Fracture, Toughness, Fatigue, and Creep. Materials Science & Manufacturing PROCESSES. Why Study Failure. In order to know the reasons behind the occurrence of failure so that we can prevent failure of products by improving design in the light of failure reasons. Mechanical Failure.
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In order to know the reasons behind the occurrence of failure so that we can prevent failure of products by improving design in the light of failure reasons
ISSUES TO ADDRESS...
• How do flaws in a material initiate failure?
• How is fracture resistance quantified; how do different
material classes compare?
• How do we estimate the stress to fracture?
• How do loading rate, loading history, and temperature
affect the failure stress?
loading from walking.
• Brittle failure:
• Ductile failure:
serve as void
• Evolution to failure:
crack occurs perpendicular to tensile force applied
304 S. Steel (metal)
316 S. Steel (metal)
Suppose an internal flaw (crack) already exits in a material and it is assumed to have a shape like a elliptical hole:
The maximum stress (σm) occurs at crack tip:
where t = radius of curvature
so = applied stress
sm = stress at crack tip
Kt = Stress concentration factor
Theoretical fracture strength is higher than practical one; Why?
Cracks propagate due to sharpness of crack tip
Effect of stress raiser is more significant in brittle materials than in ductile materials. When σm exceeds σy , plastic deformation of metal in the region of crack occurs thus blunting crack. However, in brittle material, it does not happen.
--Result 2: Design stress
dictates max. allowable flaw size.
--Result 1: Max. flaw size
dictates design stress (max allowable stress).
• Crack growth condition:
• Largest, most stressed cracks grow first!
• Material has Kc = 26 MPa-m0.5
• Two designs to consider...
--use same material
--largest flaw is 4 mm
--failure stress = ?
--largest flaw is 9 mm
--failure stress = 112 MPa
• Key point: Y and Kc are the same in both designs.
• Reducing flaw size pays off!
• Increasing temperature...
--increases %EL and Kc
• Ductile-to-Brittle Transition Temperature (DBTT)...
Low strength FCC metals (e.g., Cu, Ni)
Low strength BCC metals (e.g., iron at T < 914°C)
High strength materials (
Random Stress Cycle
Range of stress: