Composite Materials Literature Review For Car Bumber: August 2016 [PDF]

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Composite Materials Literature review for Car bumber Research · August 2016 DOI: 10.13140/RG.2.1.1817.3683



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Composite Materials Literature review Chidubem I.W. Ezekwem Department of Physics, Loughborough University



Abstract For cars, weight and fuel efficiency are two important issues. Research has shown that the best way to improve fuel efficiency is to reduce the overall weight of the car, without sacrificing the safety of the passengers. The use of composites in the production of car parts has proven to be able to balance the reducing the weight and preserving passenger safety. The car bumper is one of unique area that has benefited from the use of composite material. In this review, two bumpers made from nylon-6 nanocomposite and polyethylene/palm kernel shell-iron filings composite are analysed. The mechanical properties of these composite materials are then compared to conventional car bumper material such as aluminium and steel, this comparison showed a reduction in weight, cost and environmental impact, disadvantages such as difficulty in mass production and sophistication of the production process were also noted.



Contents Abstract ................................................................................................................................................... 1 Introduction ............................................................................................................................................ 2 Bumpers .................................................................................................................................................. 2 Bumper material requirements .......................................................................................................... 2 Aluminium bumpers ........................................................................................................................... 2 Steel bumpers ..................................................................................................................................... 3 Dimensions and Properties of existing steel Bumpers ................................................................... 3 The production of steel bumpers ................................................................................................... 3 Composite Bumper ................................................................................................................................. 3 Composite materials used .................................................................................................................. 4 Glass fibres ...................................................................................................................................... 4 Epoxy resin ...................................................................................................................................... 4 Design of a composite bumper ............................................................................................................... 4 Recycled Polyethylene/palm kernel shell-iron filings composite ................................................... 5 Nylon-6 ............................................................................................................................................ 6 Summary ......................................................................................................................................... 7 Appendix ................................................................................................................................................. 8 Bibliography ......................................................................................................................................... .10



Introduction This literature review will focus on car bumpers, the National Highway Traffic Safety Administration (NHTSA) defines a bumper as a shield made of aluminium, plastic, rubber or steel that is mounted on the front and rear of a passenger car. It is expected that in low speed collision the bumper system is to be able to absorb the shock to reduce or prevent damage to the car; some make use of energy absorbers or brackets and other foam cushioning materials [1]. Following current trends, weight reduction has become a key focus of automobile manufacturers. Sustainable use and development of natural resources is the main focus of automobile manufacturers in the present market [2]. Achieving the above calls for a more innovative approach to better materials, more effective manufacturing processes and an introduction of superior design concepts.



Bumpers Car bumper is a safety system used to counteract low speed collisions, it is placed in the car body and designed to prevent or reduce physical damage to the front and rear ends of the car during low impact collisions [1].



As stated earlier, the National highway traffic safety administration produced a set of standards which govern the base requirements of bumpers for passenger automobiles. The bumper standards imposed are [3] [4]:  



 



Front and rear bumpers on passenger cars should prevent damage to car body, Bumper should be capable of withstanding impacts at 2 mph across full width and 1mph on corners, Bumper should be able to withstand 5 mph crashes with parked cars, Bumpers are to be placed between 16 to 20 inches above road surfaces.



Satisfying these conditions during low speed impact collisions is paramount.



Bumper material requirements     



Able to absorb more energy while in collision Easy for large scale manufacturing, High resistance to rust, Light in weight, Low cost.



Figure 1: Structural Framework of car (©http://www.gettydesign.com/catalog/996/bodywork/diagram.jpg)



Figure 1 above shows a simple structural framework of passenger cars as it can be seen there are a total of two bumpers in the car structure them being the front and back bumper. Bumpers are not generally designed to be significantly contributed to the crash wordiness of the car during front or rear impact collisions; it should not be mistaken as a safety feature intended to prevent severe injury to the occupants. Bumpers also serve to protect the front fenders, trunk/deck lid, exhaust and cooling systems as well as critical safety related equipment such as headlights, taillights and indicators in the event of low speed collisions.



