Influence of uniaxial and biaxial pre-straining on the low cycle fatigue performance of DP590 steel

https://doi.org/10.1016/j.ijfatigue.2021.106260Get rights and content

Highlights

  • Pre-strain has a significant influence on the LCF performance.

  • Cyclic softening in ascending order of as-received, uniaxial and equi-biaxial pre-straining.

  • LCF life in descending order of as-received, uniaxial and equi-biaxial pre-straining.

  • Rotation of one maximum shear stress plane during equi-biaxial pre-straining and followed by fatigue cycling.

Abstract

The present work aims to examine the low cycle fatigue (LCF) behavior of dual-phase (DP590) steel subjected to different pre-straining (uniaxial and equi-biaxial) paths. The LCF tests reveal all specimens showed continuous cyclic softening throughout the fatigue life. However, the rate of cyclic softening is higher for the equi-biaxial pre-strain specimen. The rotation of one maximum shear stress plane during fatigue cycling after equi-biaxial pre-straining introduces non-proportional loading condition, thereby significantly reducing the fatigue life. The larger lattice rotation and in-grain misorientation due to the non-proportional loading is notable for the equi-biaxial pre-strain specimen.

Introduction

Recently, advanced high-strength steels (AHSS) are frequently utilized in the automotive body structures to reduce vehicle's cost, weight and fuel consumption. Moreover, large numbers of applications of these AHSS materials sheets are also found in the aeroplane, rail, naval and engineering structural members [1], [2]. Dual-phase steels (DP) are one of the most extensively used AHSS due to their excellent combination of high strength and ductility. This inherent high strength, low yield to ultimate ratio and good forming abilities of DP steels allow them to use thin gauge sheets without sacrificing the structural integrity of the automobile components [3], [4]. DP steels consist of dual-phase microstructure i.e., hard martensite islands are distributed over the soft ferrite matrix. The applications of DP steels are frequently found in the automotive body parts such as wheels, rails, doors, cross members, inner bumper reinforcement beams, suspension housing [5], [6]. These automobile parts are often exposed to fatigue loading conditions during their service period. Hence, the fatigue assessment of DP steel is highly required for the safe design of automotive components. More specifically, design in the low cycle fatigue (LCF) regime is highly relevant for automotive structural components due to the presence of stress-concentrated regions [7], [8]. Thereby, it is necessary to assess the LCF properties before finalizing the automotive structural components' design.

