Palm Cards Flashcards

Question

Explain why fibers have high strength

Answer 1

The probability of a flaw being present is proportional to the volume of the materials for a given length (and fibres have very low cross sectional area). Flaws can be minimized by proper manufacturing and coating procedures Drawing and spinning impose high strains along the fibre axis produces a more favourable orientation of crystal structure High cooling rate or rapid molecular deposition produces ultrafine grained structures with much higher properties.

Answer 2

Glass is first melted at temps between 12500 and 14000 C Flows into an electrically heated alloy bushing which contains a large number of holes at its base The glass drops emerging from the holes are drawn into fibres at speeds of up to 50 m/s They are cooled by a water spray and coated with a “size” by a rolling applicator Finally the fibres are combined into a strand (of 52, 102 or 204 fibres) and wound on to a take-up spool

Answer 3

The cooling rate experienced by the fibres is very high (greater than 10,0000 C per sec) Hence the fibre structure is different from that of bulk glass, resulting in a higher tensile strength but lower elastic modulus and chemical resistance

Answer 4

Glass fibres, being essentially monolithic, linearly elastic brittle materials, are very sensitive to flaws in the form of sub-microscopic inclusions and cracks Commercial fibres are particularly prone to abrasion against other fibres, resulting in a reduction in strength of up to 20% compared to pristine fibres Humid environments reduce the strength of glass fibres under sustained loading due to adsorption of moisture on to the surface causing “static fatigue” Individual fibre strength is reduced by about 50% in a PMC, but the bundling of the fibres overcomes this effect.

Answer 5

1. E-Glass: Calcium Alumino-Borosilicate used for electrical applications due to its higher electrical resistivity. 2. S Glass: Magnesium Alumino-Silicate, used for structural applications due to its higher strength and low cost

Answer 6

Consists of a lubricant, binders such as starch and polyvinyl alcohol (to hold filaments together) and primers to improve adhesion between fibres and matrices. Reduces damage caused by friction upon fibres.

Answer 7

1. High specific strength and stiffness 2. High Modulus, High Strength, Intermediate Modulus 3. Organic precursor materials (PAN and Pitch) by a process of carbonization.

Answer 8

PolyAcryloNitrile is an acrylic textile fibre

Answer 9

By wet or dry spinning of the polymer or co-polymer, and then stretching. Stretching of fibres during spin reduces the diameter, aligns more molecular chains along the length, increasing stiffness

Answer 10

Stage 1 – Stabilisation: PAN is first stabilised in air at about 2500C by oxidation to form a thermally stable ladder polymer, with a high glass transition temperature (Tg). Stage 2 – Carbonisation: Removal of N, O and H in an inert nitrogen atmosphere at 1200-16000C. Fibres develop full strength at 1500-16000C. Basal planes align along the fibre axis, but remain as extended 2D ribbons. Stage 3 – Graphitisation: Final heat treatment at 1500-25000C in an inert atmosphere (Argon), wherein basal layers grow and coalesce along the fibre direction, to provide higher E (up to 380 GPa) at the cost of lower strain capability (0.7%) and strength.

Answer 11

Precursor made of organic compounds Cheap but poor mechanical properties Low compression, shear and tensile properties. High electrical and thermal conductivity. No tension required to develop molecular orientation (unlike PAN).

Answer 12

Stage 1 – Isotropic to Mesophase Pitch: Isotropic pitch is subjected to prolonged heating in an inert atmosphere at 400- 4500C to form a liquid crystal phase (mesophase) which is meltspun into fibre form Stage 2 – Cross linking: To reduce relaxation, the pitch fibres are cross linked by heating for a short time at 3000C in atmosphere containing O2. Stage 3 – Pre Carbonisation Stage 4 – Carbonisation Stage 5 – Graphitisation

Answer 13

Very hard Difficult to machine Chemical vapor deposition

Answer 14

Superior Specific Properties: Higher specific properties compared to glass fibers. Tensile Properties: Good tensile properties up to 400°C. Compressive Strength: Poor compressive strength limits applications. Energy Absorption: High energy absorption during fracture. High strain-to-failure. Applications: Used in ballistic protection and engine containment rings Anisotropic Properties: Strong covalent bonds within chains and weak hydrogen bonds between chains result in anisotropic properties. Non-linear Compression: Highly non-linear under compression due to kink band formation from in-phase compressive buckling of fibrils. Thermal Stability: Strength reduces by about 20% at 180°C, with rapid decline thereafter. Hygroscopic Nature: Absorbs moisture (around 4% at 60% relative humidity for Kevlar 49), but tensile strength remains largely unaffected. Creep Characteristics: Significant short-term creep, negligible long-term creep. UV Degradation: Susceptible to UV degradation, but protected in PMCs by resin matrix.

