Glass Fiber Reinforced Polymer Samples Research Articles

Dynamic Time Analysis of Glass Fiber Reinforced Polymer – Numerical Simulation and Experiment

Composites are known for their significant damping, however, both reinforcement type or layered arrangement influence the response of material: mechanical, thermal, electrical etc. The paper provides the dynamic time analysis of glass fiber reinforced polymer samples of various layups. The mechanical response to dynamic excitation was tested by numerical simulation and experiment. The excitation function was the unit impulse force generating free damped vibrations. Experimental results were evaluated in time domain to obtain damping parameters. For numerical simulation, the commercial software was used to visualize deformed model in individual damping phases, Mises stress distribution in individual laminate layers, and comparison of that for different layups.

Nondestructive evaluation of GFRP composite including multi-delamination using THz spectroscopy and imaging

We utilize a fiber-coupled terahertz time-domain spectroscopy system (THz-TDS) based on photoconductive antennas to visualize multi-delamination and their thickness in a glass-fiber-reinforced polymer (GFRP) composite plate. We simulate a hidden multi-delamination by inserting a Teflon film in the GFRP composite plate that is fabricated using the vacuum-assisted resin transfer molding (VARTM) process. Reflected or transmitted terahertz (THz) waves are recorded from GFRP samples mounted on the x–y linear motorized stage during the investigation. The x–y linear motorized stage is driven in steps of 0.2 mm, thereby allowing us to scan the sample. We examine and compare the performances of the THz time-domain visualization (TTV) and THz frequency-domain visualization (TFV) algorithms to evaluate their characteristics for the visualization of defects. The thickness of the GFRP sample and multi-delamination is estimated using the reflection geometry method. Finally, we confirm that the hidden multi-delamination of the GFRP sample has been successfully visualized in the B- and C-scans. In addition, the thicknesses of the GFRP sample and the simulated delamination as compared with the experimental values exhibit a close consistency.

A NEW DIGITAL IMAGE CORRELATION SOFTWARE FOR DISPLACEMENTS FIELD MEASUREMENT IN STRUCTURAL APPLICATIONS

Abstract. Recently, there has been a growing interest in studying non-contact techniques for strain and displacement measurement. Within photogrammetry, Digital Image Correlation (DIC) has received particular attention thanks to the recent advances in the field of lowcost, high resolution digital cameras, computer power and memory storage. DIC is indeed an optical technique able to measure full field displacements and strain by comparing digital images of the surface of a material sample at different stages of deformation and thus can play a major role in structural monitoring applications. For all these reasons, a free and open source 2D DIC software, named py2DIC, was developed at the Geodesy and Geomatics Division of DICEA, University of Rome La Sapienza. Completely written in python, the software is based on the template matching method and computes the displacement and strain fields. The potentialities of Py2DIC were evaluated by processing the images captured during a tensile test performed in the Lab of Structural Engineering, where three different Glass Fiber Reinforced Polymer samples were subjected to a controlled tension by means of a universal testing machine. The results, compared with the values independently measured by several strain gauges fixed on the samples, demonstrate the possibility to successfully characterize the deformation mechanism of the investigated material. Py2DIC is indeed able to highlight displacements at few microns level, in reasonable agreement with the reference, both in terms of displacements (again, at few microns in the average) and Poisson’s module.

Quantitative comparison of optical transmission scanning with conventional techniques for NDE of impact damage in GFRP composites

Abstract Robust and quantitative non-destructive evaluation (NDE) is essential for damage characterization in advanced materials, such as glass fiber reinforced polymer (GFRP) composites. Previous work demonstrated the capabilities of an optical transmission scanning (OTS) system, including an advanced image processing algorithm, for rapid and non-contact NDE of healthy and impacted GFRP samples. The number of delaminations and their contours in the test samples were accurately identified. In this paper, damage was introduced in GFRP samples by low velocity impacts of different energies, and OTS was validated by comparing the inspection results with conventional NDE techniques, including pulse-echo ultrasonic testing (UT) and UV dye penetrant testing (UV DPT). In addition, quantitative comparisons of damage severity were made for OT scans and UT C-scans by converting images to a normalized damage metric. Experimental results demonstrated that the estimated extent of impact damage agreed very well for all used techniques. Thus, OTS combines the cost-effectiveness and rapidity of qualitative techniques (e.g., UV DPT) with the more robust and comprehensive data analysis provided by quantitative methods (e.g., UT).

