Manufacturing A Composite Bumper Using Thermoplastic Polymer And Chopped Carbon Fibres

Properties of Microstructures

• Discuss the differences between Pearlite, Bainite and Martensite in carbon steels. Sketch the microstructures and discuss the properties of each phase (e.g. stiffness, strength, toughness, ductility) with reference to typical applications.

• Pearlite, Bainite and martensite all contain alpha –ferrite and cementite phases. Pearlite has two phases of ferrite and cementite both in different layers. Pearlite has both these phases and they will alternate with each other. Bainite consists of very fine and parallel needles of ferrite that are separated by elongated particles of cementite. Martensite is nothing but transformed austenite to room temperature by rapid quenching process (Gensamer, 2012).

Properties of each phase of the Iron-Iron carbide diagram:

Pearlite:

Pearlite mechanical properties are between the soft ferrite and that of hard cementite. It is not too soft like ferrite and at the same time not too hard like cementite. Hardness of the material is less brittle than cementite. Pearlite and its high strength is mainly useful in enabling the material to draw into thin wires.

Bainite:

Toughness of bainite is different for upper and lower bainitic regions. The toughness of the upper bainitic regions is less tougher than the lower bainitic regions. Lower bainitic microstructure material is used for the sake of tool steel making applications where there is need for moderate toughness. Bainite enables good weldability as well is good for making forging steels and related materials.

Martensite:

The specific characteristic of martensite is its high brittleness. The material is brittle as well is also harder than other microstructures. Martensite is employed for making cutting tools employed for making utensils etc.

• With the aid of a diagram, explain the micro-structure of Ferrite-Martensite Dual Phase (DP) steels and discuss the advantages of using DP steels incar manufacturing(Sun,2002)

Ferrite and martensite dual phase steel as the name implies will contain both the ferrite and martensite constituents in the microstructure. They are actually produced by quenching low and medium carbon steel from above A1 temperature and below A3 temperatures. The process employed is a continuous cooling transformation process. Overall behavior, morphology and characteristic properties of the material are actually governed by the volume fraction, morphology and other related properties of the microstructure. The key properties of the micro structure consists of low yield strength, higher initial strain hardening rates, good fatigue resistance. The unique property profile of the material is mainly due to the presence of the micro constituents of the material. Multi material concept is equally competent like any other high strength and low density material like aluminum etc. Using such materials for car industry is equally advantageous for car making applications like sheet metal forming etc and hence multi material concept is useful for car making requirements and being used at present more commonly.

As an engineer working for a major car company you have been asked to investigate the possibility of manufacturing a composite bumper using a thermoplastic polymer and chopped carbon fibres. In you report

• Describe the manufacturing

Carbon fiber reinforced polymer composite material is the material being discussed in the current context. CFRP can be beneficial to use in the car company. The material has all the attributes required for making good components for running a car. Typical operations that can be employed for making CFRP with thermo plastic polymer and chopped carbon fibres include Wet layup as well as pre-peg processes.

• Wet-Layup ensures moderate properties to the material, typical steps in making the material include the following,

Manufacturing Processes of CFRP

Mould preparation,

Fabric cutout

Applying the patent agent

Resin preparation

Keeping the carbon fabric

Application of resin on carbon fabric

Applying resin and placing the layer

Curing of the resin will yield final material

• Prepeg process and key steps involved in,

Thawing the material

Prepeg cut into the required shape and size

Backing paper will be removed

Squeezing roller to remove the entrapped air between the sheets

Vaccum bagging is performed my removing all the file, breeder, barrier film, breather and bagging material.

Keeping into the autoclave

Cure cycling is done using the computer cycle

After cooling vaccum bag is removed and the part is taken out.

• If the fibres are 10 µm in diameter, the tensile strength of the fibres is 3500 MPa and the fibre- matrix fibre strength is 100MPa, what is the criticallength?

Equation for critical length is

Length (l) /10*10^-6 =  (3500*10⁶)/2*(100*10⁶)

Hence the length(l) = 0.175mm (critical length) or alternatively = 175 micro meters

• A number of manufacturing production routes have been developed for producing CFRP components. One that has generated a great deal of interest is Automated Tape Laying. Withthe aid of diagram explain the operation of Automated Tape Laying 

Automated tape laying operation consists of laying composite material tapes for making the composite material components.

Principally the device either will use gantry style systems or even robotic systems for the sake of delivering the fiber to the part. The tight and strong overhead of the machine will keep the tape and fibers intact on to the component. Thermoplast or Thermoset resins are employed with the machine for the sake of automated tape laying requirements.

• What are the advantages of Automated TapeLaying?
• The process is highly automated and hence precision and quality as expected will always be available.
• Manual interference will be minimized and hence the process is faster
• Contamination as well negative impacts on the human body are totally eliminated
• Applicable for diverse range of components

• Outline the property requirements of hot roller tool steel for steel rolling operation and discuss the advantages of precipitation hardening intool 

Hot roller tool steel need to have hardness, toughness and strength to take up the high loads at higher temperature. Material also should have high thermal and corrosion resistance particularly at high temperatures.

