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This study analyzes the effects of post-weld heat treatment (PWHT) on the mechanical properties and microstructures of SA-508 Gr.1a welds and proposes a new PWHT exemption criterion based on nonductile fracture evaluation considering welding residual stress. The welding coupons were prepared with submerged-arc welding, gas-tungsten arc welding, and shielded-metal arc welding, using ferritic steel, SA-508 Gr.1a. The microstructure of the heat-affected zone (HAZ) was analyzed using optical microscopy, electron-back-scatter diffraction and Vickers hardness testing. The mechanical properties of the welds were evaluated by uniaxial tensile test, transverse side bend test, Charpy V-notch impact test and side bend test. Bainite and ferrite structures formed mainly in the HAZ, and the grain size became coarser with proximity to the surface and fusion line. The mechanical properties did not depend strongly on PWHT, weldment thickness or welding techniques, and they satisfied the welding procedure qualification test specified in the ASME Boiler & Pressure Vessel code. Welding residual stresses were considered in assessing structural integrity using nonductile fracture evaluation. A margin of safety against nonductile fracture with residual stress was calculated for Korean Standard Nuclear Power Plant steam-generator welds, using its design parameters and operating conditions, and this safety margin is suggested as an acceptance criterion for residual stress for exemption from PWHT. Graphic abstract

The microstructure, mechanical properties and residual stress of flux-cored arc welded ASTM A516-70N carbon steel using a Mn-based low-temperature transformation (LTT) welding consumable were investigated. Microstructural analysis with X-ray diffraction, an electron backscattered diffractometer and a field-emission scanning electron microscope showed that the LTT weld metal was made up of ferrite, austenite, martensite, and bainite with phase fractions 50.5%, 0.2%, 40.2% and 9.1%, respectively. The increase in hardness and the decrease in absorbed impact energy of the LTT weld metal compared with conventional consumable welds were confirmed to be due to the relatively high fraction of martensite phase in the weld metal. The welding residual stress distributions in three coupons (LTT, conventional and postweld heat-treated conventional weld) were compared by the results using instrumented indentation testing. The LTT weld coupon showed compressive residual stress distributed in the weld metal and heat-affected zone (HAZ), confirming previous studies in which this residual stress was attributed to a martensitic phase transformation at relatively low temperature. PWHT in the conventionally welded coupon considerably reduced the tensile residual stress distributed in the weld metal and HAZ. The LTT consumable, however, showed a significant advantage in welding residual stress, even compared with the heat-treated conventional consumable. (Received April 6, 2021; Accepted April 29, 2021)

High-cycle fatigue testing under different stress conditions must be performed in fatigue test methodology, and this requires expenditure both of money and time. The high-cycle fatigue test methodology also has the limitation of being a statistical approach to assessing estimated life that is not based on material properties. Thus to evaluate the estimated life of structural materials in transmissions in use, we need a novel assessment method that is economical, effective, easy to apply, and based on the material properties. In this study, we derive the relation between rupture stress and tensile properties taking into account fatigue rupture time, and developed a methodology for evaluating the estimated life of structural materials of transmission. Using this methodology, we performed stress rupture and fatigue tests for automatic transmission UD drum steels.

The fracture strain of carbon fiber-reinforced plastics (CFRPs) within CFRP/steel hybrid laminate composites is reportedly higher than that of CFRPs due to transverse compressive stress induced by the steel lamina. A micromechanical model was developed to explain this phenomenon and also to predict the mechanical behavior of CFRP/steel hybrid laminate composites. First, the shear lag theory was extended to calculate stress distributions on fibers and matrix material in a CFRP under multiaxial stress condition, considering three deformation states of matrix (elastic and plastic deformation and fracture) and the transverse compressive stress. Then, the deformation behavior of CFRP was predicted using average stress in the ineffective region and the Weibull distribution of carbon fibers. Finally, the mechanical properties of CFRP/steel hybrid laminate composites were predicted by considering the thermal residual stress generated during the manufacturing process. The micromechanical model revealed that increased transverse compressive stress decreases the ineffective lengths of partially broken fibers in the CFRP and results in increased fracture strain of the CFRP, demonstrating the validity of the current micromechanical model.

A method for evaluating residual stress using an instrumented indentation test was developed some decades ago. More recently, another method was developed, using a Knoop indenter. The conversion factor ratio, which is one of the key factors in the evaluation algorithm, has been taken to be 0.34, although this value comes from an experimental result and its physical meaning has not been examined. Here we examine the physical meaning of this conversion factor from the previous residual stress model, and calculate its ratio using analytical model of the stress field beneath the indenter. In this process, we assumed that the conversion factor ratio was the ratio of the projected area of the plastic zone generated during the Knoop indentation test. An analysis of the stress field beneath the indenter was performed by FE simulation. Actual nanoindentation was conducted after Knoop indentation testing, using the interface-bonding technique, to identify the plastic zone. In addition, the conversion factor ratio was also calculated for the case where residual stress was present, and the geometric ratio of the Knoop indenter was different. A comparison of our results with those from previous studies showed that the conversion factor ratio obtained using our assumption was in good agreement with previous studies. (Received May 20, 2020; Accepted June 15, 2020)

SA213 T92 steel is generally used in power plants as a high-temperature structural material. In order to evaluate the degradation of the flow properties of T92 by heat, accelerated tests were condu...

