Pre-stressed Fiber Research Articles

The Effect of Curing Conditions on the Properties of Silica Modified Glass Fiber Reinforced Epoxy Composite

The effects of curing conditions on the mechanical properties of glass fiber reinforced epoxy composites under different fiber prestressing levels were quantitatively studied. The composite samples were prepared with silica particle modified continuous E-glass fibers, epoxy resin matrix, fiber prestressing and step curing processing. Room temperature cured and isothermal high temperature cured composite samples, with the same content and structure, were made to study the difference of three curing conditions on a quantitative level. Based on the test results of flexural, shear, and impact properties of the composite samples, it was found that the step curing procedure generates composite samples with up to 47% and 14% increased properties than room temperature cured and isothermal high temperature cured samples, respectively, when manufactured under the same fiber prestressing level. The possibility of generating an optimum residual stress within the samples during processing is proposed and discussed to explain the contribution of the step curing process to the composite properties.

2920 フラグメンテーション試験を用いた繊維強度評価(S27-1 複合材料の動向とその加工技術(1),S27 複合材料の動向とその加工技術)

An exact theory is developed to evaluate propagation of an interfacial debonding in single fibre composite by means of the energy release rate due to crack growth. The debonding propagation during fragmentation test is significantly affected by frictional slip at the interface in the debonding region. Hence, frictional dissipation of the energy release rate occurs along with the debonding propagation. An analysis considering the effect of the interfacial friction is conducted. The fragmentation tests are performed varying the amount of fibre pre-stress, and the debonding growth is observed directly. To validate the present analysis, the debonding propagation predicted by the present theory is compared with the experimental one. The theoretical prediction is proved to be in the excellent agreement with the experimental results, which illustrates the importance of including the frictional effect in the energy release rate. An application of the present theory to fibre fragmentation in single-fibre composite test that can evaluate the fibre strength is described.

Stress Relaxation in Prestressed Composite Laminates

Viscoelastic deformation caused in symmetric laminated plates by release of fiber prestress and by uniform thermomechanical loads is analyzed on the constituent, ply and overall laminate scales with the Transformation Field Analysis (TFA) method (G. J. Dvorak, Proc. R. Soc. Lond., 1992, A437, pp. 311–327). Fiber prestress is applied in individual plies prior to matrix cure and released after matrix consolidation. Linear or nonlinear viscoelastic constitutive relations are used to evaluate the inelastic deformation rates in terms of current constituent stress averages. The TFA method regards both thermal and inelastic strains as piecewise uniform eigenstrains acting in superposition with mechanical loads and fiber prestress release on an elastic laminate. Interactions between the eigenstrains at the three different size scales are described by certain influence functions derived from micromechanical analysis of the plies and laminates. Applications describe stress relaxation in two carbon/epoxy laminates after cooling from the curing temperature and release of optimized fiber prestress, that allows maximum tensile load application while keeping both interior and free-edge stresses within prescribed strength limits. Subsequent viscoelastic deformation under constant rate loading, and stress relaxation caused by a sustained application of an elevated temperature to a laminate without prestress are also analyzed. Results are presented in the form of initial failure maps that identify overall stress states which may or may not initiate a specific damage mode in the laminate.

Optimized fiber prestress for reduction of free edge stresses in composite laminates

An analytical procedure is described for evaluation of the effect of release of fiber prestress, applied prior to matrix consolidation, on stress distribution in individual plies and at free edges of laminated composite plates. Both thermal changes, piecewise uniform transformation strains in the plies and overall mechanical loads can be considered in the analysis. The thermal and transformation load contributions are decomposed into superpositions of certain uniform fields with mechanical loads. Release of fiber prestress is regarded as an equivalent uniaxial compression applied at the edges of each prestressed ply. Optimized distributions of fiber prestress are found in individual plies such that stresses in both laminate interior and at the free edges remain within allowable limits, while the applied mechanical load may change from zero to a certain maximum value. Specific results are found for cross-ply and quasi-isotropic symmetric S-glass/epoxy laminates under tension. They clearly demonstrate the substantial potential of fiber prestress in damage control and prevention in laminated composite structures.