Aluminium bumpers Aluminium bumpers are advantageous in the sense that they are lighter and stronger than steel. In contrast, they are much more expensive and prone to major fractures. It shot to fame through race cars but has since been overshadowed by carbon fibre [5] [6]. Ferrari has reported that: aluminium bumpers are easier to shape than carbon fibre ones and the weight increase is negligible [7]. In present work, the aluminium bumper used in passenger type cars is being replaced by composite materials made up of glass, carbon fibres, etc. and nanocomposite materials such as nylon-6 which is a



Nano clay-polyamide. The bumper thickness for composite bumper, when calculated through bending moment equation and other dimensions for steel, aluminium and composite bumper is considered to be the similar. Comparing the stress, weight and cost saving is therefore objective [8] [9].



Steel bumpers Steel bumpers [9] have many advantages such as their relatively high load carrying capacity and high ductility. However, this gives a low strength-to-weight ratio. Car manufacturers have stated that using steel adds to the aesthetic as well as minimizes life cycle costs.



Dimensions and Properties of existing steel Bumpers The table below gives an overview of the dimensional properties of a chromium coated steel bumper currently use Table 1: Details of an already existing steel bumper



Effective length Total length Thickness Effective breath Total breath Weight Tensile strength Density Cost



0.975m 2.055m 0.002m 0.078m 0.172m 5.16kg 460 MPa 7800 kg/m3 $3600



The production of steel bumpers 2mm thick steel sheets known as ‘blankets’ are fed through a series of dyes (7-8), depending on the bumper model. Each dye stamps the blanket to a particular shape using roughly 2000 tons of force. This progressively forms the blanket into the final bumper shape. Both front and rear bumpers go through the same process, the only difference being the dye used on each. The newly shaped blankets then travel via conveyor belt to the next phase of production where workers then clamp each blanket unto a specially designed cart. At this point the blankets are passed through a series of buffing wheels and then submerged in several cleaning tanks to remove any residue left on it. The blankets are then further inspected after which they undergo a plating regime. Ensuring no defect is present on the blanket is paramount because the plating process magnifies even the smallest of blemishes. The first process involves applying a coat on nickel to the blanket to protect it from corrosion, after which a chrome layer



is added. This is done using a standard electro plating process in water and chemical filled tanks where the particles of the plating metal are laced with a positive charge. When a negative charge is passed through, a magnetic field is created. This field draws the particles onto the blankets in even layers. The blankets are then subjected to a thorough rinse and then inspected by workers under high intensity lighting. Plastics are then pressure-injected via machinery into various moulds, these machines then fast harden the plastic using flash-freezing thus producing plastic components which are then added to the blankets. One of these plastic parts is the step pad that covers the topside of the rear. Once in place, workers then attach built-in hitch steal and steel mounting brackets, these add to the structural integrity of the bumper. Front bumpers have plastic trims which hang down slightly below the bumper helping to direct the air flow to the engine compartment due to their aerodynamic shape. Four steel reinforcement brackets are used to attach the bumper to the car’s frames before the licence plate holder and fog lamps are inserted and bolted in. All the bolts are set to specific tightness coefficient to ensure the bumper and its mounting brackets will adequately absorb the force of a Collison.



Composite Bumper In recent years, the automotive industry has advanced a great deal and composite materials have played a large role in this revolution. Various composite materials have been experimented on in most parts of cars. Manufacturers fill polymers with particles in a bid to improve the toughness and stiffness of the materials, as a means of enhancing their barrier properties as well as enhancing their resistance to fire ignition, or simply to reduce costs. The addition of particulate fillers can lead to drawbacks such as brittle or opaque composites. This has led to the production of a new type of composites known as nanocomposites. These composites are particle-filled polymers which have at least one dimension of dispersed particles in the nanometre range. They are distinguishable based on how many dimensions of dispersed particles are in their nanometre range, for example a nanocomposite with three dimensions in the nanometre range is known as an isodimentional nanoparticle, such as



spherical silica nanoparticles obtained by in situ solgel method [10] [11] or by polymerization directly on their surface [12]. Amongst all the possible nanocomposite precursors, clay-based and layered silicates have been subjected to more rigorous investigation, this is likely due to the availability of the clay material and their chemistry already being extensively known. Due to the reduction in weight, some manufacturers prefer composite materials over their steel counterpart. Some key advantages include:      



Absorbs more collision energy, Easier to achieve smooth aerodynamic profiles for drag reduction, Outstanding resistance to corrosion, High impact resistance, Rapid response to induced or released stress, Reduction of part count and production cost.



