Microwave NDT of Smart Composite Structures with Embedded Antennas

Glass fiber-reinforced polymer (GFRP) is widely used in engineering fields involving aerospace, energy, transportation, etc. If internal buried defects occur due to hostile environments during fabrication and practical service, the structural integrity and safety of GFRP structures would be severely undermined. Therefore, it is indispensable to carry out effective quantitative nondestructive testing (NDT) of internal defects buried within GFRP structures. Along with the development of composite materials, microwave NDT is promising in non-intrusive inspection of defects in GFRPs. In this paper, quantitative screening of the subsurface impact damage and air void in a unidirectional GFRP via microwave reflectometry was intensively investigated. The influence of the microwave polarization direction with respect to the GFRP fiber direction on the reflection coefficient was investigated by using the equivalent relative permittivity calculated with theoretical analysis. Following this, a microwave NDT system was built up for further investigation regarding the imaging and quantitative evaluation of buried defects in GFRPs. A direct-wave suppression method based on singular-value decomposition was proposed to obtain high-quality defect images. The defect in-plane area was subsequently assessed via a proposed defect-edge identification method. The simulation and experimental results revealed that (1) the testing sensitivity to buried defects was the highest when the electric-field polarization direction is parallel to the GFRP fiber direction; and (2) the averaged evaluation accuracy regarding the in-plane area of the buried defect reached approximately 90% by applying the microwave reflectometry together with the proposed processing methods.

In recent years, pipeline operators face many threats to the integrity of pipelines and hence, sophisticated nondestructive testing (NDT) technologies and measuring devices are being used for diagnosing the pipeline conditions. The presence of various defects is almost common during manufacturing, handling and under servicing conditions of glass-fibre reinforced polymers (GRP) composite pipes. Among various defects, severe pits or wall loss defects can be internally produced after a certain period of time due to various practical reasons. The present study has been taken up to analyse theviability and effectiveness of thermal NDT techniquein the evaluation of pits in GRP pipes. The thermal contrast evaluation profiles are obtained from number of experiments conducted on GRP pipes with artificially induced pits and the effect of pit size and its penetrationare analysed. The results show that the largerand deeper pits can be detected faster with increased thermal contrast and vice-versa. The investigation shows that the method leads a simple approach in applying thermal NDT to evaluate pits in GRP pipes.

When GFRP (Glass Fiber Reinforced Polymer) plate was utilized as as a tensile reinforcing material and permanent formwork, it is possible to greatly reduce the construction period and construction costs. In order to understand the behavior of the composite structure of HPFRCC (High Performance Fiber Reinforced Cementitious Composites) and GFRP plate, local bond stress and slip relationship is required. In this study, flexural bonding experiments were performed and the results were analysed to study the bonding behavior between GFRP plate and cast-in-placed HPFRCC, and we propose a simple experimental bond stress and slip relationship that can be easily applied to the composite structures.

A composite structure multi-parameter sensor based on an embedded polarization maintaining fiber grating (PMFG) has been studied theoretically and experimentally. A fiber Bragg grating (FBG, for comparison with the PMFG) and a PMFG are embedded in glass fiber reinforced polymer (GFRP) laminates in the experiment. The temperature and stress response characteristics of the fiber gratings embedded in corresponding GFRP are studied. The temperature sensitivity of the FBG improves from 10.64pm/°C to 21.66pm/°C. The temperature sensitivity of the PMFG improves from 10.22m/°C to15.7pm/°C (x-axis) and from 9.56 pm/°C to15.1 pm/°C (y-axis). The peak wavelengths in the reflection spectra of the two gratings all shift linearly with the temperature and/or the stress of GFRP. The two temperature-wavelength sensitivities and the two stress-wavelength sensitivities of the PMFG are all different with each other. The temperature and the stress of the GFRP can be calculated according to the two peak wavelength (in x-axis and in y-axis) shifts of a PMFG. Therefore, one PMFG is component for monitoring the temperature and the stress of a composite structure simultaneously according to the experimental results.

This paper presents on the structural behavior of the the methyl methacrylate monomer (MMA) double wide flanged the glass fiber-reinforced polymer(GFRP) pipe composite structures for the manhole raise. The evaluation of structural performance on this composite structure was conducted by the axial load, fatigue load, and ultimate load test. The assessment indicates that the MMA double wide flanged GFRP pipe composite structures was confirmed safety, durability and reliability in result as expected. It was found that this composite structure was able to short working times to around 30-50% and construction costs to around 10-23% with compare other construction methods. Also, environmental pollution and civil complaints will be prevented because there will be no longer any noises, vibrations, dust, or construction wastes.

