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This study investigates the influence of mineralogical characteristics on the mechanical properties of tight formations from the Montney Play in Canada. A series of instrumented indentation tests were conducted on rock samples cored at depths of 2068–2037 m. The mineralogical characteristics of the indents from the indentation tests were obtained from scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis. By creating digital mineral maps from EDS elemental maps, the localized mineralogical composition and pore distribution around the indents were quantified, and their variations during indentation tests were correlated to the indentation depth/contact area. This new approach can obtain a unique dataset that directly relates rock indentation to their mineralogical characteristics. Pearson correlation analysis was employed to examine the relationships between mineralogical characteristics and mechanical behaviors, such as elastic, plastic and maximum indentation depths, indentation Young's modulus, and hardness. Furthermore, machine learning models were developed using data from instrumented indentation tests and SEM–EDS analysis to predict indentation behavior. The results indicated albite and porosity both have a positive influence on elastic and maximum indentation depths, while orthoclase and pyrite displayed negative correlations. Gaussian process machine learning models were developed to simulate both the loading and unloading stages of an indentation test, which can accurately forecast the indentation force-depth relation on rocks with different mineralogical characteristics. The findings of this study provide valuable insights into the collective effects of mineralogical characteristics on rock mechanical properties. Highlights: Instrumented indentation and SEM–EDS analysis were used on Montney tight formations. Digital mineral map was created to relate their composition to indentation results. Albite/porosity positively relate to elastic and maximum indentation depth. Orthoclase/pyrite negatively correlate to elastic and maximum indentation depth. Gaussian process model predicts indentation behaviors from mineral characteristic. [ABSTRACT FROM AUTHOR]

Over the last few decades, Instrumented Indentation Test (IIT) has evolved into a versatile and convenient method for assessing the mechanical properties of metals. Unlike conventional hardness tests, IIT allows for incremental control of the indenter based on depth or force, enabling the measurement of not only hardness but also tensile properties, fracture toughness, and welding residual stress. Two crucial measures in IIT are the reaction force (F) exerted by the tested material on the indenter and the depth of the indenter (D). Evaluation of the mentioned properties from F–D curves typically involves complex analytical formulas that restricts the application of IIT to a limited group of materials. Moreover, for soft materials, such as austenitic stainless steel SS304L, with excessive pile-up/sink-in behaviors, conducting IIT becomes challenging due to improper evaluation of the imprint depth. In this work, we propose a systematic procedure for replacing complex analytical evaluations of IIT and expensive physical measurements. The proposed approach is based on the well-known potential of Neural Networks (NN) for data-driven modeling. We carried out physical IIT and tensile tests on samples prepared from SS304L. In addition, we generated multiple configurations of material properties and simulated the corresponding number of IITs using Finite Element Method (FEM). The information provided by the physical tests and simulated data from FEM are integrated into an NN, to produce a parametric mapping that can predict the parameters of a constitutive model based on any given F–D curve. Our physical and numerical experiments successfully demonstrate the potential of the proposed approach. [ABSTRACT FROM AUTHOR]

The instrumented indentation approach has become popular to characterize the mechanical properties of metals on-site. Although the evaluation of the materials' strength of metal structures is possible using the instrumented indentation approach, a few research studies have been reported to determine the strength of the steels presenting Lüders behavior. This paper presented a series of experiments including instrumented indentation and tensile tests, and proposed a method to evaluate the yield strength of steels presenting Lüders behavior. A steel truss bridge is used in the on-site benchmark experiment to verify the feasibility and reliability of the instrumented indentation approach. Tensile tests in the laboratory were conducted to obtain the accurate strength of steel from the steel truss bridge. The instrumented indentation tests were done on members of the steel truss bridge and the tensile test specimens. The strength from the instrumented indentation tests was compared with those from tensile tests. The comparison results are used to discuss the accuracy and precision of the instrumented indentation approach. The results indicate that the instrumented indentation approach is a feasible and sufficiently accurate method for measuring the strength of steel components. The difference of the yield strength is about 10%, and the difference of the tensile strength is around 5%. A method to improve the measured accuracy of yield strength is proposed. By using the proposed method, the difference of the yield strength for steels presenting Lüders behavior is less than 5%.

