An Eulerian approach to soil impact analysis for crashworthiness applications

In the processing of composites, fibre reinforcement is draped over complex three dimensional geometries, yielding local variations in the fibre volume fraction and fibre orientation with subsequent effects on the permeability of the reinforcement during resin infiltration and on the properties of the composite product. This study presents computer simulations of the draping of fabrics by following a mechanical, finite element approach. The fabric is considered as a solid continuum with mechanical properties and friction properties at interfaces. The problem is solved by applying an explicit dynamic finite element analysis. In order to incorporate the large shear deformation of fabric during draping, an updated material behaviour law was formulated on the basis of changing fibre directions. The model was implemented in a user computer subroutine and tested first in case-studies of simple in-plane shear where the predictions were also compared with experimental data from a picture frame type of shear test. Thereafter, numerical simulations focused on the draping of a fabric in a “hat” shaped mould, comprising a hemispherical cup with a wide flat rim. The predictions were compared with predictions from similar simulations without the updated material behaviour law, experimental data and predictions from a “fishnet” algorithm.

In recent decades, terrorist attacks have been increased all over the world. For this reason, the design of blast resistant structures is very crucial. On the other hand, for decades the optimum design of structures has been an interesting subject for structural engineers and researchers as it can provide cost and behavioural efficiencies. In the current research, the optimum design of steel moment frames under blast loading has been studied. Towards that end, first, a damage index is developed based on the maximum plastic strain in the members. Then, according to the damage index, a multi-objective optimisation methodology is proposed to minimise the structural weight and the damage index. The method utilises explicit nonlinear dynamic finite element analysis as the structural analysis method and NSGA-II optimisation algorithm as the optimisation technique. At the final point, based on the proposed method a framework is developed, and two numerical examples are studied. Results of this research show that the developed method is effective in achieving optimal designs with desired cost and damage.

The fatigue property of riveted lap joint is greatly related to the riveting-induced residual stress, especially the stress distribution on the faying surface. However, an accurate study of the residual stress characteristics in the riveted sheet could be very difficult. In this paper, both numerical and experimental investigations were carried out on the stress/strain characteristics in riveted aircraft lap joints. A special specimen was designed for the test of strain variations on the faying surface of the sheet by microstrain gages. For the numerical simulation, the rivet squeezing process was analyzed using the explicit dynamic finite element (FE) method, whilst a general static FE analysis was employed for the elastic springback after the squeeze force was removed. A comparison of the strain variations between the experimental results and FE simulations shows a general good agreement, although there may be some difference for points measured near the hole surface. The FE analysis reveals that both compressive and tensile residual stresses could be introduced in the riveted sheet. Massive compressive residual stress can be created in the near-surface layer of the hole. However, the stress level is not always increased with increasing the squeeze force, and so is the improvement of fatigue life observed. Further study is still necessary to account for the fatigue life decreasing effect caused by a high squeeze force.

One of the widely used geometrically complex parts in recent civil passenger aircrafts is the L-shaped composite brackets connecting ribs to skins. Due to the sharp curved geometry, interlaminar opening stresses are induced and delamination occurs under considerable mode-mixities at the corner. Dynamic phenomena during delamination initiation and propagation of L-shaped beams are investigated using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods (CZM). In ABAQUS a sequential explicit analysis followed by static (implicit) solution is used where the solution duration is considerably reduced. The thickness of the specimens is varied from 1.0 mm to 4.0 mm while the inner radius is kept same. Loading is applied parallel to one of the arms quasi-statically. Even though the crack is at the very middle of the specimen, this specific loading type yields variable traction fields and mode-mixities in the two sides of the crack in which delamination occurs under shear stress dominated loading on one crack tip and opening stress dominated loading on the other. It is observed that the delamination propagation is highly dynamic even though the loading is quasi-static. The speed of the delamination under shear dominated loading at one side can reach 800 m/s and under normal stress dominated loading is 50 m/s in dedicated thickness levels. In addition, moving elasto-dynamic radial compressive waves along the interface are observed. An important observation for design applications, a typical solution of adding more plies to the laminate might yield failure transition to a secondary crack nucleating at the arm and propagating towards the center crack.

Solid expandable tubular is an emerging and promising technology in petroleum industry. It consists of increasing the diameter of a tubular by hydraulically pushing or mechanically pulling a conical mandrel through it. In this paper, a dy namic explicit finite element analysis (FEA) has been used to study the solid tubular expansion using ABAQUS, a commercial FEA software package. The required drawing force for tubular expansion was estimated for different mandrel shapes, friction coefficients, and expansion ratios. The drawing force increases with the increase in friction coefficient and expansion ratio. It was also found that the material velocities increase on the front of the mandrel but decrease to zero in the post expansion section. Moreover, the plastic deformation shows a reduction of thickness with friction coefficient and expansion ratio. Simulation results also revealed that contact occurs at the two ends of the conical mandrel tubular interface and that the contact stress increases with friction coefficient reaches to maximum value at an expansion ratio of 20%.

