On the Fracture Toughness of Advanced Materials

Abstract

Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. In the first instance, such resistance to fracture is a function of bonding and crystal structure (or lack thereof), but can be developed through the design of appropriate nano/microstructures. However, the creation of tough microstructures in structural materials, i.e., metals, polymers, ceramics and their composites, is invariably a compromise between resistance to intrinsic damage mechanisms ahead of the tip of a crack (intrinsic toughening) and the formation of crack-tip shielding mechanisms which principally act behind the tip to reduce the effective "crack-driving force" (extrinsic toughening).Intrinsic toughening is essentially an inherent property of a specific microstructure; it is the dominant form of toughening in ductile (e.g., metallic) materials. However, for most brittle (e.g., ceramic) solids, and this includes many biological materials, it is largely ineffective and toughening conversely must be developed extrinsically, by such shielding mechanisms as crack bridging. From a fracture mechanics perspective, this results in toughening in the form of rising resistance-curve behavior where the fracture resistance actually increases with crack extension. The implication of this is that in many biological and high-strength advanced materials, toughness is developed primarily during crack growth and not for crack initiation. This is an important realization yet is still rarely reflected in the way that toughness is measured, which is invariably involves the use of single-value (crack-initiation) parameters such as the fracture toughness KIc.

On the effect of X-ray irradiation on the deformation and fracture behavior of human cortical bone

“…Traditionally, toughness has been thought of as the ability of a material to dissipate deformation energy without propagation of a crack. However, fracture is actually the result of a mutual competition of intrinsic damage mechanisms ahead of the crack tip that promote cracking and extrinsic shielding mechanisms mainly behind the tip that impede it [42,44]. We thus consider the influence of irradiation in terms of how it may affect these two mechanistic components.…”

“…The most prominent mechanism is that of plastic deformation which provides a means of blunting the crack tip through the formation of "plastic" zones. Extrinsic toughening mechanisms, conversely, operate primarily in the wake of the crack tip to inhibit cracking by "shielding" the crack from the applied driving force [41][42][43][44]. Whereas intrinsic toughening mechanisms are effective in inhibiting both the initiation and growth of cracks, extrinsic mechanisms, e.g., crack bridging, are only effective in inhibiting crack growth [42].…”

FIB/FESEM experimental and analytical assessment of R-curve behavior of WC–Co cemented carbides

“…[16][17][18][19][20][21][22]), the development of the multiligament zone implies the existence of a rising crack growth resistance (R-curve) behavior in hardmetals [23][24][25][26][27], the size of which is dependent on the width and strength of the ligaments [7,24,28]; and thus, on the microstructural arrangement of the composites under consideration. R-curve behavior may be described as the ability of a microstructure to develop toughening mechanisms on an advancing crack, which can be done, for example, by screening the crack tip from the far-field driving force [21,29,30]. In the case of ceramics toughened by ductile reinforcements, the magnitude of these stresses increases with crack extension due to the formation of new bridges at the crack wake until a plateau is reached.…”

Effect of initial notch orientation on fracture toughness in fail-safe steel

“…Since strengthening of materials reduces toughness [1][2][3], the idea for improving strength-toughness balance is always sought. It is reported that the refinement of crystal grains is an effective method for developing strength and toughness in metallic materials without the addition of alloying elements.…”