Sandwich Structures

7th March 2016

The information from this page was supplied to Enlighten by Caterham Group

SANDWICH STRUCTURES

Core materials refer to the central component of a sandwich construction. Usually a sandwich structure consists of two relatively thin, stiff and strong  faces separated by a relatively thick lightweight core, for instance, honeycomb, balsa or foam cores. The purpose of sandwich structure is to  achieve a stiff and simultaneously light component.

Depending on the purpose the materials can vary; however the most important characteristics for sandwich structures are:

• They are lightweight compared to metallics
• Have high stiffness
• Cost effective compared to other composite structures

When considering using core materials for specific applications, the production technology used for the sandwich structure is crucial, for example, wet-layup, prepreg, infusion, etc.

At Caterham Composites, we use core materials in numerous industries in order to use the advantages of stiffness and lightweight. For instance core materials have been used in our motorsport project to protect the driver (stiffness) and at the same time save weight in order to distribute the weight where it is most efficient.

Advantages
As mentioned, the advantages of core materials are its stiffness and strength on the one side and its light weight on the other side; without increasing the weight dramatically, higher stiffness and strength can be achieved. Also, sandwich constructions function as a thermal insulation or thermal transfer, depending on the materials used. Moreover, the dampening of vibration and noise is another significant benefit provided by core materials. Last but not least, the filling of hollow spaces limits water ingression.

Disadvantages
However, core materials can have some disadvantages as well depending on the material used for production. For instance, solid laminates offer better resistance to damage and damage tolerance than a core structure; however, they are significantly heavier.

Future Outlook
Core materials are essential in numerous industry segments, for example within the marine, aerospace and motorsport markets. Sandwich structures are used for producing boat hulls, car hoods and other body part, aircraft panels, etc. As various markets became aware of the benefits of using core materials, the demand increases, a trend likely  to continue.

Gordon Murray Design’s iStream based vehicles make use of composite sandwich panels

Summary
Resulting from the tremendous benefits of core materials and the possibility to vary materials according to product requirements, core materials are preferred because of its:

• Stiffness
• Lightweight
• Strength
• Thermal insulation/ thermal transfer
• Dampening of vibration and noise
• Water ingress prevention (Resulting from filling the hollow spaces)

HONEYCOMB

Honeycomb is a well known core used to build sandwich structure. The name originates from the structure of honeycombs built by bees to store honey. Honeycombs and flexcores are used in many applications, for example chassis of modern Formula One cars use honeycomb sandwich structures. The honeycomb, flexcores and nomex are sandwiched between two carbon skins with the purpose of creating a very stiff and strong structure that offers protection to the driver in case of a severe crash.

Advantages
The usage of honeycomb sandwich structures is advantageous. First and foremost the material itself is very light and a wide variety of materials can be used, for instance carbon, paper, aluminum, etc. Additionally, the core has a high strength to weight ratio and  a very good compressive strength. The sheets of honeycomb can be available in different thicknesses and densities and can be deformed to follow the shape of the tool where the component will be laminated.

Disadvantages
The major disadvantage of honeycomb cores is its price; the material is very expensive. Furthermore, corrosion can be another pitfall of honeycombs.

Future Outlook
The aluminum honeycomb for instance is available in different aluminum grades with specific material properties and thus can be applied as required. However, the material used has a significant effect on the price. The possibility of adapting the grades to required output is a tremendous advantage in diverse markets. The trend of its usage in the Formula One is increasing as well as for the aerospace segment for aircraft components.

Summary
As a result honeycomb is a preferred core material that is advantageous because of:

• High strength to weight ratio
• Good compressive strength
• Lightweight
• Various materials (carbon, aluminum, paper, etc.)

BALSA CORE

The balsa core is one of the most widely used wood cores, especially with regard to the marine industry where it is commonly used for boat hulls and decks due to its behavioral properties. Balsa is a natural product and the balsa tree grows fairly quickly and thus, can supply the market on a constant level.

Advantages
Balsa core has numerous advantages and is used in many cored composite applications as a result of its low costs. Furthermore, the material Balsa is a very strong yet light material.

Disadvantages
The major disadvantage of balsa core is the possibility of water damage as balsa will rot when it becomes wet. Additionally the repair is expensive compared to a solid fiber glass materials, often used as an alternative to balsa core in the marine industry.

Summary
Balsa core is preferred based upon:

• Low density
• Good mechanical properties
• Can be processed with a lot of composite manufacturing techniques
• Natural product (supply is guaranteed in a short period of time)

FOAM CORES

Foam cores are used in various markets to enhance the overall composite product. Cores are mainly used in sandwich structure and are rigid and closed cells. This means the gas is surrounded by resin and each cell is isolated from other cells, making the foam cores usable for the marine industry (preventing water immigration). Depending on usage temperature, resin compatibility, costs and manufacturing aspects, each one is more suitable for a specific application.

Foams are manufactures from a wide variety of materials, i.e. Polystyrene, Polyurethane, PVC (Polyvinyl Chloride), or PMI (Polymethacrylimide). All of the materials have different characteristics but a common point is lightweight.

 

Polystyrene
Polystyrene, also referred to as Thermocole, is made from petroleum and is one of the most widely used plastics worldwide with several billion kg used per year. Polystyrene can be either a thermoset or thermoplastic. Different degrees of Expanded Polystrene foam exist and are in comparison to other materials lighter and more buoyant. The variety of products Polystyrene can be used for make it versatile and interesting for many sectors, i.e. CD and DVD cases, surfboards or boxes. There are two types of Polystyrene: Open celled and closed celled. Open cell Polystyrene is cheaper but difficult to shape, paint or airbrush. Moreover, the open cells are not water resistant. The closed cell is more expensive but absorbs little water. Furthermore, the life expectancy is much longer in comparison to open celled Polystyrene.

