Micromechanical Testing Research Articles

Intrinsic-extrinsic size effect relationship for micromechanical tests

Miniaturized mechanical tests are commonly utilized to evaluate properties of materials, including thin films, nanostructured, and irradiated materials. However, the specimen size effect occurs when miniaturized sample geometries contain too few dislocation sources, resulting in elevated yield stresses. The size effect is controlled by extrinsic (specimen dimensions) and intrinsic (microstructure) factors. Here, we summarize extrinsic and intrinsic size effects from micro-compression pillar, micro-cantilever bend, and flexure studies reported in the archival literature. We find an approximately linear relationship between intrinsic and extrinsic size effects. Meaningful mechanical properties can be measured when extrinsic size dominates the intrinsic size.

Optimization of a dissimilar platinum to niobium microresistance weld: a structure–processing–property study

Dissimilar metal resistance spot welds, critical to the manufacture of medical devices, typically form brittle intermetallic compounds that are prone to failure. Here, a case study of biocompatible metals platinum and niobium using advanced analytical techniques is presented. It describes the variation of properties and microstructure using microresistance spot welding under four conditions, including a legacy process and processing conditions optimized by design of experiments. Adjustments to the electrode force, welding current, surface roughness, and pulse duration and exchanging the platinum anode contact for a cathode result in a joint with less porosity and greater uniformity in the thickness, chemistry, and microstructure of the fusion zone. The optimized microstructure contains fewer defects, with increased plasticity under deformation and a more uniform microstructure reducing the propensity for failure and variability between welds. Extensive analysis with optical, scanning electron, transmission electron microscopy coupled with nano- and micromechanical testing (such as micropillar compression) was used to characterize the weld zone.

Correlating diameter, mechanical and structural properties of poly(l-lactide) fibres from needleless electrospinning

The development and application of nanofibres requires a thorough understanding of the mechanical properties on a single fibre level including respective modelling tools for precise fibre analysis. This work presents a mechanical and morphological study of poly-l-lactide nanofibres developed by needleless electrospinning. Atomic force microscopy (AFM) and micromechanical testing (MMT) were used to characterise the mechanical response of the fibres within a diameter range of 200–1400 nm. Young’s moduli E determined by means of both methods are in sound agreement and show a strong increase for thinner fibres below a critical diameter of 800 nm. Similar increasing trends for yield stress and hardening modulus were measured by MMT. Finite element analyses show that the common practice of modelling three-point bending tests with either double supported or double clamped beams is prone to significant bias in the determined elastic properties, and that the latter is a good approximation only for small diameters. Therefore, an analytical formula based on intermediate boundary conditions is proposed that is valid for the whole tested range of fibre diameters, providing a consistently low error in axial Young’s modulus below 10%. The analysis of fibre morphology by differential scanning calorimetry and 2D wide-angle X-ray scattering revealed increasing polymer chains alignment in the amorphous phase and higher crystallinity of fibres for decreasing diameter. The combination of these observations with the mechanical characterisation suggests a linear relationship between Young’s modulus and both crystallinity and molecular orientation in the amorphous phase. Statement of significanceFibrous membranes have rapidly growing use in various applications, each of which comes with specific property requirements. However, the development and production of nanofibre membranes with dedicated mechanical properties is challenging, in particular with techniques suitable for industrial scales such as needleless electrospinning. It is therefore a key step to understand the mechanical and structural characteristics of single nanofibres developed in this process, and to this end, the present work presents changes of internal fibre structure and mechanical properties with diameter, based on dedicated models. Special attention was given to the commonly used models for analyzing Young’s modulus of single nanofibers in three-point bending tests, which are shown to be prone to large errors, and an improved robust approach is proposed.

A Hierarchical Programmable Mechanical Metamaterial Unit Cell Showing Metastable Shape Memory

Hierarchically structuring materials open the door to a wide range of unexpected and uniquely designed properties. This work presents a novel mechanical metamaterial unit cell with strain‐dependent solid–solid phase changes resultant from hierarchically structured “mechanisms” built into an auxetic unit cell, and further presents a realization of this kind. The interaction of auxetic structure and mechanism allows stable or metastable elastic energy states to be reached as a result of mechanical deformation. The result is a principally elastic analog to a shape memory material with a functional dependency on its negative Poisson's ratio. Prototypes are additively manufactured using direct laser writing, and are subsequently subjected to uniaxial compression with a customized micromechanical test set up. Experimental results depict reversible states initially triggered by deformation; the unit cell is a building block for a programmable material with a nonlinear “if… then…” relationship. Implementing interior mechanisms as a hierarchical level unlocks new directions for mechanical metamaterials research, and the authors see potential impacts or applications in multi‐scale modeling, medicine, micro‐actuation and ‐gripping, programmable matter/materials.

