Fracture toughness and damage development in limpet shells

Fracture toughness is an important property for many biological materials, but it can be difficult to obtain accurate and relevant values of toughness in such materials owing to complexities of geometry, material anisotropy, etc. Here we present the results of the first ever attempt to describe and measure cracking and fracture toughness in the shells of limpets. Three different experiments were devised. Firstly, small single-edge-notched bend specimens were machined, enabling us to measure KICfor through-thickness cracks growing in the circumferential direction in the shell walls, giving a value of 0.98 MPa√m. Secondly, radial notches were cut into intact shells which were loaded in compression through the apex. Failure occurred by crack propagation from the notch roots, and finite element analysis (FEA) was used to obtain critical K values. However the analysis gave a surprisingly high toughness value and the results were very sensitive to test variables, especially friction. The experiment demonstrated the remarkable resistance of shells to this kind of damage, but could not be used to measure KIC.Thirdly, impact tests were carried out to create internal damage in the form of delamination cracks. This allowed us to estimate toughness in terms of a crack propagation energy GICfor these cracks of 146 J/m2, equivalent to a KICof 2.59 MPa√m. Scanning electron microscopy showed that the delamination cracks had much smoother fracture surfaces than those from the through-thickness cracks, however they displayed a regular structure of folds or pleats at the 100 nm scale which may act to hinder crack face movements during shear/compression loading as occurs under impact, which is a common cause of damage for these shells in their natural surroundings.