An investigation on loading rate sensitivity s of environmentally fatigued fibrous composites

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1 ICMAT11-A-0010 An investigation on loading rate sensitivity s of environmentally fatigued fibrous composites Bankim Chandra Ray Metallurgical and Materials Engineering, National linstitute t of ftechnology, Rourkela, India

2 Focus Introduction In-service degradation Loading rate sensitivity Discussions through mechanical testing and scanning micrographs Remarks

3 COMPOSITES- the combination of two or more distinctly different materials to make an improved or superior material Service temperature (aerospace application) -50 C to 120 C Humidity 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 =57MPa Kevlar/Epoxy=38 MPa) Environmental attack can degrade fiber/matrix interface mostly by mechanochemical principle. Weak boundary layer(wbl) formation at the fiber/matrix interface and the bulk of matrix may change the locus of failure and also threshold value of failure. Promoting the interlaminar failure and/or propagation of crack through this Promoting the interlaminar failure and/or propagation of crack through this layer

4 Implications of environment exposure Exposure to elevated temperature mechanical properties( ),cracking, chalking and flaking of polymer. Mti Matrix microcrack Delamination critical failure modes Matrix microcrack damage leading to laminate failure Premature buckling Stiffness( ) Delamination Moisture intrusion( i )

INTERFACE(2D)/INTERPHASE(3D) Matrix swelling strain Interfacial stress

Weak interface Brittle failure broomlike failure debonding/fiber pull-out

polymer film, thus it may be susceptible to perish by environmental shock.

5 INTERFACE(2D)/INTERPHASE(3D) Matrix swelling strain Interfacial stress transmissibility mechanical Plasticization degradation Strong interface Weak interface Brittle failure broomlike failure debonding/fiber pull-out The interface-interphase in glass fiber composites is assumed to be a polymer film, thus it may be susceptible to perish by environmental shock. Smeared out contributions from a background of weak glass transition temperature.

6 Experimental work Changing grelative humidity yat constant temperature shock conditioning Changing gtemperature eat constant relative humidity shock conditioning Polymer composites Resin(Epoxy and unsaturated polyester) Fiber(E-glass)wt%-55,60,65 Polymer composites Resin(Epoxy and unsaturated polyester) Fiber(E-glass)wt%-55,60,65 Laminated SBS specimen (ASTM D ) Laminated SBS specimen (ASTM D ) Humidity chamber 60%RH at 50ºC-1hr 19 Cycles Another humidity chamber 95%RH at 50ºC-1hr 19 cycles Humidity chamber 60%RH at 50ºC-1hr Another humidity chamber 60%RH at 70ºC-1hr

7 Environmental and experimental variables 1 mm/min( ) 10mm/min( ) 1mm/min( ) 10mm/min( ) plain( ) frozen( ) 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. Enviromental exposure interfacial stress transmissibility( ) (due to chemical degradation,mechnaical d i degradation, d matrix plasticization) i Fiber/matrix interface interfacial strength(chemical bond, secondary forceses of attraction, mechanical keying factor at interface) Energy absorption mechanism (under high loading) (includes mode II shear, matrix cracking, translaminar fracture by fiber rupture/kinking)

Micromechanics of damage and degradation of FRP composites GFRP at

Riverlines are natural develpoment of scraps and convergence of

The directioon of crack growth is the direction in which the

$microcracking Fiber pull-out Fiber fracture GFRP at +60 C for 1 hr$ $Scraps (cleavage steps) form due to low fracture energy.$ a loss of patch bonding tends to occur over time, particularly if

a loss of patch bonding tends to occur over time, particularly if

8 Micromechanics of damage and degradation of FRP composites GFRP at -80 C for 1hr Riverline marks Toughned matrix Texture microflow Riverlines are natural develpoment of scraps and convergence of crack planes. The directioon of crack growth is the direction in which the riverlines merge. scraps Fibers Potholes Fiber imprint GFRP at -80 C for 2 hr Matrix microcracking Fiber pull-out Fiber fracture GFRP at +60 C for 1 hr Scraps (cleavage steps) form due to low fracture energy. Scraps cleavage In low temperature, t due to shrinkage hi of matrix, a loss of patch bonding tends to occur over time, particularly if freeze-thaw cycling occurs which is assisted by potholes in the matrix region Matrix microcracking is assumed to form when the total energy released by the formation of the microcrack reaches a critical value.

