Research On Sandwich Panels And Their Use In Motorsports

1. Describe the different panel options used by the group with some material performance details listed with pros and cons.

2. Reasons why you would use different cores and ply counts.
3. Specifically for the ply count and core used for your groups testing do the following:
a) Describe the method of construction as a guide.
b) Describe the test methods used.
c) Analyse the performance of testing versus the theory.

d) Analyse the shortfalls in knowledge. What else do you need to know to make engineering decisions regarding the choice of panel type?

4. Give examples from motorsports where your panel type would be used, the loading expected, the likely failure type and the cost of a component from this panel type.

Types of Sandwich Panels

This report paper is about the practising and learning the construction of sandwich panels through testing different panels to determine their sandwich core material and ply count. The objective of this paper is to discuss the process and method of manufacture, test and relation to motorsports of the sandwich panels. For these objectives to be achieved, there is need to research areas of motorsports application in which the sandwich panels will be used. Some of the subtopics discussed in this research paper include construction of diverse sandwich panel types, materials used for ply and core quantities, cons and pros of different methods, tests method used, and data gained.

The sandwich panels are types of laminated composite material. Composite structures in various forms have been used for hundreds of years. Technically, composite means a combination of materials, the early ships used the combination of metal and wood just like the early cars and aircraft. A sandwich panel is any structure made of thin skin-layer, a low-density core, and three layers bonded to each side. Sandwich panels are utilized in areas where a combination of low weight and high structural rigidity is a necessity. There are two types of sandwich panels, namely Aluminium composite panels (ACM) and Structural Insulated Panels (SIP) (Akovali, 2013).

Fibre Reinforced Plastics (FRP) have been adopted widely in the industry of motorsport for numerous motives, but the major reason being because they can be moulded into a virtually limitless variety of shapes comparatively easy. The top team have invested in a great deal of equipment to subject their composites material and components in the quest for increased knowledge that they hope will enable improved competitiveness. The equipment ranges from machines of electro-mechanical test that can accomplish relatively simple state load test in tensile, flexural and compressive tests (Allen, 2011).

A sandwich panel is any structure made of thin skin-layer, a low-density core, and three layers bonded to each side. Sandwich panels are utilized in areas where a combination of low weight and high structural rigidity is a necessity (Association, 2015). The structural insulated panels are the panels mused as a material for building. The figure below shows the structure of an I-beam and sandwich panel: 

The materials that are thermoplastic in nature are used as a skin material to produce sandwich panels so as to utilize some specific technical process of performance. The number of pre-preg layers to produce the required skin thickness are laminated together under pressure and heat. The required temperature will depend on the type of material chosen, however, it is likely to be about a range of pressure of 50 to 100 psi and temperature of 310^oC for in the majority of instances and a duration of 30 minutes. The cured flat laminates will then be bonded to a material core for some form like form or honeycomb by the use of a room temperature cure and tow part epoxy (Banerjee, 2013). 

Fibre Reinforced Plastics in Motorsport

There are numerous options for the panel depending on the material used for stiffeners of the sandwich panels, the panel options include fibre, resin, and fabrics. Fibre Reinforced Plastics (FRP) have been adopted broadly in the industry of motorsport for numerous motives, but majorly is since they can be moulded into the virtually limitless variety of shapes moderately easy. The FRP is a composite material composed of a reinforced fibre mass bonded with the matrix of plastic resin that will be involved in binding and holding them together. The characteristics of the resulting composite depend on the characteristics of the fibre components involved in the process. Some of the reinforcing fibre that can be used in making cars include glass fibre, aramid fibre, and carbon fibre (Bosch, 2007).

Glass fibre reinforced plastics which are commonly abbreviated as GFRP involves the use of glass fibre in plastics in motorsports in the construction of the bodywork. The show advantage of the GFRP is that it is readily available, result from its beneficial strength to work ratio, and it is also cheap. The major disadvantage of this glass fibre is that it can easily break after suffering from random surface flaws caused by application of stress on a piece of glass leading to the glass cracking and breaking. Glass fibres primarily exist in for of T, S, R, and E. E-glass are derived from alumina borosilicate and is characterized by its low mechanical properties, good stiffness, strength, and its cheapness (Bruce, 2012).

