Fiber Reinforced Polymer Structures Research Articles

Sensor Systems for FRP Lightweight Structures: Automotive Features Based on Serial Sensor Products.

To achieve resource efficiency and an increased performance, as well as a higher safety and more features for vehicles, lightweight composites are a central sphere of activity for automotive innovations. This becomes particularly striking if the focus is not only a reduced vehicle weight but also an efficient overall concept. In addition to compatible material technologies and component design, new electronic solutions are of interest. A research contribution at the Robert Bosch Company deals with the direct integration of a current automotive acceleration sensor in fiber-reinforced polymer (FRP) parts. The sensor is part of the passive vehicle safety. Primarily, the principal application of the currently mounted sensor as an integrated part of the vehicle structure was proven. Sensor-integrated parts were evaluated on their sensing functionality as well as their structural performance. The present research is done to use the integrated sensor for a secondary feature. The study shows that the sensor can also be an indicator for the condition of its surrounding FRP structure. Hence, the sensor integration makes it possible to derive a secondary feature for automobiles by using the current sensor for future functionalized lightweight structures.

GFRP-to-timber bonded joints: Adhesive selection

Recent research at the University of Queensland (UQ) has led to the development of a new type of structures called “Hybrid Fiber Reinforced Polymer (FRP)-Timber structures” (“HFT”). In HFT structures, FRP is combined with timber veneers to create high-performance, lightweight, easy-to-construct structural members. These HFT members take advantage (i) of the orthotropic properties of both, timber and FRP to orientate the fiber direction to produce optimal composite properties, and (ii) of the geometry of the cross sections to maximize the load bearing capacity for a given amount of material. While preliminary experimental work has revealed as such the effectiveness of HFT structural members, no work has been carried out so far to investigate the behavior of these HFT structures. Performance of these new HFT members relies significantly on the bond between FRP and timber. This paper presents the results of an experimental study aimed at selecting a suitable commercially available adhesive for glass fiber reinforced polymer (GFRP)-to-timber bonded joints. The experimental program included 393 single lap joint tests covering four different commercially available adhesives, two different curing temperatures, and two test methods (dry and moisture cycle tests). The test results revealed that both, polyurethane (PUR) and cross-linking polyvinyl acetate emulsion (PVAx) performed as the best under dry conditions, while PUR was shown to be superior to all other adhesives when subjected to moisture cycles. Epoxy and phenol resorcinol formaldehyde adhesive (PRF) commonly used in FRP structures and laminated timber structures, resp., were found to be less performing structural adhesives for HFT structures.

Simulated grating-based x-ray phase contrast images of CFRP-like objects

Because of their microstructure, carbon fibre reinforced polymers outperform conventional materials in many aspects. These materials are, given the low x-ray absorption contrast they provide, ideally suited for phase contrast x-ray computed tomography. For system prototyping and for benchmarking and testing acquisition and reconstruction procedures, computer simulations are a valuable tool. In this paper, we propose the application of an alternative simulation strategy that unites Monte Carlo simulations with numerical wave optics and involves detailed modelling of the internal carbon fibre reinforced polymer structure down to the individual fibre scale.

Structural performances of pultruded GFRP emergency structures – Part 1: Experimental characterization of materials and substructure

This paper presents an experimental study in the field of structures made of pultruded fiber reinforced polymers (FRP) elements to be used for emergency purposes. A preliminary design of a 3D pultruded glass fiber reinforced polymer structure is presented with the mechanical characterization of the constituting elements. The axial and flexural properties of laminate and I-shaped GRFP profiles are discussed considering the short term creep. In a companion paper, the benefits, the limits and the reliability of the structure analyzed for emergency applications are discussed. In details, the numerical structural analysis of the full-scale 3D model is described followed by the experimental assessment of the structural capacity by a full-scale 2D model.

