Static Indentation Research Articles

Influence of AlCoCrFeNiCuSn HEA reinforcement particles on the plastic flow behaviour of the friction stir processed composite material under constrained confined deformation conditions

In the present investigation, an AlCoCrFeNiCuSn high entropy alloy (HEA) reinforced (with 2.5, 5, 7.5 weight percentage (wt.%)) AA7050-based metal matrix composite has been developed by utilising the friction-stir processing (FSP) technique. Further, the representative volume element (RVE) based finite element analysis (FEA) model and macro-mechanical static indentation FEA model have been developed to analyze the plastic deformation of developed composite material under constrained confined deformation conditions at a load range of 4.90 to 49.03 kN. The microstructural investigation confirms the fair distribution of AlCoCrFeNiCuSn HEA particles in the AA7050 matrix. The micro-mechanical RVE-based FEA model and macro-mechanical static indentation FEA model were successfully validated with experimental and analytical models (ECM and FPM) with less than 5% percentage difference. The yield strength of matrix material was increased with 6.51%, 10.47%, and 15.19% by increasing the AlCoCrFeNiCuSn HEA reinforcement particle with 2.5wt.%, 5wt.%, and 7.5wt.% respectively. Further, the tensile strength of matrix material was increased with 1.88%, 3.93%, and 10.49% by increasing the AlCoCrFeNiCuSn HEA reinforcement particle with 2.5wt.%, 5wt.%, and 7.5wt.% respectively. Additionally, Composition 1 (AA7050/2.5wt.% AlCoCrFeNiCuSn HEA), composition 2 (AA7050/5wt.% AlCoCrFeNiCuSn HEA) and composition 3 (AA7050/7.5wt.% AlCoCrFeNiCuSn HEA) show 7.39%, 11.91% and 22.16% more Meyer’s hardness than matrix material.

Numerical investigation on rock fragmentation during indentation with a conical pick based on discrete element method

This article investigates the fragmentation of rock during the indentation process with a conical pick. The study explores the impact of indentation dip angle, indentation spacing, and confining pressure on rock fragmentation through simulation using the discrete element method. Rock models of coal and red sandstone are created and calibrated for the simulation. The findings indicate that the indentation force increases exponentially with the increase of indentation dip angle for both coal and red sandstone. The specific energy increases first and then decreases with the increase of indentation dip angle. The maximum specific energy is found in the condition of indentation dip angle of 25° for red sandstone, while it is 20° for coal. The indentation force increases logarithmically with the increase of indentation spacing tending to be an unrelieved indentation condition. The optimal indentation spacing with the lowest specific energy is determined to be 50 mm for breaking coal and 34 mm for breaking red sandstone. Additionally, the indentation force increases exponentially for coal while it increases linearly for red sandstone with the increase of the confining pressure. For both coal and red sandstone, specific energy increases with the increase of the confining pressure.

Real-time in-situ coatings corrosion monitoring using machine learning-enhanced triboelectric nanogenerator

Current methods for monitoring coating corrosion are limited by their inability to provide real-time data and dependence on external power sources. This study presents a novel in-situ corrosion monitoring system using a solid-liquid triboelectric nanogenerator (TENG) that converts mechanical energy into electrical signals for self-powered sensing. TENG signals and electrochemical impedance spectra were measured on a dopamine-modified lignin-polydimethylsiloxane coating on steel in 1 M NaCl solution under no corrosion, indentation, pitting, and broken conditions, respectively. We extract time-frequency features from the TENG signals to predict the coating’s corrosion condition by applying a customised convolutional neural network (CNN). By extracting time-frequency features from the TENG signals and applying a custom CNN, a prediction accuracy of 99 % for corrosion classification was achieved. Furthermore, the CNN regression model predicted coating impedance values with a high coefficient of determination (R² = 0.98), demonstrating its effectiveness in tracking corrosion progression. The developed TENG also facilitates defect localisation via a matrix electrode beneath the coating. Our approach introduces a promising real-time technology for in-situ corrosion monitoring.

