Inför tentan Flashcards
(172 cards)
1
Q
What does CFRP consist of?
A
Carbon fiber reinforced plastics. Carbon fibers and plastics.
2
Q
Name three advantages with PMC.
A
Specific stiffness
Specific strength
Light weighted
Corrosion resistance
3
Q
Name three disadvantages with PMC.
A
Temperature range
Long manufacturing time
Expensive
Recyclability
4
Q
Why is it easier to manufacture thermoset-based composites than thermoplastic-based composites?
A
No need to melt anything. Lower viscosity, easier to impregnate the fibers
5
Q
Name the three most common types of fibers in PMC.
A
Carbon, Aramid, Glass
6
Q
Name two advantages with carbon fiber.
A
Very strong
Very stiff
Thermal and electrical conductivity
Excellent fatigue properties
low thermal expansion
good X-ray permeability
7
Q
Name two disadvantages with carbon fiber
A
Expensive
Low failure strain / brittle
8
Q
What does the abbreviation “IM” means regarding carbon fibers
A
Intermediate modulus
9
Q
Name two advantages with glass fiber.
A
High strength
Good thermal properties
Good corrosion resistance
Good thermal and electrical insulation
Low price
10
Q
Name two disadvantages with glass fiber.
A
Low stiffness
Sensitive to moisture
Sensitive to notches
11
Q
What is the difference between S-glass fibers and E-glass fibers?
A
S-glass more expensive
12
Q
Describe E-glass
A
E-glass: Electrical Grade, most common type (99% of all continuous fibers)
13
Q
Describe S-glass
A
S-glass: High strength grade, 30% higher strength and 20% higher stiffness than E-glass. More expensive
14
Q
Name three common types of natural fibers.
A
Hemp, flax, cotton, ramie, jute, coconut, wood
15
Q
What does NCF mean?
A
Non crimp fabric
16
Q
Describe the structure of NCF.
A
Fabric that doesn’t crimp. Better compression properties? Unidirectional layers/ CSM are stitched together to prevent movement with polyester
Higher stiffness and strength compared to weaves
17
Q
Name the three most common types of thermosets in PMC.
A
Epoxi, Polyester, Vinylester
18
Q
Name two advantages with epoxy
A
Mechanical properties
Thermal resistance
Chemical resistance
Electrical insulation
Excellent adhesion
Low shrinkage at cure
19
Q
Name two disadvantages with epoxy
A
Allergic reactions
Toxicity
Poor UV resistance
Expensive
20
Q
What is important to remember when you work with epoxy?
A
Gloves and ventilation.
21
Q
Name two advantages with UP.
A
Unsaturated polyester
Good mechanical properties
Low viscosity
Easy curing
Low price
22
Q
Name two disadvantages with UP.
A
Low temperature resistance
Large shrinkage at cure
Problem with styrene evaporation. Good ventilation necessary!
23
Q
Why are glass fibers covered by sizing?
A
To improve (chemical) adhesion to the matrix.
Protect the fibers.
Avoid static electricity.
Keeping the fibers in bundles.
24
Q
Why is it normally good with a strong bonding between the fibers and the matrix?
A
Evenly disperse the force.
Preventing debonding / delamination
25
Why is it not so wise to use old glass fibers?
The sizing dries with time --> poor bonding
26
Why are residual stresses formed in PMC?
Different thermal expansion coefficients
27
Name two advantages with biocomposites.
Renewable resources
Easy to market
Low price
28
Name two disadvantages with biocomposites.
Moisture sensitive
Variances in the fibers due to different growth
Relatively low strength
Long-term properties
29
Name two common nano-reinforcements in composite materials.
Carbon nanotubes
Nanoclay
Cellulose fibrills
30
Why does unsaturated polyester normally contain styrene?
To lower the viscosity (30-35%)
31
How is curing of unsaturated polyester initiated?
Adding a free radical catalyst --> start the chain reaction of polymerization
32
What's the purpose of adding inhibitors?
