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 Experimental MechanicsJournal Prestige (SJR): 0.947 Citation Impact (citeScore): 2Number of Followers: 21      Hybrid journal (It can contain Open Access articles) ISSN (Print) 1741-2765 - ISSN (Online) 0014-4851 Published by Springer-Verlag  [2469 journals]
• Correction to: In‑situ SEM High Strain Rate Testing of Large Diameter
Micropillars Followed by TEM and EBSD Postmortem Analysis

PubDate: 2022-06-01

• Direct Measurement of the Transverse Modulus of Carbon Fibres

Abstract: Background The transverse compressive properties are integral to fully utilizing the high performing mechanical properties of carbon fibres (CF). Direct measurements of transverse properties of CFs are difficult. Objective A system that directly measures transverse compressive modulus (ET) by utilizing interference speckle patterns to measure sub-pixel resolutions is presented. Methods The compression data from an isotropic glass fibre was fitted using an elastic contact mechanics model. An initial section was observed, which was not readily fitted using the model and was systematically removed, resulting in an ET of 82 GPa, comparable to the reported longitudinal tensile modulus (EL) of 86 GPa. Results The response of Ag wire to cyclic compression was measured, with the behaviour consistent with a material undergoing typical cyclic stress–strain into a stable hysteretic loop. Several CFs were compressed and an inversely proportional relationship between ET and EL was observed. Transverse compressive moduli of 8.2 (0.8), 6.5 (0.7), 4.3 (0.3), and 2.1 (0.4) were obtained for Toray T300 and Mitsubishi Pyrofil HS40, Pyrofil TR50S, and Dialead K13312, respectively (numbers in parenthesis are standard deviations). Conclusion Hysteresis was observed for some of the polyacrylonitrile (PAN)-based CFs and a method for ensuring an accurate fitting of CFs, including hysteretic load/unload curves, was proposed.
PubDate: 2022-06-01

• A Comparison of Conventional Gel Stiffness Characterization Techniques
with Cavitation Rheology

Abstract: Background Interest in soft gels has arisen in recent years as they can be applied to many fields such as tissue engineering, food additives, and drug delivery. The importance of these technologies lies in the stiffness of applied materials and hence there is a strong need for determining the stiffness of gels precisely. Cavitation rheology, a novel experimental method, can measure the Young’s modulus in any part of a soft material. However, compared with fully developed conventional techniques, cavitation rheology is not completely exploited and needs more in-depth research conducted. Objective In this paper, four experimental approaches have been applied to determine the Young’s modulus of an ultra-soft tri-block copolymer (PMMA-PnBA-PMMA): classic shear rheology, static indentation, cavitation rheology and low-velocity impact. Although there are plenty of examples of soft gel stiffness characterization in the open literature, this is the first time (to the knowledge of the authors), that cavitation rheology and the impact pinch-off experiment have been compared with the more traditional stiffness testing approaches of classic rheology and indentation. Furthermore, the relationship between gel’s stiffness and the von Mises strain rate is investigated in the analysis. Methods Benchmark data is obtained from a classic shear rheology experiment. A modification to the previous cavitation rheology analysis is made to improve the accuracy in predicting the Young’s modulus and surface tension. The measurements of static indentation and dynamic low-velocity impact experiments are taken non-invasively by optical visualization. Gel samples with three concentrations are applied to all the experiments to investigate the feasibility of each method. Results The comparison between different experiments indicates a slight strain-rate dependence in gel stiffness across various gel concentrations. Cavitation rheology is shown to have a clear correlation with high-strain rate tests, but not quasi-static ones. Conclusions This paper has made some significant contributions in regards to broadening the knowledge of cavitation rheology. In addition, we provide an in-depth analysis of pragmatic stiffness measurement techniques and demonstrate their usefulness across various stiffness regimes in a soft polymeric gel with tunable mechanical properties.
PubDate: 2022-06-01

