Effective indenter radius and frame compliance in instrumented indentation testing using a spherical indenter

We introduce a novel method to correct for imperfect indenter geometry and frame compliance in instrumented indentation testing with a spherical indenter. Effective radii were measured directly from residual indentation marks at various contact depths (ratio of contact depth to indenter radius between 0.1 and 0.9) and were determined as a function of contact depth. Frame compliance was found to depend on contact depth especially at small indentation depths, which is successfully explained using the concept of an extended frame boundary. Improved representative stress-strain values as well as hardness and elastic modulus were obtained over the entire contact depth

Evaluation and control of mechanical degradation of austenitic stainless 310S steel substrate during coated superconductor processing

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 Science24345444

Identification of residual stress directionality using anisotropic indenter in instrumented indentation testing

Instrumented indentation testing can be used to quantitatively evaluate the local residual stress on the surface. Many studies have confirmed that indentation load-displacement curves obtained from Vickers indentation and Berkovich indentation are shifted depending on the residual stress state. Based on this, many researchers have proposed models for evaluating the residual stress by comparing indentation curves obtained from stressed and stress-free specimens of the same composition and microstructure. Though Vickers and Berkovich indenters can quantitatively evaluate the residual stresses, it is difficult to evaluate their directionality such as principal direction and principal stresses because the indenters are axisymmetric. In order to overcome these limitations, we have evaluated the residual stress directionality by using less axisymmetric indenters, such as the Knoop indenter and a modified Berkovich indenter (a conventional Berkovich indenter extended along one axis). [1] With these two sorts of indenters, the degree of shifting of the indentation curve depends on the direction of the long axis of the indenter in the non-equibiaxial stress state. We introduced a conversion factor, a proportional constant between indentation load difference and stress, and proposed a method for quantitatively evaluating the directionality of surface residual stress using this conversion factor. We applied a non-equibiaxial stress state to cruciform specimens and verified the accuracy of the proposed model using the conversion factor in Knoop and modified Berkovich indentation testing. Also, the experiments and finite-element analysis of Knoop and modified Berkovich indentations showed that the ratio of the length of the major axis and minor axis of the indenter is correlated to the conversion factor ratio; a generalized formula is proposed. REFERENCES [1] Jong-hyoungKim and Huiwen Xu, “Determination of directionality of non-equibiaxial residual stress by nanoindentation testing using a modified Berkovich indenter”, JMR 33. 3849-3856, 2018

Evaluation of tensile properties using instrumented indentation technique for small scale testing

The Instrumented indentation technique (IIT) is a useful tool for estimating various mechanical properties such as tensile properties, fracture toughness, and residual stress by analyzing the load and depth curve. Unlike conventional test such as tensile test, CTOD, since IIT makes an indent with rigid indenter and measures load and depth continuously, it requires only a localized area and small area on the target material. IIT also has merits of simple specimen preparation and experimental procedure in terms of time and cost. Also, it can be applied to in-field structures nondestructively. In this study, we introduce a method for evaluating tensile properties, primary yield strength and tensile strength using representative stress-strain beneath the rigid spherical indenter through numerous investigations of instrumented indentation curves. Analytic models and procedures for estimating the mechanical characterization of materials using IIT are proposed. The representative stress-strain method directly correlates indentation stress and strain beneath indenter to true stress and strain of the tensile test by taking into account the plastic constraint effect. The experimental results from IIT were verified by comparing results from the uniaxial tensile test. In particular, the applications of IIT in small scale and localized area of materials are presented. Reference 1) D. Tabor: Hardness of metal, (first ed. Clarendon Press, New York, 1951) 2) W.C. Oliver and G.M. Pharr, J. Mater, Res, Vol. 7, (1992), p. 1564 3) S.-K. Kang, Y.-C. Kim, K.-H. Kim, J.-Y. Kim and D. Kwon, Int. J. Plast. 49, 1 (2013

A new approach to evaluate residual stress using instrumented indentation testing at nano scale

In structural integrity, residual stress is one of the major factors affecting structure failure. In particular, tensile residual stress accelerates crack growth and reduces integrity. Hence test methods have been devised that can quantitatively evaluate residual stresses, including X-ray diffraction, hole-drilling, and contour methods. Now a relatively new technique, instrumented indentation testing, can be used to quantitatively evaluate the surface residual stress of a structure semi-nondestructively with mechanical response causing small indents. Many studies have confirmed that indentation load-displacement curves are shifted depending on the residual stress state. For the same indentation depth, a larger indentation load is required for a compressive residual stress state, and a smaller indentation load is required for a tensile residual stress state, in contrast to the stress-free state. Thus, for the same indentation depth, there is a difference in indentation load between the stressed and stress-free states. Kwon and Lee have suggested and verified experimentally that, among the surface residual stress components, a deviatoric stress term parallel to the indentation axis induces a virtual force that affects the plastic deformation occurring during indentation, and consequently also affects the indentation load-displacement curve. [1] In this paper, principle and application for measuring residual stress by IIT at multi-scale will be included. References [1] Y.-H. Lee and D. Kwon, “Estimation of biaxial surface stress by instrumented indentation with sharp indenters”, Acta Materialia 52. 1555-1563, 2004

