Toughening mechanism

Despite the fact that ceramics are inherently brittle, a variety of approaches have been used to enhance their fracture toughness and resistance to fracture. The essential idea behind all toughening mechanisms is to increase the energy needed to extend a crack, that is, in Eq. (11.11). The basic approaches are crack deflection, crack bridging, and transformation toughening. 

It is experimentally well established that the fracture toughness of a polycrystalline ceramic is appreciably higher than that of single crystals of the same composition. For example, Ki. of single-crystal alumina is about 

It can be shown that the fracture toughness of a composite due to elastic stretching of a partially debonded reinforcing phase at the crack tip with no interfacial friction is given by  

Transformation-toughened materials owe their very large toughness to the stress-induced transformation of a metastable phase in the vicinity of a 

To understand the phenomenon, it is useful to refer to Fig. 11.18, where fine tetragonal zirconia grains are dispersed in a matrix. If these tetragonal 

In this section, the role of crazing and shear yielding in conferring toughness in polymers will be discussed, as well as some aspects of the crack propagation itself. For further details the reader is referred to a recent comprehensive and critical review of crazing and fracture in polymers by Kambour (1973) and a discussion by Kambour and Robertson (1972), 

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Toughening Mechanisms. The mechanics of tetragonal-to-monoclonic transformation can effect the strength and toughness of the... 

The matrix polymers can be divided into brittie or ductile categories, each having specific requirements for achieving toughness (Table 3). Numerous variations are possible. For instance, often mbber particles that vary in both size and kind are desirable for optimum performance. In these cases, the requirements of the mbber phase and the toughening mechanisms are complex. 

The transformation toughening mechanism has been most successfully exploited in materials where the phase transformation of interest is... 

Toughening of BMIs with thermoplastics is a promising approach however, more information is required about the toughening mechanism involved in order to select the most promising polymers in terms of backbone chemistry, molecular weight, and reactive groups. 

Figure 7 shows these results schematically for both twist and tilt crack deflections. Thus, for the stress intensity factor required to drive a crack at a tilt or twist angle, the appHed driving force must be increased over and above that required to propagate the crack under pure mode 1 loading conditions. Twist deflection out of plane is a more effective toughening mechanism than a simple tilt deflection out of plane. 

Deflection rarely operates as the sole toughening mechanism in a system, although its contribution in some systems may be significant. Crack deflection, however, is a major aspect of bridge formation processes that leads to toughening via bridging ligaments. 

The process 2one toughening mechanism is seldom found to operate in isolation from other toughening mechanisms. The exception is the case of particles that undergo a phase transformation in which case the toughness is attributed to process 2one shielding alone. 

E. R. EuUer, Jr., E. P. Butler, and W. C. Carter, in NATO Advanced Kesearch Workshop on Toughening Mechanisms in Quasi-Brittle Materials, Kluwer Academic PubUshers, Dordrecht, The Netherlands, 1990. 

The notched Izod impact strength of PET at room temperature is only 45 J/m. Supertough PET with notched impact strengths up to 1000 J/m can be prepared by melt blending PET with 20wt% of a reactive elastomeric terpolymer (e.g. E-MA-GMA). Pecorini and Calvert [28] have attributed this supertoughness phenomenon to two distinct toughening mechanisms, as follows ... 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

Interleaving strips made from ductile short fibers, notably Kevlar fiber mat, and an adhesive (Browning and Schwartz, 1986) provide extra energy required during delamination crack propagation due to additional toughening mechanisms such as... 

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