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Engineering Materials - Thermal Material Properties

 

Engineering Materials


Thermal Properties

 

Thermal Properties

Thermal conductivity

The thermal conductivity is the rate of heat transfer through a material in steady state. It is not easily measured, especially for materials with low conductivity but reliable data is readily available for most common materials.

Thermal diffusivity

The thermal diffusivity is a measure of the transient heat flow through a material.

Specific heat

The specific heat is a measure of the amount of energy required to change the temperature of a given mass of material. Specific heat is measured by calorimetry techniques and is usually reported both as CV, the specific heat measured at constant pressure, or CP, the specific heat measured at constant pressure.

Melting point

The melting point is the temperature at which a material goes from the solid to the liquid state at one atmosphere. The melting temperature is not usually a design criteria but it offers important clues to other material properties.

Glass transition temp

The glass transition temperature, or Tg is an important property of polymers. The glass transition temperature is a temperature range which marks a change in mechanical behavior. Above the glass transition temperature a polymer will behave like a ductile solid or highly viscous liquid. Below Tg the material will behave as a brittle solid. Depending on the desired properties materials may be used both above and below their glass transition temperature.

Thermal expansion coefficient

The thermal expansion coefficient is the amount a material will change in dimension with a change in temperature. It is the amount of strain due to thermal expansion per degree Kelvin expressed in units of K-1. For isotropic materials " is the same in all directions, anisotropic materials have separate "s reported for each direction which is different.

Thermal shock resistance

Thermal shock resistance is a measure of how large a change in temperature a material can withstand without damage. Thermal shock resistance is very important to most high temperature designs. Measurements of thermal shock resistance are highly subjective because if is extremely process dependent. Thermal shock resistance is a complicated function of heat transfer, geometry and material properties. The temperature range and the shape of the part play a key role in the material's ability to withstand thermal shock. Tests must be carefully designed to mimic anticipated service conditions to accurately asses the thermal shock resistance of a material.

Creep resistance

Creep is slow, temperature aided, time dependent deformation. Creep is typically a factor in materials above one third of their absolute melting temperature or two thirds of their glass transition temperature. Creep resistance is an important material property in high temperature design, but it is difficult to quantify with a single value. Creep response is a function of many material and external variables, including stress and temperature. Often other environmental factors such as oxidation or corrosion play a role in the fracture process.

Creep is plotted as strain vs. time. A typical creep curve shows three basic regimes. During stage I, the primary or transient stage, the curve begins at the initial strain, with a relatively high slope or strain rate which decreased throughout stage I until a steady state is reached. Stage II, the steady state stage, is generally the longest stage and represents most of the response. The strain rate again begins to increase in stage III and rupture at tR generally follows quickly.

Different applications call for different creep responses. In situations where long life is desired minimum creep rate is the most important material consideration. Testing through stage II should be sufficient for determining minimum creep rate. Is not necessary to proceed all the way to rupture. For this type of test the longer the test the more accurate the creep rate will be. Unfortunately practicality limits most creep tests to times shorter than would be desirable for high accuracy.

For short lived applications such as rocket nozzles the time to failure may be the only consideration. The main issue is whether or not the component fails, not the amount of deformation it may undergo. For this application creep tests may be run to completion but without recording any data but the time to rupture. In this case temperatures may be elevated above expected conditions to provide a margin of safety.

The main objective of a creep test is to study the effects of temperature and stress on the minimum creep rate and the time to rupture. Creep testing is usually run by placing a sample under a constant load at a fixed temperature. The data provided from a complete creep test at a specific temperature, T, and stress includes three creep constants: the dimensionless creep exponent, n, the activation energy Q, and A, a kinetic factor.

 

 




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