On the influence of crystallography and dendritic microstructure on micro shear behavior of single crystal Ni-based superalloys

An in-situ SEM micromechanical test technique is used to investigate the response of a Ni-based single crystal superalloy to double shear loading. The present work shows that micro double shear testing can detect mechanical differences between interdendritic and dendritic regions with γ′-volume fractions of 77% and 72%, respectively, i.e., the interdendritic regions exhibit a larger flow stress than the dendritic regions. These micromechanical differences are apparent when micro double shear specimens are oriented for single-slip while they appear to be overshadowed by dislocation interactions, when multiple-slip is promoted. Sudden deformation events are observed to occur concomitantly with the formation of shear steps (localized plastic deformation) at the surface of the shear zones during single-slip. The micro double shear specimens oriented for single-slip show very low work-hardening. In contrast, much higher stresses are required to plastically deform micro double shear specimens oriented for multiple-slip which exhibit stronger work-hardening. No sudden deformation events could be detected for multiple-slip which results in a more homogeneous deformation of the shear zones (absence of localized plastic deformation).

Deformation twinning of ultrahigh strength aluminum nanowire

Recent studies have demonstrated that many metal nanowires (NWs) with low stacking-fault energies display ultrahigh strength and can accommodate large plastic strains by spreading mechanical twins throughout the entire volume of the NWs. This previous observation on the plasticity, however, is largely different from that exhibited by Al NWs. In situ tensile tests performed on the <110> Al NW revealed that the NW exhibited ultrahigh strength (∼2.7 GPa) and superelasticity (∼4.8%), while contrary to expectations, it failed quickly once plastic flow initiates and displayed a limited plasticity (∼1.2%). This low plasticity was attributed to the formation of thin-layered twins with a zigzag configuration. Upon further deformation, these twins self-locked with each other, which prevented the NWs from carrying further plastic strains. Here, by employing the in situ micro-mechanical test and the atomic simulations, we performed quantitative and comprehensive analyses to explore why Al NWs display ultrahigh strength and how twins with a zigzag configuration are formed in the Al NWs.

ANALYSIS OF TRIBOLOGICAL PROPERTIES OF SELECTED PTFE-BASED POLYMER COMPOSITES IN A SLIDING INTERACTION WITH ALUMINIUM OXIDE (Al2O3)

The paper presents the microstructure and mechanical and tribological properties of polymer composites based on polytetrafluoroethylene (PTFE) intended for use in friction couples where reciprocating motion is performed, e.g., in compressors or actuators. Micromechanical tests carried out using the Oliver-Pharr method showed that the PTFE composite with a 40% bronze content (T8B) had the most advantageous mechanical properties (hardness H, Young’s modulus E). In turn, tribological tests that were conducted using a ballon- disc tester in the linear (reciprocating) motion showed that the polytetrafluoroethylene composite with a mixture of 25% bronze powder and 15% graphite (T4GM) had the lowest tribological wear. The tribological properties of composite T5W with 25% graphite content were not much worse. Despite the most favourable mechanical parameters, the tribological wear of composites T8B and PTFE with glassy carbon (T3Ws) was nearly twice higher due to the absence of grease formed by a graphite filling. The results show that the use of composites containing a bronze-graphite filling improves the service life of lubricant-free friction couples that perform reciprocating motion.

The Tribological Behavior of Two Potential-Landslide Saprolitic Rocks

We investigate in this study the tribological behavior of two potential-landslide saprolites conducting grain-scale experiments with an advanced custom-built micro-mechanical loading apparatus. A lateritic rock from India and a completely decomposed granite from Hong Kong were used. These geological materials have been subjected to decomposition and disintegration due to weathering. The characterization of the materials at the grain scale was conducted based on energy-dispersive X-ray spectroscopy analysis, scanning electron microscope images, and interferometry analysis. Both materials showed high roughness values and their surface-morphological characteristics were relatively inconsistent. Repeated micro-mechanical shearing tests were conducted on pairs of grains, and their normal load—deflection, tangential load—deflection, tangential stiffness, and frictional responses were explored with a focus on the effect of repeating the shearing tests following the same shearing paths. It was shown that the behavior was markedly different, between shearing the grains at lower and greater normal loads; in the latter case the production of debris slightly increased the friction, but it substantially increased the tangential stiffness. These observations were also linked to the changes of the normal load—deflection behavior of surfaces subjected to pre-shearing. These findings are important in the fundamental understanding of the behavior of geological materials interfaces as well as the discrete modeling of natural and engineering problems. Digital microscopic images were also implemented to further support the observations from the study.

