New Experiments Suggest that All Shear and Some Tensile Failure Processes are Inappropriate Subjects for ASTM Standards

摘要

There are four ASTM standards which involve apparent shear failure of composites. These include the losipescu (D 5379), tube torsion (D 5448), the short beam test (D 2344), and two-rail and three-rail shear (D 4255). However, careful experiments in which polymers have been sheared show that failure is normally restricted to tensile failure with breaking of polymer chains and cross links. Moreover, it is already known that shear hackle is produced by a tensile process. In view of this, these standards should be reexamined to determine whether strengths should be reported at all. The same argument applies to the new standard being developed for mixed Mode I-Mode II in-terlaminar fracture toughness. Mode I is the only failure mode which is consistent with experimental observations. In addition, the D 3039 standard for tensile strength has been revised to include balanced and symmetric laminates. Experiments have shown that, with angle ply laminates, much greater strengths can be obtained just by making the specimen wider and shorter. These latter tests agree very well with tests on pressurized filament wound tubes, with structures equivalent to the angle ply structures. This strongly indicates that the ASTM test can severely underestimate the strength. The ASTM standard also seriously underestimates the stiffness. There appears to be an "edge softening" effect, so that apparent properties are very dependent on the aspect ratio of the test sample. Thus, true material properties are not measured. It is proposed that all ASTM D 30 mechanical tests be reexamined to determine (a) what purpose is served by having the test at all, and (b) whether it is measuring a true materials property.