L.A. Louca

Experimental and numerical studies on the response of stiffened plates subjected to gas explosions

Abstract This paper presents both experimental and numerical studies of stiffened plates subjected to hydrocarbon explosions and a parametric study of a simplified model of a stiffened plate considering different response aspects including the contribution of stiffeners under different stress state and loading levels. Boundary conditions, particularly, the in-plane restraints have a significant influence on the panel response when subjected to this kind of extreme load case. The difference in boundary conditions between a specimen and a component in an actual structural system has to be considered in order to predict the response accurately. The contribution of stiffeners is influenced by many factors, but the second moment of the cross section is the most important factor. Once buckling occurs in stiffeners, the deformed shape of a stiffened plate is very similar to that of an unstiffened plate.

Dynamic Performance of Simply Supported Rigid Plastic Circular Thick Steel Plates Subjected to Localized Blast Loading

AbstractClose-in explosive charges, such as improvised explosive devices, produce localized blast loadings that can potentially cause damage to property in military and civil structures and/or loss of life. Because the localized short-duration blast pulse affects most severely a small area of a plated structure, the plate’s boundary effects are not as influential as they would be when quasi-static or even a global blast loading is applied, and thus full plate action may not be used. Many common structural forms are composed of individual plated elements, and thus the investigation of localized blast loading effects on plates is an important aspect that leads to understanding the integral behavior. Typically, plates are made of ductile metallic materials, such as steel, which exhibit considerable postyield deformation capacity when subjected to such extreme dynamic loads. An analytical study of the dynamic plastic response of rigid plastic plated structures is the aim of the current study. A circular plate...

Pressure-Impulse Diagrams for Blast Loaded Continuous Beams Based on Dimensional Analysis

Pressure-impulse diagrams are commonly used in preliminary blast resistant design to assess the maxima of damage related parameter(s) in different types of structures as a function of pulse loading parameters. It is well established that plastic dynamic response of elastic-plastic structures is profoundly influenced by the temporal shape of applied pulse loading (Youngdahl, 1970, “Correlation Parameters for Eliminating the Effect of Pulse Shape on Dynamic Plastic Deformation,” ASME, J. Appl. Mech., 37, pp. 744–752; Jones, Structural Impact (Cambridge University Press, Cambridge, England, 1989); Li, and Meng, 2002, “Pulse Loading Shape Effects on Pressure–Impulse Diagram of an Elastic–Plastic, Single-Degree-of-Freedom Structural Model,” Int. J. Mech. Sci., 44(9), pp. 1985–1998). This paper studies pulse loading shape effects on the dynamic response of continuous beams. The beam is modeled as a single span with symmetrical semirigid support conditions. The rotational spring can assume different stiffness values ranging from 0 (simply supported) to ∞ (fully clamped). An analytical solution for evaluating displacement time histories of the semirigidly supported (continuous) beam subjected to pulse loads, which can be extendable to very high frequency pulses, is presented in this paper. With the maximum structural deflection, being generally the controlling criterion for damage, pressure-impulse diagrams for the continuous system are developed. This work presents a straightforward preliminary assessment tool for structures such as blast walls utilized on offshore platforms. For this type of structures with semirigid supports, simplifying the whole system as a single-degree-of-freedom (SDOF) discrete-parameter model and applying the procedure presented by Li and Meng (Li and Meng, 2002, “Pulse Loading Shape Effects on Pressure–Impulse Diagram of an Elastic–Plastic, Single-Degree-of-Freedom Structural Model,” Int. J. Mech. Sci., 44(9), pp. 1985–1998; Li and Meng, 2002, “Pressure-Impulse Diagram for Blast Loads Based on Dimensional Analysis and Single-Degree-of-Freedom Model,” J. Eng. Mech., 128(1), pp. 87–92) to eliminate pulse loading shape effects on pressure-impulse diagrams would be very conservative and cumbersome considering the support conditions. It is well known that an SDOF model is a very conservative simplification of a continuous system. Dimensionless parameters are introduced to develop a unique pulse-shape-independent pressure-impulse diagram for elastic and elastic-plastic responses of continuous beams.

Determining the through-thickness properties of thick glass fiber reinforced polymers at high strain rates

The use of thick fiber reinforced polymer (FRP) laminates in composite armor and naval structures requires thorough characterization of the through-thickness properties of said laminates, both quasi-statically and at high strain rates. Specimens cut from an E-Glass/vinyl ester FRP were tested in compression both quasi-statically and dynamically using a split Hopkinson pressure bar (SHPB). The SHPB tests utilized a conical striker for pulse shaping, to reduce the variation in strain rate during the test. The quasi-static through-thickness compressive strength was 417 MPa, while the SHPB tests produced a strength of 462 MPa at an average strain rate of 5.1 × 102 s−1. A single HPB configured for spalling tests was used to determine the dynamic through-thickness tensile strength (interlaminar tension). The interlaminar tensile strength was 125 MPa at an average strain rate of 1.8 × 103 s−1.