To prolong the working time. Control the curing.
| Only prolongs the reaction ~ 2 hours → need more time? change catalyst! the inhibitor can affect the properties
33
Which resins do often contain an inhibitor?
polyester resins
34
What’s the purpose of adding accelerators?
Speeding up the curing process.
| green/bluish due to cobalt
35
Why should accelerators and catalyst never be mixed directly together?
they can burn/explode
36
If you want to add both accelerator and catalyst what should you do?
1)add the accelerator 2) stir & dissolve 3) add the catalyst
37
What is the “pot-life”?
The time until curing starts
38
How does the temperature affect the pot-life?
Increase temperature → more rapid curing process → shorter pot-life
39
How does vitrification affect the curing?
It abruptly halts the curing (if you want further curing → heat it up to the rubbery phase)
40
Name five parameters that affect the pot-life and curing of vinyl esters
Catalyst used
Accelerator used
Inhibitor used
Temperature
Age of resin
Mass to be cured
Applied heat
Size and type of mould
Fillers/reinforcements
41
What is gelation?
Gelation is characteristic for thermosets, and it is of foremost significance
From a processing standpoint, gelation is critical since the polymer does
not flow and is no longer processable beyond this point.
42
What is Vitrification?
glassy states --> abrupt halt to curing
43
Describe the curing of epoxy.
1. starts with connecting monomers in a liquid state.
2. Reaches gelation (does not affect the movement of monomers)
3. Reaches vitrification (glassy state)
To cure an epoxy resin a hardener is needed.
44
What temperature is required when curing epoxy?
Some epoxy systems can be cured even at temperatures as low as 0°C. However, to obtain good mechanical properties it is normally recommended to cure the resin at +20°C and then (if needed) post cure the material at higher temperature.
45
When curing epoxy, a hardener is required. What does this affect?
Different harderners have different ideal curing temperatures → affects the curing rate.
46
How is the temperature of curing epoxy determined?
The higher the temperature is of curing epoxy, the faster it cures. The temperature of curing epoxy is determined by the ambient temperature plus the exothermic heat generated by its cure. (ambient temperature + exothermic heat)
47
What happens when heating epoxy that has not gelled?
will lower its viscosity, allowing the epoxy to run or sag more easily on vertical surfaces.
48
What can happen when heating epoxy applied to a porous substrate?
heating epoxy applied to a porous substrate (softwood or low-density core material) may cause the substrate to "out-gas" and form bubbles in the epoxy coating. To avoid out-gassing, wait until the epoxy coating has gelled before warming it.
49
How does incomplete curing effect epoxy?
Lower stiffness
Higher moisture absorption
Reduced chemical resistance
Risk of allergic reaction
Skin, red and swallowed
Stiffness not as high as expected, higher moisture, bad chemical resistance
50
Name three important substances to look out for when working with thermoset-based composite materials.
Styrene, epoxy, dust, solvents (catalysts, accelerators)
51
What is important to remember when working with uncured epoxy?
Use suitable gloves!
52
What is important to remember when working with uncured polyester?
Good ventilation!
53
How can the amount of styrene in the air be reduced when working with UP?
Add some wax? Places on the top of the material ⇒ reduce the styrene in the air
54
How can the curing of thermosets be studied?
```
Spectroscopy
Magnetic resonans (NMR)
Thermal analysis (DSC)
Dielectric measurements
Hardness tests
Tensile tes
```
55
How can the amount of styrene in the air be reduced when working with UP?
Add some wax? Places on the top of the material ⇒ reduce the styrene in the air
56
How shall waste and spill of uncured resins be taken care of?
Place it in bags
Mark the bags with thermosets
Clean floor etc.
57
Describe the manufacturing of a plug
In order to make a mould, we make a plug = master model
Wood, plywood, cover it with clay, mill it in order to get the right dimensions, finish and polish surfaces. Paint the plug and add wax to the plug in order to not get stuck
58
Describe the manufacturing of a mould when you have a plug.
