Different Levels Of Pre-stress Research Articles

Flexural Behavior of Damaged Hollow RC Box Girders Repaired with Prestressed CFRP.

In recent years, numerous studies have explored the benefits of utilizing prestressed carbon fiber-reinforced polymer (CFRP) for strengthening concrete structures. However, research on the reinforcement of prestressed CFRP on full-scale hollow RC box girders, particularly damaged bridges, remains limited. In this study, both experiments and finite element analysis (FEA) were performed to investigate the flexural behavior of full-scale hollow RC box girders with varying degrees of damage, which were strengthened using CFRP with different levels of prestress. The adhesive behavior of the CFRP-concrete interface was considered in the FEA. Numerical simulations were conducted to assess the flexural behaviors of the girders, including failure modes, yield and ultimate loads, and deflections. The results revealed that the application of prestressed CFRP efficiently increased the yield and ultimate loads of the box girders. Specifically, when the degree of damage of the hollow box girder was less than 23%, the flexural bearing capacity of the repaired girder could be recovered after being strengthened with two prestressed CFRP strips measuring 50 mm in width and 3 mm in thickness. However, the risk of premature debonding at the CFRP-concrete interface increased when the prestressing level of CFRP and degree of damage of hollow RC box girders exceeded 35% and 40%, respectively. These findings suggest that the use of prestressed CFRP may be a promising method for repairing damaged hollow RC box girders, but careful consideration of the degree of damage and prestressing level would be necessary to ensure the effectiveness and safety of the repair.

Stress-laminated timber decks in bridges: Friction between lamellas, butt joints and pre-stressing system

Stress-laminated timber (SLT) decks in bridges are popular structural systems in bridge engineering. SLT decks are made from parallel timber beams placed side by side and pre-stressed together by means of steel rods. SLT decks can be in any length by just using displaced butt joints. The paper presents results from friction experiments performed in both grain and transverse direction with different levels of pre-stress. Numerical simulations of these experiments in addition to comparisons to full-scale experiments of SLT decks presented in literature verified the numerical model approach. Furthermore, several alternative SLT deck configurations with different amounts of butt joints and pre-stressing rod locations were modelled to study their influence on the structural properties of SLT decks. Finally, some recommendations on design of SLT bridge decks are given.

Experimental testing of pre-stressed granular assemblies as a surrogate material for the dynamic analysis of launcher cryogenic tanks

This paper focuses on the experimental validation of a new methodology for substitution of Liquid Hydrogen (LH2) on a simplified model of launcher cryogenic tank in vibration analysis by using pre-stressed grains as surrogate materials. The objective of the experiments presented here is to analyze the modal behavior of the (Tank and Granular materials) system during pre-stress procedure. In these experimental tests, a cylindrical tank is fully filled with granular materials and vibrated under swept sine and random excitations. Then, the natural frequency of each vibration mode is measured at different levels of pre-stress. The results obtained from the tests show the evolution of each flexural mode as a function of applied pre- stress. Nonlinear behavior of the analyzed system during vibration was also observed. These experimental values carried out with Tank-Granular material system (TGM) are compared with numerical and analytical results to prove the potential of the new methodology. Finally, based on obtained experimental results, some solutions are proposed in order to improve the new methodology for vibration analysis of cryogenic tank which is an important thematic in aerospace engineering.

Repeated thermal shock behavior of ZrB2-SiC-graphite composite under prestress

The thermal shock resistance of ZrB2-SiC-graphite composite under nominal prestress of 0, 20, 30, 40 or 50MPa after subjected to 10 and 30 cycles of thermal shock was evaluated by measuring the residual flexural strength of the tested specimen. In each test the applied prestress kept constant and in each cycle the specimen center was heated to 2000°C within 5s through electrical resistance heating method and cooled down naturally to room temperature. A lot of broken SiO2 bubbles in the tested specimens were observed with a SEM. For the specimen subjected to 10 cycles of thermal shock, the residual flexural strength does not show big change under different levels of prestress, although the thickness of oxide layer increases at larger prestress, which is presumably attributed to the effect of the oxide layer that heals the cracks and the pores and enhances the strength. For the specimen subjected to 30 cycles of thermal shock, the residual strength decreases, in general, with the increase of prestress level. The thermal shock fatigue under different levels of prestress was also tested, and it was found that the increase of prestress may speed the failure of the specimen, indicating that the level of prestress may fatally affect the failure of the material.

