Damith Mohotti

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.

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.

Evaluation of possible head injuries ensuing a cricket ball impact

BACKGROUND AND OBJECTIVE The aim of this research is to study the behaviour of a human head during the event of an impact of a cricket ball. While many recent incidents were reported in relation to head injuries caused by the impact of cricket balls, there is no clear information available in the published literature about the possible threat levels and the protection level of the current protective equipment. This research investigates the effects of an impact of a cricket ball on a human head and the level of protection offered by the existing standard cricket helmet. METHOD An experimental program was carried out to measure the localised pressure caused by the impact of standard cricket balls. The balls were directed at a speed of 110 km/h on a 3D printed head model, with and without a standard cricket helmet. Numerical simulations were carried out using advanced finite element package LS-DYNA to validate the experimental results. RESULTS The experimental and numerical results showed approximately a 60% reduction in the pressure on the head model when the helmet was used. Both frontal and side impact resulted in head acceleration values in the range of 225-250 g at a ball speed of 110 km/h. There was a 36% reduction observed in the peak acceleration of the brain when wearing a helmet. Furthermore, numerical simulations showed a 67% reduction in the force on the skull and a 95% reduction in the skull internal energy when introducing the helmet. CONCLUSIONS (1) Upon impact, high localised pressure could cause concussion for a player without helmet. (2) When a helmet was used, the acceleration of the brain observed in the numerical results was at non-critical levels according to existing standards. (3) A significant increase in the threat levels was observed for a player without helmet, based on force, pressure, acceleration and energy criteria, which resulted in recommending the compulsory use of the cricket helmet. (4) Numerical results showed a good correlation with experimental results and hence, the numerical technique used in this study can be recommended for future applications.

Effective use of steel composites in blast and impact load damage mitigation

The current study proposes a new concept of crack analysis of reinforced concrete (RC) members. The novel philosophy behind the proposed concept is to establish the mean crack spacing and width through the compatibility of the stress-transfer and mean deformation approaches by equating the mean strains of the tensile reinforcement defined analytical techniques. The concept considers primary cracks at the stage of stabilised cracking assuming that a single RC block of a length of the mean crack spacing represents the averaged deformation behaviour of the cracked member. Based on the experimental evidence, reinforcement strain within the block is characterized by a strain profile consisting of straight lines. The latter represent three different zones that are described by different bond characteristics. Crack spacing is defined as the sum of lengths of these zones within the length of the block. The proposed concept holds the features of a simplicity and mechanical soundness: it involves the least amount of empiricism and is devoid of empirically established effective area of concrete. Aside from the predictive capabilities, the model proposes a tool for constitutive modelling. A preliminary statistical analysis of mean crack spacing using limited test data has demonstrated good predictive capabilities of the model resulting in 15% of the coefficient of variation. The proposed approach allows a critical assessment of the classical bond theory in regard to its fundamental statement relating crack spacing to /ef ratio. A preliminary study has shown that the larger are the member’s section depth and the reinforcement ratio, the more the classical approach deviates from reality. It can be deduced that crack spacing is mostly governed by four geometrical parameters given in the order of importance: section height, reinforcement ratio, bar diameter and cover. The influence of bar diameter on crack spacing is very much dependent on reinforcement ratio. For the members with large reinforcement ratio the effect of bar diameter on crack spacing is insignificant. For lightly reinforced members, variation in bar diameter may result in a significant change in spacing. The above findings strongly oppose the conventional understanding on cracking of RC structures accepted for many decades. ABOUT THE SPEAKER Gintaris Kaklauskas is Professor of Department of Reinforced Concrete Structures and Geotechnical Engineering and Director of Research Institute of Building Structures at Vilnius Gediminas Technical University (VGTU). PhD and DrSc (Habil. Dr.) degrees received from VGTU. Real member of Lithuanian Academy of Science. The recipient of ASCE best paper Moisseiff Award 2013, Lithuanian Science Prize 2013 and Marie Curie (Senior Research category) grant. Visiting professor (under Fulbright fellowship) at University of Illinois, Urbana-Champaign. Research interests include serviceability analysis and constitutive modeling of concrete structures.