Fatigue tests in reversed tension‐compression have been carried out on samples of polyethylene and polystyrene of widely varying molecular weights, extending up to 2, 000, 000. All tests on polystyrene specimens were made at 1600 rpm. For polyethylene, tests speeds had to be reduced to 100 rpm in order to avoid serious temperature effects. For both materials, increasing molecular weight leads to improved resistance to alternating loading. For polystyrene, this improvement in ultimate properties even continued well beyond molecular weight values where Tg, becomes effectively independent of molecular weight. For polyethylene, samples of high molecular weight did not fail even after 107 cycles of alternating loading at a stress level of 3000 psi.

synopsisSignificant improvements in the fatigue life of polystyrene can be realized by increase in molecular weight. I n this investigation, samples of polystyrene of varying molecular weight were subject to alternating cycles of axial tension and compression at a test speed of 1600 rpm. One set of fatigue tests was made on samples machined from commercial whole polymer rod. A second set of samples was prepared from a PS standard having an average molecular weight of 160,000 and a narrow molecular weight distribution. A third set of samples had a high average molecular weight of 860,000. For a whole polymer, the 8-log N curves tended to have the same general shape as for metals, and the endurance limit appeared to be about 1400 psi. The test, results show that the average fatigue life, a t any given stress amplitude, is significantly increased by increase in molecular weight. For example, the average life of the high molecular weight standard at any given stress leve! was found to be more than tenfold that of the low molecular weight standard.