Bioactive coatings on porous titanium for biomedical applications

Commercial pure titanium is a recognized and accepted material for cortical bone tissue substitution. However, stress-shielding phenomena and lack of osseointegration result in significant limitations. This work is focused on the achievement of an effective solution for both problems via fabrication of porous titanium substrates coated with bioactive glass. Substrates were obtained through the space holder technique giving values of stiffness and yield strength compatible with cortical bone tissue to reduce the stress-shielding phenomenon. Titanium substrates were coated with different number of layers of bioactive glass 45S5 by dripping sedimentation. The substrates porosity was characterized by different techniques. Ultrasound, compression and micro-mechanical testing were used for mechanical properties evaluation. After substrates coating, the infiltration ability, coating homogeneity and structural integrity (chipping and cracking) were evaluated for each coating layer. The chemical composition of coating and phases were studied before and after in vitro tests in Simulated Body Fluid. The results showed more homogenous coating, adherence and greater hydroxyapatite growth for the tri-layer system in both dense and porous samples. Besides, the relation of Ca/P was closed to that of stoichiometric hydroxyapatite in the human body. The coated porous titanium could be potentially used in load bearing partial implants with improved osseointegration.

In-situ high temperature micromechanical testing of ultrafine grained yttria-stabilized zirconia processed by spark plasma sintering

Yttria-stabilized zirconia (YSZ) exhibits both enhanced strength and fracture toughness attributed to transformation-induced plasticity. Recent studies show that miniaturization of YSZ to the microscale enhances the effectiveness of stress-induced phase transformation by reducing mismatch stress among incommensurate grains. However, the fundamental understandings on the high temperature micromechanical behaviors of ultrafine grained YSZ remain limited. Here, we report on the high temperature (up to 670 °C) in-situ micromechanical testing of spark plasma sintered YSZ. The mechanical behaviors of YSZ tested below 400 °C are highlighted by large inelastic strain (∼7%) due primarily to phase transformation toughening. Beyond 400 °C, martensitic transformation toughening is gradually superseded by grain boundary sliding triggered by ultrafine grains. The micropillars tested at 670 °C exhibit significantly enhanced plastic flow, arising mainly from dislocation activity along with grain boundary sliding.

The Biomechanics of the Inter-Lamellar Matrix and the Lamellae During Progression to Lumbar Disc Herniation: Which is the Weakest Structure?

While microstructural observations have improved our understanding of possible pathways of herniation progression, no studies have measured the mechanical failure properties of the inter-lamellar matrix (ILM), nor of the adjacent lamellae during progression to herniation. The aim of this study was to employ multiscale, biomechanical and microstructural techniques to evaluate the effects of progressive induced herniation on the ILM and lamellae in control, pre-herniated and herniated discs (N = 7), using 2year-old ovine spines. Pre-herniated and herniated (experimental) groups were subjected to macroscopic compression while held in flexion (13°), before micro-mechanical testing. Micro-tensile testing of the ILM and the lamella from anterior and posterolateral regions was performed in radial and circumferential directions to measure failure stress, modulus, and toughness in all three groups. The failure stress of the ILM was significantly lower for both experimental groups compared to control in each of radial and circumferential loading directions in the posterolateral region (p < 0.032). Within each experimental group in both loading directions, the ILM failure stress was significantly lower by 36% (pre-herniation), and 59% (herniation), compared to the lamella (p < 0.029). In pre-herniated compared to control discs, microstructural imaging revealed significant tissue stretching and change in orientation (p < 0.003), resulting in a loss of distinction between respective lamellae and ILM boundaries.

