The role of quasi-plasticity in the extreme contact damage tolerance of the stomatopod dactyl club.

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.