Effect Of Load Intensity Research Articles

Features of bond-slip relations: 3D finite element analysis based on tests of short RC ties

The interaction between concrete and reinforcement is highly complex and is of fundamental importance for predicting the behaviour of reinforced concrete (RC) structures. While the interaction of the two materials is often characterised by a bond – slip law, there are currently no such laws capable of accurately predicting serviceability characteristics, such as deflection, crack spacing, and width. This paper examines the effect of various parameters on the shape of bond – slip relation at loads before yielding of reinforcement. The study is based on a mesoscopic numerical analysis, utilizing the finite element method along with a friction cohesive zone model to explore the bond-slip phenomena in short RC ties. The numerical model is combined with an experimental campaign involving six short concrete ties reinforced with 20 mm bars. Steel strain profiles were monitored within the reinforcement using electrical strain gauges. These experimental steel strain profiles served as a robust tool for controlling and calibrating numerical models. The numerical models employed solid finite elements to simulate both reinforcement and concrete, with special zero-thickness plain elements applied to simulate the friction cohesive contact at the steel-concrete interface. Bond-slip relations were obtained from the numerically modelled reinforcement strain profiles. It was shown that concrete strains can be ignored when assessing slip. The effect of the embedded length of reinforcement under the fully confined conditions was demonstrated to have little effect on bond-slip relations. It was also shown that the maximum bond stress is strongly related to the thickness of the concrete cover. The effect of load intensity on the bond – slip behaviour was investigated. It was shown that under full confinement, the initial bond stiffness was not affected by load intensity. However, degradation of bond stiffness with increasing load due to internal cracking took place in the RC ties with a small cover/diameter ratio.

Performance of utility tunnels under gas explosion loads

Gas explosion accidents could lead to great losses of lives and property. Reliable prediction of structural responses under gas explosion loads is essential for developing effective technologies to mitigate the potential hazards of gas explosions. In the present study, numerical studies are conducted to investigate the performance of utility tunnels under gas explosion loads. Numerical models were developed by using LS-DYNA and the accuracy of numerical models in predicting the responses of reinforced concrete (RC) structures subjected to gas explosions was verified by comparing with the testing data. Based on the verified numerical method, the numerical models of utility tunnels under gas explosion loads were established and the effects of load intensity, concrete strength, reinforcement strength and section type on the performance of utility tunnels were analyzed and discussed. It is found that the gas explosion loads with high overpressure peak value can bring about severe damage of tunnels and the failure of inner diaphragms. The influence of concrete strength is limited. Improving reinforcement strength and the confinement of slab-slab joints of the gas compartment can enhance the performance of utility tunnels under gas explosion loads effectively.

Dynamic response and failure of carbon/epoxy composite sandwich subjected to underwater impulsive loading

In this work, the dynamic response and failure modes of carbon/epoxy sandwich structures with PVC foam cores subjected to underwater impulsive loading are experimentally investigated. The 3D digital image correlation technique with high speed photography is used to capture the dynamic response of the structures. The effects of loading intensity, core density, fluid-structure interaction and ply orientation of face sheets on the deformation and damage of the clamped sandwich structures are assessed. The results showed that the compression of PVC foam can effectively reduce the deformation and damage of the sandwich structures. The maximum deflection of rear face sheets increases with PVC core density increasing. The total deflection of sandwich plates including front sheets, cores and rear sheets increases with the decreasing of core density and fluid-structure interaction parameters. Additionally, the influence of ply orientation of carbon/epoxy face sheets on the blast resistance of the sandwich structures can be ignored in the range of normalized impulse intensity (0.08-0.29) used in the tests. With the loading intensity increasing, the failure modes of the sandwich plates present permanent deflection, fiber breakage at boundary and matrix crack of carbon/epoxy at front face-sheets, core compression and debonding between cores and face sheets including front and rear sheets. The results obtained from those tests can provide valuable reference to design the lightweight protective structures.

Numerical investigations on blast resistance of sandwich panels with multilayered graded hourglass lattice cores

This research focuses on the dynamic response of sandwich panels with multilayered graded hourglass lattice core subjected to blast loading. A three-layer lattice core configuration is proposed to improve the absorption efficient of kinetic energy resulted from blast shock wave. The relative density of each core layer is changed with sectional dimension of core truss members to regulate the energy absorption of each core layer. Three-dimensional numerical simulation analyses of dynamic response are carried out, and the applied impulsive pressure distribution on the surface of the panels is calculated using the CONWEP code. The panels are made of stainless steel AL6XN, which is assumed to follow bilinear strain hardening and strain rate-dependence. Peak back sheet deflection and energy absorption of core layers for four types of hourglass lattice panels are comparatively analyzed and the effects of load intensity on the peak deflection are discussed. Furthermore, the near-optimal configuration under blast loadings is proposed.

Molecular dynamics studies on energy dissipation and void collapse in graded nanoporous nickel under shock compression

We present systematic investigations on energy dissipation and void collapse in graded nanoporous nickel by non-equilibrium molecular dynamics simulations. It is found that void size gradient influences the time history path of the energy dissipation. Under strong shock loading conditions, when the voids are completely collapsed, the total energy dissipation is dependent only on the porosity, and is independent of the void size gradient in the shock direction. The total energy dissipation increases with the increase of porosity. However, when the porosity increases to a critical value of 6%, the total energy dissipation reaches an upper limit. Increasing the porosity beyond this critical value would not result in further increase in energy dissipation. The simulations show that voids collapse is attributed to the combined effect of transverse and longitudinal plasticity flow of void wall. Two mechanisms of voids collapse are revealed: the plasticity mechanism and internal jetting mechanism. Under relatively weaker shocks, the plasticity mechanism, which leads to transverse collapse of voids, prevails; while at the stronger shock strengths, the internal jetting mechanism, which leads to longitudinal flow, plays a more significant role. The earliest appearing dislocations in a void may either nucleate at the front half surface or on the back half surface, depending on the position of the void in the sample and the void size gradient. Moreover, the simulations provide quantitative descriptions about the effects of loading intensity on energy dissipation rate and void collapse rate. We show that the energy dissipation rate can be well represented by a quadratic polynomial function of the shock loading velocity, and the void collapse rate is a linear function of the shock loading velocity.

