A Further Study on Knoop Indentation Plastic Deformation for Evaluating Residual Stress

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.

Numerical Simulation of the Depth-Sensing Indentation Test with Knoop Indenter

Depth-sensing indentation (DSI) technique allows easy and reliable determination of two mechanical properties of materials: hardness and Young’s modulus. Most of the studies are focusing on the Vickers, Berkovich, and conical indenter geometries. In case of Knoop indenter, the existing experimental and numerical studies are scarce. The goal of the current study is to contribute for the understanding of the mechanical phenomena that occur in the material under Knoop indention, enhancing and facilitating the analysis of its results obtained in DSI tests. For this purpose, a finite element code, DD3IMP, was used to numerically simulate the Knoop indentation test. A finite element mesh was developed and optimized in order to attain accurate values of the mechanical properties. Also, a careful modeling of the Knoop indenter was performed to take into account the geometry and size of the imperfection (offset) of the indenter tip, as in real cases.

Physical, Dielectric Properties and Micro-Hardness of the (Ba0.90Ca0.10)0.90(Na0.50Bi0.50)0.10TiO3 Ceramics Prepared by Molten Salt Method

In this study, the physical properties, dielectric properties, and micro-hardness of (Ba0.90Ca0.10)0.90(Na0.50Bi0.50)0.10TiO3 or BCT-NBT ceramics prepared by molten salt method with various sintering temperatures were investigated. The powders were calcined at 500-1100°C for 4 h with heating rate of 5°C/min. It was found that the optimum calcination condition was 1000°C for 4 h. These powders were pressed and sintered at 1200-1400°C for 3 h with a heating rate of 5°C/min. The microstructure was examined by scanning electron microscope (SEM). The density of the sintered samples was measured by Archimedes method with distilled water as the fluid medium. Dielectric properties were examined by LCR meter. The micro-hardness of the BCT-NBT ceramics was determined using the Vickers and Knoop indentation techniques. The results showed that the average grain sizes increased with increasing sintering temperatures. At sintering temperatures higher than 1200°C, the fracture mode changed from partial intra-granular to mainly intra-granular. The sintering temperature at which the density, dielectric and hardness properties were maximal was 1350°C. The highest density was about 5.4 g/cm3, and the Vickers and Knoop micro-hardnesses were 6.6 and 6.4 GPa, respectively. The dielectric constant at the Curie temperature was 3682 and the dielectric loss was 0.01 at 1 kHz frequency.