A method is proposed to predict the fracture toughness of in-service structures using the instrumented indentation test. While previous studies have attempted to predict fracture toughness using spherical indenters, we propose the method to predict fracture toughness using flat-ended cylindrical indenters. Using the geometric similarity of a cylindrical indentation test and the Cracked Round Bar (CRB) fracture toughness test, fracture toughness values were derived from a single indentation test by assuming that the load–depth curve of the indentation test is the same as the load–displacement curve of the CRB fracture toughness test. To determine the crack initiation point, the concept of limit load in CRB testing is adopted, and a new load–depth curve is obtained using a suggestion in the standard of fracture toughness test. In order to apply the proposed method directly to in-service structures, the model uses mechanical parameters that can be obtained by indentation testing. The model was verified on metallic materials primarily used in nuclear power plants.

We investigated the analytic relation between hardness and compressive yield stress using an expanding cavity model (ECM) for nanoporous gold (np-Au). We prepared three np-Au samples with ligament sizes 30.61, 59.36 and 116.33 nm by free-corrosion dealloying and post heat treatment. The indentation contact morphology was examined to estimate the hardness accurately from the nanoindentation load-depth curve. Unlike conventional dense metals, the deformation was confined to the projected contact area, and the center of the residual impression was dominated by densification. The projected contact area estimated by the Oliver–Pharr method was overestimated, so that a new contact area function was proposed that considered the indentation contact morphology of np-Au. It was confirmed that a hardness value taking into account the indentation contact morphology of np-Au matches well with the hardness derived by direct measurement of the residual impression. We modeled the ratio of hardness to compressive yield stress for np-Au using an ECM. The scaling factors, which represent the extra strain-hardening in the core in the ECM, were analyzed for np-Au and dense metals. An ECM that better matches np-Au is suggested based on the scaling factor resulting from densification beneath the indenter.

Evolution of abnormally growing Goss grains in Fe–3%Si steel was observed time sequentially by electron backscattering diffraction (EBSD). For example, some specimens were heated at 1080 °C for 5 min, then cooled, polished and observed by EBSD, whose procedure was repeated for 6 times. After each heat treatment at 1080 °C, the abnormal grain growth morphology of Goss grains in Fe–3%Si steel could be observed by polishing away a thin layer of the oxidized surface. Goss grains grew abnormally in a highly irregular way. As a result, some matrix grains were isolated at the growth front, which produced island grains. The irregular growth often resulted in incomplete isolation, which produced peninsular grains. Numerous matrix grains were isolated by the impingement of abnormally growing Goss grains. Once matrix grains became isolated, island or peninsular grains, they shrank much faster than before. If the grain boundary mobility of island and peninsular grains is not low enough, they would shrink in an accelerated way because their local curvature of grain boundary becomes larger as they shrink. Therefore, the high shrinking rate of island and peninsular grains indicates that island and peninsular grains are formed not because of their low grain boundary mobility but because of solid-state wetting.

The tensile yield strength of high-density polyethylene using instrumented indentation tests with a flat-ended cylindrical indenter was evaluated. The variation in the field expressed by stress and strain beneath the flat-ended cylindrical indenter is investigated using a new expanding cavity model to study the relation between tension and indentation. This model starts from the separation of forces into the compressive force on the material and the frictional one, which is generated during indentation on the sides of indenter. The authors propose a method to correct the frictional force based on the saturation of indentation hardening and obtain load–depth curve with compressive component only. For conversion of indentation force and displacement, our new representation model is applied. By modifying Johnson’s model, the new assumption of conservation of indentation plastic volume is suggested. This model proves and supports conventional relations of the strain rates between indentation and tension theoretically. These are verified through the experiments: instrumented indentation and uniaxial tensile test. The authors find a good agreement between the tensile yield strengths at various strain rates.

We suggest a method for accurately estimating the uncertainty of indentation yield strength determined from the modified Meyer relation as a mathematical function of the measurement, taking into account Type A and Type B uncertainty. Using this method, we quantitatively compared the expanded uncertainty level of the yield strength as measured by instrumented indentation testing (IIT) and uniaxial tensile testing, and propose a dominant measurand that affects the final expanded uncertainty of the indentation yield strength. To interpret the difference in uncertainty between IIT and uniaxial tensile testing, we investigated the effect of the major sources of uncertainty in the IIT system: sample surface roughness and angular misalignment between the surface normal of the sample and the symmetric axis of the indenter. The surface roughness was controlled using 400-, 1000- and 2000-grit paper and the misalignment angle ranged over 0°, 1° and 2°. Acceptable surface roughness and standard uncertainty of misalignment angle are proposed that give the IIT similar uncertainty to uniaxial tensile testing.