Answer 15

Tough Operate best under 100 deg temps Poor creep behavior Low compressive strengths due to kink band formation Specific strengths comparable to aramids.

Answer 16

1. Mats 2. Woven Fabrics 3. Warns 4. Tows 5. Rovings 6. Braided Fabrics 7. Non-Crimp Fabrics 8. Tapes 9. 3D textile preforms

Answer 17

Provides shape Holds the fibres together Transfers load in and out of fibres Separates the fibres to prevent failure from adjacent fibres Protects fibres from environmental damage Determines “matrix dominated” properties: - Longitudinal compressive strength - Transverse tensile strength - Intra and interlaminar shear strength

Answer 18

1. Low temp thermosets (epoxies, polyesters, vinyl esters, phenolic resins) = 100 - 150 deg 2. Medium temp thermosets (Bismaleimides) = Up to 230 deg 3. High temp thermosets (Polyimides) = Up to 300 deg

Answer 19

1. Excellent chemical and mechanical properties at low temps 2. Low viscosity during curing, enabling easier forming 3. Low shrinkage 4. Good fiber adhesion 5. Tg increases with increasing cure temperature

Answer 20

Low toughness, and sensitive to impact damage Limited temp range Absorbs moisture Sensitive to UV exposure High cost compared to polyesters Less convenient curing compared to polyesters

Answer 21

1. Diluents Added to reduce viscosity before cure 2. Flexibilizers Added to reduce elastic modulus and increase elongation to failure 3. Toughening agents Increase fracture toughness and reduce crack propagation rates 4. Inert fillers, such as hollow glass microspheres: Added to alter density, resin flow and effective modulus

Answer 22

Formation of a solid solution with a more ductile polymer Precipitation of elastomeric second phase Development of interpenetrating polymer networks The inclusion of elastomeric second phase can be achieved by adding an elastomer (rubber) to form a copolymer with the base resin which then precipitates out upon curing to form a dispersed second phase Adding a very fine powder to form a dispersion

Answer 23

Initial low viscosity that permits good wetting of the fibres Low cost (inexpensive and readily available raw materials, easy long term storage) Cure conditions can be modified easily without too much expertise Easy manufacture of modifications for a variety of specific applications Excellent environmental durability

Answer 24

High exotherm and high shrinkage on cure (this leads to residual stresses and poor interfacial bonding with fibres) Lower values for mechanical properties than epoxies Systems with adequate shear strength tend to be brittle (low toughness) Toughening mechanisms are relatively ineffective Poor chemical resistance

Answer 25

Intermediate class of materials between epoxies and polyesters The major ester ingredient is produced by reaction of a standard epoxy and methacrylic acid The resins are cured by free radical reaction process like polyesters Tougher and more chemically resistant than polyesters due to less cross linking Large range with different mechanical properties, including rubber toughened resins

Answer 26

Advantages: Combines chemical resistance of epoxies with easy processing of polyesters Lower cross linking, hence better mechanical properties than polyesters Improved interfacial bond strength with fibres Disadvantages: High cost compared to polyesters Higher shrinkage levels than epoxies

Answer 27

Condensation polymerisation of phenol with formaldehyde under alkaline or strong acid conditions is used to produce a pre-polymer called “resol”. This is polymerised under heating or with acidic or basic catalysts to form densely cross-linked structure forming the matrix Water and volatile byproducts are formed in the reaction, hence high pressures are required for curing High void content

Answer 28

Advantages Excellent resistance to high temperatures, especially under oxidizing conditions Phenolics have good ablation properties, forming char readily, yielding a superficial layer of carbon, which burns away protecting the underlying composite Good as fire retardant materials Disadvantages High pressures needed for polymerisation, hence difficult to fabricate Significantly lower mechanical properties due to high void content

Answer 29

1. Condensation polyimides - Thermoplastics used as matrices or toughening mechanisms in high temperature epoxy formulations 2. Addition polyimides thermoSETs. In general they release large quantities of volatiles unless high temperatures and pressures are employed, hence fabrication is complex and expensive 3. Very high Tg

