Fracture toughness of angle ply laminates

Angle ply laminates have a very high resistance to through thickness fracture and special methods are needed for their toughness testing. Thus the standard ASTM E1922 test is not suitable because few of the fibres break during the test, and the crack does not develop in a self-similar mode, as required by the standard. Because of this, a special wide, double edge notched sample, has been developed. It had a width of 43mm and a gauge length of 20mm. Some samples were coated with photoelastic material and photoelastic patterns were recorded during testing. Balanced angle ply laminates were used with angles of 15, 30, 45, 60, and 75 degrees. Quasi isotropic carbon-epoxy laminates were also tested. The angle ply laminates were very tough. Thus the 15 degree angle ply materials had fracture toughnesses of 130 (carbonepoxy) and 110 MParootm (glass-epoxy). The corresponding works of fracture were about 1.2 MJm(-2) in both cases. By contrast the value for the quasi isotropic laminate was only 50 MParootm, i.e. a work of fracture of 54 kJm(-2). Angle ply laminates are also very strong and stiff, but this, too, can only revealed by testing wide samples. The notched strength tests showed that the 30degrees carbon-epoxy balanced angle ply laminate was 40% stronger than the quasi isotropic one, as well as being seven times as tough. Once these excellent properties are more widely appreciated we can expect to see much greater use of angle ply laminates in aerospace and other critical applications.