Composites Part 1 Flashcards
1
Q
What is a composite?
A
Two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct within the finished structure.Overall properties of the composites are superior to those of the individual components. Design goal: obtain a more desirable combination of properties (principle of combined action). E.g. low density and high strength.
2
Q
Why are metals and ceramics/polymers not good enough sometimes?
A
Metals have excellent strength and toughness combinations but they are quite dense and many corrode in use. Ceramics and polymers are lighter and more corrosion-resistant, but often lack toughness
3
Q
Examples of composites in nature?
A
Wood, bamboo, bones, muscles, tissue
4
Q
What are the different materials in concrete?
A
Cement, stones and steel armaments
5
Q
What are bricks and pottery reinforced with?
A
Plant fibers and straw
6
Q
Definition of an engineering composite?
A
multi-phase (2 or more) material that is artificially engineered
7
Q
What are the two phase types in a composite?
A
(1) Matrix - is continuous filler material
(2) Dispersed (particulate/fiber) - is discontinuous and surrounded by matrix
8
Q
What is the purpose of the matrix phase?
A
Transfer stress to dispersed phase (protect dispersed phase from environment)
9
Q
What are the different materials groupings in terms of matrix materials?
A
MMC - metal matrix composite
CMC - ceramic matrix composite
PMC - polymer matrix composite
10
Q
What are the different classifications of composites?
A
- Particle reinforced
- Fiber-reinforced
- Structural
11
Q
What are the different types of particle reinforced composites?
A
- Large particle
- Nanoscale dispersion
12
Q
What are the different types of fiber-reinforced composites?
A
- Continuous (long-aligned)
- Discontinuous (short) –> aligned or random orientation
13
Q
What are the different types of structural composites?
A
- Laminates
- Sandwich plates
14
Q
What are laminates?
A
Stacked and bonded fiver-reinforced sheets
15
Q
What is a benefit of laminates?
A
Balanced in-plane stiffness
16
Q
What are sandwich panels?
A
Honeycomb/foam core between 2 facing sheets
17
Q
What are 2 benefits of sandwich panels?
A
Low density. large bending stiffness
18
Q
What are some example applications of fiber reinforced plastics?
A
1 - Airplanes
2 - Teeth fillings
3 - Sport apparel: nike trainers, kevlar combat helmets and ballistic vests
4 - Marine current turbine and wind turbine )kevlar fiber/epoxy matrix due to high strength and low weight.
19
Q
What are the advantages of FRP composites?
A
- Good strength to weight ratio
- Complex shapes are easily accomplished e.g. double curvature
- Designer properties - can be optimised to meet stiffness, strength and manufacturing requirements
- Part consolidation to provide pre-fabricated/ pre-assembled product
- Texture and self-coloring
- Resistant to corrosion
- Resistant to fatigue damage with good damping characteristics.
20
Q
What are the disadvantages of FRP composites?
A
- Properties depend on processes (poor mechanical properties transverse to the fibers)
- Limited design skills and standards/guides
- Poor heat and fire resistance
- Expensive (high production cost tools required)
- Difficult to join (design process is more complex than metals
- Low temperature applications
21
Q
Why are fibers the most common method of reinforcing FRPs?
A
Fibers, especially long and continuous forms, provide the stiffest and strongest materials and it is for this reason that they are also the most common method of reinforcing FRPs.
22
Q
Why are the properties of FRP highly dependent on the alignment of the fiber?
A
Because the introduction of fibers into the matrix induces directionality or anisotropy into the material.
23
Q
What are the variables that have a major influence on the properties of FRPs?
A
- Fiber type, cost and ease of processing into other fabrics
- Mechanical properties
- Alignment and distribution of the fiber
- fiber/matrix interface
- Size and shape of fiber
- loading direction
24
Q
What are the 4 types of fiber materials?
A
Most common - glass. Other regularly used fiber materials: aramid, carbon and to a smaller extent, boron.
25
Do the fiber materials have a yield point/ what type of behaviour do they exhibit/ what is the strain to failure like?
All exhibit elastic behaviour up to the point of failure. There is no yield point and very low strain to failure.
26
Can you draw the tensile stress/strain graph for different fiber materials?
Yes or no.
27
What is glass fiber made up from?
A mixture of oxides making up silica with other materials such as aramid, carbon and a small amount of boron.
28
What process produces carbon fiber?
controlled pyrolysis.
29
How can a range of carbon fibers with different mechanical properties, strength and stiffnesses be achieved?
By the processes of oxidation, carbonization and graphitization.
