The deformation per unit of the original dimension is called the strain. Most materials have elastic properties, and within certain limits of loading the deformation due to simple compression or tension loading is directly proportional to the load and to the stress induced by the load. In the elastic range of deformation the material returns to its original dimension or shape when the load is removed. When subjected to sustained or long-term loading, many building materials experience additional deformation, which does not fully disappear when the loading is removed. This deformation is called creep or plastic flow and has been discussed in relation to structural deflections in CBD 54.
Most materials expand when heated and contract when cooled.
For solid materials the increase in length per unit length for one degree rise in temperature is known as the linear coefficient of thermal expansion. Values for the linear coefficients of thermal expansion for most materials used in building have been determined by various laboratories and are available in handbooks. These are sometimes related to the Centigrade scale and sometimes to the Fahrenheit scale and for definite temperature ranges and moisture contents. It is a simple matter, therefore, to determine the linear deformation due to temperature change for any building material that is unrestrained, and to use design judgement for further assessment. Moisture Content Changes.