The strength of angle-ply laminates and composites with misaligned fibres

grantor: University of TorontoIn the first part of this work, single fibre of E-glass, carbon AS4, Kevlar 49 were embedded normally in epoxy resin and cured at room temperature. The oblique tensile strength of these fibres was measured. A 50% angle, $\phi\sb{\rm h}$, was described. This was the angle of pull at which the fibre breaks at half its normal strength. These 50% angles were $20\sp\circ,\ 30\sp\circ$-40$\sp\circ$, and 45$\sp\circ$ for glass, carbon and Kevlar respectively. Also it was found that Kevlar fibres contributed more strength than carbon and carbon more than glass, if they were used to make random fibre composites. In the second part, the strength of relatively wide angle-ply laminates, ($\pm\phi\rbrack\sb{\rm s},\ \phi = 0\sp\circ$-60$\sp\circ$, with very short gauge-lengths for glass and carbon was studied. These specimens proved to be very much stronger than the results expected based on the Tsai-Hill work. It indicated that the previous theory for tensile strength involved excessive edge effects. This was confirmed by reviewing the results from tests on pressurized tubes. These tubes strength results were an order of magnitude higher than the early laminate test results. So, it can be concluded that design with angle-ply laminates, based on the earlier theory, may be unduly conservative. This work was extended to investigate the effect of test specimen width, 3-100 mm, on the tensile strength of (${\pm}45\sp\circ\rbrack\sb{\rm S}$ laminates. The strength increased monotonically by increasing the width. It again indicated that the previous theories were relevant to edge effects. Also, the relative stiffness has been estimated from the cross head movement. The widest specimen gave the highest stiffness values. The stiffness results suggest that laminate theory for elastic constants may also be influenced by edge effects. So these results indicate that the way is open for much greater use of angle-ply laminates in real structures.M.A.Sc