30
What configuration should the composite have for max tenacity (break strength) and initial modulus?
Extended chain configuration and almost perfect crystalline configuration
31
Why are kevlar fibers highly crystalline?
Due to rigid back bone that arises from the para orientation on the benzene ring?
32
In general, what happens to the strength of a material which is drawn into fiber form?
The material becomes much stronger in the drawn direction than it was in its original form
33
What happens to glass when is is melt drawn?
The resulting glass fiber is isotropic
34
How are aramid fibers formed?
By wet spinning and drawing
35
How are carbon fibers formed?
By oxidation and carbonizaton
36
What are the resulting fibers for aramid and carbon once formed?
They are anisotropic - different properties in the longitudinal (parallel to fiber) and transverse (perpendicular to fiber) directions
37
How are glass fibers manufactured?
By drawing molten glass into very fine threads and then immediately protecting them from contact with the atmosphere or with hard surfaces in order to preserve the defect free structure that is created by the drawing process.
38
What temp does silica melt at?
1720 degrees celcius
39
What is silica the basic element in?
Quartz, a naturally occurring rock. It is crystalline (rigid and highly ordered atomic structure).
40
How is silica made into quartz?
SiO2 is heated above 1200 deg. and then slowly cooled, it will crystalize and become quartz.
41
How is silica made into glass instead of quartz?
By altering the temperature and cool down rates. If pure SiO2 is heated to 1720 then cooled quicklym crystallization can be prevented and the process yields amorphous or randomly ordered atomic structure such as glass.
42
What are the advantages of E-glass?
Good electrical insulator and high strength
43
What are the advantages of S-glass?
High silica content with high temp performance, high strength and stiffness.
44
What is the most widely used type of glass fiber?
E-glass, accounts for 95% of total glass fiber production.
45
What are the main advantages of glass fiber?
- low cost
| - isotropic properties
46
What percent of composite materials are based on glass fiber?
over 90%
47
What are the different steps in making glass fibers?
```
Step 1: batching (raw materials)
Step 2: melting
Step 3: fiberization
Step 4: Coating
Step 5: Drying/packaging
```
48
What are the bonds like in aramid/kevlar fibers?
Strong covalent bonding in the fiber direction; relatively weaker hydrogen bonding in the fiber transverse direction.
49
What does the anisotropic nature of kevlar/aramid mean?
The fiber has poor compressive properties. Under compressive loads the fiber develops kink bands which eventually lead to ductile failure.
50
Name and describe 4 types of kevlar?
1. Kevlar AP - a next-generation fiber that offers advanced performance, value, and increased design flexibility in many applications.
2. Kevlar 29 - the original family of product types of kevlar. These yarns are used in ballistic applications, ropes and cables, protective apparel such as cut resistant gloves etc.
3. Kevlar 49 - high modulus type used in fiber optic cable, textile processing, plastic reinforcement, ropes, cables, and composites for marine sporting goods.
4. Kevlar 100 - producer-colored kevlar yarns used in ropes and cables, tapes, strappings, gloves and other protective apparel, and sporting goods.
51
How is carbon fiber produced?
By the controlled pyrolysis of a precursor.
52
What are the most commonly used precursors for carbon fiber production?
pitch (petroleum based) and polyacrylonitrile (PAN).
53
What are the successive stages of PAN pyrolysis?
1. Oxidation - oxidizing atmosphere at 200-250 deg.
2 - Carbonization - non-oxidizing atmosphere at 1000 deg for high strength fiber.
3. Graphitization - non-oxidizing atmosphere of 2500 to 3000 deg for high modulus fiber.
4. surface treatment
5. sizing
54
How can carbon fibers with different mechanical properties i.e strength and stiffness be produced?
By adjusting the process temperature.
55
How are carbon fibers grouped?
According to modulus and strength
56
What are the 5 different groups for carbon fiber and their corresponding modulus/strengths?
1. Ultra-high modulus, UHM. (Modulus >450GPa)
2. High modulus, HM. (Modulus between 350-450GPa).
3. Intermediate-modulus, IM. (Modulus between 200-350 GPa).
4. Low modulus and high-tensile, HT. (Modulus 3GPa).
5. Super high-tensile, SHT. (tensile strength >4.5GPa).
57
What are the main characteristics for fiber selection?
1. Drapability (conformity)
2. Compressibility (change of thickness)
3. Stackability (orientation of plies)
4. Wetability by the resin
5. Width and thickness of mat
6. mass of mat
58
What is roving?
