Pre-stressed Fiber Research Articles

Multiscale shear failure mechanisms within a prestrained composite

AbstractThe elastic fiber prestressing (EFP) technique has been developed to balance the thermal residual stress generated during curing of a polymeric composite, where continuous fibers were prestretched under either constant stress or constant strain throughout the curing process. The tension was only removed after the resin was fully cured. It has been demonstrated that EFP is able to enhance the shear properties of the composite, while the underlying mechanics is still unknown. Here, we investigated the multiscale shear failure mechanisms induced by the EFP within a carbon composite. A bespoke biaxial fiber prestressing rig was developed to apply biaxial tension to a plain‐weave carbon prepreg, where the constant strain‐based EFP method was employed to produce prestrained composites with different prestrain levels. Effects of EFP on macro‐scale shear failure were subsequently characterized through mechanical tests and micro‐morphological analysis. Both the micro‐ and meso‐scale representative volume element (RVE) finite element models were established and experimentally verified. These were then employed to reveal the underlying stress evolution mechanics induced by EFP. It is found that EFP would improve the shear performance of a composite by enhancing the fiber/matrix interfacial bonding strength. This attributes to the elastic strain recoveries of the prestrained fibers locked within a polymeric composite, which generate compressive stresses to counterbalance the external loading. The multiscale shear failure mechanisms were then proposed. These findings are expected to facilitate structural design and application of the EFP for aerospace composites.Highlights Biaxial tension is applied to produce prestrained woven composite. Prestrain effects on microstructural stress evolution mechanics are revealed. Multiscale shear failure mechanisms are proposed for prestrained composites.

In‐situ strain evolution mechanisms within a prestressed carbon composite

AbstractThermal residual stress generated during curing is known to be detrimental to mechanical performance of a carbon composite. Fiber prestressing technique has been developed for decades to counterbalance these negative effects. Although there have been some achievements in development of the prestress mechanisms, these are mainly based on the extrapolations from macroscopic mechanical characterizations, lack of direct evaluation of the in‐situ in‐plane strain or stress evolution mechanisms, in order to reveal the impact induced by different prestressing methods. Here, we investigated the in‐situ strain evolution mechanisms in producing a prestressed carbon composite. Since clamping of the uncured prepreg is the most challenging, both the strain evolutions with and without precured prepreg edges were evaluated to reveal the underlying mechanisms induced by stress relaxation during fiber prestressing. Mechanical tests in terms of Charpy impact and three‐point bending, as well as fractured morphology were carried out to evaluate the prestress effects. The underlying mechanisms were then proposed to reveal the fundamentals in producing a prestressed composite. These are expected to revolutionize industrial production and applications of prestressed polymeric composites, indicating detrimental mechanisms on precuring strip ends following conventional prestress procedures.Highlights There is an optimal prestrain level to maximize properties of a composite. Internal strain development is dependent on the fiber clamping methods. Stress relaxation is beneficial to induce compressive stress within a composite. In‐situ strain evolution mechanics is revealed for prestressed carbon composite.

Mould free laminated composites using eccentric fibre prestressing

Mould free fibre reinforced composites are an emerging manufacturing process that allows for complex shapes to be produced, independently from complex moulding. Previous examples demonstrated reconfiguration through thermal stresses induced by an asymmetrical stacking sequence within a laminate. However, asymmetry within the laminate can adversely influence the stiffness and geometric coupling which results in heavier structures. This paper presents eccentric prestressing as a novel method to produce more structurally efficient mould free laminated composites. Euler-Bernoulli beam theory and finite element methods were employed to predict the strain and deformation of laminated composites with selective fibre prestress. The internal and external strains were measured with chirped fibre bragg grating sensors, and surface mounted strain gauges respectively to validate the numerical and analytical predictions where a difference of 3.13% was reported. Finally, a comparative study was conducted to show the benefits of eccentric prestress over asymmetric 4D printed laminates.

