Fracture Mechanics (Simon Hogg) Flashcards
What is meant by periodic shearing force?
When a perfect lattice slips, there is zero stress when lattice rows align, when the top row is pulled past the bottom row by half an atom and when it reaches the rest position again so this force is periodical like a sin wave.
What does Tm represent in a lattice slip formula?
Max shear stress (where distance displaced is 1/4 of distance between atom centres)
Why do dislocations slip easier than a whole lattice shear?
Only a few atomic bonds need to be broken to move so they glide with lower stress
What is Peierls-Nabarro stress?
Minimum external stress required to move a dislocation irreversibly without assistance of lattice vibrations.
What is the relationship with closely packed and separation?
InterAtomic spacing (separation) is widthways, interplane spacing (packing) is vertically
As separation increases, packing gets closer
What does Peierls-Nabarro stress tell us about d/b relationship?
Big b (atomic spacing) = Small d (interplane spacing)
What is the Schmid factor?
Stress x cos(theta) x cos(lambda)
Slip will exist on slip system that has max value of schmid factor
What is strength?
Resistance of a material to fracture or to plastic flow (depends on text books)
How does temperature affect failure mechanism?
Room temps - usually ductile
Very low temps - usually brittle
Slow strain at high temps - usually some form of creep
How does attraction force change as you pull two atoms apart?
Under a tensile load, atomic spacing increases as the strain is greater than the attraction pulling it together, as they separate, this attraction decreases until a peak stress point when the atoms no longer attract each other.
How much do stress concentrations differ from the applied stress?
Proportional to the applied stress x sqrt (pi x 1/2 crack length / distance from crack tip)
Why is crack tip radius normally approaching zero?
Cracks are sharp
Where is maximum cohesive strength achieved in a stress concentration?
Crack tip
How do brittle and ductile fractures normally occur?
Brittle - Trans or intergranular fracture, very little energy adsorbed, no plastic deformation and rapid crack propagation
Ductile - Normally by a cavitation mechanism, lots of energy adsorbed due to plastic deformation in ‘plastic zone’ at crack tip, crack propagation is slow and stable
With reference to the grains, how does cleavage (transgranular) failure occur?
Crack propagates by breaking atomic bonds on specific planes - normally most densely packed (in BCC - {001})
Crack path will pass through grains
Steps/ridges in a grain are due to small changes in local misorientation which can change cleavage plane
How will a polycrystalline grain boundary affect transgranular cracks?
Crack path will divert when crossing a grain boundary
When and how does intergranular failure occur?
Usually when detrimental impurities segregate to grain boundaries in polycrystalline materials.
Impurities reduce the cohesion between grains so they separate with little to no plastic deformation.
What will happen if a polycrystalline material doesn’t cleave?
Most likely fail in a ductile transgranular manner
Small particles in metals act as stress concentrators which can seperate from matrix or fracture themselves
Results in an internal crack at centre of necked region.
The nucleated voids then grow and join as crack advances until final fracture
What is cup and cone failure?
Shape of the two parts that have fractured - most metals fracture by shear at a 45 angle to applied stress.
What is ductile dimpling?
Where particles have separated from matrix in ductile fracture
Describe the appearance of a fractured brittle polycrystalline metal.
Macroscopic - would look flat as plastic deformation before fracture is negligible
Microscopic - fracture surface may feature a series of steps if cleavaged - steps would correspond to the change in cleavage plane within a grain OR fracture surface may appear granular if failure was intergranular as grain boundaries will have seperated
Describe the appearance of a fractured ductile polycrystalline metal.
If material was very pure, completely ductile would mean it reduces to a point before final fracture.
Most metals though would have a cup and cone appearance with a flat central fibrous region and 45 degree shear region at edges.
Microscopically, you’d see dimples or voids from particles which may decohese or fracture if matrix is strong
Why is a brittle material more sensitive to cracks?
Yield strength is high so little to no plastic deformation to relieve the stress at the crack tip.
In a ductile material, void formation happens and absorbs energy at the crack tip which blunts it to reduce the stress concentration.
Describe the appearance of a fracture surface that has failed due to fatigue.
Macroscopic - steps called ‘beachmarks’ due to the interruption of the stress cycle
Microscopic - striations due to the avg distance the crack front moves per cycle (much tinier)
Describe the appearance of a fracture surface due to hydrogen embrittlement.
Varies from microvoids (ductile) to ‘quasi-cleavage’ (brittle) to intergranular
What is the square indent picture?
Vickers hardness indent
Annealing twins present
Wavy lines represent slip steps where dislocation emerges
Height of steps = magnitude of Burgers vector
Steps change orientation in different grains - they don’t move randomly in crystal structures