• Stretching Graphene to 3.3% Strain Using Formvar-Reinforced Flexible
Substrate

Abstract: Background As a one-atom-thick material, the mechanical loading of graphene in large scale remains a challenge, and the maximum tensile strain that can be realized is through a flexible substrate, but only with a value of 1.8% due to the weak interfacial stress transfer. Objective Aims to illustrate the interface reinforcement brought by formvar resins as a buffering layer between graphene and substrates. Methods Single crystal graphene transferred to different substrates, applied with uniaxial stretching to compare the interface strength, and finite element analysis was performed to simulate tensile process for studying the influence of Poisson’s ratio of the buffering layer for interface reinforcement. Results In this work we use formvar resins as a buffering layer to achieve a maximum uniaxial tensile strain of 3.3% in graphene, close to the theoretical limit (3.7%) that graphene can achieve by flexible substrate stretching. The interface reinforcement by formvar is significantly higher than that by other polymers, which is attributed to the liquid–solid phase transition of formvar for more conformal interfacial contact and its suitable Poisson’s ratio with graphene to avoid its buckling along the transverse direction. Conclusions We believe that these results can provide guidance for the design of substrates and interfaces for graphene loading, as well as the support for mechanics analysis of graphene-based flexible electronic devices.
PubDate: 2022-06-01

• A Dynamic Hydraulic Fracturing Test Technology Based on Split Hopkinson
Pressure Bar System

Abstract: Background The dynamic mechanical characteristics of rock under biaxial compression-tension stress states have essential engineering significance, but there is no relevant apparatus or approach. Objective A new technology to study the dynamic mechanical behavior of rock materials under compressive-tensile stress states based on SHPB devices is presented. An approximate approach is developed to determine the strength of rock material under biaxial stress states. Methods A hollow ring specimen is designed instead of the common cylindrical sample, and hydraulic oil is filled in the hollow region formed by the rock specimen and SHPB system. Once an incident stress wave is applied, the inner surface of the rock specimen firstly undergoes compressive pressure from oil, and then tensile stress arises along the circumferential direction of the ring specimen, thus, the rock is under a dynamic compression-tension stress state. To determine the strength of rock material under this stress state, theoretical analysis and numerical simulations are performed, and an approximate and practical approach is developed. Results Dynamic mechanical characteristics, including the strength, of seven marble samples under biaxial compressive-tensile stress state are obtained based on the newly proposed technology and approach. The test results show that it has a significant strain rate effect. Conclusions The new technology is effective and practical for studying the dynamic mechanical behavior of rock under biaxial compression-tension stress states, and it also provide an alternative method for characterizing the dynamic tensile strength of rock.
PubDate: 2022-06-01

• Alumina as a Computed Tomography Soft Material and Tissue Fiducial Marker

Abstract: Background The use of 3D imaging is becoming increasingly common, so too is the use of fiducial markers to identify/track regions of interest and assess material deformation. While many different materials have been used as fiducials, they are often used in isolation, with little comparison to one another. Objective In the current study, we aim to directly compare different Computed Tomography (CT and μCT) fiducial materials, both metallic and nonmetallic. Methods μCT imaging was performed on a soft-tissue structure, in this case heart valve tissue, with markers from eight different materials attached. Additionally, we evaluated the same markers with DiceCT stained tissue in a fluid medium. Results All of the metallic markers generated significant artifacts, and were found unsuitable for soft-tissue μCT imaging, whereas alumina markers were found to perform the best, with excellent contrast and consistency. Conclusions These findings support the further use of alumina as fiducial markers for soft material and tissue studies that utilize CT and μCT imaging.
PubDate: 2022-06-01

• Determining Dynamic Properties of Elastomer-Dampers by Means of Impact
Testing

Abstract: Background Damping elements made of elastomer materials are used in almost every mechanical system to prevent damage to components caused by impact-like excitations and the resulting high-frequency, large-amplitude oscillations. The dynamics of these operating conditions exceed the performance limits of conventional experimental testing methods, impeding validated predictions of the damper’s transmission behaviour. Objective A method is proposed to directly investigate the influence of impacts on the transmission behaviour of elastomeric dampers by impact testing. Methods Torsional-loaded elastomer dampers were experimentally investigated using a drop tower. During the experiment, a mass is brought into impact contact with a lever arm connected to the tested coupling. Measurements on resulting torsional oscillations and a comparison of the measurement results with a simple analytical model of the system allow for determining the coupling parameters stiffness and damping ratio. Results The characteristic parameters stiffness and damping ratio of the elastomer damper were mapped as a function of excitation-amplitude and frequency. A comparison of drop-tower test results with servo-hydraulic measurements validated the determined parameters. Conclusions Determining the transmission behaviour of elastomeric dampers from highly dynamic and impact-induced oscillation states proved to be a good approach to supplement established testing methods.
PubDate: 2022-06-01

