Showing papers on "Indentation published in 2018"

A Critical evaluation of indentation techniques for measuring fracture toughness

Abstract: The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.

Failure mechanisms and damage evolution of laminated composites under compression after impact (CAI): Experimental and numerical study

Abstract: The damage tolerance of Carbon Fibre Reinforced Polymer (CFRP) to Barely Visible Impact Damage (BVID) is a critical design limiter for composite structures. This study investigated the key driving mechanisms and damage evolution of the compressive failure of laminated composites containing BVID using compression after impact and indentation (CAI) tests. Experiments were carried out on two similar quasi-isotropic laminates: [45 2 /90 2 /0 2 /−45 2 ] 2S and [45/90/0/−45] 4S . Matrix cracking and delaminations were introduced by either low-velocity impact or quasi-static indentation tests prior to the CAI tests. The full-field displacement during CAI as well as the moment of rupture was captured by 3D Digital Image Correlation (DIC). The effect of ply-blocking and influence of factors, such as impact energy, delamination area and surface indentation, on compressive failure was studied. Previously validated high-fidelity finite element (FE) numerical models for the indentation and impact events were then used to investigate the damage evolution during CAI failure.

Ultrahard carbon film from epitaxial two-layer graphene

Abstract: Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, is resistant to perforation with a diamond indenter and shows a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp2 to sp3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than three to five layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation. Indentation in bilayer epitaxial graphene induces its reversible transformation into a diamond-like structure with stiffness and hardness comparable to diamond.

Friction stir processing of Al6061-SiC-graphite hybrid surface composites

Abstract: Friction stir processing (FSP) of Al6061-SiC-Graphite hybrid composites is studied in detail via force analysis, spectroscopic, microstructural and indentation studies. Effect of various tool rotat...

Barely visible impact damage assessment in laminated composites using acoustic emission

Abstract: Despite the key advantages of Fiber Reinforced Polymer (FRP) composites, they are susceptible to Barely Visible Impact Damage (BVID) under transverse loadings This study investigates BVID in two quasi-isotropic carbon/epoxy laminates under quasi-static indentation and Low-Velocity Impact (LVI) loadings using Acoustic Emission (AE) First, the evolution of interlaminar and intralaminar damages is studied by analyzing the AE signals of the indentation test using b-value and sentry function methods Then, the specimens are subjected to the LVI loading and the induced damages are compared with the indentation test and the percentage of each damage mechanism is calculated using Wavelet Packet Transform (WPT) In consistent with the mechanical data, ultrasonic C-scan and digital camera images of the specimens, the AE results show a considerable similarity between the induced BVID under quasi-static indentation and LVI tests Finally, the obtained results show that AE is a powerful tool to study BVID in laminated composites under quasi-static and dynamic transverse loadings

Comparative study of mechanical-electrical-thermal responses of pouch, cylindrical, and prismatic lithium-ion cells under mechanical abuse

Abstract: In the study, mechanical abuse tests mainly in the form of indentation were performed on the cylindrical cell, pouch cell, and prismatic cell The mechanical force-displacement response, open circuit voltage (OCV), and temperature distribution were recorded and compared In spherical head indentation tests of the pouch and prismatic cell and lateral indentation of the cylindrical cell, the peak force is strongly correlated with OCV drop and local temperature increase However, in flat-end cylinder indentation tests, the internal mechanical damage is progressively developed, and the OCV drop and the temperature increase occur before the peak force The fracture surfaces of the post-mortem samples were examined to investigate the correlation between fracture patterns and internal short circuit (ISC) behaviors (OCV and temperature distribution) Two distinct fracture patterns were observed that the in-plane fracture induced by biaxial stretching and inter-layers’ fracture induced by shearing A strong correlation is observed between the number of shear fractures and OCV drop An increase in the number of inter-layers’ fractures increases the rate of OCV drop Additionally, the fracture patterns influence the ISC area and location, thereby affecting the heat generation and conduction as well as the temperature distribution

Mechanical properties of the solid electrolyte Al-substituted Li7La3Zr2O12 (LLZO) by utilizing micro-pillar indentation splitting test

Abstract: Garnet structured Al-substituted Li7La3Zr2O12 (Al:LLZO) is a promising candidate as electrolyte in all-solid-state Li-ion batteries due to its chemical stability against Li-metal and high voltage cathode materials. In order to ensure long-term stable operation, electrolyte crack growth induced and/or the volume change of the active material on the cathode side needs to be avoided, requiring in particular knowledge of local and global mechanical properties of the electrolyte material. Micro-pillar splitting test was used for the first time on this material to determine the microscopic fracture toughness of single grains and compare it with conventional Vickers indentation fracture toughness (VIF), which represents macroscopic fracture toughness. Both methods yielded comparative results. In conclusion, the micro-pillar splitting test can be used as an advanced locally resolved characterization method that can open up new experimental directions for characterizing and understanding battery materials and enable a targeted approach for material improvements.

Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow

Abstract: Nanoindentation experiments performed in 5 and 18 μm thick vapor deposited polycrystalline lithium films at 31 °C reveal the mean pressure lithium can support is strongly dependent on length scale and strain rate. At the smallest length scales (indentation depths of 40 nm), the mean pressure lithium can support increases from ∼23 to 175 MPa as the indentation strain rate increases from 0.195 to 1.364 s−1. Furthermore, these pressures are ∼46–350 times higher than the nominal yield strength of bulk polycrystalline lithium. The length scale and strain rate dependent hardness is rationalized using slightly modified forms of the Nabarro–Herring and Harper–Dorn creep mechanisms. Load-displacement curves suggest a stress and length-scale dependent transition from diffusion to dislocation-mediated flow. Collectively, these experimental observations shed significant new light on the mechanical behavior of lithium at the length scale of defects existing at the lithium/solid electrolyte interface.

A simple model for indentation creep

Abstract: A simple model for indentation creep is developed that allows one to directly convert creep parameters measured in indentation tests to those observed in uniaxial tests through simple closed-form relationships. The model is based on the expansion of a spherical cavity in a power law creeping material modified to account for indentation loading in a manner similar to that developed by Johnson for elastic-plastic indentation (Johnson, 1970). Although only approximate in nature, the simple mathematical form of the new model makes it useful for general estimation purposes or in the development of other deformation models in which a simple closed-form expression for the indentation creep rate is desirable. Comparison to a more rigorous analysis which uses finite element simulation for numerical evaluation shows that the new model predicts uniaxial creep rates within a factor of 2.5, and usually much better than this, for materials creeping with stress exponents in the range 1 ≤ n ≤ 7. The predictive capabilities of the model are evaluated by comparing it to the more rigorous analysis and several sets of experimental data in which both the indentation and uniaxial creep behavior have been measured independently.

Are elastic moduli of biological cells depth dependent or not? Another explanation using a contact mechanics model with surface tension

Abstract: Atomic force microscopy (AFM) has become the most commonly used tool to measure the mechanical properties of biological cells. In AFM indentation experiments, the Hertz and Sneddon models of contact mechanics are usually adopted to extract the elastic modulus by analyzing the load–indent depth curves for spherical and conical tips, respectively. However, the effects of surface tension, neglected in existing contact models, become more significant in indentation responses due to the lower elastic moduli of living cells. Here, we present two simple yet robust relations between load and indent depth considering surface tension effects for spherical and conical indentations, through dimensional analysis and finite element simulations. When the indent depth is smaller than the intrinsic length defined as the ratio of surface tension to elastic modulus, the elastic modulus obtained by classical contact mechanics theories would be overestimated. Contrary to the majority of reported results, we find that the elastic modulus of a cell could be independent of indent depths if surface tension is taken into account. Our model seems to be in agreement with experimental data available. A comprehensive comparison will be done in the future.

Reversible phase transformation in polycrystalline TRIP steels induced by cyclic indentation performed at the nanometric length scale

Abstract: Metastable austenitic stainless steels are an interesting group of materials, which exhibit the Transformation Induced Plasticity effect In this regard, phase transformation from austenite to martensite enhances the work hardening of the metastable austenitic stainless steels affecting the deformation dynamics and mechanical properties including fatigue properties Within this context, the reversible load-induced phase transformation from ? to ?-martensite is investigated at the local scale under cyclic indentation This reversible phase transformation is manifested itself by a combination of hysteresis loops, elbow formation, and reversible pop-ins in the loading curve The initial cyclic achieved through the nanoindentation technique allows to identify three different deformation regimes for the austenitic grains Firstly, a softening effect takes place due to the dislocation activation; subsequently the phase transformation induces a hardening effect and finally, the load deformation curve reaches a plateau where no more plastic deformation is observed

Nanoindentation of high-purity vapor deposited lithium films: The elastic modulus

Abstract: Nanoindentation has been used to measure the elastic modulus of 5 and 18 μm thick high-purity vapor deposited polycrystalline lithium films at 31 °C. Over indentation depths ranging from 150 to 1100 nm, the modulus is found to vary with film thickness from 9.8 GPa ± 11.9% to 8.2 GPa ± 14.5%. These results are well within the range of lithium’s orientation dependent elastic modulus, which spans approximately 3.1 to 21.4 GPa. The measured values may also indicate (111) and (100) texture for the 5 and 18 μm thick films, respectively. The potential effects of pileup and surface contamination are found to be negligible if any at all. Small but discernible changes in damping capability near the free surface may provide insight into the subsurface defect structure and the potential for localized heating. Numerous experimental challenges are addressed and key metrics are used to validate the measured elastic modulus.