In pursuit of damage tolerance in engineering and biological materials
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oduction One of the hallmarks of modern materials science is the attempt to understand material behavior over a range of length scales, from nanoscale to macroscopic dimensions. This “mesoscale” (or multiscale) perspective is imperative, as materials, especially biological and natural materials, have characteristic structural features that span most of these length scales; moreover, it is at these differing scales where their specific properties originate. An excellent case in point is human bone, which like most biological materials has a complex hierarchical structure spanning from atomic dimensions, where the twisting of peptide chains form collagen molecules, to the osteonal structures at nearmillimeter dimensions (i.e., the roughly cylindrical structures in cortical bone through which bone remodels) (Figure 1).1,2 Distinguishing these scales is critical, as the mechanisms that contribute to strength and ductility in bone, specifically those associated with the generation of plasticity, are created largely at submicrometer dimensions, similar to the nanoscale dimensions of the Burger’s vectors that control dislocation plasticity in metals. Toughness for bone, however, is additionally generated at much coarser dimensions—at the tens to hundreds of microns—as the crack path interacts with the bone-matrix
structure. These crack/microstructure interactions primarily involve large structural features such as osteons, which serve to generate crack deflection and bridging mechanisms that “shield” the crack from the applied driving force (i.e., they lessen the local stresses and strains experienced by the crack).3 Indeed, the union of these mechanisms represents the essence of fracture resistance in materials: respectively, intrinsic toughening arising from resistance to damage ahead of the tip of a crack, motivated by plasticity mechanisms generated at the nanoscale, paired with extrinsic toughening associated with crack-tip shielding mechanisms generated at the microscale largely behind the crack tip (Figure 1).4 To explore the relevance of these multiple-scale concepts to the overall mechanical behavior of materials, we need to characterize both structure and mechanical properties at many differing dimensions. Here, we describe the use of several such techniques involving macroscale fracture mechanics, microscale in situ x-ray computed microtomography (μ-CT) and scanning electron microscopy, and nanoscale in situ small-/wide-angle x-ray scattering/diffraction (SAXS/WAXD) to examine the sources of structural damage and fracture resistance in several divergent classes of materials, specifically
Robert O. Ritchie, Department of Materials Science and Engineering, University of California, Berkeley, USA; [email protected] DOI: 10.1557/mrs.2014.197
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MRS BULLETIN • VOLUME 39 • OCTOBER 2014 • www.mrs.org/bulletin
© 2014 Materials Research Society
IN PURSUIT OF DAMAGE TOLERANCE IN ENGINEERING AND BIOLOGICAL MATERIALS
(R-curves)* can provide crucial information on crack-path interactions with relevant microstructural fea
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