• Quantifying the Uncertainty of Critical Resolved Shear Stress Values
Derived from Nano-Indentation in Hexagonal Ti Alloys

PubDate: 2022-06-01

• Metal Artifacts in Attenuation and Phase Contrast X-Ray Microcomputed
Tomography: A Comparative Study

Abstract: Background Metal artifacts arising around high-density components are a widely known problem in X-ray computed tomography (XCT) for both medical and industrial applications. Although phase contrast imaging XCT (PCI-XCT) is known to be less prone to metal artifacts caused by beam hardening, so far only little effort was made for its comparison to other, more established methods. Objective In the course of this work, this absence in literature is addressed by a quantitative comparison of PCI-XCT to attenuation contrast XCT (AC-XCT). Methods A polymer specimen including four Ti6Al4V inserts was investigated by PCI- and AC-XCT with different pre-filter settings and metal artifact reduction (MAR) algorithm. Artifacts and image quality were evaluated by a streak index which provides a quantitative metric for the assessment of streak artifacts and contrast-to-noise ratio (CNR). Results Results showed that streak artifacts are significantly reduced in PCI-XCT and only matched by AC-XCT in combination with hardware pre-filtering of the X-ray beam and post-processing by a MAR algorithm. However, hardware pre-filtering leads to worse CNR and artifacts close to the surface of metal inserts could not be removed sufficiently by the MAR algorithm. Conclusions This work demonstrates the potential of PCI-XCT for the reduction of metal artifacts and presents the first quantitative comparison to established AC-XCT methods.
PubDate: 2022-06-01

• Coupling Self-Adaptive Meshing-Based Regularization and Global Image
Correlation for Spatially Heterogeneous Deformation Characterization

Abstract: Background Image-based global correlation involves a class of ill-posed inverse problems associated with speckle quality and deformation gradients on specimen surfaces. However, the method used to simultaneously integrate the prior information related to images and deformations and effectively regularize these inverse problems still faces severe challenges, especially when complex heterogeneous deformation gradients exist over sample surfaces with locally degraded speckle patterns. Objective We propose a novel self-adaptive meshing-based regularization for global image correlation to determine spatially complex heterogeneous deformations. Methods A virtual truss system with a linearly elastic constitutive relationship is employed to self-adaptively implement surface meshing by numerically balancing the exerted virtual forces under the constraints of the local speckle image quality and deformation gradients. The 2-norm-based condition number of the local stiffness matrix is introduced to ensure numerical stability during meshing. Results The algorithms can behave as a smart regularization procedure integrating all the prior information during numerical calculations, consequently achieving an accurate, precise and robust characterization of heterogeneous deformations, as demonstrated by virtual simulations and actual experiments. Conclusions The regularization strategy coupled to image-based correlation is also promising for automatic quantification of complex heterogeneous deformations, particularly from images with locally degraded speckle patterns.
PubDate: 2022-06-01

• Algorithms for Nanoindentation Strain Rate Jump Testing and Analysis

Abstract: Background Understanding the dynamics of deformation processes is of interest for determining the dominant thermally activated processes during plasticity [1] and fracture [2, 3]. Strain rate jump (SRJ) tests have been recently introduced to investigate intrinsic deformation mechanisms and have been successfully utilized on some nanomechanical test platforms [4–6]. Objective The goal is to create standardized SRJ testing and analysis protocols for Hysitron nanoindenters, which cannot be found in literature, besides our previous work [7]. Methods Presented here is software to create SRJ test load functions for the TI980, which could be adapted to other instruments, and standardized protocols for analyzing the associated data. Results These protocols are validated using single crystal tungsten as a model material and agree well with literature values from other instruments. Conclusions New freely available software, validated on single crystal tungsten, creates opportunities for others to investigate intrinsic deformation mechanisms in more complex systems.
PubDate: 2022-06-01

