Papers by Keyword: Instrumented Indentation Technique

Abstract: High-pressure gas containers must be able to withstand high internal pressures because they store compressed gases. Otherwise, cracks or defects may lead to an explosion, which may in turn lead to a large-scale disaster. Therefore, accurate analysis of the causes of cracks or defects and various techniques for detecting cracks or defects are needed. In this research, we analyzed the failure mechanism of a high-pressure gas container through fractography using scanning electron microscopy and optical microscopy and through measurements of their mechanical and chemical properties.

Abstract: Different microstructures in the weld zone of a metal structure including a fusion zone and a heat affected zone, are formed as compared to the base material. Consequently, the mechanical properties in the weld zone are different from those in the base material to a certain degree owing to different microstructures and residual welding stresses. When a welded structure is loaded, the mechanical behavior of the welded structure might be different from the case of a structure with homogeneous mechanical properties. It is known that obtaining the mechanical properties in the weld is generally difficult owing to the narrow regions of the weld and interfaces. As an alternative way to obtain the weld mechanical properties, the weld mechanical properties of Alloy800HT, SUS316L, and Alloy617, were recently measured using an instrumented indentation technique, and the representative weld mechanical properties of these materials were estimated with a 95% lower confidence level for later structural analyses of the welded structures.

Abstract: The goal of this paper is to study the time-dependent properties of multi-walls carbon nanotubes (MWCNTs)-filled polypropylene (PP) composites using the instrumented indentation technique. Two types of the indentation test, the 3-step and the 5-step indentation tests, were considered to investigate the creep response during sharp indentation. In order to characterize the state of distribution of MWCNTs, the Scanning Electron Microscopy (SEM) technique was used. It was found that the maximum indentation depth decreases with the increase in the MWCNTs concentration. At lower MWCNTs concentration, the effect of injection pressure on the creep displacement is not significant. Comparison of the creep displacements from 3-step and 5-step loading histories indicates a noticeable decrease in creep displacement when the 5-step loading history is used.

Abstract: The instrumented indentation technique (IIT) is a powerful method for evaluating mechanical properties of materials such as elastic modulus, tensile strength, fracture toughness and residual stress. Especially, IIT is a promising alternative to conventional methods of residual stress measurement such as hole drilling, saw cutting, X-ray/neutron diffraction, and ultrasonic methods because of its various advantages of nondestructive specimen preparation, easy process, characterization of material properties on local scales and measurement of in-service structures. Evaluation of residual stress using IIT is based on the key concepts that the deviatoric-stress part of the residual stress affects the indentation load-depth curve and that the quantitative residual stress in a target region can be evaluated by analyzing the difference between the residual stress-induced indentation curve and residual stress-free curve. To verify the applicability of the suggested technique, indentation tests were performed on the welded zone.

Abstract: The instrumented indentation technique (IIT) has recently attracted significant research interest because it is nondestructive and easy to perform, and can characterize materials on local scales. Residual stress can be determined by analyzing the indentation load-depth curve from IIT. However, this technique using a symmetric indenter is limited to an equibiaxial residual stress state. In this study, we determine the directionality of the non-equibiaxial residual stress by using the Knoop indentation technique. Different indentation load-depth curves are obtained at nonequibiaxial residual stresses depending on the Knoop indentation direction. A model for Knoop indentation was developed through experiments and theoretical analysis.

Abstract: Laser-welded parts experience high local temperatures and severe heating-cooling cycles which lead to large local residual stresses. These stresses introduce unacceptable degradation of the mechanical properties of a weldment. Thermo-elasto plastic analyses with 3-D FE models, as well as experimental investigations were performed in order to predict temperature distribution and residual stresses of ND-YAG laser-welded joints with various gap widths between the dissimilar steel types of austenitic and precipitation-hardening stainless steel. The specimens have the shape of a pocket to optimize the weight of the structure, which consists of a thin skin (AISI304) and a thick skeleton (AISI630). The residual stresses at the surface of the weldments were measured using the instrumented indentation method. The residual stresses and melt-pool zone (MPZ) profiles show good agreement between the theoretical and experimental results. Considering the residual stresses, the allowable gap width range of the laser-welded joints for the pocket-shaped specimen was calculated. For a welding joint with gap widths, the longitudinal residual stress values at the yield stress level were observed. Melt-pool zone profiles described by the underfill and penetration depth also depend upon the gap size.