Polystyrene is used in various industries. The characteristic of lightweight in combination with its low acquisition costs. One industry segment benefiting from this material is the surfboard market. Once Polystyrene is sealed with epoxy resin, the surfboards last longer and emit fewer toxic gasses.

One major advantage of Polystyrene is the cost aspect; it is fairly cheap compared to other foam materials. Furthermore, Polystyrene can be used with epoxy resin and can be shaped easily by hand or using a hot wire foam. Polystyrene is moisture resistant and easier to handle. Additionally the material is valued for its insulation qualities.

Nonetheless, the material has some disadvantages as well that impact decisions significantly. The material cannot be used with polyester resin due to the temperature and chemical sensitivity. Furthermore, Polystyrene is flammable and moreover, regarded as pollutant since it takes long to decompose. Another disadvantage to be mentioned is the poor solvent resistance and the fact that homopolymers are brittle.

In summary, polystyrene is a common foam material which is used in many businesses based on its properties:

• Cost (relatively cheap)
• Lightweight
• Moisture resistance
• Transparent Color

Polyurethane
Polyurethane is a closed cell foam that is very common in the composite industry containing a low-conductivity gas mostly hydrochlorofluorocarbons (HCFC). Due to the closed cell structure the water absorption is low making the material suitable

for the marine industry. Contrary to the Polystyrene this material can be used with polyester resin. Furthermore, it is available in ready shapes and can also be cast to a specific shape. The versatility of the materials are a major advantage and make it applicable to various projects within numerous markets.

Polyurethane are used within the composites sandwich structures but also for cushioning, packaging, buoyancy/flotation, SIPs, Insulation and Models. It can be hard like fiber, squishy like upholstery foam, protective like varnish, bouncy like rubber and sticky like glue.

The versatility of Polyurethane is a major advantage and make it suitable for a variety of fields. Furthermore, the material is flexible and durable, advantageous for specific industries. It is very cost effective and fulfills the requirements of lightweight materials needed within specific areas. Moreover, Polyurethane has a high R-value and can be either sprayed, poured or formed into rigid boards.

Polyurethane has some disadvantages that can have an impact on companies’ decisions. For instance, the R-value drops over the time and the chemical release is not environmentally friendly.

In summary, polyurethanes are versatile and can be applied to numerous projects within various sectors.

• Lightweight
• Moisture resistance
• Easy to handle
• Flexible
• Durable

PVC (Polyvinyl Chloride)
Polyvinyl Chloride are largely used in the marine industry as a result of the low water absorption. PVC is one of the most widely used and produced plastics worldwide based on two aspects: the low costs and the versatility of the material itself. The range of products made from PVC is exhaustive ranging from credit cards to toys, pipelines and many more. Moreover, the PVC’s properties can be influenced by adding plasticizers and thereby, soften or harden the material depending on the amount added.

Polyvinyl Chloride is mainly used in the composite sandwich structures based on its advantageous properties. The markets using PVC ranges from marine, aerospace, sporting goods and many more resulting from the materials’ versatility. Furthermore, two types of PVC exist that even broadens the field of application; namely: rigid and ductile. The difference is mainly obvious with regard to temperature resistance, density and physical properties.

PVC has numerous advantages approving its wide usability. First and foremost the material is cheap and cost-effective. Additionally the material has excellent durability and long-life expectancy as well as good electrical and insulation properties. Additionally, PVC has easy processing characteristics to achieve desired specifications. PVC is resistant to many solvents, including styrene and most fuels. Moreover, the material is self-extinguishing and will not rot.

However, Polyvinyl Chloride has some disadvantages as well. For instance, PVC has a limited thermal capability restricting it to certain fields of application. Moreover, the foam core is sensitive to UV and oxidative degradation.

In summary, polyvinyl chloride is advantageous especially in the marine industry based on its beneficial properties.

• Lightweight
• Moisture resistance
• Resistant to many solvents
• Durable

Polymethacrylimide (PMI)
PMI belongs to the family of closed cell foams and is mainly used in high tech sandwich construction, for instance in combination with prepreg systems. The material is produced by mixing methacrylic acid and methacrylonitrile monomers. Due to its good mechanical properties, the foam core material can be processed in an autoclave so they can cope with high pressure without collapsing. As a result of its closed cell structure and thus its limited absorption possibilities, some processes are difficult to apply, for example, the resin infusion process.

As a consequence of the overall costs involved and its performance characteristics, PMI has been used in higher performance composite parts including helicopter rotor blades, ailerons and stringer profiles in pressure bulkheads. Generally speaking the foam core material PMI is used in the aerospace industry in a variety of structural sandwich application in either commercial, military and general aviation aircraft, satellites and helicopters.

PMI is advantageous for many reasons. The foam core material is more water absorbent than other foam cores. Furthermore, the foam core material can withstand processing temperatures up to 176°C. Additionally, PMI foams have some of the highest overall strengths and stiffness of foam cores and moreover, have a high dimensional stability. Its high fatigue life and its cured and elevated temperatures are of great advantage.

The main disadvantage of Polymethacrylimide is the cost aspect; in comparison to other foam cores PMI is very expensive.

In summary, PMI is beneficial for various markets, especially in the high performance segment based on its beneficial properties.

• Lightweight
• Strength
• Stiffness
• High dimensional stability
• High fatigue Life

Comments