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Lesson 24. THEORIES OF FAILURE, STRESSES IN ELEMENTARY MACHINE PARTS
Module 5. Properties of material, failures and factor of safety
Lesson 24 THEORIES OF FAILURE, STRESSES IN ELEMENTARY MACHINE PARTS 24.1 Introduction It has already been discussed in the previous chapter that strength of machine members is based upon the mechanical properties of the materials used. Since these properties are usually determined from simple tension or compression tests, therefore, predicting failure in members subjected to uniaxial stress is both simple and straight-forward. But the problem of predicting the failure stresses for members subjected to bi-axial or tri-axial stresses is much more complicated. In fact, the problem is so complicated that a large number of different theories have been formulated. The principal theories of failure for a member subjected to bi-axial stress are as follows: 24.2 Principal Theories of Failures
Since ductile materials usually fail by yielding i.e. when permanent deformations occur in the material and brittle materials fail by fracture, therefore the limiting strength for these two classes of materials is normally measured by different mechanical properties. For ductile materials, the limiting strength is the stress at yield point as determined from simple tension test and it is, assumed to be equal in tension or compression. For brittle materials, the limiting strength is the ultimate stress in tension or compression. 24.2.1 Maximum principal or normal stress theory (Rankine’s Theory) According to this theory, the failure or yielding occurs at a point in a member when the maximum principal or normal stress in a bi-axial stress system reaches the limiting strength of the material in a simple tension test. Since the limiting strength for ductile materials is yield point stress and for brittle materials the limiting strength is ultimate stress, therefore according to the above theory, taking factor of safety (F.S.) into consideration, the maximum principal or normal stress (σt1) in a bi-axial stress system is given by σt1 = σyt/F.S for ductile material = σu/F.S for brittle material Where, σyt = Yield point stress in tension as determined from simple tension test, and σu = Ultimate stress. 24.2.2 Maximum distortion energy theory (Hencky and Von Mises Theory): According to this theory, the failure or yielding occurs at a point in a member when the distortion strain energy (also called shear strain energy) per unit volume in a bi-axial stress system reaches the limiting distortion energy (i.e. distortion energy at yield point) per unit volume as determined from a simple tension test. Mathematically, the maximum distortion energy theory for yielding is expressed as (σ_{t}_{1})^{2} + (σ_{t}_{2})^{2} – 2σ_{t}_{1} × σ_{t}_{2} = (σ_{yt }/F.O.S)^{ 2} Where, σ_{yt }is yield stress F.O.S. = Factor of safety. This theory is mostly used for ductile materials in place of maximum strain energy theory. 24.2.3 Maximum shear stress theory (Guest’s Or Teresa’s Theory) According to this theory, the failure or yielding occurs at a point in a member when the maximum shear stress in a bi-axial stress system reaches a value equal to the shear stress at yield point in a simple tension test. Mathematically, ז_{max} = ז_{yt} /F.O.S. Where ז_{max} = Maximum shear stress in a bi-axial stress system, ז_{yt} = Shear stress at yield point as determined from simple tension test, F.O.S. = Factor of safety. 24.2.4 Maximum strain energy theory (Haigh’s Theory) According to this theory, the failure or yielding occurs at a point in a member when the strain energy per unit volume in a bi-axial stress system reaches the limiting strain energy per unit volume as determined from simple tension test. We know that strain energy per unit volume in a bi-axial stress system, U_{1}=1/2E[σ_{t}_{1}^{2} + σ_{t}_{2}^{2} – ((2σ_{t}_{1}* σ_{t}_{2})/M)] U_{2}=1/2E[σ_{yt}/F.O.S]^{2} According to the above theory U_{1}=U_{2} 1/2E[σ_{t}_{1}^{2} + σ_{t}_{2}^{2} – ((2σ_{t}_{1}* σ_{t}_{2})/M)] =1/2E[σ_{yt}/F.O.S]^{2} Or [σ_{t}_{1}^{2} + σ_{t}_{2}^{2} – ((2σ_{t}_{1}* σ_{t}_{2})/M)]= [σ_{yt}/F.O.S]^{2} This theory may be used for ductile materials. 24.2.5 Maximum principal strain theory (Saint Venant’s Theory) According to this theory, the failure or yielding occurs at a point in a member when the maximum principal (or normal) strain in a bi-axial stress system reaches the limiting value of strain as determined from a simple tensile test. The maximum principal (or normal) strain in a bi-axial stress system is given by Є_{max}= (σ_{t}_{1}/E)-( σ_{t}_{2}/m*E) According to the above theory, Є_{max}= (σ_{t}_{1}/E)-( σ_{t}_{2}/m.E)= Є= (σ_{yt}/E*F.O.S)…………………………. Where, σ_{t}_{1} and σ_{t}_{2} = Maximum and minimum principal stresses in a bi-axial stress system, ε = Strain at yield point as determined from simple tension test, 1/m = Poisson’s ratio, E = Young’s modulus, and F.O.S. = Factor of safety. From equation (i), we may write that σ_{t}_{1}-( σ_{t}_{2}/m)= (σ_{yt}/F.O.S) This theory is not used, in general, because it only gives reliable results in particular cases. |