What is 'Fiber Volume Fraction' (FVF)?
This is expressed as a percentage and represents the ratio of fiber reinforcement to resin. Different types of fiber reinforcements and manufacturing processes affect the FVF.

Why is 'FVF' important?
The strength, stiffness and mechanical properties of a laminate will increase in proportion to the amount of fiber, up to a practical limit of approximately 60-70%.

How does a high 'FVF' reduce weight?
A high FVF means less resin is used in a laminate and this can contribute to increased performance due to lighter weight.

How can the choice of fiber reinforcement affect 'FVF' ?
Reinforcements that have loosely bundled fibers will result in a low FVF, as resin must displace the air between the fibers. Reinforcements that feature fine, densely packed fibers will achieve a higher FVF due to the increased number of fibers. Fabrics with finer, more densely packed fibers achieve higher mechanical properties because they provide an increased surface area or 'interface' for resin to bond with the fiber.

How can manufacturing techniques affect 'FVF' ?
'Chopper gun' or 'wet' hand lay-up manufacturing techniques typically result in resin-rich, low-FVF laminates that are heavier and weaker. Optimal FVF can only be achieved using sophisticated 'pre-pregs' and high tech processing using vacuum bagging and autoclaves.

How is the performance of a FRP composite measured?
The performance of a composite is measured under four main direct loads: tension, compression, shear and flexure.

What is a 'tensile load' ?
In simple terms if you pull the ends of a piece of string between your hands the string experiences a tensile load. The ability of a composite to withstand such loads relies on the tensile stiffness and strength of the reinforcement fibers, which are much higher than the resin that binds them together.

   • Click here to see illustration of tensile load

What is a 'compressive load' ?
If you push a coil spring together between your hands it experiences a compressive load. The ability of a composite to withstand such loads relies on the resin's adhesive strength bonding the fibers together and mechanical properties, which keep the fibers straight and stop them buckling.

   • Click here to see illustration of compressive load

What is a 'shear load' ?
If you put a thick foam sponge between the palms of your hands and try to slide the sides in opposite directions it experiences a shear load. The ability of a composite to withstand such loads relies on the resin's adhesive strength and mechanical properties that stop adjacent fibers sliding over each other and transfer stresses throughout the composite. Interlaminar Shear Strength (ILSS) is often used to indicate this property in laminates that feature multiple layers of reinforcement.

   • Click here to see illustration of shear load

What is a 'flexural load' ?
If you hold the ends of a stick of celery between your hands and bend it away from your body, it experiences a flexural load. Flexure is a combination of tensile, compressive, and shear loads. The side of the celery closest to you experiences a compressive load, the opposite side experiences tensile load and the center experiences shear.

   • Click here to see illustration of flexural load

Can a material have very different 'tensile strength' and 'tensile modulus' ?
Imagine a length of rope with a heavy steel weight hanging from one end. To do this without breaking, the rope must have a tensile strength greater than the force applied by the steel weight. If it had equal 'tensile modulus' (stiffness), the rope could support the steel weight in mid air, on one end. The fact that it cannot do so illustrates the rope has very different tensile strength and tensile modulus.

   • Click here to see comparison of tensile strength of different engineering materials
   • Click here to see comparison of tensile modulus of different engineering materials

What is 'density' ?
Density refers to the mass of a substance per unit volume.

   • Click here to see comparison of density of different engineering materials

What is 'specific tensile strength' ?
This helps measure another aspect of performance based on 'tensile strength' divided by 'density'. The 'specific tensile strength' takes into consideration the weight of a material relative to its strength.

   • Click here to see comparison of specific tensile strength of different engineering materials

What is 'specific tensile modulus' ?
This helps measure performance based on 'tensile modulus' divided by 'density'. The 'specific tensile modulus' takes into consideration the weight of a material relative to its stiffness.

   • Click here to see comparison of specific tensile modulus of different engineering materials

Laminate impact strength

In high performance composites where a core is laminated on either side by very stiff carbon fiber, this 'skin' either side of the core can be extremely thin, less than 0.5mm thick. However, while such laminates can bear very high tensile and compressive loads they can be prone to impact damage. For this reason carbon fiber is sometimes combined with glass or aramid fiber in hybrid fabrics that offer greater impact resistance.

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   • Click here to see comparison of laminate impact strength

 

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