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In this study, an experimental methodology based on micromechanical testing inside a scanning electron microscope is proposed to characterise bonding of paper layers connected by wet pressing. The peeling force–displacement evolution law that characterises the delamination of micromechanical double cantilever beam specimens of paper tissue have been extracted from such peeling tests. It is observed that the force–displacement evolution curve achieves a steady-state value related to the effective adhesive energy of the interface. This behaviour is explained by examining the complex load transfer mechanism between the layers exerted by cellulose fibrils. A statistical approach is used for the computation of the effective adhesive energy. It is argued that the observed force–displacement evolution law may be satisfactory described by a stochastic model that depends on the distribution function of the fibrils strength, and on two geometrical distribution functions related to the in-plane and out-of-plane fibrils angles with respect to the undeformed interface configuration. Some applications of the proposed model are demonstrated on examples. [ABSTRACT FROM AUTHOR]

[Display omitted] • A validated PBO/EP single fiber pull-out test method is proposed. • The IFSS was found to have an inverse relationship with the test temperature. • The accuracy of single fiber pull-out was verified by COMSOL simulation results. In the micromechanical test of fiber-reinforced composites, single fiber pull-out test is the most widely used test method. However, for high performance fibers/matrix, different test equipment and sensor accuracy can affect the single fiber pull-out test results very much. In this study, an innovative single fiber pull-out test method is proposed for testing the interfacial shear strength (IFSS) of high performance fibers/resins at different temperatures. Single fiber pull-out test was carried out on PBO fiber/epoxy resin (EP) composites at room temperature and elevated temperature. The IFSS was found to have an inverse relationship with the test temperature. The IFSS value of PBO/EP decreased from 25.25 MPa at room temperature to 6.25 MPa as the temperature approached the glass transition temperature. COMSOL simulations showed that fiber pull-out with angular tilt exhibited better interfacial strength, demonstrating the validity of the method. The results of this paper provide ideas to study the effect of temperature on the mechanics of fiber-reinforced resin matrix composites and expand the way to design stiffer, stronger and more robust composites. [ABSTRACT FROM AUTHOR]

A micro-mechanical technique is used to determine values of the critical stress intensity factor for fracture for grain boundaries of various orientations in Ni alloy 600 exposed to Pressurized Water Reactor (PWR) primary water at 325 °C with a hydrogen partial pressure of 30 kPa. Pentagonal cross-section cantilevers 5 μm wide by 25 μm long were milled using a focused ion beam (FIB) at individual grain boundaries in unoxidised Alloy 600 samples and in samples that had been exposed to simulated PWR environment for 4500 h and for 1500 h. The cantilevers were notched at the grain boundaries using FIB and tested using a nanoindenter to deflect them in bending. The critical stress intensity factor for the fractured cantilevers in samples that had been exposed for 4500 h was measured to be between 0.73 and 1.82 MPa(m) 1/2 . No intergranular fracture occurred in the samples that had been exposed for 1500 h and in the unoxidised samples. No direct correlation was observed between the grain boundary misorientation angle and the critical stress intensity factor. [ABSTRACT FROM AUTHOR]

Knowledge of the fracture characteristics of individual microstructural features such as grains and grain boundaries is essential for a full understanding of the macroscopic behaviour of ceramics. We have investigated the fracture of single crystal, bicrystal and polycrystalline Al 2 O 3 on the micrometre scale by testing microcantilevers containing notches and artificial flaws introduced at features of interest (e.g. grain boundaries) using focused ion beams (FIB). The specimens were loaded to fracture using a nanoindenter and tests were performed under air, water and silicone oil. The results were influenced significantly by ion implantation at the notch tip, moisture assisted slow crack growth and finite notch tip radius. Methods of avoiding these effects or correcting for them are proposed. It is shown that using these methods, the fracture toughness ( K c ) and threshold stress intensity for subcritical crack growth ( K 0 ) can be measured in individual grains and grain boundaries of typical microstructures. [ABSTRACT FROM AUTHOR]

In situ scanning electron microscope (SEM) experiments, where small-scale mechanical tests are conducted on micro- and nanosized specimens, allow direct visualization of elastic and plastic responses over the entirety of the volume being deformed. This enables precise spatial and temporal correlation of slip events contributing to the plastic flow evidenced in a stress-strain curve. A new pin-loading methodology has been employed, in situ within the SEM, to conduct microtensile tests on thin polycrystalline metal foils. This approach can be tailored to a specific foil whose particular grain size may range from microns to tens of microns. Manufacture of the specialized pin grip was accomplished via silicon photolithography-based processing followed by subsequent focused ion beam finishing. Microtensile specimen preparation was achieved by combining a stencil mask methodology employing broad ion beam sputtering along with focused ion beam milling in the study of several metallic foil materials. Finite-element analyses were performed to characterize the stress and strain distributions in the pin grip and micro-specimen under load. Under appropriately conceived test conditions, uniaxial stress-strain responses measured within these foils by pin-load microtensile testing exhibit properties consistent with larger scale tests. [ABSTRACT FROM AUTHOR]

