Glass Fiber Reinforced Plastic Laminates Research Articles

Improvement of the mode II interface fracture toughness of glass fiber reinforced plastics/aluminum laminates through vapor grown carbon fiber interleaves

The effects of acid treatment, vapor grown carbon fiber (VGCF) interlayer and the angle, i.e., 0° and 90°, between the rolling stripes of an aluminum (Al) plate and the fiber direction of glass fiber reinforced plastics (GFRP) on the mode II interlaminar mechanical properties of GFRP/Al laminates were investigated. The experimental results of an end notched flexure test demonstrate that the acid treatment and the proper addition of VGCF can effectively improve the critical load and mode II fracture toughness of GFRP/Al laminates. The specimens with acid treatment and 10 g m−2 VGCF addition possess the highest mode II fracture toughness, i.e., 269% and 385% increases in the 0° and 90° specimens, respectively compared to those corresponding pristine ones. Due to the induced anisotropy by the rolling stripes on the aluminum plate, the 90° specimens possess 15.3%–73.6% higher mode II fracture toughness compared to the 0° specimens. The improvement mechanisms were explored by the observation of crack propagation path and fracture surface with optical, laser scanning and scanning electron microscopies. Moreover, finite element analyses were carried out based on the cohesive zone model to verify the experimental fracture toughness and to predict the interface shear strength between the aluminum plates and GFRP laminates.

COMPUTATIONAL ANALYSIS OF EFFECT OF STACKING ORIENTATION ON LOW VELOCITY IMPACT BEHAVIOR OF GFRP COMPOSITE LAMINATE

In recent years, Glass Fiber-reinforced plastic (GFRP) composite materials have been increasingly replacing conventional metallic materials in structural applications, due their excellent specific strength and stiffness properties. However, GFRP laminate having weak interfaces in normal to loading direction and they are very susceptible to impact loading. The damage of composite structures to impact events is one of the most important aspects of study to uncover the significance and the more dominant damage characteristics in terms of impactor mass, impact velocity, impact energy and different stacking orientations. This paper presents the effect of stacking sequence on low velocity impact behavior of GFRP composite laminate.

Effects of Grinding Process Parameters and Coolants on the Grindability of GFRP Laminates

Grinding is one of the mandatory methods for fabrication of precise glass fiber reinforced plastic (GFRP) parts. In this work, grinding studies were carried out on GFRP using a cubic boron nitride (CBN) wheel. The grinding process was performed at different cutting speeds, feed rates, and depth of cuts. Grinding performance was evaluated by measuring forces and surface roughness of the ground GFRP composite laminate material. The effects of coolants in grinding the composite laminate material were investigated. Test results showed that in most cases, dry grinding achieved the greatest material removal rate and the lowest surface area roughness (Sa). Higher speed tends to produce better surface finish in dry grinding. Surface area roughness produced in wet grinding shows random and drastic variations. Surfaces ground in synthetic coolant show clean, hard, and smooth matrix, and are in general of better quality than those ground with emulsion coolant. Optical and electron microscopy was used to evaluate the grinding mechanism of GFRP laminate material. Surface profile waviness is the predominating factor contributing to surface area roughness over surface peaks and valleys.

Modal Strain Energy Based Damage Detection Applied to a Full Scale Composite Helicopter Blade

The use of composites in the aircraft industry has generated a great need for structural health monitoring and damage detection systems, to allow for safer use of complex materials. Such is the case with helicopter blades - these components nowadays are mostly composed of carbon fiber or glass fiber reinforced plastics laminates, epoxy and honeycomb filled core structures. The use of composite materials on the main rotor blade also allows for more complex and efficient shapes to be designed, but at the same time, their use requires an additional effort when it comes to structural monitoring, since damage can occur and go unnoticed. This work presents experimental results for structural health monitoring method based on strain energy. The test subject is a full-scale composite helicopter main rotor blade, which is a highly flexible, slender beam that can display unusual dynamic behavior with orthotropic behavior. This damage detection method is based on the modal strain properties, and a damage detection index is used to identify and quantify damage. A test setup was built to carry out an experimental modal analysis on the main rotor blade. For that purpose, a total of 55 uniaxial accelerometers were used on the helicopter blade to measure the displacement modes of the structure. To compute the strain modes from the displacement modes, central differences approximation is used. Damage is introduced on the blade by attaching a small mass to two different locations. Experimental results show the possibility of locating damage in this case.