The S-glass was initially produced for military and aerospace applications and is characterized by its high silica content, water resistance, and is also costly. The aramid fibres have chemical and physical properties that are very extraordinary and different. This type of fibre is characterized by penetration resistance and high impact. Some of the advantages of the aramid fibres include resistance to abrasion, toughness, relatively low density, high stiffness, and high tensile strength. The major drawback of the aramid fibre is its unwillingness to be cut by anything (Davies, 2013).

Carbon fibres were initially produced for commercial use such as in the production of material for the filament in the original light bulbs of electricity. Carbon fibres are generated by organic precursor fibres thermal treatment that is controlled. The carbon fibres possess the highest specific stiffness, good resistance, and compressive strength. Polyester resin is an example of resin used for moulding and has the consistency of thin honey at room temperature that is normal. The polyester resin is normally made of petroleum derivatives and coal. Some of the advantages of the polyester resin include high strength, easy to use, cheap, and rapid curing (Fermín, 2016).

Materials for Panel Construction

The major core materials used in the construction of the sandwich panels include honeycomb material and polystyrene foam. The polystyrene foam should not be used together with polyester resin since the styrene in the compound dissolves the foam. However, the polystyrene is compatible with epoxy resin and it is characterized by shaped into different forms for stiffening constructions, can be sanded, it is weightless, and cut as well as it does not absorb resin. This core material does not have any significant properties of the structure and is applied when creating structural improvements such as strips of square sections (Institut, 2016).

Honeycomb materials are the second core material that is commonly used globally in the manufacture of components used in making composites. The honeycombs are normally made from either Nomex paper of aluminium and it is important to note that these materials are virtually weightless but effectively increases the laminate’s thickness without adding any weight. The honeycomb types which include Nomex and aluminium are appropriate for use with pre-pregs. The choice of the type to use depend on ease of use, availability, and cost (Koschade, 2016).

These core materials enormously increase the stiffness while adding flexural length significantly. The table below shows the gains which can be attained:

The figures in the table 1 above relate to a generic laminate of two-ply to the panels of the sandwich faced with skins of one-ply on both sections of two different honeycomb core thickness. The core that is thinner of the corresponding thickness provides the considerable increase in strength and stiffness whereas the core that is thicker, the combined thickness of the skin is three times and hence provides a stiffness increase for specifically 6% increase in the weight of the panel. These benefits are seriously taken in the context of their potential application on cars used for competition (Lund, 2011).

The panels of the honeycomb-cored sandwich performance depend on the bond amongst the outer skin of a laminate over the minute contact region. This is the reason why the honeycomb core sandwich panels are applicable in wet lay-up systems since the complete contact between the specific materials is not easy to attain especially when there is a degree of curvature. Honey combes are relatively cheap compared to the prices of carbon fabric and aramid on the basic of the square meter. Nomex honeycomb is laminated by the use of epoxy resin and then cure and tested in a similar manner as the former laminates so as to generate a given relative stiffness (Manbeck, 2012).

Core Materials for Sandwich Panels

Some of the techniques that are commonly used to produce structural components such as monocoque chassis include wet lay-up, dry lay-up, and constructed fibre. The wet lay-up technology is normally used especially when the structural component involves extensive material. The major drawback of the wet lay-up technique is that it is difficult to control the ratio of resin to fabric or the fraction of the fibre due to the problem of acquiring full distribution of resin in the whole of every laminate ply. This makes it difficult for the prediction of the functionality of the laminate since deviations in the fraction of the fibre leads to changes in mechanical properties (O'Rourke, 2016).