Experimental characterization of composite laminates under low-velocity multi-impact loading

The use of composite materials undergoes a significant growth in many industries including aeronautic, railway, and automotive. Laminated composites are often subjected to severe loading conditions such as hailstones on aircraft and road gravels on automobiles. The present paper deals with the damage tolerance in carbon fiber-reinforced polymer structures caused by low velocity multiple impacts. Low velocity multi-impact tests at different energy levels (10, 15, and 20 J) are performed in order to simulate the gravels' impact conditions corresponding to mass of 10–50 g for velocity of 100–150 km/h. These tests were carried out on composite laminates with three types of stacking sequences (AERO, QIQH45, and QIQH60). AERO, widely used in aviation industry, is symmetric stacking sequence, and the two others QIQH45 and QIQH60 are quasi-isotropic and quasi-homogeneous laminates based on 45° and 60° ply orientation, respectively. Two specific parameters D and d are deduced from force–displacement and force–time responses which make it possible to rank the three stacking sequences to their damage tolerances. The impact damage assessment within the composite material with non-destructive testing method using ultrasonic inspection gives a good correlation with parameter D. Better impact damage tolerance is achieved with QIQH45 than the QIQH60 and AERO laminates. The parameter D can be regarded as a failure criterion for dimensioning of composite structures subjected to repetitive impacts.

Factors governing the tensile strength of basalt fibre

Basalt fibre has attracted great attention in developing fibre-reinforced polymers (FRP) in recent years. The tensile strength of basalt fibre is one of the most important considerations when formulating FRP structures. In this study, the strength of seven commercial basalt fibres was studied and compared, and the factors governing the strength of these fibres were investigated. The results showed that the strength of fibre is mainly dominated by the elemental composition of material and sizing on fibre surface, which functioning as “backbone” and “clothing” for the filament, respectively. Correlations between the tensile strength of fibre as a function of Al2O3 content and Fe3+/Σ(Fe3++Fe2+) in the material were also established. Interestingly, as a processing step after fibre spinning, the sizing can increase the strength of material by up to 25%, illustrating the importance of such post-treatment for developing high performance basalt fibres.

Manufacturing uncertainties and resulting robustness of optimized patch positions on continuous-discontinuous fiber reinforced polymer structures

Discontinuous fiber-reinforced Sheet Moulding Compound (SMC) in combination with continuous carbon fiber patches provide a high design freedom in combination with good weight-specific properties. However, the application of these materials requires a strategic and exact positioning of the patches, necessitating the consideration of unavoidable manufacturing defects during the design phase. Therefore, a workflow is proposed to evaluate the robustness of multi-objective patch optimization results using two robustness measures, the degree of robustness and robustness index. An efficient calculation of the robustness measures is achieved by replacing computational expensive simulation models with a Kriging surrogate model. Typical manufacturing deviations occurring during the patch positioning and moulding process are determined from experiments using active thermography. Finally, the proposed workflow is applied to the multi-objective optimization of two patches on a demonstrator under four-point-bending load. The resulting robustness measures can be used as a decision criterion for the selection of the best Pareto optimal solution. Furthermore, they can be used for the determination of the maximum occurring objective variation as well as the largest permissible manufacturing tolerances.

Three-dimensional printing of hierarchical liquid-crystal-polymer structures

Fibre-reinforced polymer structures are often used when stiff lightweight materials are required, such as in aircraft, vehicles and biomedical implants. Despite their very high stiffness and strength1, such lightweight materials require energy- and labour-intensive fabrication processes2, exhibit typically brittle fracture and are difficult to shape and recycle3,4. This is in stark contrast to lightweight biological materials such as bone, silk and wood, which form by directed self-assembly into complex, hierarchically structured shapes with outstanding mechanical properties5-11, and are circularly integrated into the environment. Here we demonstrate a three-dimensional(3D) printing approach to generate recyclable lightweight structures with hierarchical architectures, complex geometries and unprecedented stiffness and toughness. Their features arise from the self-assembly of liquid-crystal polymer molecules into highly oriented domains during extrusion of the molten feedstock material. By orienting the molecular domains with the print path, we are able to reinforce the polymer structure according to the expected mechanical stresses, leading to stiffness, strength and toughness that outperform state-of-the-art 3D-printed polymers by an order of magnitude and are comparable with the highest-performance lightweight composites1,12. The ability to combine the top-down shaping freedom of 3D printing with bottom-up molecular control over polymer orientation opens up the possibility to freely design and realize structures without the typical restrictions of current manufacturing processes.

