Past Paper Theory Flashcards
(29 cards)
Compare match-die moulding and vacuum bagging manufacturing techniques and provide two differences between them.
Matched-die moulding requires two matching moulds which are heated and loaded with raw material (sheet moulding compound SMC or dough moulded compound DMC) and pressed to shape the mould and cured. Also known as compression moulding.
Vacuum bagging involves employing a negative pressure (bag moulding) to turn the composite into the shape of the mould. Only one mould is required and the vacuum consolidates the shape, which is then usually within an autoclave that controls the curing temperature.
Two main differences are only one mould is required for vacuum bagging and matched die is a quicker more effective method for high production parts.
Explain three main differences between thermosets and thermoplastics? Which one is better for higher temperature applications? Which one is better for recycling?
Thermoset plastics have a permanent molecular network due to cross linking, whereas thermoplastics can be reheated and reshaped and are therefore better for recycling.
Thermosets have a high temperature resistance and are therefore better in a high temperature application. Thermoplastics become viscous at a high temperature and have a glass transition temperature where the material turns from glassy behaviour to rubbery and then viscous behaviour.
Thermoplastics can be more rapidly processed and no chemistry is involved, whereas thermosets must be cured right first time and can take longer.
What are the two main reasons for composite fibre content being defined either as weight fraction or volume fraction?
The weight fraction is the ratio of the weight of one constituent to the total weight of all constituents.
The volume fraction is the ratio of the volume of one constituent to the total volume of all constituents.
Composites are typically manufactured by weight fraction. Composite properties are a function of the volume fraction.
If the ply construction of a multiple laminate is symmetrical what does this imply about the value of the B matrix?
The B matrix is equal to zero.
State what is understood by the Reserve Factor in terms of failure criteria for composite laminates.
The reserve factor is the magnitude of load required to cause failure of the composite lamina.
What are the five failure modes?
Longitudinal Tensile Failure Longitudinal Compressive Failure Transverse Tensile Failure Transverse Compressive Failure In-plane Shear Failure
A laminate with a balanced lay-up is one that for every layer with a + orientation contains an identical layer with a - orientation. What does this mean for the values of the components A13 and A23 of the A matrix? What does this mean for the load-deformation coupling in such a laminate?
A13=A23=0 In a balanced lay-up. A13 and A23 relate the in-plane direct forces to the in-plane shear. They also relate the in-plane shear force to the in-plane direct strains.
Therefore there will be no coupling between direct stresses and shear strains.
How could you reduce the bending-twisting in a multiple symmetrical laminate?
Decrease the ply thickness and increase the number of plies whilst maintaining the laminates overall thickness.
What is the Q bar transformed stiffness matrix?
It is determined from the stiffness coefficients and is a function of the angle between the load and the fibre direction.
Define the critical fibre length in discontinuous fibre reinforced composites.
It is the minimum fibre length required to achieve a stress level in the fibre that is equal to the fibre strength.
Give two parameters that give a good estimation of the longitudinal tensile strength of a unidirectional composite with a high volume fraction.
Fibre volume fraction
Fibre tensile strength
What is the relationship between the longitudinal modulus and the fibre volume fraction?
The longitudinal modulus is generally proportional to the fibre volume fraction.
When can a multiple composite laminate be assumed to have failed?
When the stress or strain components in any ply exceeds the appropriate strength or failure strain of that ply.
Why is Tsai-Hill often preferred to the stress and strain criterions?
It gives a better prediction than does a limit criterion for a unidirectional lamina under bi-axial and complicated stress conditions. Limit criteria are simple to use but do not consider the interaction between different failure modes. Only one criterion for failure stress has to be satisfied in Tsai-HIll.
Does the maximum stress criterion give the same results as the maximum strain criterion? Explain your answer.
The results are not always the same, if the failure mode is shear then they will be the same. However for longitudinal or transverse failure modes the results can be different. Maximum strain gives more accurate results.
How do you ensure a full suppression of shear-extension coupling in a unidirectional ply?
S13 bar = S23 bar = 0
Q13 bar = Q23 bar = 0
Stress generated along x would generate a shear stress deformation. Stress generated along y would generate a shear stress deformation. Shear stress generated along xy would generate extension along x.
If these terms were set to 0 then there would be a full suppression of any shear-extension coupling in the ply. This is known as a specially orthotropic lamina.
Give the units of the ABD matrices.
The A matrix has N/mm (KN/mm)
The B matrix has N (KN)
The D matrix has N.mm (KN.mm)
When a composite is produced it is found that the longitudinal strength of the composite is actually lower than the strength of the base polymer. Explain this result in terms of the micromechanics of the system.
When the failure strain of the matrix is greater than the failure strain of the fibre then the base polymer can have a higher strength than the composite. This happens at low volume fractions, when the failure mode is matrix dominated.
How can the fibre volume fraction be found at which the longitudinal strength is a minimum for the case where the longitudinal strength of the composite is actually lower than the strength of the base polymer.
It can be found by equating the two failure modes. Think of the graph and the appropriate failure mode equations.
In a real life composite example, what would be a principle reason for errors in values calculated using an expression for an idealised two component fibre reinforced composite? Explain what could be done to alleviate this problem.
Real life composites contain voids, which would give errors in the values obtained from an idealised relationship.
The volume fraction of the void would need to be obtained in order to account for the potential error in the expression.
The material property that could be measured is the densities of the composite, fibre and matrix. This would allow the void volume fraction to be determined.
Name two processes for manufacturing high volume fraction unidirectional composites.
Vacuum bagging with autoclave curing involves laying fibres in unidirectional sheets, which are impregnated with resin and partially cured into pre-pregs. The pre-pregs are stacked in predetermined orientations and consolidated using a vacuum within an autoclave which controls the curing temperature.
A closed mould method for high volume fraction unidirectional composites is pultrusion, where a continuous feed of fibres in predetermined orientations are impregnated with resin and pulled through a heated die to give the shape of the final component.
Describe three different manufacturing/processing methods used in the field of composite materials. Nature of the reinforcement should be different in each case. Discuss the general properties likely to be obtained and why particular reinforcement is used.
Filament winding is used for pipes and complex containers to provide reinforcement. This involves positioning continuous fibres onto a mandrel at predetermined angles in a wet resin. The resin is partially or completely cured before removing the component from the mandrel. The general properties are sufficient to deal with complex loading such as hoop and circumferential stresses seen in pipes.
Resin injection moulding can use discontinuous fibres where wet resin is injected into a closed preform mould. This is a popular technique for mass production of either continuous or discontinuous fibres typically used for car body panels. The general properties for discontinuous parts with random orientations are likely to be more isotropic than the continuous fibres or aligned discontinuous would potentially be.
An alternative to the above could be the likes of hand lay-up that uses chopped strand mats (CSM) or woven mats to make larger components like a boat hull. These have continuous fibres that are aligned bi-directional in a single layer. The dry fibre is impregnated with the wet resin with a brush and roller until the required part thickness is built up. Typical lay-ups are cross-ply, angled-ply or quasi-isotropic that all have different properties and will be selected based on the application.
What are the main factors effecting the choice of manufacturing/processing of a composite?
The cost of the technique used.
The shape of the component to be made.
The number of components required.
The required performance in the components application.
What are the main barriers to more widespread use of composites?
High cost: The raw materials are more expensive than traditional materials.
Recycling: As there are two constituents it makes it more difficult to remove and recycle them.
Repair: Once damaged it is not easy to repair a composite as it has lost its structural integrity.
