Fracture
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This document discusses fracture in materials under tensile loads. It describes two main types of fracture: ductile and brittle. Ductile fracture occurs through plastic deformation and the growth of microvoids, leading to necking of the material before failure. Brittle fracture occurs suddenly without plastic deformation through rapid crack propagation perpendicular to the tensile stress. The document outlines the stages and characteristics of ductile fracture in metals and compares it to the flat, cleavage surfaces of brittle fracture.
Fracture
- 1. Fracture
- 2. Fracture: Simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static (i.e. constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material. The applied stress may be tensile,compressive,shear or torsional. The present discussion will be confined to fractures that result from uniaxial tensile loads. Any fracture process involves two steps : i. Crack formation ii. Propagation For engineering materials, two fracture modes are possible 1.Ductile 2.Brittle
- 3. Ductile Fracture: Classification is based on the ability of a material to experience plastic deformation. Ductile materials typically exhibit substantial plastic deformation with high energy absorption before fracture. ductility may be quantified in terms of 1. %EL = Final length - Initial Length x 100 Initial Length 2. % RA = Original area- final area x 100 original area
- 4. Cont… Ductile fracture is characterized by extensive plastic deformation in the vicinity of an advancing crack. Ductile fracture is almost always preferred for two reasons. 1st brittle fracture occurs suddenly and catastrophically without any warning: This is consequence of the spontaneous and rapid crack propagation. On the other hand, the ductile fracture, the presence of plastic deformation gives warning that fracture is imminent, allowing preventive measures to be taken.
- 5. Cont…. Second, more strain energy is required to induce ductile fracture in as much as ductile materials are generally tougher. Ductile fracture surfaces will have their own distinctive features on both microscopic and macroscopic levels. Below fig shows schematic representations for two characteristics macroscopic fracture profiles.
- 6. Cont… Fig (a) is found for extremely for softy metals such as pure gold and lead at room temperature, an other metals, polymers, and inorganic glasses at elevated temperature. These highly ductile materials neck down to a point fracture, showing virtually 100% reduction in area.
- 7. Stages of fracture: The fracture process normally occurs in several stages given below.
- 8. 1st , after necking begins, in fig(b) small cavities on micro voids, in the interior of the cross-section. Next as deformation continues these micro voids in large, come together, and coalesce to form an elliptical as shown in fig ©. Which has long axis perpendicular to the stress direction. The crack continues to grow in a direction parallel to its major axis by micro voids coalescence process in fig (d). Finally fracture occurs by rapid propagation of a crack round the outer parameter of the neck as in fig (e). Shear deformation at an angle of 45 degree with tensile axis.
- 9. Brittle fracture: Brittle fracture takes place without any appreciable deformation, and by rapid crack propagation. The direction of crack motion is very nearly perpendicular to the direction of applied tensile stress and yields a relatively flat fracture surface, as indicated in fig 8.1 ©. Brittle fracture in amorphous materials such as ceramic glasses, yields relatively shiny and smooth surface. For most brittle crystalline materials cracks propagation corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes.
- 10. Cont… Such a process is termed cleavage. This type of fracture is said to be transgranular(or trans crystalline),because the fracture cracks pass through the grains. In some alloys crack propagation is along grain boundaries, this fracture is termed as intergranular.