Theories of Failure Memorial University of （纪念大学理论的失败）.pdf

Theories of Failure Strength of a material or failure of the material is deduced generally from uni-axial tests from which stress strain characteristics of the material are obtained. The typical stress-strain curves for ductile and brittle materials are shown below. Material Strength parameters are Sy OR Su Theories of Failure In the case of multidimensional stress at a point we have a more complicated situation present. Since it is impractical to test every material and every combination of stresses , , and , a 1 2 3 failure theory is needed for making predictions on the basis of a material’s performance on the tensile test., of how strong it will be under any other conditions of static loading. The “theory” behind the various failure theories is that whatever is responsible for failure in the standard tensile test will also be responsible for failure under all other conditions of static loading. Theories of Failure The microscopic yielding mechanism in ductile material is understood to be due to relative sliding of materials atoms within their lattice structure. This sliding is caused by shear stresses and is accompanied by distortion of the shape of the part. Thus the yield strength in shear Ssy is strength parameter of the ductile material used for design purposes. Generally used theories for Ductile Materials are: •Maximum shear stress theory •Maximum distortion energy theory. (von Mises-Hencky’s theory). Theories of Failure The Maximum - Shear - Stress Theory The Maximum Shear Stress theory states thatfailure occurs when the maximum shear stress from a combination of principal stresses equals or