Ludvík Kunz

Role of persistent slip bands in fatigue

Perhaps the most thoroughly studied feature of cyclic plasticity has been persistent slip bands (PSBs) formed in some metals during cyclic loading. The aim of this paper is to delimit the conditions for the occurrence of the PSBs and to discuss their role in the fatigue process in all its stages, that is in cyclic hardening resulting in a saturated stress–strain response, in initiation of fatigue microcracks and in propagation of fatigue cracks. The PSBs are zones of high cyclic slip activity. Therefore the cyclic stress–strain response depends on their volume fraction and on the ratio of cyclic slip activities within the PSBs to those outside the PSB. The cyclic plastic deformation within the PSBs leads to the formation of surface extrusions and intrusions along the traces of the active slip planes; fatigue microcracks start from the surface intrusions. Thus the PSBs play a crucial role in the process of crack initiation. Stage I fatigue cracks usually run along the PSBs. At very early stages of crack propagation these PSBs are formed before the cracks are initiated; later the PSBs are formed in the plastic zones ahead of the cracks. Stage II fatigue cracks often follow the PSBs on the microscopic level in the threshold region.

Cyclic plasticity and substructure of metals

Fatigue of metals is a consequence of cyclic plastic deformation. Fatigue properties, as well as the mechanical properties in general, are strongly dependent on the starting substructure. Moreover, the substructure undergoes changes during cyclic loading. This is valid both for the quasi-homogeneous cyclic plastic deformation in the whole cyclically loaded metal and for the highly localised cyclic plastic deformation at the crack initiation sites and later at the crack tip. The paper is devoted to the delimitation of the cyclic plasticity and its specific features, which distinguish it from the monotonic plasticity. Further, the relation of the cyclic plasticity to the substructure and damage accumulation is discussed.

Cyclic stress-strain behavior of 9Cr1Mo steel at positive mean stress

Abstract Cyclic plastic behavior of a ferritic martensite 9%Cr–1%Mo steel was studied under load controlled cycling with positive mean stresses. The results were compared with data determined under symmetrical cycling. Cyclic hardening followed by softening was found to be inherent to cycling at positive mean stress contrary to the symmetrical loading where only continuous softening was observed. For a given stress amplitude, higher mean stress results in higher plastic strain amplitude. The onset of cyclic softening and the onset of cyclic creep acceleration occur simultaneously. In the region of cyclic softening a unique relation between the instantaneous cyclic creep rate and the instantaneous plastic strain amplitude was found for cycling at constant mean stress.

Comparison of fatigue behaviour of single crystals and polycrystals

Abstract The available fatigue characteristics of single crystals and polycrystals are compared from the point of view of their convertibility using a simple conversion factor both for the stress and the plastic strain values. The S-N curves for single crystals were found to be in very good agreement with the S-N curves for polycrystals converted by the Taylor factor M = 3.06. The cyclic stress-strain curves for pure iron can also be very well converted using this factor, while the cyclic stress-strain curves for f.c.c. metals can be converted to a first approximation only. The application of the conversion factor to the Coffin-Manson curves of f.c.c. metals does not lead to any satisfactory agreement. The explanation is based on the considerably different volume fraction of zones exhibiting higher cyclic slip activity in single crystals and in polycrystals.

Fatigue properties of magnesium alloy AZ91 processed by severe plastic deformation

Fatigue properties of cast AZ91 magnesium alloy processed by severe plastic deformation were investigated and compared with the properties of the initial cast state. The severe plastic deformation was carried out by equal channel angular pressing (ECAP). The ECAP treatment resulted in a bimodal structure. The bimodality consists in a coexistence of fine grained areas with higher content of Mg17Al12 particles and areas exhibiting larger grains and lower density of Mg17Al12 particles. Improvement of the basic mechanical properties of AZ91 (yield stress, tensile strength and ductility) by ECAP was significant. Also the improvement of the fatigue life in the low-cycle fatigue region was substantial. However the improvement of the fatigue strength in the high-cycle fatigue region was found to be negligible. The endurance limit based on 10(7) cycles for the cast alloy was 80 MPa and for the alloy processed by ECAP 85 MPa. The cyclic plastic response in both states was qualitatively similar; initial softening was followed by a long cyclic hardening. Fatigue cracks in cast alloy initiate in cyclic slip bands which were formed in areas of solid solution. In the case of severe plastic deformed material with bimodal structure two substantially different mechanisms of crack initiation were observed. Crack initiation in slip bands was a preferred process in the areas with large grains whereas the grain boundaries cracking was a characteristic mechanism in the fine grained regions.

