Prestressed composite structures – Modeling, manufacturing, design
Introduction
The concept of increasing strength capacity of structural elements by introducing preliminary stresses counteracting the exploitation stresses is known for years. A large number of applications of prestressed materials in civil engineering proves that proper compression can effectively increase the strength of structural elements.
In the context of fiber-reinforced composites the term prestressing indicates applying initial tensile stress to the fibers embedded in selected layers of the composite. By the similarity to reinforced concrete structures the initial prestressing of the fibers and its further release after curing of the resin produces a clamping load which causes compressive stresses in the surrounding matrix. The entire procedure generates initial self-equilibrated state of stress of the composite, which substantially influences its mechanical properties and modifies its response to external loading. However, in contrast to a standard reinforced-concrete element a typical FRP composite has more complex construction since it is composed of many layers of various mechanical properties and lamination angles. Thus, the applied external loading may cause very complex mechanical behavior including coupling between tension, bending and twisting. It is a matter of optimal design of prestressed composites to select the plies which should be prestressed and to adjust the value of prestressing force such that global structural response is optimally mitigated.
Because of the rapid development of composite materials and growing demand for light materials with particularly high stiffness and strength properties the application of prestressed FRP composites in industry seems to be a question of time. This assumption is confirmed by increasing number of publications concerning the problem of mechanical characteristics of such materials. The positive influence of the preliminary tension on strength of the specimen made from dental composite containing tensioned glass fiber was described by Schlichting [10]. Moreover, the analysis of bending strength of prestressed composite presented by Matahhari et al. [6], [7] revealed up to 33% increase of transverse load-carrying capacity of the examined specimen. In number of publications [11], [9], [3] the authors pointed out improvement of mechanical properties and increase of stiffness of prestressed composites.
Apart from above described major objective, prestressing of fiber-reinforced composites has several important additional advantages. At first, compressive stresses generated in the matrix reduce both cure-induced and thermally-induced residual tensile stresses resulting from the cure cycle. Consequently, creation of compressive stresses prevents or, at least, impedes formation and propagation of crack in the matrix [8]. As it was suggested by Dvorak et al. [2], due to the fact that fibers are maintained prestressed during the entire curing process the misalignment of fibers, also called fiber waviness, is substantially reduced. It was also proved that prestressed composites are characterized by improvement of fatigue life and resistance to stiffness degradation in the matrix-dominated fatigue region [4].
Damage mechanisms in uni-directionally reinforced composites are extensively described by Krishnamurthy [5], where the composites are tested under quasi-tension, compression and fatigue. Basic mechanical properties of prestressed composites as resistance to tensile, flexural loading and impact, their advantages over classical composites as well as potential applications are presented in publications [1], [2], [4], [5], [8].
Section snippets
Analytical model of prestressed composite
The problem of the modeling of uni-directionally reinforced prestressed composites will be considered in micro- and macro-scale with the use of analytical and numerical approaches. The analysis will be limited to linear models based on assumptions of small deformation and elastic constitutive relations. An elementary structural part considered in this study will be inhomogeneous layer constructed of matrix and reinforcing fibers (Fig. 1a). Such element will be initially analyzed by
Experimental evaluation of the prestressing concept
For the purpose of experimental evaluation of the concept of prestressing a dedicated laboratory stand (Fig. 2) was created. The main part of the stand was plane table composed of two blocks: one was fixed to a base and the other was movable. The load screw was mounted in the moving block in such a way that the screw end was blocked in the fixed part of the stand and movable part was shifted during rotation of the screw. For alignment of the moving and fixed blocks two guide bars were used. The
Numerical model of prestressed composite
Numerical, FEM-based modeling of prestressed composites is required when applied boundary conditions or external loading cause inhomogeneous deformation of the middle surface of the composite and, consequently, inhomogeneous state of deformation and stress of the entire structure. In such case the equation of equilibrium is no longer trivial and it requires numerical solution by finite element method.
In general, two approaches for the modeling of prestressed composite structures can be
Prestressed composite design
Presented above analytical and numerical models can be effectively used in design and optimization of prestressed composites. In the process of composite design the initial prestress can be used for various purposes including: i) reduction of deformation or internal forces caused by applied external loading and ii) decrease of required layers’ thicknesses or fiber volume fraction of the composite. Since the problem of reducing generated strains and stresses was considered in the former paper of
Conclusions
The concept of prestressed composites is based on introducing appropriately adjusted tensile forces into selected layers of the reinforcement. Prestressing has a strong potential for improving mechanical properties of the composite structure including increase of strength, increase of stiffness and reduction of mass. The preliminary analysis of prestressed composites can be performed with the use of proposed analytical model, which gives results consistent with numerical simulations and
Acknowledgements
The authors would like to gratefully acknowledge the financial support through the Grant No. 2012/07/D/ST8/02661 (C.G. and A.O.) and through the Grant No. 2012/05/B/ST8/02971 (J.H.-S.), both financed by the National Science Centre – Poland.
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Fibre prestressed composites: Theoretical and numerical modelling of unidirectional and plain-weave fibre reinforcement forms
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