An investigation on loading rate sensitivity of ICMAT11-A-0010
es ga o o oad g a e se s y oenvironmentally fatigued fibrous composites
Bankim Chandra Ray
Metallurgical and Materials Engineering, N ti l I tit t f T h l R k l I diNational Institute of Technology, Rourkela, India
FIntroduction
Focus IntroductionIn-service degradationLoading rate sensitivityg yDiscussions through mechanical testing and scanning micrographs
kRemarks
COMPOSITES-‘the combination of two or more distinctly different materials to make an improved or superior material’p pService temperature (aerospace application)
50 C to 120 C-50 C to 120 CHumidity 0% RH to 96%RH
A judicious selection of fiber, matrix and engineered interface
better combination of strength and modulus(Bond strength between E-Glass/Epoxy =33MPa ,Carbon/Epoxy =57MPaKevlar/Epoxy=38 MPa)Kevlar/Epoxy=38 MPa)
Environmental attack can degrade fiber/matrix interface mostly by mechano-chemical principle.Weak boundary layer(WBL) formation at the fiber/matrix interface and the bulk ofWeak boundary layer(WBL) formation at the fiber/matrix interface and the bulk ofmatrix may change the locus of failure and also threshold value of failure.
Promoting the interlaminar failure and/or propagation of crack through thisPromoting the interlaminar failure and/or propagation of crack through thislayer
Implications of environment exposureExposure to elevated temperature mechanical properties( ) crackingExposure to elevated temperature mechanical properties( ),cracking, chalking and flaking of polymer.
M t i i kMatrix microcrackcritical failure modes
Delamination
Matrix microcrack damage leading to laminate failure
Premature bucklingStiffness( ) DelaminationM i t i t i ( )Moisture intrusion( )
INTERFACE(2D)/INTERPHASE(3D)Matrix swelling strainI f i l i ibili h i lInterfacial stress transmissibility mechanical Plasticization degradation
Strong interface Weak interface
B ittl f il b lik f ilBrittle failure broomlike failure
debonding/fiber pull-outThe interface-interphase in glass fiber
composites is assumed to be a polymer film,thus it may be susceptible to perish byenvironmental shock.
Smeared out contributions from a background of weak glass transitiontemperature.
Experimental workChanging relative humidity at constant Changing temperature at constant C a g g e a ve u d y a co s a
temperature shock conditioning
Polymer composites
C a g g e pe a u e a co s arelative humidity shock conditioning
Polymer compositesPolymer compositesResin(Epoxy and unsaturated polyester)
Fiber(E-glass)wt%-55,60,65
Polymer compositesResin(Epoxy and unsaturated polyester)
Fiber(E-glass)wt%-55,60,65
Laminated SBS specimen(ASTM D2344-84)
Laminated SBS specimen(ASTM D2344-84)
Humidity chamber60%RH at 50ºC-1hr
Humidity chamber60%RH at 50ºC-1hr60%RH at 50 C 1hr
19 Cycles Another humidity chamber
95%RH 50ºC 1h
60%RH at 50 C 1hr
Another humidity chamber60%RH 70ºC 1h
19cycles
95%RH at 50ºC-1hr 60%RH at 70ºC-1hr
Environmental and experimental variables
1 mm/min( ) plain( )1 mm/min( )10mm/min( )
1mm/min( )10mm/min( )
p ( )frozen( )
Enviromental exposure interfacial stress transmissibility( )(d t h i l d d ti h i l d d ti t i l ti i ti )
A) GFRP at 70ºC and 95%RH B) GFRP at 60ºC and 95%RH C) Comparison of ILSS values of at -6ºC for 24 hr plain moisture and frozen one.
(due to chemical degradation,mechnaical degradation, matrix plasticization)
Fiber/matrix interface interfacial strength(chemical bond, secondary forceses of attraction, mechanical keying factor at interface), y g )
Energy absorption mechanism (under high loading)
(includes mode II shear matrix cracking translaminar fracture by fiber rupture/kinking)(includes mode II shear, matrix cracking, translaminar fracture by fiber rupture/kinking)
Micromechanics of damage and degradation of FRP compositesp
Riverlines are naturaldevelpoment of scraps andconvergence of crack planes.
Th di ti f k
GFRP at -80°C for 1hr
Riverline marks
Toughned matrixTexture microflow
The directioon of crackgrowth is the direction inwhich the riverlines merge.
scraps
Fibers
Scraps (cleavage steps) form due to low fracture energy.
Scraps cleavageI l t t d t h i k fPotholes
Fiber imprint
In low temperature, due to shrinkage ofmatrix, a loss of patch bonding tends to occurover time, particularly if freeze-thaw cyclingoccurs which is assisted by potholes in thematrix region
GFRP at -80°C for 2 hr
Matrix microcrackingFiber pull-out
Matrix microcracking isassumed to form when thetotal energy released by theformation of the microcrack
GFRP at 80 C for 2 hr
pFiber fracture
formation of the microcrackreaches a critical value.
GFRP at +60°C for 1 hr
Micromechanics approachMicrocrack nucleus of macroscopic fracture.Work of Fracture/Fracture process (energy dissipated per unit area)Work of Fracture/Fracture process (energy dissipated per unit area)
Rtotal=Rsurf+Rpull-out+Rredist
Rsurf=VfRf+(1-Vf)Rm+VflcRif/d
Where Rtotal= The specific work of fracture due to new surface created, Rpull-out=Due to fiber pull-out, Rredist= Due to mechanism of stress redistrubution.
and Vf= The volume fraction of fiber, lc= The ineffective length of fiber, Rf=Specific work of fracture of fiber, Rm=Specific work of fracture of matrix, Rif= Interfacial fracture toughness, d= Fiber diameter
The fracture processes are controlled by Thermal relaxation time(τ)
Mechanical relaxation time(τ1)Mechanical relaxation time(τ1)1.Loading time > mechanical relaxation time (low strain rate)
Small isothermal, plastic deformations at the crack tip increase the crack resistance and make the fracture strain higher.