In this study, we characterize the viscoelastic behavior of polymers using nanoindentation. We applied the indentation representative stress approach and elastic solution to the Maxwell model and determined that there was a linear relationship between the inverses of the initial unloading stiffness and indentation unloading rate. From nanoindentation tests with various unloading rates on poly(dimethylsiloxane), the linear relationship between the indentation unloading rate and the initial unloading stiffness was confirmed. We have suggested two parameters such as, elastic coefficient and viscous coefficient, represent the viscosity and elasticity of the polymer material based on their relation. In order to check the dependency of the elastic and viscous coefficients on mechanical properties, we performed nanoindentation on poly(dimethylsiloxane) with different crosslinking densities by mixing different proportions of curing agent. The viscosity and elasticity depend on the crosslinking density of polymer, and it was confirmed that the elastic coefficient and viscous coefficient obtained from nanoindentation varied with the same trend depending on crosslinking density. (Received November 14, 2018; Accepted March 27, 2019)

Residual stress is generally evaluated using indentation by comparing the indentation curves of stressed and stress-free states. Here, we suggest a new method that can evaluate surface residual stress without indentation testing on stress-free specimen using stress-independent indentation parameters and an analysis of indentation contact morphology for the stress-free state. We found that several indentation parameters are independent of the stress by Vickers indentation testing on various stress states. The indentation contact morphology can be represented by indentation parameters including stress-independent ones, and by applying the stress-independent parameters obtained from the stressed state to the indentation contact depth function, we can estimate an indentation curve for stress-free state. The estimated curve matches well with the experimental stress-free indentation curve, and it was also confirmed that the applied stress values evaluated by comparing the estimated curve with the stressed indentation curve agree well with the reference values obtained from strain gauge.

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.

Invar alloy sheet was welded by resistance seam welding (RSW) with a constant electrode force and three different welding currents. Tensile properties were evaluated using instrumented indentation testing (IIT) with a spherical indenter and microstructure observations were obtained under an optical microscope. IIT performed on the base material at room temperature (RT) and &minus, 163 &deg, C, a cryogenic temperature (CT), gave results in good agreement with those of tensile testing. The strength of each zone was higher in the order of heat-affected zone (HAZ) &lt, weld nugget (WN) &lt, base material (BM) because the amount of cold working was least in the BM, heavy metal elements and carbon vaporized during melting, and the WN was formed more tightly than the HAZ, effectively constraining the plastic zone generated by the indentation. As for the welding current, the nugget, which becomes larger and tighter as the current increases, more effectively constrained the plastic zone in the indentation, and this soon increased the strength. Generally, Invar is known to consist of single-phase austenite, and microstructure observations have confirmed that the average grain size is ordered as BM &lt, HAZ &lt, WN. Fan-like columnar grains developed in the direction of the temperature gradient, and equiaxed grains were observed near the BM. It was confirmed that the grain size in the WN also increases as the current is increased. Interestingly, the constraint effect with increasing nugget size was more important for strength than the grain size.

This study derives a uniaxial tensile flow from spherical indentation data using an artificial neural network (ANN) combined with finite element (FE) analysis. The feasibility of the FE-based simulations is confirmed through experimental indentation for various steels. Parametric studies of the FE simulation are performed to generate an ANN training database. An encoding for feature extraction and a hyperparameter optimization is implemented to design the ANN with high predictive performance. The indentation load–depth curves are converted into hardening parameters through the trained ANN. The predictive performance of the FE–ANN model using real-life indentation data is investigated in-depth with thorough error evaluation, and verified by uniaxial tensile tests. The emphasis is made that the mean absolute percentage error between the experimental and simulated indentation data is required to be meticulously controlled below 1% to accurately predict the tensile properties. The validations demonstrate that the applied FE–ANN modeling approach is very robust and captures the tensile properties well. Furthermore, the Taguchi orthogonal array (OA) method that can achieve high efficiency and fidelity with less training data is discussed. The FE–ANN model is concisely designed using the Taguchi OA method and can predict elasticity as well as plasticity.

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.

