Sudharshan N. Raman

Elastomeric polymers for retrofitting of reinforced concrete structures against the explosive effects of blast

The main distinction of blast load from other types of dynamic loadings is its impulsive nature, where the loads usually act for a very short duration but transmit very high impulsive pressures. This paper presents an overview of the present retrofitting techniques in use to enhance the capacity of structural elements to withstand the effects of blast loads, and introduces an alternative retrofitting approach by utilizing polymer coatings. The authors have demonstrated the positive effects of this approach by conducting a numerical investigation on the behavior of an unretrofitted reinforced concrete panel subjected to the blast load from a 2 kg charge at 1.6 m stand-off distance, and subsequently comparing its performance with several polymer coated panels. The analysis was performed by using an explicit nonlinear finite element (FE) code. The results demonstrate the contributions of this technique in terms of panel displacement control and energy dissipation. Considering that the polymer coating can also act as a protective layer in improving the durability of structural materials, this technique can also be optimized favorably to enhance the overall sustainability of structures.

Low-cost evaluation techniques for information retrieval systems: A review

For a system-based information retrieval evaluation, test collection model still remains as a costly task. Producing relevance judgments is an expensive, time consuming task which has to be performed by human assessors. It is not viable to assess the relevancy of every single document in a corpus against each topic for a large collection. In an experimental-based environment, partial judgment on the basis of a pooling method is created to substitute a complete assessment of documents for relevancy. Due to the increasing number of documents, topics, and retrieval systems, the need to perform low-cost evaluations while obtaining reliable results is essential. Researchers are seeking techniques to reduce the costs of experimental IR evaluation process by the means of reducing the number of relevance judgments to be performed or even eliminating them while still obtaining reliable results. In this paper, various state-of-the-art approaches in performing low-cost retrieval evaluation are discussed under each of the following categories; selecting the best sets of documents to be judged; calculating evaluation measures, both, robust to incomplete judgments; statistical inference of evaluation metrics; inference of judgments on relevance, query selection; techniques to test the reliability of the evaluation and reusability of the constructed collections; and other alternative methods to pooling. This paper is intended to link the reader to the corpus of ‘must read’ papers in the area of low-cost evaluation of IR systems.

Quasi-Static Behavior of Palm-Based Elastomeric Polyurethane: For Strengthening Application of Structures under Impulsive Loadings

In recent years, attention has been focused on elastomeric polymers as a potential retrofitting material considering their capability in contributing towards the impact resistance of various structural elements. A comprehensive understanding of the behavior and the morphology of this material are essential to propose an effective and feasible alternative to existing structural strengthening and retrofitting materials. This article presents the findings obtained from a series of experimental investigations to characterize the physical, mechanical, chemical and thermal behavior of eight types of palm-based polyurethane (PU) elastomers, which were synthesized from the reaction between palm kernel oil-based monoester polyol (PKO-p) and 4,4-diphenylmethane diisocyanate (MDI) with polyethylene glycol (PEG) as the plasticizer via pre-polymerization. Fourier transform infrared (FT-IR) spectroscopy analysis was conducted to examine the functional groups in PU systems. Mechanical and physical behavior was studied with focus on elongation, stresses, modulus, energy absorption and dissipation, and load dispersion capacities by conducting hardness, tensile, flexural, Izod impact, and differential scanning calorimetry tests. Experimental results suggest that the palm-based PU has positive effects as a strengthening and retrofitting material against dynamic impulsive loadings both in terms of energy absorption and dissipation, and load dispersion. In addition, among all PUs with different plasticizer contents, PU2 to PU8 (which contain 2% to 8% (w/w) PEG with respect to PKO-p content) show the best correlation with mechanical response under quasi-static conditions focusing on energy absorption and dissipation and load dispersion characteristics.

Dynamic Properties of High Volume Fly Ash Nanosilica (HVFANS) Concrete Subjected to Combined Effect of High Strain Rate and Temperature

The study aims to determine the dynamic properties of high volume fly ash nanosilica HVFANS concrete exposed to strain rates between 30.12 to 101.42 s‐1 and temperatures of 25, 400, and 700 oC by using split Hopkinson pressure bar SHPB machine. The static and dynamic compressive strengths of HVFANS concrete were slightly lower than plain concrete PC at room temperature, while its values were higher at 400 and 700 oC. The results proved that the CEB model of dynamic increase factor is more reliable to estimate the behaviour of HVFANS concrete at studied temperatures. The toughness, critical strain, and damage of HVFANS concrete recorded a superior performance than PC under studied strain rates and temperatures that would reflect the possibility of use HVFANS concrete in structures to improve its resistant of fire and impact loads, as well as to decrease the demand on Portland cement which could lead to restrict the risks of liberated gases during cement production. Furthermore, equations were proposed to estimate the dynamic increase factor, toughness, and critical strain of both concretes under investigated conditions.

