a higher amount of crushed particles have a greater strength due to a higher packing characteristics. The investigations on critical state locus on q-p′ plane showed that by increasing the roundness, sphericity and regularity of the specimens the critical friction angle (ϕ cs ) decreased. Also, studies on flow liquefaction in undrained instability state (UIS) showed that by increasing the particle shape descriptor values, the specimens are more prone to be liquefied.

Stable Speckle Patterns for Nano-scale Strain Mapping up to 700 °C

Abstract: The digital image correlation (DIC) of speckle patterns obtained by vapour-assisted gold remodelling at 200 – 350 °C has already been used to map plastic strains with submicron resolution. However, it has not so far proved possible to use such patterns for testing at high temperatures. Here we demonstrate how a gold speckle pattern can be made that is stable at 700 °C, to study deformation in a commercial TiAl alloy (Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB2). The pattern is made up of a uniformly sized random array of Au islands as small as 15 nm in diameter, depending on reconstruction parameters, with a sufficiently small spacing to be suitable for nano-scale, nDIC, strain mapping at a subset size of 60 × 60 nm2. It can be used at temperatures up to 700 °C for many hours, for high cycle fatigue testing for instance. There is good particle attachment to the substrate. It can withstand ultra-sound cleaning, is thermally stable and has a high atomic number contrast for topography-free backscatter electron imaging.

Mechanical Properties of Hard W-C Coating on Steel Substrate Deduced from Nanoindentation and Finite Element Modeling

Abstract: Mechanical properties of a hard and stiff W-C coating on steel substrate have been investigated using nanoindentation combined with finite element modeling (FEM) and extended FEM (XFEM). The significant pile-up observed around the indents in steel substrate caused an overestimation of hardness and indentation modulus. A simple geometrical model, considering the additional contact surfaces due to pile-up, has been proposed to reduce this overestimation. The presence of W-C coating suppressed the pile-up in the steel substrate and a transition to sink-in behavior occurred. The FEM simulations adequately reproduced the surface topography of the indents in the substrate and coating/substrate systems as well. The maximum principal stresses of the indented W-C/steel coated system were tensile; they were always located in the coating and evolved in 3 stages. Cohesive cracking occurred during loading in the sink-in zone (stage III) when the ultimate tensile strength (σ max ) of the coating was reached. The obtained hardness (H c ), indentation modulus (E c ), yield stress (Y) and strength (σ max ) of the W-C coating were H c = 20 GPa, E c = 250 GPa, Y = 9.0 GPa and σ max = 9.35 GPa, respectively. XFEM resulted in fracture energy of the W-C coating of G = 38.1 J · m-2 and fracture toughness of K IC = 3.5 MPa · m0.5.

Stresses and Strains in Cruciform Samples Deformed in Tension

Abstract: The stress and strain relationship in the gauge region of six cruciform geometries is studied: the ISO standard geometry with slits in arms, two geometries with thinned gauge areas, two geometries with thinned gauge areas and slits in arms, and one modified ISO standard geometry with slits in arms and a thinned gauge area. For all the geometries, finite element simulations are performed under uniaxial loading to compare the plastic strain, the von Mises stress distribution and the in-plane stress evolution. Results show that less plastic strain can be achieved in the gauge of the two ISO standard geometries. For the remaining cruciform geometries, a strong non-linear coupling between applied forces in arms and gauge stresses is generated. The evolution of this non-linear coupling depends on the geometry type, applied biaxial load ratio and the elastic-plastic properties of the material. Geometry selection criteria are proposed to reduce this non-linear coupling.

Experimental and Numerical Analysis of Initial Plasticity in P91 Steel Small Punch Creep Samples

Abstract: To date, the complex behaviour of small punch creep test (SPCT) specimens has not been completely understood, making the test hard to numerically model and the data difficult to interpret This paper presents a novel numerical model able to generate results that match the experimental findings For the first time, pre-strained uniaxial creep test data of a P91 steel at 600 ∘C have been implemented in a conveniently modified Liu and Murakami creep damage model in order to simulate the effects of the initial localised plasticity on the subsequent creep response of a small punch creep test specimen Finite element (FE) results, in terms of creep displacement rate and time to failure, obtained by the modified Liu and Murakami model are in good agreement with experimental small punch creep test data The rupture times obtained by the FE calculations which make use of the non-modified creep damage model are one order of magnitude shorter than those obtained by using the modified constitutive model Although further investigation is needed, this novel approach has confirmed that the effects of initial localised plasticity, taking place in the early stages of small punch creep test, cannot be neglected The new results, obtained by using the modified constitutive model, show a significant improvement with respect to those obtained by a ’state of the art’ creep damage constitutive model (the Liu and Murakami constitutive model) both in terms of minimum load-line displacement rate and time to rupture The new modelling method will potentially lead to improved capability for SPCT data interpretation

On the Correlation of FEM and Experiments for Hyperelastic Elastomers

Abstract: Correlation of modern finite element methods (FEM) with advanced experimental techniques for elastomers, biomedical materials, and living organs requires study and modification of the behavior of these materials In this study, the mechanical behavior of a commonly-used elastomer, silicone rubber, which provides excellent biocompatibility, was examined under different applied loading configurations, and large deformations were investigated through both experiment and simulation The stress-strain behaviors of silicone rubber were tested, using multiple homogeneous experiments, including uniaxial extension and equibiaxial tension, the load-apex displacement response, and digitized deformed shapes of two of the most-used structures for nonlinear hyperelasticity—the inflation of a clamped circular membrane, and indentation of the membrane by a spherical indenter Uniaxial and equibiaxial data were evaluated simultaneously, characterized by various constitutive models for implementation in the FE simulation These constitutive models examined the prediction of the FE simulations for the inflation and indentation tests in comparison to the results of experiments at various load-apex displacement levels The results showed that the constitutive models calibrated with the uniaxial and equibiaxial tests, predicted nearly the same results as the actual experimental results, particularly for the applied loads that generated moderate strain However, when the FE simulations based on the constitutive models were adjusted, employing only uniaxial or equibiaxial tests, they predicted different results, where the degree of their correlations with experimental results was incomplete or in some states simply poor The simulations suggested that the inverse FE procedure should not be restricted to the choice of material models, while more attention should be given to the choice of ranges of deformation

