The principle of the method specified is to deflect the test specimen, supported as a beam, at constant rate at the midspan until the specimen fractures or until the deformation reaches some predetermined value. During this procedure the force applied to the specimen is measured. The method is applicable to rigid thermoplastics moulding and extrusion materials, rigid thermosetting moulding materials, fibre-reinforced thermoset and thermoplastics composites, thermotropic liquid-crystal polymers.