J. Polák

On the role of point defects in fatigue crack initiation

Abstract Analysis of inhomogeneous cyclic straining, observation of surface relief at emerging persistent slip bands (PSBs) and data on point defect generation and migration in fatigued metal single cyrstals and polycrystals are the origin of a new model for fatigue crack initiation in PSBs. Because of the high plastic strain amplitude, non-equilibrium point defects are generated in PSBs. They are distributed inhomogeneously according to the inhomogeneous sink distribution. Point defect migration over short distances causes mass transport and results in the formation of extrusions and intrusions on the metal surface. The distribution of extrusions and intrusions is related to the dislocation substructure of the PSB. Cyclic straining under stress together with the strain concentration at sharp intrusions result in environment-assisted formation of new surfaces and therefore in crack initiation.

High cycle fatigue life of metals

Abstract In eight different materials with U.T.S. ranging from 5 to 80 kpmm−2 the fatigue life curves were determined both for cycling with constant stress amplitude and for cycling with constant plastic strain amplitude in the high cycle region (number of cycles to fracture > 105). The results made it possible (1) to check directly the validity of the Manson - Coffin law in the high cycle region; (2) to show how far the different fatigue life curves can be mutually transformed via the cyclic stress - strain curve; (3) to determine the threshold values of the plastic strain amplitude corresponding to the endurance limit.

Cyclic stress-strain response and dislocation structures in polycrystalline copper

Abstract Polycrystalline copper was cyclically strained at constant total strain amplitudes and a constant strain rate up to fracture. Cyclic hardening-softening curves and the cyclic stress-strain curve were measured. Microstructural observations by means of transmission electron microscopy, optical microscopy and scanning electron microscopy after etching procedures were used to reveal the dislocation arrangement and its dependence on the plastic strain amplitude. Two distinct regions in the cyclic stress-strain curve differing in both the stress-strain behaviour and the dislocation structures were identified. In specimens cycled in the low ampliture region a high density of persistent slip bands at the surface and in interior grains was found. Fatigue softening was discussed in connection with the observed interlinking of these persistent slip bands and with the formation of softer areas.

Surface topography and crack initiation in emerging persistent slip bands in copper single crystals

Abstract The surfaces of copper single crystals oriented for single slip and fatigued at room temperature at a constant plastic strain amplitude were observed by scanning electron microscopy. Both well-developed and embryonic stages of persistent slip band formation were found. The surface relief at the emerging persistent slip band is characterized by tongue-and ribbon-like intrusions and extrusions. The correlation of the surface relief with the dislocation arrangement in the persistent slip band was determined. The mechanism of fatigue crack initiation by interconnecting tongue-like intrusions is discussed.

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.

High cycle plastic stress-strain response of metals

Abstract In eight different materials, constant plastic strain amplitude tests and constant stress amplitude tests in the high cycle region ( N > 10 5 cycles) were performed. By simultaneous recording of the plastic strain amplitude and the stress amplitude the hardening or softening behaviour of the materials was followed. The response of the materials is similar during both types of test, and cyclic stress - strain curves found using both procedures are in good agreement. In stress cycling of materials exhibiting a yield point the inhomogeneity of the plastic strain amplitude may occur and a real stress - strain curve cannot be found from constant stress amplitude cycling.

On the cyclic stress-strain curve evaluation in low cycle fatigue

Abstract Constant strain amplitude tests, strain amplitude changes and short cut procedures for cyclic stress-strain curve determination in low carbon steel were analysed in order to understand the differences or similarities they show in the cyclic stress-strain response. These observations were supplemented by electron microscope observations of the resulting dislocation structure after constant amplitude loading and strain amplitude changes. It was found that the previous cyclic history considerably influences the cyclic stress-strain response. Constant amplitude cycling after an amplitude change has a tendency to bring both the stress-strain response and internal dislocation structure into a steady state identical with that reached by cycling the virgin specimen at the same (final) strain amplitude. The actual stress-strain response during variable amplitude loading depends on the cyclic history. The lower bound of this response is the cyclic stress-strain curve obtained from tests on companion specimens and the upper bound is the stabilised hysteresis loop at the highest strain level. Between these extremes lies the cyclic stress-strain curve obtained from the incremental step test. This cyclic stress-strain curve approximates reasonably well the stabilised cyclic stress-strain response in variable amplitude loading.

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.

Resistivity of fatigued copper single crystals

Abstract The electrical resistivity of copper single crystals oriented for single glide and cyclically strained at liquid helium temperature has been measured. Annealing experiments allow separation between the contribution from dislocations and point defects and the total resistivity increment. The measurement of resistivity within a cycle reveals the development of reversibility with continued cycling. It indicates simultaneous defect production and annihilation within a cycle. Intermediate annealing reduces the excess point defect concentration and results in a drastic increment in the production rate of point defects. The mechanisms of dislocation and point defect production and annihilation are considered and the implications for the formation of extrusions and intrusions are discussed.

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.