Failure Of Materials Flashcards
Define toughness.
Toughness is the energy dissipated per unit volume of the material up to failure.
Uc* = 0.5 x stress x strain (when linear! Otherwise integral for area under curve).
Describe the stress-strain response of a ductile material.
Ductile materials produce a non-linear response. The toughness remains the same, but the strain energy changes when past the elastic limit.
Once a material has plastically deformed, U is the energy recovered when the material is unloaded.
What is ‘inelastic strain energy’?
The energy absorbed by the material through plastic deformation, denoted by U_inelastic.
Define ‘elastic strain energy’ and ‘elastic strain energy density’. How are they related?
Elastic Strain Energy, U:
- The energy input into a system when e.g. loading a bar.
- Area under the curve of a load-displacement plot.
- Can drive a crack to grow.
Elastic Strain Energy Density, U*:
- The elastic strain energy per unit volume material.
- Area under the curve of a stress-strain plot.
U = U* . V
What are the two general classes of fracture seen in engineering materials?
- Brittle (low energy absorption, <5% strain, absorbed by transgranular cleavage fracture).
- Ductile (high energy absorption, absorbed by microvoid coalescence).
What are microvoids? How and where are they formed?
- Microvoids are pockets of air or foreign particles with little to no bond strength.
- Easily formed at inclusions, inter metallic or second-phase particles and grain boundaries.
- Microvoids grow and coalesce as the applied load increases.
Describe how ductile failure under uniaxial tensile force arises and what may be seen at a macro- and microscopic level.
Microscopic:
- Necking caused by dislocation movements/polymer chain sliding.
- Atomic deboning and microvoid initialisation.
- Microvoid coalescence to form larger cracks.
- Cracks eventually propagate normal to tensile axis.
Macroscopic:
- Crack propagation through the periphery along the shear plane at 45o.
- Cup and cone pattern.
Describe microvoid shape for the following:
A) Uniaxial tensile loading
B) Shear
C) Tensile tearing
Uniaxial Tensile Loading:
- Equiaxed dimples
Shear:
- Elongated and parabolic dimples pointing in opposite directions on matching fracture surfaces.
Tensile Tearing:
- Elongated dimples pointing in the same direction on the matching fracture surface.
Describe how brittle failure arises and what may be seen at a micro- and macroscopic level (the three steps of cleavage fracture).
Cleavage fractures have three key steps:
- Plastic deformation to produce dislocation pile-ups.
- Crack initiation.
- Crack propagation to failure.
Macroscopic:
- Absence of gross plastic deformation.
- Grainy or faceted texture.
- River marking or stress lines.
Describe the two cleavage fracture modes. What strength is each?
Cleavage fracture is the breaking of atomic bonds along crystallographic planes (transgranluar):
- Rough texture with river and feather patterns.
- Moderate to high strength.
This may occur along the grain boundaries (intergranular):
- Sharp and 3D faceted grains.
- Moderate to low strength.
Describe size effects.
Size effects are the change in strength with specimen dimensions. They are induced by flaws and defects, further leading to stress concentrations.
Describe the ductile to brittle transition temperature.
- Plotted as the absorbed energy against temperature behaviour.
- Increasing temperature allows more slip system to operate, yielding plastic deformation to occur prior to failure.
Briefly describe four other key types of failure (not including fracture).
Fatigue:
- Fracture by slow crack growth.
- Occurs when material is subject to many repetitions below the static crack growth stress.
Corrosion Fatigue:
- Combined effects of cyclic stress and corrosive environments.
- Fatigue resistance decreases in presence of aggressive chemical environment.
Stress Corrosion Cracking:
- As above, but for non-cyclic stress (but still below yield stress).
Creep Failure:
- The result of a static load applied over long periods of time.
What is basic view of fracture mechanics?
Elastic stress analyses assume perfectly homogeneous and flawless materials, which is not suitable for designing high-strength materials.
Relationships are established between the material’s inherent resistance to crack growth and the far-field stress.
When a crack reaches a certain critical length, it can propagate catastrophically through the structure.
What is Inglis’s solution?
Stress = R(1+2sqrt(a/rho_r)) Where rho_r = b^2/a
The maximum stress at the tip of a notch. However, as the crack becomes perfectly sharp, the stress tends to infinity.