Question1 (a) State the main categories of structural composites and give two ex
ID: 2073301 • Letter: Q
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Question1 (a) State the main categories of structural composites and give two examples of commercial applications/products within each category 6 marks (b) Discuss how the interfacial chemistry between the phases of a composite material is of critical importance. Your answer should include a description of the individual roles of each phase of a typical composite. 7 marks (c) Calculate the modulus of elasticity in both the longitudinal and transverse directions for a continuous and aligned fibre-reinforced composite consisting of 30 vol% aramid (Kevlar 49) fibre, having a modulus of elasticity of 130 GPa, and an epoxy resin that when hardened displays a modulus of 5000 MPa. Ec Where Eand V are elastic modulus and volume fraction, and subscripts c, m and f represent composlte, matrix and fibre phases. 6 marks (d) Describe the structure of "discontinuous fibre reinforced composites". Why is this type of composite receiving attention and how could the production of such materials help address recycling issues within the sector? 6 marks TOTAL 25 marks)Explanation / Answer
Va)
A composite is a material which is composed of two or more materials in order to obtain a new product which has the best properties of both. Here basically we try to impart the best properties of each material.
For example, Polymer glass fiber composite, the glass fibers are extremely brittle and hard, so more susceptible to crack. Whereas matrix material polymer is soft and ductile. The combined composite will have best characteristics of both such as stiff, strong and ductile.
Composites are broadly classified based on reinforcement material used.
1)Particle reinforcement composite.
2)Fibre reinforcement composite.
Particle reinforcement composite
They are cheapest and most widely used composites in today's technology.
They again divided into two types.
The best example is concrete. Concrete is made up of ceramic matrix and this matrix holds reinforcement particles known as discontinuous fibers.
here we use very hard and small particles to strengthen. Mostly we use oxides or nitrides so that they act as barriers to crack propagation in the matrix as well as they are suited for high-temperature applications.
Fiber reinforced composites
In many applications like racing cars, in aerospace industries or in automobiles we see that reducing weight without affecting strength and stiffness is desired and is challenging for a designer since if we reduce weight by 10 kgs for an aircraft or racing car that improves fuel efficiency and reduces operating expenses.
The designers made it possible by using composites in aerospace and automotive applications. By using a matrix material which has very low density reinforced with strong and stiff fibers. The strength of the composite depends on the length of the fiber, the orientation of the fiber with loading axis. The interfacial bond also plays an important role, it should be defect free.
Examples
Kevlar and aramid fiber composite can be used as textile fiber, in some cases, it is also used as a bulletproof jacket
Wood consists natural strong and flexible cellulose fibers. surrounded and held together by matrix material called lignin. can be used for home furniture.
b)
The bonding between matrix and fiber material should be strong and defect free.This is the region where load transfer takes place from matrix material to fiber material. They should protect fiber material from oxidation in the high-temperature range. Let's suppose the bonding across the interface is too strong Stress cracking may occur in the matrix material. If bonding across the interface is very weak, The load cannot be transferred from matrix material to the fiber material effectively.
c)
Elastic modulus in longitudinal direction:
E=EmVm + Ef Vf .
The elastic modulus of fiber(Kevlar) = 130 Gpa.
Volume fraction = 0.3
Elastic modulus of matrix(resin) = 5 Gpa
Volume fraction = 0.7
Elastic modulus of composite in longitudinal direction = 130*0.3+5*0.7 = 42.5 Gpa
Elastic modulus in transverse direction = EmEf /( VmEf + Vf Em)
Elastic modulus of composite in transverse direction = (5*130)/(0.7*130+0.3*5) = 7.02 Gpa.
d)
They are cheapest and most widely used composites in today's technology.
They again divided into two types.
The best example is concrete. Concrete is made up of ceramic matrix and this matrix holds reinforcement particles known as discontinuous fibers.
here we use very hard and small particles to strengthen. Mostly we use oxides or nitrides so that they act as barriers to crack propagation in the matrix as well as they are suited for high-temperature applications.
As we know that the composites are highly anisotropic in nature for a composite with long fibers, the modulus of elasticity in the transverse direction is 5 times less compared to the longitudinal direction. So these composites cannot be used for transverse loading or they are not effective and leads to failure of the component of we subject it to transverse loading. If we have an application where we need the significant strength of composite in transverse direction too, we can use particulate composite where small particulate fibers in the order of 10 micrometers to 50 micrometers are randomly arranged within the composite. Owing to the small size of particulate fibers, large volume fractions are possible so large loads can be carried by composites and random orientation brings homogeneity and isotropic behavior to the composite. So the composite now is capable of taking loads in longitudinal and transverse directions.
Mechanical recycling is adapted for recycling discontinuous fibers. This technique consists of grinding materials more finely after a first crushing or shredding step into smaller pieces. It is difficult to recycle continuous fibers compared to discontinuous fibers.
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