Surajit Kumar Paul

Non-linear Correlation Between Uniaxial Tensile Properties and Shear-Edge Hole Expansion Ratio

Stretch flanging of steel sheets is an important formability issue for automobile industry. Finite element simulation study confirms that the edge of the hole deforms in a uniaxial tensile manner during the hole expansion process. To understand the effect of various tensile properties on hole expansion ratio, current experimental data and collected data from published work have been used. Yield stress, ultimate tensile stress, coefficient of normal anisotropy, total elongation, and post uniform elongation are closely related to hole expansion ratio. A non-linear relationship between hole expansion ratio and tensile properties (ultimate tensile stress, coefficient of normal anisotropy, and total elongation) is developed in the present investigation.

Theoretical analysis of strain- and stress-based forming limit diagrams

Forming limit diagram is extensively used in the analysis of sheet metal forming to define the limit of deformation of materials without necking or fracture. This article contributes in construction of strain- and stress-based forming limit diagrams by theoretical analysis of several available instability criteria, including bifurcation analysis of diffuse and through-thickness neck formation, shear stress-based criteria onset of through-thickness necking, and so on, and comparative studies among different available theories are done with earlier published work on forming limit in different literatures. The scope of Tresca criterion in forming limit diagram is also investigated in this article. It is found that apart from stress-based forming limit diagram, conventional failure criterion equivalent failure strain versus triaxiality (η) plot is also strain path independent.

True stress-controlled ratcheting behavior of 304LN stainless steel

Cyclic plastic deformation response of materials under asymmetric stress cycling is known as ratcheting. Combined effect of fatigue and permanent tensile strain accumulation results in early failure of materials during ratcheting. For this reason, ratcheting should be emphasized in the safety assessment and life estimation of engineering structures. Engineering and true stress-controlled ratcheting behavior of 304LN stainless steel has been carried out at room temperature. Effects of stress amplitude, mean stress, and their histories (i.e., step loading) on the ratcheting behavior are analyzed in this investigation. It is noticed that under true stress-controlled ratcheting experiments, ratcheting life increases in presence of mean stress, and hysteresis loop area and plastic strain energy decreases with the increasing mean stress. A comparison has also been drawn in between the true and engineering stress-controlled tests, and massive differences in ratcheting life and strain accumulation were found. Ratcheting strain accumulation ceases in descending step loading, is noticed in this work.

Prediction of entire forming limit diagram from simple tensile material properties

The purpose of this study is to develop a phenomenological model for prediction of the entire forming limit diagram from simple tensile material properties. The phenomenological model is based on the necking and ductile damage theories. In the proposed model, void nucleation is described as a function of the equivalent plastic strain, and void growth is a function of the stress triaxiality. The forming limit curves calculated from the proposed phenomenological model matched reasonably well in the region of uniaxial tension to balance biaxial tension with the experimental forming limit curves generated on C-Mn 440 steel, interstitial-free 340 steel, and interstitial-free steel sheets.

A Multiaxial Low Cycle Fatigue Life Prediction Model for Both Proportional and Non-proportional Loading Conditions

This paper has presented a life prediction model in the field of multiaxial low-cycle fatigue. The proposed model is generally applied for constant amplitude multiaxial proportional and non-proportional loading. Depending upon applied strain path the equivalent strain varies within a cycle. Equivalent average strain amplitude is considered as fatigue damage parameter in the proposed model. The model has requirement of only two material constants and no other tuning parameters. The model is examined by the proportional and non-proportional low-cycle fatigue life experimental data for eight different types of materials. The model is successfully correlated with multiaxial fatigue lives of eight different materials.

Low Cycle and Ratchetting Fatigue Behavior of High UTS/YS Ratio Reinforcing Steel Bars

Cyclic deformation behavior of high UTS/YS rebars has been studied employing both symmetric strain-controlled and asymmetric stress-controlled cycles in an attempt to understand the influence of UTS/YS ratio on fatigue life. While strain-controlled cyclic deformation did not exhibit a pronounced influence of UTS/YS ratio, a substantial life enhancement is noted for the asymmetric stress-controlled cycle. Reasons for life enhancement were found to be due to the ratchetting strain development and the associated hardening behavior. An equivalent stress-based model has been used to predict both the symmetric and asymmetric fatigue lives of rebars.

Local Ratcheting Response in Dissimilar Metal Weld Joint: Characterization Through Digital Image Correlation Technique

Ratcheting fatigue behavior of different alloys and their weld joints has been a topic of interest in the past few decades. However, little information is available about the ratcheting response in different zones, when a composite weld section is subjected to asymmetric loading cycle. This work aims at understanding the local ratcheting response in various zones of a dissimilar metal weld (DMW) joint. Digital image correlation technique has been used to measure local ratcheting strain components of a DMW joint. Accumulation of ratcheting strain in various zones of a DMW joint was found to be different, harder region accumulating negligible and softer region accumulating larger local ratcheting strain. Fatigue crack initiation or neck formation occurred in soft region owing to high ratcheting strain accumulation.

Effect of mean stress on multiaxial ratcheting life: A simplified life prediction model based on average equivalent stress amplitude

This article proposes a model to predict uniaxial and multiaxial ratcheting life by addressing the three primary parameters that influence failure life: fatigue damage, ratcheting damage and the multiaxial loading path. These three factors are addressed in the present model by (a) the stress amplitude for fatigue damage, (b) mean stress-dependent Goodman equation for ratcheting damage and (c) an inherent weight factor based on average equivalent stress to account for the multiaxial loading. The proposed model requires only two material constants which can be easily determined from uniaxial symmetric stress-controlled fatigue tests. Experimental ratcheting life data collected from the literature for 1025 and 42CrMo steel under multiaxial proportional and nonproportional constant amplitude loading ratcheting with triangular sinusoidal and trapezoidal waveform (i.e. linear, rhombic, circular, elliptical and square stress paths) have shown good agreement with the proposed model.

Identification of Post-necking Tensile Stress–Strain Behavior of Steel Sheet: An Experimental Investigation Using Digital Image Correlation Technique

The stress–strain behavior of sheet metal is commonly evaluated by tensile test. However, the true stress–strain curve is restricted up to uniform elongation of the material. Usually, after the uniform elongation of the material the true stress–strain is obtained by extrapolation. The present work demonstrates a procedure to find out the true tensile stress–strain curve of the steel sheet after necking using digital image correlation (DIC) technique. Hill’s normal anisotropic yield criteria and local strains measured by DIC technique are used to correct the local stress and strain states at the diffuse necked area. The proposed procedure is shown to successfully determine the true tensile stress–strain curve of ferritic and dual-phase steel sheets after necking/uniform elongation.