define fatigue
it is a progress, localized permanent structural damage that occurs when a material is subjected to cyclic or fluctuating loading
maximum stress values are less than the ultimate tensile stress limit and may be below the yield stress limit of a material…. 90% materials fail from fatigue
fracture
is a seperation or fragmentation of a solid body into two or more parts under actions of stress
fracture can occur in how many ways?
- slow application of external loads (tension)
- rapid application of external loads (impact)
- cyclic or repeated loading (fatigue)
- time-dependent deformation under constant load (creep)
fatigue cracks are generally _________
initiated flaws on a free surface… once initiated cracks grow at accelerated rate
when crack become unstable it can cause
sudden catastrophic failure
fatigue failure can be divide into the following stages:
- cyclic plastic deformation
- nucleation
- short crack or small crack phase
- crack propagaion or growth rates
- final instability or failure
the following factors are necessary to cause fatigue fracture:
- applied stress of sufficent magnitude
- large variation or fluctuation in applied stress
- sufficiently large number of cycles of applied stress
fatigue is a _____ process
stochastic (random)
Name factors that affect fatigue
- type of loading
- surface roughness, scratches, dents
- type of material
- grain size- fine grained parts have longer fatigue life
- environmental conditions
- residual stress- heat and deformation
- direction of loading
- temperature
- geomentry
direction of loading is important in case of
non isotropic material
define stress concentrator
is a location in an object where stress is concentrated.
infinite lifetime concept
design to keep stress below threshold of fatigue limit
finite lifetime concept
design for a fixed life after which the user is instructed to replace the part with a new one
damage tolerant design
replace part once crack proceeds…“retirement for cause”
safe-life design based on the infinite-life criterion reflects the
classic approach to fatigue
it is generally categorized as a
high cycle fatigue
strain-life criteria is primarily intended to address the
low cycle fatigue area
types of fatigue testing
tension-compression
tension-tension
irregular or random
sheet and plate bending