ESTIMATING SHEAR STRENGTH OF RC AND PRC MEMBER WITH FIBER SHEETS

連続繊維シートで補強したRC部材のせん断耐力の評価を行うために, ノッチでひび割れ位置を限定したRC部材を使用して, せん断ひび割れを強制的に導入して, せん断耐力に有効的に寄与するシートの有効付着面積を求めている. この有効付着面積を既報の研究で得た連続繊維シートとコンクリートの付着強さ式に適用することによって, 連続繊維シートによるせん断耐力の向上効果を評価できることを明らかにしている. また, より実際の桁に近いRC桁の側面に接着した連続繊維シートの補強効果と一部のシートに導入したプレストレス力の効果を検討している. 実験の結果, シート, プレストレス力によるせん断補強効果が明らかにされており, 算定式により実験値を評価できている.

The effect of fiber pre-stress on residual stresses and the onset of damage in symmetric laminates

Stresses and deformations caused by fiber pre-stress are analyzed in symmetric elastic laminates that are loaded by uniform in-plane tractions, temperature changes, and uniform eigenstrains induced by various physical processes in symmetrically located pairs of plies. The pre-stress is applied to all fibers of selected symmetric pairs of plies prior to matrix cure, and is released after cooling to room temperature. The effect of the pre-stress on residual stress distribution is illustrated by failure maps constructed in overall stress planes of ply strength branches, derived from the computed local stresses and the maximum stress criterion. Examples are presented for 9-ply (0̄/90) 2s and (0/±45/90/0̄) s S-glass/epoxy laminates, both in the stress-free state, and after −150°C cooling from the curing temperature, with different magnitudes of pre-stress applied either to all plies or to the 0° plies. Releasing the pre-stress is shown to be equivalent to applying a sustained compression in the axial direction of each ply. The ply strength branches and initial failure envelopes, therefore translate in the overall stress space in the direction and by the magnitude of the overall stresses that are equivalent to the applied pre-stress. In systems with compliant polymer matrices, fairly small fiber stress changes are needed to support large translations of the ply strength branches. The implication is that matrix damage can be retarded or avoided altogether by moderate pre-stress magnitudes that are within the operating capacity of existing filament-winding fiber or placement devices.

Design and fabrication of submerged cylindrical laminates—II. Effect of fiber pre-stress

This is the sequel to the first part of this paper ( Dvorak et al., 1999, Int. J. Solids Structures 36, 3917–3943) , concerned with modeling and analysis of laminated composite cylinder fabrication procedures, such as filament winding or fiber placement, which involve fiber pre-stress for waviness reduction as well as overall or local heating to and cooling from matrix curing temperatures. The fiber pre-stress applied in individual plies is shown to cause a self-stress in the respective plies, and relaxation stresses in the already completed plies and in the supporting mandrel. The final residual stress state is reached after mandrel removal. Influence functions that relate the ply stresses to the applied pre-stress forces are derived. Direct problems are solved for ply stresses caused by prescribed constant or linearly or parabolically changing pre-stress magnitudes in the layers. A superposition of the constant and parabolic distributions is shown to lead to nearly uniform stresses through the cylinder wall. The magnitudes depend on the radial stiffness of the mandrel that supports the structure during fabrication. Inverse problems are formulated as nonlinear optimizations and solved by quadratic programming. The goal is to determine fiber pre-stress distributions through the wall thickness such that the total stresses due to external hydrostatic pressure and fiber pre-stress are as uniform as possible through the wall thickness and confined by the ply strength magnitudes.

