Fiber Reinforced Polymer Structures Research Articles

Image enhancement methods for inspection of planar and non-planar FRP structures using a noise-based microwave NDT inspection system

Microwave nondestructive testing (MNDT) includes inspection techniques that assess a particular material’s health status using low-power and contactless inspection systems. In near-field microwave inspections, imaging results are heavily influenced by the standoff distance parameter, i.e., the physical separation between the microwave sensor and the sample under test (SUT). Variations in the standoff distance during an inspection tend to cause defect masking of disbonds and delaminations in fiber-reinforced polymer (FRP) materials, causing defects to go undetected frequently. An MNDT near-field inspection system using noise waveforms is used to identify engineered internal defects within carbon fiber-reinforced polymer (CFRP) samples. Tactics utilizing Principal Component Analysis (PCA), Stacked Sparse Autoencoders (SSAEs), and an autoencoder network trained in a manner for anomaly detection are used to minimize the effects of standoff distance, reduce defect masking, and increase the ability to identify hidden defects. The samples tested are constructed to possess planar and non-planar geometries, such that the viability of the data-driven image enhancement and standoff distance correction methods are demonstrated with respect to a wide variety of in-situ inspection applications.

Axial Impact Response of Carbon Fiber-Reinforced Polymer Structures in High-Speed Trains Based on Filament Winding Process.

The continuous increase in the operating speed of rail vehicles demands higher requirements for passive safety protection and lightweight design. This paper focuses on an energy-absorbing component (circular tubes) at the end of a train. Thin-walled carbon fiber-reinforced polymer (CFRP) tubes were prepared using the filament winding process. Through a combination of sled impact tests and finite element simulations, the effects of a chamfered trigger (Tube I) and embedded trigger (Tube II) on the impact response and crashworthiness of the structure were investigated. The results showed that both triggering methods led to the progressive end failure of the tubes. Tube I exhibited a mean crush force (MCF) of 891.89 kN and specific energy absorption (SEA) of 38.69 kJ/kg. In comparison, the MCF and SEA of Tube II decreased by 21.2% and 21.9%, respectively. The reason for this reduction is that the presence of the embedded trigger in Tube II restricts the expansion of the inner plies (plies 4 to 6), thereby affecting the overall energy absorption mechanism. Based on the validated finite element model, a modeling strategy study was conducted, including the failure parameters (DFAILT/DFAILC), the friction coefficient, and the interfacial strength. It was found that the prediction results are significantly influenced by modeling methods. Specifically, as the interfacial strength decreases, the tube wall is more prone to circumferential cracking or overall buckling under axial impact.

A Proposed Non-Destructive Method Based on Sphere Launching and Piezoelectric Diaphragm.

This work presents the study of a reproducible acoustic emission method based on the launching of a metallic sphere and low-cost piezoelectric diaphragm. For this purpose, tests were first conducted on a carbon fiber-reinforced polymer structure, and then on an aluminum structure for comparative analysis. The pencil-lead break (PLB) tests were also conducted for comparisons with the proposed method. Different launching heights and elastic deformations of the structures were investigated. The results show higher repeatability for the sphere impact method, as the PLB is more affected by human inaccuracy, and it was also effective in damage detection.

Models analyzing the response of the adhesive joint between FRP and concrete: a comparative analysis

Fiber-reinforced polymers (FRP) for structural elements’ strengthening and rehabilitation are increasingly gaining popularity because of their proven efficiency and ease of application. The response of the adhesive joint formed between the FRP and concrete is a crucial factor influencing the overall behavior of FRP structures. The analysis of the interface crack propagation is a complex task involving the consideration of multiple interacting mechanisms. This work aims to provide a background to a recently proposed model that analyzes the behavior of the adhesive joint formed at the interface between the FRP and concrete, by creating its counterpart. The study presented herein discusses a model based on fracture mechanics to obtain a reference point needed to assess the output provided by an algorithm utilizing a damage-based model for concrete and an empirical-based procedure for the loss of bond action. Typically, model validation is performed via a comparison against experimental data. However, additional insights into the interacting phenomena can be provided by comparing the results of various applicable models.

Understanding the Effect of Drilling Parameters on Hole Quality of Fiber-Reinforced Polymer Structures.