Instrumented indentation testing is an efficient approach for measuring mechanical properties such as equivalent elastic modulus and hardness. Recently, the indentation-based approach has been extended to estimate the stress–strain curve corresponding to tensile or compression tests. However, estimation performance is decreased in high work-hardening alloys because of the limitation of the functional expressiveness in a simple power-law hardening model containing two material constants. Although a modified constitutive model can be employed to improve the expressiveness, additional experimental data are required to adequately determine these material constants. In this study, a method for determining the additional material constant in a modified work-hardening model combining power-law and linear hardening was proposed without additional experimental data, based on the two stress–plastic strain curves of the power-law and linear hardening models. Consequently, the material constants can be determined using an existing approach. In this study, a data-driven estimation approach using the response surfaces of the loading curvature of a load–depth curve and the pile-up height of an indentation impression is established. The proposed approach was applied to high-work-hardening steel for validation.

Instrumented Indentation Test (IIT) is a non-conventional mechanical characterization technique to evaluate hardness, Young modulus, creep and relaxation of materials. In the macro range, it represents a cheaper and faster alternative to conventional tensile-based tests. IIT is a metrological scale; thus, to establish traceability, calibration is essential. Frame compliance calibration is critical because it is a major contribution to the measurement uncertainty. This work discusses the limits of the current state-of-the-art and proposes a novel methodology for frame compliance calibration. The introduced approach demonstrates the source of common systematic errors in the mechanical characterization reported in the literature, i.e. edge effect, while first highlighting a relevant frame compliance nonlinearity. The proposed procedure is cost-effective and relies upon constitutive spring modelling of IIT and calibration of reference block by nanoindentation. Results show that the novel approach corrects systematic trends in the characterization and yields a relative measurement uncertainty of 5%. • Macro-Instrumented Indentation can conveniently replace conventional tests. • Frame compliance calibration is critical for accurate and precise characterization. • Current calibration methods neglect frame compliance nonlinearity. • A method is proposed and applied to calibrate frame compliance nonlinearity. • An approach to calibrate reference material by nanoindentation is demonstrated. [ABSTRACT FROM AUTHOR]

The brazing of structural high-temperature-resistant nickel alloys is a predominant method in manufacturing jet engines in the aircraft industry. Ni-Cr-base brazing filler metals (BFMs) containing B and Si as the melting point depressants are used for this purpose. The presence of the latter can lead to the formation of brittle constituents in the joints, decreasing their strength, toughness and creep resistance. The structures of Hastelloy X nickel superalloy joints brazed with Palnicro 36M BFM are presented in this paper along with the mechanical properties of their particular phases as a function of brazing time. Indentation hardness, Martens hardness, reduced modulus and creep coefficient were measured using the instrumented indentation method. The elastic part of the indentation work was also calculated. Pd forms an unlimited solution with Ni, but its high content in BFM does not fundamentally change the general joint structure known from other Ni-superalloy–Ni-BFM systems. However, new Pd-containing phases are emerging. The hardest components were Ni-B and Cr-B boride phases and Pd-Ni-Si phase in MZ and the boundary of DAZ and BM. MZ reduces the plasticity of a joint to the highest extent. The hardness of particular parts in the joints and the elastic portion of the indentation work decreased with the increase in brazing time, while the reduced modulus of the indentation contact and indentation creep increased. The results of indentation creep measurements indicate that all structural components of the joints were less susceptible to creep than the parent material at room temperature. [ABSTRACT FROM AUTHOR]

A data-driven estimation method for the plastic properties of alloys was proposed using indentations at neighboring positions. An instrumented indentation test is an efficient approach to measure mechanical properties such as equivalent elastic modulus and hardness. In the mechanical test, subsequent experiments are generally performed apart from existing indentations to avoid the interaction effect caused by the dependency on the deformation history of metal plasticity. In this study, the interaction effect was utilized to estimate the plastic properties, based on the difference of load–depth curves between the first and second indentations at neighboring positions. Using finite element simulations of the neighboring indentation tests, the effective experimental conditions were examined, and response surfaces of the loading curvatures were characterized to determine two material constants of a simple constitutive model of plasticity. Finally, the proposed approach was validated for application to aluminum alloys and stainless steel. It can be also applied to various alloys characterized by different elastic moduli.

