Failure Chapter 9
Cleavage- when cracks propagate along specific crystallographic planes
Transgranular (transcrystalline)- fracture passes through the grains.
Intergranular- then cracks propagate along grain boundaries
Stress raisers- imperfections in a solid that cause measured fractured strength to be
More significant in brittle materials rather than ductile
Fracture toughness (K) - measure of a materials resistance to brittle fracture when a
crack is present, brittle materials have lower K values than ductile materials.
decreases with increasing strain rate, and decreasing temp.
Plain strain- when thickness of specimen doesn’t matter, I.E. the thickness is much
greater than crack dimensions
NDT(non-destructive test) techniques detect cracks, leaks, etc.
Brittle fracture of ceramics
Static fatigue/delayed fracture- slow propagation of cracks
Origin—mirror region where the crack is accelerating to its critical velocity----
Fracture of Polymers
Crazing- when very small holes form instead of a line crack, eventually holes
combine and form a crack
Impact Fracture Testing
Charpy, Izod- both measure impact energy (notch toughness).draw pg 311
Ductile-to-Brittle transition- occurs with decreasing temp. during transition features
of both exist up looking
a broken specimen, fibrous or dull indicates
granular = brittle. Usually a range of temps.
Most common type are low-strength
Fatigue- when stresses lower than yield strength are repeated over and over again
Cyclic stresses- reversed stress cycle- when goes from max tensile to max
Repeated stress cycle- when it goes to max tensile and back to zero/max C back to
Fatigue limit- for some ferrous and titanium alloys it is the limiting stress level,
where below it fatigue failure will not occur.
No such thing for nonferrous alloys
Fatigue strength- the stress level at which failure will occur for some specified # of
Fatigue life- number of cycles to cause failure at a specified stress level.
Fatigue in polymers
Similar to metals, sometimes failure occurs due to localized heating & softening
Crack Initiation and Propagation
Beachmarks and striations, both are markings of crack propagation from fatigue,
beachmarks you can see with unaided eye, striations are microscopic, neither
appear at rapid failure areas
Factors that affect fatigue life
Mean stress, surface effects, design factors (fillets, sharp corners, etc), surface
treatments (finishes, etc).
Case Hardening- carburizing or nitriding steel alloys, increases hardness and
Thermal fatigue- occurs at elevated temps. Caused by fluctuating thermal stresses,
expansion, contraction etc.
Creep- constant load/stress, slow time-dependent deformation
Primary/transient creep- decreasing rate due to strain hardening
Secondary/steady state creep- linear plot, rate constant
Tertiary creep- rate increases to failure