Influence of confining prestress on the transition from interface defeat to penetration in ceramic targets

Abstract Replica scaled impact experiments with unconfined ceramic targets have shown that the transition velocity, i.e., the impact velocity at which interface defeat ceases and ceramic penetration occurs, decreased as the length scale increased. A possible explanation of how this scale effect is related to the formation of a cone crack in the ceramic has been presented by the authors in an earlier paper. Here, the influence of confinement and prestress on cone cracking and transition velocity is investigated. The hypothesis is that prestress will suppress the formation and growth of the cone crack by lowering the driving stress. A set of impact experiments has been performed in which the transition velocity for four different levels of prestress has been determined. The transition velocities as a function of the level of confining prestress is compared to an analytical model for the influence of prestress on the formation and extension of the cone crack in the ceramic material. Both experiments and model indicate that prestress has a strong influence on the transition from interface defeat to penetration, although the model underestimates the influence of prestress.

Active confinement of weakened concrete columns

This paper investigates the advantages of using of external active confinement to improve the compression load capacity of concrete cylinders using mild steel band clamps. For this, passive and actively pre-stressed steel bands have been attached to concrete cylinders to mimic real structural columns which may otherwise have to be removed and replaced due to poor strength performance. The experimental programme included compression load testing a number of unreinforced concrete cylindrical specimens to establish the effect the confinement had on the compressive load-carrying capacity and the stress–strain behaviour. A number of parameters were investigated in the study including the effect of varying the band spacing, volumetric ratio and passive and active confining forces. It was found that confining the concrete did produce an increase in compressive strength. However, the different levels of pre-stress used had no significant effect on strength but did influence the confined concrete stress–strain behaviour with greater lateral stresses at peak strength observed. Ductility increases were observed for confined specimens and significant axial strains achieved with noticeable peak strength enhancements. However, due to initial misfit between the bands and the concrete in the passive confinement state, confining forces generated only contained the cracked specimens. The actively pre-stressing confinement yielded an increase in load capacity of 53 % as the bands were fully activated.

Parametric reduced-order models of battery pack vibration including structural variation and prestress effects

The goal of this work is to develop a numerical model for the vibration of hybrid electric vehicle (HEV) battery packs to enable probabilistic forced response simulations for the effects of variations. There are two important types of variations that affect their structural response significantly: the prestress that is applied when joining the cells within a pack; and the small, random structural property discrepancies among the cells of a battery pack. The main contributions of this work are summarized as follows. In order to account for these two important variations, a new parametric reduced order model (PROM) formulation is derived by employing three key observations: (1) the stiffness matrix can be parameterized for different levels of prestress, (2) the mode shapes of a battery pack with cell-to-cell variation can be represented as a linear combination of the mode shapes of the nominal system, and (3) the frame holding each cell has vibratory motion. A numerical example of an academic battery pack with pouch cells is presented to demonstrate that the PROM captures the effects of both prestress and structural variation on battery packs. The PROM is validated numerically by comparing full-order finite element models (FEMs) of the same systems.

The SMC (Short Model Coil) ${\rm Nb}_{3}{\rm Sn}$ Program: FE Analysis With 3D Modeling

The SMC (Short Model Coil) project aims at testing superconducting coils in racetrack configuration, wound with cable. The degradation of the magnetic properties of the cable is studied by applying different levels of pre-stress. It is an essential step in the validation of procedures for the construction of superconducting magnets with high performance conductor. Two SMC assemblies have been completed and cold tested in the frame of a European collaboration between CEA (FR), CERN and STFC (UK), with the technical support from LBNL (US). The second assembly showed remarkable good quench results, reaching a peak field of 12.5 T. This paper details the new 3D modeling method of the SMC, implemented using the ANSYS Workbench environment. Advanced computer-aided-design (CAD) tools are combined with multi-physics Finite Element Analyses (FEA), in the same integrated graphic interface, forming a fully parametric model that enables simulation driven development of the SMC project. The magnetic and structural model is described and the results are compared with the experimental data from the strain gauges, which monitor the mechanical strain of the structure.