In this paper a numerical study using the finite element method is undertaken to predict the settlement response of a footing, considering plane strain conditions, resting on a reinforced granular bed on soft soil. The granular fill, soft soil and geosynthetic reinforcement are considered as non linear materials. The geosynthetic reinforcement is modelled with interface elements for allowing slip between the soil and reinforcement. The results obtained from the present investigation showed close agreement when compared with the results of finite element analysis and lumped parameter modelling carried out by previous researchers, assuming no slip conditions. The number of reinforcement layers was taken as one or three (multilayer). A parametric study has been carried out to illustrate the effect of slippage of the reinforcement layer on the settlement response. The increase in the settlement is not significant when the slippage of the reinforcement is considered.

Dynamic delamination modelling using interface elements

Elastic support is a commonly used method of vibration isolation and vibration reduction. The research on the mode of underground arch structure with elastic support can improve the battlefield survival ability of personnel and equipment in underground protection engineering. In this paper, the ANSYS/LS-DYNA nonlinear explicit dynamic finite element analysis program is used to numerically simulate the mode of the elastically supported underground arch structure, and the natural vibration circle frequency variation curve and mode diagram of the structure are obtained. The results show that the elastic support reduces the natural frequency of the structure, prolongs the natural vibration period of the structure, and has a good vibration isolation and vibration reduction effect; the critical stiffness coefficient causes the first and second modes of the structure to be easily excited at the same time, which should be avoided in structural design.

Introducing the hierarchy into cellular materials has attracted increasing attention in the effort to pursue improved absorbed-energy abilities and impact resistance. In this paper, the dynamic crushing properties and energy absorption capacities of joint-based hierarchical honeycombs with different topologies were explored by means of explicit dynamic finite element (FE) analysis using ANSYS/LS-DYNA. Four types of joint-based hierarchical honeycombs with uniform cell-wall thickness were firstly constructed by substituting each vertex of regular honeycombs with a smaller self-similar cell (hexagon or square). The respective influences of hierarchical parameters and impact velocities on in-plane dynamic deformation modes, mechanical characteristic and energy absorption of joint-based hierarchical honeycombs were discussed. Research results showed that the hierarchy had a far greater influence on the in-plane deformation modes of honeycombs. Compared with regular honeycombs, the dynamic plateau stress and specific energy absorption of joint-based hierarchical honeycombs can be improved if the proper hierarchical parameters were chosen. Adding the joint-based hierarchy into regular honeycombs can enhance the crushing stress efficiency (CSE) of the specimens. In addition, by introducing a non-dimensional dynamic sensitivity index, the dynamic shock enhancement of hierarchical honeycombs was also investigated. These researches are useful for the multi-objective dynamic optimization design and controllable properties of cellular materials.

Although uplift motion of flat-bottom cylindrical shell tanks has been considered to contribute toward various damages to the tanks, the mechanics were not fully understood. As well as uplift displacement of the tanks, fluid pressure accompanying the uplift motion of the tanks may play an important role in the cause of the damage. An accurate estimate of the fluid pressure induced by the uplift motion of the tanks is indispensable in protecting the tanks against destructive earthquakes. As a first step of a series of research, this study mathematically derives the fluid pressure on a rigid rectangular tank with a unit depth accompanying angular acceleration, which acts on a pivoting bottom edge. The rectangular tank employed herein is equivalent to a thin slice of the central vertical cross section of a rigid flat-bottom cylindrical shell tank. Assuming a perfect fluid and velocity potential, a continuity equation is given by the Laplace equation in Cartesian coordinates. The fluid velocities accompanying the motions of the walls and bottom plate constitute the boundary conditions. Since this problem is set as a parabolic partial differential equation of the Neumann problem, the velocity potential is solved with the Fourier-cosine expansion. The derivative of the velocity potential with respect to time gives the fluid pressure at an arbitrary point inside the tank. A mathematical solution for evaluating the fluid pressure accompanying the angular acceleration acting on the pivoting bottom edge of the tank is given by an explicit function of a dimensional variable of the tank, but with the Fourier series. The proposed mathematical solution well converges with a few first terms of the Fourier series. Values of the fluid pressure computed by the explicit finite element (FE) analysis well agrees with those by the proposed mathematical solution. For the designers’ convenience, diagrams that depict the fluid pressures normalized by the maximum tangential acceleration given by the product of the angular acceleration and diagonals of the tank are also presented. Consequently, the mathematical solution given by the Fourier series converges easily and provides accurate evaluation of the fluid pressures on a rigid rectangular tank accompanying the angular acceleration acting on the pivoting bottom edge. Irregularity in the fluid pressure distribution increases as the tank becomes taller.

