Cracks 4 Flashcards
What are the limitations of LEFM theory?
-Stresses cannot physically be infinite at crack tip
-Plastic deformation will start when sigmayy reaches yield strength of material
What is the equation for sigmayy?
sigmayy = KI/((2pi*x)^1/2)
What happens with stable crack extension (increasing crack length with constant or increasing load)?
Increasing toughness with crack growth
What does the crack driving force increase with?
-higher applied loads
-longer cracks
What is the crack driving force measured by?
The elastic term K
Elastic plastic parameters can also be used, but are not covered in this module
What happens when K = KIC?
Fracture starts, but can stop again due to increasing toughness
What happens at the crack tip in ductile fracture?
-Cracks begin ‘blunting’
-Voids begin to form ahead of tip (usually around inclusions)
-Voids join up as crack grows
-Cycle repeats
What are the energy dissipation mechanisms which give materials toughness (the ability to consume work during crack advancement)?
-Crack tip plasticity
-Ductile fracture mechanisms (void formation/growth, shear localisation)
-Crack deflection (grain to grain misorientation, strengthening fibres/particles used)
What is the plastic zone radius?
ry = (1/2pi)*(KI/sigmaY)^2
How is stable fracture achieved?
-Match increasing crack driving parameter to increasing material toughness
(As crack extends then arrests when KI = KIC)
What is the point of instability?
When material toughness cannot keep up with the increasing crack driving parameter
What refinement accounts for plasticity?
Add the crack tip plastic zone radius to a in the equation for K.
This simplifies to
KIe = KIelastic + KIplastic
