Behaviour_of_materials2.doc.docVIP

Behaviour of Materials 1. Introduction When a force is applied on a body it suffers a change in shape, that is, it deforms. A force to resist the deformation is also set up simultaneously within the body and it increases as the deformation continues. The process of deformation stops when the internal resisting force equals the externally applied force. If the body is unable to put up full resistance to external action, the process of deformation continues until failure takes place. The deformation of a body under external action and accompanying resistance to deform are referred to by the terms strain and stress respectively. 2. Stresses Stress is defined as the internal resistance set up by a body when it is deformed. It is measured in N/m2 and this unit is specifically called Pascal (Pa). A bigger unit of stress is the mega Pascal (MPa). 1 Pa = 1N/m2, 1MPa = 106 N/m2 =1N/mm2. 2.1. Three Basic Types of Stresses Basically three different types of stresses can be identified. These are related to the nature of the deforming force applied on the body. That is, whether they are tensile, compressive or shearing. 2.1.1. Tensile Stress Consider a uniform bar of cross sectional area A subjected to an axial tensile force P. The stress at any section x-x normal to the line of action of the tensile force P is specifically called tensile stress pt . Since internal resistance R at x-x is equal to the applied force P, we have, pt = (internal resistance at x-x)/(resisting area at x-x) =R/A =P/A. Under tensile stress the bar suffers stretching or elongation. 2.1.2. Compressive Stress If the bar is subjected to axial compression instead of axial tension, the stress developed at x-x is specifically called compressive stress pc. pc =R/A = P/A. Under compressive stress the bar suffers shortening. 2.1.3. Shear Stress Consider the section x-x of the rivet forming joint between two plates subjected to a tensile force P as shown in figure. The stresses set up at the section x-x