Cyclic strengthening of polypropylene under strain-controlled loading

Experimental data are reported on isotactic polypropylene in uniaxial tensile cyclic tests with a strain-controlled program (oscillations between fixed minimum and maximum strains) at room temperature. The stress–strain diagrams demonstrate that (i) maximum and minimum stresses per cycle decay with number of cycles, (ii) the rate of decrease in minimum stress strongly exceeds that in maximum stress, (iii) secant elastic modulus increases (cyclic strengthening) or decreases (cyclic softening) with number of cycles depending on maximum strain. A physical mechanism is suggested to describe strengthening of semicrystalline polymers that accounts for (i) lamellar fragmentation and (ii) alignment of broken lamellar pieces under cyclic deformation. With reference to this concept, several conclusions are formulated that are confirmed by additional tests on pre-loaded samples.