Overview of mechanical properties of polymer matrix composites

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1 Overview of mechanical properties of polymer matrix composites Dr. Suhasini Gururaja Assistant Professor Aerospace Engineering, IISc, Bangalore (Parts of the material for this presentation has been borrowed from lecture notes of Prof. K. Lin and Dr. Patrick Stickler at University of Washington, Seattle. Some figures have been reproduced from open literature and used here for purely pedagogical purposes.) 1

2 History Composite materials as a scientific/engineering discipline is approximately 60 years old. Composites have been useful for thousands of years Animal hair was added to pottery to improve strength Straw-reinforced clay was used to make bricks (Exodus 5:7) Bitumen was embedded with papyrus reeds to build boats Achilles s shield was a composite laminate design (Homer s Illiad, xviii: ) Composite materials found in nature*: Wood: cellulose fibers in a lignin mating Bone: Collagen fibers in an apatite matrix *Mechanical Design in Organisms, Wainwright et al,

3 Introduction What is a composite material? Combination of two or more chemically distinct materials on the macroscopic scale tailored to achieve improved properties that neither constituents individually possess. Improved properties achieved include Improved specific strength, stiffness, durability, corrosion resistance etc. Classification of composites Polymer Matrix Composites (PMCs) Metal Matrix Composites (MMCs) Ceramic Matrix Composites (CMCs) 3

4 Potential structural advantages of advanced composites High Specific Strength (strength/density) High Specific Stiffness (modulus/elasticity) Tailored properties in load application direction Tailored CTE for critical components Excellent fatigue performance Depending on resin/matrix combination and design Corrosion resistant UV resistant Good dielectric 4

5 Specific property comparison 5

6 Key Differences between Composites and Metal Anisotropy Tailored Properties Fatigue and Corrosion Lighting protection Discontinuous stresses Delamination Damage Tolerance Environmental Effects Repairability Reduction in parts counts 6

7 Tailored Properties Composites Properties CAN be tailored Properties can be tailored by combining different percentages of 0 o, 45 o, -45 o and 90 o plies Optimal use of material Material properties can be tailored per loading requirements to meet design allowables while reducing overall weight Metals Properties CANNOT be tailored Properties are represented in fixed values that cannot be tailored Material CANNOT be optimized Structural performance can only be improved through changes in geometry, such as thickness, which adds to weight 7

8 Fatigue and corrosion Composites Better fatigue performance than metals in tension (relatively flat S-N curves) Compressive fatigue properties are not as good as those in tension Superior corrosion resistance for CFRP Galvanic corrosion occurs between CFRP and Al, Mg, Cd plate and steel. Metals Relatively poor fatigue properties in both tension and compression (more steep S-N curve) Poor corrosion resistance, especially in a cracked structure 8

9 Discontinuous Stresses In-plane strains: Continuous throughout the laminate thickness Constant under uniform extensional forces Distributed linearly under bending In-plane stresses: Generally discontinuous throughout the laminate thickness because each ply has different stiffness values Stresses due to stretching Stresses due to bending The strain-based design criteria are generally used in industry 9

10 Delamination Occurs in laminated composites Causes local bending and buckling in compressively loaded structures Can grow under normal and shear loads Careful designs needed at locations prone to delamination 10

Damage tolerance/repairability Composites Critical damage Impact, delamination Compression after Impact (CAI) strength is a key design parameter Damage growth Complicated by multiple damage types and

11 Damage tolerance/repairability Composites Critical damage Impact, delamination Compression after Impact (CAI) strength is a key design parameter Damage growth Complicated by multiple damage types and failure modes Current design is for no damage growth Damage assessment is more difficult surface and internal damage Most repairs are bonded repairs time consuming and require highly skilled labor Repair materials and adhesives are time sensitive Metals Critical damage Fatigue crack, stress corrosion Damage growth Crack growth can be reasonably well predicted using fracture mechanics approach Refer to FAR , Damage tolerance and fatigue evaluation of structures Damage detection techniques are well defined and surface damage can be easily found Most repairs are bolted repairs relatively easier and cheaper No shelf life of repair materials Quality of repair is easier to control 11

12 Environmental Effects/Thermal Stresses Composite properties are strongly affected by moisture, temperature, sunlight, microbes, release agents, solvents etc. Property reduction factors (knockdown factors) are used appropriately Due to mismatch in CTEs, thermal residual stresses exist in structures. Thermal stresses must be considered in composite tool design and manufacturing. 12

Classifications of reinforcements Continuous fibers: lengths are in effect infinite - Unidirectional Tape, Woven or braided Fabric Whiskers, short

