MIME 260 lecture 7

Ductile fracture

Accompanied by significant plastic deformation absorbing high energy before fracture

Brittle fracture

little or no plastic deformation; catastrophic, rapid crack propagation

Fracture process steps

1. crack formation

2. crack propagation

Pipe failures

ductile: one piece, large deformation, brittle failure: many pieces, small deformations

Moderately ductile vs brittle failure

cup and cone vs flat surface

Moderately Ductile failure stages

necking, void nucleation, void growth and coalescence, shearing at surface, fracture

Brittle fracture cleavage

crack propagation(cleavage), either transgranular(through grains), intergranular(along grain boundaries)

longer the wire, the smaller the load for failure

flaws cause premature failure, larger samples contain longer/more flaws

Flaws are…

stress concentrators

Griffith Crack

elliptical hole through plate perpendicular to the applied stress

stress amplification is not restricted to microscopic defects

may occur at macroscopic internal discontinuities, at sharp corners, notches in large structures

How to drive crack growth

opening or tension mode, sliding mode, tearing mode

crack propagates if…

applied stress exceeds critical stress

crack propagation brittle materials

creates new surfaces, crack tip remains sharp, retain a high stress concentration factor Kt

crack propagation for ductile materials

plastic deformation blunts crack tip(reduces Kt)

Fracture Resistance: Bending test

3 or 4 point bending, easy to control experimental conditions, directly and quantitatively measure fracture toughness

Fracture Resistance: Impact testing

severe testing case, material more brittle during impact test, measure impact energy

impact energy

only qualitative indication of material toughness, rather than quantitative measure

fracture behaviors are dependent on loading conditions:

materials are more brittle under high loading rates, lower temperatures, subject to certain triaxial stress states

Fatigue

form of fracture, can occur below yield strength, structures subjected to cyclic loads, fracture occurs after millions of cycles of loading

fatigue fracture

brittle, no shape change, fatigue deteriorates material

Fatigue=

failure under applied cyclic stress

Fatigue responsible for

90% of mechanical engineering failures

Improving fatigue life

impose compressive surface stresses(to prevent surface cracks from growing), remove stress concentrators(fillet)

shot peening

improves fatigue life by shooting balls on surface of material causing compression on the surface.

creep

very slow strain rate plastic deformation, conditions: applied stress less than yield stress, elevated temperatures

primary creep

slope decreases with time

secondary creep

steady state, linear slope

tertiary creep

slope increases with time

creep(fracture)

brittle

creep is dependent on…

temperature

polycrystalline materials more prone to creep because…

grain boundary migration/slip at elevated temperature