Post Processing of Thermographic NDT Data Using Dirac Comb

In the present study, a new method is proposed for the post processing of the thermographic non-destructive testing (TNDT) data. In this method, the temperature data of the thermograms is first converted into a pulse train using the Dirac comb. The pulse train is then transformed to the frequency domain using the Discrete Fourier Transform (DFT). The real part of the transformed data has a number of cycles; showing the shifting and duality property of the DFT. At the beginning as well as at the end of the cycles the contrast of the data is sharp enough to form images showing distinctly the defect and the sound areas of the sample. The method is named as DFT of Comb Data, in short DCD. The technique showed promising results even for small (O5mm) defects located at the depth of 10 mm below the surface. Results from the DCD method are compared with the two of the established post processing techniques, namely the Pulsed Phase Thermography (PPT) and the Thermographic Signal Reconstruction (TSR). An industrial grade long wave infrared (LWIR) camera, FLIRTM T440, is used in the present study. Three samples with synthetic defects are tested here that include, two samples of glass fibre reinforced polymer (GFRP) and one sample of honeycomb. The GFRP samples are 18 and 8.5 mm thick and have defects in the form of flat bottomed holes. The honeycomb sample has three square shaped debonding defects between its aluminium core and one of the face-sheets (1.5 mm thick Carbon Fibre Reinforced Polymer - CFRP).

Thermal cycling effects on the durability of a pultruded GFRP material for off-shore civil engineering structures

This paper investigates the effects of thermal cycles on the structural integrity of a pultruded Glass Fibre Reinforced Polymer (GFRP). Through a critical review of current literature alongside a comprehensive experimental campaign, the material’s response to cyclic thermal loading has been ascertained, defined by the rate of degradation of its physical, mechanical and visco-elastic properties. Matching sets of both dry and soaked samples conditioned in distilled water for 224days. Freeze–thaw cycling of both dry and soaked samples was conducted between 20°C and −10°C for a total of 300 cycles. Computed Tomography scanning (CT-scan) was undertaken to assess the microstructural physical changes throughout freeze–thaw cycling. After exposure, GFRP samples exhibited a minor decrease in glass transition temperature (Tg) which indicated minor structural degradation. Dry GFRP sample’s mechanical response exhibited negligible changes in both tensile and in-plane shear properties. However, as a result of the higher induced thermal stresses, soaked samples showed a significant degradation of their tensile and shear strengths. Yet, the soaked material’s stiffness remained largely unaffected due to the potential reversible nature of plasticization, which acts to increase the material’s molecular mobility when initially moisture-saturated, but is later recovered as the soaked samples lose moisture throughout freeze–thaw cycling.

High-speed frequency-domain terahertz coherence tomography.

High-speed frequency-domain terahertz (THz) coherence tomography is demonstrated using frequency sweeping of continuous-wave THz radiation and beam steering. For axial scanning, THz frequency sweeping with a kHz sweep rate and a THz sweep range is executed using THz photomixing with an optical beat source consisting of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using coherent homodyne detection employing signal averaging for noise reduction and used as axial-scan data via fast Fourier transform. Axial-scan data are acquired while scanning a transverse range of 100 × 100 mm(2) by use of a THz beam scanner with moving neither sample nor THz transmitter/receiver unit. It takes 100 s to acquire axial-scan data for 100 × 100 points with 5 averaged traces at a sweep rate of 1 kHz. THz tomographic images of a glass fiber reinforced polymer sample with artificial internal defects are presented, acquired using the tomography system.

High-speed terahertz reflection three-dimensional imaging using beam steering.