Influence of a fiberglass layer on the lightning strike damage response of CFRP laminates in the dry and hygrothermal environments

This paper experimentally investigates the lightning strike response of a fiberglass/carbon woven hybrid laminate ([(0/90)g(45/02/−452/902)c]s) under dry and hygrothermal conditions. Damage characteristics are evaluated using various inspection methods and possible damage mechanisms are discussed. Observations demonstrate that as the increase of lightning amplitude, notably enlarged carbon fiber breakage and fiberglass delamination areas as well as fiber bulging are presented in the in-plane and in-depth directions. Compared to a [452/02/−452/902]s carbon woven specimen, the carbon fiber damage area, internal delamination area and damage depth for the hybrid one are 4.94 times, 2.26 times, 3.21 times greater under 22 kA lightning strike and 3.34 times, 1.97 times, 1.67 times greater under 32 kA strike, respectively. Hygrothermal aging significantly intensifies lightning strike damage on the hybrid specimen with complete detachment of fiberglass layer and massive internal delamination. The possible mechanism for the present damage behavior is analyzed. These results suggest that the fiberglass layer is not expected to protect the integrity of the carbon fiber reinforced polymer structure from lightning strike damage.

Effect of curing conditions on mode-II debonding between FRP and concrete: A prediction model

The rehabilitation and strengthening of concrete structures using Fiber-Reinforced Polymer (FRP) materials have been widely investigated. As a priority issue, however, the effect of curing conditions on the bonding behavior between FRP and concrete structures is still elusive. This study aims at developing a prediction model to accurately capture the mode-II interfacial debonding between FRP strips and concrete under different curing conditions. Single shear debonding experiments were conducted on FRP-concrete samples with respect to different curing time t and temperatures T. The J-integral formulation and constrained least square minimization are carried out to calibrate the parameters, i.e., the maximum slip s- and stretch factor n. The prediction model is developed based on the cohesive model and Arrhenius relationship. The experimental data are then analyzed using the proposed model to predict the debonding between FRP and concrete, i.e., the interfacial shear stress-slip relationship. A Finite Element (FE) model is developed to validate the theoretical predictions. Satisfactory agreements are obtained. The prediction model can be used to accurately capture the bonding performance of FRP-concrete structures.

Seismic Design of Pultruded FRP Structures as Ancillary and/or Independent Solution

The civil engineering fields of FRP (Fiber Reinforced Polymers) pultruded profiles are the structural rehabilitation of the existing constructions and the new buildings. In the first case the FRP intervention is ancillary to masonry construction as beams and trusses for roofs and floors; while, in the second case, the all-FRP structures are for over elevation frame, emergency and specific structures in complex conditions. The non-linear responses of masonry structures with truss beams made of pultruded FRP profiles and traditional materials have been compared through finite element models subjected to the seismic forces. Furthermore, the seismic response of all-FRP building with concentric diagonal braces has been analysed. For both applications it is possible to assert that despite the absence of ductile behaviour of FRP pultruded material, the new technology guarantees a dissipative response through the global ductility and the effect of the low self-weight related to the mechanical performances.

Constitutive modelling of the strain-rate dependency of fabric reinforced polymers

Among the various mechanisms which occur during impact, the strain rate effect plays a significant role on the mechanical response of layered carbon fibre reinforced polymer structure. In this work, the viscoelastic behaviour of the material is studied to introduce a strain-rate dependency. To preserve numerical efficiency the generalised Maxwell model, formulated in the strain-space, is taken as a basis. The non-linear viscoelastic behaviour is introduced by coupling the generalised Maxwell model with a pre-existing intralaminar matrix continuum damage model. The fact that the Maxwell model preserves the explicit scheme of the damage model leads to an efficient material model for impact simulations. This paper proposes a complete framework to implement the strain-rate sensitive damage model in an explicit finite element code (for low-speed impact simulations). For this purpose, the procedure of parameter identification, based on DynamicMechanical Analysis, is given. Furthermore, a challenging experimental procedure on high-speed jack device with a particular attention paid to the consistency of the results is proposed to validate the developed model.

