Alagappan Ponnalagu

Developing a ground motion model (GMM) for Fourier amplitude spectrum (FAS) is essential in seismology and engineering for generating response spectrum and synthetic time histories. Despite data-driven techniques being efficient in modeling complex relations, very few GMMs are developed for FAS using them. An efficient hybrid data-driven algorithm combining genetic algorithm and artificial neural network is implemented using the GeoNet database with 905 records from 77 events in the current work. The input parameters of the model are moment magnitude, Joyner–Boore distance, shear wave velocity, depth to the top of the rupture plane, fault, and tectonic flags. The developed FAS model is statistically tested to be robust and has good agreement with the recorded data and other available GMMs. The developed GMM to FAS has an overall correlation coefficient in the range of 0.8108–0.9298 and sigma in the range of 0.26–0.4 (in log10 units). Further, synthetic time histories are generated from the predicted FAS values and are consistent with various ground motion parameters and the response spectra.

The development of shock tubes and understanding of shock wave propagation and its interaction with a model is of significant interest in various domains. In this context, shock tubes effectively recreate the field explosion in controlled laboratory conditions and ensure safety, low cost and repeatability. The blast wave simulators (BWS) are operated in a reflective (for barrier wall, blast absorbent material, etc.) and diffractive (for biofidelic head and torso, In-vivo, etc.) mode. The side wall reflections in refractive mode and end wall reflections from the model in reflective mode shock tube cause secondary loading to the model. In this study, a reflection wave eliminator (RWE) with a flap assembly was developed to minimize secondary loading in closed-end shock tubes, and its performances are discussed. As the first cycle of shock wave crosses the RWE, it will open the flap assembly and helps in minimizing the successive cycles of shock waves. The effect of RWE location and the number of flap openings on shock wave parameters, such as positive peak overpressure and impulse, for the case of two different shock tubes length, such as 3.3 m and 5.3 m, has been studied. It was observed that the peak overpressure reduction in the secondary shock wave because of single flap RWE at the model location is 71.31% and 88.12% for 3.3 m and 5.3 m long shock tubes, respectively. The secondary loading of the model in closed-end shock tubes can be significantly reduced by tuning the standard shock tube using the RWE proposed in this study.

In this work, pavement cross-section as stipulated by the Indian Roads Congress code of practice for pavement design was used in the stress-analysis. Typically, two types of asphalt layers are laid over three to four layers of granular materials for pavements constructed for heavy traffic volume in India. Asphalt mixtures pertaining to these top two layers were fabricated with polymer modified and unmodified asphalts in the laboratory. These mixes were tested in the simple performance test equipment at temperature of 60°C under a wide range of frequencies. A four parameter Burgers' model was used for fitting the experimental data for both the asphalt layers. A two-dimensional finite element model of the pavement structure was used within the ABAQUS computing environment. The granular base, subbase and subgrade granular layers were assumed to be elastic. The pavement model was subjected to cycles of periodic loading and the stresses and strains were monitored at all the critical locations. This was used to quantify the influence of modifiers on the mechanical response of the pavement and parametric analysis was carried out to find out enhanced service life of the asphalt layers that could be achieved due to the use of high-performance materials. © 2011 ASCE.