• Effect of Confinement on the Impact Response of a Granular Array

Abstract: Background Plastic dissipation at inter-granular contacts during elasto-plastic wave propagation plays a significant role in wave attenuation. However, it is unknown if plastic dissipation during impact is enhanced if the granular medium is initially in an unconfined ’fluid-like’ state or that of a more rigid ‘solid-like’ state caused by applying a confining pressure. Objective The goal of this work is to investigate both experimentally and numerically the impact response of a two-dimensional hexagonal granular array consisting of metallic spheres enclosed in a polymeric membrane subjected to different levels of confining pressure. We seek to quantify the granular trajectories, the effect of the membrane, and the ratio of the dissipated plastic energy to the net input energy between the unconfined and confined states. Methods We perform experiments using a modified split Hopkinson pressure bar on a specimen of monodisperse brass spheres confined by a polymeric membrane and record the impact event using high-speed photography so that particle tracking can be used to track granular motion. After impact, the sphere surfaces are examined to measure plastic contact areas, allowing the dissipated plastic energy to be estimated. To support the experiments, capture the lateral confining effect of the membrane and applied pressure, and investigate larger arrays, discrete element simulations are conducted. Results When the granular array is confined, we observed shorter and consistent granular trajectories between trials, a greater dissipated energy to net input energy ratio, and a stiffer membrane response. Conclusion Experimental and numerical results indicate that the external confining pressure increases plastic dissipation.
PubDate: 2022-06-01

• An Inverse Identification Method for the Characterization of Elastic
Conforming Contact Behavior During Flat Punch Indentation

Abstract: Background The flat punch indentation problem is a typical prototype of conforming contact which can be frequently encountered in many applications. Its characterization is of great difficulty as the interface is embedded and the stress is highly concentrated at the contact boundary. Objective In this work, an inverse identification method is developed for reconstructing the interfacial stress during flat punch indentation from the measured displacement field. Methods This method consists of a modeling procedure and a characterization procedure. In the modeling, the basic tendencies of interfacial stress distributions are established and the relationship between the stress and displacement are formulated. In the characterization, the parameters in the model, including the profiles of contact interface and the stress distributions are optimized by matching the calculated displacement to the experiment data. Segmentation-aided digital image correlation is implemented for the displacement acquisition near the contact interface. An optimization framework is developed so the different kinds of parameters can be accurately obtained. Results Both of simulated experiment and real-world experiment are carried out, and results show that the proposed method can accurately characterize the elastic contact behavior. Conclusions The modeling merely constrains the tendencies rather than gives a close-form solution of interfacial stress and the identification only requires the measurement of local displacement, which can greatly increase its applicability in applications.
PubDate: 2022-06-01

• Particle-Assisted Laser-Induced Inertial Cavitation for High Strain-Rate
Soft Material Characterization

Abstract: Background While there are few reliable techniques for characterizing highly compliant and viscoelastic materials under large deformations, laser-induced Inertial Microcavitaton Rheometry (IMR) was recently developed to fill this void and to characterize soft materials at high to ultra-high strain rates ( $$O(10^{3}) \sim O(10^{8})$$ s $$^{-1}$$ ). Yet, one of the current limitations in IMR has been the dependence of the cavitation nucleation physics on the intrinsic material properties often generating extreme deformation levels and thus complicating material characterization procedures. Objective The objective of this study was to develop an experimental approach for modulating laser-induced cavitation (LIC) bubble amplitudes and their resulting maximum material deformations. Lowering the material stretch ratios during inertial cavitation will provide an experimental platform of broad applicability to a large class of polymeric materials and environmental conditions. Methods Experimental methods include using three types of micron-sized nucleation seed particles and varying laser energies in polyacrylamide hydrogels of known concentration. Using a Quadratic law Kelvin-Voigt material model, we implemented ensemble-based data assimilation (DA) techniques to robustly quantify the nonlinear constitutive material parameters, up through the first, second, and third bubble collapse cycles. Fitted values were then used to simulate bubble dynamics to compute critical bubble collapse Mach numbers, and to assess time-varying uncertainties of the full cavitation dynamics with respect to the current state-of-the art theoretical model featured in the IMR model. Results While varying laser energy modulated bubble amplitude, seed particles successfully expanded (more than doubled) the finite deformation regime (i.e., maximum material stretch, $$\lambda _{max} \approx$$ 4 - 9). Comparing experimental data to IMR simulations, we found that fitting beyond the first bubble collapse, as well as increasing laser energy, increased the bubble radius fit error, and larger $$\lambda _{max}$$ values exhibited increasingly violent bubble behavior (marked by increasing collapse Mach numbers greater than 0.08). Additionally, time-varying analysis showed the greatest model uncertainty during initial bubble collapse, where bubbles nucleated at lower laser energies and resulting $$\lambda _{max}$$ had less uncertainty at collapse compared to higher laser energy and $$\lambda _{max}$$ cases. Conclusions This study indicates IMR’s current theoretical framework might be lacking important additional cavitation and/or material physics. However, expanding the finite deformation regime of soft materials to attain lower stretch regimes enables broader applicability to a larger class of soft polymeric materials and will enable future, systematic development and incorporation of more complex physics and constitutive models including damage and failure mechanisms into the theoretical framework of IMR.
PubDate: 2022-05-17

