Definition Composite Materials
A composite Material is made up of two or more constituents. The constituents have different properties, and when combined, have better characteristics.
What are typical values for the modules E?
Steel: 210 GPa
Aluminium: 70 GPa
UD carbon-epoxy composite: 165 GPa (at 60% FVC)
Historic examples of composites
-Wood (Lignin matrix with cellulose fibers)
-Straw-reinforced mud
Typical properties of CFRP
Modulus E ~ 200 GPa
Specific Modulus ~ 130 Gpa/(g/cm³)
Tension Strength X ~ 2.2 GPa
Specific Strength ~ 1.4 GPa/(g/cm³)
Typical properties of carbon fibers?
Tensile strength up to 6370 MPa (HT)
Modulus up to ~950 GPa (UHM)
Typical Fibers have E of ~300 GPa at X of ~5500 MPa
Density of 1.74-1.9g/cm³
Typical properties of glass fiber?
Tensile strength of 3400 MPa
Modulus of 73 GPa
Density of 2.6 g/cm³
What constitutes the design flexibility of composite materials?
Layup, Number of layers, FVC, fiber orientation, type of reinforcement
Positive charactetistics of composites, other than strenght and modulus
dimensional stability, fatique resistance, corrosion resitance, low life-cycle energy balance
What are FRP, GFRP and CFRP
fiber reinforced plastic, glass fiber reinforced plastic, carbon fiber reinforced plastic
what are WPC, MMC, and CMC
wood reinforced plastic, metal matrix composites, ceramic matrix composites
Whats the difference between Laminae and Laminate
Laminae: single play
Laminate: multiple plies (lay-up)
Whats the difference between filaments and a roving?
Filament: single strand of fiber
Roving: bundle of fibers (Up to 50000 for carbon)
What is the role of the fibers in a composite? What are their requirements?
They carry loads acting on the composite, and need high strength and stiffness (-> strong atomic bonds)
How are molecular bonds oriented into the right direction?
During the manufacturing process (spinning) the fibers are stretched
What types of fibers are there?
Natural, Organic, Inorganic and Metalic
What are some natural fibers used in composites?
Hair, wool, silk, cotton, flax, hemp
What are some organic fibers used in composites?
polyethylene, polyporpylene, polyamide, polyacrylonitrile
What are some inorganic fibers used in composites?
carbon, glass
What are some metalic fibers used in composites?
Steel, aluminium
What is Sizing?
SUbstance that’s applied to the surface of fibers to improve adhesion to the matrix and ease of handling
What are the functions of the matrix of a composite?
-fixation of the fiber
-load transfer among fibers
-support of fiber under compression
-joining of plies
-protection of fibers from the environment
What are the three types of polymers?
thermosets, elastomers and thermoplastics
What are advantages of thermosets?
-high modulus
-low creep
-thermal and chemical stability
-low viscosity during processing
What are disadvantages of thermosets?
-melting and welding not possible
-recycling is difficult
What are advantages of elastomers?
-very high strains >100%
-reversible deformation
What are disadvantages of elastomers?
-welding not possible
-not usable for structural composites
What are advantages of thermoplastics?
-meltable and weldable
-recyclable
-ideal for forming processes
What are disadvantages of thermoplastics?
-creep
-infiltration is difficult due to their high viscosity
-moisture absorption (PA)
What are dry semis and what three types of them are there?
fiber products that aren’t yet infiltrated with matrix material.
Types:
-woven fabrics
-Non-Crimp Fabric (fibers are stacked and fixated with stitching instead of woven)
-braided fabrics
What are advantages of thermoset prepregs?
-high fiber volume content
-homogenous fiber distribution
-good alignement of fibers
-low porosity (especially after autoclave curing)
-perfect mixing of resin and hardener
Most load-bearing structures in aerospace are made from them.
What are disadvantages of thermoset prepregs?
-storage at -18°C required
-limited shelf life
-expensive material
What are organo sheets?