On dimensionless loading parameters for close-in blasts

Close-range blasts pose a threat through severe damage to structures and injury or death. In this work, the spatial and temporal descriptions of a localised blast load are presented using 6 non-dimensional parameters. These are found to be solely functions of the charge stand-off distance to diameter ratio for a cylindrically-shaped charge. Numerical simulations of a localised blast are performed using AUTODYN, where the pressure variation on a rigid barrier for various charge stand-off/diameter combinations is obtained. The least-square regression is then utilised to obtain the relationship between stand-off/diameter ratio and dimensionless loading parameters. The relevant expressions and dimensionless charts are presented. The proposed equations are verified by comparing experimental data with numerical results obtained by finite element analysis (FEA) of blast loaded steel plates (using the user-defined subroutine VDLOAD implemented in the FEA package ABAQUS/Explicit). Excellent correlation of the measured permanent displacement with numerically predicted results is obtained.

Characterization of the Mechanical Behavior of a Polymer-Based Laminate and Constituent Fibers at Various Quasi-Static Strain Rates

AbstractPredictive computational modeling of the response of armor systems to dynamic threats such as blast and impact requires understanding and quantification of the behavior of the armor materials. This paper describes the mechanical characterization of Dyneema HB26. The in-plane tensile, compressive, and shear stress-strain behavior and strength of the laminate at low rates has been determined experimentally. The tensile behavior of the Dyneema SK76 fibers, which comprise 83% of the laminate has been determined, including the effect of temperature and rate.

A simplified method in the static plastic analysis of corrugated steel panels

Using normality theory and a simplified beam model, a theoretical method is built up to evaluate the relationship between the central deflection of a corrugated panel and external uniformly distributed pressure load. The interaction between the bending moment and the membrane force in the panel is included, as well as the influence of connections. This theoretical model is calibrated against finite element analyses. The results show encouraging agreement.

BLAST RESPONSE OF FIELD OBJECTS

During the last few decades, several major industry accidents occurred across the world. These include the Buncefield event in the United Kingdom (UK) back in 2005. There were a number of steel switch boxes in the site located within the area covered by the vapour cloud. The present work reports numerical studies on the steel boxes covering both detonation and deflagration scenarios and assessing the response of those boxes in order to aid the investigation of the explosion. New Eulerian capabilites in ABAQUS has are together with existing Lagrangian formulations to create three numerical models with increasing complexity: (1) Pure Lagrangian model; (2) Uncoupled Eulerian – Lagrangian model; (3) Coupled Eulerian – Lagrangian model. Results from different modelling approaches are discussed and parametric studies are carried out based on Pure Lagrangian models to investigate the response of the switch box to a series of combination of pressures and impulses. Findings from the parametric study are summarised in the form of pressure – impulse diagrams and residual deformation of selected boxes are presented. The results confirmed the estiamted minimum overpressure level of 200kPa and it can also be concluded that the overpressure wave inside the cloud is most likely to be of a deflgrative form. The forensic study described in this paper has gives a good insight into the likely loading scenarios. The study presents a systematic approach for analysing the response of small objects to various blast loadings and the results have shown sthat the forensic studies can be undertaken by using the pressure – impulse diagrams in conjunction with damaged residual deformations.

Response of Armour Steel Plates to localised Air Blast Load – A Dimensional Analysis

We report on the results of dimensional analyses on the dynamic plastic response of square armour steel plates due to detonation of proximal cylindrical charges and ensued air blast loading. By assuming a generic function for the blast load, which is multiplicative comprising its spatial and temporal parts, a set of 14 dimensionless parameters, representative of the load and plate deformation, were identified and recast in the form of dimensionless functions of stand-off to charge diameter ratio. Parametric studies were performed using commercial code ABAQUS’s module of Finite Element hydrocode using MMALE and MMAE techniques, and combined with regression analyses to quantify the dimensional parameters and the expressions for dimensionless functions. A few numerical studies with various FE mesh types were also performed to validate the transient deflections against the small-scale experiments. For pulse loading due to proximal charges of small orders of stand-off/charge diameter ratio, the magnitude of the transverse deflection increased abruptly with incremental decrease in stand-off, in contradistinction to the plate deformations at higher stand-offs where variations in displacement are smooth. This confirmed the existence of a stand-off at which a transition in behaviour takes place. For stand-off values less than charge diameter, a dimensionless energy absorbing effectiveness factor was considered to investigate the prediction of rupture in the plate corresponding to different charge masses. This factor is measured as a baseline parameter to predict, using solely numerical means, the blast loads which ensue rupture on full-scale prototypes.

Hybrid steel-to-composite joint behavior under tensile loading

Hybrid steel-to-composite joints are being used more commonly for load bearing applications. However, these hybrid joints usually entail geometry and material discontinuities which can induce stiffness mismatch and cause local stress concentrations. The shock impedance mismatch caused by the different wave propagation characteristics can also be crucial to the structural response of the hybrid joints under impulsive loads. Recent research at Imperial College London (ICL) and the U.S. Naval Academy (USNA) has focused on characterizing the behavior and ultimate load capacity of hybrid steel-to-composite joints with different configurations under various loading conditions. This paper presents results from tensile strength testing of steel-to-composite double lap joints, comparing pseudostatic strength with dynamic strength and comparing joints that exploit perforated steel plates with those manufactured with non-perforated steel plates. An intentional manufacturing flaw also was incorporated into half of the joints, both perforated and non-perforated joints, in order to assess the effect of this flaw type on joint strength. Finite Element Analysis (FEA) results are compared to experimental results for both perforated and non-perforated joints.