Precipitation hardening will provide high strength and good corrosion resistance. Also the material will provide high fatigue strength, good resistance to galling. Also the material will possess good corrosion resistance as well.

• Explain how precipitated particle (e.g. carbides) will enhance the strength of a material with reference to particledislocation 

Precipitated particle will contribute to the increase in the strength requirement for the movement of the particle in the lattice. Actual precipitation will act as an obstacle for the dislocation motion. Once precipitation occurs, there it will be required to apply more stress to move the dislocation through the lattice. This will make the alloy harder and stronger in the material (Martin, 2012).

• B1 is a Ternary phase diagram of stainless steels. Sketch a simple diagram onto the answer sheet and illustrate how the point of the 18-8 stainless steel on the diagram isdetermined.

 

 Fig. B1

The figure above can be quite comfortably employed to determine the location of the 18-8 point on the curve. 18% chromium is the percentage of chromium in the tool steel and 8% is the percentage of nickel in the material.  Hence for determining the point, opposite to the vertex of Cr, a line is plotted parallel to that side and passing through 18% chromium composition line. At the same time another line is drawn opposite to the vertex of Ni and parallel to the line indicating the percentage compositions of iron. The point of intersection of these two lines will provide the point 18-8, which is 18% chromium and 8% Nickel percentage. The corresponding line passing through this point and parallel to the chromium percentage line will provide the percentage of iron contained inside. Percentage of iron is dependent on both the percentages of chromium and percentages of nickel. In the above figure line1 and line2 will indicate the two lines described in the description.

• With the aid of diagram, describe: (i) the molecule structure difference between thermoplastic and thermosets, and (ii) the deformation of a linear and cross linked polymer structure under tension load.

Equation for Critical Length

Thermo plastic materials can be remelted again and again for different shapes and component requirements whereas thermo setting materials are once for all set for the final shapes, once they are provided with certain shape that is final and that can be remodeled back to its original shape.

Following are the molecular structures of thermoplastic and thermo set materials,Thermo set polymers: Thermo set polymer molecules are cross linked in all the three dimensions.  Such a cross linking will actually result in very strong structure of the material. Thermo set plastic materials will actually soften after heating for the first time. When they are allowed to soften they can take a different shape, As they are set in a permanent and stiff molecular structure, they will not change further by reheating or even by reshaping like in the case of thermo plastic materials.

Thermoplastic materials do have linear as well as branched molecular structures. They are quite flexible at all the ordinary operating temperature ranges. They will become pasty and liquid masses when they are heated. This is the limitation that will make the thermo plastic materials applicable only at the room temperature.

Linear and cross linked polymer structures under tension loads:

Cross linking in the polymers will result in the formation of the stiffer and stronger polymers. They will get highly stronger by chemical bonding in actual practice. Linear polymers consist of single continuous chain of repeated units, they will form of weak bonding when compared with the cross linking and hence with the same tension applied the strain generated will be higher in the linear linked polymer structures.

• B2 is a diagram showing the structure of semi crystalline plastic. (i) Sketch a diagram on the answer sheet and label the region for crystal and amorphous structures. (ii) With the aid of a heat flow vs. temperature curve, describe the difference in thermal behavior of crystalline structure and amorphous structure upon heating and, discuss at what temperature range, moulding process shouldbe performed(Kubaschewski,2013).

 

Fig. B2

The crystalline and amorphous regions in the structures are shown as in the following figure,

Thermal behavior of the crystalline and the amorphous substance is quite different and the following

Details will illustrate the difference in the thermal behavior of the two,

The melting point of the crystalline substance is very sharp, the bonds in the material will all break at the same time. Hence the structural change of the material will happen at a specific point of time and hence the melting point of the material is said to be quite fixed and sharp. The melting point of the amorphous substances is not sharp, this is due to the reason that the bond strength is not uniform in the material, different atoms, molecules in the structure will have different type of strengths and hence the melting point of these materials is not very sharp.

Automated Tape Laying Operation

Moulding process of semi crystalline substance need to be performed above T_g, the glass transition temperature. When it is heated above T_g and less than the melting point temperature, the material will get sufficient time for the sake of crystallization. Hence higher than T_g is preferred, at the same time, when the material is crystallized above T_g, there is also sufficient provision for the material for increasing its elastic modulus or stiffness of the material. The stiffness of the material molded above T_g will be higher than the material molded at below the glass transition temperatures.

• Discuss the main factors (at least 3) that affects the propertiesof  

Factors that effect the properties of polymers,

Chain length of the polymer,

Side group of the material

Branching contained in the material

Also cross linking and the temperature of the polymers will also impact the properties of the polymer materials.

• Outline the properties of Ceramics in comparison to metals and give THREE typical examples of ceramics and their applications, briefly discuss why the ceramic material is being

Ceramics materials do have high thermal resistance when compared to the metals. Also ceramics materials do possess higher compressive strength when compared with metals. Also ceramic materials will possess higher brittleness when compared with metals(Munz,2013).

