Fracture

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Engineering | Finite Element Analysis | Stress

Value Design

Consulting

Further, fatigue is affected by the raw stock type (rod, bar, or plate), the raw stock size, and the direction of vibration relative to the material's grain. For nominally equivalent material, fatigue may also vary from heat-to-heat (especially for titanium) or among different manufacturers.

Fatigue is also affected by machining, which can leave residual tensile stresses that shorten fatigue life. These stresses are difficult to predict, since they depend on such factors as material removal rate, the type of machining coolant, the tool sharpness, etc. FEA cannot predict these surface stresses.

Thus, unless the material's fatigue properties and the effects of machining are well known, the stresses predicted by FEA probably cannot be used to predict fatigue life. However, the FEA stress data can be used to redesign fatigues that have known failure problems. Although this is a limitation of FEA, it is also a limitation of any other method of fatigue analysis. The fatigue life cannot be predicted from stress unless the material's fatigue characteristics are known.

Fracture

Fracture occurs when new cracks appear or existing cracks become extended. This phenomena is very complex in crystalline solids. Fractures are generally categorised as being brittle or ductile.

Brittle fracture manifests itself as a very rapid propagation after little or no plastic deformation. The speed at which cracks propagate rises rapidly to a terminal velocity which is usually around one third the speed of sound in the material. In polycrystalline materials the crack front proceeds along clevage planes within each crystal, giving the fracture surface a granular appearance. If brittle fracture proceeds along grain boundaries it is referred to as inter granular fracture.

Ductile rupture is fracture that takes place after extensive plastic deformation. It proceeds by slow propagation resulting from the formation & coalescence of voids. The fracture surface is dull & fibrous in appearance. Three distinct stages occur in polycrystalline materials. The component begins to neck-down locally & small discrete cavities appear in the neck region. Next, the cavities coalese into a crack in the centre of the cross-section. Finally, the crack spreads to the surface along shear planes oriented at 45° to the tensile axis.