A bundle of continuous filaments. The bundle may be in the form of an untwisted strand or twisted yarn. The roving is available in a range of weights or 'tex'; weighed in grams per kilometer.
59
What is chopped strand mat (CSM)?
This is a sheet of reinforcement material comprised of randomly dispersed chopped fibers (usually 25-50 mm in length) held together with a resinous binder. CSM is produced in a variety of widths, lengths and weights.
60
What is woven fabric? Explain weft and warp?
This is produced from yarns. Woven fabrics are described by the weave pattern; the number of yarns in warp and weft and the yarn count. In weaving the weft is the term for the yarn which is drawn through the warp to create cloth. Warp is the lengthwise or longitudinal thread in a roll, while weft is the transverse thread.
61
What is a plain weave?
The simplest form with warp and weft yarns alternatively passing under and over each other.
62
What is a balanced weave?
The number and count of warp and weft yarns are equal.
63
What is a satin weave?
Each warp and weft yarn goes over one yarn then under a number of yarns. Made by floating a yarn over 4+ yarns then under one yarn.
64
What are the advantages of satin weave?
Makes a very smooth, shiny and drapable fabric.
65
What are the disadvantages of satin weave?
Not durable, floats tend to break or snag.
66
What does a 5-harness satin weave have?
One warp yarn passing over 4 weft yarns before passing under the fifth yarn.
67
What is a twill weave?
One in which the warp and weft yarns which pass over each other are varied, recognizable by diagonal lines.
68
What does a 2x2 twill weave have?
2 over by 2 under.
69
Why are satin and twill weaves more suitable for moulding complex curvatures than plain weaves?
Because they have better drape characteristics.
70
What is woven roving?
Coarse heavy weight woven fabric produced from strand rovings. A range of filament diameter, no of strands and weave styles are available. Plain weave woven roving is the most common. Tapes are usually woven in a variety of weave styles and widths, normally less than 150mm.
71
How are knitted fabrics produced?
Produced using conventional textile knitting processes. Using modern computer controlled knitting machines the process is capable of producing a variety of patterns and shapes at high rates of production.
72
Why are knitted fabrics more pliable than woven?
Because they don't have any crimped fibers.
73
Why do knitted fabrics have to be tailored to individual customer requirements?
Because there is a wide variety of yarn orientations and fabric weights
74
What are the disadvantages of knitted fabrics?
- The reinforcement materials are easily damaged by the process
- The high proportion of looping in the resulting fabrics lead to poor mechanical properties.
75
What type has a greater strength to weight ratio: woven or braided fabric?
Braided
76
Why are braided fabrics more expensive than woven fabrics?
Because of its complex manufacturing process. However, manufacturing costs have decreased making braided fabrics more cost-competitive.
77
Where do braided fabrics get their strength?
From intertwining three or more yarns without any two yarns being twisted around each other, continuously woven on the bias so that at least one axial yarn is not crimped. This arrangement distributes the load efficiently throughout the braid.
78
What configurations do braids come in?
Flat or tubular
79
What are flat braids used for?
Selective reinforcement, such as strengthening specific areas in pultruded parts.
80
What are tubular cross sections used in?
Tubular braid can be pultruded over a mandrel, producing hollow cross sections for use in windsurfer masts, lamp and utility poles, and other parts.
81
What is non-crimp fabric?
Continuous reinforcement, usually roving, laid in planar form in straight lines usually at some specified angle to the weft direction and is held together with a light weight warp or linking yarn (knit)
82
Why can non-crimp fabric drape into complex shapes?
The absence of reinforcement loops reduces kink stresses and allows the fabric to shear easily and therefore drape into complex shapes.
83
What varieties of non-crimp fabrics can you get?
Unidirectional, biaxial, triaxial, etc.
84
What are tissues, felts and veils?
Very lightweight, random reinforcement mats primarily used to improve the surface appearance of mouldings.
85
Where are tissues, felts and veils typically placed?
either between the gel coat (if used) and the heavier laminating reinforcement, or as the final lamination to block out the fiber pattern of the underlying reinforcement and provide smooth surface finish.
86
What is a hybrid?
A hybrid is a reinforcement, with two or more different types of fiber or fiber form to achieve cost and/or performance benefits/
87
What are the 2 distinct categories of hybrid?
1. combination laminates
| 2. mixed fibers
88
What are preforms?
Dry fiber mats, usually CSM or CFRM, that have been preformed into a desired shape prior to resin impregnation. The mats are a special preformable grade coated with a thin thermoplastic binder which may be in emulsion (or film) form or in powder form.