Hole quality in composite materials is gaining interest in aerospace, automotive, and marine industries, especially for structural applications. This paper aims to investigate the quality of holes performed without a backup plate, in thin plates of glass fiber-reinforced polymer (GFRP). The samples were manufactured by two different technologies: vacuum bagging and an innovative method named vacuum mold pressing. Three experiments were designed choosing the control factors that affect the maximum cutting force, delamination factor, and surface roughness of drilled holes in composite materials based on twill fabric layers. Quality analysis of the hole features was performed by microscopy investigations. The effects of the main factors on the targets are investigated using the statistical design of experiments, considering control factors, such as support opening width, weight fraction (wf), feed per tooth, and hole area. The results showed that the feed per tooth and hole area had a more significant influence on the delamination factors and surface roughness (Sa). The best quality of the holes drilled in twill-based GFRP was achieved for a lower feed rate of 0.04 mm/tooth and used a support opening width of 55 mm.

Design, manufacturing and experimental validation of additive manufacturing cores for bearing seats in carbon fibre reinforced polymer structures

The integration of carbon-fibre reinforced polymers (CFRP) in structural applications offers significant advantages due to their high strength-to-weight ratio. However, these materials exhibit limitations under out-of-plane loads, particularly in bearing applications. This study explores an innovative approach to enhance the performance of CFRP structures in such scenarios by incorporating annular polyamide inserts manufactured via additive manufacturing (AM).To evaluate the mechanical performance of the AM inserts, a novel ring tensile test is designed to emulate the bearing load conditions. This test also enables the analysis of the impact of several design and manufacturing parameters of the AM inserts, including surface geometry, surface treatment, internal structure, and curing process. These are then compared with specimens made of carbon fibre sheet moulding compounds (CF-SMC), commonly used to support bearing loads. The study reveals that AM inserts provide a viable alternative to state-of-the-art CF-SMC, offering a significant enhancement in mechanical properties under specific bearing loading conditions. The test results indicate a 17.7% improvement in the first failure limit and an 8.6% increase in the ultimate strength for AM inserts compared to CF-SMC.Additionally, the study develops a simplified analytical model to predict stress distributions and potential failure mechanisms, validating its efficacy through experimental data with discrepancies of less than 6%. Economic analysis underscores the cost benefits of AM inserts due to reduced labour and higher repeatability. This research demonstrates the potential of AM inserts to improve the stability and strength of CFRP structures, paving the way for their broader application in demanding load-bearing environments.

A micromechanical study on sand − FRP interface subjected to cyclic loading

Fibre-reinforced polymer (FRP) is a promising composite to be used in construction in coastal and marine environments to resist seawater corrosion that deteriorates the properties of conventional civil engineering materials. A classical ground or seabed − structure interaction problem that is involved in the design of FRP structures, is however less understood when the soft nature of FRP is in subjection to the cyclic loadings from traffic, wind, wave and currents, causing penetration and abrasion at the soft FRP − soil interface. This study has downscaled the preceding problem into a micromechanical study at a benchmark sand grain − FRP interface. A large number of cyclic loading is applied, for the first time, at the sand − FRP composite interface, focusing on the development of the elastoplastic behaviour in the normal direction and the evolution of friction and energy dissipation in the tangential direction. The study combines the understanding from the tribology with the knowledge of civil engineering involved in the sand − FRP interaction, suggesting that a larger stick zone at the contact subjected to cyclic shearing is a key triggering of the simultaneous occurrence of the increased coefficient of friction and reduced damping ratio.

The mechanical characteristics of an aluminum foam winding CFRP composite structure under axial compression

To enhance the energy absorption properties of the energy-absorbing structure, carbon fiber-reinforced polymer (CFRPs) with higher specific energy absorption and porous material aluminum foam with better compressive characteristics were organically combined, and a lighter aluminum foam winding carbon fiber-reinforced polymer structure (CFRP-FA-FW) was designed. Through quasi-static compression testing, the deformation mode and energy absorption properties of CFRP-FA-FW under axial load were examined. The energy absorption and specific energy absorption of CFRP-FA-FW are both increased by 113.55 % and 60.73 %, respectively, compared to the simple composite structure CFRP-FA. Finite element simulation was used for the parametric analysis of the CFRP-FA-FW structure to assess the effects of the relative density of the aluminum foam, the fiber lay-up angle, and the thickness. The results reveal that the change in the relative density of aluminum foam has little impact on the failure deformation mode of CFRP-FA-FW under axial load; the structure has a higher energy absorption capacity and a smoother energy absorption process when the fiber lay-up angle is [0°/90°]ns and [45°]ns; the energy absorption capacity of CFRP-FA-FW is significantly improved by increasing the thickness of the carbon fiber lay-up, and the procedure is also more efficient.

