Creep, fatigue and impact

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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)
• "creep of glass fibres is considered
  insignificant" (13 refs) but no stress levels
  given!
• "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".
• "... 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
• maximise fibre orientation (?o)
• maximise fibre length (?l)
• maximise fibre volume fraction
• maximise fibre Youngs modulus
• use highly cross-linked polymers
• stay well below glass transition temperature

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Creep IV (Liao reporting Thomas)

• long-term stress-rupture behaviour of
  unidirectional fiber/epoxy systems
• calculations for static load 50 ultimate
  stress, the respective probabilities of
  survivalover a 30 year period arecarbon/epoxy K
  evlar/epoxy glass/epoxy 99.99 99.8 22.
• "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

mean strength
negative defects do not exist
reduced strength due to defects
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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/wee
  kly/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
• not a true indication of the material response
  unless the crack runs parallel to the striker
  motion.
• drop weight (vertical)
• energy varied by change of weight or height
• high strain rate hydraulic machines
• Hopkinson-bar techniques
• ballistic (horizontal and high-energy)
• normally conducted in the horizontal plane

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Compression after impact (CAI)

• CRAG test method 403
• impact the laminateover a range of energy levels
• monitor the type and size of damage
• 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 counters
• WR UD UD

Staff side Villain side
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Real impacts III rail

• British Rail High Speed Train (HST)
• still used by FGW on Penzance-Paddington route
• problem of impactors hung from bridges
• three candidate materials for cab body
• 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 shock
• USS Osprey (MHC-51)http//www.youtube.com/watch?v
  plAAuk9VwLsfeatureplayer_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 testsCheck the system,before
  lighting the blue touchpaper