Improvement of Tensile and Flexural Properties of Fiber Pre-Stressing Composite by Using Nano Graphene

Fiber pre-stress composites are used nowadays in many applications that need extra strength without adding more reinforcements. Nano graphene platelets are selected to be added to the composite as it has high tensile properties. This work studied the effect of adding Nano graphene platelets with different weight ratios of 0.5%,1%, and 2% to the resin during the curing process in the presence of fiber pre-stressing of 1MP and 2MPa. The results show improvement in most properties of composites like tensile strength, modulus of elasticity, flexure strength, and modulus of flexure with the ratio of (43-57%),(35-55%),(100-134%), (60-90%) respectively. A numerical model was built to simulate tensile and flexure tests using ANSYS software. The divergence between the experimental and numerical results did not exceed 5%. Adding Nano graphene with a weight ratio of more than 2% will have a negative effect on the properties as the Nano graphene will agglomerate and lose most of its high properties that match with most literature reviewed.

Strengthening of the Existing Structural Beam Member by using Pre-Stressed Fibre Reinforced Polymer

Reinforced concrete beam members are load-carrying members in structures. The beams performance is largely depending on the condition of the tensile reinforcement in members and quantity in the beam. Focus is put on flexural strengthening, which currently is the most common application field for composite materials in structural engineering. Generally, pre-stressed FRP will be demonstrated to follow the principle of conventional pre-stressed concrete by resulting in higher cracking, yielding, and bearing loads. Especially under service loads, the structural behavior is improved. Civil structures in several situations require strengthening of the structural members due to lack of stiffness, strength, serviceability, durability. Some situations where the members are in need of rehabilitation in their lifespan. There are many situations where the requirement of strengthening is must for structures such as Retrofit for seismic effected building

Monitoring of mechanical properties of prestressed glass fiber epoxy composites over 12 months after fabrication

Fiber prestressing during matrix curing can significantly improve the mechanical properties of fiber-reinforced polymer composites. One primary reason behind this improvement is the generated compressive residual stress within the cured matrix, which impedes cracks initiation and propagation. However, the prestressing force might diminish progressively with time due to the creep of the compressed matrix and the relaxation of the tensioned fiber. As a result, the initial compressive residual stress and the acquired improvement in mechanical properties are prone to decline over time. Therefore, it is necessary to evaluate the mechanical properties of the prestressed composites as time proceeds. This study monitors the change in the tensile and flexural properties of unidirectional prestressed glass fiber reinforced epoxy composites over a period of 12 months after manufacturing. The composites were prepared using three different fiber volume fractions 25%, 30%, and 40%. The results of mechanical testing showed that the prestressed composites acquired an initial increase up to 29% in the tensile properties and up to 32% in the flexural properties compared to the non-prestressed counterparts. Throughout the 12 months of study, the initial increase in both tensile and flexural strength showed a progressive reduction. The loss ratio of the initial increase was observed to be inversely proportional to the fiber volume fraction. For the prestressed composites fabricated with 25%, 30%, and 40% fiber volume fraction, the initial increase in tensile and flexural strength dropped by 29%, 25%, and 17%, respectively and by 34%, 26%, and 21%, respectively at the end of the study. Approximately 50% of the total loss took place over the first month after the manufacture, while after the sixth month, the reduction in mechanical properties became insignificant. Tensile modulus started to show a very slight reduction after the fourth/sixth month, while the flexural modulus reduction was observed from the beginning. Although the prestressed composites displayed time-dependent losses, their long-term mechanical properties still outperformed the non-prestressed counterparts.

The construction process for pre-stressed ultra high performance concrete communication tower.

