# Mechanical Characterisation

Fibre based composites demonstrate improved mechanical properties by sharing strain between the matrix and the fibre reinforcement [16]. The reinforcement can be calculated using the Hashin-Shtrikman model [17].

Vr is volume fraction of reinforcement component; Er is elastic moduli and Eg is gel matrix. EC,U and EC,L corresponds to the fibres aligned and perpendicular to the direction of loading. Randomly aligned fibre reinforced composites fall in the region between the two bounds. Fibres aligned in the direction of loading exhibit higher stiffness even at relatively low volume fractions. To quantify the effect of reinforcement an amplification factor, A, is introduced to facilitate comparison between studies, Ec is simple bounds on the modulus.

By combining Eqs. 3.1 and 3.3, the volume fraction V_{r} is influenced by A and there is modulus mismatch between the reinforcement and gel matrix, Er/Eg.

It has been reported that fibre reinforcement polyvinyl alcohol (PVA) hydrogels demonstrate an increased tensile modulus from 0.23 MPa without any reinforcement to 260 MPa when reinforced with 29 wt.% ultra high molecular weight polyethylene (UHMWPE) fibres [18], whereas PVA covalently grafted onto the UHMWPE demonstrated increased interfacial shear strength from 11 kPa to over 220 kPa [19].