Composite materials used Glass fibres Fibre reinforced plastics combine the strength and stiffness of fibrous materials. Materials produced through this means possess very high resistance to corrosion, low density and easy moulding capability. Majority of reinforced plastics produced recently are either polyester resins or glass reinforced epoxy. Glass fibres make good reinforcing agents, due to the relative ease at which high strength can be obtained in using a few microns in diameter.



Properties of Glass Fibres The properties of glass fibres are:     



Corrosion resistance, Electrical properties, Resistance to impact, Low density, Specific strength.



Types of Glass Fibres The most commonly used glass fibres are E-glass, Sglass, C-glass and D-glass. E-glass stands for electrical glass as it was designed for electrical applications, Eglass fibres are high quality glass fibres used for standard reinforcement for resin systems which comply with the necessary mechanical properties. The S in S-glass stands for high silica content. It retains its strength at high temperatures and has high fatigue strength. It is largely used in aerospace applications.



The C in C-glass stands for corrosion. It is designed to have an improved surface finish with a high resistance to corrosion. In D-glass, the D stands for the dielectric which is used for applications requiring low electric constants. Advantages of Glass Fibres Glass fibres are most widely used as reinforcements for composites due to the following advantages:   



Easy to fabricate, Molten glass can be easily drawn into highstrength fibres, Relatively strong fibres produce very high strength in its composite form.



Epoxy resin These are low molecular weight organic liquids which contain epoxide groups. Epoxides have 1 oxygen and 2 carbon atoms in its rings and are formed by most reactions between epichlorohydrin and aromatic amines. Hardeners, plasticizers and fillers can be added to produce epoxies with wide ranges of properties from viscosity to impact. Even with its high cost when compared to other polymer matrices, its popularity overshadows the rest. The main reasons are:    



Availability and diversity, Good compatibility with glass fibres, High strength, Low viscosity.



Epoxy resin is more expensive than aluminium and steel, roughly retails at $1 to $10 per pound. With glass fibres starting at around $1 they are pricecompetitive with aluminium and steel only when being used in small quantities. Also, production of these materials one a large-scale (i.e. volumes of at least 30,000 units per year) requires large investments in technology.



Design of a composite bumper For the design of a composite bumper two cases will be considered: one involving nylon-6; a nanocomposite designed by Toyota; and a recycled polyethylene/palm kernel shell-iron filings (CPKS) composite manufactured by students at the Ahmadu Bello University Zaria, Nigeria.



Recycled Polyethylene/palm kernel shell-iron filings composite A project carried out by the research students at the department of mechanical engineering Ahmadu Bello University in Zaria, Nigeria, produced a new composite material with properties suitable for car bumper manufacturing. A matrix made up of empty water sachets was reinforced using carbonized palm kernel shell particulates (CPKS) and iron fillings, using a percentage composition of 5 wt%1 for iron fillings with CPKS varied from 5-20 wt% at 5% intervals. The physical and mechanical properties of the composite were tested alongside current bumper materials samples. Empty water sachets, CPKS and iron fillings are all classed as waste products in Nigeria and as such pose huge environmental challenges. Research into composite material production using palm kernel shell (PKS) had already been ongoing with varying results [13] [14]. Materials AND METHOD The manufactured composite was made out of recycled low density polyethylene, CPKS and iron fillings. Pre-analysis were carried out on each material, with the aid of a PW 00 X-ray spectrometer, an x-ray fluorescence test was carried out on the CPKS after which the CPKS were ground into powder form and weighted and a binder was then added to the sample, which subsequently mixed and pressed into pellets. Next, the tensile strength of the RLDP was recorded. Table 2 below displays the percentage composition by weight for the RLDP and the CPKS. Table 2: Formulation of Carbon Material [15]



S/n



1 2 3 4 5



Carbonized Palm Kernel 0 5 10 15 20



Iron Filings (wt %) 5 5 5 5 5



RLDP (wt %) 100 90 85 80 75



Sample Label CPKS 0 CPKS 5 CPKS 10 CPKS 15 CPKS 20



The iron fillings percentage was kept constant due to results from previous research noting this as an optimal amount.