We suggest a new method to evaluate stress directionality, the ratio of principal stresses, using nanoindentation by introducing a modified Berkovich indenter that is extended in one direction from the Berkovich indenter. In a nonequibiaxial stress state, the indentation load-depth curves are shifted differently as the extended axis of the indenter is placed in accordance with each principal direction. The indentation load-difference is proportional to each principal stress and the slopes are defined by the normal and parallel conversion factors whose ratio is constant at 0.58. The suggested method was verified by indentation tests using five nonequibiaxial stressed specimens. The evaluated stress directionality results show agreement with the applied reference values within ±20%. Furthermore, we calculated the conversion factor ratios for other modified Berkovich indenters extended to different degrees through finite element analysis and confirmed that the conversion factor ratio was inversely proportional to the extension of the modified Berkovich indenter.

Twist-spun carbon nanotube (CNT) yarns exhibit a large and reversible rotational behavior under specific boundary conditions. In situ polarized Raman spectroscopy revealed that a tension-induced twist provides reversibility to this rotation. The orientation changes of individual CNTs were followed when twist-spun CNT yarns were untwisted and subsequently retwisted. Twist-spun CNT yarn, when untwisted and subsequently retwisted under the one-ended tethered boundary condition, showed irreversible orientation changes of the individual CNTs due to snarls formed during the untwisting operation, which resulted in macroscopic irreversible rotational behavior of the CNT yarns. In contrast, the orientation changes of the individual CNTs in twist-spun CNT yarn, when operated under the two-ended tethered boundary condition, were hysteretically reversible due to a tension-induced twist, which has not been reported previously. Indeed, the tension-induced twist was observed by following the orientation change of individual CNTs in elongated CNT yarns, which simulated the deformational behavior of the CNT yarn rotated under the two-ended tethered boundary condition.

The superconductor industry considers cold-rolled austenitic stainless 310S steel a less expensive substitute for Hastelloy X as a substrate for coated superconductor. However, the mechanical properties of cold-rolled 310S substrate degrade significantly in the superconductor deposition process. To overcome this, we applied hot rolling at 900 A degrees C (or 1000 A degrees C) to the 310S substrate. To check the property changes, a simulated annealing condition equivalent to that used in manufacturing was determined and applied. The effects of the hot rolling on the substrate were evaluated by analyzing its physical properties and texture. Web of Science 24 3 454 448

We investigated the initiation and propagation of fiber fracture in unidirectional (UD) carbon fiber-reinforced composites (CFRPs). The statistical fiber breakage of CFRPs and multiple fiber fracture (the propagation of fiber breakage near the broken fiber) have been studied and observed in specimens after fracture. To date, however, this propagation phenomenon has been rarely observed directly, and investigations have depended mainly on prediction. This direct non-destructive evaluation of fiber fracture phenomenon was performed via X-ray computed tomography of UD CFRPs. Specimens were loaded under various stress levels and imaged at low voltage (20 keV). The fiber breakages were analyzed from these images, which revealed multiple fractures leading to large clusters of broken fibers. Quantitative analyses after image processing revealed a marked increase in the number of multiple fractures, meaning the number of broken fibers in a broken fiber cluster, near the final fracture. Finally, the trend in multiple fractures was compared with the predicted results, showing reasonable agreement.

In instrumented indentation testing (IIT), residual stress can be evaluated by shift in indentation load-depth curves for stress-free and stressed states. Although the average surface residual stress is able to be evaluated with Vickers indenter, in order to know stress directionality, another indentation tests with two-fold symmetric indenter, for example, Knoop indenter, are needed. As some necessities for evaluating nonequibiaxial residual stress within small indent area, we suggest a novel way to evaluate directionality of residual stress, p, using wedge indenter characterized by two parameters, edge length and inclined angle. We develop wedge-indentation-mechanics model based on predetermined conversion factors which are determined by IITs for various uniaxial stressed states combining with finite element analysis simulations. By utilizing the developed model, directionality of residual stress is evaluated through two serial wedge IITs with respect to principal directions. We find good agreements between applied residual stress and residual stress evaluated by the developed model for biaxial tensile stress states.

While nanoindentation is a very useful tool to examine the mechanical properties of ion irradiated materials, there are some issues that should be considered in evaluating the properties of irradiated layer. In this study, in order to properly extract the hardness of only-irradiated layer from nanoindentation data, a new procedure is suggested in consideration of the geometry of indentation-induced plastic zone. By applying the procedure to an ion irradiated Fe-12Cr alloy, the reasonable results were obtained, validating its usefulness in the investigation of practical effect of irradiation on the mechanical behavior of future nuclear materials.

Despite advantages of organic light-emitting diode (OLED) displays over liquid crystal displays, reliability concerns persist. These concerns must be addressed before OLED displays are widely adopted. In particular, existing methods are unable to reliably estimate the lifetime of large OLED displays (i.e., displays of 55 in or larger). This study proposes a novel model that incorporates physical and statistical uncertainty to estimate the lifetime of large OLED panels under normal usage conditions. A likelihood-ratio-based validation method is presented to determine the validity of the calculated model parameters. A bivariate acceleration model with two critical factors—temperature and luminance—is presented. The lifespan predicted by the proposed lifetime model shows a good agreement with the experimental results.