Towers are important structures for installing radio equipment to emit electromagnetic waves that allow radio, television and/or mobile communications to function. Feasibility, cost, and speed of the construction are considered in the design process as well as providing stability and functionality for the communication tower. This study proposes the new design for construction of segmental tubular section communication tower with ultra-high-performance fibre concrete (UHPFC) material and prestress tendon to gain durability, ductility, and strength. The proposed mix design for UHPFC in this study which used for construction of communication tower is consisted of densified Silica Fume, Silica fine and coarse Sand and hooked-ends Steel Fiber. The prestressed tendon is used in the tower body to provide sufficient strength against the lateral load. The proposed design allows the tower to be built with three precast segments that are connected using bolts and nuts. This paper presents a novel method of construction and installation of the communication tower. The advantages of proposed design and construction process include rapid casting of the precast segment for the tower and efficient installation of segments in the project. The use of UHPFC material with high strength and prestress tendon can reduce the size and thickness of the tower as well as the cost of construction. Notably, this material can also facilitate the construction and installation procedure.

Development of beneficial residual stresses in glass fiber epoxy composites through fiber prestressing

The undesired residual stresses in fiber reinforced polymer composites are developed during their manufacturing processes due to the thermal and chemical shrinkage of the polymer matrix, which negatively affect the performance of the composites. Applying tensile stress to the reinforcement fibers during the curing of the matrix can reduce or eliminate the undesired residual stresses. Furthermore, the undesired tensile residual stress within the matrix can be replaced with beneficial compressive residual stress which can improve the mechanical properties of the composites. In this study, theoretical stress analysis was performed in order to determine the value of the optimum compressive residual stress that is associated with the highest tensile strength. This value was determined based on the tensile testing result of prestressed composites with a 40% fiber volume fraction. For the composites with different fiber volume fractions, a new approach was introduced to estimate the optimum fiber prestressing level which generates the optimum compressive residual stress within the matrix and consequently, the highest tensile strength can be achieved. In order to validate this approach experimentally, prestressed composites with 25 and 30% fiber volume fraction were prepared by applying the estimated optimum fiber prestressing levels during curing of matrix and tensile testing was performed. The result of the stress analysis showed that the value of the optimum compressive residual stress corresponding to the highest tensile strength is 2.5 MPa approximately. The tensile testing results confirmed the validity of the new approach in estimating the optimum fiber prestressing level for the composites with different fiber volume fractions. For both prestressed composites with 25 and 30% fiber volume fraction, the highest tensile strength was achieved when the theoretically estimated optimum fiber prestressing level was applied to the fibers during the curing of the matrix.

Love Waves in a Pre-Stressed Fiber Reinforced Medium Rest upon a Monoclinic Half Space

Love waves in a pre-stressed fiber-reinforced medium lying above a monoclinic half-space have been investigated. Upper surface of fiber reinforced layer remains stress free and interface of half space and layer satisfies continuity conditions .The dispersion equation for Love waves propagation has been derived. Effect of anisotropy parameter of half space and initial stresses of reinforced layer on Love waves propagation have been observed from dispersion curves. Some particular cases have also been developed by using the dispersion equation. Further, the range of the existence of Love waves is calculated. The cut-off periods for three nodes of Love waves with variation of anisotropy parameter and compressive stress are presented in tabular form.

Matrix-induced pre-strain and mineralization-dependent interfibrillar shear transfer enable 3D fibrillar deformation in a biogenic armour