An interlaminar fatigue damage model based on property degradation of carbon fiber-reinforced polymer composites

Interface delamination is a significant damage mechanism in fiber-reinforced polymer (FRP) laminated composites caused by interlaminar normal and shear stresses. This is due to the low interlaminar properties including interface stiffness and strength that continuously degrade to cause fatigue crack nucleation and growth. In this respect, an interlaminar fatigue damage model is proposed to accurately address the reliability aspect of the FRP composite materials and structures. The model considers a damage process that consists of two stages: (1) fatigue damage evolution due to interlaminar properties degradation to the onset of crack nucleation. A cyclic cohesive zone model with a bilinear softening behavior is introduced along with the fatigue life model to capture the mean stress effect of the interface. This is followed by (2) the crack nucleation process leading to the physical separation/debonding of the interface material point, as governed by the cycle-dependent fracture energy dissipation. The mechanical prediction of the interlaminar fatigue damage process is illustrated through finite element simulation of a mixed-mode flexural test of a carbon fiber-reinforced polymer (CFRP) composite beam subjected to fatigue loading. The results indicate that the interlaminar normal and shear crack opening displacement increases exponentially with the larger number of load cycles. In addition, the high-stress gradient is limited to the interface crack front zone with the normal and shear stress peaks at 69.2 and 16.5 % of the respective interlaminar strength. Moreover, controlled interlaminar crack growth is initially foreseen at 7.0 × 10-6 mm/cycle, followed by an abrupt crack “jump” at 287,000 load cycles.

An anisotropic topology optimization procedure for continuous fiber reinforced polymer structures with biaxial fiber layout for improved intersection point design

Fiber placement technologies allow for manufacturing of complex-shaped composite parts with load-adapted fiber orientation. For design of corresponding structures, topology optimization is well-established. However, anisotropic topology optimization approaches are often limited to unidirectional fiber orientations. In truss-like topology optimized structures, the intersections between the individual trusses are subject to multiaxial stress states. For those, a unidirectional fiber orientation is not appropriate and may not be suitable for manufacturing.In this work we propose a topology optimization procedure that allows for unidirectional but also biaxial fiber orientations. The choice between unidirectional and biaxial fiber layout is based on the stress state within the finite elements. For the biaxial fiber layout, the first fiber angle is aligned with the maximum absolute principal stress direction, while the second fiber orientation is calculated using composite net theory.Results show that considering biaxial fiber orientations leads to fiber orientations that can be better manufactured by fiber placement technologies and are considered suitable for automatic derivation of manufacturing-tolerant placement paths. Furthermore, the stress state at intersections is improved reducing the probability of inter fiber failure. In addition, consideration of biaxial fiber orientations could increase the stiffness at constant weight compared to purely unidirectional topology optimization.

Vibration-based prediction of residual fatigue life for composite laminates through frequency measurements

Fiber reinforced polymer (FRP) structures may experience cumulative fatigue damage during their service life which can lead to structural failure. This paper focuses on predicting the residual fatigue life of FRP structures using vibration parameters. The relationship between residual fatigue life and natural frequencies is examined through modal testing and fatigue measurements on FRP beam specimens. Two prediction methods; semi-empirical models and machine learning (ML) algorithms, are utilized. The semi-empirical models are derived from existing “residual stiffness” models based on the relationship between bending stiffness and flexural frequencies. ML algorithms Support Vector Machine (SVM) and Artificial Neural Network (ANN), are developed for fatigue life prediction. Experimental validation is performed using measured frequencies during fatigue testing of FRP beams. The ML algorithms can use multimode frequencies unlike single mode of semi-empirical models. The verification results show that the ML algorithms can be used to predict the residual fatigue life with the selection of the appropriate mode of frequency. The results show that ML algorithms outperform single-mode frequency inputs, and the use of higher modes of measured frequencies improves the precision of fatigue life prediction. An inverse algorithm based on SVM exhibits higher prediction accuracy and stability, even with limited training samples.

A novel damage detection method for carbon fibre reinforced polymer structures based on distributed strain measurements with fibre optical sensor

This paper presents a novel approach for damage detection in carbon fibre reinforced polymer (CFRP) structures based on local strain feature matching. The proposed approach utilizes distributed in-plane strain measurements for plate structures under load (in-service application) to define a local point autocorrelation coefficient based on Delaunay triangulation and Hausdorff distance. This coefficient effectively identifies the discontinuities in distributed in-plate strain, enabling accurate capture of damage information through the damage index which is defined as the integral of this coefficient. Notably, the proposed approach requires no prior knowledge of the non-damaged strain distribution and yields excellent detection results for both Barely Visible Impact Damage (BVID) and Visible Impact Damage (VID). Leveraging the advantages of distributed strain sensing, coupled with image processing technology for the purpose of in-service damage detection of CFRP structures, the proposed novel method is a promising technology. The efficacy of this technology has been demonstrated in this paper through theoretical analysis and experimental investigations, offering valuable insights and reference for future research in this domain.