Detection of Delamination in GFRP and CFRP by Microwaves with Focusing Mirror Sensor

A technique to detect delamination in composite materials by noncontact, rapid and high sensitive microwave reflectometry with a focusing mirror sensor was proposed. The focusing mirror sensor, which has high sensitivity and resolution, is expected to detect delamination sensitively. In this paper, the ability of microwave inspection to detect delamination in glass fiber reinforced plastic (GFRP) and carbon fiber reinforced plastic (CFRP) was verified. As the results, the existences of 100 μm thick delamination in 3 mm thick GFRP laminate and 2 mm thick CFRP laminate were detected.

Modelling and analysis of factors influencing surface roughness and delamination of milling of GFRP laminates using RSM

PurposeThe purpose of this paper is to develop a mathematical model for surface roughness and delamination through response surface methodology (RSM) and analyse the influences of the entire individual input machining parameters (cutting speed, fibre orientation angle, depth of cut and feed rate) on the responses in milling of glass fibre reinforced plastics (GFRP) composites with solid carbide end mill cutter coated with PCD.Design/methodology/approachFour factors, five level central composites and a rotatable design matrix in response surface methodology were employed to carry out the experimental investigation. “Design Expert 8.0” software was used for regression and graphical analysis of the data were collected. The optimum values of the selected variables were obtained by solving the regression equation and by analyzing the response surface contour plots. Analysis of variance (ANOVA) was applied to check the validity of the model and for finding the significant parameters.FindingsThe developed second order response surface model was used to calculate the surface roughness and delamination of the machined surfaces at different cutting conditions with the chosen range with 95 per cent confidence intervals. Using such a model, remarkable savings in time and cost can be obtained.Originality/valueThe effect of fibre orientation during milling of GFRP laminates using RSM has not been previously attempted for analysis.

Impact Damage Detection in GFRP Laminates through Ultrasonic Imaging

Composite materials are increasingly used in aerospace, naval and automotive vehicles due to their high specific strength and stiffness. In the area of Non destructive testing, ultrasonic C-scans are used frequently to detect defects in composite components caused during fabrication and damage resulting from service conditions. Ultrasonic testing uses transmission of high frequency sound waves into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo and through transmission wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections. Under low-velocity impact loading delaminating is observed to be a major failure mode. This report presents the use of above two techniques to detect the damage in glass fiber reinforced plastic (GFRP) laminates. Pulse echo is used to locate the exact position of damage and through transmission is used to know the magnitude of damage in composite. This paper work will be carried out on two different thicknesses and at impact energy levels varying from 7 to 53J. The ensuring delamination damage will be determined by ultrasonic C-scans using the pulse-echo immersion method for through transmission. Delamination areas were quantified accurately by processing the raw image data using a digital image processing technique. Based on the data obtained, correlation will be established between the delamination area and the impact energy.

Identification of Failure Modes in Composites from Clustered Acoustic Emission Data Using Pattern Recognition and Wavelet Transformation

Acoustic emission (AE) is widely used to characterize damage occurring in composite materials: however, the discrimination between AE signatures due to different damage mechanisms is still an open issue. In this study, the various failure mechanisms in bidirectional glass/epoxy laminates subjected to uni-axial tension are identified using AE monitoring. AE data recorded during the tensile testing of a single-layer specimen are used to identify matrix cracking and fiber failure. In contrast, delamination signals are characterized using a two-layer specimen with a pre-induced defect, produced by artificially inserting a 10 mm wide Teflon tape in the middle portion of the two layers. Twelve-layer Glass fiber reinforced plastics laminates were also tested as a reference for the comparison of results. The procedure leading to signal discrimination involves a number of steps. First, Fuzzy C-means clustering associated with principal component analysis are used to discriminate between failure mechanisms, while parametric studies using AE count rate and cumulative counts allowed damage discrimination at various stages of loading. The two above methods led to AE waveform selection: on the selected waveforms, fast Fourier transform (FFT) enabled calculating the frequency content of each damage mechanism. Continuous wavelet transform (WT) allowed identifying frequency range and time history for failure modes in each signal, while noise content associated with the different failure modes is calculated and removed by discrete WT. Short time FFT (STFFT) finally highlighted the possible failure mechanism associated with each signal.