The cuticle of stomatopod is an example of a natural mineralized biomaterial, consisting of chitin, amorphous calcium carbonate and protein components with a multiscale hierarchical structure, and forms a protective shell with high impact resistance. At the ultrastructural level, cuticle mechanical functionality is enabled by the nanoscale architecture, wherein chitin fibrils are in intimate association with enveloping mineral and proteins. However, the interactions between these ultrastructural building blocks, and their coupled response to applied load, remain unclear. Here, we elucidate these interactions via synchrotron microbeam wide-angle X-ray diffraction combined with in situ tensile loading, to quantify the chitin crystallite structure of native cuticle – and after demineralization and deproteinization – as well as time-resolved changes in chitin fibril strain on macroscopic loading. We demonstrate chitin crystallite stabilization by mineral, seen via a compressive pre-strain of approximately 0.10% (chitin/protein fibre pre-stress of ∼20 MPa), which is lost on demineralization. Clear reductions of stiffness at the fibrillar-level following matrix digestion are linked to the change in the protein/matrix mechanical properties. Furthermore, both demineralization and deproteinization alter the 3D-pattern of deformation of the fibrillar network, with a non-symmetrical angular fibril strain induced by the chemical modifications, associated with loss of the load-transferring interfibrillar matrix. Our results demonstrate and quantify the critical role of interactions at the nanoscale (between chitin-protein and chitin-mineral) in enabling the molecular conformation and outstanding mechanical properties of cuticle, which will inform future design of hierarchical bioinspired composites. Statement of SignificanceChitinous biomaterials (e.g. arthropod cuticle) are widespread in nature and attracting attention for bioinspired design due to high impact resistance coupled with light weight. However, how the nanoscale interactions of the molecular building blocks – alpha-chitin, protein and calcium carbonate mineral – lead to these material properties is not clear. Here we used X-ray scattering to determine the cooperative interactions between chitin fibrils, protein matrix and biominerals, during tissue loading. We find that the chitin crystallite structure is stabilized by mineral nanoparticles, the protein phase prestresses chitin fibrils, and that chemical modification of the interfibrillar matrix significantly disrupts 2D mechanics of the microfibrillar chitin plywood network. These results will aid rational design of advanced chitin-based biomaterials with high impact resistance.

Effect of fiber prestressing on mechanical properties of glass fiber epoxy composites manufactured by vacuum-assisted resin transfer molding

The mechanical properties of fiber-reinforced polymer composites depend on several aspects such as the characteristics of constituents, fiber volume fraction, and manufacturing techniques. Fiber prestressing is considered a very attractive manufacturing technique that can be used to produce fiber-reinforced polymer composites with high mechanical properties. This technique has the potential to eliminate or reduce some manufacturing problems like fiber waviness. In the present study, a new approach was used to prepare prestressed fiber-reinforced polymer composites. Unidirectional E-glass fiber-stitched mats were impregnated with epoxy matrix through vacuum-assisted resin transfer molding process. Once the infusion was done, a pre-calculated tensile force was applied to the fiber mats through a hydraulic tensile machine. The impregnated fiber mats were left under tension and vacuum during curing of the epoxy matrix (24 h). Five prestressed samples were prepared by using five different prestressing levels 20, 40, 60, 80, and 100 MPa. In addition, non-prestressed (control) sample was prepared for the purpose of comparison. The influence of fiber prestressing on fiber waviness, fiber volume fraction, and void content was investigated. Flexural, tensile, and compression tests were performed to observe the effect of fiber prestressing on the mechanical properties. The results have shown the success of this new approach in producing prestressed fiber-reinforced polymer composites with high mechanical properties comparing to non-prestressed composites. The microstructure analysis has shown dramatical reduction in fiber waviness for the prestressed samples over control sample. All prestressed samples have shown higher fiber volume fraction and lower void content comparing to the control sample. Also the results have shown as the prestressing level increases, fiber volume fraction increase and void content decreases. Prestressing levels of 40 and 60 MPa were found to be the best candidates, they have led to an increase in tensile strength, compressive strength, and flexural strength by 24.2%, 72.5%, 28% and 28.6%, 100.4%, 26.1%, respectively, comparing to the non-prestressed sample. Ease of implementation and promising results of this new approach would attract the attention toward it. Automotive industry is one potential nominee to apply this approach during manufacturing of fiber-reinforced polymer leaf spring.

