Department of Applied Mechanics

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

A composite of two different medium entropy alloys (MEAs, i.e., CoCrFeNi and AlCoCrFe) was synthesized using ball milling and spark plasma sintering. The composite microstructure contains a homogenous distribution of fcc and bcc phases with submicron-sized grains and exhibits excellent microstructural and phase stability even after 100 h heat treatment at 800 °C. The composite provides a combination of high compressive strength, adequate plastic strain, and multiple strain-hardening stages at room temperature. This first exploratory study on a MEA composite can be used as a template to other systems and illustrates the feasibility of combining two or more MEAs. © 2020 Acta Materialia Inc.

World University root node

Department of Metallurgical and Materials Engineering

Scripta Materialia

Composite of medium entropy alloys synthesized using spark plasma sintering

The compression after high-velocity impact (CAI) behavior of pseudo-elastic shape memory alloy (PE-SMA) embedded glass/epoxy composite laminates manufactured via vacuum-assisted resin infusion (VARI) technique was investigated experimentally. High-velocity impact tests were performed on the glass/epoxy composites using an in-house build gas-gun set-up at four different velocities (70, 85, 95, and 105 m/s). To elucidate, the series of damage events undergone on the composites under the high-velocity impact, a high-speed camera was used. Quasi-static compression-after-impact (CAI) tests were performed to evaluate the damage tolerance of different composites subjected to high-velocity impact. Real-time damage progression during the CAI as well as the extent of damage was characterized by using a 2D Digital Image Correlation (DIC) technique. Three different types of embedment configurations of PE-SMA wire, namely, with length variation (35, 70, and 150 mm) and also with and without anchors, were used. The impact test results show that the ballistic limit of anchored PE-SMA composites with an embedment length of 35 mm is 72.72% higher than that of the homogeneous glass/epoxy composites. CAI results indicate that the PE-SMA based composites have better damage tolerance with respect to the homogeneous ones at 70 m/s. In contrast, at higher velocities, the delamination area decreases in the homogenous glass/epoxy composites due to more localized damage and present a higher compressive strength. © 2020 Elsevier Ltd

Department of Engineering Design

Composite Structures

Compression after high-velocity impact behavior of pseudo-elastic shape memory alloy embedded glass/epoxy composite laminates

The numerical investigation of 2D insect wing kinematics in an inclined stroke plane is carried out using an immersed boundary solver. The effect of vortex shedding and dipole jet on the vertical force generation by the flapping wing due to change in the stroke plane angle is investigated in the vicinity of the ground. The results of instantaneous force and vorticity contours reveal the underlying lift enhancement mechanisms for the inclined stroke plane flapping wing. Moreover, they aid in the understanding of the wake-ground interaction and the associated shear layers. The calculated average vertical force delineates different force trends for the inclined stroke plane flapping near the ground. Furthermore, the dipole jet patterns are analyzed for different heights. These patterns are found to be a better tool to assess the kinematics for the vertical force enhancement and reduction, especially at intermediate heights. Vertical force enhancement is the critical parameter in the design of the micro aerial vehicle (MAV). Through this study, it is certain that the dipole jet has the potential to be used as a lift modification mechanism in MAVs. In summary, the study gives a holistic view of the physics of the inclined plane kinematics near the ground and serves as the basis for the design of MAVs. © 2021 IOP Publishing Ltd.

Bioinspiration & biomimetics

Can the ground enhance vertical force for inclined stroke plane flapping wing?

Surface coatings have proved to be one of the most effective methods to protect the concrete structures in aggressive environments. However, coating the structures with polymer after 28 days of curing/hydration to get the best interfacial performance has little scientific backing. The measured surface free energy of the hydrating cement paste showed that the surface energy is high for a 2-day hydrated paste compared to cement paste cured for 7, 28 days. This opens up the possibility for exploration into coating an acrylic based polymer on a concrete surface before it reaches 28 days of hydration, which has been investigated in this study. The practical work of adhesion between the hydrating cement paste and polymer as measured from macro mechanical tests showed to have a dependency on the surface evolution of the cement paste. However, it was also observed that it depends equally on the polymer functional groups. Poly (methyl methacrylate) (PMMA), when coated on a 2-day hydrated cement paste, showed higher interfacial strength than when coated on later days (7, 14, 28 days). In the case of epoxy, irrespective of when the polymer was coated, it had the same interfacial strength. The influence of the evolving cement paste and the polymer functional group (PMMA in particular) on the interfacial strength at the micro- and molecular-length scales are discussed. It is found that the free/adsorbed calcium form a metal-complex with the ester functional group of the polymer in addition to the physical interactions, resulting in higher interfacial strength. The observations shed light on the importance of interfacial science and act as a guideline for the design of effective polymer-cement coatings. © 2020