Lifetime prediction of woven GFRP laminates under constant tensile loading in hydrothermal environment

This study aims to investigate the effects of a hydrothermal environment on the creep behavior of woven glass fiber reinforced plastics (GFRPs) and to propose a method for predicting their lifetime. Toward this end, experiments were carried out in air and deionized water at 40, 60, 80 and 95 ∘C. Static tensile tests of woven GFRP were conducted in air and in deionized water to evaluate its mechanical properties and to determine suitable experimental conditions for subsequent constant tensile load tests. The mechanical properties of the woven GFRP decreased with an increase in temperature and with water immersion. Constant tensile load tests were also conducted in air and in deionized water to investigate the creep behavior and fracture time. The fracture time decreased with an increase in stress and water temperature and demonstrated the possibility of a threshold stress for fracturing. In addition, the fracture time during each constant tensile load test was predicted using a modified Reiner–Weissenberg (R-W) criterion, which is a failure criterion for linear viscoelastic materials based on the accumulation of dissolved energy within the GFRP. In this study, the R-W criterion was modified to consider the effects of degradation and its acceleration, which are due to the applied stress and immersion in a solution. The predicted results were in good agreement with the experimental data when considering the effects of hydrothermal aging.

NDE inspection of terahertz waves in wind turbine composites

An investigation of terahertz waves was made for the nondestructive evaluation of composite materials and structures in the wind turbine. The modalities of the terahertz radiation used were time domain spectroscopy (TDS) and continuous wave (CW). The composite materials and structures investigated include both non-conducting polymeric composites and conducting carbon fiber composites. Terahertz signals in the TDS mode resembles that of ultrasound; however, unlike ultrasound, a terahertz pulse was able to detect a smaller crack hidden behind a larger crack. This was demonstrated in thick GFRP (glass fiber reinforced plastics) laminates of the wind turbine using saw slots. In carbon composites the penetration of terahertz waves is quite limited and the detection of flaws is strongly affected by the angle between the electric field direction of the terahertz waves and the intervening fiber directions. The structures used in this study included both solid laminates and the parts in the wind turbine. The defects and anomalies investigated by terahertz radiation were foreign material inclusions, simulated disband, delamination and mechanical impact damage. The effectiveness and limitations of terahertz radiation are defined and noted for the NDE of composites.

Numerical modelling of GFRP laminates with MSC.Marc system and experimental validation

The study considers numerical modelling and simulation of static processes, including progressive failure, for GFRP (glass fibre reinforced plastic) laminates, of a mixed sequence of laminas reinforced with E-glass plain weave fabric or E-glass mat. The laminate has been manufactured on polyester matrix. The purpose of the research is to determine the options/values, recommended in engineering calculations, of parameters for numerical modelling and simulation of static processes including failure for beam, plate and shell structures built of composites undertaken, using FE code MSC.Marc. The effective elastic and strength constants of homogeneous laminas have been determined experimentally at room temperature according to standard procedures. The 3-point bending test of composite beams has been conducted experimentally and simulated numerically. The experimental validation of numerical modelling and simulation of the bending test is positive in both the quasi-linear and the progressive failure zones.

Experimental Investigation of Failure Mechanisms in GFRP Lap Joint using Acoustic Emission

In this paper, Acoustic Emission (AE) based damage mode identification of Glass Fibre Reinforced Plastic (GFRP) lap joints is detailed using laboratory scale experiments. Bonded single lap joint specimens are fabricated from GFRP laminates. These specimens are subjected to tensile test with on-line AE monitoring. The study of failure mechanisms is facilitated by the choice of different oriented lap joint specimens in which one or two such mechanisms predominate. Range of peak frequencies in each orientation is investigated using frequency analysis. Fast Fourier Transforms enabled calculating the frequency content of each damage mechanism. Parametric plots are used to discriminate the different stages of occurrence of failure mechanism in lap joints. Results have shown that predominate of failure modes in each orientation is used as a key in the study of discrimination of failure modes from AE data in lap joints.