Because frit bonding material is brittle, sudden impacts such as dropping may damage the bonding significantly. Strain-rate-controlled in-situ nano-indentation testing, which can determine localized material properties, was carried out on the frit-bonded specimen, especially on the frit bonding matrix and the filler. The results were compared with the drop-impact fracture behavior to understand the fracture characteristics. Mechanical properties at static condition or low strain rate did not show proper relationship with the fracture tendency of the drop tested result of the frit bonding. From the relationship between fracture toughness and the ratio of modulus/hardness, fracture characteristics at the drop impact situation could be estimated by the values at the high strain rate nano-indentation. The ratio between modulus and hardness on frit matrix showed close relationship with drop impact fracture. Though crack propagation path deflected at filler interface, filler property gave less influence on fracture tendency of drop impact fracture due to its small volume fraction. The properties of frit matrix were crucial to the fracture characteristics of the frit bonding.

Since materials used in or exposed to high-temperature environments can undergo variation or degradation of mechanical properties, it is important to evaluate mechanical properties at high temperature, in particular for structural applications and aerospace materials. Instrumented indentation testing (IIT) is widely used to evaluate such mechanical properties of materials as tensile properties, residual stress, fracture toughness, etc., exploiting theoretical approaches to indentation mechanics. In this study, we used IIT to evaluate variations in tensile properties with temperature of the Ti alloy Ti-6Al-4V, a candidate material for aerospace applications, using a high-temperature chamber and a modified representation method. Comparison of our results with conventional uniaxial tensile test results showed good agreement (within a 10% error range) in yield strength and ultimate tensile strength. This confirms the potential of IIT for evaluating to evaluate high-temperature tensile properties of metallic materials and for research on material behavior in various temperature conditions.

Instrumented indentation testing is more advanced than conventional hardness testing in measuring various mechanical properties of materials. To evaluate these mechanical properties, information about indentation contact area is required. In particular, the deformation behavior of metals at high-temperatures seems to differ from that at room temperature, and thus for accurate evaluation of mechanical characteristics at high-temperatures, the high-temperature contact area should be used. In this study, an instrumented indentation system for high-temperatures was developed and measurement-errors caused by equipment temperature were calibrated. In addition, the pileup effect during indentation was studied at different temperatures. A new equation for the high-temperature contact area is proposed. For verification, conventional hardness testing was performed to compare the results with high-temperature instrumented indentation testing.

Surface residual stress is usually characterized by two parameters; (1) magnitude and (2) direction of two principal residual stresses. We propose a novel way to evaluate these parameters by instrumented indentation testing (IIT) using a Knoop indenter, which has two-fold symmetry. Non-equibiaxial surface stress causes a shift from residual stress in the force-indentation depth curve of IIT that depends on the Knoop indentation in-plane angle between residual stress and long diagonal of Knoop indenter. We develop a theoretical model to evaluate surface residual stress using only experimental parameters obtained by four Knoop IITs at 45° rotated angles, by introducing mathematical definition of conversion factor, a, which ratio of normal and parallel conversion factors was known as constants (~0.34) on previous studies. We verify this model by Knoop IITs on four metallic cruciform samples in which the surface residual stress is controlled by bending. Experimental results of principal stress and direction show good agreements with applied stress and directions.

The instrumented indentation technique (IIT) is a novel method for evaluating mechanical properties such as tensile properties, toughness and residual stress by analyzing the indentation load-depth curve measured during indentation. It can be applied directly on small-scale and localized sections in industrial structures and structural components since specimen preparation is very easy and the experimental procedure is nondestructive. We introduce the principles for measuring mechanical properties with IIT: tensile properties by using a representative stress and strain approach, residual stress by analyzing the stress-free and stressed-state indentation curves, and fracture toughness of metals based on a ductile or brittle model according to the fracture behavior of the material. The experimental results from IIT were verified by comparing results from conventional methods such as uniaxial tensile testing for tensile properties, mechanical saw-cutting and hole-drilling methods for residual stress, and CTOD test for fracture toughness.

Residual stress is a major factor in failure and fracture in structures or electronic components. Various testing methods are used to measure residual stress: there are saw-cutting, holedrilling, X-ray diffraction and layer-removing methods. In particular, instrumented indentation testing (IIT) has many advantages: it is a simple and non-destructive procedure that can be used for in-field testing. In previous research, we proposed an algorithm for evaluating the magnitude and directionality of residual stress using an asymmetric Knoop indenter with long and short axes in the ratio 7.11:1. Indenting in different directions with a Knoop indenter creates different indentation load-depth curves depending on the residual stress state. In addition, the directionality of the residual stress can be expressed as a function of the load difference ratio calculated from these load-depth curves. However, When the Knoop indentation test is performed at small indentation depths, experimental issues such as surface preparation or indentation normality can become significant as the load difference decreases. In order to solve these issues, we introduce a wedge indenter, that makes it possible to select the edge length independent of indentation depth. We can thus decrease indent size when working in a small testing area. The load difference between the stress-free and stressed state is related to the sensitivity of residual stresses, and a wedge indenter can maximize the sensitivity to residual stress. In this study, we suggest a way to use the wedge indenter and verify the model using cruciform bending specimens and finite element analysis.