Polymeric materials are widely used in structural components and systems, and the accurate prediction of their complex time‐dependent behavior is critical. Several constitutive models are available for different types of mechanical behaviors and loading conditions. However, selecting the model and the correct calibration of its parameters is often a challenge. This paper evaluates the applicability of the two‐layer viscoplasticity (TLV) model to predict the viscoplastic behavior of polyvinylidene fluoride (PVDF) in a large range of strain rates and complex states of strain. The setting of parameters and model validation was made by performing experimentally and numerically uniaxial compression tests and relaxation tests. The model parameters were also analyzed in performing microindentation tests. The TLV model using the same set of parameters selected well predicted the PVDF behavior in a wide variety of compressive loading conditions. [ABSTRACT FROM AUTHOR]

A data-driven estimation method for the plastic properties of alloys was proposed using indentations at neighboring positions. An instrumented indentation test is an efficient approach to measure mechanical properties such as equivalent elastic modulus and hardness. In the mechanical test, subsequent experiments are generally performed apart from existing indentations to avoid the interaction effect caused by the dependency on the deformation history of metal plasticity. In this study, the interaction effect was utilized to estimate the plastic properties, based on the difference of load–depth curves between the first and second indentations at neighboring positions. Using finite element simulations of the neighboring indentation tests, the effective experimental conditions were examined, and response surfaces of the loading curvatures were characterized to determine two material constants of a simple constitutive model of plasticity. Finally, the proposed approach was validated for application to aluminum alloys and stainless steel. It can be also applied to various alloys characterized by different elastic moduli. [ABSTRACT FROM AUTHOR]

The instrumented indentation test (IIT) is an attractive non-destructive testing technique. Determining accurate strength properties of steel using IIT is still challenging. In this paper, a new methodology is proposed to acquire the yield and ultimate tensile strength from a single IIT. This method extracts true stress-strain curves from IIT results. Acquired stress-strain curves indicate that the initial yield stress is not repeatable. This is caused by the inhomogeneous deformation of IIT specimens. Based on the obtained true stress-stain curves, corresponding yield strength, and ultimate tensile strength are calculated through theoretical derivation. The results show that the strength has a convergent tendency. On basis of this phenomenon, the strength is determined with an extrapolating method. Finally, the strength properties of Q345R are investigated to verify the reliability of this method. It is found that the strength determined from IIT and conventional tensile tests shows good agreement. The proposed method is effective in predicting strength properties from a single instrumented indentation test. [ABSTRACT FROM AUTHOR]

Friction stir welding has the advantages of producing less material deformation and of simple joining of relatively thick materials, but the disadvantages of forming relatively large heat-affected zones around weld areas. In addition, in research on reductions in bulletproof performance in heat-affected zones, it is necessary to evaluate the ballistic limit velocity (V50), which measures bulletproof performance, and this requires specimens of a certain minimum size and has only limited application to local areas such as heat-affected zones. Instrumented indentation testing (IIT), a method of measuring material properties by utilizing load-depth curves measured by a small indenter leaving fine marks on the material, has no specimen size requirements; in addition, it has simple test procedures and is nondestructive. Here a theoretical model is proposed for evaluating V50 of aluminum alloys by ductile hole formation using IIT. Heat-affected zones generated after friction stir welding of for 7000-series aluminum alloys were simulated, and the model was validated through comparison with the conventional V50 test. In addition, there is currently no way to directly evaluate V50 for welded areas in 7000-series aluminum, but here IIT was used to assess V50 for each local part of the welded specimen. The conventional V50 test takes 15 min per shot and cannot be reused of specimens, but the test using IIT takes 1 min to complete one test and has benefits in terms of time, cost and safety. [ABSTRACT FROM AUTHOR]