89
How is the preforming operation normally carried out?
In a heated press which melts the thermoplastic binder present on this grade of mat and forms the desired shape. On removal of heat and pressure the mat retains its formed shape. The handleable preform is removed from the press and taken to another mould for the resin impregnation operation. The preforms have sufficient integrity to be stacked in quantity prior to subsequent moulding.
90
What is the role of the matrix in FRP?
To maintain the structural integrity by bonding the reinforcement together. The matrix also serves to provide environmental protection for the reinforcement.
91
What does the choice of matrix depend on?
the mechanical, thermal and chemical properties required as well as the processing methods available and permitted cost
92
What are the two polymer matrix classifications?
1. thermoplastic
| 2. thermoset
93
What is the difference between thermoplastics and thermosets?
Thermoplastics remain permanently fusible so that they will soften and eventually melt when heat is applied, whereas cured thermoset polymers do not soften, and will only char and break down at high temps.
94
Why do thermoplastics melt and thermosets don't?
Thermoplastics have relatively weak forces of attraction between the chains, which are overcome when heat is applied.
Thermosets have strong chemical bonds cross-linking the molecules so it is effectively one large molecule with no crystalline structure.
95
What are the difference in properties between thermoplastics and thermosets?
Compared with thermoplastics, thermosets are generally harder, more rigid and more brittle and their mechanical properties are not heat sensitive. They are also less soluble in organic solvents.
96
What is the most common choice for the matrix?
thermoset
97
What are common types of thermosets chosen?
polyesters, epoxies and phenolics
98
What is the selection criteria for resin?
1. Stress and strain limits
2. Heat distortion temperature
3. shrinkage
4. adhesion to fibers
5. fire resistance
6. cost of materials
7. ease of processing
8. shelf life and storage
9. safe handling and processing
99
How are polyester resins formed? How is it cross linked?
by a condensation reaction between glycol and an unsaturated dibasic acid. The unsaturated resin is dissolved in a monomer solvent, usually styrene, and cross linked by the addition of a catalyst and heat.
100
How can the cross linking of polyester be speeded up?
An accelerator may be added to speed up the reaction
101
What is the shrinkage of polyesters on curing typically?
Between 4 and 7% by volume
102
How are vinyl-esters similar to polyesters?
In that they cure by radical initiated polymerization.
103
How are vinylesters formed?
From the reaction of an epoxy resin with acrylic or methacrylic acid and properties can be varied using different epoxy resins.
104
What are the advantages of vinyl-esters?
They are generally tougher, have improved chemical resistance and are capable of higher operating temps than polyesters. They fall between polyesters and epoxies in terms of performance and cost.
105
How are epoxies formed?
By condensation of epichlorhydrin and polyhydroxy compounds.
106
How are epoxies normally supplied?
As a single constituent, the resin, with a second constituent, the hardener or cross-linking agent, that has to be added.
107
How are a range of properties possible with epoxies?
By combining different resins and hardeners
108
What are the advantages of epoxies?
- higher strength and adhesion to fibres than polyesters.
109
What is the shrinkage of epoxies on curing?
between 0.25 and 2% by volume.
110
How are phenolics formed?
By condensation of phenol and aldehyde. The condensation reaction is usually promoted by heat but can also be initiated using a strong acid catalyst.
111
What are the advantages of phenolics?
Good fire resistance with low smoke and toxic fume emission characteristics
112
What are the disadvantages of phenolics?
During cure they produce highly volatile contents, predominantly water and are unstable at room temperature. The pot life of the resin is therefore low and a high proportion of the mass of the hardened resin is made up of water. The resin cannot be pigmented, the color being unstable. The shrinkage upon curing is between 8 and 10%
113
What is gel coat?
A dense, void-free layer of resin on the exterior of moulding to improve the surface finish.
114
How is gel coating applied?
It is applied to the mould and then partly cured prior to applying the resin.
115
What are the advantages of gel coat?
Tough, resilient films, most can be pigmented
116
What is pre-preg?
A thin sheet of partially cured, or b-staged, resin containing reinforced fibers. The fibers can be woven or unidirectional. The pre-preg sheet usually comes with a plastic backing sheet or release film which is removed prior to laminating. Normally supplied as rolls.
117
What are the parameters that can typically be specified for pre-pregs?
- fiber type and grade
- surface treated?
- resin type
- resin content
- cured ply thickness
118
What are the disadvantages of pre-pregs?
- limited shelf life due to partially cured state
| - has to be stored at low temps in a freezer
119
How are pre-pregs processed?
