Question

You are designing a composite using short glass fibres and an epoxy resin. You can choose between fibres with critical fibre lengths of 0.4, 0.8, or 1.2 mm, all of which have the same strength and stiffness. Which fibre should you choose to achieve the strongest and stiffest composite? Explain your choice

Answer 1

The critical length of the fiber is defined as 'the minimum length of the fiber at which the center point of that fiber reaches the ultimate tensile strength. This happens when the matrix of that fiber achieves the maximum shear strength.

Now, in order to achieve the effective strength and stiffness, the fiber length must be greater than the critical length. Fiber with too small critical length does not transfer the load and fails easily. Fiber with too large critical length costs too much without addition of any extra benefit. Hence we should choose the fiber with optimum critical length. Therefore, we should choose the fiber with critical length of 0.8 mm.