Conventional Vickers and true instrumented indentation hardness determined by instrumented indentation tests

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

Subjective optimality in finite sequential decision-making

Author summaryIn many real-life decisions, such as hiring an employee, the current candidate is the only option decision-makers can choose among sequentially revealed options, while past options are forgone and future options are unknown. To make the best choice in such problems, decision-makers should set appropriate criteria considering the distribution of values and remaining chances. Here, we provide behavioral and physiological evidence for subjective valuation that explains how individuals set criteria deviating from optimality. The extent to which individuals expect from candidates, how sensitive they are to the value of candidates, and how costly it is to examine each candidate determine the way how individuals make choices. Our results suggest that seemingly suboptimal decision strategies in finite sequential decisions may be optimal in subjective valuation. Many decisions in life are sequential and constrained by a time window. Although mathematically derived optimal solutions exist, it has been reported that humans often deviate from making optimal choices. Here, we used a secretary problem, a classic example of finite sequential decision-making, and investigated the mechanisms underlying individuals' suboptimal choices. Across three independent experiments, we found that a dynamic programming model comprising subjective value function explains individuals' deviations from optimality and predicts the choice behaviors under fewer and more opportunities. We further identified that pupil dilation reflected the levels of decision difficulty and subsequent choices to accept or reject the stimulus at each opportunity. The value sensitivity, a model-based estimate that characterizes each individual's subjective valuation, correlated with the extent to which individuals' physiological responses tracked stimuli information. Our results provide model-based and physiological evidence for subjective valuation in finite sequential decision-making, rediscovering human suboptimality in subjectively optimal decision-making processes

Overlapping Ablation Using a Coaxial Radiofrequency Electrode and Multiple Cannulae System: Experimental Study in ex-Vivo Bovine Liver

Objective To assess the sizes and configurations of thermal zones after overlapping ablations using a coaxial radiofrequency (RF) electrode and multiple cannulae in ex-vivo bovine liver. Materials and Methods For ablation procedures, a coaxial RF electrode and introducer set was used. Employing real-time ultrasound guidance and overlapping techniques in explanted, fresh bovine liver, we created five kinds of thermal zones with one (n=10), two (n=8), four (n=3), and six ablation spheres (n=3). Following ablation, MR images were obtained and the dimensions of all thermal zones were measured on the longitudinal or transverse section of specimens. The shape of the composite ablation zones was evaluated using three-dimensional MR image reconstruction. Results At gross pathologic examination of ten single-ablation zones (spheres), the long-axis (transverse) and short-axis lengths of zones ranged from 3.7 to 4.4 (mean, 4.1) cm and from 3.5 to 4.0 (mean, 3.7) cm, respectively. The long-axis (transverse) and short-axis lengths of double-ablation zones (cylinders) with 23% overlap ranged from 7.0 to 7.7 (mean, 7.3) cm and from 3.0 to 3.9 (mean, 3.5) cm, respectively; those with 58% overlap ranged from 6.0 to 6.4 (mean, 6.2) cm and from 3.8 to 4.6 (mean, 4.3) cm, respectively. The long-axis (diagonal) and short-axis lengths on a transverse section of four-ablation zones (cakes) ranged from 8.5 to 9.7 (mean, 9.1) cm and from 3.0 to 4.1 (mean, 3.7) cm, respectively. Gross pathologic examination of three composite six-ablation zones (spheres) showed that the long-axis (diagonal) and short-axis lengths of zones ranged from 9.0 to 9.9 (mean, 9.4) cm and from 6.8 to 7.5 (mean, 7.2) cm, respectively. T2-weighted MR images depicted low-signal thermal zones containing multiple curvilinear and spotty regions of hyperintensity. Conclusion Using a coaxial RF electrode and multiple cannulae, together with ultrasound guidance and precise overlapping ablation techniques, we successfully created predictable thermal zones in ex-vivo bovine liver.ope

Radiofrequency Ablation of Liver Cancer: Early Evaluation of Therapeutic Response with Contrast-Enhanced Ultrasonography

The early assessment of the therapeutic response after percutaneous radiofrequency (RF) ablation is important, in order to correctly decide whether further treatment is necessary. The residual unablated tumor is usually depicted on contrast-enhanced multiphase helical computed tomography (CT) as a focal enhancing structure during the arterial and portal venous phases. Contrast-enhanced color Doppler and power Doppler ultrasonography (US) have also been used to detect residual tumors. Contrast-enhanced gray-scale US, using a harmonic technology which has recently been introduced, allows for the detection of residual tumors after ablation, without any of the blooming or motion artifacts usually seen on contrast-enhanced color or power Doppler US. Based on our experience and reports in the literature, we consider that contrast-enhanced gray-scale harmonic US constitutes a reliable alternative to contrast-enhanced multiphase CT for the early evaluation of the therapeutic response to RF ablation for liver cancer. This technique was also useful in targeting any residual unablated tumors encountered during additional ablation