Fracture Mode

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

To explain the mechanism by which the formation of Ni3S2 scale can result in the fatigue failure of the blade disc, the effect of sulfur on the mechanical properties of nickel base alloys must be understood. Since the fracture modes are totally intergranular with evidence of... 

Heterogeneous compatible blends of preformed elastomers and brittle plastics are also an important route for the development of blends of enhanced performance with respect to crack or impact resistance. Polycarbonate blends with preformed rubber particles of different sizes have been used to provide an insight into the impact properties and the fracture modes of these toughened materials. Izod impact strength of the blends having 5-7.5 wt% of rubber particles exhibits best overall product performance over a wide range temperature (RT to -40°C) [151-154]. 

The fracture mode of stress-corrosion cracks in austenitic stainless steels can be transgranular, intergranular or a mixture of both. One of the earliest environments found to cause problems was solutions containing chlorides or other halides and the data due to Copson (Fig. 8.30) is very informative. The test solution for that data was magnesium chloride at 154°C the alloys contained 18-20alloy with a composition of approximately 18Cr-8Ni has the least resistance to cracking in this environment. 

There are a number of fracture modes, the most important of which are ductile overload, which is fairly well understood and can be predicted reasonably accurately, and brittle fracture, which is less predictable from an engineering viewpoint and can cause catastrophic failures due to the speed of the fracture. 

The early study of brittle failures, notably those of the Liberty ships, indicated a temperature dependence. This can be illustrated by plotting both fracture stress (of) and yield stress (Oy) against temperature (Fig. 8.81). Below a certain temperature some materials exhibit a transition from ductile to brittle fracture mode. This temperature is known as the ductile-brittle transition temperature DBTT. 

Although the creep behavior of a material could be measured in any mode, such experiments are most often run in tension or flexure. In the first, a test specimen is subjected to a constant tensile load and its elongation is measured as a function of time. After a sufficiently long period of time, the specimen will fracture that is a phenomenon called tensile creep failure. In general, the higher the applied tensile stress, the shorter the time and the greater the total strain to specimen failure. Furthermore, as the stress level decreases, the fracture mode changes from ductile to brittle. With flexural, a test specimen... 

B. The direct identification of such microscopic fracture modes as transgranular cleavage, radial fracture, intergranular separation or ductile dimpled rupture, was shown to be considerably more dependable by SEM than by OM... 

Whilst these extremes of behaviour can be readily distinguished, there are transitional types of fracture behaviour that lie between them, and in these cases, judgement about fracture mode can be difficult. 

The term fracture implies fragmentation of a solid body into two or more bodies under the action of stress. Two main types of fracture mode are observed in solids. The first is ductile fracture which is the failure of a material after it has undergone a considerable amount of plastic deformation. The other is brittle fracture which is the failure of a material without undergoing practically any plastic deformation. The type of failure which occurs depends largely on the nature of the material and its condition however, failure is also affected by... 

Impact modifiers for PET are generally elastomeric compounds that increase impact strength and elongation while usually decreasing modulus. An effective way to enhance the impact strength and to induce a brittle/ductile transition of the fracture mode, is by the dispersion of a rubber phase within the PET matrix. The... 

Fig. 9.12 Results of molecular mechanics simulations (a) A Stone-Wales defect (A mode) in a zigzag SWCNT, (b) a Stone-Wales defect (B mode) in a zigzag SWCNT, the bonds with highest potential energy are indicated by arrows. Propagating cracks in (c) A defect-fiee zigzag tube, and (d) defect-lfee armchair tube. Fracture mode of armchair tube with (e) Stone-Wales defect (A mode), and (f) Stone-Wales defect (B mode). Fracture mode of zigzag tube with (g) Stone-Wales defect (A mode), and (h) Stone-Wales defect (B mode) (Huynh et al., 2002. With permission from Wiley)...

The fracture mode of both forgings was by the microvoid nucleation and growth process (Figure 4). Microvoids nucleate at nonmetallic inclusions in the steel (sulfides, oxides, etc) and grow under strain until they coalesce at fracture (10). While the CF heats had a fairly uniform microvoid size distribution on the fracture surface, the HERF steel fracture surface had a bimodal distribution of microvoids with large microvoids surrounded by clusters of fine microvoids. 

The fracture modes of the tritium-exposed alloys are shown in Figures 12 and 13 for the CF steels and in Figure 14 for the HERF steels. The fracture mode was still microvoid coalescence but now the voids were even smaller ( l um) than those in the hydrogen-exposed alloys. 

One final test was conducted on a heat treated sample to elucidate the effect of heat treatment and sensitization on the fracture behavior of this steel. A CF sample was heated for 24 hours at 650°C to see if the fracture mode would continue to change with an even larger amount of carbide precipitation. Note in Figure 15-a this heavily sensitized steel has a fracture appearance that is completely dominated by small microvoids associated with carbides. The bimodal distribution of microvoids like those in Figure 7 has been eliminated. In fact, the fracture appearance is remarkably similar to that of the tritium-exposed-and-aged steels albeit at a different magnification (Figure 15-b). It appears that carbides in the microstructure affect the fracture mode in a similar manner as the decay helium bubbles but on a different scale. 

Fracture modes of the forged steels were dominated by the dimpled rupture process in unexposed, hydrogen-exposed and tritium-exposed steels and welds. Heavily sensitized steels had a similar fracture appearance as tritium-exposed-and-aged steels... 

The failure processes in thermoplastics composites with aligned glass fibers of sub-critical transfer length have been characterized (Lauke and Schultrich, 1983, 1986a, b Lauke et al., 1985 Lauke and Pompe, 1988) in terms of matrix fracture mode which is determined mainly by the ductility of the matrix material, loading rate and temperature. The total specific work of fracture, / t, is expressed as the sum... 

Regarding the effects of shear band structure on the fracture mode in glassy polymers, Wu and Li [170] concluded that when microshear bands propagate in a specimen cross section, a shear failure is produced with a very small overall de-... 

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