Material Failure Flashcards
What is simple fracture
The separation of a body due to an imposed stress that is a static and at a temp low relative to melting point. Two fracture modes: ductile and brittle,
Ductile vs brittle fracture
Ductile sustains substantial plastic deformation with high energy absorption before fracture. brittle doesn’t, crack progresses quickly and spontaneously
What are the two steps involved in any fracture process?
Crack formation
Crack probation
What is meant if a crack is stable (typical of ductile fracture)
crack resists propagation unless the applied stress increases
What is meant by unstable crack (typical of brittle fracture)
Crack, once formed, probates quickly without needing added stress
Appearance of ductile vs brittle cracks
Ductile: rough deformed surfaces with signs of stretching
Brittle: flat clean surface with little to no deformation
Why is ductile fracture preferred over brittle
Brittle occurs without warning. furthermore, more strain is required to induce ductile fracture as those materials are usually tougher.
Highly ductile fracture appearance vs moderate
Highly: Material neck down to point of fracture, with virtually 100% area reduction
Moderate: moderate necking then rough surface
Fracture process for moderately ductile
1) necking
2) microvoids (small cavities) form
3) microvoids enlarge, come together and coalesce to form an elliptical crack perpendicular to stress direction.
4). Crack continues to grow by microvoid coalescence process
5) rapid probation of a crack around perimeter due to shear deformation. This is at 45 degrees to the tensile axis where shear stress is a max
What is meant by a cup and cone fracture
A classic ductile fracture shape where one side is like a cup and the other a cone
Why is measured fracture strength for more brittle material lower than theoretically predicted
Small, microscopic flaws or cracks or internal discontinuities at shard corners and notches. applied stress may be amplified or concentrated at the tip. Magnitude of amplification dependent on crack orientation and geometry.
Why is the effect of a stress raiser more significant for brittle than ductile materials
For ductile, plastic deformation ensues when the max stress exceeds yield strength. This leads to a more uniform distribution of stress and to the development of a max stress concentration less than the theoretical value.
Fracture toughness
How resistant a material is to fracture
Fatigue
A form of failure that occurs in structures subjected to dynamic and fluctuating stresses. Under these circumstances it is possible for
failure to occur at a stress level considerably lower than the tensile or yield strength for a static
load. Fatigue failure is brittle like in nature, even for usually ductile materials. Process occurs by initiation and propagation of cracks, usually perpendicular to applied stress
Reversed stress cycle (know graph too)
Regular sinusoïdal Time dependence where amplitude is symmetric about zero stress level
Repeated stress cycle (know graph)
Sinusidol. Maxima and minima are asymmetrical relative to zero stress level.
Mean stress
Average between min and max stresses in the cycle
Stress range
Difference between min and max
Stress amplitude
Half reg stress range
Stress ratio
Ratio of min over max stress
Fatigue limit
The largest number of fluctuating stress that will not cause failure for essentially an infinite number of cycles
What types of metals may have a fatigue limit
Some ferrous metals
Know how to draw an S-N graph for materials with or without a fartgue limit
Fatigue strength
The stress level at which failure will occur for some specified number of cycles
