Karel Obrtlík

Cyclic stress-strain response of polycrystalline copper in a wide range of plastic strain amplitudes

Abstract Polycrystalline copper was cyclically strained with constant plastic strain amplitude from a very wide interval (3×10 −6 −1×10 −2 ). Fatigue hardeing-softening curves and the plot of the loop shape parameter vs. the number of cycles were evaluated. Fatigue softening observed in low amplitude cycling was studied in detail. Measurements of the loop shape parameter which indicated the cyclic slip localization were accompanied by surface relief observations and electronmicroscope observations of the dislocation structure. Several mechanisms of the fatigue softening were considered. The basic cyclic stress-strain curve was evaluated and the difference between this curve and curves using apparent saturation values of the stress amplitude were discussed.

Nucleation stress for persistent slip bands in fatigued copper single crystals

Abstract Copper single crystals oriented for single slip were cyclically strained at a constant plastic strain amplitude up to fracture. The stress amplitude, the loop shape parameter and the average hardening coefficient were measured using a computer-controlled electrohydraulic system. The nucleation stress amplitude for persistent slip bands was found and compared with the saturation stress amplitude. The long-term steady decrease in the loop shape parameter found for all amplitudes indicates hardening within the persistent slip bands. No oscillations in the loop shape parameter before the minimum was reached and no support for avalanche-type cyclic straining were found.

Cyclic strain localization in polycrystalline copper at room temperature and low temperatures

Abstract Copper polycrystals have been cyclically strained with constant plastic strain amplitudes at temperatures of 295, 178 and 88 K. Hysteresis loops were recorded, and stress amplitude and loop shape parameter were plotted vs. number of cycles. In room temperature cyclic straining, the surface relief and dislocation structure evolution was studied. The mechanical measurement and surface and internal structure observations indicate the presence of the characteristic stages in the fatigue life of polycrystals. The fatigue mechanisms maps were drawn for polycrystalline copper, demarcating the region of homogeneous straining, the region of progressive localization and the region of stabilized repeated localized cyclic straining, leading to the initiation and growth of primary fatigue cracks.

Cyclic softening in annealed polycrystalline copper

Abstract Cyclic straining of annealed polycrystalline copper with constant plastic strain amplitudes results in initial hardening which is followed by long-term softening in the low amplitude region. The logarithmic softening rate was measured in dependence on the plastic strain amplitude, and the dislocation structures and the surface relief evolution were studied in order to explain the observed softening.

Hardness of fatigued copper polycrystals and their relation to their dislocation structure

Abstract The Vickers hardness was determined for copper polycrystals cyclically strained at a constant plastic strain amplitude up to fracture. The dependence of the hardness on the plastic strain amplitude exhibits a minimum at a strain of 1 × 10 −3 followed by a sharp increase. This is explained in terms of the observed dislocation structure produced during fatigue. Using polishing and sectioning techniques, the same value of hardness has been found on the surface and in the bulk of the material.

LOW CYCLE FATIGUE IN WC-Co CEMENTED CARBIDES

Smooth specimens of WC-9wt.%Co cemented carbides were loaded under a symmetrical push-pull cycle. Young’s tensile modulus and the hysteresis loop were recorded with high sensitivity. Hardmetals exhibit an early hardening followed by saturation behaviour for the rest of life. Basic cyclic stress-strain curve (CSSC) is in good agreement with that obtained by the multiple step test procedure. CSSC has a high hardening exponent and is shifted to high stress amplitudes in comparison with metals. Fatigue life curves are presented in the form of Wohler and Manson-Coffin curves. Defects in the structure are responsible for the appreciable scatter of the experimental data and represent the sites of fatigue crack initiation.