Typical applications of ceramics include high temperature resistant components like space craft outer casing elements, applications like tool steels where there will be exposure to very high operating temperatures, Also ceramic materials are used in the making of composite materials like cermets, where ceramics will be combined with metals for making variety of components as well as tool steel materials. Ceramic lining is employed in automobile cylinders for high temperature resistant applications(Lewis,2013).

Ceramic material is a polymer and unlike metal will not possess corrosion and there is high thermal resistance and thermal endurance possible. Due to these reasons ceramics are employed for variety of applications.

• Describe the two following toughening mechanisms ofceramics

• TransformationToughening

Transformation toughening process consists of  impeding the ceramic structures using stress induced martensite like materials as second phase for the sake of toughening the structural ceramic materials. Transformation of such dispersed phase of martensite material will provide the necessary toughening of the material.

• MicroCracking

Single phase polycrystalline ceramics are normally subjected to the micro cracking process. The grains in these materials are anisotropic. In these materials tiny cracks open up on either way of the main cracks in these materials. Formation of micro cracks in both these sides will work on to cause two effects, the actual energy required for the formation of the first crack will be increased. This in turn will limit the tendency of the formation of the crack. Secondly when the main crack is propagating the secondary cracks formed behind the main crack as well behind the main crack front will work on to increase the volume of the material, the increase in the volume of the material will try to close the formation of the main cracks in the material. This inturn will work to increase the resistance to the propagation of the main crack.

• Outline two typical applications of single crystal materials. In each case, briefly discuss why the single crystal isbeing 

Hot Roller Tool Steel

Single crystal lattice will be contained in such single crystal material. The crystal lattice is unbroken till the edge of the material and will not contain any grain boundaries. As there are no any grain boundaries the material will offer unique mechanical, optical and electrical properties.

Some typical applications include applications in optics, electronics and ornaments like gems. Unique properties of better optical outlook and better physical and mechanical properties will be make them fit for these applications.

The aerospace industry is using Carbon Fiber Reinforced Plastics (CFRP) for an increasing number of its applications. The reason for this is that CFRP has the material properties that are of great interest to the aerospace sector, due to its high strength to weight ratio.

• With the aid of diagrams explain the tensile stress-strain behavior of CFRP component in a longitudinal loading and what useful behavior does CFRP have with respect toits failure mode?

Tensile stress strain behavior of CFRP will be like as shown in the following figure, the stress variation will be directly proportional to the strain rate and the following figure will illustrate this change,

CFRP will have more consistent phase during which the elastic modulus will remain constant. There will be well defined failure of the material at the yield point, the following figure will illustrate the elastic failure response of each of the materials carbon, Glass, Glare and caral fibre reinforced composite materials(Owen,2016). 

• What will be the modulus of elasticity of a composite in the longitudinal direction, given the following(Anderson,2017):

Em: Moduli of Elasticity for the Matrix = 6.9 X 103 MPa Ef : Moduli of Elasticity for the fibres = 72.4 X 103 MPa Vm: Volume fraction of matrix = 40 %
Elastic modulus of the composites will be given by,

= E_c = 0.4 * 6.9*10³ + 0.6 * 72.4*10³ = 46.2 * 10³ MPa.

• With the aid of diagrams describe the 4 possible orientations of fibres in a CFRP

Following are the four orientations of the fibers in the CFRP,

Figure 1 Fiber orientations

Unidirectional fibers consists that all fibers running in single and parallel directions in the composites. If supposes the arrangement of the fibers in the matrix composition is arranged in layers and if the composite contains one layer it is termed as unidirectional single layer composite and alternatively it is called as bi-layered unidirectional composite material.

Depending on the angle between the two adjacent layers, fiber orientation directions, they are again termed as Bi directional if having different directions or unidirectional bilayers, if having same direction. In bidirectional composites, the angle between fibers can be 90⁰ or 45⁰ as applicable. If incase there is no any specific direction for the fibers and they are randomly oriented, then the direction is termed as random orientation. All the above descriptions shown in the figure.

References:

Anderson, T. L. (2017). Fracture mechanics: fundamentals and applications. CRC press.

Gensamer, M., Pearsall, E. B., Pellini, W. S., & Low, J. R. (2012). The tensile properties of pearlite, bainite, and spheroidite. Metallography, Microstructure, and Analysis, 1(3-4), 171-189.

Kubaschewski, O. (2013). Iron—Binary phase diagrams. Springer Science & Business Media.

Lewis, M. H. (Ed.). (2013). Glasses and glass-ceramics. Springer Science & Business Media.

Martin, J. W. (2012). Precipitation hardening: theory and applications. Butterworth-Heinemann.

Munz, D., & Fett, T. (2013). Ceramics: mechanical properties, failure behaviour, materials selection (Vol. 36). Springer Science & Business Media.

Owen, M. Y. (2016). Fatigue of carbon-fiber-reinforced plastics. Composite materials, 5, 341-369.

Sun, S., & Pugh, M. (2002). Properties of thermomechanically processed dual-phase steels containing fibrous martensite. Materials Science and Engineering: A, 335(1), 298-308