13 Classifications of reinforcements Continuous fibers: lengths are in effect infinite - Unidirectional Tape, Woven or braided Fabric Whiskers, short fibers, and continuous fibers all have very small diameters relative to their length (high aspect ratio) *Prof K. Lin AA532 Notes, University of Washington 13

diameters (typically 1-100 mm) Whiskers: lengths < 10mm

14 Classifications of reinforcements Advanced composites modern Particulates: roughly spherical particles with diameters (typically mm) Whiskers: lengths < 10mm Short (or chopped ) fibers: length mm SMC and Preforms 14

Ready-to-cure part on mandrel Very good quality Excellent repeatability ~8mm

15 Laminate construction Fibers appear as ovals because they were cut at an angle to the 0º direction. Ready-to-cure part on mandrel Very good quality Excellent repeatability ~8mm dia carbon fiber Stacking cut plies into a desired sequence *Prof K. Lin AA532 Notes, University of Washington 127 mm ply thickness 15

16 Tensile Stress (ksi) Tensile properties of Fiber, Matrix and Composite Fiber Composite Matrix Strain (%) 16

Manufacturing Methods - Bag Molding Process 1. Mold surface covered with nonstick Teflon-coated glass fabric separator. 2. Prepreg plies laid up in desired fiber sequence and orientation. 3.

17 Manufacturing Methods - Bag Molding Process 1. Mold surface covered with nonstick Teflon-coated glass fabric separator. 2. Prepreg plies laid up in desired fiber sequence and orientation. 3. Porous release cloth and a few layers of bleeder papers placed on top of prepreg stack. 4. Complete lay-up covered with another sheet of Teflon-coated glass fabric separator, caul plate, and thin, heat-resistant vacuum bag. 5. Entire assembly placed inside autoclave where a combination of heat, external pressure, and vacuum is applied to consolidate and densify separate plies into a solid laminate. Note: To prevent moisture pickup, prepreg roll on removal from cold storage should be warmed to room temperature before use., 17

18 Bag Molding Process Typical two-stage cure cycle for a carbon fiber-epoxy prepreg : 1. First stage Increasing temperature up to 130 C (266 F). Dwelling at this temperature for nearly 60 minutes until the minimum resin viscosity is reached. During the temperature dwell, external pressure applied to prepreg stack that causes excess resin to flow out into bleeders. 2. End of temperature dwell Autoclave temperature increased to actual curing temp. of resin. Cure temperature and pressure maintained for 2 hours or more, until predetermined level of cure has occurred. At end of cycle, temperature slowly reduced while laminate still under pressure. Flow of excess resin from the prepreg is extremely important in reducing the void content in the cured laminate., 18

19 Bag Molding Process Typical two-stage cure cycle for a carbon fiber-epoxy prepreg, 19

20 Mechanical Properties of PMCs A material property is a measurable constant characteristic of a particular material, which can be used to relate disparate quantities of interest. Key Mechanical properties include: Stress tensor to strain tensor Temperature/Moisture to strain tensor Stress (or strain) to failure/cycles to failure Crack growth to failure 20

21 Anisotropic behavior Composites Anisotropic Properties are dependent upon directions Inhomogeneous Properties are different in different plies Mostly Brittle Linear stress-strain relation and low strain to failure Metals Isotropic Properties are the same in all directions Homogeneous Properties are the same in all directions Mostly Ductile Nonlinear stress-strain relation with a large plastic deformations 21

22 Anisotropic versus isotropic z Specimen 2 Specimen 1 Specimen 3 y x Three specimens machined at different orientations from a single parent block. 22

23 Anisotropic versus isotropic Specimen 1 (E xx ) Specimen 2 (E yy ) Specimen 3 (E zz ) Tensile tests of three individual specimens 23

24 Anisotropic versus isotropic Isotropic Anisotropic E E E xx yy zz E E E xx yy zz Anisotropic materials The value of Young s modulus depends on the direction within the material the modulus is measured A similar dependence on direction can occur for other mechanical properties (n s, CTEs, ultimate strengths, etc) PMCs are anisotropic at the structural level 24

25 Principal Material Coordinate System A thin uni-directional (UD) composite panel Two specimens machined from the UD panel Note: The coordinate system is the principal material coordinate system 25

26 Principal Material Coordinate System A thin braided composite panel Two specimens machined from the braided panel Note: In this case the principal material coordinate system is not aligned with the fiber direction 26

Anisotropic behavior PMCs are anisotropic at the structural level One of the most unusual features of anisotropic materials is that they can exhibit

27 Anisotropic behavior PMCs are anisotropic at the structural level One of the most unusual features of anisotropic materials is that they can exhibit coupling Coupling between normal stresses and shear strains Coupling between shear stress and normal strains Coupling exists between s xx and g xy 27

Coupling of bending and stretching deformations The extensional force can cause shear deformation for unbalanced laminates The extensional force can induce bending curvature for asymmetric laminates

