The true breaking stress of thermoplastics

Abstract

In an attempt to compare the measured breaking stresses of thermoplastics with the theoretical strength computed from the known structure, the difficulty arises that there is no reliable theory of the absolute magnitude of the breaking stress. An upper limit is provided by the stress needed to break a covalent carbon-carbon bond, which is about 2 x 10 6 Lb. /in. 2 (Mark 1943). In comparison, the tensile breaking stress of thermoplastics is generally accepted to be in the region of 10 4 Lb. / in. 2 at room temperature. This discrepancy, a factor of about 200, can be decreased by reducing the test temperature; during a programme of work on thermoplastics (Vincent 1960), in which many thousands of specimens were broken in flexure near –180 °C, it was found that breaking stresses over 3 x 10 4 Lb. /in. 2 were com­mon and that the strengths of some specimens approached 5 x 10 4 Lb. /in. 2 . The residual discrepancy, a factor of about 40, is less surprising when it is noted that the elastic modulus found in practice is also much less than the equivalent upper limiting value. The modulus of highly oriented cellulose in the chain direction has been computed to be (6 to 38) x 10 6 Lb. /in. 2 (Mark 1943) and modulus of a single crystal of polythene, also in the chain direction, has been computed to be 5 x 10 7 Lb. /in. 2 (Shimanouchi, Asahina & Enomoto 1962).