Deterioration of strength

This boron- and carbon-doped SiC exhibits excellent strength and stiffness, extreme hardness, and thermal and chemical resistance. The strength of this system is not affected by temperatures up to I650°C. Creep is virtually nonexistent up to I400°C. CycHc durabiUty testing conducted at I370°C in air showed no deterioration of strength after 3500 h (94). 

The thermal shock damage resistance of 0.5 pm in situ formed composites will be expected to be much less than that of preformed composites. For 3 pm and 11 pm spinel composites, addition of 30 10% spinel may provide some improvements. Further deterioration of strength in the coarsest spinel composite as a result of thermal shock should be a minimum at 20% (Fig. 14.12). In general coarser (22 pm) preformed spinel powders appear to be more beneficial than finer powders, but there is no obvious advantage with additions of more than 30%.74,103... 

The strength decreases cited are attributed to the conversion reaction. It is influenced by several factors such as temperature, water-cement ratio, stress, and the presence of alkalis. The precise mechanism responsible for the deterioration of strength is not completely understood. 

Wool—Polyester Fibers. The 45/55 wool—polyester blend is the most common fiber combination in the worsted industry. Strength and exceUent dimensional stabiHty of the polyester fiber enable the creation of lightweight wear fabrics not obtainable before. Economy has modified the fiber ratio and 30/70 and 20/80 wool—polyester blends are as common as the classical 45/55 blend. Disperse dyes for polyester and acid or neutral premetaUized dyes for wool are employed in a one-bath process. Should cationic dyes be used for the wool portion, a one-bath procedure can only be employed for light to medium shades, whereas dark shades require a one-bath two-step process. Wool blends should not be dyed above 105°C in order to avoid deterioration of the fiber quaHty. 

For statie design to be valid in praetiee, we must assume situations where there is no deterioration of the material strength within the time period being eonsidered for the loading history of the produet. With a large number of eyelie loads the material will eventually fatigue. With an assumed statie analysis, stress rupture is the meehanism of failure to be eonsidered, not fatigue. The number of stress eyeles in a problem eould... 

The absorbed water has a plasticising effect and thus will cause a reduction in tensile strength and modulus, and an increase in impact strength. As has already been mentioned the presence of absorbed water also results in a deterioration of electrical properties. 

The deterioration of a bond line can occur due to (1) the failure of the resin (low hydrolysis resistance, degradation of the hardened resin causing loss of bonding strength) (2) the failure of the interface between resin and wood surface (replacement of secondary forces between resin and reactive wood surface sites by water or other non-resin chemicals) (3) the breaking of bonds due to mechanical forces and stresses (the influence of water will cause swelling and therefore movement of the structural components of the wood-based panels). 

When a load is initially applied to a specimen, there is an instantaneous strain or elongation. Subsequent to this, there is the time-dependent part of the strain (creep), which results from the continuation of the constant stress at a constant temperature. In terms of design, creep means changing dimensions and deterioration of product strength when the product is subjected to a steady load over a prolonged period of time. 

In RPs, insufficient compaction and consolidation before plastic solidification or cure will result in air pockets, incomplete wet-out and encapsulation of the fibers, and/or insufficient fiber or uniform fiber content. These deficiencies lead to loss of strength and stiffness and susceptibility to deterioration by water and aggressive agents. 

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