Characterisation and Application of Bio-Inspired Hybrid Composite Sensors for Detecting Barely Visible Damage under Out-of-Plane Loadings.

Traditional inspection methods often fall short in detecting defects or damage in fibre-reinforced polymer (FRP) composite structures, which can compromise their performance and safety over time. A prime example is barely visible impact damage (BVID) caused by out-of-plane loadings such as indentation and low-velocity impact that can considerably reduce the residual strength. Therefore, developing advanced visual inspection techniques is essential for early detection of defects, enabling proactive maintenance and extending the lifespan of composite structures. This study explores the viability of using novel bio-inspired hybrid composite sensors for detecting BVID in laminated FRP composite structures. Drawing inspiration from the colour-changing mechanisms found in nature, hybrid composite sensors composed of thin-ply glass and carbon layers are designed and attached to the surface of laminated FRP composites exposed to transverse loading. A comprehensive experimental characterisation, including quasi-static indentation and low-velocity impact tests alongside non-destructive evaluations such as ultrasonic C-scan and visual inspection, is conducted to assess the sensors' efficacy in detecting BVID. Moreover, a comparison between the two transverse loading types, static indentation and low-velocity impact, is presented. The results suggest that integrating sensors into composite structures has a minimal effect on mechanical properties such as structural stiffness and energy absorption, while substantially improving damage visibility. Additionally, the influence of fibre orientation of the sensing layer on sensor performance is evaluated, and correlations between internal and surface damage are demonstrated.

Atomistic simulation of the influence of semi-coherent interfaces in the V/Fe bilayer system on plastic deformation during nanoindentation

Semi-coherent interfaces can have a strong influence on the mechanical behavior of bilayer systems, which is seen very clearly under indentation conditions where a well-defined plastic zone interacts directly with the interface. The main aim of this work is to study the influence of a semi-coherent bcc/bcc interface in the V/Fe bilayer system with molecular dynamics (MD) simulations. In particular, the influence of the V layer thicknesses on the apparent hardness of bilayer system is investigated. Our results show that the deformation behavior of pure V and pure Fe resulting from the MD simulations is in good agreement with the literature. Moreover, the MD simulations reveal a significant enhancement of the hardness of V/Fe bilayer system for thinner vanadium layers, resulting from the crucial role of the semi-coherent interface as a barrier to dislocation propagation. This is seen from a detailed analysis of the interaction of mobile dislocations in the plastic zone with misfit dislocations in the interface. Our work shows that dislocation pile-ups at the interface and formation of horizontal shear loops are two key mechanisms dominating the rate and magnitude of plastic deformation and thus contributes to our understanding of mechanical behavior of bilayer systems with semi-coherent interfaces.

Determination of operational properties of paint coating materials

Problem. Varnishes are used in all branches of industry and economy. With their help, you can protect metals from the effects of aggressive environment, prevent from the negative effects of corrosion, preserve consumer properties, and give the treated surface an aesthetic look. One of the methods of protecting metal from corrosion is the application of coatings, including paints and varnishes, which form a protective film on the surface of the metal and prevent corrosion processes on the surface. Methodology. The research was conducted on the operational characteristics of enamel-type paint coatings: paint powder UD-1028-07040-T and soil FL-03K. Determination of moisture absorption of the free coating film was carried out using the desiccator method. In order to determine the penetration time of aggressive solutions through the paint coating film, an electrochemical method was used, whose purpose is to determine the change in the electrochemical potential of the object under study during the penetration of aggressive solutions through the paint coating film to the metal. The study of the hardness of paint coatings was carried out by the method of static indentation, respectively, using a microhardness meter. Originality. The research results showed the advantage of the UD-1028-07040-T powder paint coating over the FL-03K soil, which can be explained by such factors as the structure of the paint coating, the porosity of the surface, and the phenomenon of adhesion between the paint and the surface on which it is applied. Practical value. Powder paint UD-1028-07040-T absorbs water much less (up to 2.12%) compared to FL-03K soil (up to 5.14%), this indicates a relatively higher porosity of FL-03K soil; it is confirmed by the results of porosity determination, which was 1 point. When determining the penetration time of aggressive solutions through the paint film, it was found that FL-03K soil quickly passes aggressive solutions through its surface (in 144 hours) compared to powder paint (192 hours). This indicates that the use of powder paint in conditions involving contact with aggressive substances will be more appropriate.

