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Physical and Mechanical Properties of Dental Materials Stress When a force (external) acts on the body, tending to produce deformation, a resistance is developed within the body to this external force. Stress: it is the internal resistance of the body to the external force. Stress is equal in magnitude but opposite in direction to the external force applied. The external force is also known as load. For simple tensile or compressive the stress is given by the expression: F: the applied force, and A: the cross-sectional area. Types of stresses 1- Tensile stress. 2- Compressive stress. 3- Shear stress. Tensile stress is a result in a body when it is subjected to two sets of forces that are directed away from each other in the same straight line. The load tends to stretch or elongate a body. Compressive stress is a result in a body when it is subjected to two sets of forces in the same straight line but directed towards each other. The load tends to compress or shorten a body. Figure (1-3). Shear stress is a result of two forces directed parallel to each other. The load tends to twist or slide of one portion of a body over another. Figure (1-4). Strain The application of an external force to a body results in a change in dimension (shape) of that body (deformation). For example, when a tensile force is applied the body undergoes an extension, the magnitude of which depends on the applied force and the properties of the material. The numerical value of strain is given by the expression: Figure (1-5): Diagram indicating how the magnitudes of (a) compressive and (b) tensile stresses and strains. Figure (1-6): Universal testing machine It is difficult to induce just a single type of stress in a body. Whenever force is applied over a body, complex or multiple stresses are produced. These may be a combination of tensile, compressive, or shear stresses. Figure (1-7): Complex stress pattern developed in cylinder subjected to compressive stress If we take a cylinder and subject it to a tensile or compressive stress, there is simultaneous axial and lateral strain. Within the elastic range the ratio of the lateral to the axial strain is called Poisson's ratio.