Creep Behavior of Resin Matrix and Basalt Fiber Reinforced Polymer (BFRP) Plate at Elevated Temperatures

Pre-stressed fiber reinforced polymer (FRP) has great application potential in structural strengthening. However, the elevated temperature resistance of FRPs is always a key concern due to the poor thermal stability of its resin matrix. In this study, the effects of temperature on the creep behavior of the resin matrix and basalt fiber reinforced polymer (BFRP) was experimentally investigated. The tensile stresses were set at 2.6 MPa for the resin matrix and 522 MPa (35% of its ultimate tensile strength (fu)) for BFRP, and the exposure temperatures were 25 °C, 80 °C, 120 °C, and 160 °C. The short-term strain of the resin matrix and BFRP exposed to different exposure temperatures was measured. The variation of the thermal property and interlaminar shear strength (ILSS) of the BFRP were studied. The results indicated that molecular chain disruption and post-cure coexisted. The resin matrix is sensitive to the exposure temperatures, and a remarkable increase of the strain was observed when the exposure temperature exceeded its glass transition temperature (107.5 °C). The resin matrix fractured within 50 seconds when it was exposed to 160 °C. BFRP showed excellent temperature resistance even though the exposure temperature exceeded its glass transition temperature (123.7 °C). Sustained loading led to stress transferring to the basalt fiber in BFRP specimens, especially at elevated temperatures. Stress redistribution caused interfacial damage, and ILSS decreased by 0.5%, 13.6%, and 14.6% for 80 °C, 120 °C, and 160 °C exposure from its original value of 73.5 MPa. Dynamic mechanical thermal analysis (DMTA) was used to explain the post-curing and interface damage of BFRP.

Effect of prestress level on the strength of CFRP composite laminate

The effect of fiber prestressing has been investigated on the tensile strength, modulus strength, flexural properties and residual stresses of Carbon fiber reinforced polymer (CFRP) composites. Unidirectional carbon fiber in an epoxy resin has been examined under different prestressing levels (0, 5, 10, 15, 20, 30, 45, 60, 80 MPa) at ambient temperature and 50 % fiber volume fraction. A new method was implemented to produce the prestressed laminates for several standard tests. The results showed that prestressing on 3-ply unidirectional carbon fibers improved the fiber tensile strength to 99 % and the tensile modulus to 31 %. Similarly on 8-ply unidirectional carbon fibers has improved the fiber flexural strength to 63 % and flexural modulus to 81 %. A new technique was used to measure the residual stresses and tensile modulus of the composite laminate by recording the final extension and the remaining load directly after the curing process and releasing the applied load.

A wrinkle in flight: the role of elastin fibres in the mechanical behaviour of bat wing membranes.

Bats fly using a thin wing membrane composed of compliant, anisotropic skin. Wing membrane skin deforms dramatically as bats fly, and its three-dimensional configurations depend, in large part, on the mechanical behaviour of the tissue. Large, macroscopic elastin fibres are an unusual mechanical element found in the skin of bat wings. We characterize the fibre orientation and demonstrate that elastin fibres are responsible for the distinctive wrinkles in the surrounding membrane matrix. Uniaxial mechanical testing of the wing membrane, both parallel and perpendicular to elastin fibres, is used to distinguish the contribution of elastin and the surrounding matrix to the overall membrane mechanical behaviour. We find that the matrix is isotropic within the plane of the membrane and responsible for bearing load at high stress; elastin fibres are responsible for membrane anisotropy and only contribute substantially to load bearing at very low stress. The architecture of elastin fibres provides the extreme extensibility and self-folding/self-packing of the wing membrane skin. We relate these findings to flight with membrane wings and discuss the aeromechanical significance of elastin fibre pre-stress, membrane excess length, and how these parameters may aid bats in resisting gusts and preventing membrane flutter.

Mathematical Aspects Regarding Cracks Behaviour in Wood Composites

In this paper we study the interaction of two unequal collinear cracks in a pre-stressed fiber reinforced wood composite material in anti-plane mode of fracture. A mathematical model is associated to the mechanical problem. Starting from the boundary, constitutive and far field conditions we obtain the representation of the incremental displacement, stress and strain fields using two complex potentials. Using Sih s generalized fracture criterion we determine the critical incremental shear stress which produce crack propagation and the crack tips propagation direction in a particular case of a Pine wood composite.