High-speed terahertz (THz) reflection three-dimensional (3D) imaging is demonstrated using electronically-controlled optical sampling (ECOPS) and beam steering. ECOPS measurement is used for scanning an axial range of 7.8 mm in free space at 1 kHz scan rate while a transverse range of 100 × 100 mm(2) is scanned using beam steering instead of moving an imaging target. Telecentric f-θ lenses with axial and non-axial symmetry have been developed for beam steering. It is experimentally demonstrated that the non-axially symmetric lens has better characteristics than the axially symmetric lens. The total scan time depends on the number of points in a transverse range. For example, it takes 40 s for 200 × 200 points and 10 s for 100 × 100 points. To demonstrate the application of the imaging technique to nondestructive testing, THz 3D tomographic images of a glass fiber reinforced polymer sample with artificial internal defects have been acquired using the lenses for comparison.

Advanced birefringence measurements in standard terahertz time-domain spectroscopy.

Polarization-sensitive (PS) terahertz (THz) technology can be used for investigating anisotropic materials that are opaque for visible light. A full characterization of an anisotropic material requires the extraction of the birefringence as well as the orientation of the optical axis from the measurement data. We present an approach based on THz time-domain spectroscopy (TDS) that exploits the spectral content of the THz signal for determining these two parameters from only two measurements. In contrast to an earlier approach with a more sophisticated PS-THz system and quasi-circularly polarized THz radiation, now a simple standard THz-TDS system can be employed. After a description of the mathematical model for data analysis we demonstrate the applicability of our method for a lithium niobate crystal and furthermore for a glass-fiber reinforced polymer sample, for which the orientation of the optical axis and birefringence are obtained in a spatially resolved way, showing the potential of the method also for PS-THz imaging. As no specialized setup or components are required, our approach can be easily and extensively applied for the analysis of anisotropic samples at THz frequencies.

Effects of reinforcement configuration and densification on impact strength of wood veneer/polyester composites

Current energy absorbers in industrial applications are made of metals or fiber-reinforced polymers using glass and carbon fibers. These materials are extremely stiff and strong but exhibit low-energy absorption when subject to the impact load. Other issues in the use of these materials are their high cost (fiber-reinforced polymer) and weight (metal). Wood reinforcements on the other hand are light weight and economic but less stiff. This study investigated the impact resistance and fracture patterns of wood-reinforced polyester composites using a drop-weigh impact test and considers the potential of using wood as a natural reinforcement in the manufacturing of polymer composites. Densified and un-densified Douglas-fir veneers were used to create three different mat configurations: woven, cross, and unwoven (unidirectional) mats. A total of 350 specimens were tested following ASTM D5420, and their impact resistance was calculated using the staircase method. Scanning electron microscopy was used to examine the resin distribution and its penetration into the reinforcement. Additionally, light micrographs of the veneers before and after densification were examined to determine the effect of densification on the cell-wall structure. Glass fiber-reinforced polymer samples had significantly higher impact resistance than the wood composites. Densification of the veneer did not significantly improve the composite performance. The effect of reinforcement configuration on the final performance of the wood–polyester composites, however, was significant with the woven, and cross configurations having notably higher impact energy than unidirectional composites.

Investigation of glass–fibre reinforced polymers by polarisation-sensitive, ultra-high resolution optical coherence tomography: Internal structures, defects and stress

In this paper, we report on optical coherence tomography (OCT) for the investigation of glass–fibre reinforced polymer (GFRP) materials. OCT, a contactless and non-destructive imaging technique, provides depth-resolved information on internal structures within semi-transparent materials. In our study, we apply a highly advanced OCT technique: we use a transversal scanning, ultra-high resolution, polarisation-sensitive (UHR-PS)-OCT for the analysis of various GFRP samples. UHR-imaging with resolution in the micron range is shown to provide information on the scale of single fibres. PS-OCT, an extension of classical OCT, is sensitive to birefringence and consequently to anisotropies and stress within a material. Our system is therefore applied to the investigation of defects located within the fibre layers of loaded and damaged GFRP samples. Imaging of matrix fracture, cracks, and internal stress, demonstrates the high potential of UHR-PS-OCT for the investigation of damage in GFRP materials, with special emphasis on detecting the early stages of defect formation.

