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.

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.

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.

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.

Peak ground motions and spectral accelerations estimated from the prediction equations are highly significant in earthquake hazard studies. Recently, these predictive relationships developed for higher-order parameters obtained paramount importance as they describe different ground motion characteristics. The northeastern region of India experiences extreme seismicity due to the Indian plate subduction under the South Asian plate. However, only a few ground motion prediction equations (GMPEs) are available for such tectonic environments due to insufficient ground motion data. In this regard, it is noticed that the tectonic environment experienced by New Zealand is similar to that of northeast India. So, in this paper, two GMPE models for New Zealand are developed with the help of the artificial neural network (ANN) technique using the GeoNet database. Model-1 corresponds to various higher-order parameters, whereas model-2 developed for spectral accelerations (Sa) between 0.01 and 5s. Further, these models are compared against global and region-specific GMPEs. The developed models shows good agreement with other GMPEs and the data but slightly over predicts at distances greater than 300 km. Additional consideration of site-to-site variability in the current models reduced the total standard deviations of model-1 by 19–22 % and model-2 by 20%–23 %. Further, the estimates of these developed models are compared with some of the significant earthquakes in northeast India, and from these results, it is concluded that the current models can be adapted in such regions to estimate ground motion.

We study the initiation of damage in a polymeric body in which there is a line defect due to the formation of a "weld line" that occurs when two polymer streams join together and then solidify. We show that damage initiates in the region of weakness, namely the "weld line" based on a criterion for damage that was developed earlier. We also show that if there are other stress concentrators also additionally present, such as a hole, then there is a competition between the stresses induced due to the weakness and the stress as a consequence of the stress concentrator (in this instance a hole). This study adds more credence to the criterion for the initiation of damage that is based completely on knowledge of information at the current configuration of the body, that is, the criterion for damage is not based on the value of quantities that also need information based on a reference configuration such as the stress or strain.

Damage in concrete has been modelled using various approaches such as fracture mechanics, continuum damage mechanics and failure envelope theories. This study proposes a new approach to model the initiation of damage in concrete that addresses some limitations associated with the existing approaches. The proposed approach defines damage in terms of changes in the density of the material at the microscopic level, where such changes are induced by mechanical loading. The suggested approach is used to simulate the response of 2D concrete bodies to uni-axial tension and uni-axial compression. The simulation results indicate that the proposed model, by means of a single constitutive function, is able to correctly predict failure patterns and aptly capture the damage mechanisms under both uni-axial tension and uni-axial compression loadings using only the information related to the microstructure, the density field and the stiffness field. As a continuation, in Part II, the ability of the D3-M approach to model fully coupled chemo-mechanical damage in concrete using a single constitutive equation will be demonstrated.

Blast-induced traumatic brain injury (bTBI) is a rising health concern of soldiers deployed in modern-day military conflicts. For bTBI, blast wave loading is a cause, and damage incurred to brain tissue is the effect. There are several proposed mechanisms for the bTBI, such as direct cranial entry, skull flexure, thoracic compression, blast-induced acceleration, and cavitation that are not mutually exclusive. So the cause-effect relationship is not straightforward. The efficiency of protective headgears against blast waves is relatively unknown as compared with other threats. Proper knowledge about standard problem space, underlying mechanisms, blast reconstruction techniques, and biomechanical models are essential for protective headgear design and evaluation. Various researchers from cross disciplines analyze bTBI from different perspectives. From the biomedical perspective, the physiological response, neuropathology, injury scales, and even the molecular level and cellular level changes incurred during injury are essential. From a combat protective gear designer perspective, the spatial and temporal variation of mechanical correlates of brain injury such as surface overpressure, acceleration, tissue-level stresses, and strains are essential. This paper outlines the key inferences from bTBI studies that are essential in the protective headgear design context.