Computational micromechanics of fiber kinking in unidirectional FRP under different environmental conditions

The determination of ply properties of Fiber Reinforced Polymers (FRP) for particular operational environmental conditions in aeronautical applications is mandatory in order to fulfill current industry stringent certification requirements. However, the traditional experimental approach requires massive investments of resources and time. From the behaviour obtained experimentally, constitutive equations including failure criteria are then devised to be used in the design of FRP structures. The ply longitudinal behaviour under compression is generally the most difficult to measure and characterize. In this work, an alternative coupled experimental-computational micromechanics approach is proposed to determine the longitudinal compression properties of unidirectional FRP plies under different environmental conditions. This methodology includes experimental characterization of matrix and fiber/matrix interface, combined with numerical simulations of realistic microstructures. The interface decohesion is simulated using cohesive-frictional interactions. A pressure dependent, elasto-plastic model that includes tensile damage is employed to capture the matrix nonlinear behaviour. The numerical predictions match the experimentally-obtained ply properties available in the literature in a remarkable way and suggest that virtual ply property characterization is a mature and reliable approach to conduct screening of materials.

Machinability of Continuous-Discontinuous Long Fiber Reinforced Polymer Structures

In the present paper a new material system which combines the properties of continuous and discontinuous fibers (CoDiCoFRP) is investigated. The machinability and the induced damage effects of six material variations are investigated for a milling process at various cutting speeds and feed rates with an uncoated cemented carbide tool. The resulting forces and cutting torque were measured and analyzed for each material. After the milling process, the surface layers of the specimens were analyzed to quantify damage introduced during the milling process. The results show a strong influence of the material structure on the machining forces and damage.

Study on Performance of Fiber Reinforced Concrete Composite Beam Structure

FRP (fiber reinforced polymer material) having a high specific strength and specific modulus, good corrosion resistance and other advantages. FRP materials in civil engineering industry has been more and more popular, and gradually become a hot issue in the world. In order to take full advantage of a variety of materials and overcome the problems in the FRP structure, this paper mainly studies the composite structure of FRP and traditional materials, namely FRP-concrete composite beam structure. The mechanical parameters of FRP (mainly including CFRP and GFRP) were selected. And the stress -strain diagram of FRP materials are drawn. Through tensile tests on FRP (including CFRP and GFRP), FRP was found to belong to brittle materials. As well as the mechanical properties of FRP materials, the ultimate load analysis, the decision to use CFRP as a composite beam structural stiffness of the research materials. When considering concrete shrinkage, creep, temperature difference effect, the stiffness of composite beam meets the requirement. The deflection of FRP- concrete composite beam is verified by mechanical formula. The change of the concrete stiffness will affect the change of the structural stiffness of the FRP- concrete composite beam. As well as through an example, it is found that the concrete shrinkage and temperature can affect the change of the stiffness of the composite structure.

Fiber-reinforced polymers with integrated shape memory alloy actuation: an innovative actuation method for aerodynamic applications

This contribution focuses on the application potential of active fiber-reinforced polymer (FRP) structures with integrated shape memory alloy (SMA) elements for new aerodynamic functions. The advantages of hybrid SMA FRP structures are highlighted, and promising application concepts are discussed. To illustrate the applicability of the technology, the developed manufacturing, design, and control approaches are presented. Main focus is the development of a new concept for an active aerodynamic airfoil and the realization of a hardware demonstrator. Beginning with the idea of an adaptive airfoil, able to bear an application relevant down force at a relatively high deflection, the design process starts with an evaluation of different actuation concepts. An SMA-powered cantilever is a part of the profile itself. Applying the finite-element method with a suitable model for the active hybrid material, an effective selection of material and design is possible. After manufacturing and assembling of the active hybrid airfoil, a comparison of experimental results and simulation is the first proof of success. Finally, the installation of an integrated hardware setup with power source, control, and the active hybrid structure, demonstrating actuation on demand, verifies the potential and the distinct advantages of the new approach using SMA FRP structures.

