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Failure criteria - DESIGN AGAINST MATERIAL FAILURE A major...

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DESIGN AGAINST MATERIAL FAILURE A major objective in ME 211 is to develop methods to design a component against the possibility of material failure under load. Failure is very much a local process. It is the weakest link that fails and it doesn’t make any difference how strong the rest of the structure may be. Brittle and ductile materials Materials may be broadly characterized as either brittle or ductile . Brittle materials typically fail by crack propagation resulting in fracture and are therefore most at risk in tensile stress fields, since tensile stress causes the cracks to open, causing a high stress at the crack tip. By contrast, ductile materials do not fracture, but rather yield , meaning that the shape of the component is permanently changed. Yield is predominantly determined by the local shear stresses. Based on these considerations, we shall adopt the criterion that brittle fracture occurs when the maximum tensile stress reaches a critical value, whereas ductile materials yield when the maximum shear stress reaches a critical value 1 . Finding the stress components at a particular point In a typical design problem, a component of a given shape and dimensions is supported in some way and loaded by prescribed external forces. This results in stresses that vary throughout the body. In ME 211, we introduce a variety of ways in which such stress fields can be generated. For example, in a beam, the bending moment will vary with position along the beam and at any given position, the tensile stress will vary with distance from the centroidal axis. The stresses at a given point can usually be determined by sectioning
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