Short-Term Exposure to Temperature

Ideally, short-term behavior under thermal loading is described in terms of mechanical properties as a function of temperature. Useful pointers when determined as a function of temperature are the modulus and damping values, modulus of shear G, and mechanical loss factor tan(5. 

These thermally relevant characteristic values can be obtained by dynamic mechanical analysis (DMA). This analysis returns information about the way mechanical properties change under slight, usually sinusoidal dynamic loading as a function of temperature, time, and/or frequency. The periodic force (input signal) causes a corresponding response signal, namely deformation or strain in the viscoelastic test specimen, which can be evaluated for dynamic-elastic characteristic values in terms of amplitude and phase offset [36]. 

FIGURE 2.6 Temperature-dependent mechanical plastic behavior 

DMA curves are also the point of departure for determining the glass transition temperature 7g. 7g is determined hy evaluating the step-like drop evinced by the storage modulus with linear curve imposition across temperature hy the half-step 

The dimensional stability of a plastic when exposed to heat is another crucial property. The Vicat method of determining a plastic s softening point and the heat deflection temperature (HDT) method are two ways of rapidly determining which polymers have suitable characteristic values. In both methods the test specimens are heated under defined load with a certain heating rate while deformation is measured. Vicat softening temperature and HDT are each defined as the temperatures at which deformation reaches a given value [8]. 

---

As mentioned above in section 8.2 and listed in Tables 8.1 and 8.3, heat resistant, organic fibres have chemical structures that are little changed physically or chemically by short-term exposures to temperatures above the 200 and even the 300 °C levels and may be used continuously at temperatures as high as 150 °C. For technical textiles used in high temperature industrial processes, such as gas and liquid filtration, longterm exposures to temperatures of about 100-150 °C are often required. Typical properties of these fibres listed in Table 8.3 are shown in detail in Table 8.4. ... 

A cure cycle of 15-30 min at 300-500°F and 50-200 psi pressure gives the best results. A short-term exposure to temperatures as high as 600°F can be tolerated, and the adhesive is suitable for continuous use to 250°F. Durability studies have been reported by Minford and Bethune. " Superior fatigue resistance and complete resistance to outdoor exposure in the jungles of Panama has been shown by Bodner and Wegman for nitrile-phenolic-bonded aluminum specimens. 

Thermal analysis data provide a means for predicting composite performance for short-term exposure to temperatures. This, in conjunction with data derived from mechanical properties at various temperatures, should lead to the selection of appropriate composites for high temperature applications. 

Fig. 14. Deterioration of the intrinsic coercive force of several RE-TM alloy powders during short-term exposure to air at elevated temperature. (After Stmat 1970.)...

The thermal stability of high-temperature plastics can be further enhanced by reinforcing additives. Such reinforced high-temperature plastics are used in rocket construction techniques where short-term heavy thermal attack needs to be resisted (the nose of a rocket, liner of the nozzle, etc.). Undergoing this load, a part of the polymer is consumed either by volatilization or by carbonization, resulting in a protection of the subjacent layers from thermal effects. This consumption of the polymer by virtue of a short-term exposure to a very high temperature is called ablation. 

In applications involving no undue mechanical stresses, PP articles will withstand 100 °C for a long period of time, depending on the stabiliser systems. Consequently, the heat and thermal stability of PP is closely related to its maximum continuous use temperature (Section 4.2.2). Short-term exposure to 140 °C is also possible. It has been observed that a properly heat stabilised and properly processed material ean undergo up to five processing cycles without noticeable reduction in molecular weight or the level of antioxidant content. 

Type 317L stainless steel is used for welding, brazing and other short-term exposures to high temperatures. 

This chapter is concerned with the short-term mechanical properties — moduli and strengths — of glass, aramid and carbon fibres in a thermosetting resin matrix. A little information on reinforced thermoplastic matrix systems is also included. The data mainly refer to the room temperature properties of 55-65 v/o fibre, unidirectional, systems. The effects of the variation in fibre volume loading, method of test and instantaneous and long term exposure to temperature are briefly mentioned. Longitudinal properties tend to be fibre dominated, and so are compressive properties to some extent for glass and carbon fibres. The anisotropy of unidirectional materials is noticeable. 

Behavior under conditions of long-term exposure to temperature cannot be ascertained in short-term mechanical or thermo-mechanical tests, because time-dependent aging and relaxation processes are involved. 

---