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Creep, fatigue and impact

Creep, fatigue and impact. John Summerscales. Creep I. creep is a deformation process occurring under quasi-static loading that can lead to mechanical failure. Creep II. Liao et al (1998) "creep of glass fibres is considered insignificant" (13 refs) but no stress levels given!

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Creep, fatigue and impact

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  1. Creep, fatigue and impact John Summerscales

  2. Creep I creep is a deformation process occurring under quasi-static loading that can lead to mechanical failure

  3. 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".

  4. Creep III • Consider rule-of-mixtures to reduce creep: • maximise fibre orientation (ηo) • maximise fibre length (ηl) • maximise fibre volume fraction • maximise fibre Young’s modulus • use highly cross-linked polymers • stay well below glass transition temperature

  5. 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 are:carbon/epoxy Kevlar/epoxy glass/epoxy 99.99% 99.8% 22%. • "under a load of 40% ultimate stress, the survival probability for glass/epoxy is 97%"

  6. 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

  7. Fatigue II • probability of failure: not Poisson distribution mean strength negative defects do not exist reduced strength due to defects

  8. …. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. Real impacts II: criminal damage • Bank/Post-Office counters • WR UD UD Staff side Villain side

  16. 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

  17. Real impacts IV: naval mines • Minesweepers exposed to explosive shock • USS Osprey (MHC-51)http://www.youtube.com/watch?v=plAAuk9VwLs&feature=player_detailpage :

  18. 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).

  19. Apocryphal tales ? • Bird strike tests: US requests help from UK • Bird strike tests:Check the system,before lighting the “blue touchpaper”

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