Instrumented indentation tests are an efficient approach for the characterization of stress–strain curves instead of tensile or compression tests and have recently been applied for the evaluation of mechanical properties at elevated temperatures. In high-temperature tests, the rate dependence of the applied load appears to be dominant. In this study, the strain-rate-dependent plasticity in instrumented indentation tests at high temperatures was characterized through the assimilation of experiments with a simulation model. Accordingly, a simple constitutive model of strain-rate-dependent plasticity was defined, and the material constants were determined to minimize the difference between the experimental results and the corresponding simulations at a constant high temperature. Finite element simulations using a few estimated mechanical properties were compared with the corresponding experiments in compression tests at the same temperature for the validation of the estimated material responses. The constitutive model and determined material constants can reproduce the strain-rate-dependent material behavior under various loading speeds in instrumented indentation tests; however, the load level of computational simulations is lower than those of the experiments in the compression tests. These results indicate that the local mechanical responses evaluated in the instrumented indentation tests were not consistent with the bulk responses in the compression tests at high temperature. Consequently, the bulk properties were not able to be characterized using instrumented indentation tests at high temperature because of the scale effect.

• An optimization was used to determine the material mechanical properties. • Mechanical properties were directly extracted from multiple loading–unloading data. • The uniqueness and adequately precision can be assured with the proposed method. A methodology is proposed to obtain mechanical properties of bulk materials from multiple loading-unloading curves of an instrumented indentation test (IIT). The mechanical properties, including strain-hardening exponent (n) and strain-hardening rate (K), were extracted from IIT data using iterative finite element simulation coupled with a simulated annealing swarm particle optimization. A systematical study was carried out to discuss the uniqueness of the identified properties. Results showed that the mechanical properties obtained in this way are unique and accurate adequately. It was found that the maximum errors of K and n generated from the macro indentation tests are 8.2% and 6.3% respectively. The identified mechanical properties from ten loading-unloading curves was compared with those obtained from a single loading-unloading curve. We found that ten loading-unloading cycles of an indentation test will result in more precise mechanical properties. [ABSTRACT FROM AUTHOR]

Residual stresses, existed in engineering structures, could significantly influence the mechanical properties of structures. Accurate and non-destructive evaluation of the non-equibiaxial residual stresses in these structures is of great value for predicting their mechanical performance. In this work, investigating the mechanical behaviors of instrumented spherical indentation on stressed samples revealed that non-equibiaxial residual stresses could shift the load-depth curve upwards or downwards and cause the residual indentation imprint to be an elliptical one. Through theoretical, experimental, and finite element (FE) analyses, two characteristic indentation parameters, i.e., the relative change in loading curvature and the asymmetry factor of the residual indentation imprint, were found to have optimal sensitivity to residual stresses at a depth of 0.01R (R is the radius of spherical indenter). With the aid of dimensional analysis and FE simulations, non-equibiaxial residual stresses were quantitatively correlated with these two characteristic indentation parameters. The spherical indentation method was then proposed to evaluate non-equibiaxial residual stress based on these two correlations. Applications were illustrated on metallic samples (AA 7075-T6 and AA 2014-T6) with various introduced stresses. Both the numerical and experimental verifications demonstrated that the proposed method could evaluate non-equibiaxial surface residual stresses with reasonable accuracy.