Pre-impregnated fabrics which are also abbreviated as pre-pregs are impregnated already with a system of pre-catalysed resin which cures with the heat application. The pre-pregs is done as an industrial process that is precisely controlled through either hot-melt technique where there is coating of fabric with a weight that is controlled by melted resin per unit area, or by solvent dip method where the fabric that is dry is dipped in a resin solvent and bath before drying in an oven to remove the solvent. Any of the two methods can give the user a variety of fibre choices and fabric styles and types with a maximum quality per unit area (Oscar, 2012).  

The primary principle of contact moulding of wet lay-up is that a product with specifically a single smooth section is created by lamination of fibre reinforced resin contacting the mould. There is no external pressure utilized but there is an application of an external heat. In case there is need of using pre-pregs that cure up to approximately 80 degrees, then there is need of using resin moulds based on wet lay-up polyester. Temperatures above 90 degrees would result in the resin beginning to soften. The dry lay-up technique is a method used in making reinforced products through the application of dry resin system while putting reinforcement in place (Plantema, 2015).

The primary considerations of lay-up of the number of pliers, fabric weight, weave type, and fibre type have to be determined before the beginning of the process of wet lay-up. Resin infusion is another method that begins with fabric that is dry being laid onto a mould or onto surface coat skin that is specialized in the gel coat. The process by which the impregnation of resin is done is to carry the dry fabric lay-up and then remove resin through the fabric by the use of suction or vacuum pressure. Dry lamination by the use of pre-impregnated enables the laminator and designer to attain the maximum performance from the materials chosen which becomes critical when the component planned is a structure that is load-bearing (Plantema, 2013). 

Construction Techniques and Testing

There are numerous standardized and standardized procedures that are used for testing composites and plastic materials in form of minute specimens. These test methods are essentially put in place for quality control and development of materials. The specification of ASTM covers thermal stability, melting points, wear resistance, fracture toughness, bending and tensile impact, notch toughness, crack-growth rates, fatigue strength under numerous loading types, stress relaxation, creep, deflection under load, torsional strength, shear, flexural, compressive, and tensile (Ravi, 2011).

The major consideration in this section is taken on the testing of components to evaluate the impacts of manufacturing and shape methods on the material properties from which they are prepared from. The stiffness and strength are proportional to the density of the material similar to the compression and shear. The load applied on the sandwich beam creates a bending moment which is optimum at the end that is fixed and the shear force along the beam’s length. In the sandwich panel, the forces create compression in the lower skin and tension in the upper skin. The spaces of the core facing the skins and transfers shear amongst them to make the panel make of composite function as a structure that is homogeneous (Standard, 2015). 

Some of the failure modes that designers of the sandwich panels should put into consideration during the analysis of the composite material include local compression, intracell buckling, skin wrinkling, shear crimping, panel buckling, stiffness, and strength. The strength of the core compression should be enough to prevent local loads on the surface of the panel which may lead to local compression failure mode. For any skin material, the size of the core cell should be minute enough to preclude intracell buckling. The compressive strength of the core and the compressive modulus of the facing skin should be sufficient enough to preclude a skin wrinkling failure (Standard, 2015).

The sandwich panel that was used in the laboratory for the purposes of the experiment was Nomex + 1 layer of glass fibre pre-preg. The sandwich core material or this composite is made up of the aluminium core + adhesive + fibreglass single layer.

The graph of extension against the load for the specimen 1 above was predicted to be linear based on the elastic analysis because the objective was to evaluate specifically the initial shear stiffness of the sandwich specimen 1. The relationship between the aluminium core + adhesive + fibreglass and the load applied based on the measured values for a 250mm by 250mm piece of Nomex thick specimen is a curve shown above. The graph shown that the extension of the sandwich specimen increases with an increase in the load applied until a maximum load of 100N is applied from which the extension increases while the load is constant (Taylor, 2011).