Flexural behaviour of glass fibre-reinforced plastic laminates with drilled hole

Drilling is one of the most commonly used process for hole making. Drilling of polymer matrix composites (PMCs) causes substantial damage around the drilled hole. Drilling-induced damage not only decreases the strength of the composite laminate with hole, but it also deteriorates the long-term performance of the PMC laminates under different loading conditions. In the present research investigation, the flexural behaviour of the glass fibre-reinforced plastic laminates with drilled hole was experimentally investigated under three-point loading conditions. The results of the experimental investigation were compared with those of the finite element model and were found to be in close agreement.

Compressive Behavior of Glass Fiber Reinforced Plastic Laminates with Drilled Hole

The polymer matrix composites (PMCs) are increasingly being used in structural applications owing to their superior mechanical properties. The in-service performance of composite materials requires them to perform under a variety of loading environments. Although a lot of research work has been done towards the mechanical characterization of PMCs, a lot remains to be done in special cases such as characterization of glass fiber reinforced plastics (GFRP) laminates with drilled hole. It is an established fact that drilling in composite laminates causes damage around the drilled hole and the damage influences the mechanical behavior of GFRP laminates under different loading conditions. The present investigation aims at studying the compressive behavior of GFRP laminates with drilled holes. The holes have been drilled at various input parameters (cutting speed, feed rate and drill point geometry) and the effect of these parameters on the compressive behavior of the GFRP laminates has been investigated using statistically designed experiments. It has been found that the drilling parameters significantly affect the compressive behaviour of laminates with drilled holes.

Experimental and numerical study of fastener pull-through failure in GFRP laminates

An experimental and numerical study of the fastener pull-through failure mode in glass–fiber reinforced plastic (GFRP) laminates using both phenolic and vinylester resins is presented. It is shown that the type of resin does not affect the mechanical response of the joint when a pull-through test is performed because similar values of the sub-critical initial and final failure loads are obtained. Moreover, considering that the joint is considering to fail when the sub-critical failure load is reached, a methodology to predict the pull-through failure mode is proposed. It is observed that the main failure mechanism is the delamination of the plies; therefore, the prediction of the sub-critical initial failure load is performed using a three-dimensional finite element model where cohesive elements are used to simulate delamination. The predictions agree remarkably well with the experimental results.

An experimental assessment of open-hole tension–tension fatigue behaviour of a GFRP laminate

The drive to improve predictive models and design methodologies of fibre-reinforced polymer composites subjected to fatigue loading is reliant on more accurate and reliable methods for monitoring and assessing material degradation and fatigue life. This paper presents the results of an experimental study carried out to evaluate the applicability of various measurement techniques including digital image correlation (DIC) and multiplexed fibre Bragg grating (FBG) sensors for this purpose. The study focused on the open-hole tension (OHT) fatigue behaviour of a quasi-isotropic glass fibre-reinforced plastic (GFRP) laminate under constant and block amplitude loading. Constant amplitude tests were conducted at two different R-ratios ( R = σ min/ σ max) and the block amplitude tests involved two or more stress levels. Longitudinal strain and stiffness, and surface temperature are used to assess damage progression and fatigue life. These parameters are shown to be interrelated and potentially suitable for predicting notch fatigue performance.

Delayed Fracture of GFRP Laminates under Constant Tensile Load in Hydrochloric Acid

