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Unformatted text preview: ME 382 Lecture 19 24/ii/06 1 F RACTURE Confused terms (1): Stiffness vs . strength Stiffness: Deformation caused by an applied load or stress Strength: Maximum load or stress that can be supported without yield or fracture (i) local yield (ii) net-section yield (iii) plastic collapse (iv) fracture Confused terms (2): Yield vs. fracture Yield : Permanent deformation in response to a shear stress (by slip, twinning or martensitic transformation) Fracture: Separation of material into two or more bits Usually a response to a normal stress (Although, it can occur in response to a shear stress) Ductile fracture Fracture that occurs after macroscopic yield May occur before or after necking In very soft ductile materials, so much necking can occur that the final area 0. Occasionally, shear fracture can occur (plastic collapse along shear planes) ME 382 Lecture 19 24/ii/06 2 Often fracture results from growth and nucleation of voids. Voids can be nucleated by (i) normal stress at interfaces with precipitates (ii) strain mismatch between precipitates and matrix (iii) hydrostatic stresses Voids grow by plastic flow Region between voids acts like small tensile bars that fracture Dimpled fracture surface (often with precipitates in dimples) In a tensile bar - classical cup and cone fracture Depending on location of voids, fracture can either be Transgranular through the middle of grains Intergranular along grain boundaries Ductility if easy void growth and fracture (compared with plastic flow)...
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