Abstract
Instrumented indentation is a widespread tool for characterising the mechanical properties of biological materials. Here, we show that the ratio between indentation hardness and modulus is approximately constant in biological materials. A simple elastic-plastic series deformation model is employed to rationalise part of this correlation, and criteria for a meaningful comparison of indentation hardness across biological materials are proposed. The ratio between indentation hardness and modulus emerges as the key parameter characterising the relative amount of irreversible deformation during indentation. Despite their comparatively high hardness to modulus ratio, biological materials are susceptible to quasiplastic deformation, due to their high toughness: quasi-plastic deformation is hence hypothesised to be a frequent yet poorly understood phenomenon, highlighting an important area of future research.
Highlights
In search for inspiration for the design of novel materials with enhanced properties, biological materials receive an increasing amount of attention (e. g. 1–5)
Instrumented indentation has become the dominant tool for material characterisation on small scales
The primary use of indentation is to relate the measured properties to ultrastructure and function, which requires a detailed understanding of their mechanical significance
Summary
In search for inspiration for the design of novel materials with enhanced properties, biological materials receive an increasing amount of attention (e. g. 1–5). The structural arrangement and properties of these components are tailored to specific functional demands. A thorough understanding of how the arrangement and properties of individual components determine the performance of the ‘bulk’ materials is a prerequisite for any attempt at replicating their functionality. In biological materials, instrumented indentation is frequently used as a tool to relate structural changes, for example degree of mineralisation, to variations in material properties, such as fracture toughness, elastic modulus, and hardness. These properties, in turn, determine performance and the adaptive value of the structural changes in ques-
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