creep, fatigue and impact john summerscales. creep i creep is a deformation process occurring under...
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Creep, fatigue and impact
John Summerscales
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Creep I
creep is a deformation process occurring under quasi-static loading that can lead to mechanical failure
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Creep II
• Liao et al (1998)o "creep of glass fibres is considered insignificant"
(13 refs) but no stress levels given!o "0° laminates exhibit a minimal amount of creep
at low stress 6.2 MPa (900 psi) and moisture content (0.5-0.94% by mass) at room temperature".
o "... the major cause of creep of FRP comes from creep of the polymer matrix, creep of glass fibers is considered insignificant".
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Creep III
• Consider rule-of-mixtures to reduce creep:o maximise fibre orientation (ηo)
o maximise fibre length (ηl)
o maximise fibre volume fractiono maximise fibre Young’s modulus
o use highly cross-linked polymerso stay well below glass transition temperature
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Creep IV (Liao reporting Thomas)
• long-term stress-rupture behaviour of unidirectional fiber/epoxy systemso calculations for static load = 50% ultimate
stress, the respective probabilities of survivalover a 30 year period are:carbon/epoxyKevlar/epoxy glass/epoxy
99.99% 99.8%22%.
o "under a load of 40% ultimate stress, the survival probability for glass/epoxy is 97%"
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Fatigue I
fatigue is the progressive, localised, and permanent structural damage that occurs when a material is subjected to cyclic or fluctuating strains at nominal stresses that have maximum values less than (often much less than) the static yield strength of the material.
• from Wikipedia, the free encyclopedia
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Fatigue II
• probability of failure: not Poisson distribution
reduced strength due to defects
mean
streng
th
negative defects do not exist
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…. Weibull statistics for composites
• normal distribution is symmetric:every weak sample has a corresponding strong
sample.
• Weibull distribution is skewed towards the weak side:more weak samples than strong samples.
Figure from http://composite.about.com/library/weekly/aa060997.htm
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Fatigue III
• usually tested on a servo-hydraulic machine:
• sinusoidal waveform • square wave• triangular wave• replay of a random waveform
• circuits of a test-track• also earthquake simulation laboratories
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Fatigue IV (from Liao)
• "... cyclically loaded at 20-30% quasi-static strength, unidirectional glass/epoxy can last for about a million cycles"
• "Dharan also suggested that loading below the matrix micro crack initiation stress (equivalent to ~0.75% strain level) for glass/epoxy will not lead to fatigue failure“.
... but remember these are UD cases,off-axis strengths will NOT be so good
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Impact (definition and considerations)
impact is the transfer of energy to a target normally over a very short timescale.
• the deformation processes may act faster than the normal response times for the material.
• the glass transition temperature is a function of loading rate and may seem to be significantly reduced during high rate loading
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Impact (techniques)• pendulum methods, e.g. Charpy/Izod
o not a true indication of the material response unless the crack runs parallel to the striker motion.
• drop weight (vertical)o energy varied by change of weight or height
• high strain rate hydraulic machines• Hopkinson-bar techniques • ballistic (horizontal and high-energy)
o normally conducted in the horizontal plane
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Compression after impact (CAI)
• CRAG test method 403o impact the laminate
over a range of energy levelso monitor the type and size of damageo test coupons for residual compression
strength
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Real impacts I: criminal damage• Cash-in-transit (CIT) or
Cash/valuables-in-transit (CVIT) boxes: physical transfer of banknotes, coins and items of value from one location to another.
• Attack by axe, car, guillotine, hammer, etc
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Real impacts II: criminal damage
• Bank/Post-Office counterso WR UD UD
Staff side
Villain side
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Real impacts III: rail
• British Rail High Speed Train (HST)o still used by FGW on Penzance-Paddington route
• problem of impactors hung from bridges• three candidate materials for cab body
o aluminium, titanium or GRP/foam sandwich
• metals petal and let impactor through• sandwich has multiple “nets” to catch item
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Real impacts IV: naval mines
• Minesweepers exposed to explosive shocko USS Osprey (MHC-51)
http://www.youtube.com/watch?v=plAAuk9VwLs&feature=player_detailpage :
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Real impacts V: bird-strike
• As before net closing speed is the issue• Aircraft industry uses standard impactor• Comtek Advanced Structures worked with
Bombardier in 1996 to develop repair methods for the Dash-8 leading edge. The improved repair method was tested for its ability to withstand bird-strikes:"bullet time" video (2.98 MB .WMV file).
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Apocryphal tales ?
• Bird strike tests: US requests help from UK
• Bird strike tests:Check the system,before lighting the “blue touchpaper”