Creep of CMSX-4 single crystals of different orientations in tension and compression

Abstract Creep tests of γ′ strengthened superalloy CMSX-4 single crystals of different orientations were performed at 950 °C, in both tension and compression at constant load (initial stress 250 MPa) as well as at constant stress (250 MPa). It was found that the creep resistance depends strongly on the crystal orientation. For creep in tension it decreases in the sequence 〈111〉, 〈001〉, 〈011〉 and for creep in compression it decreases in the sequence 〈001〉, 〈111〉, 〈011〉. Constant stress tests in tension generally reveal a higher creep resistance than the constant load tests in tension. In compression the differences in the mode of testing lead only to small and unsystematic differences in creep behavior. Comparison of constant stress tests in tension and compression shows that the creep curves practically coincide at the early stages of the test. In tension there is later tertiary creep terminating in fracture. In compression no tertiary creep occurred within the test run up to a strain of about 2%. In all the tests a massive rafting, i.e. directional coarsening of γ′ particles, was observed.

Cyclic slip localisation and fatigue crack initiation in fcc single crystals

Initiation of fatigue cracks both in model fcc single crystals and in fcc single crystals used in engineering practice is discussed. The emphasis is placed on the role of different types of cyclic slip localisation. It is shown that the phenomenon of persistent slip bands is confined to single crystals of sufficiently high stacking fault energy cycled at a relatively narrow range of loading conditions. The necessary prerequisites for the microcrack initiation are discussed and accessed.

Small cracks—nucleation, growth and implication to fatigue life

The nucleation and early propagation of fatigue cracks are smoothly consequential processes, which can occupy a major part of the fatigue life of structural materials. Nevertheless, their quantitative description is at present not yet sufficient to perform reliable life calculations. The paper summarizes the present state of the art in the recent advancement of nucleation and early propagation of cracks in metals both as for the mechanisms and as for their quantitative description. The following items are treated: Sites of crack nucleation, prerequisites and mechanisms of the nucleation, end of nucleation stage and transition to propagation stage, plasticity of small cracks and growth of microstructurally and mechanically small cracks.

Effect of loading history on cyclic stress–strain response

Abstract An attempt is made to rationalize the effect of loading history on cyclic plasticity in terms of interaction between the cyclic plastic strain and the unidirectional plastic strain, which is either pre-set (pre-deformed materials) or inevitably arises during fatigue tests (cycling with nonzero mean stress, effect of start-up procedure, ramp loading). In the case of easy-cross-slip metals, the relation between the cyclic and unidirectional plastic strain components is shown to be non-monotonous. This forms a basis for the unifying discussion of the effect of pre-strain, mean stress and start-up procedure of fatigue tests on the cyclic stress–strain response.

Stress–strain response and fatigue life of copper single crystals cyclically loaded with a positive mean stress

Abstract Copper single crystals oriented for single slip were cyclically loaded under load control with a non-zero tensile mean stress. No plateau was found in the cyclic stress–strain plot, and no persistent slip bands were found in the dislocation structure. The dislocation structure consists of cells arranged into slabs lying along the primary slip plane. The S / N curve is shifted towards shorter lives with respect to the S / N curve for symmetrical stress controlled tests. Microcracks initiate — as in the case of symmetrical cycling — at coarse slip markings with a sharp notch-peak topography. This leads to the conclusion that cyclic slip localisation always plays the crucial role in the initiation of fatigue microcracks, but the modes of this slip localisation are different for different loading conditions. In the case studied, the localisation is due to relative sliding of the slabs of cells.