1. Apply a gelcoat
2. Apply laminate layer of glass fibre (CSM) + temperature resistant esin
3. Apply structural laminate layer (in steps to avoid shrinkage) CSM or spray- up of glass fiber and polyester
4. Add stiffeners: built up by fibre-reinforced plastics
59
Describe a good way of de-moulding a composite motorboat hull
Start the de-moulding process by trimming the edges.
Then release the edges by the use of wooden or plastic wedges.
Apply compressed air carefully along the released edges.
Remove the component!
Store the component appropriately until it has fully cured
60
What different manufacturing methods are Fosieplast using?
Hand lay-up (poor mechanical properties) and Vacuum Injection Moulding (large components)
61
Name 5 advantages with vacuum injection moulding compared to hand lay-up
```
better mechanical properties (less voids)
better working environment
faster process
less material needed (spill)
cheaper
```
62
Why are different types of flow media used in vacuum injection moulding
to be able to impregnate a long piece
to speed the process up for large components
63
Describe the structure for three different types of flow media
Surface medium
allows very fast distribution of large quantities of resin,
and provides good control over the flow front
Core channels
most effective on flat surfaces,
and a very effective, waste-free way to infuse parts
Interlaminar medium
highly porous
layer in the laminate, such as a continuous roving material or a nonwoven
breeder-like material
64
What is a “skin coat”?
Normally, a skin coat consists of a few layers of CSM with a high quality, low-shrink resin.
flat shiny surface (not fibers), made from hand lay-up → prevent fiber print-through
65
What is a barrier coat?
similar… but made out of a resin layer.
| more effective than skin coat
66
What different types of manufacturing methods are “Composite Design” using?
prepreg (low voids, high fibres)
67
Pros with prepreg?
high fibre content
| low void content → good mech. properties
68
Cons with prepreg?
touch when not cured: allergic reactions
| expensive
69
Describe the different steps in prepreg lay-up
1. need a mould
2. apply a release agent (to be able to remove it later on)
3. take out the material from the freezer (prevents it from curing)
4. cut number of samples of prepreg
5. place the samples in the mould (apply only a few layers)
6. debulking: apply vacuum bag to remove embedded air
7. add more layers ...etc…
8. bleeder put it in an autoclave (to cure it)→ press out the air (can tolerate <1% =good quality)
9. remove from autoclave → trim it
10. Post work
70
What does it mean when a prepreg material is “Non-bleed”
would like as much fibre as possible and as little epoxy as possible
the bleeder takes up the extra amount of epoxy
when we don’t have any extra epoxy in the material, we don’t need the bleeder → non-bleed
71
What is meant by out-of-autoclave technique?
autoclave is very expensive
manufacture it without the autoclave
Use other methods such as RTM or vacuum + oven
not as good properties but much cheaper!
72
How high amount of fibres can be obtained in a prepreg laminate?
65-75%
73
What is the theoretical maximum amount of fibre in a laminate?
90-91% due to the placement
74
Describe the different steps in RTM.
1. Dry fibers in mould
2. Press the fibers in the mould to shape
3. Inject the resin
4. Let cure
Nice surfaces
75
What is the difference between HP RTM and LP RTM?
HIgh/low pressure
| High: good tolerances, more expensive
76
What type of property is described by the permeability?
easy for the resin to flow and impregnate
| how easy it is to flow through a porous media
77
What does “race-tracking” mean?
flows easier in the edges → in the middle we get dry spots
78
What does does SMC sheet moulding compound consist of?
chalk
glass fibers
unsaturated polyester
79
What does does AMC advanced moulding compound consist of?
similar as for SMC
```
remove the chalk
epoxy
carbon fibers
fast cycle time
lower density
```
80
Describe how wet moulding works
1) make mould place dry fibers
2) spray the fibers with resin
3) close the mould→ wet out the fibers
81
Name 3 typical applications for filament winding
pressure vessels
pipes
masts
82
Describe the difference between pultrusion and extrusion
Pull fibers through the die
slow process (not cost efficient)
better insulation by pultrusioned composites
83
What is GMT?
