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)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".

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

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%"

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

Fatigue II

• probability of failure: not Poisson distribution

reduced strength due to defects

mean

streng

th

negative defects do not exist

…. 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

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

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

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

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

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

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

Real impacts II: criminal damage

• Bank/Post-Office counterso WR UD UD

Staff side

Villain side

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

Real impacts IV: naval mines

• Minesweepers exposed to explosive shocko USS Osprey (MHC-51)

http://www.youtube.com/watch?v=plAAuk9VwLs&feature=player_detailpage :

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).

Apocryphal tales ?

• Bird strike tests: US requests help from UK

• Bird strike tests:Check the system,before lighting the “blue touchpaper”