THE EFFECT OF FIBER ALIGNMENT ON COMPOSITE STRENGTH .1. SINGLE-FIBER STUDIES

Tests have been carried out in which a fibre was embedded normally in a polymer, and pulled at an angle to the normal to the polymer surface . It was found that some fibres, especially Kevlar, had to be pulled a t quite large angles in order for the strengths to be much reduced. A criterion, the 50% angle, was developed. This is the angle of pull at which the fibre breaks at half its normal strength. 50% angles were: 2 0 degrees for glass, 30-40 degrees for carbon and 45 degrees for Kevla r. Thus, although Kevlar fibres are weaker than glass, they should mak e stronger random-fibre composites as long as the fibres are long enou gh so that they are broken in the fracture process. Testing at 30 degr ees gives information about the strength of extremely small volumes of fibre material and indicates that AS4 carbon may have an intrinsic st rength as high as 30 GPa. (C) 1997 Elsevier Science Limited.