Glass mat thermoplastics
84
How does Composite laminate with UD fibers in the loading direction fail in compression?
Fiber micro-buckling (the fibers will buckle) → formation of kink band
Happens quite early. Compression strength is ALWAYS lower than tensile strength
85
Which is the first damage mechanisms in tensile loading of a cross- ply laminate?
matrix/transverse cracking
controlled by ductility, sizing
(0.4% strain)
86
Why are overloads more detrimental for composite materials than for metals?
Can lead to matrix cracking → will grow → cause delamination
87
Name 5 parameters that influences the size of an impact damage
geometry of the impactor
amount of fibers
sizing
reinforcement in Z direction
ductility of matrix
thickness of laminate
speed of impact
88
What different types of damages can appear when a composite component is hit by an object?
1. matrix cracking
2. + delamination
3. + fiber failure/fracture
89
How can the impact tolerance of a composite component be improved?
good adhesion between fiber/matrix
possibility to move and doesn’t need to take up all the energy
reinforcement in Z direction
placement of fibers
90
What is the biggest problem with composites?
impact!!
91
Why is it interesting to test the shear properties of composite materials?
lammellaes will like to slide → delamination
have to restrict this with high shear strength
need high shear strength between the lamellas
92
Why is it not good to combine aluminium and carbon fiber- reinforced plastics?
galvanic corrosion → aluminum will disappear
| apply thin layer of glass fiber to prevent contact between carbon fiber and aluminum
93
What different types of defects can appear during secondary processing?
delaminations will form on the back side when drilling a hole
microcracks in resin
matrix crack
reorient the fibers
debonding between the fiber/matrix
94
Name 2 pros with water jet cutting of composite laminates
no HAZ
fast
nice cuts
no big health concerns
can add sand to be able to get through thicker plates
95
Name 2 cons with water jet cutting of composite laminates
some materials are sensitive to water → fibers can rotten
costly technique
96
Why is strain, and not the stress plotted on the y-axis in fatigue diagrams for composite laminates?
stress varies for different directions, and for fiber/matrix
97
Why is it not good if fibers shift and move during manufacturing?
huge reduce of stiffness and strength for just a small angle difference
98
Which is the minimum acceptable radius in composite components?
5 mm, but rather 10
| otherwise resin rich area
99
Why are residual stresses formed in fiber-reinforced plastics?
matrix and fiber will not have the same shrinkage
fiber less than matrix
both chemical and thermal shrinkage
chemical → curing
Absorption of moisture
can lead to warpage/cracking
100
Why is it important with low out-of plane stresses in composite materials?
poor properties of matrix compared to fibers.
| if this direction is loaded → delaminations & failures
101
Why is the design/strain limit often as low as 0.3% for composite components in the aircraft industry?
0.5- 0.6 % matrix cracking ( is not allowed, stay below)
static loading
fatigue loading, matrix cracking can occur earlier than 0.5%
stay below fatigue limit
102
How is a symmetric lay-up created?
90-45-45-90 (symmetric according to the mid-plane)
shrinks the same at the top/bottom
avoid warpage (same residual stresses at top and bottom)
103
Why is a symmetric lay-up favorable?
```
shrinks the same at the top/bottom
avoid warpage (same residual stresses at top and bottom)
```
104
How much does glass fiber approximately cost per kilo?
25 kr/kg
105
How much does carbon fiber approximately cost per kilo?
> 150 kr/kg
106
How much does carbon fiber prepreg approximately cost per square meter?
250-275 kr/m2 (200 g/m2 weave)
107
Suggest a suitable material for a component which shall be stiff and cheap.
Steel, aluminum, plywood
108
What does E2 describe in laminate theory?
Transverse stiffness (Young’s modulus)
109
How can the stiffness in the fiber direction for a UD laminate be calculated?
E = E1*V1 + E2*V2
110
Explain how the weight fraction fiber can be changed to volume fraction.
Use the density
111
How shall the fibers in a laminate be placed to obtain the maximum bending
stiffness?