The Mg-Al-Ca system has been shown to be a promising alloy system since it combines the low density of Mg with a high creep resistance at high alloying contents and a high ductility for dilute alloys, while simultaneously avoiding the requirement of alloying with costly rare earth elements. Nevertheless, the adequate preparation of the alloy system for subsequent microstructural, mechanical, electrochemical and defect analysis is challenging. Therefore, within this publication, we present and apply methods for the structural, mechanical, electrochemical and defect analysis with a focus on the corresponding required metallographic preparation methods in order to support research on the alloy system. • For solid solution Mg-Al-Ca and Mg-Al-Ca composites, electropolishing is required for EBSD, slip trace analysis and micromechanical tests • No electropolishing was applied to avoid chemical modification of the specimen surface if corrosion testing was performed • The CaMg 2 Laves phase required electropolishing for subsequent EBSD analysis and micromechanical testing, but would result in an undesired topography for the CaAl 2 phase [ABSTRACT FROM AUTHOR]

Crack initiation governs high cycle fatigue life and is sensitive to microstructural details. While corresponding microstructure-sensitive models are available, their validation is difficult. We propose a validation framework where a fatigue test is mimicked in a sub-modeling simulation by embedding the measured microstructure into the specimen geometry and adopting an approximation of the experimental boundary conditions. Exemplary, a phenomenological crystal plasticity model was applied to predict deformation in ferritic steel (EN1.4003). Hotspots in commonly used fatigue indicator parameter maps are compared with damage segmented from micrographs. Along with the data, the framework is published for benchmarking future micromechanical fatigue models. [Display omitted] • Workflow for validation of micromechanical fatigue simulations against experiments. • A validation approach on measured microstructures for damage localization. • Comparatively large specimen areas enable validation in the challenging HCF regime. • Supervision through deep learning segmentation for validating protrusions & cracks. • Multimodal dataset and code published to test other fatigue damage formation models. [ABSTRACT FROM AUTHOR]

Abstract: This study performed microtensile testing to investigate the mechanical properties of cyclic fatigue in freestanding nanocrystalline copper thin films of sub-micrometer thickness. We observed the mechanical response associated with tension–tension fatigue under various stress amplitudes and mean stress conditions at cyclic loading frequencies of up to 10Hz. Experiments were carried out using feedback to provide load control for sputter deposited 300, 500, and 700nm Cu thin films. Cu films were deposited on the Si substrate and then separated from the substrate following the completion of processing. The feedback control maintains the stress within the film at a constant value, even when the stress threatens to drop due to plastic deformation during the course of the experiment. As anticipated, the number of loading cycles to failure exceeded 106 under low mean loads at low amplitudes with an increase in the load leading to a decrease in the number of cycles to failure. These results provide clear evidence of cyclic creep rate dependence and changes in the failure mechanism from crack formation to extended plasticity following a decrease in the mean load or load amplitude. Moreover, we observed how the fatigue and cyclic creep of the tested films depended on the length scale. [Copyright &y& Elsevier]

Abstract: The development of single fibre pull-out and microbond tests for characterising interfacial strength in thermoplastic composites is reviewed in detail. Manufacture of an experimental jig and sample preparation regimes for both tests are described. The challenges addressed in the sample preparation include the measurement of embedded fibre length for pull-out samples and the low yield rate of axisymmetric resin droplets obtained during sample preparation under nitrogen. The applications of these laboratory developed techniques are demonstrated by characterisation of the interfacial shear strength (IFSS) of glass fibre-polypropylene (GF-PP) and natural fibre-polylactic acid (NF-PLA). The comparison of the IFSS between neat and modified GF-PP showed that both methods were sensitive to the interfacial performance change despite the poor agreement between them for the absolute IFSS values from the same composite. The effect of the material modification was also reflected in load-displacement curves with different behaviour of the frictional motion after complete debonding. When a high level of fibre-matrix adhesion was realised in the composites with weak fibres, the microbond test showed higher feasibility for characterising the IFSS. This was clearly shown in its application to NF-PLA. [Copyright &y& Elsevier]