A new type of reinforced concrete slab wrapped with glass fibre reinforced polymer (GFRP) is proposed, which is composed of GFRP profile slab wrapped with reinforced concrete slab. In order to master the anti-explosion performance of the new composite slab structure under the action of explosion, the dynamic response results of the new wrapped glass fibre reinforced concrete slab structure and the ordinary concrete slab structure under the action of explosion are analyzed and compared by using ANSYS/LS DYNA and fluid structure coupling algorithm. The results show that the new wrapped glass fibre reinforced concrete slab structure, which through the sand bonding treatment of GFRP ribbed slab and inner surface, GFRP slab is closely combined with concrete, giving full play to the superposition effect of GFRP and concrete on explosion impact resistance, which has better explosion impact resistance than ordinary reinforced concrete slab structure, and provides important support for the application of GFRP reinforced concrete composite structure.

NDE of glass fiber reinforced composite structures using single sided solid-state proton NMR

This chapter reviews blast experiments that have been carried out on composite sandwich panels with glass-fiber reinforced polymer (GFRP) face-sheets, carbon-fiber reinforced polymer (CFRP) face-sheets and face-sheets containing a mixture of fiber fabrics. The panels were subjected to explosive charges ranging from 1.28 to 100 kg TNT equivalent during the air and underwater blast tests. The difference in panel response was recorded using high-speed photography and digital image correlation (DIC) during the air blast tests. More conventional instrumentation using strain gauges was required for the underwater blast tests. Following each experiment, the panel damage was analyzed and compared either visually or using X-ray computed tomography (CT) scanning. The addition of polypropylene (PP) Innegra interlayers into a GFRP front face-sheet has been shown to be advantageous during air blast loading. Due to the increased thickness of the front face-sheet, the panel deflects less and experiences less front face-sheet damage. The replacement of GFRP plies with aramid plies during underwater blast loading was shown to be detrimental to the panel performance. The panel suffered from more severe damage compared to a fully GFRP panel. The comparison of CFRP against GFRP panels during underwater blast revealed that the CFRP has a greater stiffness but this result in greater face-sheet debonding due to its lower strain to failure. These experiments have shown that an optimal blast resilience response could be achieved through the combination of different fiber fabrics.

The detection of impact and depth defects in Glass Fiber Reinforced Polymer (GFRP) composites has been extensively studied to develop effective, reliable, and cost-efficient assessment methods through various Non-Destructive Testing (NDT) techniques. Challenges in detecting these defects arise from varying responses based on the geometrical shape, thickness, and defect types. Long Pulse Thermography (LPT), utilizing an uncooled microbolometer and a low-resolution infrared (IR) camera, presents a promising solution for detecting both depth and impact defects in GFRP materials with a single setup and minimal tools at an economical cost. Despite its potential, the application of LPT has been limited due to susceptibility to noise from environmental radiation and reflections, leading to blurry images. This study focuses on optimizing LPT parameters to achieve accurate defect detection. Specifically, we investigated 11 flat-bottom hole (FBH) depth defects and impact defects ranging from 8 J to 15 J in GFRP materials. The key parameters examined include the environmental temperature, background reflection, background color reflection, and surface emissivity. Additionally, we employed image processing techniques to classify composite defects and automatically highlight defective areas. The Tanimoto Criterion (TC) was used to evaluate the accuracy of LPT both for raw images and post-processed images. The results demonstrate that through parameter optimization, the depth defects in GFRP materials were successfully detected. The TC success rate reached 0.91 for detecting FBH depth defects in raw images, which improved significantly after post-processing using Canny edge detection and Hough circle detection algorithms. This study underscores the potential of optimized LPT as a cost-effective and reliable method for detecting defects in GFRP composites.

