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!
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".
o
Creep III
• Consider rule-of-mixtures to reduce creep:
maximise
o maximise
o maximise
o maximise
o
fibre
fibre
fibre
fibre
orientation (ηo)
length (ηl)
volume fraction
Young’s modulus
use highly cross-linked polymers
o stay well below glass transition temperature
o
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 survival
over a 30 year period are:
carbon/epoxy Kevlar/epoxy glass/epoxy
99.99%
99.8%
22%.
o "under a load of 40% ultimate stress, the survival
probability for glass/epoxy is 97%"
o
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
mean strength
reduced strength
due to defects
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 403
impact the laminate
over a range of energy levels
o monitor the type and size of damage
o test coupons for residual compression strength
o
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 counters
o
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 shock
o
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”