Use of Coupled Smooth-Particle Hydrodynamics/Lagrangian Method in the Simulation of Deformable Projectile Penetration

Mesh-dependent finite element (FE) analysis poses considerable limitations in terms of applying it in large deformation problems. As simulations such as projectile penetration are largely dependent on the material failure criterion, the artificial element erosion technique, which is usually incorporated in mesh-dependent FE techniques, may result in considerable inaccuracies. Therefore, over the last few decades, computational mechanists and engineers have focussed on implementing a mesh-independent analysis method to overcome the numerical instabilities that occur in mesh-dependent FE methods. The smooth-particle hydrodynamics (SPH) technique is one of the methods that is becoming popular among computational mechanists and engineers. The knowledge and understanding of different parameters involved in such simulation is essential prior to its application. In this study, a comprehensive numerical investigation of projectile penetration through monolithic aluminium plates using the SPH technique has been performed. While there have been studies reported in published literatures on the application of the SPH technique on projectile simulations, very limited attention has been placed on investigating the influence of different parameters on the analysis results, especially on deformation of the projectiles. One of the main objectives of this study is to investigate the contribution of different numerical parameters on the simulation of complete penetration of deformable projectiles (5.56 mm x 45 mm NATO standard) through 16 mm AA5061-H116 aluminium plates. The effects of particle density, smooth length, different particle sorting options, and scale factor for smooth length, have been parametrically studied and presented. In addition, the penetration mechanism of projectiles through a metallic target has been numerically and experimentally studied. Numerical simulations show very good agreement with the experimental results. The velocity time histories for monolithic aluminium plates show a “dip” in its velocity reduction, which is considerably difficult to observe in mesh-dependent methods. Three stages of the penetration process have been numerically identified and discussed.

Blast damage assessment of symmetrical box-shaped underground tunnel according to peak particle velocity (PPV) and single degree of freedom (SDOF) criteria

This study aimed to determine the reliability of the damage criteria that was adopted by the peak particle velocity (PPV) method and the single degree of freedom (SDOF) approach to assess the damage level of a box-shaped underground tunnel. An advanced arbitrary Lagrangian Eulerian (ALE) technique available in LS-DYNA software was used to simulate a symmetrical underground tunnel that was subjected to a surface detonation. The validation results of peak pressure into the soil revealed a good consistency with the TM5-855-1 manual within differences that were much less than previous numerical studies. The pressure contours revealed that the blast waves travelled into the soil in a hemispherical shape and the peak reflected the pressure of the tunnel that occurred immediately before the incident pressure reached its highest value. The assessment results proved that the criteria of the above methods could efficiently predict the damage level of a box-shaped tunnel under different circumstances of explosive charge weight and lining thickness at a depth of 4 m within slight differences that were observed during van and small delivery truck (SDT) explosions. However, the efficiency of both the methods was varied with the increase of burial depth. Whereas, using the PPV method significantly underestimated or overestimated the damage level of the tunnel, especially during SDT and container explosions with a lining thickness of 250 mm at burial depths of 6 and 8 m, respectively, the damage level that was obtained by the SDOF method greatly matched with the observed failure modes of the tunnel. Furthermore, new boundary conditions and equations were proposed for the damage criteria of the PVV method.

Bio-Based Polyurethane Elastomer for Strengthening Application of Concrete Structures Under Dynamic Loadings

Feasibility of application of a bio-based elastomeric polyurethane (PU) coating to improve the dynamic resistance of concrete specimens by enhancing their energy absorption capability was investigated. A series of experimental investigation were conducted using scaled concrete specimens with dimensions of 160 × 40 × 40 mm, which were coated with eight different coating configurations by varying the coating thickness and location. Three-point bending test was conducted under quasi-static and dynamic conditions, by varying the strain rates (0.00033 s−1 and 0.067 s−1). The maximum flexural stress, failure strain, and strain energy density characteristics were used to assess the effectiveness of the proposed retrofitting technique. Polymer layers of 1–4 mm thick provided 2.9–8.9 times enhancement in failure strain, 3.0–11.3 times enhancement in strain energy density, and a marginal enhancement in the maximum flexural stress under dynamic conditions compared to the dynamic response of uncoated concrete specimens. In addition, the dynamic response of concrete specimens was improved when the thickness of the PU coating was increased and when the coating was applied on both faces.