ENB331 Lecture Materials and Manufacturing 2
Most failures in Engineering Structures are due to:
Poor design Defects introduced during processing and
Poor material selection fabrication
Assembly and service conditions
Imperfections in materials
Classifications of Failure Mechanisms
1. Fracture: Failure of materials under static (constant) loading:
a. Brittle fracture: breaking of materials following elastic deformation (fracture dominant failure)
b. Ductile fracture: Failure due to elongation beyond point of necking (yielding dominant failure)
2. Fatigue: Time dependent failure due to repeated stress/strain (cyclic loading)
3. Creep: Time dependent strain (deformation under static load at high temperatures
Yielding Dominant Failure: Formation of micro-voids and cavities at grain boundaries and/or
interfaces between material and impurities. Coalescence of voids and cavities to form elliptical
crack, in doing this, extensive plastic deformation around crack tip until critical crack length.
Rapid propagation of crack by shear deformation at ~45° to tensile axis.
CUP and CONE fracture Fibrous central region consisting of multiple “dimples”
(left over from micro-voids). Shear lip (tearing) of the
material at the outer region. Defects in “Yielding
dominant failure” are microscopic: dislocations,
interstitials, grain boundaries, precipitates.
Fracture Dominant Failure: Crack propagates rapidly and perpendicular to tensile axis. Little or no plastic
deformation around crack tip. Very flat fracture surface (Cleavage). Flaws are macroscopic i.e. weld defects, porosity
(holes), inclusions, steel corrosion, cracks etc.
Fracture through crystal Cracks propagate along
grains transgular. the grain boundaries:
In polycrystalline materials
this can give a faceted e.g. solidification of brittle
appearance as different films along grain
crystallographic planes are boundaries can lead to
exposed. intergranular fracture.
Ductile Fracture Brittle Fracture
One piece Many pieces
Large deformation Small deformation
1 Week 4 Tuesday, 27 August 2013 ENB331 Lecture Materials and Manufacturing 2
The longer the wire, the smaller the
load for failure.
This is because:
Flaws cause premature failure, larger
samples contain more flaws
The stress ahead of perpendicularly orientated elliptical notich in an
The stress is concentrated due to notch and the maximum stresm σ
(fracture stress) depends upon the radius of the notch tip, ρ.
σm≈ σ * √
Griffith formulated that failure of materials can be occurred beyond a critical crack length (Griffith Crack Length).
The theoretical critical stress for crack propagation in a brittle material:
E = modulus of elasticity
γ = specific surface energy
2 Week 4 Tuesday, 27 August 2013 ENB331 Lecture Materials and Manufacturing 2
Geomertical Effect of Cracks
Geometrical changes of section, defects, flaws, holes etc
concentrate stress differently.
e.g. maximum stress at a circular hole in a plate is three
times the nominal stress. Whereas an elliptical flaw
concentre much higher stresses.
Limitation of Stress Concentration Factor
As the elliptical notch turns into a crack, ρ goes to m and σ tends to infinity.
This suggests that the stress at the tip of all cracks is infinite, if stress at crack tip is infinite why don’t all cracks cause
fracture at very low stresses? Material at the crack tip yield