Conclusion

The material properties for the facing sheets were gotten from the tensional test results for every specimen of the sandwich panels. The elastic modulus of the specimen, E was gotten to be 489.464117Mpa. The test results shows that increasing the density of the whole thickness of the fibres leads to the creation of waviness and imperfection zone among the fibres hence leading to the reduction in elastic modulus and also tensile strength. The tensile strength of the sandwich specimen was found to be 109.45232N. The main reason for flexural testing is to determine flexural behaviour as well as flexural stiffness and shear stiffness.

The flexural strain at tensile strength was found to be 0.02010mm/mm and the flexural stress at the tensile strength of the sandwich panel specimen was found to be 6.56714Mpa.

The relationship between the aluminium core + adhesive + Nomex and the load applied based on the measured values for a 250mm by 250mm piece of Nomex thick specimen is a curve shown above. The graph showed that the extension of the sandwich specimen 2 increases with an increase in the load applied until a maximum load of 120N is applied from which the extension increases while the load is constant and then start reducing from 120N to 60N as the extension increases. The elastic modulus of the specimen, E was gotten to be 7244.08918Mpa. The tensile strength of the sandwich specimen 2 was found to be122.01978N. The flexural strain at tensile strength was found to be 0.00970mm/mm and the flexural stress at tensile strength of the sandwich panel specimen was found to be 43.55257Mpa (Wright, 2010). 

Sandwich beam has been adopted in the in the industry of motorsports for numerous reason, however, the major reason being that these composites can be moulded into the virtually limitless variety of shapes comparatively effortlessly. Bodywork curvature that is complex can easily be attained at low cost using less equipment that is specialized. After the mould have been generated from the main pattern, it is very easy to produce many replicas that are identical compared to manufacturing the original. The techniques of moulding do not give provisions for the mass production since it is labour intensive despite the cost of making moulds and patterns are low making minute production quantity to be commercially sustainable (Zenkert, 2014).

If this is compared with the injection moulding where the unit cost is very low and the cost of tooling is very high making it more appropriate to be used in mass production of specific components of plastics in some applications like those found on and in motorsport production. These are some of the reasons why sandwich beams moulding methods are currently being used in motorsports where volume required of specific components are normally minute, and where tool requirements and designs frequently vary (Bosch, 2007).

Some of the failure modes that designers of the sandwich panels should put into consideration during the analysis of the composite material include local compression, intracell buckling, skin wrinkling, shear crimping, panel buckling, stiffness, and strength. The excessive deflection failure occurs when the sandwich panel lacks sufficient shear and bending stiffness as shown in the figure below:

The shear crimping failure occurs when shear modulus and core thickness is not sufficient enough to avoid the core from failing hastily in shear loads of end compression. 

The skin wrinkling failure occurs when the compressive modulus of the core compression strength and the facing skin is not sufficiently high to avoid failure of skin wrinkling

The intracell buckling failure occurs when the size of the core cell for a given skin material is not small enough to prevent buckling of intracell

The local compression failure occurs when the core compressive strength is not sufficient enough to resist local loads on the surface of the load.

The cost of honeycombs materials are not cheap, but when compared to the cost of carbon fabrics and aramid fabrics on the basis of the square metre. The normal cost of 3mm thick Nomex honeycomb core for 1.25m by 2.25m sheet is approximately 125 Pounds, this is representing approximately 80 Pounds per square unit. The honeycomb made of aluminium is a little bit cheaper than the Nomex one (Taylor, 2011).   

Conclusion

This report paper is about the practising and learning the construction of sandwich panels through testing different panels to determine their sandwich core material and ply count. The sandwich panels are a types of laminated composite material. Composite structures in various forms have been used for hundreds of years. Fibre Reinforced Plastics (FRP) have been adopted broadly in the industry of motorsport for numerous motives, but possibly since they can be moulded into the practically limitless variety of shapes moderately easy.

A sandwich panel is any structure made of thin skin-layer, a low-density core, and three layers bonded to each side. Sandwich panels are utilized in areas where a combination of low weight and high structural rigidity is a necessity. There are numerous options for the panel depending on the material used for stiffeners of the sandwich panels, the panel options include fibre, resin, and fabrics. The major core materials used in the construction of the sandwich panels include honeycomb material and polystyrene foam. The polystyrene foam should not be used together with polyester resin since the styrene in the compound dissolves the foam.