Most constitutive relations that are used to capture the mechanical response of concrete are based on the assumption that concrete is homogeneous. However, it is a well-known fact that concrete has three distinct regions, namely - aggregate, mortar and Interface Transition Zone (ITZ). One of the reasons for assuming that concrete is a homogeneous material is to simplify the problem for finite element analysis (FEA). The damage behaviour of concrete depends upon the interaction of these three regions. Hence it is necessary to have a clear demarcation of the three regions for discretization and further analysis. It is well known that damage is initiated in the ITZ, which is a weak, porous and heterogeneous region of cement paste around aggregates whose thickness ranges from 9–50 µm. Due to the vast scale difference in the ITZ with aggregate, the ITZ of concrete cross-section is either computer-generated or manually inserted. While generated images are very limited by the algorithms/procedures employed, manual insertion of ITZs on real images takes time and is prone to certain uncertainties. An algorithm that processes the images of concrete for more reliable identification of the location of the ITZ and provides control on its thickness, colour and the minimum size of the aggregates to be included can drastically reduce human-induced error and enable faster and more reliable processing of existing images. In this work, an image processing algorithm is developed using OpenCV and NumPy, which are open source libraries in Python. The concrete image is processed by multiple means like log transformation, erosion, dilation, bilateral filtering and adaptive gaussian thresholding, which significantly improve the identification of different regions in concrete which further enhances appropriate FEM meshing. A contour feature extraction tool called canny edge detection is used to identify the aggregate and to draw the ITZ. The damage predicted through the FEM analysis of the problem domain that is processed by the proposed algorithms is validated by comparing it with the experimentally obtained damage patterns. The proposed algorithm performs better on computer-generated images than the images of actual concrete cross-sections. The accuracy of this algorithm on computer-generated images is over 75%, and it achieves over 90% accuracy on real images. The resulting image is also comparable to images that are computer programmed to have ITZs. Our algorithm enhances the accuracy of FEM analysis of images through the inclusion of ITZ and enhancement of the features of the image.

Fatigue and damage are the least understood phenomena in the mechanics of solids. Recently, Alagappan et al. (“On a possible methodology for identifying the initiation of damage of a class of polymeric materials”, Proc R Soc Lond A Math Phys Eng Sci 2016; 472(2192): 20160231) hypothesized a criterion for the initiation of damage for a certain class of compressible polymeric solids, namely that damage will be initiated at the location where the derivative of the norm of the stress with respect to the stretch starts to decrease. This hypothesis led to results that were in keeping with the experimental work of Gent and Lindley(“Internal rupture of bonded rubber cylinders in tension. Proc. R. Soc. Lond. A 1959; 249, 195–205 :10.1098) and agrees qualitatively with the results of Penn (“Volume changes accompanying the extension of rubber”, Trans Soc Rheol 1970; 14(4): 509–517) on compressible polymeric solids. Alagappan et al. considered a body wherein there is a localized region in which the density is less than the rest of the solid. In this study, we show that the criterion articulated by Alagappan et al. is still applicable when bodies have multiple localized regions of lower density, thereby lending credence to the notion that the criterion might be reasonable for a large class of bodies with multiple inhomogeneities. As in the previous study, it is found that damage is not initiated at the location where the stresses are the largest but instead at the location where the densities tend to the lowest value. These locations of lower densities coincide with locations in which the deformation gradient is very large, suggesting large changes in the local volume, which is usually the precursor to phenomena such as the bursting of aneurysms.

In this paper we study the state of stress and strain in infinite elastic slabs of nonlinear viscoelastic solids containing elliptic holes subject to an uni-axial as well as a bi-axial state of stress. The geometry affords one to get some inkling concerning the states of stress and strain in bodies containing a crack by obtaining the limit of the solutions as the aspect ratio (in this case the ratio of the minor axis to the major axis) of the ellipse tends to zero. We consider two classes of non-linear viscoelastic bodies, the classical incompressible Kelvin–Voigt solid (Thomson in R Soc Lond 14:289–297, 1865; Voigt in Ann Phys 283(12):671–693, 1892) and a generalization of a compressible model due to Gent (Rubber Chem Technol 69(1):59–61, 1996). We also study for the sake of comparison the case of a nonlinear neo-Hookean elastic solid with an elliptic hole.

Blast wave simulators (BWS) are shock tubes capable of generating shockwaves with Friedlander profile (typical profile observed during free field explosion).They have primary importance in blast-related research.The pressure–time profile parameters, such as peak overpressure, positive time duration, and decay coefficient of the shockwave, depend on the shock tube parameters (STPs) such as driver length, driven length, and burst pressure.We can generate shockwaves with the desired pressure–time profile by effectively tuning the STP.This study experimentally investigates the effect of driver length on the pressure–time profile of a shockwave generated by a blastwave simulator.Increasing the driver length increases the positive phase duration and peak overpressure at all probe locations.Also, it increases Friedlander profile formation distance.Further, a finite element model for shock tube is developed in ABAQUS/Explicit and the numerical results are compared with the experimental observations.The developed numerical model can predict the observed pressure–time profile with reasonable accuracy, so that it can be used for further parametric studies in the design of blast wave simulators.

