Characterization and investigation of size effect in nano-impact indentations performed using cube-corner indenter tip
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The traditional macro-scale form of dynamic indentation measures the dynamic deformation behavior of a material by simulating impact conditions. Similarly, the nano-impact indentation technique, with small-scale contacts and high spatial resolutions, is a novel technique for obtaining mechanical properties of materials at dynamic strain rates (.102 s1). Nano-impact hardness values display a decreasing trend or size effect that continues for several micrometers of indentation depth, compared to the primarily sub micrometer depth range of size effect in quasistatic nanoindentations. For the first time, the factors behind the enhanced size effects for dynamic micro-scale indentations have been investigated by the current work: non-uniform loading and resulting instability using strain rate profiles, plastic wave behavior during loading using resistance force versus indentation depth profiles, quantification of energy of the dynamic plastic wave, and localization of impact strain using electron backscattered diffraction (EBSD) mapping of the strain affected vicinity of indentation imprints.
I. INTRODUCTION
Nano-scale and micro-scale indentations, with axially symmetric indenter/contact shapes have been widely used to probe the mechanical properties of solids.1 The application of load, tracking of deformation depth, and measurement of hardness with a very high spatial resolution are the primary advantages of such indentations.2 Moreover, the simplicity of obtaining mechanical properties and the relative non-destructive nature of testing makes hardness measurement by indentation an accessible technique for mechanical testing of material surfaces.3 Although the increasing loading rate effect on nanoindentations have been investigated in the past, strain rates of deformation for such studies were mostly confined to the quasistatic regime.4–6 Indentation mechanics at dynamic regime of strain rates, 102–104 s1, are yet to be understood entirely. Understanding materials at this strain rate regime is critical for applications such as the protection of aircraft and automotive structures during collision, for ballistic armours, and for walls in nuclear power plants, among several others.7 For macro-scale mechanical tests, the instability caused by high strain rate dynamic deformation and the subsequent promotion of localized strain has been widely reported in the literature.8–11 Such localization of strain results in the generation and accumulation of thermal energy at the point(s) of contact.12,13 Localization of thermal energy during dynamic deformation has been Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.170
shown to cause adiabatic softening at sufficiently high _ 14,15 Similarly, the response of strains, e, and strain rates, e. materials toward micro-scale dynamic or impact indentations needs investigation. To study the material response to dynamic indentations, the loading rates during indentation should be such that it generates pla
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