Nanoindentation induced anisotropy of deformation and damage behaviors of MgF2 crystals

The competition mechanism between the slip motions and cleavage fractures is related to the anisotropy of deformation behaviors, which is essential to manufacture complex optical components. To identify competition mechanism between the slip motions and cleavage fractures and reveal the anisotropy of deformation and damage behaviors of MgF2 crystals, the nanoindentation tests were systematically conducted on different crystal planes. In addition, the stress induced by the nanoindentation was developed and decomposed along the slip systems and cleavage planes, and cleavage factors and Schmid factors were calculated. The stress, cleavage factors and Schmid factors indicated that the activation degree of the slip motions and cleavage fractures determined the indentation morphologies. Under the same indentation conditions, the nanoindentation of the (001) crystal plane activated most slip motions, so the plastic deformation is most prone to occur on this crystal plane. The nanoindentation of the (010) crystal plane activated less slip motions and most cleavage fractures, resulting in the severest brittle fractures on the (010) crystal plane. The theoretical results consisted well with the experimental results, which provides the theoretical guidance to the low-damage manufacturing of MgF2 components.

Efficient modelling of progressive damage due to quasi-static indentation on multidirectional laminates by a mesh orientation independent kinematically enriched continuum damage model

This work proposes a computationally efficient high-fidelity modelling framework for analysis of damage in composites subjected to quasi-static indentation. A ply-by-ply modelling approach is adopted, and a new semi-discrete continuum damage model is used for intra-ply cracking that allows for cracks to grow independent of the ply mesh pattern. This feature greatly simplifies the meshing effort since the requirement of a ply-oriented mesh and imposition of tie constraints at mesh-mismatched ply interfaces is eliminated. The model is further enhanced to realise kinematic interactions between the cracked and uncracked material domains at the constitutive level. Inter-ply delamination is simulated using cohesive elements. Through a challenging problem of static indentation on a quasi-isotropic laminate, it is shown that the model can capture solution-dependent multiple discrete ply cracks and detailed crack-delamination interaction, but with reduced computational time and complexity, compared to a reference model with ply-oriented mesh and pre-seeded cracks at known locations.

The study on the influence of different damage introduction methods on the damage characteristics of aircraft composites

Composite materials have advantages incomparable to traditional metal materials. However, they are more sensitive to impact damage and their mechanical properties decrease significantly after impact damage. Therefore, it is of great significance to study the energy absorption, mechanical response and damage mechanism of materials in different ways. To study the influence of different punch diameter sizes on the material damage characteristics, three kinds of punch diameters of 12.7 mm, 16 mm and 25.4 mm are selected to carry out the quasistatic indentation test and drop hammer impact damage test on the test pieces respectively. The material damage caused by different punch sizes under two damage introduction methods is analyzed. Damage introduction was carried out on the test piece with the same layering through two methods: quasi-static indentation and drop hammer impact. The damage depth, damage area and residual strength were compared after damage introduction. The difference between quasistatic indentation and drop hammer impact damage introduction was verified by comparing the damage characteristics to each other. The results show that with the increase of punch diameter size, the damage area decreases and the residual strength of the material increases. In addition, the test results of different punch diameter sizes under the two damage introduction methods have the same change trend, which proves that the impact of punch size on the material damage characteristics has no obvious relationship with the damage introduction method. Quasi-static indentation and drop hammer impact damage have no significant difference in the residual strength of materials after damage, which verifies the equivalence of static indentation and damage introduction.