Design and fabrication of submerged cylindrical laminates—I

This is the first of a two-part paper concerned with both structural and fabrication process design of a closed-end laminated composite cylinder intended for service in deep sea environment. The cylinder is made of many different orthotropic layers and is loaded by uniform, axisymmetric surface tractions. In addition, piecewise uniform eigenstrains and residual stresses may be caused in the layers during fabrication, by fiber prestress for waviness reduction and by piecewise uniform changes in temperature. The overall goal is to assure efficient use of the composite structure under a prescribed hydrostatic pressure, and to select fiber prestress distribution such that the total stresses in the plies do not exceed certain strength magnitudes. Mechanical and residual stresses in the layers are evaluated with mechanical and transformation influence functions. For the proportional loading applied by a hydrostatic pressure, a procedure is outlined for design of several layups such that the cylinder wall experiences an isotropic in-plane strain and, therefore, all layers support the same compressive normal stresses, regardless of fiber orientation. The results are applied in design and analysis stress fields in a specific structure. Fabrication process design is discussed in the second part of the paper ( Srinivas et al., 1999, Int. J. Solids Structures, 36, 3945–3976) .

Demonstration of a Dynamic Polymer Composite Connector

This paper introduces the dynamic polymer composite (DPC) connector concept for creating structural joints of tubular composite members. DPC connectors are fiber-reinforced tubular elements featuring pretensioned circumferential fibers and a thermoplastic matrix. Application of heat softens the thermoplastic, allowing the circumferential fibers to clamp the connector onto the tubes to be joined. A laboratory demonstration of the DPC connector concept is described. General design relationships are presented to relate the basic performance measures to material properties and prestress levels. The need to maximize certain properties of the prestressing fiber and the thermoplastic matrix is discussed.

Self-monitoring structural materials

Self-monitoring (or intrinsically smart) structural materials, including concrete containing short carbon fibers, and polymer-matrix and carbon-matrix composites containing continuous carbon fibers, were reviewed. Each material is capable of monitoring its own reversible strain and damage through the effects of these on the electrical resistance of the material. This capability is valuable for structural control and structural health monitoring. Among these three materials, the concrete gives the highest strain sensitivity or gage factor (up to 700), while the carbon-carbon composite gives the highest damage sensitivity (i.e., sensitivity even to the damage after the first cycle of tensile loading within the elastic regime). The origin of the self-monitoring ability differs among the three materials. For the concrete, it is related to slight fiber pull-out during strain and fiber and matrix fracture during damage. For the polymer-matrix composite, it is related to the increase in the degree of fiber alignment and reduction of fiber pre-stress during tension in the fiber direction and to fiber fracture and delamination during fatigue. For the carbon-carbon composite, it is related to dimensional changes during strain and fiber and matrix fracture during damage. © 1998 Elsevier Science S.A.

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.

Controlling Thermal Stresses in Composites by Means of Fiber Prestress

A new method of applying a prestress to the fibers in composite laminates prior to and during cure is described, and several [0 P/90] T laminates produced at varying prestress levels are discussed. Experimental measurements described in this paper confirm that fiber prestressing can reduce or eliminate residual thermal stresses. This is evidenced by a reduction in the thermal warpage of unsymmetric [0 P/90] T laminates. An existing thermal analysis of unsymmetric laminates (developed by Hyer) is modified slightly in this paper to account for fiber prestressing, and is then used to predict out-of-plane deformations of the [0 P/90] T panels. Measurements compare well with theory. Specimens machined from the prestressed panels were also subjected to tensile loadings and inspected for matrix cracks in the 90° ply. It was found that fiber prestressing greatly reduced matrix cracking in [0 P/90] T specimens, but had little effect on [0/90 P] T specimens.

Thick-walled composite cylinders with optimal fiber prestress

A thick-walled composite cylinder consisting of many different cylindrically orthotropic layers is loaded by uniform, axisymmetric surface tractions and by piecewise uniform eigenstrains in the layers. A theoretical framework is established for evaluation of internal stress fields. An optimization procedure is implemented to find eigenstrain distributions that adjust the stresses in the layers to selected levels, while allowing the application of certain ranges of fiber prestrain magnitude to reduce fiber waviness. A particular fabrication process that utilizes fiber prestress as a source of the layer eigenstrains is analyzed, and a distribution of the fiber prestress forces is found that produces a desired distribution of both hoop and axial stresses in the cylinder wall. Both mandrel stiffness and the chosen prestress magnitude in the first layer influence the distribution. It is shown that application of a high fiber prestress that is constant through the wall thickness can be harmful as it may generate very high hoop stress gradients.