1



Wt%- mass fraction



Raw palm kernel shells (PKS) were sourced from local palm oil processing plants; the shells were heated in a furnace to about 800°C turning it into carbon ash using a process known as ashing. The ash was then sieved to remove unwanted contaminants. This was done in order to remove most of the moisture in the shell whilst retaining its carbon content.



Figure 2: Palm Kernel Ash particulate [15]



Figure 3: Iron Filings [15]



The iron fillings were gathered and sieved. The iron fillings, carbonised palm kernel shell and polyethylene were compounded into a two roll mill at a temperature of 130°C forming a homogeneous mixture. 400g of each composition was compounded and labelled. In a 150 nm sized square mould, the mixture was placed and subjected to a pressing pressure of 0.4MN/m2 until they cured, the temperature of each plate was kept at 150°C during this process, at the end of each press cycle the boards were removed from their moulds and left to cool before being cut into separate pieces for characterization. This process is known as pressing and it serves to increase the compatibility for the material. The physical and mechanical aspects of the material were studied using a range of tests and compared to that of three prominent conventional car bumpers in Nigeria Stress-strain properties The tensile strength indicates the compounds ability to withstand forces that pull it apart as well as its stress before break point; tensile tests were performed using a Hounsfield tensiometer, with a maximum load of 250 KN. After measuring the ultimate tensile strength, breaking stress, tensile modulus and percentage elongation of all composite and conventional car bumper types, the composite bumpers failed to surpass their conventional counterparts (See appendix figure 7-13). The conventional samples had tensile strengths ranging from 10.08 to 14.92 N/mm2 compared to the composite materials who’s highest was 7.94 N/mm2.



The tensile modulus for the composite material increases to a peak value of 29.92 N/mm2 at 15 wt% of CPKS, which then fell when more particulates was introduced. The introduction of reinforcement also resulted in a reduction of percentage elongation of the composite material; this can be attributed to the presence of two hard and brittle phases in the matrix. Hardness properties The hardness properties were measured at room temperature and recorded. The data shows an increase in hardness number in relation with the increase in percentage composition of reinforcement (see appendix figure 12); this can be attributed to the percentage of hard and brittle phases of the ceramic body in the polymer matrix. The large variation of hardness number of the composite materials is as a result of the distribution of the reinforcements in the matrix and can be solved by ensuring a more uniform distribution of reinforcements in the matrix. Impact Properties A reduction in the composites’ impact energy was noted as the concentration of CPKS increased. This is largely due to the reduction in elasticity of the material due to the addition of particles which reduce the deformability of the matrix, thereby reducing the matrix ability to absorb impact energy.



Figure 4: Schematic illustration for synthesis of Nylon-6/clay [20]



Figure 5: Formation of Nylon-6 Nanocomposite by situ polymerisation [21]



Nylon-6 Toyota Central Research Laboratory first reported their work on Nylon-6 in the early 1990’s [16] [17]. It was reported that small amounts of Nano-filler loading, results in a pronounced improvement in thermal and mechanical properties. The properties of Nylon-6 is not only a factor of its individual parent components i.e. Nano-filler and nylon, but also its morphology and characteristics [18]. Materials and Method Under appropriate thermodynamic interactions, polymers can spontaneously intercalate the galleries of organ clays. However the static diffusion cannot lead to full exfoliation [19]. Toyota disclosed an improved method for producing nylon-6/Clay nanocomposites using an in situ polymerization that exfoliates the alum inosilicate layers through a chemical mechanism. The Toyota process can be seen in figure 5 below. Also shown below in figure 4 is the Schematic illustration for synthesis of Nylon-6/C lay.



As shown in the figure above, sodium montmorillonite is mixed with an aminolauric acid in an aqueous hydrochloric acid to protonate the aminolauric acid which then exchanges with the sodium counter ions. Alkyl units of the resulting organ clay have terminal carboxyl groups. Under certain conditions, these carboxyl groups initiate ring-opening polymerization of caprolactam forming nylon-6 chains which are ionically bonded to the alum inosilicate platelets. Driven by the free energy from the polymerization, the chains grow forcing the platelets apart until exfoliation is accomplished. According to a report written by a team from the chemical engineering department of Texas Materials institute, nanocomposites would have been more widely used if they could be formed from existing polymers using conventional melt processing techniques such as injection moulding and extrusion [21].