Glancing-incidence focussed ion beam milling: A coherent X-ray diffraction study of 3D nano-scale lattice strains and crystal defects

This study presents a detailed examination of the lattice distortions introduced by glancing incidence Focussed Ion Beam (FIB) milling. Using non-destructive multi-reflection Bragg coherent X-ray diffraction we probe damage formation in an initially pristine gold micro-crystal following several stages of FIB milling. These experiments allow access to the full lattice strain tensor in the micro-crystal with ∼25 nm 3D spatial resolution, enabling a nano-scale analysis of residual lattice strains and defects formed. Our results show that 30 keV glancing incidence milling produces fewer large defects than normal incidence milling at the same energy. However the resulting residual lattice strains have similar magnitude and extend up to ∼50 nm into the sample. At the edges of the milled surface, where the ion-beam tails impact the sample at near-normal incidence, large dislocation loops with a range of Burgers vectors are formed. Further glancing incidence FIB polishing with 5 keV ion energy removes these dislocation loops and reduces the lattice strains caused by higher energy FIB milling. However, even at the lower ion energy, damage-induced lattice strains are present within a ∼20 nm thick surface layer. These results highlight the need for careful consideration and management of FIB damage. They also show that low-energy FIB-milling is an effective tool for removing FIB-milling induced lattice strains. This is important for the preparation of micro-mechanical test specimens and strain microscopy samples.

Probing elementary dislocation mechanisms of local plastic deformation by the advanced acoustic emission technique

To gain a deeper insight into fundamental processes of localised plastic deformation, we propose a new strategy for micro-mechanical testing combining the micro-scale scratching and analysis of low amplitude acoustic emissions (AE) accompanying dynamic dislocation processes in solids. The analysis of the AE spectral density reveals its strong dependence on crystallographic orientations of the grains along the indenter path, thus reflecting the finest features of the local dislocation activity. Adding a temporal dimension with a sub-microsecond resolution, the proposed methodology substantially enhances the existing capacity of micro-mechanical testing and can be used in a wide range of testing schemes.

In-situ elastic-plastic fracture mechanics on the microscale by means of continuous dynamical testing

Measuring the local behaviour of a propagating crack in a quantitative manner has always been a challenge in the field of fracture mechanics. In-situ microcantilever testing inside a scanning electron microscope (SEM) is one of the most promising techniques for the investigation thereof. However, quantifying such experiments is fairly challenging. Here, for the first time we utilize a continuous measurement of the dynamic compliance in-situ to permit evaluation of the crack length. Microcantilever experiments have been performed on brittle single crystalline Si and nanocrystalline Fe to assess the stability of the setup, the applicability of the technique inside an SEM and to establish a correlation between stiffness and crack length. Subsequently, micromechanical fracture tests were performed on single crystalline, 〈001〉{001} oriented tungsten as a model material and continuous J-Δa curves were measured. The gathered data was evaluated with close relation to standardized fracture mechanics testing and showed an overall agreement with literature data. This novel possibility to measure J-Δa curve behaviour continuously and locally while also following the crack extension through in-situ imaging inside an SEM is generally applicable and will allow new insights in the crack propagation of modern materials.

Nanomechanical testing at high strain rates: New instrumentation for nanoindentation and microcompression

Micromechanical testing is normally limited to the quasi-static strain rate regime: 10−5 to 10−2s−1. In this work, a nanomechanical testing device has been developed that allows indentation and microcompression at strain rates from the quasi-static regime continuously up to the high strain rate regime. To reach the highest strain rates, the conventional, strain gage-based load cell was replaced with a new piezo-based sensor. The sensor's concept, calibration methods, and measurement strategies are detailed. It is shown, using nanocrystalline Nickel as a case study material, that this new high strain rate device can measure precisely mechanical properties at strain rates up to 1000s−1 by nanoindentation, and strain rates up to 100s−1 by microcompression, enabling the device to measure strain rates over 9 orders of magnitude.

Static assessment of nanoscale notched silicon beams using the averaged strain energy density method

This paper extends the averaged Strain Energy Density (SED) method to the static assessment of notched components at the nanoscale. First, in situ micromechanical testing of notched nano-cantilever beams made of single-crystal silicon is briefly reviewed. Then, an alternative strategy based on the Theory of Critical Distances is employed to evaluate the control volume and the critical SED. The method is later verified against experiments and FE analyses. The SED method successfully estimates the load at fracture of nanoscale notched specimens with a maximum discrepancy of 4.7%. Moreover, the method is mesh-independent, and therefore very coarse meshes can be employed in numerical analyses. Finally, the results are discussed on the basis of the breakdown of continuum fracture mechanics at the nanoscale. The extension of the SED approach to the micro- and nanoscales provides a fast and simple tool for the design of micro- and nanodevices.