Flexible photonics in carbon and glass fiber reinforced polymers for new multifunctionality: Exploring the advances, challenges, and opportunities

Flexible photonics, characterized by their planar design and integrated features, have surfaced as a promising technology to unlock new possibilities for multifunctionality within fiber reinforced polymer composite materials. A comprehensive review of current progress, challenges, and opportunities associated with flexible photonic integration into carbon and glass fiber reinforced polymers is provided. A systematic examination of the literature has revealed several flexible photonic technologies that have demonstrated potential for integration in composite components to monitor performance in manufacture, service, and reuse. The review highlights the advantages and limitations of the current state-of-the-art in flexible integrated photonics for making assessments of compatibility with carbon and glass fiber reinforced polymer structures. By examining proof-of-concept demonstrations, the improved performance and novel functionalities that can be achieved for industrial applications are identified. The challenges associated with the integration process, such as durability and scalability are discussed in the context of the manufacturing processes required to create composite components. The concept of integrating flexible photonics in composite structures is relatively new, hence the paper closes by highlighting opportunities for further research and development in this field.

FRP–soil interfacial mechanical properties with molecular dynamics simulations: Insights into friction and creep behavior

AbstractThe fiber‐reinforced polymer (FRP) has attracted much attention in civil engineering due to its durability and cost‐effectiveness. The soil–FRP structure interface plays an essential role when the FRP is adopted in geotechnical engineering, but its fundamental interfacial behavior remains unclear. In the present study, the atomistic models of silica representing sand, water film representing the lubricated condition, and cross‐linked epoxy representing FRP are constructed to investigate the FRP–sand interfacial properties at the nano‐scale through molecular dynamics (MD) simulations. The epoxy model geometry and forcefield are first validated by comparing the thermodynamic and mechanical parameters with experimental and simulation measurements. The silica–epoxy interfacial tribological and rheological behavior is then explored by conducting friction and creep simulations under dry and lubricated conditions. The friction force has been found linearly dependent on the normal load and increases with the sliding velocity while decreasing against water content. The modified Amontons law for the adhering surface could describe the silica–epoxy interfacial friction well. The shear stress level influences the creep characteristics with primary, secondary, and tertiary (or rupture) creep modes. The results of this study at the nano‐scale can be further developed to enhance the current contact laws of sand–FRP structure in micromechanics‐based modeling approaches.

Direct embedment of RVE-based microscale into lab size coupons to research fracture process in unidirectional and bidirectional composites

The structural applicability of fibre-reinforced polymer (FRP) composites requires testing for their resilience against fracture. The paper presents a virtual tool to configure the microscale geometrical model constituting the representative volume element embedded in the homogeneous macroscale region, providing an alternative to the tedious experimental procedures. The conformal meshing strategies in the microregion are also elucidated in the paper. The microscale model is enriched with the constitutive material models exhibiting complex phenomena of individual constituents of the composite and cohesive damage properties. The elastoplastic and elastic–viscoplastic material models coupled with the energy-based and rate-dependent damage initiation criteria, respectively, are applied to the matrix. At the same time, the fibre-matrix interface is treated with the weak traction–separation law. Through the virtual tool, the microscale region allows to make an insightful study of the toughness mechanism and visualise the damage initiation and propagation in the FRP structure at different fracture modes.

Mechanical properties of FRP materials at elevated temperature – Definition of a temperature conversion factor for design in service conditions

One of the main concerns about the use of fibre-reinforced polymer (FRP) materials in civil engineering structures is the potential reduction of their stiffness- and strength-related properties when exposed to elevated temperature. This concern, mostly due to the glass transition process underwent by polymeric matrices, needs to be duly reflected in design standards. In the scope of the development of the European Technical Specification CEN/TS 19101: 2022, “Design of Fibre-Polymer Composite Structures”, this paper presents the definition of a temperature conversion factor for in-service conditions to take into account the reduction of FRP mechanical properties with temperature. The first part of the paper briefly describes the basis of design of CEN/TS 19101: 2022 and the framework to consider the effects of environmental conditions on material properties. The second part presents an assessment of recommendations about the reduction of mechanical properties with temperature in existing design guidelines for FRP structures. The third part describes a survey of test data available in the literature concerning mechanical tests at elevated temperature of FRP materials produced with different fibres, resins, shapes and manufacturing methods. The survey shows that the provisions available in existing design guidelines are not always precise and, in some cases, they are non-conservative. Based on this assessment, the fourth and final part of the paper presents the method used to define the values of the temperature conversion factor included in CEN/TS 19101: 2022, which is consistent with the partial factor method of the Eurocodes. The method takes into account (i) the maximum service temperature experienced by the FRP material, (ii) its glass transition temperature, and (iii) the type of mechanical property, namely if it is either fibre- or matrix-dominated. The proposed temperature conversion factor values proved to be adequate considering the test data available in the literature.