We propose a novel method for estimating stress directionality p and principal direction θp using four Knoop indentations. Indentation load-depth curves with a Knoop indenter are shifted with respect to indenter orientation to stress direction. Based on this phenomenon, we derived p and θp theoretically in terms of the indentation load differences between a stressed and an unstressed sample. Plastic zone beneath the Knoop indenter was theoretically estimated based on expanding cavity model for confirming separate plastic zones of four Knoop indentations. The proposed models and algorithms were verified by indentation experiments under various applied stress states that are generated using a stress-generating jig with two independent orthogonal loading axes. Estimated principal directions showed good agreement with applied principal directions. For small target area when considering in-field application, we proposed an estimation method of one load difference with other three load differences, and it was verified as feasible method with experimental results.

In this paper we propose a novel method, spherical indentation, for evaluation of the plastic properties of combined structures. Three-dimensional (3D) printed products, for example gradient metal alloys consisting of different kinds of material, contain interfaces that can act as weak points and threaten the mechanical reliability of products. Combined structures containing an interface between Cu alloy and Ag were prepared for testing. Samples were heat-treated at 100°C and 200°C for 3 h to optimize processing conditions. The indentation tensile properties of the samples were estimated by analyzing multiple loading–unloading curves obtained by use of the representative stress and strain method. A continuous increase in both yield strength and tensile strength was observed for the Cu alloy and the Cu/Ag interface after heat treatment at up to 200°C, because of precipitation hardening. These experimental results show that mechanical characterization of combined structures by spherical indentation is highly useful on the nano and micro scales.

Generally, post-weld heat treatment is applied to decrease welding residual stress and improve the mechanical properties and microstructure of weldment, and its performance has been recommended for many years [1, 2]. However, current steel-making technology has improved significantly and, steel toughness levels have generally improved substantially [1]. Additionally for several quenched and tempered steels, it is reported that in some cases, mechanical properties such as tensile strength and impact toughness are degraded after post-weld heat treatment [3]. In addition, for large steel assemblies, post-weld heat treatment can be expensive, so that there is an economic incentive to avoid post-weld heat treatment [2]. The research presented here suggests a way to exempt post-weld heat treatment for SA-508 Grade 1A material, which is used for pressure vessels in nuclear power plants, by considering both mechanical properties and residual stress to simplify the welding procedure. Weldments made of 120 mm thick SA-508 Grade 1A should be post-weld heat treated, according to current ASME BPV Code. In order to increase the PWHT exemption thickness to 120 mm, we performed mechanical tests using welding coupons without PWHT; the test results satisfied current mechanical property criteria. We present a residual stress acceptance criterion based on brittle fracture criteria in this research.

We describe an optical three dimensional (3D) measurement method using white-light dispersive interferometry in the range of 5 μm–150 μm. A spectrometer was employed for real-time wavelength scanning and the distance information was obtained using a simple peak detection method in the spectral domain. Fast measurement time enabled us real-time distance measurement of the specimen. The piezoelectric actuator and capacitance distance sensor was employed for nanometer positioning and measuring distance reference value. This technique can be implemented as a distance measurement unit in white-light scanning interferometry by simply attaching a spectrometer to the conventional system. The results showed an accuracy of better than 60 nm for any position within 5 μm–150 μm. The proposed technique can be used as shape-measuring tool for 3D printed products. Measurement results using the proposed method are also presented. These experimental results show that pthe roposed method can be used as a measurement tool for 3D printed products.

We derive a general contact-depth function for the Vickers indenter by modifying a scaling relation between yield strain and indentation depth ratio, which is comprised of indenter angle, plastic constraint factor, and indentation depth ratio. The validity of this function is demonstrated by using various indenters of different angles. A method for calibrating the actual contact area of an imperfectly shaped Vickers indenter is suggested that yields a better evaluation of Vickers hardness in the instrumented indentation test.

We investigate compatibilities of three constitutive equations, the Hollomon, the Swift, and the Voce equations for determination of yield and ultimate tensile strengths based on tensile true stress–strain curves of 27 metal alloys including those with power-law type and linear-type strain-hardening. We analyze each constitutive equation in terms of yield strength determined by the intercept of the linear elastic loading curve and plastic flow curve and ultimate tensile strength evaluated by the concept of instability in tension. We found that the describing plastic flow is very sensitive in determination of the yield strength and tensile strength from parameters of constitutive equation. Voce equation gives estimate yield strength and tensile strength better than Hollomon and Swift equations.