Fully consolidated composite sheets with thermoplastic matrix
What are advantages of thermoplastic prepregs?
-can be stored at room temperature
-short cycle times
-in-situ consolidation
-local reinforcements possible by combining consolidation of laminate with back-injection molding process
What are disadvantages of thermoplastic prepregs?
-prone to creep
-high material cost
What’s the typical density of carbon fibers?
1.7-1.9 g/cm³
What are the positive characteristics of carbon fibers?
-high modulus (especially pitch based)
-high strength (especially PAN based)
-good thermal stability and conductivity
-excellent creep resistance
What are negative characteritics of carbon fibers?
-high cost
-low strain to failure
-oxidation at temperatures >450°C
What are possible precursors for the production of carbon fibers?
-PAN (Polyacrylonitrile)
-Pitch
-Natural fibers (cellulose, …)
What are the main steps in the process of manufacturing carbon fibers from PAN or Pitch fibers?
Oxidation/stabilization of the precursor at high temperatures ~300°C
Carbonisation at 1200°C-1400°C under nitrogen atmosphere-> thermal degradation of non-carbon atoms leads to ~50% weight loss.
Graphitization at up to 3000°C. High temperatures produce high modulus fibers, for high strenghts temperature is limited to ~1400°C
Surface treatment/sizing
How are PAN fibers produced?
1 Acrylonitrile is polymerised into Polyacrylonitrile
2 Pan is spun (usually solution spinning - wet spinning) and stretched
What are characteristics of carbon fibers produced from pitch and PAN?
Pitch: 80% carbon yield, can be graphitized for ultra-high modulus
low to medium strenght, market share <5%
PAN: 50% carbon yield, high orientation of molecular chains and low
amount of defects lead to high/ultimate strength, >95% market
share
What are positive characteristics of glass fibers??
-high dimensional stability
-good fire resistance
-moisture and chemical resistance
-good strenght-to-weight ratio
-low price
-high elongation (compared to carbon)
What are negative characteristics of glass fibers?
-low youngs modulus
-higher heat expansion than carbon or aramid
What are the main steps in the manufacturing process of glass fibers?
Batching/blending of raw materials
Melting (~1400°C)
Fiberization (through platinum/rhodium alloy bushing, “nozzle pulling”)
coating (lubricants, binders, coupling agents)
Drying and packaging
What are the main raw components of glass?
Silica/quartz (SiO2)
Limestone
Koalin
What are typical properties of glass fibers (E-Glass)?
-density of ~2.6 g/cm³
-tensile strength of ~3400 Mpa
-Modulus of ~73 GPa
What are the two types of aramid fibers? What are their applications?
Meta-aramids and Para-amids.
Meta: -fire resistand clothing
-hot gas filtration
-sandwich core material
Para: -body armour
-cut resistant gloves
-reinforcement in composites
What are positive characteristics of aramids?
-high stiffness
-high tensile strength
-low densitiy
-resistance to organic solvents
-non-conductive
What are negative characteristics of aramids?
-relatively low compressive strenght
-moisture absorption
-sensitive to radiotion (UV)
-difficult to handle (e.g. cutting)
How are aramid fibers manufactured?
Meta-aramids: dry spinning
Para-aramids: dry jet - wet spinning
How do the properties of the constituents of a composite compare?
E_f>E_m: fiber should have higher modulus
e_f<e_m: no damage of the matrix allowed before the fiber starts to fail
Rf>R_m: Fiber has higher strength -> reinforcement
What are polymers, what are plastics?
Polymer: Molecule consisting of several repeat units
Plastics: Polymer compound or blend with additives
What is the glass transition temperature T_g?
Temperature, at which the biggest change of material properties occurs during the glass transition state from the energy-elastic state to the enthalpy-elastic state
What properties increase or decrease during the glass transition?
Increase of:
thermal expansion coefficient
specific heat capacity
failure strain
Decrease of:
Elastic modulus and shear modulus
strength
What are the service temperature ranges of polymers?