28 Coupling of bending and stretching deformations The extensional force can cause shear deformation for unbalanced laminates The extensional force can induce bending curvature for asymmetric laminates A balanced laminate can develop twisting curvature under extensional forces Unbalanced and asymmetric laminates result in a larger bending deformation, lower natural frequencies of vibration, and lower critical buckling loads., 28

Uniaxial tensile test ASTM 3039 - Adhesively bonded tabs

tensile strain - Modulus of Elasticity - Poisson s ratio

29 Uniaxial tensile test ASTM Adhesively bonded tabs Properties - Ultimate Tensile Strength - Ultimate tensile strain - Modulus of Elasticity - Poisson s ratio *Vassilopoulos and Keller, Fatigue of FRCs, Springer,

30 Designation D3039 D5450 D695 D3410 D5467 D5449 D3518 D5379 D4255 Title Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials Standard Test Method for Transverse Tensile Properties of Hoop Wound Polymer Matrix Composite Cylinders Standard Test Method for Compressive Properties of Rigid Plastics Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials with unsupported Gage Section by Shear Loading Standard Test Method for Compressive Properties of Unidirectional Polymer Matrix Composites Using a Sandwich Beam Standard Test Method for Transverse Compressive Properties of Hoop Wound Polymer Matrix Composite Cylinders Standard Practice for In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a 45 Laminate Standard Test Method for Shear Properties of Composite Materials by the V- Notched Beam Method Standard Test Method for In-plane Shear Properties of Polymer Matrix Composite Materials by the Rail Shear Method D5448 Standard Test Method for In-plane Shear Properties of Hoop Wound Polymer Matrix Composite Cylinders 30

31 Yielding and Fracture of Composites Predicting fracture of multiangle composite laminates under general load conditions remains a challenging area of research. Three distinct "materials" regions may be defined The fiber The matrix The fiber-matrix interphase The mechanical properties exhibited by the polymer in the interphase region differ from bulk properties. The initial nonlinear deformations exhibited by a PMC are therefore almost entirely initiated within the polymeric matrix. The fracture process is initiated when one or more microcracks are formed in the matrix. 31

32 Yielding and Fracture of Composites Matrix Cracks Cracks that occur in the polymeric matrix, at some distance from the fiber/matrix interface. Matrix cracks generally occur in planes either parallel or perpendicular to the fiber direction. Fiber-Matrix Debonding The crack has formed in the interphase region, and a (non-planar) crack extends around the periphery of the fiber. Fiber Cracks Cracks that occur in the fiber itself. Fiber cracks almost always occur in a plane perpendicular to the axis of the fiber, and extend across the entire width of the fiber. 32

33 Yielding and Fracture of Composites Viscoelastic behavior: yielding and crack growth in polymers is a time-dependent phenomenon called creep. An increase in temperature and/or an increase in moisture content further accentuate the time-dependency. If a tensile stress is applied and held constant the composite may eventually fail due to slow crack growth (often called a "creep-torupture" failure). Chemical aging: polymers aging occurs due to ultra violate light. 33

34 Failure of Multi-angle Composite Laminate Multi-angle laminates are subject to failure modes that do not exist in unidirectional laminates. The initiation of delamination failures is often attributed to free-edge stresses. Free-edge stresses occur whenever adjacent plies possess differing Poisson ratios or coefficients of mutual influence. Pre-existing thermal and/or moisture stresses occur in multi-angle laminates. Due to a mismatch in effective thermal expansion and moisture expansion coefficients from one ply to the next. 34

35 Failure of Multi-angle Composite Laminate Additional damage mechanisms in composites include: Fiber-matrix debonding: a crack forms around the periphery of a fiber. Load can no longer be transferred from the matrix to the fiber. Fiber micro-buckling: fibers within a ply that experiences compressive stresses in the fiber direction buckle. Reduces the compressive stiffness exhibited by the ply. Leads to failure of the fibers due induced bending stresses. 35

36 Failure of Multi-angle Composite Laminate Failure modes for multiangle laminates. Matrix cracking/splitting (microcracks). Delamination. Fiber fracture. Fiber/matrix debond. Fiber kinking (microbuckling). Global laminate buckling. Failure should be verified experimentally. 36

37 Failure of Multi-angle Composite Laminate Experimental observations of the evolution of damage in a quasiisotropic laminate (monotonically increasing uniaxial load, N xx ) 37

38 Failure of Multi-angle Composite Laminate The 90 o plies yield as N xx increases to a critical level. Cracks begin to form in the 90 o plies at load levels above the first-ply failure stress. As the effective stress is further increased, cracks eventually begin to form within the ±45 plies As the effective stress is increased further, delaminations begin to develop. Matrix cracks begin to form between plies, and these new matrix cracks lie within planes that are parallel to the x-y plane 38