Stress-strain properties One notable benefit of adding high aspect ratio, Nano scale platelets to the polymer is the increase in modulus per unit mass of reinforcement. This results in the material demonstrating higher strength, hardness and scratch resistance [21], as well as a sizably increased stress at break. This is explained using the presence of polar and ionic interactions between the polymers and its layers. Figure 13 show the relationship between modulus and molecular weight of the nylon 6 matrix. The graph shows the higher level of modulus at given MMT loading. The stress at break was found to be sizably strong. A car bumpers ability to function at relatively high stress and strain conditions is essential; and the stress-strain properties of nylon-6 make it a good choice. Thermal expansion Behaviour Polymer nanocomposites are expected to have improved thermal expansion properties, while retaining the processing and surface characteristic of its matrix owing to the small size and low content of the Nano-filler [21]. Through the use of a high resolution transmission electron microscopy (TEM), the orientation of the clay platelets in the nylon 6 nanocomposite was viewed. As seen in figure 14, the platelets are better aligned in the FD axis than their TD counterparts with there being little alignment in the ND axis. The thermal expansion of the high molecular weight nylon-6 nanocomposites possesses thermal expansion coefficient in the rubbery state on par with that of those below the glass state, this improved thermal expansion means deformation in the car bumper due to temperature conditions is kept as a minimum. Impact properties The formation of Nylon-6 nanocomposites does not result in significant reduction in the impact properties of the material, the stiffness and strength of the nanocomposite are greatly improved as the amount of organo-clay is increased. However, the IZOD2 impact strength is reduced from 20.6 to 18.1 J/m when 4.7 wt. % of organo-clay is incorporated. This is still a relatively good impact resistance value for low speed impacts. The impact density ratio values for the nanocomposite further supports the use of Nylon-6 for the production of car bumpers. More mainstream bumper materials offer better impact resistance properties at low and high speed impact conditions.



2



ASTM standard method of determining the impact resistance of materials.



Effect of clay on nylon-6 crystallization Isothermal crystallization studies at 197°C shows that small crystal platelets act as nucleating agents for crystallization for the nylon-6 matrix. At this temperature, the crystallization half-life, 𝑡1⁄ is 2



normalized by that of the extruded matrix polymer without any clay. This property is of particular commercial interest. While clay increases the number nuclei, high clay loading retards polymer crystal growth.



Summary This research work has reviewed two key composite materials along with their various production routes, advantages and disadvantages. In the case of the Palm kernel Shell-Iron filing composite, it was noted that most of the composite materials’ mechanical properties were lower than that of their conventional counterparts. In addition, the composite material with 5wt% of and 10 wt% CPKS were recommended for use in the production of car bumpers due to their high impact energy to density ratios of 0.2 and 0.19 respectively, which puts them close to that of the standard failure mode exhibited during their testing. In addition, the materials used are  



Environmental impact, Low costs (roughly a 77.2% reduction); steel bumper cost $3600 whilst its CPKS counterpart was valued at $820.



Nylon-6 nanocomposites present a many possibilities: these nanocomposites not only exhibit excellent mechanical properties, but also display outstanding combination of optical, electrical, thermal, magnetic and other physico-chemical properties. One advantage of nanocomposites is that the strength, shrinkage, warpage, viscosity and optical properties of the polymer matrix are not significantly affected; another advantage is their mechanical, electrical, thermal, barrier and mechanical properties such as increased tensile strength, improved heat deflection temperature, flame retardant, etc., which. can be achieved with typically 3-5 wt.% loading. However, there are huge limitations in producing them, such as costs, processing constraints, oxidative and thermal instability and unstable market share [22].



Appendix



Figure 9:Tensile modulus for the PKS composite material compared with conventional material [15] Figure 6:Breaking point for PKS composite material compared with conventional material [15]



Figure 10:Percentage elongation for the PKS composite material compared with conventional material [15]



Figure 7:Ultimate tensile strength for the PKS composite material [15]



Figure 11:Impact strength of PKS compared with conventional materials [15] Figure 8:Density of PKS composite materials compared with conventional materials [15]



Figure 12: Hardness number for PKS composite material [15]



Figure 14:Orientation of clay platelets in nylon-6 nanocomposites as determined by TEM [21]



Figure 13: Effect of wt% on Modulus [10] [21]



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