Evaluation of multi-scale indentation tensile properties of metallic materials considering strain hardening characteristics and stress field analysis

Abstract

Assessing structural components for safety and reliability is important because their failure or breakdown entails extensive damage, and rebuilding requires huge effort and expense. In particular, unexpected events in use combined environment can make the performance and lifetime of structures worse than designed, leading to severe accident and failure. Thus, understanding the degradation in mechanical properties is a major issue in appraising industrial fitness for service. However, measuring the properties of materials in service is not easy because of the destructive nature of conventional mechanical tests. The primary mechanical property used in safety assessment is strength. Tension testing requires specimens of a specific geometry and size, and thus cannot be applied to in-use structures or small-scale materials. Therefore, to assess the safety of in-use structures, an alternative nondestructive way to measure in-service material properties is highly desirable. Instrumented indentation testing (IIT) is a powerful tool for measuring mechanical properties by analyzing the indentation load-depth curve. It is simple to perform and does not require a sample of any specific shape or size: for testing, it is enough to polish the surface to be tested. Its ability to localize property measurement makes it easy to find weak points nondestructively. In addition, IIT can be used as a multi-scale mapping tool for the mechanical properties of composite materials and constituent phases for material design. The tensile properties of metallic materials can be determined from the indentation parameters in a spherical indentation test following the representative stress and strain method. Here an algorithm for evaluating the tensile properties is proposed based on the expanding cavity model. The stress-strain relation is determined by comparison of constitutive equations in the uniaxial stress-strain curve. The stress distribution beneath the indenter for strain-hardening materials is developed using an analytical solution for an internally pressurized spherical shell. The plastic constraint factor is newly defined from the stress distribution beneath the indenter as depending on material properties such as yield strain, and the Voce equation parameters. In determining representative strain, it is expressed only in terms of the contact angle between indenter and specimen, which rarely depends on material properties. The values of tensile properties were estimated for more than 20 different metallic materials and were compared with the values from standard uniaxial tension tests. One of the difficulties in measuring macroscopic properties using micro/nano indentation is that harndess depends on size: it is commonly observed to increase with decreasing indentation depth (this is the indentation size effect observed in numerous micro/nanoindentation experiments on various materials). By combining the representative stress and strain model and the Nix-Gao model, which developed a mechanism-based ISE model by calculating GND density and has been accepted as the most general model, the macroscopic stress can be estimated through micro/nano indentation. In addition, recently developed nano-indentation systems with piezo-actuation method show improved perfomance over previous commercial systems.