Room-temperature strength

The aim of toughening a matrix by adding second-phase particles is essentially to increase its strength. For cracks in homogeneous materials under uniform 

The preceding reasons for strength reductions are a consequence of microstructural deficiencies, but even perfect microstructures, in which the critical flaw size is the same as for the unreinforced matrix material, can yield smaller strength increases from particle-induced toughening than would 

6 Relationship between strength and toughness for glass/alumina composites with best fit straight line to eq. (4.2). Data from [29], 

Combining the information in the previous two paragraphs enables the following common observations regarding strength-toughness relationships in particulate composites to be explained straightforwardly  

7 Plot of strength versus indentation load for alumina with various grain sizes and alumina/aluminium-titanate composite. The data points and solid line are from the composite. The other lines are for alumina with various grain sizes A, 2.5 pm, experimental B, 6 pm, interpolated C, 80 pm, experimental (reproduced from Bennison et al. [7] by kind permission of Taylor and Francis Ltd (http // www.tandf.co.uk/journals). 

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Heat resistance is an important characteristic of the bond. The strength of typical abrasive stmctures is tested at RT and at 300°C. Flexural strengths are between 24.1 and 34.4 MPa (3500—5000 psi). An unmodified phenoHc resin bond loses about one-third of its room temperature strength at 298°C. Novolak phenoHc resins are used almost exclusively because these offer heat resistance and because the moisture given off during the cure of resole resins results in undesirable porosity. Some novolaks modified with epoxy or poly(vinyl butyral) resin are used for softer grinding action. 

Polyolefins. Interest has been shown in the plasticization of polyolefins (5) but plasticizer use generally results in a reduction of physical properties (12), and compatibiHty can be achieved only up to 2 wt %. Most polyolefins give adequate physical properties without plasticization. There has been use of plasticizers with polypropylene to improve its elongation at break (7) although the addition of plasticizer can lower T, room temperature strength, and flow temperature. This can be overcome by simultaneous plasticization (ca 15 wt % level) and cross-linking. Plasticizers used include DOA. 

The beryllides, being intermetaUic compounds, are hard, strong materials which exhibit Httie ductihty at room temperature. Strength properties increase gradually as a function of temperature up to about 870°C, above which a sharp increase in strength occurs, peaking in the region of 1260°C the modulus of mpture values exceed 280 MPa (40,000 psi) at this latter temperature. 

Considerable recent activity in the area of ceramic processing is aimed toward the formulation of materials with high strengths, comparable to the room temperature strength of metal alloys, at high temperatures (of the order of 2000 K). The impetus comes from the significant gains made in the last 20 years with materials formed from submicron powders of silicon nitride and silicon carbide and the promise of similar improvements in the near future. 

Laurich-Mclntyre, S.E. and Bradt, R.C., Room temperature strengths of individual tabular alumina and sintered spinel grains (aggregates) , UNITECR 93 Congress, Sao Paulo, Brazil, 1993. 

Some prominent features of these steels are relatively high room temperature strength (high proof and tensile stresses) and high fatigue strength. 

T5 140 235 5 Sand-castings, good room temperature, strength and ductility... 

As mentioned previously, the main body of research on whisker-reinforced composites was concerned with alumina, mullite, and silicon nitride matrix materials. None the less, selected work examined zirconia, cordierite, and spinel as matrix materials.16-18 The high temperature strength behavior reported for these composites is summarized in Table 2.5. As shown, the zirconia matrix composites exhibited decreases in room temperature strength with the addition of SiC whiskers. However, the retained strength at 1000°C, was significantly improved for the whisker composites over the monolithic. Claussen and co-workers attributed this behavior to loss of transformation toughening at elevated temperatures for the zirconia monolith, whereas the whisker-reinforcement contribution did not decrease at the higher temperature.17,18... 

FIGURE 6.17. Optimum values of room-temperature strength and fiacture elongation and their correlation with the contiguity factor for tungsten heavy alloys [6.1,6.31]. 

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