SiO2–polyimide nanocomposites were successfully fabricated by surface modification of silica nanoparticles. In order to create structural similarity between the polymer and the SiO2 surface and to generate interfacial interaction with the polymer chain, (3-trimethoxysilylpropyl)diethylenetriamine was attached to the surface, and then, the surface amines were reacted with phthalic anhydride. The modified silica nanoparticles in polyamic acid solution were subjected to thermal imidization to obtain SiO2–polyimide (PI) nanocomposite films. Cross-sectional transmission electron microscopy results showed no significant aggregation in any of the SiO2–PI nanocomposite films having up to 20 wt% of silica nanoparticles. The effects of silica nanoparticles as a filler material on the thermal, optical, and mechanical properties of the SiO2–PI nanocomposite films were studied in comparison with those of PI by UV-vis spectrometry, thermal gravimetric analysis, thermal mechanical analysis, and nanoindentation.

The effect of indentation using hardness testers on abnormal grain growth (AGG) of non-Goss grains were studied in Fe-3%Si steel. The primary recrystallized specimen was locally deformed by indentation under loads of 0.01, 0.05, 0.1, 0.2, 0.5, 2, 3, 6, 12 and 18 kgf and then heated to 860 °C and held for 10 min for recrystallization or recovery. When these specimens were heated to 1080 °C, AGG of non-Goss grains occurred around the area deformed by indentation loaded above 0.1 kgf. One abnormally growing grain was observed in the specimen indented under the loads of 0.2, 0.5, 2, 3 and 6 kgf, whereas three and eight abnormally growing grains were observed respectively in the specimen indented under loads of 12 and 18 kgf. Analyses by synchrotron X-ray microdiffraction show that all abnormally growing grains had sub-boundaries with the misorientation angles below 0.6o whereas no matrix grain had sub-boundaries.

We propose an extended expanding cavity model (ECM) in instrumented spherical indentation to evaluate flow properties measured in uniaxial mechanical testing. We describe the mean pressure of the projected surface from radial stress at the hemispherical core boundary with a scaling factor for strain-hardening metals. Plastic constraint factors determined by the strain-hardening exponent, yield strain and scaling factor successfully illustrate flow stress–strain points in uniaxial tension tests. We suggest a novel way to determine the strain-hardening exponent from the ratio of indentation loading slope, and a modified Meyer relation to measure yield strengths.

Power generating unit structures are designed and built to meet standard to secure its safety for expected life time. As the structures have been exposed to combined environment, degradation of structure material is accelerated and it can cause unexpected damage; evaluating precise mechanical properties of weak site like welded area is an essential research area as it is directly connected to safety issues. Existing measuring technique like tensile test requires specific size in testing specimen yet it is destructive method which is hard to apply on running structures. To overcome above mentioned limitation, IIT is getting limelight as it is non-destructive and simple method. In this study, latest technique is introduced to evaluate tensile property and residual stress by analyzing stress field occurs under the indenter while IIT is performed. Test on welded area, the weak site of nuclear structures have been practiced and confirmed that IIT can be usefully applied to evaluate integrity in industry.

We evaluate representative stress and strain of austenitic stainless steels using instrumented indentation tests with a spherical indenter by taking into account the real contact depth and effective radius. We investigate the relation between material pileup underneath the spherical indenter and the strain-hardening exponent in uniaxial tensile tests for these steels. We evaluate the suitability of three constitutive equations, the Hollomon, Ludwigson, and Swift equations, for describing linear-type strain-hardening of austenitic stainless steels. Using the real contact depth and effective radii developed for the austenitic stainless steels, we find good agreement between representative stress and strain in instrumented indentation and uniaxial tensile tests.

The experimental approach to analyze the direct correlation between the plastic pileup and the plastic strain of the Vickers indentation test was presented. The correlation between the yield strain and the plastic pileup was analyzed through the interrupted tensile test of SUS316L, the stainless steel in which the strain hardening phenomenon one of the two major factors affecting the plastic pileup is fixed. Finally, it was checked that the plastic strain is the parameter directly affecting the plastic pileup and that the relation through the linear function can be analyzed. Moreover, as for the depth of the indentation test, it was also checked that the plastic pileup can be corrected through the factor.

The instrumented indentation test is an important alternative in quantifying residual stresses. Various indentation models have been developed to determine residual stresses, but no previous models can be applied to a nonequibiaxial residual stress state. To overcome this limitation, a Knoop indentation technique was developed to use the asymmetric characteristics of the Knoop indenter. With the ratio of conversion factors and equations for the relation between the load differences and the residual stress, a model of Knoop indentation was developed to determine the stress directionality p. This model was verified and compared through experiments on various biaxial tensile stress states. Imperfections in the Knoop tip change the conversion factor ratio. Using finite element simulations, this change in the conversion factor was expressed by a fitting equation.

