6 Flashcards
What does a notch do to stress?
- Increase average stress by reducing x-section
- Concentrates Stress (Kt)
What is Nominal Stress?
The average stress in the net-section higher that the applied stress
Stress Concentration factor, Kt=
σpeak/σnominal
For a circular hole, Kt=
3
For an elliptical hole, Kt=
1+2(a/r)½
What is Fracture Mechanics?
The general analysis of the failure of structural materials with existing flaws. The main outcome is fracture toughness
Why is there such a variation between theoretical and experimental bond strength?
Stress concentration at microscopic flaws. It is amplified at tips of microscratches, voids, notches, surface scratches, corners etc.
What are Stress Raisers?
- Microcracks
- Voids
- Notches
- Surface Scratches
- Corners
What does the magnitude of ampification of Stress depend on?
Microcrack orientation, geometry and dimensions
Define Fracture toughness?
Critical value of stress intensity factor at a crack-tip necessary to produce catastrophic failure under simple uniaxial load
Stress Intensity Factor, KI=
fσ(πa)½
a= crack length
KI is a variable
Fracture Toughness, KIC=
fσmax(πamax)½
This is a property and increases with refinement of grain size
Design Criterion for KIC
KI <=KIC
Brittle materials have
Low KIC
KIC decreases when
- Strain rate increases
- Temperature decreases
KIC increases when
Grain size decreases
Cracks will suddenly grow if
Load is too high, leading to fracture in a brittle manner
Ductile fracture occurs in a
Transgranular manner- Through the grains
Brittle fracture occurs in a
Intergranular manner- along grain boundaries. Enhanced by impurities
How does ductile fracture occur in a simple tensile test?
- Nucleation, growth and coalescence of microvoids at necked region
- The stress then causes seperation of grain boundaries
- As local stress increases, microvoids grow and coalesce into larger cavities
- Eventually metal-metal contact is too small to support load thus fracture
Which is more notch-sensitive under tensile loading; Brittle or Ductile?
Brittle
What is Fatigue?
The lowering of strength or failure of a material due to repetitive stress
What are the 3 stages of fatigue?
- A small crack nucleates at the surface and can include scratches, pits, sharp corners, inclusions, grain boundaries or dislocation concentrations
- Crack gradually propagates as the load continues to cycle
- Sudden fracture when remaining x-section is too small to support applied load
For Fatigue to occurs at least part of the load must be
Tensile