NEGATIVE POISSONS RATIOS IN ANGLE-PLY LAMINATES - THEORY AND EXPERIMENT

A series of composite panels has been prepared by laminating unidirectional prepreg tapes of epoxy resin reinforced with continuous carbon fibres. Each panel was a balanced, symmetrical laminate with the plies alternating at +/- theta to a reference direction where theta = 0, 10, 15, 20, 25, 30 and 40-degrees. The full set of nine elastic constants was determined for each panel using ultrasonic velocity measurements. The experimentally determined elastic constants were then compared with theoretical predictions obtained using standard laminate theory. The Poisson's ratios of the composites were of particular interest in showing negative values for theta in the range between 15 and 30-degrees, as predicted by the theory.