Abstract: Many biomaterials and tissues are complex multilayered structures in which the individual layers have distinct mechanical properties that influence the mechanical behavior and define the local cellular microenvironment. Characterization of the mechanical properties of individual layers in intact tissues is technically challenging. Micropipette aspiration (MA) is a proven method for the analysis of local mechanical properties of soft single-layer biomaterials, but its applicability for multilayer structures has not been demonstrated. We sought to determine and validate MA experimental parameters that would permit measurement of the mechanical properties of only the top layer of an intact multilayer biomaterial or tissue. To do so, we performed parametric nonlinear finite-element (FE) analyses and validation experiments using a multilayer gelatin system. The parametric FE analyses demonstrated that measurement of the properties of only the top layer of a multilayer structure is sensitive to the ratio of the pipette inner diameter (D) to top layer thickness (t top), and that accurate measurement of the top layer modulus requires D/t top <1. These predictions were confirmed experimentally by MA of the gelatin system. Using this approach and an inverse FE method, the mean effective modulus of the fibrosa layer of intact porcine aortic valve leaflets was determined to be greater than that of the ventricularis layer (P <0.01), consistent with data obtained by tensile testing of dissected layers. This study provides practical guidelines for the use of MA to measure the mechanical properties of single layers in intact multilayer biomaterials and tissues. [Copyright &y& Elsevier]

Single-crystal α-Ti microcantilevers, with various pre-selected crystal orientations, have been manufactured from a polycrystalline commercially pure Ti sample using a focused ion beam. A nanoindenter was used to conduct microbending tests. Relatively small deflections of ∼500 nm were used to measure the stiffness of the beams. Young's modulus along the beam's length direction was determined from the measured compliance by means of both simple beam theory and finite-element analysis. The results show that when the effects of flexure at the 'fixed' end were accounted for, by finite-element modelling, the values of Young's modulus were in excellent agreement with available macroscopic data. [ABSTRACT FROM AUTHOR]

Abstract: Tendon is a hydrated multi-level fibre composite, in which time-dependent behaviour is well established. Studies indicate significant stress relaxation, considered important for optimising tissue stiffness. However, whilst this behaviour is well documented, the mechanisms associated with the response are largely unknown. This study investigates the sub-structural mechanisms occurring during stress relaxation at both the macro (fibre) and nano (fibril) levels of the tendon hierarchy. Stress relaxation followed a two-stage exponential behaviour, during which structural changes were visible at the fibre and fibril levels. Fibril relaxation and fibre sliding showed a double exponential response, while fibre sliding was clearly the largest contributor to relaxation. The amount of stress relaxation and sub-structural reorganisation increased with increasing load increments, but fibre sliding was consistently the largest contributor to stress relaxation. A simple model of tendon viscoelasticity at the fibril and fibre levels has been developed, capturing this behaviour by serially coupling a Voigt element (collagen fibril), with two Maxwell elements (non-collagenous matrix between fibrils and fibres). This multi-level analysis provides a first step towards understanding how sub-structural interactions contribute to viscoelastic behaviour. It indicates that nano- and micro-scale shearing are significant dissipative mechanisms, and the kinetics of relaxation follows a two-stage exponential decay, well fitted by serially coupled viscoelastic elements. [Copyright &y& Elsevier]

To study the intragranular mechanical behavior of a Sn-Ag-Cu solder, a focused ion beam system was used to manufacture a notched microspecimen at a eutectic area within a grain. A low-speed tensile test was performed on the microspecimen using an Instron 5848 microtester at room temperature. The results show that, besides an obvious shearing deformation, a large number of subgrains are formed in the β-Sn matrix. Subgrain boundaries are composed of a large number of dislocations. Besides the heterogeneous deformation due to the notch, Ag3Sn intermetallic compounds influence the evolution path of subgrain boundaries. [ABSTRACT FROM AUTHOR]

Abstract: Adhesion forces between tetrahydrofuran (THF) hydrate particles in n-decane were measured using an improved micromechanical technique. The experiments were performed at atmospheric pressure over the temperature range 261–275 K. The observed forces and trends were explained by a capillary bridge between the particles. The adhesion force of hydrates was directly proportional to the contact force and contact time. A scoping study examined the effects of temperature, anti-agglomerants, and interfacial energy on the particle adhesion forces. The adhesion force of hydrates was found to be directly proportional to interfacial energy of the surrounding liquid, and to increase with temperature. Both sorbitan monolaurate (Span20) and poly-N-vinyl caprolactam (PVCap) decreased the adhesion force between the hydrate particles. [Copyright &y& Elsevier]

