Effect of temperature on the mechanical properties

The tensile strength and elastic modulus of metals decrease with increasing temperature. For example, the tensile strength of mild steel (low carbon steel, C 0.25 per cent) is 450 N/mm1 2 3 4 at 25°C falling to 210 at 500°C, and the value of Young s modulus 

000 N/mm2 at 25°C falling to 150,000 N/mm2 at 500°C. If equipment is being designed to operate at high temperatures, materials that retain their strength must be selected. The stainless steels are superior in this respect to plain carbon steels. 

Creep resistance will be important if the material is subjected to high stresses at elevated temperatures. Special alloys, such as Inconel (International Nickel Co.), are used for high temperature equipment such as furnace tubes. 

The selection of materials for high-temperature applications is discussed by Day (1979). At low temperatures, less than 10°C, metals that are normally ductile can fail in a brittle manner. Serious disasters have occurred through the failure of welded carbon steel vessels at low temperatures. The phenomenon of brittle failure is associated with the crystalline structure of metals. Metals with a body-centred-cubic (bcc) lattice are more liable to brittle failure than those with a face-centred-cubic (fee) or hexagonal lattice. For low-temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic stainless steel (fee) or aluminium alloys (hex) should be specified see Wigley (1978). 

V-notch impact tests, such as the Charpy test, are used to test the susceptibility of materials to brittle failure see Wells (1968) and BS 131. 

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Thermal Properties. Before considering conventional thermal properties such as conductivity it is appropriate to consi r briefly the effect of temperature on the mechanical properties of plastics. It was stated earlier that the properties of plastics are markedly temperature dependent. This is as a result of their molecular structure. Consider first an amorphous plastic in which the molecular chains have a random configuration. Inside the material, even though it is not possible to view them, we loiow that the molecules are in a state of continual motion. As the material is heated up the molecules receive more energy and there is an increase in their relative movement. This makes the material more flexible. Conversely if the material is cooled down then molecular mobility decreases and the material becomes stiffer. 

K. Higashi, T.G. Nieh, and J. Wadsworth, "Effect of Temperature on the Mechanical Properties of Mechanically-Alloyed Materials at High Strain Rates," Acta Metall. Mater., 43 3275 (1995). 

The effect of temperature on the mechanical properties has definite place in specification of a material for any particular application. Some materials become dangerously brittle under arctic conditions, and all metals exhibit at elevated temperatures a phenomenon called creep. 

The effect of temperature on the mechanical properties of a liquid can be investigated using a special type of dynamic mechanical analyser called an oscillatory rheometer. In this instrument the sample is contained as a thin film between two parallel plates. One of the plates is fixed while the other rotates back and forth so as to subject the liquid to a shearing motion. It is possible to calculate the shear modulus from the amplitude of the rotation and the resistance of the sample to deformation. Because the test is performed in oscillation, it is possible to separate the shear modulus (G) into storage (G ) and loss modulus (G") by measuring the phase lag between the applied strain and measured stress. Other geometries such as concentric cylinders or cone and plate are often used depending on the viscosity of the sample. 

FIGURE 2.16 Effect of temperature on the mechanical properties of the EB fibers collected from the water bath. (Reprinted from Yang, D., Fadeev, A.G., Adams, RN., and Mattes, B.R., Proc. SPIE, 4329, 59, 2005. With permission... 

The mechanical properties of polymers are further influenced by external conditions such as temperature, the pressure of the test environment and the presence of other additives or impurities such as moisture and solvents. The tensile strength, modulus and hardness decrease with an increase in temperature, but the elongation at break will increase. The effect of temperature on the mechanical properties of polymers is much greater than in any other category of materials. 

The mold temperature, on the other hand, affects the thickness of skin layers [82,83,87]. In general, the effect of temperature on the mechanical properties is relatively small compared to the injection speed. Mathematical modeling and computer simulations based on Doi s molecular theory for the motion of rigid rods were recently used in an attempt to predict the orientation and flow behavior of TLCPs in the mold as a function of processing conditions. A reasonably good agreement between the simulation and the experiment was reported [88]. 

The effect of temperature on the mechanical properties of plastic materials has a fundamental role in the selection of materials. Unlike metals and ceramics, plastics are extremely sensitive to the slightest changes in temperature. The selection of plastics for applications under different temperatures is a complex task. The plastic material must be able to support a stress under operating conditions without getting distorted. The effect of temperature on geometrical stability and mechanical properties in general can be studied following different procedures and methods like at constant temperature or with a temperamre ramp. 

Table 3.1-98 Effect of temperature on the mechanical properties of wrought Ni-based superalloys...

V. N. Krivobok, and A. M. Talbot, "Effect of temperatures on the mechanical properties. characteristics, and processing of austenitic stainless steel," Proceedings ASTM, Vol. 50 (1950). 

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