For the specimen of sandwich panel used in this research, there is need of each group carrying out all the five groups of the sandwich panels so that proper analysis and the trend of the flexural tests of the sandwich panel can be determined and evaluated for the consistencies to come out clearly. These categories of the specimen that should be used by every group for this analysis include Nomex + 2 layers of carbon fibre pre-preg, Nomex+ 2 layers of carbon fibre pre-preg, Nomex + 2 layers of glass fibre pre-preg, Nomex+ 1 layer of carbon fibre pre-preg, and Nomex + 1 layer of glass fibre pre-preg.

Proper analysis of the sandwich panels could also be made effective by analysing the other properties of the composite such as its thermal stability, melting points, wear resistance, fracture toughness, bending and tensile impact, notch toughness, crack-growth rates, fatigue strength under numerous loading types, stress relaxation, creep, deflection under load, torsional strength, shear, flexural, compressive, and tensile.  

Akovali, G., 2013. Polymers in Construction. Paris: Smithers Rapra Publishing.

Allen, G., 2011. Analysis and Design of Structural Sandwich Panels. Chicago: Pergamon Press.

Association, A. T. E. W., 2014. Panel Design Specifications. Toledo: APA The Engineered Wood Association.

Association, A. T. E. W., 2015. Mechanical Properties of APA Structural Panels. Toledo: APA The Engineered Wood Association.

Banerjee, N., 2013. Flexural Performance of Insulated Sandwich Panels. New York: Southern Illinois University Carbondale.

Bosch, R., 2007. Automotive Handbook, 7th Edition. Michigan: Springer.

Bruce, H., 2012. Evaluation of Load-bearing Honeycomb Core Sandwich Panels. Melbourne: Construction Engineering Research Laboratory.

Davies, J., 2013. Lightweight Sandwich Construction. London: John Wiley & Sons.

Fermín, G., 2016. Design and Construction of New Honeycomb Sandwich Panels Using Superplastic Forming and Vacuum Forming Technique. Perth: Georgia Institute of Technology.

Institut, N. R. C. (. B. R., 2016. Sandwich Panel Design Criteria. Michigan: National Academies.

Koschade, R., 2016. Sandwich Panel Construction: Construction with factory engineered sandwich panels, consisting of metallic facings and a foamed polyurethane core. Moscow: Wiley.

Lampman, M., 2014. Paper Honeycomb Sandwich Panels as Materials for the Construction of Stage Scenery. Colorado: University of Wisconsin-Madison.

Lund, E., 2011. Alternatives to Lumber and Plywood in Home Construction. Chicago: DIANE Publishing.

Manbeck, H., 2012. Plywood Design Specification Supplement 4: Design and Fabrication of Plywood Sandwich Panels. Manchester: Document U814-H.

O'Rourke, B., 2016. Competition Car Composites. New York: IEEE.

Oscar, L., 2012. Performance of sandwich panels in FPL experimental unit. New York: U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory.

Plantema, F., 2013. Sandwich Construction: The Bending and Buckling of Sandwich Beams, Plates, and Shells. Paris: Jon Wiley and Sons.

Plantema, F., 2015. Sandwich Construction. Michigan: John Wiley and Sons.

Ravi, B., 2011. Composite Materials: Testing and Design. California: ASTM.

Standard, A., 2015. Standard Test Method for Density of Sandwich Core Materials. Tacoma: ASTM International.

Taylor, S., 2011. Modeling Structural Insulated Panel (SIP) Flexural Creep Deflection. California: Structural Engineering.

Wright, D., 2010. Testing Automotive Materials and Composites. London: Colorado.

Zenkert, D., 2014. An Introduction to Sandwich Construction. London: Engineering Materials Advisory Services Ltd.