Stress is defined as the internal resistance or counter force a material offers to applied external load. These counter forces tend to return the atom to their original position. The total resistance developed is equal to external loads. This resistance is known as Stress. 1SI unit of stress is N/m2.Stresses are developed inside a material by different mechanism, such as Body Forces (Like Gravity) Surface contact force. 2Types of StressesAccording to type of forces acting on component, Stresses can be classified as: Direct Stress Indirect Stress 3Direct StressesWhen a force is applied which is parallel or collinear to axis of component, Direct Stresses are developed.Stress= Force/Area?=F/AExamples of Direct Stresses are: Tensile stress Compressive Stress Shear StressTensile Stress:When two equal and opposite forces act on a body, then stresses developed are called Tensile Stresses.It tends to increase the length of the material.These stresses are perpendicular to surface. 4Compressive Stresses:When two equal and opposite forces act on a body, the stresses developed are compressive stresses.It tends to decrease the length of material. These stresses act perpendicular to surface. 5Shear Stress:When parallel and opposite forces act on a body, Shear stresses are developed.Shear stresses are also known as Tangential Stress. 6Indirect StressStress Developed due to moment and torsion are called indirect stresses.Bending stress:Bending stress is the measure of the internal stress produced by external force or moment causing it to bend. 7Torsion:Shear stress produced when we apply the twisting moment to the end of the shaft about its axis is called torsional stress. Poison RatioWhen a body is subjected to tensile force not only it elongates in axial direction but also contracts in lateral direction. 8For example: A rubber when subjected to tensile force it elongates as well as it contracts in lateral direction.Consider a bar having length l and radius r, is subjected to tensile force p. due to tensile force it length will increase by amount and radius contract by amount Strain in axial and longitudinal direction is:within elastic range the ratio of these strain is constant. And the constant is called poison ratio. Consider a structural component subjected to loads in the direction of the three coordinated axes and producing normal stresses .this condition is referred to as multiaxial loadingsConsider an element of an isotropic material. Assume that each side of the cube is equal to unity, since it is always possible to select the side of the cube as a unit of length. Under the given multiaxial loading, the element will deform into a rectangular parallelepiped of sides equal, respectively, to 1 + ?x, 1 + ?y, and 1 + ?z, where ?x, ?y, and ?z denote the values of the normal strain in the directions of the three coordinate axes. In order to express the strain components ?x, ?y, and ?z in terms of the stress components ?x, ?y, and ?z, the effect of each stress component will be considered separately, and the obtained results will be combined. The approach proposed here is based on the principle of superposition. This principle states that the effect of a given combined loading on a structure can be obtained by determining separately the effects of the various loads and combining the results obtained, provided that the following conditions are satisfied: Each effect is linearly related to the load that produces it. The deformation resulting from any given load is small and does not affect the conditions of application of the other loads. The stress component ?x causes a strain equal to ?x/E in the x direction, and strains equal to ???x/E in each of the y and z directions. The stress component ?y, if applied separately, will cause a strain ?y/E in the y direction and strains ???y/E in the other two directions. The stress component ?z causes a strain ?z/E in the z direction and strains ???z/E in the x and y directions. Combining the results obtained, it may be concluded that the components of strain corresponding to the given multiaxial loading are: The above equations are referred as the Hook’s Law for the multiaxial loading for homogenous isotropic material.