Krishnaswamy Hariharan

Foil Optimization in Tailor Welded Blank of an Automotive Floor Component

In the present work, double foil butt resistance seam welding in an automotive floor component has been investigated. The possibility of reducing foil from the existing configuration is evaluated. Mechanical properties of the single and double foil weld configuration are compared using tensile tests. The fractography of failed location is analyzed, and the force distribution in base metal and welded specimen is derived analytically to explain the observations in tensile tests. The microstructure and hardness profile of both the weld configurations and their influence on the mechanical properties are analyzed. The weld defect is analyzed using ultrasonic inspection technique. Based on the evaluation of metallurgical characterization and mechanical properties evaluation, the number of foils in an automotive floor component has been optimized. Cost savings of around 30% in the welding process is achieved by foil optimization.

Decoupling Thermal and Electrical Effect in an Electrically Assisted Uniaxial Tensile Test Using Finite Element Analysis

Application of intermittent electric pulses during uniaxial tensile test changes the mechanical behavior owing to electroplastic effect. The electric current increases the temperature of the specimen due to Joule heating. It is, therefore, necessary to decouple the thermal effect from the overall behavior to understand the contribution of electric current in the mechanical behavior. In the present work, an electro-thermo-mechanical finite element study of an electrically assisted uniaxial tensile test of Al5052 alloy is performed to isolate the thermal effect. The simulated results yielded the thermal effect due to the electric current. By comparing the experimental and simulated results, the contribution of electric current is decoupled from that of thermal effect. It is found that the thermal component contributes significantly to the instantaneous stress drop and long-range permanent softening observed in experiment. The electric current, in addition to the instantaneous stress drop and permanent softening, affects the reloading behavior. The present work can be utilized to develop simpler constitutive models for the mechanical behavior of metals subjected to pulsed electric current.

A Novel Multi-objective Genetic Algorithms-Based Calculation of Hill’s Coefficients

The anisotropic coefficients of Hill’s yield criterion are determined through a novel genetic algorithms-based multi-objective optimization approach. The classical method of determining anisotropic coefficients is sensitive to the effective plastic strain. In the present procedure, that limitation is overcome using a genetically evolved meta-model of the entire stress strain curve, obtained from uniaxial tension tests conducted in the rolling direction and transverse directions, and biaxial tension. Then, an effective strain that causes the least error in terms of two theoretically derived objective functions is chosen. The anisotropic constants evolved through genetic algorithms correlate very well with the classical results. This approach is expected to be successful for more complex constitutive equations as well.

Application of Cost-Effective Stainless Steel for Automotive Components

High-strength nickel free austenitic stainless steel material as a light-weight substitute for commercial automotive deep drawn steel has been studied with an automotive bumper as an example. The stainless steel gave a weight saving as high as 50% at the same cost that of deep drawn steels. The study examines the techno-economic merits of value engineering automotive material. The bumper made from stainless steel stamping was reduced in thickness to 1.2 mm from 2.5 mm of conventional Extra Deep Draw (EDD) grade steel. Forming behavior of the stainless steel bumper was theoretically simulated and was found to match with experimental observation. The Forming Limit Diagram (FLD) behavior of the stainless steel material shows superior formability. Typical static and impact load cases based on functional requirements of the bumper was simulated, and the light weight stainless steel bumper was found to perform better than the existing EDD bumper. The bumpers made qualified the vibration test requirement.

Influence of Yield Criteria in the Prediction of Strain Distribution and Residual Stress Distribution in Sheet Metal Formability Analysis for a Commercial Steel

In this study, Hill 48 and Barlat 89 yield criteria are evaluated by simulating the Limiting Dome Height test of the Numisheet 96 benchmark problem [1] for a commercial D-grade steel and the results are validated with experimental data. Three strain modes, viz. biaxial, plane strain, and uniaxial strain modes, are used in the simulation studies. The yield criteria are evaluated based on the strain distribution in forming and residual stress distribution after springback. The results show that the Hill 48 and Barlat 89 yield criteria do not show a good match with predicted results of strain distribution in the plane strain mode. The difference between predicted results and experiments can be attributed to the combined influence of friction coefficient and draw bead boundary condition. The prediction of residual stress distribution differed considerably with the yield criteria. The residual stress predicted is validated for selected locations using X-ray diffraction techniques.

