What is a 'laminate' ?
When multiple layers of resin and reinforcing fibers are successively added on top of each it forms a 'laminate'. The successive layers or plies add strength and stiffness to a composite and can also include a different type of layer known as a 'core.'

What is a 'balanced' laminate?
To avoid the risk of warping a composite should be 'balanced', so that the reinforcing fibers used are isotropic, exhibiting the same physical and mechanical properties in all directions and mirroring each other through the component's mid-plane. This does not necessarily mean the layers must be composed of identical materials, but should exhibit identical properties.

Laminate mechanical properties
FRP composites rely on a combination of fiber and resin to form a laminate, which has mechanical properties that are greater than the sum of the component parts. For example, resin has lower mechanical properties than commonly used fibers, however when combined the unique mechanical properties of each compliment each other to result in a higher performance material. The ultimate mechanical properties of a laminate are dictated by the type of resin; how it interacts with the fiber; the type of fiber; the reinforcing textile's weave style; fiber orientation; and fiber density relative to the resin matrix.

   • Click here to see comparison of tensile characteristics of uni-directional pre-preg laminate
   • Click here to see comparison of compressive characteristics of uni-directional pre-preg laminate

What is a core?
In engineering theory the flexural stiffness of a material is proportional to the cube of its thickness. Applying this rule to FRP composites, the stiffness of a laminate can be dramatically increased by adding a material between the layers of a laminate. This is called a 'core' and cores can be made from low-density materials.

   • Click here to see diagram of core function

What are the benefits of using cores?
It seems astounding but using a core to double the thickness of a laminate will increase its flexural stiffness by a factor of eight - and this can be achieved with marginal extra weight. Cores can result in lighter weight laminates of equivalent stiffness and stiffer laminates of equivalent weight. Cores can achieve cost savings too, by reducing the amount of laminate required to achieve a specific flexural stiffness.

What are cores made from?
Cores are usually made from either synthetic polymer foams or honeycomb structures. Foam cores include PVC, polystyrene, polyurethane, polymethyl methacrylamide, and styrene acrylonitrile (SAN) co-polymer foams. Honeycomb structures can be made from aluminium, thermoplastic, 'Nomex' and paper. Lightweight timbers such as balsa and cedar can also be used as core material but are susceptible to water ingress if not sealed inside the laminate.

   • Click here to see comparison of honeycomb and foam core structure
   • Click here to see comparison of core material cost
   • Click here to see comparison of core density vs. compressive strength
   • Click here to see comparison of core density vs. shear strength

What is 'sandwich' construction?
When core materials are incorporated in a composite it is sometimes referred to as 'sandwich' construction, as layers of laminate encapsulate either side of the core.

What is 'Witnessing'

Fabricating complex composite structures often requires separate sections to be molded and subsequently joined together. This is usually achieved using adhesives rather than conventional fasteners like bolts and rivets, as loads can be spread across a wider interface area where adjoining laminates meet. 'Witnessing' can occur when adhesives or additional resin and reinforcement is added to the rear surface of a laminate and often becomes visible only some time after. It is similar in some respects to 'print through', where the underlying layers of reinforcement fiber work their way through to the surface and the texture of the fiber becomes visible. 'Witnessing' happens for a variety of reasons including the different mechanical properties of the bonding agent, or because the 'balance' of the laminate is disturbed in a small area. The addition of material subtly alters the mechanical properties compared to the surrounding laminate. Like 'print through', 'witnessing' is extremely difficult to eliminate and can occur when core materials are added to the rear of automotive panels to increase their stiffness..........read more

 

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