Bioinspired hybrid helical structure in lobster Homarus Americanus: Enhancing penetration resistance and protective performance

The high impact and penetration resistance of the cuticle in the lobster Homarus americanus can be attributed to its unique twisted plywood structure. Hybridization of two helical structures was observed in the exoskeleton, with different pitch angles and ply thickness of the exocuticle and endocuticle within the exoskeleton. Given the survival challenges posed by the natural environment, it is worthwhile to study why it evolved into a hybrid helical rather than a uniform helical structure. In light of this, we have proposed an innovative bionic hybrid helical structure design strategy that goes beyond simply reducing the pitch angle. In this paper, we replicated complex biological microstructures by introducing two types of pitch angles and ply thicknesses for regional configurations. Quasi Static Indentation (QSI) full-penetration experiments were conducted. Experimental results revealed that the exocuticle with a large pitch angle and small ply thickness possesses a higher knee load under the penetration test, which provides a higher initial damage threshold. On the other hand, the endocuticle with a small pitch angle and large ply thickness induces higher damage dissipation, which provides superior protective performance. These research findings demonstrate that ply hybridization can be a promising method for microstructure design to improve the penetration resistance of thin laminates and serve as a reference for developing more refined gradient helical structure designs.

EARLY-STAGE SELF-ASSEMBLING PEPTIDE HYDROGEL-BASED INTERVENTION FOR ARTICULAR PATELLA CARTILAGE REPAIR

AbstractObjectivesThe aim of this study was to develop an in vitro GAG-depleted patella model and assess the biomechanical effects following treatment with a SAP:CS self-assembling hydrogel.MethodsPorcine patellae (4–6 month old) were harvested and subject to 0.1% (w/v) sodium dodecyl sulfate (SDS) washes to remove GAGs from the cartilage. Patellae were GAG depleted and then treated by injection with SAP (∼ 6 mM) and CS (10 mg) in Ringer's solution through a 30G needle. Native, GAG depleted and SAP:CS treated patellae were tested through static indentation testing, using 15g load, 5mm indenter over 1hr period. The degree of deformation of each group was assessed and compared (Mann-Whitney, p<0.05). Native, GAG depleted, sham (saline only) and SAP:CS treated paired patellae and femurs were additionally characterized tribologically through sequential wear testing when undergoing a walking gait profile (n=6 per group). The cartilage surfaces were assessed and compared (Mann-Whitney, p<0.05) using the ICRS scoring system, surface damage was illustrated through the application of Indian ink.ResultsStatic indentation tests indicated significant increase in indentation deformation of GAG depleted group compared to native group (n=6, p<0.01) and significant reduction in deformation of SAP:CS treated group compared to GAG depleted group (n=6, p<0.05). Sequential wear tests indicated a significant increase in the cartilage damage on the both surfaces of the patellofemoral joint in the GAG depleted group, compared to the native group (n=6, p<0.001), Following SAP:CS treatment, significant protection from damage was observed on femoral surface (n=6, p<0.005), with some non-significant reduction in damage on the patella surface. Sham injections showed no significant increase in damage compared to the native and treated samples.ConclusionsThe ∼50% reduction of GAGs represented a moderate osteoarthritic patella cartilage model. This same loss transferred to the dynamic wear tests with significant changes in the damage on the femoral counter face associated with the GAG loss. SAP:CS treatment showed promise in restoring cartilage stiffness to treat Chondromalacia patella in static indentation tests. Sequential wear tests showed that the SAP:CS treatment protects the cartilage layer of both surfaces in the patellofemoral joint from damage in an extreme degeneration model. The sham injections showed that injecting cartilage with a 30G and saline does not cause any significant damage to the cartilage layer.Declaration of Interest(a) fully declare any financial or other potential conflict of interest

Representative volume element (RVE) based FEA investigation on the plastic flow behaviour of friction-stir processed AA7075/Si3N4composite under constrained deformation conditions

In the present investigation, the Si3N4reinforced (with a weight percentage of 2.5%, 5% and 7.5%) AA7075-based metal matrix composite has been developed by friction-stir processing. Further, a micro-mechanical representative volume element (RVE)-based finite element analysis (FEA) model has been developed for the evaluation of mechanical properties and plastic flow behaviour of developed composite material in uniaxial conditions. Moreover, a macro-mechanical-based static indentation finite element model has been developed for predicting the plastic flow behaviour, Meyer's hardness, constraint factor (CF) and lip height around indentation as a function of average strain under constrained conditions. The results revealed that Meyer's hardness, yield strength, tensile strength and lip height were increased with the increase in Si3N4reinforced weight percentage. Further, the lip height was inversely proportional to the strain hardening exponent. It has been also observed that the CF of all three developed composite material compositions increased with the increase in average strain ( εavg) up to the transition strain ( εtr) only, after that its value was constant with further increase in average strain ( εavg).