Cement and Concrete Composites

Interfacial adhesion mechanism between organic polymer coating and hydrating cement paste

The objective of this work is to determine the interfacial strength between cement paste and epoxy, coated at the different instant of time during the hydration. The factors influencing the magnitude of interfacial adhesive strength were also studied. To pursue this, the interfacial adhesive strength between cement paste and epoxy coated at different days of hydration was determined by performing macro-mechanical experiments such as slant shear and pull-off adhesion test. The various instant of hydration when epoxy was coated are 2, 7, 14, 21, and 28 days from the point of initiation of hydration. The possible influencing factors considered were water-cement ratio, curing time, and thickness of the coating. Results of macro-mechanical tests show that the average interfacial adhesive strength between cement paste and the epoxy coating was in the same range, irrespective of the instant of coating. Although the higher surface energy and work of adhesion should result in a higher interfacial strength for epoxy coated on 2nd day, the measured value of interfacial strength, however, did not show such. One of the possible reasons why interfacial strength of epoxy coated on the 2nd day is similar to 28th day coating in spite of higher surface energy and work of adhesion, is the presence of the larger amount of surface water during the earlier days. The effect of this excess water balances out the contributory effect of the physical interactions and mechanical interlocking. From the parametric study, it was observed that the factors considered in the present study play a major role, particularly when coated during the early days of hydration. © 2021 Elsevier Ltd

Department Of Civil Engineering

Construction and Building Materials

Evaluation of interfacial strength between hydrating cement paste and epoxy coating

Polymeric gels, when made into thin films, are reported to depict a reversible folding behavior on exposure to favorable solvents as a result of the coupled diffusion–deformation mechanism. In this work, we develop a model based on the coupled diffusion–deformation mechanism which can predict the large deformation caused during this reversible folding behavior of a polymeric thin film. The formulation incorporates a concentration dependent Young's modulus variation. It is found that the incorporation of concentration dependent Young's modulus of the film in the formulation is essential to predict the folding curvatures accurately. The results obtained using this model are validated with the experimental results of the Chitosan–Water system by comparing the folding characteristics — intermediate folding stages and curvature, while accurately explaining the mechanism of reversible folding. The model is applied to other material systems such as PVA–Water and PDMS–Hexane systems. Furthermore, we demonstrate that the developed model successfully predicts the buckling and wrinkling phenomenon observed experimentally in stimuli-responsive polymer thin films. © 2020 Elsevier Ltd

Department of Mechanical Engineering

Mechanics of Materials

Coupled diffusion–deformation behavior of stimuli-responsive thin polymer films

In this paper, the development of modified beamforming methods, Filtered Delay Weight Multiply and Sum (F-DwMAS) and Filtered Delay Euclidian-Weighted Multiply and Sum (F-DewMAS), are reported. These methods were investigated on a minimum-redundancy synthetic aperture technique, called as 2 element Receive Synthetic Aperture Focusing Technique (2R-SAFT), which uses one element on transmit and two consecutive elements on receive, for reducing hardware complexity without compromising much on the image quality. The performance of the developed F-DwMAS and F-DewMAS methods were compared with Delay and Sum (DAS) and recently introduced F-DMAS beamforming methods. Notably, in the proposed methods, an additional aperture window function is designed and incorporated to the F-DMAS method. The different methods of F-DwMAS, F-DewMAS, F-DMAS and DAS were compared in terms of the resulting image quality metrics, Lateral Resolution (LR), Axial Resolution (AR), Contrast Ratio (CR) and contrast-to-noise ratio (CNR), in simulation and experiments on tissue-mimicking phantoms. Experimental results show that (F-DwMAS) and F-DewMAS resulted in improvements of AR by (46.32% and 23.51%), 43.56% and 17.78%, LR by (47.81% and 30.27%), 44.26% and 26.14% and CR by (45.68% and 17.15%), 42.16% and 9.87% compared to those obtained using DAS and F-DMAS, respectively. However, CNR of F-DwMAS and F-DewMAS was found to be 31.19% and 21.16% less compared to DAS, but 4.89% and 18.64% more than F-DMAS, respectively. Hence, it can be concluded that the image quality improved by both F-DwMAS and F-DewMAS compared to DAS and F-DMAS. Also, between F-DwMAS and F-DewMAS, the later has the advantage of ready applicability to different acquisition schemes and settings compared to the former also having an additional advantage of better CNR compared to both F-DMAS and F-DewMAS. © 2020 Elsevier B.V.