Particle–particle pulloff adherence forces were measured as a function of temperature in the ice/n-decane/ice and tetrahydrofuran (THF) hydrate/n-decane/THF hydrate systems using a newly developed micromechanical testing technique. Experiments using ∼200 μm radius particles were performed at atmospheric pressure over the temperature range 263–275 K. The ice and hydrate particles displayed very similar behavior. While the measured adherence forces had significant variation, the shapes of the cumulative force distribution curves were similar among the different sets of experiments. The measured adherence forces distributions shifted to lower force values as the temperature was decreased from the solid melting temperature. The observed forces and trends were explained by the capillary cohesion of rough surfaces, with the capillary bridging liquid being stabilized below its freezing point by the negative curvature of the bridging liquid/n-decane interface. [Copyright &y& Elsevier]

The stiffness and fracture toughness of a thermally grown oxide (TGO) on a MCrAlY coating were measured by the microcantilever bending technique. TGO cantilevers were prepared by a focused ion beam and bent in-situ by a nanoindenter in a scanning electron microscope. The Young's modulus and fracture toughness of the TGO were determined to be 352 GPa and 4.4 MPa·m1/2, respectively. The measurement results were compared with mechanical properties of the TGOs and bulk α-Al 2 O 3 ceramics reported in the literature to develop a mechanistic understanding of the microstructural features governing the stiffness and fracture toughness of the TGO. • Mechanical properties of a TGO have been measured by micro-beam bending. • Basal texture reduces the stiffness of the TGO. • The TGO has higher fracture toughness than bulk α-Al 2 O 3 ceramics. [ABSTRACT FROM AUTHOR]

The main goal of this work is to obtain a reliable value of the fracture toughness of single grains of tungsten carbide (WC). It is attempted by testing microcantilever and micropillar samples shaped out, by means of focused ion beam milling, of individual WC particles embedded within a WC-Co cemented carbide grade. Experimental testing included the use of a nanoindenter for inducing microcantilever bending and micropillar splitting, as well as sampling WC grains with basal and prismatic orientations. Experimental results are compared with those previously assessed through implementation of the indentation microfracture technique. Bending of notched microcantilevers yielded more consistent results than those measured out of micropillar splitting tests. In this regard, the average value of fracture toughness for single WC grains, within the two-phase interpenetrated network existing in cemented carbides, is found to be 5.6 ± 0.8 MPa·m1/2. Such a relatively high value – coherent with local plastic features evidenced in nanoindentation imprints - is in satisfactory agreement with results indirectly estimated from other macromechanical tests. • Fracture toughness of single WC grains is measured by micromechanical methods. • Microcantilevers produce the most consistent results. • Fracture toughness of WC grains of cemented carbides, is found to be 5.6 MPa·m1/2. [ABSTRACT FROM AUTHOR]

Cells sense and respond to the heterogeneous mechanical properties of their tissue microenvironment, with implications for the development of many diseases, including cancer, fibrosis, and aortic valve disease. Characterization of tissue mechanical heterogeneity on cellular length scales of tens of micrometers is thus important for understanding disease mechanobiology. In this study, we developed a low-cost bench-top microindentation system to readily map focal microscale soft tissue mechanical properties. The device was validated by comparison with atomic force microscopy nanoindentation of polyacrylamide gels. To demonstrate its utility, the device was used to measure the focal microscale elastic moduli of normal and diseased porcine aortic valve leaflet tissue. Consistent with previous studies, the fibrosa layer of intact leaflets was found to be 1.91-fold stiffer than the ventricularis layer, with both layers exhibiting significant heterogeneity in focal elastic moduli. For the first time, the microscale compressive moduli of focal proteoglycan-rich lesions in the fibrosa of early diseased porcine aortic valve leaflets were measured and found to be 2.44-fold softer than those of normal tissue. These data provide new insights into the tissue micromechanical environment in valvular disease and demonstrate the utility of the microindentation device for facile measurement of the focal mechanical properties of soft tissues. [ABSTRACT FROM AUTHOR]