Every sheet metal parts are manufactured by multistage forming processes such as stamping to fabricate the automotive structural components [9]. Forming operation experiences large plastic deformation, complex stress history, and environment temperature, which results in work hardening due to strengthening effect, change in shape, thinning of the component and development of residual stresses [8], [10]. The effect of forming plays a significant role in the LCF performance as work hardening taking plays in conjunction with the toughness and ductility reduction. Thus, the influence of forming on the LCF properties must be investigated before being the component is put into service. In the present investigation, forming strain is introduced by means of uniaxial and equi-biaxial pre-straining. Although many researchers examined the effect of uniaxial and biaxial pre-strain on the tensile properties, there are relatively few studies reported on the cyclic behavior of DP steels post biaxial pre-straining operation. Fredriksson et al. [11] performed fully reversed strain-controlled fatigue experiments for different deep-drawing steels like DP400, HSLA500, DDQ and DP600 under the as-received, uniaxial pre-strain and equi-biaxial stretching conditions. Tensile pre-strain results in a linear increment in flow stress and the ultimate tensile strength. While the cyclic softening phenomenon was observed in all the pre-strain specimens under fully reversed strain cycling. Le et al. [12] found a decrease in the uniaxial pre-strain 304L stainless steel's fatigue life compared to the as-received condition under strain-controlled cycling. The decrease in the specimen's fatigue life was attributed to the reduction in ductility after the pre-straining operation. The reduction in fatigue life of uniaxial pre-strain DP steel was observed by Gustavsson et al. [13] due to the presence of tensile mean stress under fully reversed strain-controlled fatigue loading. Le et al. [14] studied the influence of uniaxial and plane strain pre-strain on the high cycle fatigue (HCF) and LCF performance of DP600 steel and reported degradation in fatigue parameters in the LCF region. Yan et al. [15] noticed that a considerable amount of strain hardening occurred along with the reduction of ductility after biaxial stretching of interstitial free (IF) steel. They have explained the improvement in the material's strength by strain hardening. The rise in yield strength after biaxial stretching improves crash resistivity of the material [16]. Chiriac et al. [17] explained that during plastic deformation of DP steel, strain partitioning takes place among ferrite and martensite phases. The soft ferrite matrix continuous yielding increases the material's yield strength, and the hard martensitic matrix starts to deform after 4% of biaxial stretching, resulting in strain hardening of the biaxial stretched DP980 steel. Generally, the fatigue life of the pre-strain material mainly depends upon the applied strain amplitude, the nature of pre-strain and the material [10], [18], [19]. Strain controlled fatigue test was performed by Yan et al. [15] for the as received and 24% biaxial pre-strain IF steel. The fatigue life of the biaxial pre-strain specimen was reduced in the LCF region. The biaxial pre-strain IF steel's fatigue life improved in the high cycle fatigue (HCF) region compared to the as-received material. Parker et al. [20] investigate the effect of 40% balanced biaxial stretching on hot rolled low carbon steel SAE1008. They have reported that fatigue resistance starts to degrade in the LCF region compared to the as-received condition due to the cyclic softening nature of the 40% balanced biaxial stretched low carbon steel. The pre-straining results decrease in ductility, which can cause poor fatigue performance in the LCF region. In contrast, it was found that fatigue resistance improves in the HCF region. The same research group carried out strain-controlled fatigue experiments of 12.5% uniaxial pre-strain DP600 steel under different strain path changes. They have also observed the continuous cyclic softening of the uniaxial pre-strain material, while HCF experiments result in considerable improvement in fatigue lives of the pre-strain material [8]. Very limited literature is available related to the effect of uniaxial and biaxial pre-strain on the strain-controlled LCF properties of DP590 steel and summarized in Table 1. Thus, a thorough study of the LCF behavior of DP590 steel exposed to different pre-strain conditions is investigated in the present work.

The present study concerns the influence of uniaxial and equi-biaxial pre-strain on the monotonic and the cyclic plastic deformation response of DP590 steel. DP590 sheet is subjected to uniaxial and equi-biaxial tensile pre-strain up to 10% of the equivalent strain followed by the uniaxial tensile and LCF experiments to assess the tensile and fatigue performance of the material. Fractography of the failed LCF tested specimen is carried out to explain the fatigue damage mechanism during the LCF experiment. With the help of the electron backscattered diffraction (EBSD) technique, the correlation has been made by analyzing the change in misorientation of the grains, lattice rotation maps, and local plastic strain during fatigue loading of the DP590 steel under different pre-strain conditions.

Section snippets

Experimental methodology

Dual-phase steel (DP590) used in the present investigation is a cold-rolled sheet of 1.4 mm thickness. The chemical composition of DP590 steel is given in Table 2. Tensile and fatigue (LCF) are performed on a servo-hydraulic 25 kN BISS machine. The tensile and LCF specimen geometry and dimensions are shown in Fig. 1. All tensile tests are carried out in accordance with the ASTM E8 at a strain rate of 0.001 s−1. A contact extensometer having a gauge length of 12.5 mm is mounted on the tensile

Initial microstructure of the material

The typical microstructure of the polished and etched as-received DP590 steel is shown in Fig. 4. The polishing is done according to the standard mechanical procedure, i.e., paper polishing followed by the diamond paste, and 2% nitral solution is employed to reveal the microstructure of the material. The dual-phase microstructure of DP590 steel consists of hard martensite continuously surrounding individual ferrites grains. The average grain size of ferrite and martensite are 6.47 μm and

Conclusions

The effect of forming on the low cycle fatigue behavior of dual-phase (DP590) steel is comprehensively investigated in the present work. To introduce the forming effect on DP590 steel sheet, 10% uniaxial and equi-biaxial pre-strains are imposed on the sheet material. After that, the as-received and pre-strain specimens are subjected to the fully reversed strain-controlled cyclic loading at different strain amplitudes. The tensile pre-strain results in higher work hardening and exhaustion of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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