Influence of artificial pre-stressing during the curing of VIRALL on its mechanical properties

A method for improving the mechanical properties of VIRALL has been proposed on the basis of the tensile behavior of Vinylon strands. The influence of artificial pre-stressing during the curing of VIRALL on its tensile properties has been experimentally investigated. It is revealed that fiber pre-stressing may lead to a dramatic increase in the elastic limit, yield strength and failure strength of VIRALL laminates. These results can be referred to the strengthening effect of Vinylon fibers and the modifications and recreations of the ply stress states in VIRALL laminates.

Micromechanics of crack bridging in fibre-reinforced concrete

The stress-crack width relationship has been determined experimentally for concretes reinforced with two types of fibres, steel and polypropylene, of various fibre volume fractions. A micromechanics-based theoretical model is proposed which captures the essential features of the stress-crack width relationships at small crack widths (less than 0.3 mm). Micromechanisms accounted for include the bridging actions due to aggregates and fibres, Cook-Gordon interface debonding and fibre pre-stress. The fibre bridging action involves interface slip-dependent friction as well as snubbing friction for fibres bridging at inclined angles. Theoretical predictions based on independent parametric inputs compare favourably with experimental measurements of the stress-crack width relationship. Findings in this research provide confidence in the use of the proposed model for materials engineering targeted at prescribed structural performance.

Loading Tests on Concrete Beams Prestressed With Glass Fiber Tendons

First, a brief survey of the development and characteristics of glass fiber prestressing bars is given. The system was developed in Germany and has been applied in three highway bridges and one pedestrian bridge. The main part of the paper deals with loading tests on beams prestressed with glass fiber tendons. The tests were performed in the Magnel Laboratory of the University of Ghent. Three short beams, each with a span of 6 ft 7 in (2 m), were loaded to failure. In the glass fiber bars, optical sensors and copper wire sensors were integrated, thus allowing long-term monitoring of the tendons. Although the paper deals mainly with the structural behavior of the beams, the test program was also used to evaluate these non-destructive techniques by comparing the results with conventional deformation measurements.

Câbles à base de fibres pour la précontrainte

Successful use of high performance composite materials was first made in the aircraft industry and, to a lesser extent, in the automobile industry. The use of these materials in civil engineering is now a reality, particularly for prestressing applications. This state-of-the-art paper deals with composite materials used for cables. It describes the composition of fiber prestressing cables and presents their mechanical properties. The anchorages, the advantages and disadvantages of these cables, their field of applications, and three successful applications in structural engineering are also discussed. Key words: composite materials, prestressing, fiber cables, concrete.

An Experimental Study of the Effect of Prestressed Loose Carbon Strands on Composite Strength

The purpose of this work is to demonstrate experimentally the strength behavior of composites reinforced with prestressed fiber strands. In a preceding article, a theoretical analysis of the feasibility of strength enhancement by fiber prestressing has been demonstrated. In this paper, fiber prestressing is achieved by bending of the loose bundles and the weak points of the fibers are thus eliminated. The experimental results show that after prebending the fibers to an appropriate level the strength of composites can be enhanced. Although the increase in composite mean strength is small, the varia tion of composite strength is greatly reduced, and the strength at high survivability level is enhanced most significantly.

An investigation of the compressive strength of Kevlar 49/epoxy composites

Tests were performed to evaluate the effect of a wide range of variables including matrix properties, interface properties, fiber prestressing, secondary reinforcement, and others on the ultimate compressive strength of Kevlar 49/epoxy composites. Scanning electron microscopy is used to assess the resulting failure surfaces. In addition, a theoretical study is conducted to determine the influence of fiber anisotropy and lack of perfect bond between fiber and matrix on the shear mode microbuckling. The experimental evaluation of the effect of various constituent and process characteristics on the behavior of these unidirectional composites in compression did not reveal any substantial increase in strength. However, theoretical evaluations indicate that the high degree of fiber anisotropy results in a significant drop in the predicted stress level for internal instability. Scanning electron microscope data analysis suggests that internal fiber failure and smooth surface debonding could be responsible for the measured low compressive strengths.