Nondestructive Testing (NDT) assessment of materials’ health condition is useful for classifying healthy from unhealthy structures or detecting flaws in metallic or dielectric structures. Performing structural health testing for coated/uncoated metallic or dielectric materials with the same testing equipment requires a testing method that can work on metallics and dielectrics such as microwave testing. Reducing complexity and expenses associated with current diagnostic practices of microwave NDT of structural health requires an effective and intelligent approach based on feature selection and classification techniques of machine learning. Current microwave NDT methods in general based on measuring variation in the S-matrix over the entire operating frequency ranges of the sensors. For instance, assessing the health of metallic structures using a microwave sensor depends on the reflection or/and transmission coefficient measurements as a function of the sweeping frequencies of the operating band. The aim of this work is reducing sweeping frequencies using machine learning feature selection techniques. By treating sweeping frequencies as features, the number of top important features can be identified, then only the most influential features (frequencies) are considered when building the microwave NDT equipment. The proposed method of reducing sweeping frequencies was validated experimentally using a waveguide sensor and a metallic plate with different cracks. Among the investigated feature selection techniques are information gain, gain ratio, relief, chi-squared. The effectiveness of the selected features were validated through performance evaluations of various classification models; namely, Nearest Neighbor, Neural Networks, Random Forest, and Support Vector Machine. Results showed good crack classification accuracy rates after employing feature selection algorithms.

Concurrent multi-material and multi-scale design optimization of fiber-reinforced composite material and structures for minimum structural compliance

Fourier-domain (or spectral-domain) Infrared Optical Coherence Tomography (FDOCT) can penetrate into the scattering material non contactly and therefore it can be used to characterize and image the inner structure of material in a non-invasive manner. In the paper, glass-fibre reinforced polymer (GFRP) materials were quantitatively characterized by using home-made FDOCT system. In the developed FDOCT system, a 50W Tungsten halogen lamp and a high performance spectrometer are, respectively, employed as the light source and the detector. The broadband of the spectrometer and thermal light source results in the FDOCT system with high robust performance and an excellent depth resolution of 0.7 um. From the experimental results, the developed FDOCT system could be recommended in nondestructive testing or health monitoring of GFRP materials.

Composite structures have become increasingly popular in civil engineering due to many advantages, such as light weight, excellent corrosion resistance and high productivity. However, they still lack the strength, stiffness, and convenience of constructions of fastener connections in steel structures. The most popular fastener connections in steel structures are slip-critical connections, and the major factors that influence their strength are the slip factors between faying surfaces and the clamping force due to the prevailing torque. This paper therefore examined the effect that changing the following parameters had on the slip factor: (1) replacing glass fiber reinforced plastic (GFRP) cover plates with stainless-steel cover plates; (2) adopting different surface treatments for GFRP-connecting plates and stainless-steel cover plates, respectively; and (3) applying different prevailing torques to the high-strength bolts. The impact on the long-term effects of the creep property in composite elements under the pressure of high-strength bolts was also studied with pre-tension force relaxation tests. It is shown that a high-efficiency fastener connection can be obtained by using stainless-steel cover plates with a grit-blasting surface treatment, with the maximum slip factor reaching 0.45, while the effects of the creep property are negligible.

Wave propagation in glass fibre-reinforced polymer (GFRP) bars subjected to progressive damage—Experimental and numerical results

Glass fiber reinforced polymer (GFRP) composite laminates are considered the key material in many industries such as the infrastructure industries and the aerospace sector, and in building structures due to their superior specific strength and lightweight properties. The prediction of specimens’ nominal strength with open holes is still an attractive and questionable field of study. The specimen size effect is referred to its strength degradation due to the presence of holes when specimen geometry gets scaled. The non-destructive test used to measure the nominal strength of such material is a great tool for fast selection purposes, but not secure enough for several purposes. Furthermore, the destructive tests which are more expensive and time-consuming should be avoided in such structures. The present work aims to predict the nominal strength of open-hole GFRP’s composite using modal analysis of their natural frequency as non-destructive tests. At this end, the natural frequency, which is measured using modal analysis procedures, is combined with both linear elastic fracture mechanics (LEFM) and the theory of elasticity to predict the nominal strength of open-hole composite laminates. This advanced model employs two parameters of surface release energy resulting from a simple tension test and Young’s modulus based on vibration modal analysis. It is well established that these types of materials are also subjected to a size effect in dynamic response. Inversely to the known static loading size effect, the size effect in dynamic response increases with specimen size. The novel model gives excellent and acceptable results when compared with experimental and finite element ones. Size effects curves of a nominal strength of these laminates have a very close relative value with those obtained from finite element and analytical modeling. Moreover, the received design tables and graphs would be highly applicable when selecting suitable materials for similar industrial applications.