Answer 30

Advantage: Stability at high temperatures (up to 3000C) Resistance to most chemicals Can be formulated to give better mechanical properties Disadvantages High Cost Difficulty in processing

Answer 31

To orient the fibres in the appropriate directions and proportions to obtaine the desired 2D or 3D properties Produce the required shape of the component Verify properties To ensure that the fibres state and orientation Ensure free of defects

Answer 32

Fibres laid up in open mould, with resin added by brushing or spraying Labour intensive, with challenging quality control Cost effective

Answer 33

“Pre-pregs” are sheets of fibre reinforcement pre-impregnated with partially cured resin. Safer, cleaner, more consistent, less labor intensive Costly with limited shelf life

Answer 34

1. Wet Lay Up 2. Pre-Preg 3. Wrapping 4. Compression Die Moulding 5. Open Die Moulding

Answer 35

Refridgerated

Answer 36

Resin Transfer Moulding Resin Film Infusion Vacuum Assisted RTM

Answer 37

Advantages: Can be used for manufacturing complex shapes and parts Materials can be selected to obtain the desired properties Process can be tailored to each individual type of component to maximise efficiency Disadvantages: Tooling is very costly Initial set up time is time consuming and expensive Process is time consuming and laborious High operator skill and experience required Very low through put

Answer 38

Process that enables continuous reinforcement to be laid down at high speed and precision in predefined paths The process involves impregnating continuous fibres with resin and winding them on to a stationary or rotating mandrel.

Answer 39

Advantages: High speed and high precision process Fully automated High fibre volume ratios can be achieved Can be employed to make very large components Disadvantages: Tooling is very costly Initial set up time is time consuming and expensive

Answer 40

Highly automated, continuous linear process for manufacturing of components with constant cross-sectional profiles with fibre reinforced PMCs The process essentially involves impregnating continuous fibres with a thermosetting resin and pulling them through a heated die to shape and cure

Answer 41

Advantages: Continuous process Fully automated High throughput Can use inexpensive forms of reinforcement Can be employed to make very long components No lamination or autoclave curing is required No requirement to refrigerate raw materials Disadvantages: Tooling can be costly Initial set up time is time consuming and expensive Process can only be applied to parts with constant cross section

Answer 42

- Very viscous, so liquid resin moulding techniques don't work - Requires only heat and pressure over a short period of time - High temp and High Pressure

Answer 43

1. Has the same material properties in all directions (infinite number of planes of symmetry) 2. Has different material properties in different directions with no planes of symmetry 3. Has three planes of symmetry for its material properties 4. Material with one plane (at every point) in which the properties are the same in all directions

Answer 44

Longitudinal: Stiffness in fiber direction = E1 Transverse: Stiffness across fiber direction = E2

Answer 45

Major = v12 = -E2/E1 Minor = v21 = -E1/E2

Answer 46

(V12/E1) = (v21/E2)

Answer 47

1. Mechanics of materials approach 2. Theory of elasticity approach 3. Finite Element Modelling

Answer 48

The fibres are assumed to be homogeneous, linearly elastic, isotropic, regularly spaced, perfectly aligned and of uniform strength The matrix is assumed to be homogeneous, linearly elastic and isotropic The fibre matrix interface is assumed to be perfect with no voids or dis-bonds The lamina is orthotropic along its principal directions The composite is considered to be homogeneous

Answer 49

E1 = Ef*Vf + Em * Vm For: - E1 = Longitudinal Modulus - Ef = E of fibres - Vf = Volume fraction of the fibers - Em = E of Matrix - Vm = Volume fraction of matrix (1-Vf)

Answer 50

In Plane Shear Modulus

Answer 51

Tensile strength in fiber direction (1)

Answer 52

1. E-Glass: moderate strength and stiffness, low cost, non-structural and semi-structural applications 2. S-Glass: higher strength and higher stiffness than E-Glass, cost comparable to that of Carbon/epoxy, employed for some structural components 3. D-Glass: better dielectric properties than E-glass, protection against lightning strikes

Answer 53

Cheaper for non or less load bearing structures Density comparable to that of Boron/epoxy (B), higher than aramid/epoxy (A) and carbon/epoxy (C) Lower specific stiffness than all three (A,B and C) Not employed for weight critical load bearing structures Better dielectric properties than other PMCs

Answer 54

Poor comparative fatigue performance Particularly vulnerable in hot and wet conditions due to low conductivity and moisture ingress tendencies

