Pre-stressed Composite Research Articles

Parametric investigations on shear behavior of perforated transverse angle connectors in steel–concrete composite bridges

The perforated transverse angle connectors (PTACs), that posses excellent shear capacity and good ductility, have been proposed to address the casting obstruction of the bottom concrete decks in prestressed composite (PC) box-girder with corrugated steel webs (CSPWs). The structural performance of steel–concrete composite bridges largely depends on the behavior of the mechanical shear connectors. In this study, numerical analyses on the shear behavior of the PTAC were performed. The finite element (FE) model was developed using ABAQUS and validated by comparing its predictions with test data available in the literature. The comparisons indicated that the numerical model could reflect the experimental results closely. Parametric studies were then carried out to determine the influential factors on the PTAC shear capacity. It was found that the shear strength of the PTAC increased linearly with the increase of the concrete strength. The flange and web thickness had great effect on the PTAC shear capacity, while the flange and web height slightly influenced. The group PTAC arrangement had a significant influence on the shear strength and this effect can be ignored when the spacing was more than 400 mm. In addition, the perforating rebar contributed little to the shear capacity but obviously enhanced the ductility. The FE analysis results were used to verify the applicability of the current design provisions for the PTAC shear strength. It was noted that the formulas in these provisions conservatively predict the shear strength due to the effect of the flange welded on the steel girder was ignored. An empirical formula was developed to evaluate the shear strength of angle shear connectors. The mean of the ratio of the predicted results to test ones ranged from 0.981 to 1.018, which denoted the proposed design formula could accurately be used to predict the PTAC shear capacity.

Shape memory alloy–actuated prestressed composites with application to morphing automotive fender skirts

This work presents smart laminated composites that enable morphing vehicle structures. Morphing panels can be effective for drag reduction, for example, adaptive fender skirts. Mechanical prestress provides tailored curvature in composites without the drawbacks of thermally induced residual stress. When driven by smart materials such as shape memory alloys, mechanically-prestressed composites can serve as building blocks for morphing structures. An analytical energy-based model is presented to calculate the curved shape of a composite as a function of force applied by an embedded actuator. Shape transition is modeled by providing the actuation force as an input to a one-dimensional thermomechanical constitutive model of a shape memory alloy wire. A design procedure, based on the analytical model, is presented for morphing fender skirts comprising radially configured smart composite elements. A half-scale fender skirt for a compact passenger car is designed, fabricated, and tested. The demonstrator has a domed unactuated shape and morphs to a flat shape when actuated using shape memory alloys. Rapid actuation is demonstrated by coupling shape memory alloys with integrated quick-release latches; the latches reduce actuation time by 95%. The demonstrator is 62% lighter than an equivalent dome-shaped steel fender skirt.

Monitoring Pre-Stressed Composites Using Optical Fibre Sensors.

Residual stresses in fibre reinforced composites can give rise to a number of undesired effects such as loss of dimensional stability and premature fracture. Hence, there is significant merit in developing processing techniques to mitigate the development of residual stresses. However, tracking and quantifying the development of these fabrication-induced stresses in real-time using conventional non-destructive techniques is not straightforward. This article reports on the design and evaluation of a technique for manufacturing pre-stressed composite panels from unidirectional E-glass/epoxy prepregs. Here, the magnitude of the applied pre-stress was monitored using an integrated load-cell. The pre-stressing rig was based on a flat-bed design which enabled autoclave-based processing. A method was developed to end-tab the laminated prepregs prior to pre-stressing. The development of process-induced residual strain was monitored in-situ using embedded optical fibre sensors. Surface-mounted electrical resistance strain gauges were used to measure the strain when the composite was unloaded from the pre-stressing rig at room temperature. Four pre-stress levels were applied prior to processing the laminated preforms in an autoclave. The results showed that the application of a pre-stress of 108 MPa to a unidirectional [0]16 E-glass/913 epoxy preform, reduced the residual strain in the composite from −600 µε (conventional processing without pre-stress) to approximately zero. A good correlation was observed between the data obtained from the surface-mounted electrical resistance strain gauge and the embedded optical fibre sensors. In addition to “neutralising” the residual stresses, superior axial orientation of the reinforcement can be obtained from pre-stressed composites. A subsequent publication will highlight the consequences of pres-stressing on fibre alignment, the tensile, flexural, compressive and fatigue performance of unidirectional E-glass composites.

Memorizing maximum strain in carbon-fiber-reinforced plastic composites by measuring electrical resistance under pre-tensile stress

Electrical resistance change of fiber-reinforced plastics including continuous carbon fibers has been characterized in tensile tests, in order to provide the composite with the ability to memorize maximum strain. The composites were designed to exhibit obvious residual resistance resulting from fiber fracture, by which maximum strain was memorized after loading–unloading cycles. The composite with a lower volume fraction of carbon fibers was found to be suitable for showing larger resistance change in wider strain range. In order to maintain the increased resistance even after unloading, it was proposed to utilize the composites under pre-stressed condition. The cyclic tensile tests revealed that the pre-stressed composites indicated good sensitivity concerning lower detectable strain limit of less than 0.05% and remarkable residual resistance which brought about memorizing accuracy within ±1%. The applicability of the memorizing capability was demonstrated in a bending test for a concrete beam including the pre-stressed composite.

Fibre Prestressed Composites: Improvement of Flexural Properties through Fibre Prestressing

The effects of fibre prestressing have been investigated on the flexural properties of glass-epoxy composites. Fibre prestressed composites were made by applying and maintaining a known amount of tension on the fibres during the curing process of the epoxy resin. In the next step, bending tests were conducted on a tensile machine using four point bending. In this study the modulus and strength of the fibre prestressed composite increased up to 33%. The experimental data also indicated that there existed a fibre prestressing level at which the flexural properties reached their maximum values. This prestressing level was found to be a function of the curing temperature. A mechanism has been proposed to explain the improvement of the flexural properties by the use of fibre prestressing during the cure process.

Impact Strength of Fiber Pre-Stressed Composites

By applying the appropriate tension to the fibers during the curing process, the impact strength of glass-epoxy composites is improved up to 33%. Charpy tests show that the increase of the fiber prestressing increases the impact strength up to a particular level. Beyond this level, however, the increase of prestressing reduces the impact strength. The fiber prestressed samples indicate splitting breakage within the polymer that creates more new surfaces as compared with unprestressed composites. This results in the sample absorbing more energy during the impact. The splitting breakage is explained by the formation of the residual stresses in the polymeric matrix. Those stresses are controlled and promoted by fiber prestressing during the curing process.