Amorphous thermoplastics: Tservice < Tg
Semy-crystalline thermoplastics: Tservice<Tg
Elastomers: Tg<Tservice<T_decompostion
Thermosets: Tservice<Tg
What are general mechanical behaviours of polymers?
Thermosets: brittle
Thermoplastics: elastic-plastic
Elastomers: elastic
What is rheology?
Generalized study of the deformation of materialsm, focused on viscous behaviours and the measurement of viscosity
What material properties are measured using rheology?
Dynamic viscosity, complec viscosity and kinematic viscosity
What are the pot life and gel point of thermosets?
Gel point: point in time where the resin mass consists of one single molecule. Storage and loss modulus are equal.
Pot life: time wherein no apparent curing occurs and the resin can be used for infilitration
What are commonly used thermoset resin systems?
Epoxy resins (EP)
Unsaturated polyester resins (UP)
Vinyl ester resins (VE)
What are positive characteristics of epocy resins?
-low shrinkage during curing
-excellent fiber-matrix adhesion
-good fatique strength
-excellent elcetrical isolating properties
What are negative characterisitcs of epoxy resins?
-more expensive than UP-resins
-longer curing times than UP-resins
What are typical thermoplastic matrices?
Semi-crystalline:
-Polypropylene (PP)
-Polyamide (PA)
-Polyether ether ketone (PEEK)
Amorphous:
-Poly etherimide (PEI)
Whats the definition of micromechanics?
Study of composite material behaviour wherein the interactions of the constituent materials is examined on a microscopic scale to determine their effect on the properties of the composites.
What is the definition of macromechanics?
Study of composite material behaviour wherein the material is pressumed homogenous and the effects of the constituent materials are detected only as averaged apparent macroscopic properties of the composite
Which stress-states result in linear/non-linear response?
Matrix dominated stress states: non-linear response
FIber dominated stress states: linear response
Formulas for volume and weight fractions
FIber weight fraction: Wf=mf/mc
Matrix weigh fraction: Wm=mm/m_c
Fiber volume fraction: φ=vf/vc=Wf*ρc/ρf
Matrix volume fraction: vm/vc=Wm*ρc/ρm
void content: vp/vc=1-(vf+vm/v_c)
Rule of mixture
E||=Ef||*φ+Em*(1-φ)
Etrans=(φ/Etransf+(1-φ)/Em)^-1
How can failure modes of composite materials be differentiated?
Intralaminar failure: fiber failure and inter-fiber failure
Interlaminar failure: delamination
Failure mechanisms are different in tension, compression and shear
What are the fiber dominated failure modes?
Fiber failure under tension:
brittle failure of the fiber
failure of the fiber-matrix interface
Fiber failure under compression:
kinking of the fiber iniated by local defects such as fiber misalignment or waviness
What are the matrix-dominated failure modes?
Matrix failure under tension:
cohesive failure of the matrix
Matrix failure under compression:
failure offset by 53±2° to action plane
catastrophic wedge failure of the laminate
Matrix failure under in-plane shear:
two possible fracture planes
generally parallel to fiber direction
What is the purpose of mechanical testing?
Determining mechanical properties of composites
Determining material suitability for the design of composites
Material qualification during manufacturing
Determining input paramters for structural design
Determining the influence of holes, fasteners and damages
Determining the influence of temperature, moisture, strain rate
Experimental validation of structures
Material science and development
What’s measured with a Universal Testing Machine (UTM)?
Load
Displacement
Strain (if strain data acquisition system is available)
What are methods for measuring strains?
Strain Gauges: variable resistances adhered to the sample
Video Extensometer: camera detects increasing distance between lines on the sample
Digital Image Correlation: 2d or 3d cameras detect displacement of random patterns on the surface and create a strain field
What values are measured in tension testing?
Modulus E1t, poisson ratio v12, strenght Xt
Modulus E2t, strength Yt
What values are measured in compression testing?