Determination of the best microstructure and titanium alloy powders properties using neural network

Purpose: Create a software product using a probabilistic neural network (PNN) and database based on experimental research of titanium alloys to definition of the best microstructure and properties of aerospace components. Design/methodology/approach: The database creation process for artificial neural network training was preceded by the investigation of the granulometric composition of the titanium powder alloys, study of microstructure, phase composition and evaluation of micromechanical properties of these alloys by the method of material plasticity estimation in the conditions of hard pyramidal indenters application. A granulometric analysis was done using a special complex of materials science for the images analysis ImageJ. Metallographic investigations of the powder structure morphology were carried out on the scanning electron microscope EVO 40XVP. Specimens for micromechanical testing were obtained by overlaying the titanium alloy powders on the substrate made of the material close to chemical composition. Substrates were prepared by pressing the powder mixture under the load of 400 MPa and following sintering at 1300°C for 1 hour. Overlaying was performed by an electron gun ELA-6 (beam current – 16 mA). Findings: According to the modelling results, a threshold of the PNN accuracy was established to be over 80%. A high level of experimental data reproduction allows us a full or partial replacement of a number of experimental investigations by neural network modelling, noticeably decreasing, in this case, the cost of the material creation possessing the preset properties with preserved quality. It is expected that this software can be used for solving other problems in materials science too. Research limitations/implications: The accuracy of the PNN algorithm depends on the number of input parameters obtained experimentally and forms a database for the training of the system. For our case, the amount of input data is limited. Practical implications: Previously trained system based on the PNN algorithm will reduce the number of experiments that significantly reduce costs and time to study. Originality/value: A software product on the basis of the PNN network was developed. The training sample was built based on a series of laboratory studies granulometric composition of the titanium powder alloys, study of microstructure, phase composition and evaluation of micromechanical properties of powder materials. The proposed approach allows us to determine the best properties of the investigated material for the design of aerospace components.

Interfacial and wetting properties between glass fiber and epoxy resins with different pot lifes

Considerable research has been devoted to the study of interfacial adhesion, optimal wetting and interfacial properties of composites. In these studies, epoxy hardeners with different pot lifes were used for the manufacture of composites and their molding processes. Wettability and micromechanical testing revealed that these factors effected wetting, interfacial and mechanical properties. FT-IR studies revealed differences in the spectra with different 'C–N type' curing additives contained in the four different amine-type hardeners. For measurement of impregnation rate, a droplet of the different epoxy/hardener liquids was placed on glass fiber mat and the spreading behavior was measured as a function of time. The surface tension on PTFE was measured using the pendant drop method and the work of adhesion calculated for this surface tension was related to the spreading distance. The Drzal equation, based on the single fiber fragmentation test, was implemented to calculate the interfacial shear strength (IFSS) of the different composite materials. To relate the IFSS, the surface tension and the work of adhesion, static contact angle measurements were made using four solvents of known surface energy. A goal of the study was to identify an optimal epoxy hardener in terms of; pot life degree of curing, residual enthalpy, wettability and interfacial properties. From a practical standpoint the measured rate of resin transfer, and interlaminar shear strength (ILSS), mechanical and other properties were consistently affected by these manufacturing and processing differences.

Temperature dependence of interfacial strength of carbon-fiber-reinforced temperature-resistant polymer composites

In the present study, the temperature-dependence of fiber/matrix interfacial strength is discussed through temperature-controlled microbond testing and its numerical simulation. The tests are carried out from room temperature to 100 °C. The micromechanical testing machine debonds the interface and the load at which debonding occurs is measured. In the numerical simulation, thermal residual stress is calculated by considering the relaxation modulus and the time–temperature superposition principle for the resin, i.e. thermo-viscoelasticity, in order to obtain an accurate thermal residual stress. In order to reproduce the damage of resin unique to the microbond test, continuum damage mechanics and Christensen’s failure criterion for multi-axial stress states are applied from ambient temperature to 100 °C. Interfacial debonding is simulated using a cohesive zone model. The interfacial strength is determined by comparing the analysis results with the experimental results. Results show that there is no obvious temperature dependence on interfacial strength; similar interface strengths are obtained over the range of temperatures studied.

Composite Properties and Micromechanical Analysis of Highly Ductile Cement Composite Incorporating Limestone Powder

This paper presents the results of an experimental study on the effect of limestone powder on the compressive strength, tensile behavior, and micromechanical parameters of a highly ductile cement composite incorporating limestone powder. Four mixtures were determined according to the replacement ratio of cement with limestone powder. A series of experiments including compressive strength, uniaxial tension, single fiber pullout, and matrix toughness tests were performed. Test results showed that the strength (compressive, initial cracking, and tensile strength) decreased, and that the tensile strain capacity increased, with an increase of the replacement ratio of cement with limestone powder. Micromechanical tests and analysis supported the uniaxial tension test result.