The effect of porosity on fragmentation statistics of dynamically loaded ZrO2 ceramics

The effect of load intensity and porous material structure on the fragmentation statistics of ZrO2(MgO)-based ceramics is studied. Cylindrical samples were fragmented under dynamic compression. Experimental data processing showed that the shape of stress-strain curves, the fragment size distribution and distribution of time intervals between the fractoluminiscense impulses depend on the sample porosity and load intensity. The X-ray Computed Tomography (CT) study of porous material structures allowed us to link the fragmentation statistics with pronounced porosity clustering (about 97% of the total pore volume) formed due to sintering.

Novel experimental and 3D multiphysics computational framework for analyzing deformation and failure of composite laminates subjected to water blasts

The load-carrying capacity of composite structures under water-based impulsive loads is studied in relation to different materials and loading conditions. The analysis focuses on the role of fiber orientation, fiber stiffness and angle of structure obliquity relative to load direction on the deformation and failure in monolithic carbon-fiber and glass-fiber/epoxy composite plates of similar mass and thickness. Structures are subjected to impulsive loads of different intensities generated using the Underwater Shock Loading Simulator (USLS), a novel projectile-impact-based impulsive loading facility. In-situ high-speed digital imaging is used to study the deformation and failure, focusing on the effects of load intensity, failure modes and material heterogeneity. The experiments are combined with fully dynamic 3D Coupled Eulerian Lagrangian (CEL) finite element simulations accounting for the effects of fluid-structure interactions (FSI) and in-ply and inter-ply cracking and failure. It is found that the carbon-fiber laminates provide higher blast resistance, but transmit a greater fraction of the incident impulse to the supports than the glass-fiber laminates. Damage through in-ply and inter-ply cracking in the carbon-fiber laminates is ∼25% of that in the glass-fiber laminates. Higher angles of load obliquity trigger localized deformation at multiple locations, leading to more extensive in-ply damage and progressively shear-dominated rupture.

Scaling laws of structure and fragmentation parameters of ZrO2 ceramics.

The effects of loading intensity and material structure on statistical regularities of fragmentation were studied. The material under investigation is zirconia-based ceramics. The samples with different pore structure (10%, 20%, 30%, 45%, 60% - the porosity of the powder used for ceramic sample sintering) were tested. The experimental data on dynamical fragmentation of ZrO2 ceramics allowed us to construct the fragment size distribution (spatial scaling) and the distribution of time interval between fractoluminiscense pulses (time scaling). The power law exponent of spatial scaling depends on ceramics porosity and intensity of loading, which has a considerable influence on the power law exponent for samples with highest porosity (60%). Two types of the distribution function for time interval were observed. For ceramics of low porosity (less than 60%) the distribution is fitted by two power laws with different exponents, and the distribution of time intervals for ceramics of highest porosity (60%) obeys one power law. An increase in porosity leads to the growth of the number of single fracture events (the number of middle and short intervals), as a consequence of missing the second power law exponent. Statistics of pore systems in two samples with 20% and 60% porosity was studied using X-ray Computed Tomography (CT). Analyzing more than a thousand of cross section images for every sample, we get a cumulative pore area distribution, and a relationship between area and perimeter. The best fitting for pore area distribution are: 1) two power laws for sample with highest porosity (60%); 2) double exponential law for low porosity (20%). The area – perimeter relation satisfies a power law.

Seismic Behavior of Soil-Pile-Structure Interaction in Liquefiable Soils: Parametric Study

Nonlinearity of the soil medium plays a very important role on the seismic behavior of soil-pile-structure interaction. The problem of soil-pile-structure interaction is further complicated when the piles are embedded in liquefiable soil medium. A finite-element code was developed in MATLAB to model three-dimensional soil-pile-structure systems. Frequency dependent Kelvin elements (spring and dashpots) were used to model the radiation boundary conditions. A work-hardening plastic cap model was used for constitutive modeling of the soil medium. The pore pressure generation for liquefaction was incorporated by a two-parameter volume change model reported in the literature. In this paper, a 2×2 pile group in liquefiable soil is considered and a parametric study is conducted to investigate its seismic behavior. The effects of loading intensity and stiffness of the soil on the seismic behaviour of the soil-pile system are investigated, considering nonlinearity and liquefaction of the soil medium for a wide ran...

Simulation of Void Collapse in an Energetic Material, Part 2: Reactive Case

In Part 1 a numerical methodology was used to study collapse of voids in energetic materials by ignoring the contributions of the gas phase and chemical reaction. In this paper, the thermomechanical response of solid phase is combined with the vapor-phase compression and chemical reactions to study their effects on the energy deposition mechanisms. The formulation and numerical treatment of the coupled solid-gas interactions, including heat release caused by chemical reactions, are discussed. The effects of loading intensity and void size are studied. The results show that for the micron-size voids under consideration significant gas-phase chemical reactions occur. However, their influence on the void collapse itself is minimal. Results indicate that when voids completely collapse a significant amount of reaction products are created, but the collapse time is too short to set up thermal runaway at the hot spot before void collapse.