Innovative Strengthening Technique Using Post-Tensioned Fibre Composite Wraps for Bridge Headstocks

SummaryEngineered fibre composites offer an extremely versatile option to strengthen existing structures to solve structural problems arising from environmental exposure, inadequate designs, increased traffic loads and continuous aging of existing structures. While the behaviour of structures by external post-tensioning using fibre composites for flexure is well understood, there is limited research on shear strengthening, specifically using pre-stressed fibre composites. An innovative system using fibre composites has been investigated for strengthening bridge headstocks with deficient structural performance. The innovative system uses fibre composite wraps that forms an infinite loop around the headstocks and together with anchorage blocks, these are pushed-out and the gap packed with a suitable spacer to maintain the applied pre-stressing force. This paper discusses the research, development and construction of this innovative fibre composite post-tensioning system, and testing it on a model bridge headstocks in Tenthill Creek Bridge. Issues related to the effect of existing shear cracks and the importance of repairing such cracks will be discussed. With further investigation, this strengthening system could be implemented in actual structures to enhance the capacity of bridge headstocks.

Bistable Prestressed Symmetric Laminates

The bistability of unsymmetric cross-ply [0n/90n] T laminates has already been investigated in much detail. In this work a new type of bistable laminate is presented which has a symmetric lay-up. Bistability derives from an unsymmetric fiber prestress applied to the laminate. The experimental procedure used to apply this fiber prestress is presented in detail. Experimental results are compared with analytical and finite element models which have the ability to model fiber prestress accurately. As well as having minimal hygrothermal variability, it is noted that the snap-through loads for a prestressed symmetric laminate can be much higher than its unstressed [0n/90n]T equivalent.

Dynamic response of pre-stressed fibre metal laminate (FML) circular cylindrical shells subjected to lateral pressure pulse loads

Dynamic response of fiber metal laminate cylindrical shells subjected to initial combined axial load and internal pressure were studied in this paper. First order shear deformation theory (FSDT) was utilized in the shell’s equilibrium equations and strain–displacement relations were based on Love’s first approximation theory. Equilibrium equations for buckling, free and forced vibration problems of the shell were solved using Galerkin method. The influences of FML parameters such as material properties lay up, Metal Volume Fraction (MVF), fiber orientation and initial stresses on dynamic response were investigated. The results were indicated that the FML lay up, has a significant effect on natural frequencies as well as transient dynamic response with respect to various values of MVF as well as pre-stress.

Mathematical modelling of the interface crack. A new improved numerical method

Abstract A pre-stressed fiber elastic composite containing an anti-plane interface crack is considered. The boundary conditions of our problem are leading to a homogeneous and a non-homogeneous problem. Using the theory of Cauchy’s integral ( [1] , [2] , [3] , [4] ), Guz’s representation ( [1] , [3] ) and the numerical analysis we determine the incremental fields in the vicinity of the crack tips. In [5] we presented a numerical method for calculating the complex integrals which appear in the non-homogeneous complex differential equation for the complex potential. In the present paper we shall improve the method, providing an appropriate treatment of the singularities. We shall deliver a very good formula for calculating numerically the complex integrals employing Plemelj’s fomulas and Cauchy’s integral. A numerical example regarding the propagation of an anti-plane interface crack in glass-epoxy is studied.

Mathematical modelling of the interface crack propagation in a pre-stressed fiber reinforced elastic composite

Abstract A pre-stressed fiber reinforced elastic composite containing an interface crack in the mode three of classical fracture is considered. The boundary conditions of our problem are leading to a homogeneous and a non-homogeneous Riemann–Hilbert problem. Using the theory of Cauchy’s integral and the numerical analysis we determine the incremental fields in the vicinity of the crack tips. The cases of elliptical type and parabolical type of incremental antiplane shear forces are considered. Extending the validity of Sih’s criterion we obtain the critical values of the incremental stresses which are producing crack propagation and the crack propagation direction. A numerical example in the case of constant antiplane shear stress regarding the propagation of an antiplane interface crack in graphite–epoxy is considered.