Study of damage behavior and repair effectiveness of patch repaired carbon fiber laminate under quasi-static indentation loading

Damage caused due to low-velocity impacts in composites leads to substantial deterioration in their residual strength and eventually provokes structural failure. This work presents an experimental investigation on the effects of different patch and parent laminate stacking sequences on the enhancement of impact strength of Carbon Fiber Reinforced Polymers (CFRP) composites by utilising the adhesively bonded external patch repair technique. Damage evolution study is also performed with the aid of Acoustic Emission (AE). Two different quasi-isotropic configurations were selected for the parent laminate, viz., [45°/45°/0°/0°]s and [45°/0°/45°/0°]s. Quasi Static Indentation (QSI) test was performed on both the pristine laminates, and damage areas were detected by using the C-scan inspection technique. Damaged laminates were repaired by using a single-sided patch of two different configurations, viz., [45°/45°/45°/45°] and [45°/0°/0°/45°], and employing a circular plug to fill the damaged hole. Four different combinations of repaired laminates with two configurations of each parent and patch laminate were produced, which were further subjected to the QSI test. The results reveal the effectiveness of the repair method, as all the repaired laminates show higher impact resistance compared to the respective pristine laminates. Patches of [45°/0°/0°/45°] configuration when repaired by taking [45°/45°/0°/0°]s and [45°/0°/45°/0°]s as parents exhibited 68% and 73% higher peak loads, respectively, than the respective pristine laminates. Furthermore, parent and patch of configuration [45°/0°/45°/0°]s and [45°/0°/0°/45°], respectively, attain the highest peak load, whereas [45°/45°/0°/0°]s and [45°/45°/45°/45°] combinations possess the most gradual decrease in the load.

Probing soft fibrous materials by indentation.

Indentation is often used to measure the stiffness of soft materials whose main structural component is a network of filaments, such as the cellular cytoskeleton, connective tissue, gels, and the extracellular matrix. For elastic materials, the typical procedure requires fitting the experimental force-displacement curve with the Hertz model, which predicts that f=kδ1.5 and k is proportional to the reduced modulus of the indented material, E/(1-ν2). Here we show using explicit models of fiber networks that the Hertz model applies to indentation in network materials provided the indenter radius is larger than approximately 12lc, where lc is the mean segment length of the network. Using smaller indenters leads to a relation between force and indentation displacement of the form f=kδq, where q is observed to increase with decreasing indenter radius. Using the Hertz model to interpret results of indentations in network materials using small indenters leads to an inferred modulus smaller than the real modulus of the material. The origin of this departure from the classical Hertz model is investigated. A compacted, stiff network region develops under the indenter, effectively increasing the indenter size and modifying its shape. This modification is marginal when large indenters are used. However, when the indenter radius is small, the effect of the compacted layer is pronounced as it changes the indenter profile from spherical towards conical. This entails an increase of exponent q above the value of 1.5 corresponding to spherical indenters. STATEMENT OF SIGNIFICANCE: The article presents a study of indentation in network biomaterials and demonstrates a size effect which precludes the use of the Hertz model to infer the elastic constants of the material. The size effect occurs once the indenter radius is smaller than approximately 12 times the mean segment length of the network. This result provides guidelines for the selection of indentation conditions that guarantee the applicability of the Hertz model. At the same time, the finding may be used to infer the mean segment length of the network based on indentations with indenters of various sizes. Hence, the method can be used to evaluate this structural parameter which is not easily accessible in experiments.