Abstract The feature of instrumented indentation test (IIT) makes it suitable for the characterization of local constitutive behavior in a small volume of solids. In the present study, the relationship between the shape of the normalized loading P - h curve and the indenter tip rounding radius, and the influences of the constitutive parameters of a Ludwik-type isotropic hardening model on the shape and the initial slope of the normalized unloading P - h curve were investigated by theoretical analysis and sophisticated numerical sensitivity analysis. From the analytical results, a two-stage analytical approach that can inversely identify multiconstitutive parameters simultaneously was proposed. In the first stage, by considering the least-square distance between the experimental and the simulated normalized loading P - h curves (g (h/h max)) as the objective and C 1 in the tip area function as the input variable, the tip rounding was identified by a single-objective optimization procedure. With the optimal tip rounding, a multiobjective optimization procedure was then performed to identify the constitutive parameters of the target materials. The initial slope of the normalized unloading P - h curve (Δ S ' (P / Pmax-h / hmax)), absolute value of the differences between the simulated and the experimental maximum load (Δ P max), initial slope of the unloading P - h curve (Δ S), and contact hardness (Δ H) were set as objectives, while all the material parameters except the Poisson's ratio (v) were considered as input variables. Results of the numerical verification with a set of arbitrarily selected constitutive parameters and the experimental verification on Ti-18 (a near- β high-strength titanium alloy) fit fairly well with uniaxial tensile parameters. With regard to T40, the identified true stress-true plastic strain curve is about 20% higher than the uniaxial tensile curve. One possible explanation is the occurrence of the strength differential and anisotropy effects during the plastic deformation process of α -phase titanium. It is worthy to be noticed that the present study may propose a new way to calibrate the tip area function directly on the target material without any standard indentation sample and improve the quality of the identification of the material constitutive parameters by IIT. [ABSTRACT FROM AUTHOR]

Abstract Instrumented indentation tests enable evaluation of several material parameters. Preliminary investigations underlined the influence of contact stiffness in the nano-range. ISO 14577-1 recommends two methods for contact stiffness evaluation, which, however, present shortcomings mainly related to the disregard of actual shape of unloading indentation curve. Given the relevance not only at the extent of material characterization, but also at testing machine calibration, alternative evaluation procedures have been developed, showing better agreement with experimental data and lower uncertainty in the evaluation of contact stiffness. In this work, shortcomings of standard and alternative methods in the evaluation of material parameters are highlighted, and innovative procedures are proposed to overcome them. [ABSTRACT FROM AUTHOR]

• Methodology to extract bulk equivalent mechanical properties that are usually measured by tensile or compression tests (i.e., yield stress, yield strain, work hardening parameter and elastic modulus) by means of automated large nanoindentation map. • Novel post-processing approach to automatically identify yielding point of materials from stress strain curves assures high throughput analysis. • Outstanding correlation between mechanical properties and microstructural changes across the bonded cross-section, in dissimilar metal/metal and metal/ceramic diffusion bonding joint interfaces. • Qualification protocol for material assemblages of observation windows in the International Thermonuclear Experimental Reactor (ITER) fusion reactor. The International Thermonuclear Experimental Reactor (ITER) is an international nuclear fusion research and engineering project with the aim to prove the feasibility of nuclear fusion as a large-scale carbon-free source of energy. The reactor vessel contains several diagnostic windows that provide a line of sight to the plasma. Design and qualification of such windows is challenging due to severe in-service conditions and the necessity to combine glasses with metals. One specific window is composed of fused silica glass that is connected to an Inconel ferrule via an aluminium bonding layer, where the joint is consolidated through a diffusion bonding process. The optimization of design and production process parameters of the windows is object of continuous improvements via dedicated three-dimensional finite element model (FEM) simulations. Importantly, these FEM simulations necessarily require stress-strain curves as input parameters. Such curves are difficult to obtain, especially from complex systems with dissimilar joints. In the present paper a methodology based on dynamic spherical nanoindentation has been developed and applied to map the stress-strain characteristics across the diffusion bonded joints. The methodology takes inspiration from the well-known theory to extract stress-strain curves from spherical nanoindentation. Here, it is applied to automated large indentation mapping with more than 1000 indentations including fully automated post-processing. The results provide bulk equivalent mechanical properties including yield stress, yield strain, work hardening parameters and elastic modulus at a micrometre resolution. Results show outstanding correlation with microstructural changes across the bonded cross-section, including grain refinement and twinning at the Inconel/aluminium interface. [ABSTRACT FROM AUTHOR]

This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone. Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials’ plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve; (2) profile of the indented surface; (3) analyzing of the 3D-measurement image; and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface.