The effect of model dimensionality on compression strength of fiber reinforced composites

Strength of fiber reinforced polymers (FRPs) under compression loads is typically limited by a shear localization failure mode called microbuckling which is highly sensitive to fiber misalignment. In addition to the magnitude of fiber misalignment, the dimensionality of fiber misalignment also plays a prominent role in the prediction of compression strength. Therefore, a comparison between 1D, 2D, and 3D fiber misalignment is carried out in a finite element setting with a homogenized representation of fiber and matrix materials. In real FRP structures, fiber misalignment is spread in a correlated random manner throughout the material volume resulting in a distribution of compression strength. Spectral representation method is used for developing the volumetric representation of fiber misalignment in numerical models, thus preserving the spatial correlations of fiber misalignment. As an input to the spectral representation method, two different functional forms of spectral density of the fiber misalignment are considered. The results of model series based on functional forms of spectral density are also compared against a reference model series based on experimental measurements of the fiber misalignment. Finally, a simple relation is proposed for prediction of compression strength at different percentiles of distribution of failure strengths with regard to scaling of mean square spectral density.

Influence of a Flat Polyimide Inlay on the Propagation of Guided Ultrasonic Waves in a Narrow GFRP-Specimen.

Structural health monitoring systems for composite laminates using guided ultrasonic waves become more versatile with the structural integration of sensors. However, the data generated within these sensors have to be transmitted from the laminate to the outside, where polyimide-based printed circuit boards play a major role. This study investigates, to what extent integrated polyimide inlays with applied sensor bodies influence the guided ultrasonic wave propagation in glass fiber-reinforced polymer specimens. For reasons of resource efficiency, narrow specimens are used. Numerical simulations of a damping-free specimen indicate reflections of the -mode at the integrated inlay. This is validated experimentally with an air-coupled ultrasonic technique and a 3D laser Doppler vibrometry measurement. The experimental data are evaluated with a method including temporal and spatial continuous wavelet transformations to clearly identify periodically occurring wave packages as edge reflections and distinguish them from possible inlay reflections. However, even when separating in-plane and out-of-plane movements using the 3D measurement, no reflections at the inlays are detected. This leads to the conclusion that polyimide inlays are well suited as substrates for printed circuit boards integrated into fiber-reinforced polymer structures for structural health monitoring, since they do not significantly influence the wave propagation.

Probabilistic fatigue life analysis considering mean stress effects of fiber reinforced polymer (FRP) composites

In general, the S-N curve, which describes the relationship between fatigue life and stress level with a typical guarantee rate, is employed during the design of FRP structures. However, the effect of mean stress on fatigue life is typically ignored. In this paper, a revision term is proposed to consider the mean stress effects on the basis of the common S-N equation. The third material parameter of the revised S-N-R equation increased the difficulty to predict the probabilistic fatigue life with the traditional analysis method during the design of FRP structures. Hence, a failure probability evaluation method is proposed to obtain the probabilistic fatigue life with three material parameters by using Markov chain Monte Carlo (MCMC) and Bayesian inference. The results show the applicability and effectiveness of the method in evaluating failure probabilities.

Electromechanical modeling of energy harvesting for FRP composite structures coupled with piezoelectric transducers

Supplanting of metals by composites is on the rise for the last three decades in the aerospace, marine and automotive industry following the trend of electrification and indigenous design approaches. In parallel, piezoelectric (PZT) sensors and energy harvesters have gained significant attention due to their applicability and efficacy for microscale power generation systems. From a new perspective, embedding PZT sensors into composite structures will be beneficial in many aspects. Condition monitoring can be performed by using the sensing capability of PZTs while vibration can be controlled by means of its excitation capability. Besides, energy harvesting can be employed due to the mechanical forces exerted on the coupled structure. It is critical to create an accurate numerical modeling of electromechanical coupling for the investigation of efficiency of PZT sensors. In this paper, electromechanical modeling of a Fiber Reinforced Polymer (FRP) composite structure with an embedded PZT patch is presented and validated with an experimental setup. Afterwards, the energy harvesting capability of a PZT patch embedded in the FRP structure is investigated.