Effect of prestrain on tensile properties and ratcheting behaviour of Ti-stabilised interstitial free steel
Introduction
Uniaxial asymmetric cyclic loading of metals and alloys in the elastic–plastic domain results in the evolution of inelastic strain in the direction of mean stress. This inelastic strain, known as ratcheting strain, is accumulated in specimens or in components over a period of cycles. Such progressive accumulation of strain may continue until fracture [1]. Accumulation of ratcheting strain usually degrades fatigue life of structural components [2], [3]. The extent of such degradation depends upon the imposed stress parameters and the nature of the material. It is, therefore, essential to understand the ratcheting behaviour of different materials for structural integrity purpose. Over the last three-to-four decades numerous investigations have been carried out worldwide to understand the ratcheting behaviour of different materials. All these studies can be broadly classified into two groups: (i) experimental studies and (ii) constitutive modelling based on the original Armstrong–Frederick [4] nonlinear kinematic hardening model in order to investigate the effect of stress parameters and test conditions, and to qualify the ratcheting behaviour of different materials under uniaxial and multiaxial loading conditions. The phenomenological nonlinear kinematic hardening models, however, do not take account of the origin and mechanisms involved in the ratcheting deformation process.
The dislocation features associated with ratcheting deformation has been explored for copper and austenitic stainless steel [5], [6], [7], [8]. According to these investigations ratcheting occurs through breakdown of cell walls producing localised areas of high uniform dislocation density from where new cells are formed and grow during subsequent cycling. Further, cross slip of primary mobile screw dislocations aided by fluctuations of the long-range internal stress field (increase of back stress) is considered responsible for the enhanced ratcheting deformation. Hence, it is very much necessary to examine the evolution of back stress and effective stress for a complete understanding of the ratcheting deformation process. In some publications the effect of back stress on ratcheting behaviour of austenitic stainless steels and also for Cr–Mo steels has been discussed in detail [7], [8], [9], [10].
Despite numerous investigations the effect of prior cold deformation on the ratcheting response has rarely been looked into, though most of the engineering components receive different degrees of deformation in the fabrication stage. Studies on the effect of prior deformation on subsequent cyclic behaviour of different materials [11], [12], [13], [14] are mostly limited to symmetrical stress or strain cycling conditions. In all these studies it has been observed that evolution of inelastic ratcheting strain occurs in the direction opposite to prior deformation direction.
Pioneering studies of Feltner and Laird [15], [16], and Christ et al. [17] provide valuable information on the effect of different amounts of prior deformation on completely reversed strain-control fatigue behaviour of copper. Feltner and Laired [15], [16] observed that cyclic stress–strain curve in the region of high plastic strain amplitude (Δεpl ⩾ 2 * 10−3) was not influenced by low to intermediate amounts of prior deformation. Dislocation structure formed during intermediate level of prior deformation is changed to persistent slip bands (PSBs). It is also reported that with higher amount of deformation (∼20%) the prior dislocation structure is retained with more domination [17]. Jia and Fernandes [18] also observed that with increasing prestrain complete modification of the equiaxed and disorganised cell structure in copper formed during prestraining is resisted during subsequent fatigue cycling.
Paul et al. [14] observed the evolution of compressive ratcheting strain in prestressed SA333 Grade6 C–Mn steel during initial cycles of asymmetric cyclic loading with positive mean stress. However, limited study of Paul et al. could not confirm the effect of different amounts of prestress and cyclic stress combinations on the ratcheting strain evolution pattern. Chai and Laird [11] reported that two distinct phenomena (softening and ratcheting) occurred simultaneously in pre-deformed AISI 1018 low carbon steel through reorganization of the prior dislocation structure. However, effect of small plastic prestrain (0.3% and 1.6%) did not affect ratcheting strain accumulation behaviour with much dominance during asymmetric stress cycling of SAE 5160 spring steel [19], although cyclic softening behaviour was noticed.
Interstitial Free (IF) steels find extensive applications in automotive industries for their very good cold-forming characteristics. Components made of this variety of steel are often subjected to cyclic loading during service. Hence, an understanding of the cyclic deformation behaviour of such an industrially important class of material seems relevant. Although limited, recently interests among researchers, steel producers and car manufacturers in understanding fatigue performance and fatigue deformation mechanisms of IF steels are slowly growing [20], [21], [22], [23], [24]. But, the response of this variety of steel under asymmetric cyclic loading condition in the elastic–plastic domain has received very little attention. To the authors’ best knowledge it is only in three very recent publications [25], [26], [27] the ratcheting behaviour of interstitial free steels has been discussed. However, neither of these studies looked into the effect of cold deformation on the ratcheting behaviour of this variety of steel. Since, automotive components made of interstitial free steels receive various levels of cold deformation and are also subjected to cyclic loading during service; the effect of such prior deformation on the ratcheting response is worth investigating.
Present investigation has been carried out with an objective to provide a consistent mechanical database correlating the effect of different levels of cold deformation on the tensile properties and ratcheting behaviour of a titanium stabilised interstitial free (Ti-IF) steel at two different stress amplitudes over constant mean stress. In this investigation cold deformation has been simulated by tensile prestraining. The ratcheting behaviour under prestrained conditions has primarily been discussed with reference to cyclic loading at higher stress amplitude. The evolution of back stress and effective stress during cyclic loading of prestrained specimens including zero prestrain condition has been examined. Post-ratcheting tensile tests have been done for a macroscopic understanding of the internal stress state of the steels after constant amount of ratcheting strain accumulation in the specimens with and without tensile prestrain.
Section snippets
Material and experimental details
The present investigation has been done with cold rolled and batch annealed 2.3 mm thick Titanium stabilised interstitial free (Ti-IF) steel received from TATA Steel Ltd., Jamshedpur India. The chemistry of the steel in weight percent is: C-0.002, Mn-0.04, Si-0.004, S-0.007, P-0.015, Al-0.0359, Nb-0.001, Ti-0.063, N-0.024 and balance Fe. Annealing after cold rolling was done at 700 °C for 16 h under industrial batch annealing conditions. As shown in Fig. 1, the texture of the steel under cold
Effect of prestrain on tensile properties
Engineering stress–strain curves of the investigated Ti-IF steel with and without prestrain are shown in Fig. 2. The corresponding tensile properties are shown in Table 1. It is observed that although the steel used in the present investigation is almost free of interstitial contents, yield point, though not sharp, appears even at room temperature on reloading the specimens with higher amounts of prestrain. The appearance of yield point has been shown in the inset of Fig. 2 as an example.
The
Conclusions
From the results and discussion presented above the conclusions are drawn.
- (1)
Increase of yield strength with the amount of tensile prestrain occurs in two stages. Very rapid increase followed by more gradual increase in yield strength with the amount of tensile prestrain is observed. The transition in the nature of variation of yield strength occurs at ∼7.5 pct tensile prestrain. Tensile strength increases linearly with increase in the amount of prestrain. Extrapolated tensile strength at zero
Acknowledgements
The work has been carried out with the research grant received from TATA Steel Ltd. One of the authors (PSD) acknowledges CSIR, New Delhi for the award of Senior Research Fellowship to carry out the present investigation at Jadavpur University, Kolkata, India.
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2020, Materials Science and Engineering: ACitation Excerpt :Total elongation, however, is not reduced to that extent as it occurs in case of uniform ductility. In previous investigations [28,32] similar variation of uniform and total elongation with tensile pre-strain has also been observed in both hot and cold rolled IF steels. Results of insitu tensile tests are included Table 1.