Durability of glass‐fiber‐reinforced polymer nanocomposites in an alkaline environment

AbstractThis study was conducted to determine the effect of clay content in GFRP (glass‐fiber‐reinforced polymer) composite samples as they were aged in an alkaline solution. Two kinds of GFRP composite samples were prepared. One was E‐glass‐fiber‐reinforced vinyl ester polymer, and the other was nano‐GFRP composites prepared with the addition of 1 and 2 wt% of montmorillonite clay to the polymer matrix. These samples were aged in alkaline solution of pH 13.2 with and without sustained load. The load was 1335 N or 18.7% of the tensile strength of the composite samples. The aging was evaluated by measuring the reduction in tensile strength after 6 months. Also, absorption of alkaline solution into the plain and nano‐GFRP samples was investigated so as to elucidate the diffusion behaviors. It was found that for a short exposure time (e.g. 1 month) and without sustained load, dispersing 2 wt% of the nanoclay in the polymer matrix of the GFRP samples reduces the diffusivity by 39%. However, with the application of sustained load, the glass fiber composite samples deteriorate more with increasing clay content. The reduction in tensile strength was 7.1%, 12.1%, and 18.1% for the samples containing 0, 1, and 2 wt% of clay, respectively. J. VINYL. ADDIT. TECHNOL., 12:25–32, 2006. © 2006 Society of Plastics Engineers

Mechanical/bonded joints for advanced composite structures

The strengths of mechanical-connected and adhesive-bonded joining systems are combined to develop an improved connection system for advanced composite materials. The principle of resistance enhancement is based on the insertion of small diameter pins through an adhesive joint. The pins act as local reinforcement in the most highly stressed regions of the joint, enabling an adhesive joint to carry an increased load prior to failure. In a series of single-lap shear experiments using glass fibre reinforced polymer samples, adhesive-only and pinned-only specimens failed in the joint region. In contrast, many of the pinned/bonded specimens failed away from the joint at a significantly higher load. Insight was gained regarding the influence of the pin material, diameter, number and configuration on the resulting performance. The results demonstrated the potential of pinned/bonded systems as a new method for connecting composite materials. However, further work is necessary to confirm the sensitivity of the results to changes in parameters such as the fibre architecture, adherend geometry, adhesive interface and joint configuration.

Mechanical/bonded joints for advanced composite structures

The strengths of mechanical-connected and adhesive-bonded joining systems are combined to develop an improved connection system for advanced composite materials. The principle of resistance enhancement is based on the insertion of small diameter pins through an adhesive joint. The pins act as local reinforcement in the most highly stressed regions of the joint, enabling an adhesive joint to carry an increased load prior to failure. In a series of single-lap shear experiments using glass fibre reinforced polymer samples, adhesive-only and pinned-only specimens failed in the joint region. In contrast, many of the pinned/bonded specimens failed away from the joint at a significantly higher load. Insight was gained regarding the influence of the pin material, diameter, number and configuration on the resulting performance. The results demonstrated the potential of pinned/bonded systems as a new method for connecting composite materials. However, further work is necessary to confirm the sensitivity of the results to changes in parameters such as the fibre architecture, adherend geometry, adhesive interface and joint configuration.

Measurements of the clamped capacitance C o and the electromechanical coupling coefficient K t of ceramic transducers under mechanical load

Robust and quantitative non-destructive evaluation (NDE) is essential for damage characterization in advanced materials, such as glass fiber reinforced polymer (GFRP) composites. Previous work demonstrated the capabilities of an optical transmission scanning (OTS) system, including an advanced image processing algorithm, for rapid and non-contact NDE of healthy and impacted GFRP samples. The number of delaminations and their contours in the test samples were accurately identified. In this paper, damage was introduced in GFRP samples by low velocity impacts of different energies, and OTS was validated by comparing the inspection results with conventional NDE techniques, including pulse-echo ultrasonic testing (UT) and UV dye penetrant testing (UV DPT). In addition, quantitative comparisons of damage severity were made for OT scans and UT C-scans by converting images to a normalized damage metric. Experimental results demonstrated that the estimated extent of impact damage agreed very well for all used techniques. Thus, OTS combines the cost-effectiveness and rapidity of qualitative techniques (e.g., UV DPT) with the more robust and comprehensive data analysis provided by quantitative methods (e.g., UT).