When forces act on a body, internal resisting forces are developed within the body by stresses in the material of the body.A perfectly plastic material is a material that does not return to its original dimensions when the load causing deformation is removed.Whenever a body is subjected to a force, there is a change in the shape or size of the body. These changes in dimensions are called deformation.Timber, unlike steel or concrete, is not a processed material but an organic material generally used in its natural state.