The uniaxial tensile properties of metallic materials can be determined by representing the indentation stress and strain by the load/depth parameters from instrumented indentation tests using a spherical indenter. Here we propose a modified algorithm for evaluating the tensile properties of Ti alloys with low E/σy values by taking into account the plastic constraint effect in low-strain regions. The ratio of elastic modulus to yield strength, E/σy, can be interpreted as a measure of the amount of deformation plastically accommodated during indentation. For low E/σy materials, deformation is elastic-dominated, so less constraint occurs than in high E/σy materials. Finite element methods and spherical indentation tests were used to explore the constraint effect for a variety of materials characterized by different E/σy. The constraint factors were plotted as (E/σy)(a/R) based on expanding cavity model. The modified approach was experimentally verified by comparing tensile properties from uniaxial tensile test and instrumented indentation tests.

Instrumented indentation testing (IIT), an advanced version of conventional hardness tests, is widely used to evaluate the mechanical behavior of materials. Compared to conventional mechanical tests such as tensile testing, saw-cutting, CTOD, and so on, it is a relatively simple method that measures penetration load and depth continuously. In addition, IIT needs little sample preparation and can be applied from macro- to nano-scales by controlling the load and depth range, so that it is used by many researchers on metallic materials and bio-materials at all scales. Here we introduce the fundamentals of evaluating tensile properties and residual stress using IITs as published in ISO/TR 29381. Since indentation means contact between an indenter and surface of specimen, calibrating the contact area is the key issue in estimating tensile properties. By defining a representative stress and strain, indentation load-depth data can be converted to corresponding stress and strain points from the tensile curve. In evaluating residual stress, the load difference is calculated using a stress-insensitive contact hardness model.

Various kinds of polymer passivation and associated equipment have been developed for inter-level insulation, buffering, and @a-rays shielding layers in semiconductor devices. Among these, photosensitive polyimide (PSPI) has thermal and dielectric characteristics that are highly compatible with the requirements of recent devices, and its applications have been increasing. However, PSPI passivation, whose mechanical properties are strongly governed by processing, is implicated in many physical failure modes. It is very difficult to characterize PSPI via fracture mechanics because it behaves viscoelastically and is structurally entangled with variously processed ceramic oxides such as SiN and SiON; in addition, its mechanical characteristics involve some ambiguous fracture modes. In this study, we compare the chip surface toughness of several devices using micro-instrumented indentation testing (MIIT). To do this, we defined a collapse propagation rate in viscoelastic polymer-ceramic composite thin film and verified this parameter by empirically reproducing phenomena involved in chip surface collapse. We used ANSYS to simulate the elastoplastic behavior beneath the indenter tip as the PSPI layer was varied. To discuss all these issues, we obtained by nano-instrumented indentation testing (NIIT) mechanical properties that are closely related to fracture behavior. This study gives some guidance for checking the mechanical resistance of the chip surface to filler-induced attack and thus for developing new technologies for semiconductor process integration.

We evaluate Vickers hardness and true instrumented indentation test (IIT) hardness of 24 metals over a wide range of mechanical properties using just IIT parameters by taking into account the real contact morphology beneath the Vickers indenter. Correlating the conventional Vickers hardness, indentation contact morphology, and IIT parameters for the 24 metals reveals relationships between contact depths and apparent material properties. We report the conventional Vickers and true IIT hardnesses measured only from IIT contact depths; these agree well with directly measured hardnesses within ±6% for Vickers hardness and ±10% for true IIT hardness.

The remaining life prediction of those is accomplished by accelerated degradation tests such as creep testing. However, creep testing is both time-consuming and expensive and hence impossible to apply under industrial conditions, where prompt management and analysis are necessary. While residual lifetime predictions and safety assessments of power plant facilities are made using standard mechanical testing methods such as uni-axial tensile and fracture mechanics tests. However, these tests cannot be applied to in-service structure components because of their destructive nature. A nondestructive instrumented indentation technique is thus attractive for investigating the mechanical properties of in-service systems. This technique can measure flow properties by analyzing indentation load-depth curves reflecting the deformation behavior of the material beneath the rigid spherical indenter. In this study, heat treatment was used to produce degraded samples of X20CrMo12.1V and SA-213 T23, materials widely used in power plant pipelines, and their mechanical properties versus degree of degradation were evaluated by instrumented indentation. We assess the degradation of mechanical properties by microstructural analysis and discuss the possibility of appraising the safety of power plant facilities by in-situ monitoring of mechanical properties using instrumented indentation testing.