We study fatigue (weakness induced by cyclic loading) in a viscoelastic body described by a generalization of the Kelvin-Voigt constitutive relation, employing a novel damage initiation criterion developed by Alagappan et al. [13-15]. The main premise is that damage is a consequence of the inhomogeneity of the material which leads to some locations in the body being naturally weaker, say for instance due to the density being lower and the material moduli depending on the density and decreasing with density, leading ultimately to failure at that location. This approach has been used successfully for polymers, elastomers and concrete subject to monotonic loading. In this study, we consider the initiation of damage due to cyclic loading, which is referred to as fatigue. Since the body under consideration is viscoelastic, it dissipates energy in each cycle which leads to an increase in temperature. We shall not take the effect of the temperature of the material moduli, instead we assume that the material moduli depend on the density and the rate of dissipation. In the case of our specific study the shear modulus of the material depends on the density and dissipation (in the case of the constitutive relation considered the shear rate), and the structure of the shear modulus is such that it decreases with decrease in density and decreases with increase in dissipation (tantamount to the assumption that it decreases with increasing shear rate for the constitutive relation under consideration) leading to damage of the material. We find that after sufficient number of cycles, the body under consideration undergoes significant loss in load carrying capacity due to fatigue.

We extend the methodology introduced for the initiation of damage within the context of a class of elastic solids to a class of viscoelastic solids (Alagappan et al. 2016 Proc. R. Soc. Lond. A: Math. Phys. Eng. Sci. 472, 20160231. (doi:10.1098/rspa.2016.0231)). In a departure from studies on damage that consider the body to be homogeneous, with initiation of damage being decided by parameters that are based on a quantity such as the strain, that requires information concerning a special reference configuration, or the use of ad hoc parameters that have no physically meaningful origins, in this study we use a physically relevant parameter that is completely determined in the current deformed state of the body to predict the initiation of damage. Damage is initiated due to the inhomogeneity of the body wherein certain regions in the body are unable to withstand the stresses, strains, etc. The specific inhomogeneity that is considered is the variation of the density in the body. We consider damage within the context of the deformation of two representative viscoelastic solids, a generalization of a model proposed by Gent (1996 Rubber Chemistry and Technology 69, 59-61. (doi:10.5254/1.3538357)) for polymeric solids and a generalization of the Kelvin-Voigt model. We find that the criterion leads to results that are in keeping with the experiments of Gent & Lindley (1959 Proc. R. Soc. Lond. A: Math. Phys. Eng. Sci. 249, 195-205. (doi:10.1098/rspa.1959.0016)).

Bridges are key components of transportation network, especially in strategic border areas in a country, and consequently are susceptible to subversive blast attacks. Hence in this study, dynamic response of a reinforced concrete (RC) bridge (single span) consisting of a deck slab supported on longitudinal girders along with transverse ones placed symmetrically has been numerically investigated when subjected to blast loading using ABAQUS/CAE 2020. The effects of an explosive charge of 226.8 kg (TNT) at 1 m standoff distance have been analyzed using the CONWEP algorithm. Three different locations of the bursting charge along the cross section at mid span of the bridge above the deck, such as on the central girder, between two adjacent longitudinal girders, and on the cantilever part, have been considered. Concrete damage distribution in terms of concrete spalling and cracking has been studied with concrete damage plasticity (CDP) model. Also, the response in terms of damage dissipation energy, maximum displacements, and stresses has been compared for the blast scenarios. Furthermore, AASHTO: LRFD Bridge Design Specifications (2017) provisions have been used to compare obtained maximum displacement values.