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The stress relaxation rate in irradiated high purity polycrystalline titanium was studied at room temperature. Titanium specimens of grain size 40 µm were irradiated with 18 MeV electrons for 12 min at 300 K to a dose of 0.01 dpa. The stress relaxation rate in the specimens was studied using a universal testing machine. The comparison of irradiated samples with un-irradiated ones, annealed and tested in similar conditions, revealed that the stress relaxation rate is lower in the irradiated titanium specimens than in the un-irradiated ones. The results are analysed in terms of a single barrier model of stress relaxation. The intrinsic height of the energy barrier to the movement of dislocations was found to be higher in the case of irradiated specimens than in the un-irradiated ones. This is attributed to the local pinning of dislocations with the irradiation induced defects acting as obstacles.

Stress relaxation rate in unirradiated and electron-beam-irradiated polycrystalline titanium (99.994%) was studied in the temperature range 300–100 K. Titanium specimens were irradiated with 12 MeV electrons to a dose of 0.01 dpa for 12 min. at 300 K. Tensile tests of the specimens were performed using a Universal Testing Machine in the given temperature range. To measure the relaxation of stress with time, the crosshead of the machine was arrested at different fixed loads. Stress relaxation rate s for a given stress level σ0 was found to be temperature dependent, i.e., it decreased with decreasing temperature both in unirradiated and irradiated specimens. However, the decrease was more pronounced in irradiated specimens than that of unirradiated ones. The observed decrease in s values with decrease in temperature is ascribed to the retarding effect of unrelaxed dislocations pinned at defects, especially at the twin boundaries in the course of deformation, which became more conspicuous in irradiated specimens due to the interaction of glide dislocations with radiation-induced defects, in addition to mechanical twins. The activation energy for the movement of dislocations, calculated using the single-barrier model of stress relaxation, was found to be higher in irradiated specimens than that of unirradiated ones at all test temperatures.

The present paper is devoted to the existence of limit cycles of planar piecewise linear (PWL) systems with two zones separated by a straight line and singularity of type “focus‐focus” and “focus‐center.” Our investigation is a supplement to the classification of Freire et al concerning the existence and number of the limit cycles depending on certain parameters. To prove existence of a stable limit cycle in the case “focus‐center,” we use a pure geometric approach. In the case “focus‐focus,” we prove existence of a special configuration of five parameters leading to the existence of a unique stable limit cycle, whose period can be found by solving a transcendent equation. An estimate of this period is obtained. We apply this theory on a two‐dimensional system describing the qualitative behavior of a two‐dimensional excitable membrane model.