Determination of Anisotropic Yield Coefficients by a Data-Driven Multiobjective Evolutionary and Genetic Algorithm

The texture induced anisotropy of yield strength in cold rolled sheet metals is modeled using anisotropic yield criteria. The classical and other optimization methods used so far to determine the yield coefficients are limited by fixed set of experimental data, initial guess values, and pre-determined weight factors. A robust multiobjective optimization based on evolutionary algorithm proposed in this paper minimizes the error in yield stress and plastic strain ratio simultaneously and thereby overcomes the limitations in the approaches used before. The new approach is tested using Hill48 and Barlat89 yield criteria for five different materials from literature. The new approach is observed to improve the prediction capability of yield coefficients when compared to earlier approaches. The Pareto frontier obtained in the new approach can serve as a comparative tool to evaluate the accuracy of different yield criteria.

Integrating effect of forming in fatigue life prediction: review of present scenario and challenges

Plastic deformation of sheet metals during forming induces complex residual stresses in the component owing to its geometry and material anisotropy. The residual stresses affect the durability of the components. Fatigue life estimation incorporating the forming-induced residual stresses is limited. In the present work, the state-of-the-art in modelling sheet metal forming, residual stress estimation and fatigue life prediction are reviewed. The challenges in integrating the residual stress owing to forming and fatigue life prediction are brought out. The parameters influencing the sheet metal forming, residual stresses and fatigue are discussed to emphasize the complexity of the present problem.

Stress Relaxation for Formability Improvement

Improved formability has been reported due to stress relaxation when the continuous forming cycle is interrupted with steps by adjusting the punch motion. The contribution of stress relaxation and its parameters on the ductility of materials has not been established so far. In the present work, the stress relaxation behavior of three materials, low carbon steel, DP and TRIP steels are studied. The influence of strain rate and strain on the ductility enhancement due to stress relaxation is analyzed. It is observed that stress relaxation improved the ductility of materials in all the cases and therefore can be used as a potential method to improve formability in sheet metal forming.

Investigation of stress relaxation mechanisms for ductility improvement in SS316L

Abstract Stress relaxation during plastic deformation has been reported to improve ductility and alter the mechanical properties of metallic materials. The aim of the present study is to investigate the role of various mechanisms responsible for this in stainless steel SS 316L. The fractography of the tested samples is analysed using an image analyser and the void fraction at failure is correlated with the corresponding mechanisms. The parametric studies on stress relaxation at different pre-strain and relaxation time correlate well with the fractography results supporting the proposed mechanisms. TEM investigation of dislocation structures and void characterisation further confirm the role of dislocation annihilation. Moreover, a novel indentation technique combining micro- and nano-indentation techniques is used to verify the role of stress homogenisation mechanism.

Machining parameters optimization for satisfying the multiple objectives in machining of MMCs

ABSTRACT The metal matrix composites (MMCs) have gained acceptance in an extensive range of applications owing to their high strength to mass ratio. Machining of such complex MMCs is often challenging. It is essential to optimize the controllable machining parameters to simultaneously attain manifold objectives. In the current work, response surface design is created for experiments, and Genetic algorithm (GA) combined with Principal Components Analysis (PCA) coupled Grey Relational Analysis (GRA) is employed to improve the straight turning process of MMCs. The procedure is demonstrated by machining aluminum-based MMC with 25% SiC particulates. The procedure aims at identifying optimal combination of machining parameters to obtain high surface quality at lower cutting force without increasing the specific power consumption. PCA is helpful in providing the individual uncorrelated quality characteristics called as quality indices that do not have any influence on other responses. Individual quality indices have been utilized to obtain the grey relational grade through GRA. GRA has been used to alter manifold quality indices into singular column of grey relational grade as a means to change the manifold objective problem into a sole objective problem. Then, GA has been used to get the optimal parameters combination. The novelty present in this work is the avoidance of correlation existing among the quality characteristics and combining of the GRA and GA. This is an endeavor to augment the performance and accuracy of GA to solve the optimization problem associated with the turning operation.