Modulus E1c, strength Xc
Modulus E2c, strength Yc
What’s the normalization of test results?
By normalization, raw test values are adjusted to a single reference or specified FVC 𝜑𝑟𝑒𝑓. Normalization can be understood as the prediction of the mechanical properties of a material for the reference FVC.
What values are determined in shear testing?
Shear modulus G, Strength S_l
What tests are there for shear testing?
-shear-tension test with ±45° laminate
-Iosipescu test
-v-notched shear test
-shear frame test
What are advantages of the shear-frame test?
-no 5% shear strain limit
-pure shear
-usable for composites with thermoplastic matrix with failure shear strains above 5%
What values are detmined with bending tests?
-Flexural/bending modulus
-Bending strength
also: validation of tension or compression tests
What other variations of tension and compression testing are there?
-Open-hole tension and compression (for allowables of notched laminates)
-compression after impact
-tests with sample in a temperature chamber to test at varying temperatures and humidities
What’s a test for properties at high strain rates called?
Split Hopkinson Bar Test
What methods of Thermal Analysis are there?
-Thermo-mechanical analysis (TMA)
-Dynamic (Thermo) mechanical analysis (DMA, DTMA)
-Differential Scanning Calorimetry (DSC)
What is thermal analysis?
The measurement of geometrical change or change of mechanical properties of a sample as a function of temperature under a defined mechanical load
->thermal expansion coefficients, material softening
What is Dynamic (Thermo) Mechanical Analysis (DMA, DTMA)?
Measurement of the mechanical response of a sample as a fuction of frequency, temperature, and time.
-> storage modulus, loss modulus, glass transition temperature, visco-elastic material response
What is Differential Scanning Calorimetry?
Measurement of heat flow difference between a reference and a sample as a function of temperature and time.
-> Glass-transition, melt enthalpy, curing enthalpy, degree of curing, specific heat capacity
What are goals of non-destructive testing of composites?
-Safety and improves service life of parts and structures
-resource efficiency
What are the two main types of NDT of composites?
Ultrasound and Radiography
What are applications of ultrasound in NDT?
-material characterizations
-locating of geometrical deviations
part dimensions
cracks, damages
-analysis of imperfection
in adhesive bonds
in welded joints
How are ultrasonic waves generated and introduced into parts?
-generated using a piezoelectric transducer
-introduced into parts using coupling agents that reduce the difference in acoustic impedance (water, gel, air)
What are A-, B- and C-scans?
A-scan: single signal from ultrawaves in one spot
B-scan: Line scan - row of a scans
C-scan: Area scan - row of B-scans
What is computed tomography?
-2d scans of an object rotating around its axis, combined to form a 3d image with high resolution
What is Thermography?
Analysis of the infrared radiation of a sample,often in response to heating using optical light (Active thermography)
How are defects detected with thermography?
Area above cavities heat up -> hot spots in the image
What are performance characteristics of NDT techniques?
Sensitivity: how small a defect can be detected
Resolution: how easly closely located defets can be separated
Accuracy: how precisely a defect can be detected
Repeatability: how frequently the same detection or measurements
are obtained
Reliability: How many times the technique provides the correct answer
Applicability: practical use and usefulness of the tests
What are defect types in monolithic composites caused during manufacturing?
-delaminations
-porosity
-undulations (in-plane and out-of-plane waviness of rovings)
-fiber and ply misalignement
-fiber and matrix cracking
-contaminations
-incompletly cured matrix
-dry zones (incomplete impregnation of fibers)
What are defect types in monolithic composites caused during service?
-delamination
-failure of fiber-matrix interface
-fatique cracking
-degradation of matrix and/or fiber material
ingess of moisture
exposure to high temperature (lightning strikes, fire, engine exhaust)
UV-Radiation (critical for aramid fibers)
How to get volume fractions from weight fraction?
𝑣𝑖 = 𝑊𝑖 × 𝜌_𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 / 𝜌_i
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