Structural interpretation of endurance of polymeric fibers

Endurance results in cyclic tension of several polymeric fibers are reviewed and analyzed in terms of Prevorsek-Lyons theory. It is shown that the plots of log (cycles to failure) against strain amplitude can be divided into regions of short and long term endurance. An expression is derived correlating the inflection point on the S-N curve with the structural parameters: fracture surface energy p, tensile modulus E, activation energy associated with the process of crack growth F*, and stress concentration factor, q. Manson-Coffin endurance plots expressed in terms of log (strain amplitude), log (stress amplitude), and log (strain energy amplitude) are examined. From the log Nf—log (strain energy amplitude) are derived the relative values of fracture surface energy of these fibers. Experimental data are shown which reflect the structural changes which occur in fibers during endurance experiments and an analysis is carried out leading to an estimate of the corresponding changes in p, E, and q.