Department of Mechanical Engineering

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While predicting the welding distortion for a complex welded structure like the guide blade carrier assembly of power generation turbine, the traditional thermo-mechanical models commonly utilized for geometrically simple welded butt and T joints may not be adequate. For large and complex welded structures,the thermo-mechanical elasto-plastic analysis can be computationally prohibitive. Other than simplistic butt and T joints, the aforementioned distortion prediction technique is not so suitable to provide reliable results when applied to large complex 3-D welded structures. Hence, inherent strain method is proposed here to predict distortion in an outer ringof GBC (Guide Blade Carrier) of a steam turbine. Such welded outer rings of GBC are large welded structures having many welded joints of different weld geometries at several planes. In the present study, inherent strain based technique was utilized for the aforementioned welded structure in elastic finite element analysis for the accurate prediction of 3-D distortion patterns with reduced computational time. The predicted and measured values of 3-D distortions of GBC were observed to be in good agreement with the ones provided from the shop floor, indicating the adequacy of inherent strain based method for the prediction of welding distortion in large and complex welded structures. As part of this investigation, a suitable welding fixture was also designed and welding sequence was modified, which were verified through inherent stain method to aid in minimizing the distortion of welded GBC. © 2017, Korean Society of Steel Construction and Springer-Verlag Berlin Heidelberg.

World University root node

International Journal of Steel Structures

On the estimation and control of welding distortion of guide blade carrier for a 660 MW turbine by using inherent strain method

The present paper discusses the effects of friction stir processing (FSP) on properties of Mg–SiC nanocomposite. The magnesium-based RZ5 alloy, widely used for aerospace applications, was used in the present investigation as the matrix for the purpose of nanocomposite fabrication. SiC powders of nanosize were reinforced into the RZ5 matrix by FSP with varying process parameters. The process was studied with respect to the effects of parameters such as rotational speed, traverse rate, and number of passes on the nanocomposite. With appropriate parameters, uniform distribution of nanoSiC reinforcements was accomplished in the metal matrix. The abrasive wear property of the Mg–SiC nanocomposite was verified with respect to the FSP process parameters. A mathematical model was also developed to predict the strength and abrasive wear characteristics of the nanocomposite with respect to the FSP parameters. © 2017, The Brazilian Society of Mechanical Sciences and Engineering.

Journal of the Brazilian Society of Mechanical Sciences and Engineering

Estimation of strength and wear properties of Mg/SiC nanocomposite fabricated through FSP route

Steels 9-12% Cr, having the high creep rupture strength are advocated for the modern low polluting thermal power plants. In the present investigation, the P91 pipe weldments have been characterized for microstructural responses in as-welded, post-weld heat treatment (PWHT) and ageing conditions. The PWHT of welded samples were carried out at 760 °C for time of 2 h and ageing at 760 °C for 720 h and 1440 h, respectively. The effect of time has been studied on precipitates size, distribution of precipitates and grain sizes present in various zones of P91 steel weldments. The impact toughness and hardness variation of heat affected zone (HAZ) have also been studied in as-welded condition as well as at different heat treatment condition. A significant change was observed in grain size and precipitates size after each heat treatment condition. The maximum impact toughness of HAZ was obtained after PWHT at 760 °C for 2 h. The main phase observed in weld fusion zone in as-welded, PWHT and ageing conditions were M23C6, MX, M7C3, Fe-rich M3C and M2C. The unwanted Z-phase (NbCrN) was also noticed in weld fusion zone after ageing of 1440 h. © 2017, The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht.

Metals and Materials International

Characterization of microstructure of HAZs in as-welded and service condition of P91 pipe weldments

In a steam power plant, several components such as boiler tube, condenser and steam lines are made of high creep resistant steel. The P91 steel pipes are generally used in steam power plant because of high creep strength at service temperature of approximately 600oC. In the present research work, the study about shrinkage stresses and their distribution in the four quadrants of P91 pipe weld of 11 mm thickness is reported. The conventional-V and narrow-groove welds were prepared by using the gas tungsten arc welding (GTAW) process. Welding current, arc voltage, groove design, and straining length were the main parameters that affect the transverse shrinkage stresses. In the present research work, the effect of groove design on transverse shrinkage stresses has been evaluated. It also describes the effect of the number of passes on shrinkage. It is concluded that, for a given heat input, the narrow groove pipe weld joint exhibits comparatively less transverse shrinkage stress. Scanning electron microscope (SEM) with field emission gun and optical microscope has been used to characterize the weld fusion zone and HAZs of P91 pipe weldments. © 2017, Korean Society of Steel Construction and Springer-Verlag GmbH Germany.

Microstructure and transverse shrinkage stress analysis in GTA welds of P91 steel pipe

The work presented in this study was performed with the intent to characterize the microstructure evolution for short term creep exposure of cast-forged P91 steel. The short-term creep test was performed at temperature range of 620–650 °C and stresses ranging from 120 to 200 MPa. To characterize the sample after creep exposure, field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDS), optical microscope and micro-hardness testing were utilized. Creep tests were performed on round creep specimens. For low temperature service condition, longer creep life was obtained. The fracture surface of creep ruptured specimen were characterized by using the FESEM. The transgranular fracture mode was noticed in all the tests condition. The creep rupture life was found to be decreased with increase in applied stress. The maximum rupture life was measured about to be 3329.28 h for the sample exposed at 620 °C for 120 MPa. A negligible microstructural change was measured in gripping area compared to the gauge area (necking area) of crept sample. The laves phase formation was also noticed along the grain boundaries for creep exposure life of 3329.28 h. © 2017 Elsevier B.V.