Answer 55

Superior ability to absorb impact energy Degrades significantly in extreme environments

Answer 56

Advantages of GFRP Low cost Superior Impact Resistance Better dielectric Properties Disadvantages of GFRP Lower specific stiffness than all other PMCs Poorer fatigue performance Susceptible to Stress rupture Degradation of Properties due to moisture and other aggressive environments

Answer 57

Advantages: - Low density - Mid-range specific strength and stiffness - Low creep rate - Good tensile properties - High impact resistance Disadvantages: - Poor compressive strength - Poor fiber adhesion - Difficult to machine - High absorption

Answer 58

1. High Strength 2. High Modulus 3. Intermediate Modulus 4. Ultra High Modulus

Answer 59

1. PAN: Low Cost, higher strain to failure. Less stiff 2. Pitch: More conductive, More stiff. More expensive

Answer 60

1. Thermoplastics (*?) 2. Epoxy Resins 3. Bismaleimides 4. Polyimides

Answer 61

1. Strain-to-Failure 2. Compressive Strength 3. Interlaminar Shear

Answer 62

Carbon fibres are normally surface treated to develop adequate interfacial bonding. Too strong bonding is not desirable as the fibre should be capable of disbonding to alleviate local stress concentrations such as at microcracks

Answer 63

1. GFRP 2. Aramid 3. CFRP (least)

Answer 64

- Barely Visible Impact Damage -Can significantly affect the residual compressive strength and buckling resistance by causing significant reductions in the bending stiffness of the laminate

Answer 65

Maintain high fibre to matrix stiffness ratio so that the matrix strains are kept low to avoid matrix cracking

Answer 66

- Generally, fatigue resistant - At moderate strain levels fatigue resistance of CFRP is strain dependent with fibre bridged matrix cracking and disbonding - Fatigue of CFRP at low strain levels causes only a small amount of matrix cracking which can lead to environmental damage.

Answer 67

Glass fibres exhibit degradation of strength under fatigue Low modulus of glass fibres causes high matrix strains leading to matrix cracking, which exacerbates fatigue sensitivity by causing strain concentration and exposing fibres to environment

Answer 68

Between GFRP and CFRP

Answer 69

Allows damaged fibres to be isolated from the good fibres by matrix cracking, disbonding and formation of delaminations.

Answer 70

1. Cracking in the off axis plys accumulate leading to saturation of matrix cracks 2. Crack coupling and delaminations develop which slowly increase the stresses carried by the on-axis ply 3. Zero degree fibres begin to fail under the tensile load leading to final fracture

Answer 71

Minimal due to formation of microcracking and delamination

Answer 72

GFRP: Frequencies over 5 Hz can reduce fatigue performance due to accumulation of heat damage due to the poor thermal conductivity of glass fibres CFRP: Minimal

Answer 73

Depends on the ability of the matrix to support the fibres to avoid micro buckling which depends on interfacial bond strength and the presence of matrix cracks

Answer 74

Hot/wet conditions, due to lowering of the Tg and softening of the matrix

Answer 75

CFRP - The BVID delaminations grow marginally under fatigue loading, which leads to a small drop in fatigue strength or fatigue life. - Greater effect on static residual strength

Answer 76

hydrogen bonding with polar sites in the polymer molecule

Answer 77

1. Epoxies most 2. Phenolics least

Answer 78

Fick's Law

Answer 79

1. High Temp: vaporizes leading to matrix cracking 2. Low temp: freezing = expansion 3. Room temp: plasticiser of the resin system and degrades the mechanical properties at room temperature

Answer 80

Reduces. Lower high temp performance

Answer 81

1. Fiber: Compression and buckling 2. Matrix: Tensile strength and stiffnes

Answer 82

1. Control of raw materials 2. Monitoring of manufacturing 3. QA of finished product 4. In service damage monitoring

Answer 83

longitudinal tensile strength, elastic modulus, elongation, yield, density, twist, creep and sizing content

Answer 84

1. Mechanical 2. Chemical 3. Xray 4. Pressure 5. Optical 6. Ultrasonic

Answer 85

Measurement of temperature variations and electrical conductivity KNOW GRAPH (slide 13)

Answer 86

1. Fiber breaks 2. Matrix cracks 3. Delaminations

Answer 87

1. Pulse-Echo 2. Through transmission

Answer 88

A and C scans