Various methods have been designed to determine the elasto-plastic properties of metals. Instrumented indentation test (IIT) is considered to be a good candidate to determine local properties after manufacturing operations. In order to acquire elastoplastic properties from IIT, either dimensional analysis or inverse analysis of the force-displacement curve is performed. However, the major drawback of those methods is the uniqueness of the solution. Some materials may exhibit almost identical force-depth curves, although they have different elastoplastic properties. Those materials are referred as “mystical materials”. In this contribution, topological features of the indentation surfaces, i.e. indent size, pile-up and sink-in behaviour, are investigated to find a differentiating property. According to the results, indent size, pile-up and sink-in behaviour may help to find the unique solution to the inverse problem. [ABSTRACT FROM AUTHOR]

The mechanical properties of Fe-Mn-Al-C steel are significantly enhanced after κ-carbide precipitation via aging; however, most aging treatments are energy demanding because they require relatively high temperatures and extended holding times. This research determined that the precipitation of these carbides can also occur within a few seconds of thermomechanical treatments (TMTs). This behaviour has not been reported post-TMTs for this steel group. Hot compression tests were performed on Fe-21Mn-11Al-1.5C-2Si wt% specimens at test temperatures ranging from 900 °C to 1150 °C and strain rates varying from 0.01 s −1 to 1 s −1 . The effects of strain rate and test temperature on dynamic recrystallization behaviour were evaluated. The microstructures were characterized by scanning electron microscope and electron backscatter diffraction. Hardness tests were performed before and after applying processes studied i.e., TMT and aging treatment to determine the change in hardness induced. Particularly, nanoindentation tests were also used to collect indirect evidence about the deformation mechanisms. The load-displacement curves P-h and (P/h)-h showed the occurrence of several pop-ins and slope changes related to the nucleation of dislocations and strain-induced phase transformations. The occurrence of these phenomena is discussed. [ABSTRACT FROM AUTHOR]

Instrumented spherical indentation has been used to extract information on the deformation behaviour of (0001) oriented grains in commercial purity magnesium. Reproducible discontinuities were found in the loading and the unloading curves and these have been interpreted as the nucleation and growth of { 10 1 ¯ 2 } extension twins during loading and their subsequent retreat during unloading. Detailed three-dimensional electron backscatter diffraction-based orientation microscopy (3D-EBSD) revealed that the presence of residual deformation twins depended on the depth of the indent. Analysis of residual geometrically necessary dislocation density using the EBSD data showed that residual basal 〈a〉 dislocations were dominant in the deformation zone. The crystallographic misorientation observed below the indent can be rationalized by the presence of these basal 〈a〉 dislocations and this is shown to be consistent with crystal plasticity finite element method calculations. Finally, a schematic view of the different stages of deformation during loading and unloading is proposed to rationalize the experimental observations. [ABSTRACT FROM AUTHOR]

Surface preparation, which may induce considerable sample disturbance, plays an important role in instrumented indentation test (IIT). In this study, the sample disturbance (mainly divided into residual stresses and plastic strain) induced by the surface preparation process of instrumented indentation test specimens were investigated with both experimental tests and numerical simulations. Grazing incidence X-ray diffractions (GIXRD) and uniaxial tensile tests were conducted for characterizing the residual stresses and high plastic strain in the top surface layers of a carefully mechanically polished indentation sample, which, in the present work, is made of commercially pure titanium. Instrumented indentation tests and the corresponding finite element simulations were performed as well. For comparison, a reference sample (carefully mechanically polished & electrolytically polished) which represents the raw material was prepared and tested. Results showed that a careful mechanical polishing procedure can effectively reduce the level of residual stresses induced by this process. However, the high plastic strain in the surface region imposed by the polishing process is significant. The induced plastic strain can affect a depth up to 5 µm, which is deeper than the maximum penetration depth h max (3 µm) used for the instrumented indentation tests. In the near surface layer (in the range of depth about 350 nm), the plastic strain levels are fairly high. In the very top layer, the plastic strain was even estimated to reach more than 60%. The simultaneous use of indentation tests and numerical simulations showed that the existence of high plastic strain in the surface region will make the load vs depth ( P – h ) curve shift upwards, the contact hardness ( H ) increase and the contact stiffness ( S ) decrease. [ABSTRACT FROM AUTHOR]