2.Loading time < mechanical relaxation time(medium strain rate)Time is not sufficient for plastic deformations at the crack tip, polymer becomes more brittle and fracture strain is decreased.
3. Loading time > thermal relaxation time (High strain rate) Increased heat power generation at the crack tip by deformation and fracture ,but most of the heat is removed by thermal conductivitythermal conductivity.
4.Loading time < thermal relaxation time(Very high strain rate) Heat generation is faster than its removal at the crack tip occurs which enhances plastic deformation, fracture stress and strain are increased.
Glass transition temperature (Tg) and relaxation time A glass transition is characterized by unfreezing of molecular mobilities upon g y g p
warming.The physical cause of glass transitions of polymers are molecular parts
become mobile and take part in a weak glass transitionbecome mobile and take part in a weak glass transition.amorphous polymers allows 2 positions change:
1.Tunneling2. jumping
Place changes by tunneling/jumping processesare statistical processes which take some timethe so-called relaxation time
(a) Jumping (b) tunnelingthe so-called relaxation time.
Relaxation time temperature(*become longest at low temperature)
At each glass transition the free volume is changed.
Failure modesHumid aging is recognized as one of the main causes of long-term failure ofHumid aging is recognized as one of the main causes of long-term failure of FRP’s.
Embrittlement of macromolecular skeleton by Differential
lliOsmotic cracking yhydrolysisswellingOsmotic cracking
Humid aging
Hygrothermic shockPlasticization of matrixFiber /matrix
interface failure
Nature of moisture uptake kinetics
Fig 1 :Percentage of moisture gain against the number of changing humidity cycles for 55% (●), 60% (▲), and 65% (■) weight percentage of reinforcement glass/epoxy composites.
Fig 2:Percentage of moisture gain against the number of changing humidity cycle for 55% (●), 60% (▲), and 65% (■) weight percentage of reinforcement glass/polyester composites.
Fig 3:Variation of ILSS with number of humidity shock ( at constant temperature)Fig 3:Variation of ILSS with number of humidity shock ( at constant temperature)cycle at 2mm/min (▲) and 50mm/min (♦) crosshead speeds for 0.60 weightfraction glass fiber reinforced epoxy laminated composites.
Fig 4:Variation of shear strength with number of hygrothermal shock (at constant RH) cycle at 2mm/min (▲) and 50mm/min (♦) crosshead speeds for 0.60 weight fraction glass fiber reinforced epoxy laminated composites.
Environmental exposure results in reduced interfacial stresstransmissibility due to matrix plasticization chemical degradation andtransmissibility due to matrix plasticization, chemical degradation andmechanical degradation. Matrix plasticization reduces matrix modulus.
Ch i l d d ti h d l i f i t f i l b dChemical degradation hydrolysis of interfacial bondMechanical degradation matrix swelling strainMoisture uptake rate ( )
higher temperature duringLocal stress threshold for delamination hygrothermal ageing
The rate of moisture diffusion controlled by diffusivity.Diffusivity Temperature(strong function)
H idit ( k f ti )Humidity (weak function)
Fig 5:Moisture absorption kineties of carbon/epoxy composites at 60°C temperature and 95% RH, and at 70°C temperature and 95% RH.
Fig 6:Variation of ILSS values of carbon/epoxy composites with the absorbed moisture at two different hygrothermal conditions
Fig 7:Moisture absorption kineties of glass/epoxy composites at 50°C temperature and 95% RH, and at 70°C temperature and 95% RH.
Fig 8:Variation of ILSS values of glass/epoxy composites with the absorbed moisture at two different hygrothermal conditions
Fig 9:Comparison of ILSS values of carbon/epoxy and glass/epoxy composites with the absorbed moisture at two different hygrothermal conditions.
SEM Analysis
Matrix cracking Fiber breakage
Interfacial crackingInterfacial cracking
Fiber /matrix debondingGFRP at +60°C for 2 hr
g
Riverline marking
Fiber pull-out
Good adhesion fiber/matrixFiber
Th fib / i i i l b d l i fl h h i l b h i fThe fiber/matrix interacial bond greatly influences the mechanical behavior of composite material.
The state of fiber/matrix interface after humid ageing may introduce more g g ycomplications in evaluating the strain rate sensitivity of aFRP’s.
Epoxy resin is more ductile at low strain rate but the failure strain of a matrix at hi h d b li iti f t f th it t thhigh speed may become a limiting factor for the composite strength.
SummeryBoth short term and long term properties of a composites dependBoth short-term and long-term properties of a composites dependdecisively on the microstructure and properties of interface/interphasebetween the fiber/matrix.
The rate of degradation of mechanical properties of a composite laminatecould be higher than that of the individual constituents due to the synergyamong the different degradation mechanismsamong the different degradation mechanisms.
Composite structures must be designed to withstand the great diversity ofi t h l i ti i t t d i tenvironments, such as large variations in temperature and moisture.
New causes of failure in composite materials are still being uncovered asservice experience is gained. The rapid advancement of these materials hasoutstripped the understanding of appropriate failure analysis techniques.
A need probably exists for an assessment of mechanical performance ofsuch potentially promising materials under the influence of changingenvironment and loading rate.
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