• Uniaxial Magnetic Pulse Tension of TiNi Alloy with Experimental Strain
Rate Evaluation

Abstract: Background Magnetic pulse methods are known since the 80 s and have become widespread for revealing the patterns of fracture processes. The magnetic pulse method can be modified for uniaxial high strain rate tension and be used to investigate the mechanical and functional properties of materials. Objective The paper shows capabilities of the magnetic pulse method modified for uniaxial high strain rate tension, the scheme of experimental estimation of strain accumulation time and reveals the influence on the basic functional properties of the TiNi shape memory alloy. Method The special shaped TiNi alloy specimens were deformed in tension mode using the modified magnetic pulse method. The one-way shape memory effects were measured and compared with ones after quasi-static tension. We used COMSOL Multiphysics to evaluate possible heating of the specimens during tests. Results The technique resulted in a wide range of plastic strain rates from 2000s−1 to 10000 s−1, depending on the specimen’s mass and residual strain. COMSOL Multiphysics simulation did not show the presence of induced currents or heating in the working parts of the specimens during the tests. The shape memory effect after magnetic pulse tension was lost compared to the shape memory effect after quasi-static deformation. Conclusions The method allows obtaining various strain rates at the same residual strains without changing in the loading system or dimensions of the working parts of the specimens. The shape memory effect depends on the time for pre-strain accumulation: the shorter the time, the less the shape memory effect upon subsequent heating.
PubDate: 2022-05-13

• Imposition of Constraints on the Regularized Integral Method of
Incremental Hole-Drilling

Abstract: Background Incremental hole-drilling (IHD) and X-ray diffraction (XRD) are two of the most commonly used methods to measure through-thickness residual stress variation. IHD readily provides interior stress data, but the technique is prone to large uncertainty near the surface when steep stress gradients exist. XRD provides excellent near-surface data but insufficient penetration to readily define the stress distribution below the surface. To exploit the best features of each of these complementary residual stress measurement techniques, a means of combining measurements from XRD and IHD was recently demonstrated through the imposition of constraints on a series expansion solution. The regularized integral method is, however, the industry standard. A need therefore exists for a similar approach using the latter method. Objective Develop and demonstrate an approach to constrain the solution of the regularized integral method using data from complementary measurement techniques. Methods Constraints are enforced using the Lagrange multiplier method. The resulting equations are of closed form and make use of readily available information. The method is demonstrated on an aluminium alloy 7075 specimen of 10 mm thickness subjected to laser shock peening. Results Residual stress distributions obtained using the constrained and standard regularized integral methods compare well throughout the hole depth. The effect of incorporating XRD data into the constrained solution is localized to the near-surface region where the uncertainty is reduced. Conclusions Incorporation of XRD data into IHD results is readily achieved and allows the advantages of both techniques to be utilised while minimizing their shortcomings.
PubDate: 2022-05-11

• On the Cover: A Comparison of Conventional Gel Stiffness Characterization
Techniques with Cavitation Rheology