Along the bend [0 degrees]
112
What does the [A] matrix in laminate theory describe?
Extensional stiffness matrix - This matrix influences extensional strains in the laminate
113
What does the [B] matrix in laminate theory describe?
Bending-extension coupling matrix - This matrix couples extensional response to the bending response in the laminate
114
What does the [D] matrix in laminate theory describe?
Bending stiffness matrix - This matrix influences the bending response in the laminate
115
Describe the stacking of a symmetric laminate.
```
90
0
-45
------------
-45
0
90
```
116
Why is it normally good with symmetric laminates?
Decreases warping, residual stresses
117
Why do the strains in a laminate vary linearly through the thickness while the
stresses do not?
The stresses in a laminate varies from layer to layer
118
What is the stiffness transverse to the fiber direction (E2)?
E2 = Ef * Em / (Vf Em + Vm Ef)
119
WHat are the 3 ways of applying a force to make a crack propagate?
Mode I fracture
Mode II fracture
Mode III fracture
120
Explain Mode I fracture
Opening mode (a tensile stress normal to the plane of the crack)
121
Explain Mode II fracture
– Sliding mode (a shear stress acting parallel to the plane of the crack and perpendicular to the crack front)
122
Explain Mode III fracture
– Tearing mode (a shear stress acting parallel to the plane of the crack and parallel to the crack front)
123
What is Gc?
the fracture energy
124
What does the quantity Gc describe?
The energy release rate failure criterion states that a crack will grow when the available energy release rate G is greater than or equal to a critical value Gc
125
How is G1c normally measured for composite laminates?
Double cantilever beam test
| pull laminates apart
126
How is G2c normally measured for composite laminates?
End notch flexural test
127
How is End notch flexural test performed?
put a laminate on two holders, push down the middle part, the laminates will shear on each other
128
What does the effectiveness of toughening mechanisms in composite materials depend on?
Size, morphology and volume fraction of the reinforcement
Interfacial bond strength
Properties of the fibers and matrix (e.g. thermal expansion)
129
Name 3 toughening mechanisms
1. Fibre bridging
2. Microcracking
3. Crack deflection
4. Debonding (The process of fiber debonding creates new surfaces in the composite material
and therefore requires energy)
130
Describe how energy is consumed by fiber pull-out.
W = force* distance
the force needed to overcome is due to friction.
if length of fiber > critical length --> the fiber will break instead
131
How long are the fibers normally in “long fiber thermoplastics” before injection
moulding?
(12.5-25 mm) As long as the pellets used
132
How long are the fibers normally in “long fiber thermoplastics” after injection
moulding?
4-6 mm
133
How long is the “critical” fiber length for GFRP normally?
normall around 2mm?
134
How does the fiber length affect the mechanical properties of short fiber
composites?
Longer fibers = better properties
| critical length to be able to transfer the load and use the fiber
135
How does the fiber/matrix bonding affect the mechanical properties of short fiber
composites?
Better bonding results in better mechanical properties
136
What can be calculated by the Halpin-Tsai equations?
Halpin- Tsai : calculate stiffness of short fiber, aligned
137
Name two advantages with natural fibers compared to
| glass fibers.
Cheaper (?)
| Environmentally friendly
138
Name two disadvantages with natural fibers compared to
| glass fibers.
Can rot
Bad long term properties
Not as strong
139
How can composites with continuous fibers be manufactured by injection molding?
Add the fibers in a controlled manner to the mould before injecting with plastic
140
How is G3c normally measured for composite laminates?
Mixed mode bending test
141
Name two common failure criteria for composite materials.
Maximum stress
Maximum strain
Tsai-Hill
Tsai-Wu
142
What 2 groups can Failure criteria to predict lamina failure be divided into?
Failure criteria not associated with failure modes
Failure criteria associated with failure modes
143
What does "Failure criteria associated with failure modes
| " consider? give examples on such failure modes
These criteria consider the different failure modes of the constituents.
Fiber fracture
Transverse matrix cracking
Shear matrix cracking
144
What can "Failure criteria associated with failure modes" further be divided into?
