Lecture19-Notes - ME 382 Lecture 19 FRACTURE Confused terms...

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ME 382 Lecture 19 1 F RACTURE Confused terms (1): 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 after a lot of slip) Confused terms (2): Failure Failure is defined by constraint(s) imposed by the design Design will always imply no fracture Failure occurs if load results in fracture Design may state “no local yield” or "no net section yield" ("plastic collapse") Design may state “limit on stiffness” Failure occurs if load results in excessive deformation 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.
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ME 382 Lecture 19 2 Occasionally, shear fracture can occur (plastic collapse along shear planes) 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
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Lecture19-Notes - ME 382 Lecture 19 FRACTURE Confused terms...

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