Fishnet model for failure probability tail of nacre-like imbricated lamellar materials

Abstract

Nacre, the iridescent material of the shells of pearl oysters and abalone, consists mostly of aragonite (a form of CaCO3), a brittle constituent of relatively low strength ([Formula: see text]10 MPa). Yet it has astonishing mean tensile strength ([Formula: see text]150 MPa) and fracture energy ([Formula: see text]350 to 1,240 J/m2). The reasons have recently become well understood: (i) the nanoscale thickness ([Formula: see text]300 nm) of nacre's building blocks, the aragonite lamellae (or platelets), and (ii) the imbricated, or staggered, arrangement of these lamellea, bound by biopolymer layers only [Formula: see text]25 nm thick, occupying [Formula: see text] of volume. These properties inspire manmade biomimetic materials. For engineering applications, however, the failure probability of [Formula: see text] is generally required. To guarantee it, the type of probability density function (pdf) of strength, including its tail, must be determined. This objective, not pursued previously, is hardly achievable by experiments alone, since [Formula: see text] tests of specimens would be needed. Here we outline a statistical model of strength that resembles a fishnet pulled diagonally, captures the tail of pdf of strength and, importantly, allows analytical safety assessments of nacreous materials. The analysis shows that, in terms of safety, the imbricated lamellar structure provides a major additional advantage-∼10% strength increase at tail failure probability [Formula: see text] and a 1 to 2 orders of magnitude tail probability decrease at fixed stress. Another advantage is that a high scatter of microstructure properties diminishes the strength difference between the mean and the probability tail, compared with the weakest link model. These advantages of nacre-like materials are here justified analytically and supported by millions of Monte Carlo simulations.