9 Micromechanics approach Microcrack nucleus of macroscopic fracture. Work of Fracture/Fracture process (energy dissipated per unit area) R total =R surf +R pull-out +R redist R surf =V f R f +(1-V f )R m +V f l c R if /d Where R total= The specific work of fracture due to new surface created, R pull-out =Due to fiber pull-out, R redist = Due to mechanism of stress redistrubution. and V f = The volume fraction of fiber, l c = The ineffective length of fiber, R f =Specific work of fracture of fiber, Rm=Specific work of fracture of matrix, R if = Interfacial fracture toughness, d= Fiber diameter The fracture processes are controlled by Thermal relaxation time(τ) 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 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.

10 Glass transition temperature (Tg) and relaxation time A glass transition is characterized by unfreezing of molecular mobilities upon warming. The physical cause of glass transitions of polymers are molecular parts become mobile and take part in a weak glass transition. amorphous polymers allows 2 positions change: 1.Tunneling 2. jumping Place changes by tunneling/jumping processes are statistical processes which take some time the so-called relaxation time. (a) Jumping (b) tunneling Relaxation time temperature (*become longest at low temperature) At each glass transition the free volume is changed.

11 Failure modes Humid aging is recognized as one of the main causes of long-term failure of FRP s. Embrittlement of Differential Osmotic cracking macromolecular skeleton by swelling hydrolysis Humid aging Fiber /matrix interface failure Plasticization of matrix Hygrothermic shock

12 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.

13 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.

14 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 weight fraction glass fiber reinforced epoxy laminated composites.

15 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.

16 Environmental exposure results in reduced interfacial stress transmissibility due to matrix plasticization, chemical degradation and mechanical degradation. Matrix plasticization reduces matrix modulus. Chemical degradationd hydrolysisof interfacial i lbond Mechanical degradation Moisture uptake rate ( ) Local stress threshold for delamination matrix swelling strain higher temperature during hygrothermal ageing The rate of moisture diffusion controlled by diffusivity. Diffusivity Temperature(strong function) Humidity (weak function)

17 Fig 5:Moisture absorption kineties of carbon/epoxy composites at 60 C temperature and 95% RH, and at 70 C temperature and 95% RH.

18 Fig 6:Variation of ILSS values of carbon/epoxy composites with the absorbed moisture at two different hygrothermal conditions

19 Fig 7:Moisture absorption kineties of glass/epoxy composites at 50 C temperature and 95% RH, and at 70 C temperature and 95% RH.

20 Fig 8:Variation of ILSS values of glass/epoxy composites with the absorbed moisture at two different hygrothermal conditions

21 Fig 9:Comparison of ILSS values of carbon/epoxy and glass/epoxy composites with the absorbed moisture at two different hygrothermal conditions.

22 SEM Analysis Matrix cracking Fiber breakage Interfacial cracking

The state of fiber/matrix interface after humid ageing g may introduce more complications in evaluating the strain rate

23 GFRP at +60 C for 2 hr Fiber /matrix debonding Riverline marking Fiber pull-out Good adhesion fiber/matrix Fiber The fiber/matrix interacial bond greatly influences the mechanical behavior of composite material. The state of fiber/matrix interface after humid ageing g may introduce more complications 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 high h speed may become a limiting iti factor for the composite strength. th

24 Summery Both short-termterm and long-term properties of a composites depend decisively on the microstructure and properties of interface/interphase between the fiber/matrix. The rate of degradation of mechanical properties of a composite laminate could be higher than that of the individual constituents due to the synergy among the different degradation mechanisms. Composite structures must be designed to withstand the great diversity of environments, such as large variations iti in temperaturet and moisture. it New causes of failure in composite materials are still being uncovered as service experience is gained. The rapid advancement of these materials has outstripped the understanding of appropriate failure analysis techniques. A need probably exists for an assessment of mechanical performance of such potentially promising materials under the influence of changing environment and loading rate.

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

26 Thanks Questions and Queries Please

Thermal Shock and Thermal Fatigue on Delamination of Glass Fiber Reinforced Polymeric Composites

Thermal Shock and Thermal Fatigue on Delamination of Glass Fiber Reinforced Polymeric Composites B. C. RAY Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela-769008,