Instrumented indentation tests are an efficient approach for the characterization of stress–strain curves instead of tensile or compression tests and have recently been applied for the evaluation of mechanical properties at elevated temperatures. In high-temperature tests, the rate dependence of the applied load appears to be dominant. In this study, the strain-rate-dependent plasticity in instrumented indentation tests at high temperatures was characterized through the assimilation of experiments with a simulation model. Accordingly, a simple constitutive model of strain-rate-dependent plasticity was defined, and the material constants were determined to minimize the difference between the experimental results and the corresponding simulations at a constant high temperature. Finite element simulations using a few estimated mechanical properties were compared with the corresponding experiments in compression tests at the same temperature for the validation of the estimated material responses. The constitutive model and determined material constants can reproduce the strain-rate-dependent material behavior under various loading speeds in instrumented indentation tests; however, the load level of computational simulations is lower than those of the experiments in the compression tests. These results indicate that the local mechanical responses evaluated in the instrumented indentation tests were not consistent with the bulk responses in the compression tests at high temperature. Consequently, the bulk properties were not able to be characterized using instrumented indentation tests at high temperature because of the scale effect. [ABSTRACT FROM AUTHOR]

HY-80 and HY-100 steels, widely used in constructing large ocean vessels and submarine hulls, contain mixed microstructures of tempered bainite and martensite and provide high tensile strength and toughness. Weld integrity in HY steels has been studied to verify and optimize welding conditions. In this study, the T-joint weld coupons, HY80 and HY100, were fabricated from HY-80 and HY-100 steel plates with a thickness of 30 mm as base metals by submerged-arc welding. Flux-cored arc welding was performed on an additional welding coupon consisting of HY-100 to evaluate the effect of repair welds (HY100RP). Microstructures in the heat-affected zones (HAZ) were thoroughly analyzed by optical observation. Instrumented indentation testing, taking advantage of local characterization, was applied to assess the yield strength and the residual stress of the HAZ and base regions. The maximum hardness over 400 HV was found in the HAZ due to the high volume fraction of untempered martensite microstructure. The yield strength of the weld coupons was evaluated by indentation testing, and the results showed good agreement with the uniaxial tensile test (within 10% range). The three coupons showed similar indentation residual stress profiles on the top and bottom surfaces. The stress distribution of the HY100 coupon was comparable to the results from X-ray diffraction. HY100RP demonstrated increased tensile residual stress compared to the as-welded coupon due to the effect of the repair weld (323 and 103 MPa on the top and bottom surfaces). This study verifies the wide applicability of indentation testing in evaluating yield strength and residual stress. [ABSTRACT FROM AUTHOR]

Residual stresses, existed in engineering structures, could significantly influence the mechanical properties of structures. Accurate and non-destructive evaluation of the non-equibiaxial residual stresses in these structures is of great value for predicting their mechanical performance. In this work, investigating the mechanical behaviors of instrumented spherical indentation on stressed samples revealed that non-equibiaxial residual stresses could shift the load-depth curve upwards or downwards and cause the residual indentation imprint to be an elliptical one. Through theoretical, experimental, and finite element (FE) analyses, two characteristic indentation parameters, i.e., the relative change in loading curvature and the asymmetry factor of the residual indentation imprint, were found to have optimal sensitivity to residual stresses at a depth of 0.01R (R is the radius of spherical indenter). With the aid of dimensional analysis and FE simulations, non-equibiaxial residual stresses were quantitatively correlated with these two characteristic indentation parameters. The spherical indentation method was then proposed to evaluate non-equibiaxial residual stress based on these two correlations. Applications were illustrated on metallic samples (AA 7075-T6 and AA 2014-T6) with various introduced stresses. Both the numerical and experimental verifications demonstrated that the proposed method could evaluate non-equibiaxial surface residual stresses with reasonable accuracy. [ABSTRACT FROM AUTHOR]

This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone. Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials' plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve; (2) profile of the indented surface; (3) analyzing of the 3D-measurement image; and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface. [ABSTRACT FROM AUTHOR]