Non-interactive
| Interactive
145
Describe the non-interactive Failure criteria associated with failure modes
The non-interactive criteria do not take into account interactions between
stresses/strains acting on a lamina. This often leads to errors in the strength
prediction when multiaxial states of stress occur in a structure.
146
Give 2 examples of the non-interactive Failure criteria associated with failure modes
Maximum stress
Maximum strain
147
Name some toughness tests
Charpy test used for plastics
| Drop-weight impact often used for composites
148
How can you get information on
Energy for damage initiation
Energy for penetration of the material/laminate
Initiation and propagation of delaminations for different energy levels
Drop-weight impact test
149
What is a TGA?
Thermogravimetric analysis
| measure changes in material weight over T
150
How can you get information on:
Quality control
Measurement of amount of additives
Measurement of amount of fibers
TGA
151
What is a DSC?
Differential Scanning Calorimetry
difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature
152
How can you get information on:
Measurement of glass transition temperature (Tg)
Measurement of crystallization
Measurement of degree of cure
DSC
153
How can you:
Measure the stiffness as a function of temperature
Measure the stiffness as a function of loading frequency
Measure the glass transition temperature (Tg)
DMTA
| Dynamic Mechanical Thermal Analysis
154
How does Dynamic Mechanical Thermal Analysis work?
Sinusoidial stress applied and strain measured
155
What is NDT and what is it used for?
Non-destructive Testing
evaluate the properties of a material, component or system without causing damage. Generally not on the microscopic level.
156
Name some examples of NDT
Visual inspection
magnifying glass
“Coin tapping”
look for delamination
Acoustic emission
Ultrasound
finds disbonding / delaminations / bigger voids
X-ray
Thermography
Shearography
Penetrant liquid
157
What is important when it comes to Mechanical joining?
achieve load distribution over a large area
Beware of galvanic corrosion
158
Describe Adhesive joining
```
Wipe with solvent
Rinse with water or alkaline solution
Dry in oven
Lightly abrade to roughen matrix surface without exposing the reinforcement
Remove the abraded particles
Wipe with solvent
Rinse with water
Dry in oven
Bond as soon as possible
```
159
Describe an alternative way of Adhesive joining
peel ply
160
What is Important to make sure of when repairing?
that the repair is loaded in shear
161
Bevelling ratio for marine and aircraft industry?
marine industry, 1:10
aircraft industry 1:50
162
What's the first step of repairing?
All damaged material must be removed
163
What can you use when repairing?
1. different laminates layers, finish up with a grinding layer
2. use sandwich
- take away as much core material as needed
- can use glass microballoons in epoxy
164
A sandwich laminate consists of three parts:
Adhesive, surface sheets, core:
```
Surface Sheet
Adhesive
Core material
Adhesive
Surface Sheet
```
165
Name some Advantages with sandwiches
High specific stiffness and strength
Good thermal and acoustic insulation
High energy absorption
Buoyancy (flytkraft)
166
Name some disdvantages with sandwiches
Manufacturing often difficult
Complicated quality control
Difficult with load introduction
Difficult to mechanically join
167
What happens when you increase the amount of core material?
the Flexural rigidity and Bending strength increases
168
Name some requirements on the surface material?
High stiffness
High tensile and compressive strength
good impact tolerance
good surface finish
good environmental resistance
good abrasion resistance
169
Name some requirements on the core material?
Low density
High shear stiffness
High shear strength
High stiffness perpendicular to the surface sheets
Good thermal insulation
170
Describe Load introduction
To disperse the force evenly:
1. drill a hole
2. take away more core material
3. fill it with something
4. insert the plug
171
Name some advantages of Aramid fibers
```
+ High stiffness and strength
+ Good impact properties
+ Good insulation properties
+ Good fatigue properties
+ Good corrosion resistance
```
172
Name some disadvantages of Aramid fibers
- Poor compression strength
- Poor UV-resistance
- Sensitive to moisture
- Expensive
