Issue 10

D.Taylor et alii, Frattura ed Integrità Strutturale, 10 (2009) 12-20 ; DOI: 10.3221/IGF-ESIS.10.02 17 It is perhaps not surprising that there should be a relationship between L and d, since in most materials the microstructure plays a strong role in determining properties related to crack growth, such as toughness and fatigue behaviour. Thus, knowing a value of L for a particular material may shed light on the physical mechanism of failure and may give hints about how changing microstructural parameters could affect performance. (a) (b) Figure 4 : Two SEM images showing cracks in bone which display bridges consisting of uncracked ligaments across the crack faces. Photo (a) from tests conducted in our laboratories by Stewart Mahoney; photo (b) from [19]. Figure 5 : Image taken using optical fluorescence microscopy of a transverse section of bone, showing a crack (C), of length approximately 100  m, whose left-hand tip has stopped growing on reaching the boundary of an osteon (O). From [21]. Figure 6 : Values of L and d in various classes of materials. Critical Distance, L Microstructure Size, d 1nm 1  m 1mm 1m 1nm  m 1mm 1m L=d L=10d Metals, brittle fracture Metals, fatigue Amorphous Polymers Ceramics Nanomaterials? Fibre Composites Concretes Bone