Application of nanoindentation technique in mechanical characterization of organic matter in shale: Attentive issues, test protocol, and technological prospect

The rise of global shale oil and gas exploration has prompted scholars to pay attention to the mechanics of organic matter (OM) in shale reservoirs. The depth-sensing nanoindentation, a rather mature micromechanical testing technique in materials science, was gradually introduced into the mechanical data acquisition of shale matrix OM in recent years. Although this introduction is commendable, there are still some attentive issues relating to the indentation condition (mainly the reasonable setting of indentation depth parameter) and interpretation of the measured mechanical data, considering the micron-sized and nanoporous nature of OM and its close contact with surrounding minerals in shale composite. This study carried out a series of OM-positioned indentations on a typical shale sample collected from Silurian Longmaxi Formation in South China, in which we specially used the dynamic mechanical analysis that determines the mechanical data as a continuous function of indentation depth. Through it, we evidenced two main indentation effects (i.e., the size and substrate) during indentation of OM, and quantified the depths they appear (at less than 200 nm and greater than 1/20 of OM thickness, respectively). They together limit the indentation depth to a narrow range and also restrict the use of nanoindentation for OM less than 2 μm. Additionally, the porous OM is measured to be in lower mechanical values, suggesting that the porous effect on the indentation response cannot be ignored, especially when making mechanical comparison between different OM particles. Furthermore, we compared nanoindentation with the newly developed AFM-based nanomechanical mapping technique, and proposed to combine the both to achieve a full-scale characterization of shale matrix OM. Overall, this study standardized the indentation conditions to measure the inherent mechanical data of shale matrix OM, which lays the foundation for the forseeable increasing demand for testing and research of OM mechanics in the future.

Simulation and dimensional analysis of instrumented dynamic spherical indentation of ductile metals

Finite element (FE) modeling of instrumented dynamic indentation experiments in a miniature Kolsky bar is undertaken. Geometry, including an output bar with machined spherical indenter tip, and velocity history boundary conditions are extracted directly from experimental diagnostics. The test material (i.e., substrate) is polycrystalline aluminum alloy Al 6061-T6. The constitutive model used in simulations accounts for isotropic elasticity and isotropic plasticity with strain hardening, strain-rate hardening, and thermal softening under adiabatic conditions. The FE model, with representative material parameters culled from the literature, accurately reproduces the curvature of the experimental load versus depth data for three different experimental indentation velocity histories. A framework for dimensional analysis of instrumented dynamic spherical indentation is set forth, improving upon prior work. Parametric FE simulations reveal sensitivity, or lack thereof, of the predicted response to variations in the proposed independent dimensionless variables encompassing material properties. For the indenter size, maximum depth, and maximum strain rate imposed experimentally on the order of 103/s, force-depth predictions are nearly unaffected by realistic variations in mass density, melting temperature, and thermal softening parameters when the sample is initially at room temperature. Predictions are affected by elastic constants (the elastic modulus and to a lesser extent, Poisson’s ratio), initial yield strength, two strain hardening parameters, and strain rate sensitivity. Predictions are also notably affected by initial temperature, with thermal softening prominent at high enough initial temperature or much higher loading rates. Based on the dimensional analysis, static indentation and elevated temperature indentation experiments are proposed for extraction of quasi-static and thermal material properties from previously uncharacterized metals, and dynamic indentation is proposed for extraction of rate sensitivity that cannot be obtained from static tests. Rate sensitivity obtained in this way from the novel instrumented dynamic spherical indentation experiments and FE simulations produces a parameterized stress–strain response for Al 6061-T6 reasonably validated by external studies for strain rates up to the order of 103/s.