A Miniaturized Biaxial Deformation Rig for in Situ Mechanical Testing

Abstract: A novel miniaturized biaxial deformation rig is presented. It allows one to apply in-plane biaxial stress states with arbitrary stress ratios and to perform strain path changes on thin-sheet metals. The device is optimized for in situ usage inside a scanning electron microscope and at synchrotron beam lines. The sample has a cruciform shape and the geometry is optimized with the aid of finite element simulations. A proof-of-principle experiment confirms the successful operation of this rig.

In situ μ CT-scan Mechanical Tests: Fast 4D Mechanical Identification

Abstract: A recently proposed “Projection-based Digital Volume Correlation” (P-DVC) method is extended in this work to a cone-beam lab-tomograph in which a mechanical test is performed. This consists of a crack propagation test in an elastic-brittle gypsum specimen. Kinematic analysis is performed based on a reduced finite element modeling for which the appropriate boundary conditions and crack propagation stage are determined from the radiographs. By considering only two projections per loading step, an integrated model-based analysis of the entire test provides a full space and time identification of the kinematics, including the crack position and the determination of two material parameters. This is achieved with a drastic reduction in the acquisition time compared to classical digital volume correlation analysis. In the examples presented, the acquisition time was reduced by a factor of 350.

A Method of Detecting the Onset of Localized Necking Based on Surface Geometry Measurements

Abstract: To more accurately capture the onset of localized necking and obtain necking limit strains, this paper proposes a method of detecting the onset of localized necking in the Marciniak test (ie under in-plane deformation) The method is merely based on the measured surface geometry of the test specimen using digital image correlation (DIC) techniques It was inspired by the observation of a sudden increase of the surface curvature obtained from 2D curvature fits along the direction across the surface of the sheet This increase of the surface curvature is detected just before the dimples, which form the final localized neck, become obvious in the DIC measurements The appearance of this signal is explained by a neck expansion theory defined by propagation of the instability along the direction of the neck, which is a physical behavior of materials

In-Situ CT Damage Analysis of Metal Inserts Embedded in Carbon Fiber-Reinforced Plastics

Abstract: Carbon-fiber-reinforced plastics (CFRPs) are gaining increasing applicability to lightweight structures (e.g., automotive applications) due to their outstanding mechanical properties. High-performance parts can be fabricated from CFRPs, but they have the disadvantages of low shear and bearing strength. To achieve detachable connections and introduce loads without decreasing the load-bearing capacity of the composite, it is important to use mechanical fasteners without drilling into the parts. To accomplish this, metal elements called inserts are embedded in the CFRP laminate. Damage behavior in a CFRP under tensile conditions has several different mechanisms, depending primarily on the deformation of the insert. This research investigates the in-situ failure behavior of the composite under tensile loads by investigating the deformation of the insert via computed tomography (CT). The results are also used for validation of the insert’s deformation using a finite-element model (FEM).

Design, Manufacture, and Quasi-Static Testing of Metallic Negative Stiffness Structures within a Polymer Matrix

Abstract: A composite material system comprised of a monostable negative stiffness (NS) structure within a polymer matrix was designed, fabricated, and experimentally evaluated. The monostable negative stiffness (NS) structure was designed using a combination of analytical and numerical models and manufactured in stainless steel. The NS structure was arranged in parallel with different polymer matrices to experimentally evaluate the effects of the matrix properties on the overall stiffness and energy dissipation of the composite NS-matrix system when loaded in uniaxial compression. A strong influence of the matrix properties on the stiffness and energy absorption capacity of the composite system was observed. Unlike conventional composites for which there is a natural tradeoff between stiffness and energy absorption capacity, the composite NS-matrix system enhanced stiffness while simultaneously improving energy absorption relative to a neat matrix, but only when the stiffness of the matrix was carefully matched to the stiffness of the NS structure.

Synchronous Full-Field Strain and Temperature Measurement in Tensile Tests at Low, Intermediate and High Strain Rates

Abstract: Tensile tests with simultaneous full-field strain and temperature measurements at the nominal strain rates of 0.01, 0.1, 1, 200 and 3000 s−1 are presented. Three different testing methods with specimens of the same thin and flat gage-section geometry are utilized. The full-field deformation is measured on one side of the specimen, using the DIC technique with low and high speed visible cameras, and the full-field temperature is measured on the opposite side using an IR camera. Austenitic stainless steel is used as the test material. The results show that a similar deformation pattern evolves at all strain rates with an initial uniform deformation up to the strain of 0.25–0.35, followed by necking with localized deformation with a maximum strain of 0.7–0.95. The strain rate in the necking regions can exceed three times the nominal strain rate. The duration of the tests vary from 57 s at the lowest strain rate to 197 μs at the highest strain rate. The results show temperature rise at all strain rates. The temperature rise increases with strain rate as the test duration shortens and there is less time for the heat to dissipate. At a strain rate of 0.01 s−1 the temperature rise is small (up to 48 °C) but noticeable. At a strain rate of 0.1 the temperature rises up to 140 °C and at a strain rate of 1 s−1 up to 260 °C. The temperature increase in the tests at strain rates of 200 s−1 and 3000 s−1 is nearly the same with the maximum temperature reaching 375 °C.