Heat-load structures, materials

For the TFTR heat load structures, more than twenty different materials were surveyed for thermal conductivity and thermal shock resistance (64) a result of these tests several grades of... 

Except for a lew thermoset materials, most plastics soften at some temperatures, At the softening or heat distortion temperature, plastics become easily deformahle and tend to lose their shape and deform quickly under a Load. Above the heat distortion temperature, rigid amorphous plastics become useless as structural materials. Thus the heat distortion test, which defines The approximate upper temperature at which the material can be Safely used, is an important test (4,5.7.24). As expected, lor amorphous materials the heat distortion temperature is closely related to the glass transition temperature, hut tor highly crystalline polymers the heat distortion temperature is generally considerably higher than the glass transition temperature. Fillers also often raise the heat distortion test well above... 

Heavily crosslinked polymers, by contrast, tend to be very brittle and, unlike thermoplastics, this brittleness cannot be altered much by heating. Heavily crosslinked materials have a dense three-dimensional network of covalent bonds in them, with little freedom for motion by the individual segments of the molecules involved in such structures. Hence there is no mechanism available to allow the material to take up the stress, with the result that it fails catastrophically at a given load with minimal deformation. 

Decomposition temperatures, whether sharp or vague, are seldom actually reached in service life. Reinforced composites are preeminently load-bearing materials, and it is their temperature-dependent mechanical properties, such as T, or the closely related heat distortion temperature, that usually determine the maximum use temperature, at least for short or intermediate term use. Strength, yield stress and modulus all decline with increasing temperature, reflecting the increasing mobility of the molecular structure, and unacceptable levels of physical property loss will often occur well before the onset of thermal or thermo-oxidative degradation. 

Likewise, SiC has been considered a suitable material for the coolant channels of the blankets of fusion reactors created from a SiC composite (Ward and Dudaev, 2008), and also as a low-activation structural material to protect against the excessive heat loads of the metal first wall of a potential fusion reactor (Hopkins, 1974). The key figure of merit for the latter application is a high thermal shock resistance, R", which is necessary to withstand the stresses introduced by startups and plasma disruptions, together with the thermal cycling associated with normal pulsed mode operation. In this case, Rf = Ob k (l - t lE-a, where CTj, is the flexural strength, k the thermal conductivity, v the Poisson ratio, E the modulus of elasticity, and a the coefficient of thermal expansion. 

Characteristic of this type of corrosion is low-deformation separation, in most cases without formation of visible corrosion products and without measurable mass losses. The necessary tensile stresses can result from either internal stresses in the structural element or from operational stress loads. A material resistant to stress corrosion cracking (SCC) when delivered can be sensitised for stress corrosion cracking by means of improper heat treatment, e.g. during welding or from deformation. 

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