Flaw tolerance

Padture, N.P., Bennison, S.J. and Chan, H.M. Flaw-tolerance and crack-resistance properties of alumina-aluminum titanate composites with tailored microstructures ,... 

LGMs of the AT/alumina and AT/ZTA displayed some very interesting properties which include excellent machinability, low thermal expansion coefficient, improved thermal shock resistance, low hardness (about 5 GPa), low Young s modulus (E) (250 GPa) and excellent flaw tolerance [Pratapa, 1997 Pratapa Low, 1998 Skala, 2000 Manurung, 2001], These materials appeared to display a large degree of near-surface quasi-plasticity under the Hertzian or the Vickers indenter which effectively inhibits the formation and propagation of cracks. The ductile behaviour of these materials was... 

Runyan, J.L. Bennison, S.J. (1991) Fabrication of flaw-tolerant aluminium-titanate-reinforced alumina. J. Eur. Ceram. Soc. 7, 93-99. 

In some applications the lack of toughness of ceramics or CMCs prohibits their use. In cases where enhanced stiffness, wear resistance, or elevated temperature capabilities greater than those provided by metals are necessary, metal matrix composites (MMCs) offer a reasonable compromise between ceramics or CMCs and metals. Typically, MMCs have discrete ceramic particulate or fiber reinforcement contained within a metal matrix. In comparison to CMCs, MMCs tend to be more workable and more easily formed, less brittle, and more flaw tolerant. These gains come primarily at the expense of a loss of high-temperature mechanical properties and chemical stability offered by CMCs. These materials thus offer an intermediate set of properties between metals and ceramics, though somewhat closer to metals than ceramics or CMCs. Nonetheless, like ceramic matrix composites, they involve physical mixtures of different materials that are exposed to elevated temperature processes, and therefore evoke similar thermodyamic considerations for reinforcement stability. 

Figure 11.19 (a) Functional dependence of fracture toughness on flaw size for a ceramic exhibiting R curve behavior (top curve) and one that does not (lower curve), (h) Effect of R curve behavior on strength degradation as flaw size increases. Ceramics exhibiting R curve behavior are more flaw-tolerant than those that do not. 

Hydrogen Cycling, Bonfire, Severe Drop Impact Test, Flaw Tolerance, Acid Environment, Gunfire Penetration,... 

This extreme capacity for deformation mainly results from the highly nonlinear properties of the coiled-coil structures by sacrificing individual protein filaments [65]. Also, the flaw-tolerant characteristic results from the stiffening behavior during the secondary structural transitions for a single filament. In addition to the properties of a single filament, the crosslinks between intermediate filaments are also critical to the mechanical behavior of the intermediate filament network [42]. 

Ackbarow T, Sen D, Thaulow C, Buehler MJ (2009) Alpha-helical protein networks are self-protective and flaw-tolerant. PLoS One 4(6) e6015... 

J. A. Begley, The Transition Crack Length An Engineering Estimate of a Material s Flaw Tolerance, Westinghouse Scientific Paper 70-1E7-EFLAW-P1, Pittsburgh, Pennsylvania (1970). 

FIGURE 18.7 A material showing R curve behavior (bold curve) exhibits a region of stable crack growth and flaw tolerant behavior. The lighter curves Gi and 02 represent typical Griffith behavior. 

Ce-TZP ceramics are characterized by relatively low critical stresses for the stress-induced t-m transformation (ffc,t fff)- Pronounced transformation plasticity is observed, due to which the Ce-TZP ceramics are relatively flaw-tolerant and their strength is not controlled by the initial flaw size, but rather by the critical transformation stress, the zone size and the strain-hardening effect. 

Fig. 8.62 Comparison of materials exhibiting non-R-curve and R-curve behavior a for non-R curve materials, the fracture strength (<7f) decreases with increasing flaw size. R-curve materials, however, exhibit a range of crack sizes over which the fracture strength is invariant, i.e., they are flaw tolerant" b for non-R-curve materials, the toughness (T) is a constant, independent of crack size. For R-curve materials, the toughness increases with crack size. Cf denotes the crack size below which the fracture stress is constant (redrawn after Harmer et al.) [21]. With kind permission of Dr. Jorge Lino...

There are numerous examples of the application of fracture mechanics to structural adhesive systems. Most notable are those of Mostovoy and his coworkers which have already been mentioned. " Bascom and coworkers have made significant contributions to the understanding of the effect of bondline thickness on fracture toughness. Kinloch and Shaw extend the work of Bascom to include rate effects and to develop mathematical models of the fracture resistance of adhesives. Hunston et al have used these methods to study viscoelastic behavior in the fracture process of structural adhesives.Mostovoy and Ripling used these techniques to determine the flaw tolerance of several adhesives,while Bascom and Cottington have studied the effect of flaws caused by air entrapment in structural adhesives." Finally it must be mentioned that one of the most simple, most widely used tests for strucural adhesives, the peel test, is actually a version of the double cantilever beam test. 

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