Materials Science and Engineering A

Microstructure-based assessment of creep rupture behaviour of cast-forged P91 steel

In this paper, a least-squares spectral element method for parabolic initial value problem for two space dimension on parallel computers is presented. The theory is also valid for three dimension. This method gives exponential accuracy in both space and time. The method is based on minimization of residuals in terms of the partial differential equation and initial condition, in different Sobolev norms, and a term which measures the jump in the function and its derivatives across inter-element boundaries in appropriate fractional Sobolev norms. Rigorous error estimates for this method are given. Some specific numerical examples are solved to show the efficiency of this method. © 2017, Springer Science+Business Media New York.

Journal of Scientific Computing

Spectral Element Method for Parabolic Initial Value Problem with Non-Smooth Data: Analysis and Application

Design management of a thermal energy system is a critical part of identifying basic designs that meet large scale user demand under certain operating characteristics. Fin and tube heat exchangers are among the most commonly used thermal energy systems which are generating considerable interest in terms of advanced design development trends. Sufficient performance data with different fin designs can help to achieve a cost-effective operation. The aim of present work is to examine three material efficient fin patterns, namely- rectangular, polynomial and sinusoidal in an application specific fin and tube heat exchanger. Computational fluid dynamic models of fin and tube heat exchanger with different fin patterns are developed to investigate the fin pattern behavior on heat transfer and pressure loss performance data. In addition, the numerical results are utilized to analyze the engineering design scale-up heat exchanger configurations with each fin pattern focusing on the application of chosen fin and tube heat exchanger in marine exhaust gas boiler. The analysis highlights the impact of material efficient fin patterns investigated and predicts that the polynomial and sinusoidal fin patterns in fin and tube heat exchanger furnish auxiliary beneficial outcomes by up to 2% and 4% improvement in performance and weight reduction, respectively, in comparison to conventional rectangular fin. The study demonstrates, contextualizes and communicates the advantages of structural and material efficient fin designs in conjunction with energy efficiency and environmental benefits from large-scale applications. © 2017 Elsevier Ltd

Applied Thermal Engineering

Investigation of material efficient fin patterns for cost-effective operation of fin and tube heat exchanger

In this paper we propose a least-squares spectral element method for three dimensional elliptic interface problems. The differentiability estimates and the main stability theorem, using non-conforming spectral element functions, are proven. The proposed method is free from any kind of first order reformulation. A suitable preconditioner is constructed with help of the regularity estimate and proposed stability estimates which is used to control the condition number. We show that these preconditioners are spectrally equivalent to the quadratic forms by which we approximate them. We obtain the error estimates which show the exponential accuracy of the method. Numerical results are obtained for both straight and curved interfaces to show the efficiency of the proposed method. © 2016 Elsevier B.V.

Computer Methods in Applied Mechanics and Engineering

Spectral element method for three dimensional elliptic problems with smooth interfaces

This paper proposes a vertical planar 2PRP-2PPR parallel manipulator along with a serial planar RRR passive orthosis (exoskeleton/supporting system) for performing sitting/lying type (stationary trainer) lower limb rehabilitation therapies in the sagittal plane. Proposed system's kinematic arrangement along with forward and inverse solutions is presented. System dynamics is derived to understand the behavior of the over-constrained manipulator-orthosis system. Further, a robust motion control scheme is proposed and the motion control scheme is based on non-singular fast terminal sliding mode control along with a nonlinear disturbance observer. The effectiveness and usefulness of the proposed manipulator is shown with the implementation of the motion controller through computer based numerical simulations using a clinical gait motion pattern. The proposed motion control scheme is also validated on an in-house fabricated prototype through motion control experiments. The controller parameter sensitivity and controller robustness are further analyzed at different working conditions. In comparison to the conventional controllers, the proposed control scheme possess few advantages namely better robustness, less chattering, high precision and fast finite time convergence, and can work in the presence of parameter uncertainties. From the demonstration, the proposed manipulator has certain advantages over existing stationary lower limb rehabilitation trainers namely, simple design, larger workspace, higher stiffness, modular design and low cost. © 2017 Elsevier Ltd

Mechanism and Machine Theory

Design, development and control of a 2PRP-2PPR planar parallel manipulator for lower limb rehabilitation therapies

This paper presents the development of a framework for establishment of virtual environment for testing and tuning of attitude controller for rotary wing Unmanned Aerial Vehicles (UAVs). A flybarless mini-helicopter UAV is used as the platform for exposition of the proposed framework. A hardware-in-the-loop simulation (HILS) framework is established using a physics based flight dynamics simulation to enable controller design for rotary wing UAVs. The HILS setup includes the flight dynamics model, physical servo actuators and actual UAV autopilot. A computationally light real-time flight dynamics simulation is developed by using the properties estimated using series of simple ground-based experiments to simulate the small unmanned helicopter. The simulation is validated by performing flight tests on the actual UAV. It is demonstrated that accurate physics based simulations can be done without performing system-identification experiments, which can be an issue for an unstable rotary-wing vehicles with unknown dynamics. The utility of the HILS setup has been established by using the tuned PI attitude controller developed in the virtual environment for stabilization of the actual UAV under hovering condition. The validated HILS setup obviated the need for carrying out flight testing for system-identification, as all the relevant parameters required for the real-time simulation could be estimated using ground based tests. © 2017 by the American Helicopter Society International, Inc.