Recently, increasing traffic in urban areas has led to a dramatic increase in collisions between speeding vehicles and structural columns. An impact applies a greater force to a column than its regular static or dynamic load because of the mass acceleration effect of the vehicle. Vehicle impact can cause catastrophic damage to structural columns and ultimately cause them to collapse; therefore, an in-depth study of their structural resistance to vehicle impact is needed. This paper reports the behavior of a reinforced concrete (RC) compression member or column under a lateral impact load. The study quantitatively assessed the columns' resistance capacity and developed an impact-resistance capacity evaluation procedure. Because it is extremely difficult and costly to experimentally perform a parametric study for column impact scenarios, this analytical study was carried out using LS-DYNA, a commercial explicit finite element (FE) analysis program that simulates the effects of a high strain rate from impact or blast loading on structural and material behavior. The parameters used for this case study were cross-section shape variation, impact load angle, axial load magnitude ratio, concrete compressive strength, longitudinal and lateral reinforcement ratios, and slenderness ratio. Using the analysis results, an impact resistance capacity evaluation procedure using a probabilistic approach is proposed.

<div class="htmlview paragraph">The Hybrid III dummy is an anthropomorphic (humanlike) test device, generally used in crashworthiness testing to assess the extent of occupant protection provided by the vehicle structure and its restraint systems in the event of vehicle crash. Lumped-parameter analytical models are commonly used to simulate the dummy response. These models, by virtue of their limited number of degrees of freedom, can neither represent accurate three-dimensional dummy geometry nor detailed structural deformations. In an effort to improve the state-of-the-art in analytical dummy simulations, a set of finite element models of the Hybrid III dummy segments - head, neck, thorax, spine, pelvis, knee, upper extremities and lower extremities - were developed. The component models replicated the hardware geometry as closely as possible. Appropriate elastic material models were selected for the dummy “skeleton”, with the exterior “soft tissues” represented by viscoelastic materials. The impact response of each individual component was determined by explicit integration finite element analysis.</div> <div class="htmlview paragraph">Subsequently, a whole dummy model was assembled from the various segments with appropriate joint characteristics prescribed to allow for dummy articulation. Finally, the thoracic impact force-deformation response was calculated from a ballistic impactor with a mass of 23.4 kg, launched from 2 initial velocities of 6.71 m/s and 4.3 m/s, respectively. The predicted response was compared with standard dummy and human cadavers experimental data. In all calculations, the model response compared favorably with experimental data, and provided a reasonable level of confidence of the model biofidelity.</div>

Behavior of shallow tunnel in soft soil under seismic conditions

A series of drop tests was implemented in the present study in order to allow the reproduction of a single impact identical to the high frequency mechanical impact (HFMI) under monitored conditions in the laboratory. Therewith, characterization of the investigated material’s mechanical behavior by explicitly considering possible irregularities concerning the present deformation modes would be enabled. Main goal was the determination of the investigated material’s dynamic yield stress for various strain rates inside the spectrum of interest, so that the Cowper–Symonds viscous material model would be calibrated for the subsequent HFMI simulation. The values of the dynamic yield stress extracted by the present drop tests show good agreement with other experimental methods regarding the investigated material S355. The introduction of the calibrated material behavior on the present drop tests in the finite element (FE) analysis of HFMI led to reduced preciseness though, in comparison with the FE analysis, which considered high strain rate tensile tests found in literature. A series of conclusions was drawn from both the experimental and numerical investigations, confirming most of the initial expectations. Further work is proposed, in order to clarify an incompatibility met during the numerical investigations.

We compared the quasi-static and dynamic simulation responses on elastic-plastic deformation of advanced alloys using Finite element (FE) method with an explicit numerical algorithm. A geometrical model consisting of a cylinder-on-flat surface contact under a normal load and sliding motion was examined. Two aeroengine materials, Ti-6Al-4V and Super CMV (Cr-Mo-V) alloy, were employed in the FE analysis. The FE model was validated by comparative magnitudes of the FE-predicted maximum contact pressure variation along the contact half-width length with the theoretical Hertzian contact solution. Results show that the (compressive) displacement of the initial contact surface steadily increases for the quasi-static load case, but accumulates at an increasing rate to the maximum level for the dynamic loading. However, the relatively higher stiffness and yield strength of the Super CMV alloy resulted in limited deformation and low plastic strain when compared to the Ti-6Al-4V alloy. The accumulated equivalent plastic strain of the material point at the initial contact position was nearly a thousand times higher for the dynamic load case (for example, 6.592 for Ti-6Al-4V, 1.0 kN) when compared to the quasi-static loading (only 0.0072). During the loading step, the von Mises stress increased with a decreasing and increasing rate for the quasi-static and dynamic load case, respectively. A sudden increase in the stress magnitude to the respective peak value was registered due to the additional constraint to overcome the static friction of the mating surfaces during the sliding step.