Assessment of embedded fiber Bragg gratings for structural health monitoring of composites

This work provides a system-level investigation into the use of embedded fiber Bragg grating optical sensors as a viable sensing architecture for the structural health monitoring of composite structures. The practical aspects of the embedding process are documented for both carbon fiber–reinforced polymer and glass fiber–reinforced polymer structures manufactured by both oven vacuum bag and vacuum-assisted resin transfer method processes. Initially, embedded specimens were subject to long-term water submersion to verify performance in an underwater environment. A larger, more complex jointed specimen was also fabricated with a fully embedded sensor network of fiber Bragg gratings and subjected to incrementally induced bearing damage. Using commercially available interrogation hardware, a damage detection structural health monitoring algorithm was developed and deployed. The results permit statistically precise detection of low levels of connection damage in the composite specimen.

Improving the performance of bolted joints in composite structures using metal inserts

As the use of fibre reinforced polymer materials in bridge construction is becoming more popular, appropriate joining techniques, particularly for field joints, are necessary. Bolted joints are a common method for joining fibre reinforced polymer structures. The main advantage of bolted joints is their detachability, but they have a number of shortcomings. On the one hand, hole clearances, which are needed to facilitate on-site assembly, reduce the stiffness and joint efficiency. On the other hand, it is not possible to rely on the beneficial effects of bolt pre-defined tension loads, i.e. load transfer in friction, due to the considerable losses of bolt tension caused by the creep deformation in the composite material. To tackle these problems, a solution utilising metallic inserts in the hole is proposed in this paper. A series of experimental tests have been conducted to investigate the effect of inserts on the bolt-tension relaxation, the stiffness and the load-bearing behaviour of joints. Finite element analyses were also employed. The study demonstrates several benefits of the inserts: the bolt tension relaxation is minimised, the load transfer by friction may be feasible to be utilised in the bridge service state and the joint efficiency is increased in terms of stiffness and strength.

Feature-selective clustering for ultrasonic-based automatic defect detection in FRP structures

Ultrasonic inspection technique has been commonly used in defect detection of fiber reinforced polymer (FRP) materials, which transmits sound waves into materials and analyses the reflection signals. Although a variety of signal processing methods have been applied to highlight the defect features contained in ultrasonic signals, the most usual way to identify the defective regions is still not automatic, which is not only time-consuming but also critically dependent upon operator's experience. In order to solve this problem, a feature-selective unsupervised clustering method is adopted for automatic defect detection. By adaptively choosing the useful subset of features, the sizes and locations of the defective regions can be identified accurately, with the defect depths estimated at the same time. The feasibility of the proposed method is illustrated by the experiment on a carbon fiber reinforced polymer (CFRP) specimen constructed with known artificial defects by resin transfer molding.

Static and fatigue performance of resin injected bolts for a slip and fatigue resistant connection in FRP bridge engineering

This paper presents test results to evaluate the slip and fatigue performance of Resin Injected Bolted Joints (RIBJs) for pultruded Fibre Reinforced Polymer (FRP) material. The objective of the test series is to provide a robust method of connection for structural engineering that is both fatigue and slip resistant. Forty-six joints (using 23 specimens) were subjected to either static or combined static/cyclic loading at ambient room temperature. Ten specimens (five batches of two) had bolted connections without injected resin and were included to provide baseline static joint strengths. Sikadur®-30 and RenGel®-SW404 were the two cold-curing epoxy based resins used to fabricate the 13 RIBJ specimens. Testing was conducted with double lap-shear joints in accordance with modified guidance from Annex G and Annex K in standard BS EN 1090-2:2008. The specimen's geometry was established using this British Standard and an American Society of Civil Engineers pre-standard for pultruded thin-walled structures. Rectangular plates for the lap joints were cut from either a wide flange section of size 254×254×9.53mm or a flat sheet of 6.35mm thickness. Bolting was with either M16 or M20 steel threaded bolts of Grade 8.8. Sixteen specimens, for eight batches of two specimens were failed in a short duration for static strength. Four RIBJ specimens had static load cycling to an assumed service load level. Three specimens out of 23 were subjected to stage static and cyclic fatigue loadings to determine stiffness changes, life-time ‘slip’ load and residual joint strength. The reported results are evaluated for slip and fatigue performance and the main finding is that resin injection shows much promise as a mechanical method of connection in pultruded FRP structures.