MSE 2090
Failure, Fatigue,
Creep (39
questions with
complete
verified
solutions)
3 modes of failure
(1) Fracture
-without a crack - stress exceeds tensile strength
-with a crack - stress intensity exceeds fracture toughness
(2) Fatigue
-cyclic loading - stress below yield strength
(3) Environmental Effects
-corrosion-fatigue - cyclic stress below the yield strength in an aggressive
environment
, -creep - static stress below yield strength and High T (increased strain at
constant stress)
Fracture
Separation of a body into two or more pieces in response to a static stress
at T<<Tm
2 types of fracture
(1) ductile
(2) brittle
Ductile Fracture
-extensive plastic deformation proceeds fracture
-fracture occurs slowly: "stable cracking"
-large energy absorption required
evolution of moderately ductile fracture:
necking -> void nucleation -> stable void growth and linkage (micro-void
coalescence) -> shearing at surface (unstable rapid cracking at 45˚) ->
fracture
Why is unstable rapid cracking at 45˚ along edges?
diagonal shear
Brittle Fracture
-very little plastic deformation before fracture
-fracture occurs rapidly: "unstable cracking"
-small energy absorption required
2 types of brittle fracture morphology
(1) transgranular: across the grains (most common)
(2) intergranular: along the grain boundaries (less common)
Flaws are stress concentrators
There is an elliptical hole in a plate and a stress distribution that exists in
front of a hole. Maximum stress is along the edge of the hole.
stress concentration factor (when is it bad??)
Kt (bad when greater than 3, large stress conc. factor promotes failure)
Failure, Fatigue,
Creep (39
questions with
complete
verified
solutions)
3 modes of failure
(1) Fracture
-without a crack - stress exceeds tensile strength
-with a crack - stress intensity exceeds fracture toughness
(2) Fatigue
-cyclic loading - stress below yield strength
(3) Environmental Effects
-corrosion-fatigue - cyclic stress below the yield strength in an aggressive
environment
, -creep - static stress below yield strength and High T (increased strain at
constant stress)
Fracture
Separation of a body into two or more pieces in response to a static stress
at T<<Tm
2 types of fracture
(1) ductile
(2) brittle
Ductile Fracture
-extensive plastic deformation proceeds fracture
-fracture occurs slowly: "stable cracking"
-large energy absorption required
evolution of moderately ductile fracture:
necking -> void nucleation -> stable void growth and linkage (micro-void
coalescence) -> shearing at surface (unstable rapid cracking at 45˚) ->
fracture
Why is unstable rapid cracking at 45˚ along edges?
diagonal shear
Brittle Fracture
-very little plastic deformation before fracture
-fracture occurs rapidly: "unstable cracking"
-small energy absorption required
2 types of brittle fracture morphology
(1) transgranular: across the grains (most common)
(2) intergranular: along the grain boundaries (less common)
Flaws are stress concentrators
There is an elliptical hole in a plate and a stress distribution that exists in
front of a hole. Maximum stress is along the edge of the hole.
stress concentration factor (when is it bad??)
Kt (bad when greater than 3, large stress conc. factor promotes failure)