Fully 3D‐Printed, Stretchable, and Conformable Haptic Interfaces

AbstractIntegrating rich cutaneous haptic feedback enhances realism and user immersion in virtual and augmented reality settings. One major challenge is providing accurately localized cutaneous stimuli on fingertips without interfering with the user's dexterity. This sub 200 µm thick, fully printed, stretchable Hydraulically Amplified Taxels (HAXELs) enable both static indentation and vibrating haptic stimuli, localized to a 2.5 mm diameter region. The HAXELs are directly bonded to the user's skin, are soft enough to conform to any body part, and can be fabricated in dense arrays with no crosstalk. All functional materials (elastomers, stretchable conductors, and sacrificial layers) are deposited by inkjet printing, which allows rapid prototyping of multi‐material, polymer‐based structures. The actuators consist of oil‐filled stretchable pouches, whose shape is controlled by electrostatic zipping. The 5 mm wide actuators weigh <250 mg and generate cutaneous stimuli well above reported perception thresholds, from DC to 1 kHz. They operate well even when stretched to over 50%, allowing great freedom in placement. The 2 × 2 arrays are tested on the fingers of human volunteers: the actuated quadrant is correctly identified 86% of the time. Printing soft actuators allows tailoring dense and effective cutaneous haptics to the unique shape of each user.

Toward Material Property Extraction from Dynamic Spherical Indentation Experiments on Hardening Polycrystalline Metals

Static indentation and dynamic indentation are reviewed, with a focus on extraction of material properties of isotropic strain-hardening polycrystalline metals that may be rate- and temperature-sensitive. Static indentation is reviewed first, followed by dynamic indentation, since the former is regarded as a specialization of the latter with inertia, rate dependence, and adiabatic heating excluded. Extending concepts from the literature review, a treatment of dynamic indentation using dimensional analysis is forwarded, and a general framework for extraction of material property information (i.e., constitutive model parameters) from instrumented dynamic spherical indentation experiments is set forth. In an example application of the methodology, experimental data obtained from instrumented spherical indentation in a miniature Kolsky bar apparatus are evaluated via dimensional analysis. The substrate material is aluminum alloy Al 6061-T6. Several definitions of indentation strain proposed for static indentation are assessed for dynamic indentation, as are indentation strain rates. While the fidelity of the experimental method and inertial effects could inhibit extraction of elastic properties, extraction of certain plastic constitutive properties may be feasible. Current data are insufficient to enable determination of a complete and unique set of all physical properties. Motivated by the present review and analysis, new experiments and simulations are proposed that would identify influences of material properties, facilitating their extraction from data.

Indentation size effect and hardness of materials

In recent decades, the Indentation Size Effect (ISE), which manifests itself as a change in fixed hardness depending on indentation conditions, has become one of the most urgent problems of modern materials science. Research efforts are aimed at identifying and eliminating the causes of ISE, because the discrepancy between actual and true hardness values greatly complicates the task of finding the optimal material for the manufacture of parts. The proposed paper gives a brief review of the theories explaining ISE and its relationship with hardness of materials. A more detailed analysis of the effect of ISE on hardness is presented using an energy model based on the determination of the specific work of plastic deformation during indenter indentation. In all cases of ISE manifestation, a proportional relationship was established between the fixed hardness and the specific work of plastic deformation. Thus, the main cause of ISE is the hardening of the material in the centre of deformation under the indenter. The degree of influence of ISE on the fixable hardness as a function of material pre-hardening was determined. At a high level of pre-hardening (at ε>0.25) the influence of the ISE becomes insignificant, so it cannot be of practical importance. The proposed method of hardness determination will allow to assign material for manufacturing of parts to a greater extent corresponding to the operating conditions.

Прогноз несущей способности свай в билинейной постановке по результатам испытаний свай

The article analyzes the possibility of using the analytical method for forecasting the bearing capacity of piles in a bilinear formulation for prefabricated piles. The pile bearing capacity forecasting method in a bilinear formulation is an alternative pile bearing capacity forecasting technique that allows obtaining a load-settlement graph for a single pile, which makes it possible to forecast the pile bearing capacity in an analytical formulation. There were carried out 12 full-scale tests of bored piles and 19 factory-made piles for static indentation load. For each test, there was performed a numerical simulation of pile testing for a static indentation load and an analytical calculation of the bearing capacity of the pile in a bilinear formulation. The results of the analytical calculation are compared with the results of field tests of piles and the results of numerical simulation. Based on a comparison of calculated and experimental data, conclusions have been made about the possibility of using an analytical calculation of the bearing capacity of piles in a bilinear formulation.