PubDate: 2022-05-10

• Subsurface and Bulk Residual Stress Analysis of S235JRC + C Steel TIG
Weld by Diffraction and Magnetic Stray Field Measurements

Abstract: Background Due to physical coupling between mechanical stress and magnetization in ferromagnetic materials, it is assumed in the literature that the distribution of the magnetic stray field corresponds to the internal (residual) stress of the specimen. The correlation is, however, not trivial, since the magnetic stray field is also influenced by the microstructure and the geometry of component. The understanding of the correlation between residual stress and magnetic stray field could help to evaluate the integrity of welded components. Objective This study aims at understanding the possible correlation of subsurface and bulk residual stress with magnetic stray field in a low carbon steel weld. Methods The residual stress was determined by synchrotron X-ray diffraction (SXRD, subsurface region) and by neutron diffraction (ND, bulk region). SXRD possesses a higher spatial resolution than ND. Magnetic stray fields were mapped by utilizing high-spatial-resolution giant magneto resistance (GMR) sensors. Results The subsurface residual stress overall correlates better with the magnetic stray field distribution than the bulk stress. This correlation is especially visible in the regions outside the heat affected zone, where the influence of the microstructural features is less pronounced but steep residual stress gradients are present. Conclusions It was demonstrated that the localized stray field sources without any obvious microstructural variations are associated with steep stress gradients. The good correlation between subsurface residual stress and magnetic signal indicates that the source of the magnetic stray fields is to be found in the range of the penetration depth of the SXRD measurements.
PubDate: 2022-05-02

• Near Surface Residual Stress Measurement Using Slotting

Abstract: Background There are various experimental measurement techniques used to measure residual stress and this work describes one such method, the slotting method, and its application to measure near surface residual stresses. Objective This work examines its application to macro-scale specimens. Methods A series of numerical experiments were performed to understand the size required to assume that the specimen is infinitely large, namely the thickness, width, and height. To assess measurement repeatability, 12 slotting measurements were performed in a shot peened aluminum plate. Results The numerical experiments determined the specimen should have a thickness greater than or equal to 21.6 mm (0.85 in), a total specimen width (normal to the slot length) greater than or equal to 44.5 mm (1.75 in), and total height (parallel to the slot) greater than or equal to 38.1 mm (1.5 in) for the specimen to be assumed to be infinite. Slotting measurement repeatability was found to have a maximum repeatability standard deviation of 30 MPa at the surface that decays rapidly to 5 MPa at a depth of 0.3 mm from the surface. Comparison x-ray diffraction measurements were performed. Conclusions Infinite plate dimensions and slot length were determined as well as measurement repeatability. Slotting was shown to have significantly better repeatability than X-ray diffraction with layer removal for this application.
PubDate: 2022-04-29

• Size-Dependent Fracture Characteristics of Intermetallic Alloys

Abstract: Background Lightweight alloys such as intermetallic titanium aluminide (TiAl) alloys are poised to be a potential candidate for replacing heavier nickel based super alloys in an aero engine. However, before an industry wide implementation is possible, it is indispensable to develop physically accurate computational material models which account for essential deformation and fracture mechanisms. This assists the virtual prototyping required for the new product development using TiAl components. Objective The objective of this work is to determine the effect of size of tested specimens on their fracture energy and provide a physically motivated scaling law. Methods In this work, the quasi-brittle behavior of TiAl alloys is experimentally and numerically investigated. A total number of 29 geometrically identical TiAl specimens of three different sizes are tested in a three-point bending setup. Since the final abrupt failure of each specimen is preceded by plasticity, a theoretical and numerical framework which accounts for both elastic and plastic work densities is applied in simulations. Results The fracture energy density for each tested size is calculated numerically which is found to be lower for larger volumes, thereby, confirming the size effect in intermetallic TiAl alloys. A novel size effect law is proposed which is based on two physically motivated coefficients. Conclusions The work concludes with the quantitative knowledge of the size-dependent fracture energy of intermetallic alloys and an empirical scaling law to predict the same. Excellent predictive capability of the proposed law is successfully established with data of various quasi-brittle materials from literature.
PubDate: 2022-04-04

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