Fiber reinforced composite materials are typically comprised of two phases, i.e., the reinforcing fibers and a surrounding matrix. At a high volume fraction of reinforcing fibers, the matrix is confined to a microscale region in between the fibers (1–200 µm). Although these regions are interconnected, their behavior is likely dominated by their micro-scale. Nevertheless, the characterization of the matrix material (without reinforcing fibers) is usually performed on macroscopic bulk specimens and little is known about the micro-mechanical behavior of polymer matrix materials. Here, we show that the microscale behavior of an epoxy resin typically used in composite production is clearly different from its macroscale behavior. Microscale polymer specimens were produced by drawing microfibers from vitrifying epoxy resin. After curing, tensile tests were performed on a large set of pure epoxy microfiber specimens with diameters ranging from 30 to 400 µm. An extreme ductility was observed for microscale epoxy specimens, while bulk scale epoxy specimens showed brittle behavior. The microsized epoxy specimens had a plastic deformation behavior resulting in a substantially higher ultimate tensile strength (up to 380 MPa) and strain at break (up to 130 %) compared to their bulk counterpart (68 MPa and 8%). Polarized light microscopy confirmed a rearrangement of the internal epoxy network structure during loading, resulting in the plastic deformation of the microscale epoxy. This was further accompanied by in-situ electron microscopy to further determine the deformation behavior of the micro-specimens during tensile loading and make accurate strain measurements using video-extensometry. This work thus provides novel insights on the epoxy material behavior at the confined microscale as present in fiber reinforced composite materials. [ABSTRACT FROM AUTHOR]

Compression tests at low strains were performed to theoretically analyze the effects of anisotropic elasticity, misorientation, grain boundary (GB) stiffness, interfacial dislocations, free surfaces, and critical force on dislocation pile-ups in micro-sized Face-Centered Cubic (FCC) Nickel (Ni) and α -Brass bi-crystals. The spatial variations of slip heights due to localized slip bands terminating at GB were measured by Atomic Force Microscopy (AFM) to determine the Burgers vector distributions in the dislocation pile-ups. These distributions were then simulated by discrete pile-up micromechanical calculations in anisotropic bi-crystals consistent with the experimentally measured material parameters. The computations were based on the image decomposition method considering the effects of interphase GB and free surfaces in multilayered materials. For Ni and α -Brass, it was found that the best predicted step height spatial profiles were obtained considering anisotropic elasticity, free surface effects, a homogeneous external stress and a certain critical force in the material to equilibrate the dislocation pile-ups. [ABSTRACT FROM AUTHOR]

Photolithographically defined thin film Au dots were used as micro fiducial markers for digital image correlation (DIC), to enable two-dimensional strain measurement of 200 μ m-thick LIGA (Lithographie, Galvanformung, Abformung) nickel alloys. Due to the sensitivity of electrodeposited films' microstructure and properties on the processing conditions, characterization of LIGA mechanical properties continues to be necessary for microsystems commercialization. DIC offers advantages over laser-based strain measurement techniques but creating suitable speckle patterns on specimens with dimensions under a millimeter is challenging. The material surface roughness itself is often used as the speckle pattern, or micro- or nanoparticles are applied to the surface. But for materials with highly polished surfaces, such as commercial LIGA alloys, the surface roughness is not always suitable, while application of particles still poses technical challenges in uniformity and reproducibility. We fabricated freestanding tensile specimens, with gauge sections 700 μ m wide × 3 mm long × 200 μ m thick, from electrodeposited Ni-10% Co using a commercial LIGA process, and conducted microtensile tests at strain rate 0.001 s -1 . Designing and fabricating arrays of randomly oriented 1.5 μ m-thick Au dots on the specimens provided a suitable way to obtain full-field surface strains over the entire gauge lengths and was reproducible from one specimen to another. Microfabricated fiducial markers therefore can be a useful surface-preparation approach for investigating micromechanical behavior, particularly plasticity and fracture, of LIGA films using DIC.

Tensile strength is a critical design parameter to ensure reliability of silicon carbide devices. However, SiC's tensile strength depends on the dominant flaw subjected to maximum tensile stress. Typically, processing related flaws (i.e. voids or inclusions) dominate the tensile response thereby masking role of the underlying microstructure. To probe the intrinsic microstructural flaws in a hot-pressed SiC microscale tensile bars were machined using a custom femtosecond laser micromachining apparatus from thin sections. When tested in tension, the fracture strength was dramatically higher compared to bulk tensile strengths. An analysis of strength-size scaling suggested a transition in the dominant strength-determining flaw at the microscale. Analysis of fracture surfaces and critical flaw sizes suggest failure is governed by isolated large grains. These observations have important implications for parameterizing ceramic failure models. [ABSTRACT FROM AUTHOR]

A photolithographic process for the rapid fabrication of thin-film tensile-test structures is presented. The process is applicable to various physical vapor deposition techniques and can be used for the combinatorial fabrication of thin-film tensile-test structure materials libraries for the high-throughput characterization of mechanical properties. The functionality of the fabrication process and the feasibility of performing high-quality measurements with these structures are demonstrated with Cu tensile-test structures. In addition, the scalability from unary structures to libraries with compositional variations is demonstrated.