39 Failure of Multi-angle Composite Laminate The delaminated regions grow in size as the stress is increased and eventually coalesce, such that a delaminated region may extend across the entire width of the specimen. At still higher effective stress levels matrix cracks begin to form within the 0ºplies (often referred to as splitting ). These cracks lie within a plane perpendicular to the x-y plane. Final laminate fracture is precipitated by fiber failures within the 0ºplies. The effective stress level at which final fracture occurs is often called the last-ply failure stress. At final fracture the laminate fractures into fragments. Extensive and pre-existing matrix cracks and delamination that occurred at lower stress levels. Large amount of energy release associated with fiber failure. 39

40 Failure of Multi-angle Composite Laminate Reifsnider et al studied damage progression of multiangle composite laminates under fatigue loading. Material: graphite/epoxy. Layup: [0/±45/90] s Tension-tension fatigue spectrum. σ max = 0.62 σ ult σ min = σ ult R = (σ min / σ max ) = 0.1 s 0 s m N xx s a s max Ds s min 40

41 Failure of Multi-angle Composite Laminate Experimental observations of damage sequence under tension-tension fatigue load spectrum. Matrix cracks in 90ºplies. Matrix cracks in ±45ºplies. Delaminations. Matrix cracks 0ºplies (splitting). Fiber failure. Final fracture. Development of characteristic damage state Significant reduction in stiffness. 41

42 Failure of Multi-angle Composite Laminate Transverse Matrix Cracks Tension-tension fatigue 42

43 Constant Life Diagrams 0 o 45 o 90 o ASTM E *Vassilopoulos and Keller, Fatigue of FRCs, Springer,

composites shall be presented *Results from T.Briggs and M.

44 Case Study*: Tension-tension fatigue properties of chopped GFRPs SMC R27 SMC R37 Preform R25 Preform R40 Fatigue behavior of these four Compression molded composites shall be presented *Results from T.Briggs and M. Ramulu, "An Experimental Characterization of the Failure Mechanisms Activated in GFRP Composites" IMECE07, Seattle 44

45 Composite Material Composition Material Component SMC-R27 SMC-R37 Resin base: PG Maleate/PVA low profile Polyester Polyester Filler material Calcium Carbonate (MgO thickeners) Calcium Carbonate (MgO thickeners) Glass content by weight 27% 37% Material Component Preform R25 Preform-R40 Resin base w/lpa of thermoplastic Polyester Vinyl ester Filler material Clay Calcium Carbonate Glass content by weight. 25% 40% Fine glass veil 0.76 mm thick 0.76 mm thick 45

46 Compression Molding Process SMC Preform 46

47 Burn out Virgin Sample SMC R-27 SMC R-37 y x PreForm - R25 PreForm R40 47

48 UTS and tension-tension Fatigue Test Setup Ambient air thermocouple Fatigue specimen thermocouple MTS 89-KN static tensile and tension-tension fatigue load frame 48

49 Stress vs. Strain to failure Knee 49

50 Specimens after UTS Experiments SMC-R27 SMC-R37 Preform-R25 30 mm Preform-R40 50

51 Deg C Temperature versus Frequency HZ 5 HZ Minutes 51

52 Maximum Stress (MPa) Stress life plots R = 0.05, Room Temp y = ln(x) y = ln(x) y = ln(x) y = ln(x) Log Cycles SMC-R37 Preform-R40 SMC-R27 Preform-R25 52

53 Tensile Modulus (MPa) Tensile Modulus (MPa) Tensile Modulus (MPa) Tensile Modulus (MPa) Tensile Modulus degradation (a) SMC-R (c) Preform-R25 Log Cycles 70% UTS 60% UTS 50% UTS 40% UTS 30% UTS Log Cycles 70% UTS 60% UTS 50% UTS 40% UTS 30% UTS ,000 5,000 (b) SMC-R (d) Preform-R40 Log Cycles 70% UTS 60% UTS 50% UTS 40% UTS 30% UTS Log Cycles 70% UTS 60% UTS 50% UTS 40% UTS 30% UTS 53

54 Fractography - 70% UTS fatigue test (R = 0.05) Fiber bundle Pull-out Matrix fracture surface SMC-R27 Fiber fracture SMC-R37 Fiber bundle damage Preform-R25 Preform-R40 54

55 Low-Velocity Impact Effect of LVI on residual compression and tensile strengths of HTA/913 and HTA/982 CFRP laminates and E-glass/913 (normalized wrt undamaged material). [(±45,0 2 ) 2 ] s (Courtesy: Brian Harris, Fatigue in Composites, CRC Press, 2003.) 55

Introduction to Composite Materials

Structural Composite Materials Copyright 2010, ASM International F.C. Campbell All rights reserved. (#05287G) www.asminternational.org Chapter 1 Introduction to Composite Materials A composite material