INCLINED DOUBLE NOTCH SHEAR TEST FOR IMPROVED INTERLAMINAR SHEAR-STRENGTH MEASUREMENTS

A novel method to measure interlaminar shear strengths of composites i s presented. This method uses the double notch shear (DNS) specimen an d modifies the load introduction by use of a fixture. The main concept is to eliminate the compressive stress intensity at the notch roots, arising from the external load, by imposing a counteracting tensile lo ad field there. This field is caused by a bending moment acting over t he notched region. Proportional loading is achieved by holding the spe cimen in an inclined position with supports near its ends and behind t he notches. The inclination angle determines the ratio between the com pressive normal field and the tensile bending field. The required incl ination angle depends on the notch distance, specimen thickness ratio, and to a lesser degree on any material anisotropy. Two test series, w ith four different notch distances, were carried out to compare the st andard DNS test to the new method. The new method produced consistentl y higher strengths at all notch distances. For the shortest notch dist ance, which incidentally gives the highest strength values, the measur ed average interlaminar shear strength was almost doubled with the new method. As with the standard method, the new method still gives lower strength values with increasing notch distances and notable experimen tal scatter. Causes for this are discussed and some possibilities for improvements are outlined.