The structure of the stomatopod dactyl club--an ultrafast, hammer-like device used by the animal to shatter hard seashells--offers inspiration for impact-tolerant ceramics. Here, we present the micromechanical principles and related micromechanisms of deformation that impart the club with high impact tolerance. By using depth-sensing nanoindentation with spherical and sharp contact tips in combination with post-indentation residual stress mapping by Raman microspectroscopy, we show that the impact surface region of the dactyl club exhibits a quasi-plastic contact response associated with the interfacial sliding and rotation of fluorapatite nanorods, endowing the club with localized yielding. We also show that the subsurface layers exhibit strain hardening by microchannel densification, which provides additional dissipation of impact energy. Our findings suggest that the club's macroscopic size is below the critical size above which Hertzian brittle cracks are nucleated.

The surface force apparatus (SFA) comprises thin molecularly smooth mica sheets glued to glass cylinders, which are pressed into contact with their axes at right angles. It is frequently used, in conjunction with the Johnson - Kendall - Roberts (JKR) adhesion theory, to extract the surface energy of the contacting sheets. This procedure is open to possible error since the JKR theory is based on the contact of homogeneous, isotropic elastic cylinders. This paper extends the JKR theory to the layered structure of the SFA. Two approaches have been followed: (i) direct calculations for prescribed values of the layer thickness and elastic moduli; (ii) an experimental calibration procedure for an existing apparatus.

The Johnson, Kendall and Roberts (JKR) adhesion theory is frequently applied to study the adhesion mechanics of thin compliant layers coated on to glass substrate in an atomic force microscope (AFM). These compliant layers are subjected to micro indentation in the AFM using a flat or spherical indenter. But, JKR theory is based on the contact of homogeneous, isotropic solids. The finite element analysis used by Sridhar et al (1997) (J. Phys. D: Appl. Phys. 30 1710-19) to study the adhesion mechanics of the three-layered crossed-cylinder arrangement of the surface force apparatus (SFA) has been extended to this AFM geometry. The effect of indenter elasticity is included for both spherical and flat punch probes. Computations of contact size and contact compliance each as a function of load are presented for a range of values of adhesion energy and elastic modulus ratio of layer and substrate.

No abstract

As length scales decrease, adhesive forces become increasingly important. These adhesive forces contribute to the normal load in experiments conducted on thin layered systems using micro-probe instruments such as the surface force apparatus (SFA) and the atomic force microscope (AFM). Adhesion between these thin-layer systems was analysed by Sridhar et al (1997 J. Phys. D: Appl. Phys. 30 1710) for the SFA geometry and Johnson and Sridhar (2001 J. Phys. D: Appl. Phys. 34 683) for AFM using a numerical SJF (Sridhar–Johnson–Fleck) version of the JKR (Johnson–Kendal–Roberts) theory. In this paper, adhesion mechanics between a compliant elastic coating and a spherical probe is investigated using the SJF model in detail. When the substrate is rigid, the non-dimensional pull-off force may differ from the JKR value of −0.5 by as much as 90%. Computations of the contact size at zero load and pull-off force are presented for a range of values of adhesion energy. Finally, empirical relations for the contact load and contact compliance as a function of contact radius were obtained from the numerical data for practical layer-substrate material systems.