This paper aims to clarify the delayed fracture mechanism for glass fiber reinforced plastics (GFRP) in corrosive environments. The GFRP under study is composed of plain NCR-glass cloth and vinylester resin, which both possess high corrosion resistance. In this study, the experimental conditions were performed in air, deionized water, and hydrochloric acid at 40°C. Static tensile tests of woven GFRP were performed to evaluate the mechanical properties and determine the experimental conditions for the constant tensile load tests in each environment. The mechanical properties of the woven GFRP decreased with its immersion into deionized water and hydrochloric acid. The stress-strain curve decreased intensely after the knee point especially in hydrochloric acid, which is possibly because of the damage accumulation generated by the solution and applied stress. Constant tensile load tests of the woven GFRP were performed to investigate the creep behavior and fracture time in each environment. The strain and strain rate increased in the tests in deionized water and hydrochloric acid, which are the result of decrease in the stiffness owing to immersion in each solution. In addition, delayed fracture occurred in deionized water and hydrochloric acid, and the lifetimes in hydrochloric acid were shorter than those in deionized water. Moreover, it was suggested from fracture surface observations that the delayed fracture of the woven GFRP under a constant tensile load in a corrosive environment is dominated by degradations in the fiber reinforcement and fiber/matrix interface.

Fatigue behaviour of a GFRP laminate by thermographic measurements

Composite materials are widely used to build structural components, thanks to their mechanical properties. Those are generally considered ‘engineering materials’, since they are tailored to meet specific requirements. Due to their use for structural components, it is important to know their mechanical behaviour, especially under cyclic loads. At present, there is a common interest, among researchers, to study the mechanical behaviour of composites, by means of both traditional and innovative techniques, with the final purpose of making previsions regarding their service life. In fact, due to their composite nature, they behave in a different mode compared to homogeneous materials. This study is focused on a glass fibre-reinforced plastic (GFRP); the aim of this work is to study its fatigue behaviour, from both the mechanical and the thermal points of view. The main reason is that there is a lack of knowledge, in the literature, about the fatigue of composites. In this study, a GFR laminate was characterized under static and dynamic loading conditions; during the experimental tests, thermal measurements were carried out by means of an IR-thermal camera. Temperature measurements were done during the static tests, whereas in the dynamic tests the dissipated energy was measured, by using the dissipation method (D-mode). Then, various criteria for fatigue life estimation were applied fitting the experimental data. Since different thermographic techniques have been used to estimate the fatigue behaviour, a final comparison between the experimental data and the predicted fatigue behaviour is proposed and discussed, showing a good agreement.

Environmental stability of GFRP laminated composites: an emphasis on mechanical behaviour

PurposeThe main objective of this experimental investigation is to assess the effect of thermal and cryogenic treatment on hygrothermally conditioned glass fibre reinforced epoxy matrix composites, and the impact on its mechanical properties with change in percentage of individual constituents of the laminates.Design/methodology/approachThe present investigation is an attempt at evaluating the performance of the laminates subjected to different thermal and cryogenic treatments for varying time with prior hygrothermal treatment. The variability of hygrothermal exposure is in the range of 4‐64 h. Glass fibre reinforced plastics laminates with different weight fractions 0.50‐0.60 of fibre reenforcements were used. The ILSS, which is a matrix dominated was studied by three‐point bend test using INSTRON 1195 material testing machine.FindingsThe post‐hygrothermal treatments (both thermal and cryogenic exposures) resulted in an increase in the rate of desorption of moisture. It is noted that the hygrothermal treatment prior to the exposure to thermal or cryogenic conditioning is the major attribute to the variations in the ILSS values. The extent of demoisturisation of the hygrothermally conditioned composites due to a thermal or a cryogenic exposure is observed to be inversely related to its ILSS, independent of the fibre‐weight fractions. Also the ILSS is inversely related to the fibre‐weight fraction irrespective of the post‐hydrothermal treatment.Originality/valueThe reported data are based on experimental investigations.

Solar and Environmental Degradation Effect on Tensile Strength and Glass Transition Temperature of Glass Fibre Reinforced Plastic Laminates

In this study, solar and environmental degradation were performed for glass fibre/epoxy and glass fibre/vinyl ester composites. Tensile and differential scanning calorimetry (DSC) tests were applied before and after degradations. Considering tensile strength, it is concluded that vinyl ester composite samples used in this study were more durable against environmental degradation compared to the epoxy based samples. On the contrary, epoxy based samples seemed to be more durable than those of the vinyl ester under solar degradation. Moreover, the glass transition temperature loss % of both two resins was very low after solar degradation. It was observed that the destructive effect of environmental degradation was more dominant than solar degradation, according to measurements of tensile strength and glass transition temperature values.