- hand lay-up, vacuum bad and autoclave moulding
120
What are the two types of composite manufacturing processes?
- open moulding
| - closed moulding
121
What is open moulding?
the gel coat and laminate are exposed to the atmosphere during the fabrication process
122
What is closed moulding?
The composite is processed in a two-sided mould set
123
Examples of open moulding?
- hand lay-up or contact moulding
- spray-up
- filament winding
124
Examples of closed moulding?
- Compression moulding
- pultrusion
- resin transfer moulding
- vacuum bag moulding
- centrifugal casting
- continuous lamination
125
What processes can be used for low volume production?
- hand lay up
- vacuum bag molding
- vacuum infusion processing
126
What processes can be used for medium volume production?
- filament winding
- wet lay-up compression moulding
- resin transfer molding
- centrifugal casting
127
What processes can be used for high volume production?
- compression molding
- pultrusion
- reinforced reaction injection molding
- continuous lamination
128
Discuss the hand lay-up process?
The hand lay-up method, sometimes called contact molding, is simple and versatile and the most widely used FRP process. The process consists of applying layers of reinforcing material against a single sided mold and working resin into the material with a brush and roller. After a suitable period of time to allow the resin to cure, the molding, with one smooth surface, is removed from the mold and trimmed to size.
129
Can you draw a diagram of the hand lay-up process?
Yes or no
130
What the issues with hand lay-up molding?
- It is slow and highly labour intensive.
- because it depends on the skill of the laminator, the process has the inherent problem of variability in the finished moldings.
131
How can the hand-lay-up process be sped up?
The resin and reinforcement material can be simultaneously applied by spraying on to the surface of the mold. After consolidating with a roller and curing, the molding can be removed and trimmed as before.
132
What is a positive aspect of hand lay-up?
The size and complexity of the moldings that can be produced by this method are endless.
133
What are the different catergories of hand lay-up?
- wet laminating
- dry/pre-preg laminating
- vacuum baggin
- vaccum infusion may also fall into this category
134
What are some applications of spray-up (or chopping) molding?
Boats, tanks, transportation components and tub/shower units in a large variety of shapes and sizes.
135
What are the advantages of spray-up molding?
A chopped laminate has good conformity and is sometimes faster than hand lay-up in molding complex shapes.
136
What are the disadvantages of spray-up molding?
In the spray-up process the operator controls the thickness and consistency, therefore the process is more operator dependent than hand lay-up.
137
What type of moldings is vacuum bag molding used on?
Moldings which are large or of complex shape.
138
What type can the mold be in vacuum bag molding?
either male or female
139
What type of resin system is the vacuum bag technique usually used with?
pre preg resin systems although there are techniques for wet resin systems
140
How does vacuum bag molding work?
Consolidation is achieved by covering the molding with an airtight membrane, or bag, from which the air is subsequently removed with a vacuum pump providing up to one atm of pressure.
141
What is required if pressures higher than 1atm are needed in vacuum bag molding?
The whole assembly is placed inside an autoclave or heated pressure vessel.
142
What are the disadvantaged of vacuum bag molding?
- cycle times tend to be long, often several hours
- labor content is high
- process is highly dependent on skill of laminator
143
How does resin transfer molding work?
Dry reinforcement is placed between 2 matched molds. Resin is injected into the cavity and through the reinforcement which is trapped or pinched at the edges of the mold. The effect of the pinching is to provide a path for air to escape but block the passage of resin. There are variations to this which lead to more accurate moldings with less material wastage.
144
What are the advantages of resin transfer molding?
- Accurate fiber management with reproducible moldings.
| - With automation, this process is capable of high vol production with cycle times down to minutes using reactive resin
145
How does pultrusion work?
Continuous fibers pulled through resin tank to impregnate fibers with thermosetting resin.
Impregnated fibers pass through steel die that preforms to the desired shape.
Preformed stock passes through a curing die that is precision machined to impart final shape and heated to initiate curing of the resin matrix.
146
How does filament winding work?
Continuous reinforcing fibers are accurately positioned in a predetermined pattern to form a hollow (usually cylindrical) shape.
Fibers are fed through a resin bath to impregnate with thermosetting resin.
Impregnated fibers are continuously wound (typically automatically) into a mandrel. After appropriate number of layers is added, curing is carried out either in an oven or at room temperature. The mandrel is removed to give the final product.
147
Advantages of filament winding?
- Modern machines are capable of winding non-axisymmetric and branched shapes
- Accurate reproducible parts
148
Disadvantages of filament winding?
- Relatively slow