Ultrasonics

Weighted non-linear beamformers for low cost 2-element receive ultrasound imaging system

In turbulent premixed flames, the fractal dimension of flame iso-surface is argued to be D=7/3 for Damköhler's large-scale limit (Da&gt;&gt;1) and D=8/3 for Damköhler's small-scale limit (Da∼O(1)) based on heuristic scaling arguments. However, such scaling arguments do not consider the effect of the intermittent nature of turbulent kinetic energy dissipation on the flame surface. In this paper, we account for the effects of intermittent dissipation on the fractal dimension of low Da turbulent premixed flames. Intermittent dissipation leads to variability in the inner cut-off, which then affects the scalar flux and total interface area. We account for these variabilities through two approaches: coarse-grained approach based on the moments of the dissipation and fine-scale analysis by adopting the multifractal formalism. We derive two corrections to the upper-limit of fractal dimension – D=8/3+3/4(1−D1/4) and D=8/3+2/3(3−D1/3). We further show that the second correction leads to an explicit dependence of the fractal dimension (D) on the scaling exponent (ξ) of the velocity structure function through the relation: D=7/3+ξ. Thus, we explicitly quantify the effect of the intermittent nature of turbulence upon low Da premixed combustion. © 2020 The Combustion Institute

Department of Aerospace Engineering

Combustion and Flame

Fractal dimension of premixed flames in intermittent turbulence

The current study provides a discussion on two aspects of size-effects which is known to be a characteristic behavior of micro-scaled structures. First it provides the experimentalists with a novel micro-bend technique wherein the micro-cantilever is loaded at different points along its length. This eases the experimental process by reducing the efforts of the experimentalists in sample preparation (as the same sample can be used for obtaining results corresponding to different L/H ratio). Also, precision cutting and mounting of the samples to obtain the micro-cantilever of desirous overhang length is not required. Further, the above described technique is used to test nickle micro-cantilevers of three thicknesses (15, 25, 50 μm) and different L/H (=200, 300, 400) ratios. They are elastically bent to undergo small deflections. Apart from reporting the obvious size-effects for smaller thicknesses and fitting the experiments with a model for determining the length scale parameter, size-effects for beams with different thicknesses and varying L/H ratios are presented. Secondly, a discussion on the widely fluctuating nature of numerical values that the length scale parameter assumes is presented. And it is postulated that the numerical value of length scale parameter is a function of material, model and geometry of beam cross-section. © 2021 Taylor & Francis Group, LLC.

Mechanics of Advanced Materials and Structures

Novel nickle foil micro-bend tests and the need for a relook at length scale parameter’s numerical value

The specific role of disorder in the cell-wall material of a two-dimensional cellular solid on its deformation and crushing failure under uni-axial compression was examined numerically and validated with existing experimental data. A random spring network model with nearest and next-nearest neighbour interactions was developed for the representative cellular architecture of staggered-square honeycomb. The springs were taken to be quasi-brittle in nature and to account for the disorder, spring failure properties were taken to be statistically distributed. The model is shown to be capable of reproducing the non-linear behaviour of honeycombs resulting from the damage accumulation and results are shown to be in agreement with experimental measurements in terms of stiffness, strength and energy absorption. Further, the role of disorder in the material properties on the types of failure modes was investigated. Increasing disorder in the spring failure threshold was shown to result in decrease in the peak strength but a more stable crushing response resulting from the increased separation between the two primary cell wall failure modes. © 2021