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.

References Kelly A Zweben C Comprehensive Composite Materials Oxford U K ElsevierKelly A, Zweben C., Comprehensive Composite Materials. Oxford U.K, ElsevierScience Publication, 2000Jang B Z., Advanced Polymer Composites: Principle and Applications. ASMInternational, Materials Park, OH, 1994Kim K, Mai Y W., Engineered Interfaces in Fiber Reinforced Composites.Kidlington, Oxford ,U.K, Elsevier Publication, 1998Ray B C. Thermal shock on interfacial adhesion of thermally conditioned glassfiber/epoxy composites rates, Materials Letters, 58(2004):pp.2175-2177Ray B C. Effect of crosshead velocity and sub-zero temperature on mechanicalbehavior of hygrothermally conditioned glass fiber reinforced epoxy composites,Material Science and Engineering A 397(2004): pp 39 44Material Science and Engineering A 397(2004): pp.39-44Ray B C. Temperature effect during humid ageing on interfaces of glass andcarbon fiber reinforced epoxy composites, Journal of Colloid and InterfaceScience,298 (2006): pp. 111-117., ( ) ppRay B C. Effects of Changing Environment and Loading Speed on MechanicalBehavior of FRP Composites, Journal of Reinforced Plastics and Composites ,25(2006): pp. 1227-1240Greenhalgh E S. Failure analysis and fractography of polymer composites, Cambridge,UK ,CRC Publication, Woodhead Publishing, 2009

Th kThanksQ i d Q i PlQuestions and Queries Please