Effects of Inclusions and Porosity on the Indentation Response
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Effects of Inclusions and Porosity on the Indentation Response E. S. Ege and Y.-L. Shen Department of Mechanical Engineering, University of New Mexico Albuquerque, NM 87131, U.S.A. ABSTRACT A combined numerical and experimental study was undertaken to investigate the effect of microstructural heterogeneity on indentation response. Finite element analyses were carried out to simulate the stress-strain behavior and the indentation response of two model heterogeneous systems: one with hard particles embedded within a soft matrix and the other with a porecontaining ductile material. For the particle-containing system, the indentation response consistently overestimates the overall strength of the composite. This is largely due to the localized increase in particle concentration directly underneath the indent. For the porous system, the indentation response consistently underestimates the overall strength due to the porecrushing effect. Experiments on metal-ceramic composites confirmed the non-correspondence between the indentation and stress-strain responses, even when the indent size is much greater than the microstructural feature size. Implications of the present findings in utilizing indentation to quantify surface mechanical properties are discussed. INTRODUCTION Indentation techniques are extensively employed for characterizing surface mechanical properties. Researches on extracting material properties from indentation have been largely based on the treatment that the material follows, for instance, a continuum elastic-plastic response, at least for metallic and many polymeric systems [1-5]. In the majority of engineered surface in structural components and thin-film devices, heterogeneity at the microscopic level exists. Examples include nano-composite coatings for low-friction and wear resistant applications, nano-porous dielectric films in microelectronics, and functionally graded surface layers. When the size of indentation is much greater than the feature dimension of the heterogeneity such as particle or pore sizes, the indentation response is normally conceived to be amenable to continuum-based analyses. The overall mechanical properties of such heterogeneous materials determined by indentation, however, are questionable, for the deformation is expended in redistributing the hard particles or crushing the pores. In this article recent computational and experimental analyses of this problem are presented. APPROACH Figure 1 shows a schematic depicting the computational approach. A square computational domain containing discrete particles was used as a two-dimensional model system. The particles were assumed to be linearly elastic, and the matrix an elastic-plastic solid. The finite element modeling consists of two steps. Uniaxial compression was first simulated to obtain the overall stress-strain response of the composite. This stress-strain relation then served as the input properties for a homogeneous material, termed “homogenized” in Fig. 1, which was then subject to indentation. Indentation was si
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