Microstructure creep

The Metravib Micromecanalyser is an inverted torsional pendulum, but unlike the torsional pendulums described earlier, it can be operated as a forced-vibration instrument. It is fully computerized and automatically determines G y G"y and tan 8 as a function of temperature at low frequencies (10-5 1 Hz). Stress relaxation and creep measurements are also possible. The temperature range is —170 to 400°C. The Micromecanalyser probably has been used more for the characterization of glasses and metals than for polymers, but has proved useful for determining glassy-state relaxations and microstructures of polymer blends (285) and latex films (286). 

Sintered silicon carbide retains its strength at elevated temperatures and shows excellent time-dependent properties such as creep and slow crack growth resistance. Reaction-bonded SiC, because of the presence of free silicon in its microstructure, exhibits slightly inferior elevated temperature properties as compared to sintered silicon carbide. Table 2 (11,43) and Table 3 (44) show selected mechanical properties of silicon carbide at room and elevated temperatures. 

In inert atmospheres the mechanical properties of RBSN are constant up to 1200-1400 °C because of the absence of a glassy grain boundary phase, which is also the reason for the excellent thermal shock and creep behaviour. The thermal shock resistance, hardness and elastic constants depend on the microstructural parameters but are much lower than for dense Si3N4 ceramics [539]. 

For second-phase sintered ceramics, these phases control the plasticity and they are responsible for the asymmetric behaviour when deformed in tension or compression, because there is a crucial difference in the microstructure evolution associated with tension and compression creep. There are few explanations for this asymmetry. 

More, K.L., Koester, D.A. and Davis, R.F., (1991), Microstructural characterization of a creep-deformed SiC whisker-reinforced Si3N4 Ultramicroscopy, 31, 263-278. 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

Creep Mild overheating and or mild overstressing at elevated temperatures unstable microstructures and small grain size tend to increase creep rates ruptures occur after long exposure periods verify proper alloy... 

The enhancement of creep by anodic dissolution is well known, for copper in acetic acid153 and austenitic stainless steels and nickel-based alloys in pressurized water reactor (PWR) environments. The initial vacancy injection from the surface is followed by vacancy attraction to the inside dislocations, which promotes easier glide, climb, and crossing of microstructural barriers. This mechanism illustrates the corrosion-enhanced plasticity approach.95... 

The need to develop fibers with better microstructural stability at elevated temperatures and ability to retain their properties between 1000-2000°C. The requirements of fiber properties for strong and tough ceramic composites have been discussed by DiCarlo.83 A small diameter, stoichiometric SiC fiber fabricated by either CVD or polymer pyrolysis, and a microstructur-ally stable, creep-resistant oxide fiber appear to be the most promising reinforcements. 

In this chapter, we discuss the creep behavior of ceramic matrix composites in terms of their microstructure. The role of interparticle interactions during creep is emphasized. The relationship between creep rate and lifetime is also discussed. 

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