Elastic wave simulation in ground anchors for the estimation of pre-stress

Application of pre-stress on strand wires inside a structure is a way to reinforce the low tensile strength of concrete and soil materials. After years in service, the strands may lose the pre-stress due to corrosion or other reasons endangering the whole structure. Since there is no accessibility to the strands inside the structure, any inspection should be conducted on the anchor head. In this study, elastic waves on the anchor head are numerically simulated. Parameters like the wave amplitude and propagation time are influenced by the stress of the strand wires which affects the contact between the threaded steel rod, and the surrounding anchor head influencing the acoustic impedance mis-match on the interfaces. The change of contact pressure between the strands and anchor head is simulated by modifying the rigidity of a model interphase material. Results are compared with experimental under different levels of pre-stress. Therefore, an easy and fast procedure for non-destructive inspection of the pre-stress load on the strands is discussed.

Experimental Research on Flexural Behavior of Load-Bearing Characteristics on Reinfoeced Concrete Beams Strengthened with near Surface Mounted Prestressed CFRP Tendons

In order to improve flexural behavior of concrete beams, the 7mm-diameter carbon fiber reinforced plastic (CFRP) tendons applied different levels of prestress were put in to 20×20mm grooves opened in the concrete surface of the tensile region in the concrete beams.Grooves were filled with building structural adhesive, when it solidified bending test of strengthened beams would carry out. Flexural behavior of nine RC beams strengthened with prestressed near surface mounted CFRP tendons applied different levels of prestress and one unstrengthened RC beam were studied. In these tests, bending stiffness and load-bearing characteristics of test specimens were studied. Experimental results show that the high-strength performance of CFRP have been made full use; the stiffness of strengthened beams risen notably, the maximal crack load increased by 303.17%, the maximal ultimate load increased by 237.92%.

Magnetic Design and Code Benchmarking of the SMC (Short Model Coil) Dipole Magnet

The Short Model Coil (SMC) working group was set in February 2007 within the Next European Dipole (NED) program, in order to develop a short-scale model of a dipole magnet. The SMC group comprises four laboratories: CERN/TE-MSC group (CH), CEA/IRFU (FR), RAL (UK) and LBNL (US). The SMC magnet is designed to reach a peak field of about 13 Tesla (T) on conductor, using a 2500 Powder-In-Tube (PIT) strand. The aim of this magnet device is to study the degradation of the magnetic properties of the cable, by applying different levels of pre-stress. To fully satisfy this purpose, a versatile and easy-to-assemble structure has been realized. The design of the SMC magnet has been developed from an existing dipole magnet, the SD01, designed, built and tested at LBNL with support from CEA. The goal of the magnetic design presented in this paper is to match the high field region with the high stress region, located along the dipole straight section. For this purpose, three-dimensional nonlinear parametric models have been implemented using three codes (CAST3M, ANSYS, and OPERA). This optimization process has been an opportunity to cross-check the codes. The results of this benchmarking are presented here, along with the final design which incorporates the use of end spacers and a surrounding iron structure to deliver a nominal field of 13 T uniformly distributed along the cable straight section.

New test method to evaluate the impact behaviour of biaxially confined concrete

A new experimental technique for imposing uniaxial or biaxial confinement on specimens subjected to dynamic load has been developed. A drop weight impact machine was used to “split” the confined specimens. Different levels of pre-stress were chosen to evaluate the change in the impact behaviour due to the confinement. The results showed that the variations in the impact energy and in the maximum impact load due to the confinement were small. To quantify the damage of the impacted specimens, a splitting tension test was performed on the already impacted specimens under quasi-static loading conditions. The values of the residual splitting tensile strength showed that the damage depends on the level of confinement. The use of such a testing method may lead to a better understanding of the impact behaviour of cementitious materials.