The application of fiber-reinforced polymer (FRP) composites is continuously increasing due to their superior mechanical properties and the associated weight advantage. However, they are susceptible to more complex types of damage, and advanced damage characterization systems are required to prevent catastrophic failures. Various non-destructive testing and evaluation (NDT&E) and in-situ structural health monitoring (SHM) techniques have been applied for damage detection in FRP composites. These techniques have been continuously developed to achieve reliable inspections, especially for safety-critical applications such as the aerospace industry. This review presents recent advances in NDT&E techniques and SHM techniques, particularly for damage diagnosis in FRP composites. For selecting the most suitable NDT technique based on specific criteria, the analytical hierarchy process is applied as a decision-making tool to evaluate and rank the NDT techniques. The size of the specimen is found to be the most important criterion that significantly affects technique selection. Finally, the importance of developing in-situ SHM systems is outlined, and different in-situ SHM systems are then reviewed and discussed. This review provides progress of the recent damage characterization techniques and enables researchers to devise selection criteria to select the most appropriate technique for their own work.

Advancements in fiber-reinforced polymer composite materials damage detection methods: Towards achieving energy-efficient SHM systems

Applications of sandwich-structured composites gained increasing interest in aviation and aerospace industries as well as in modern lightweight design. In order to improve the reliability of computational models for the dimensioning of such structures, experimental data are indispensable prerequisites. In the current manuscript, the essential outcomes of experimental and numerical investigations of strain rate effects on the stiffness of honeycomb sandwich T-joints are presented. A series of static and dynamic tensile tests is carried out to understand the influence of different loading conditions on failure and fracture of honeycomb sandwich T-joints within strain rates up to 5000 s−1. A digital image correlation technique was employed to characterize the fracture behavior as well as to verify the strain gauges measurements. Failure modes of adhesively bonded sandwich T-joints are determined by a detailed fractographic analysis. In addition, numerical simulation was performed via three-dimensional finite element models using ABAQUS software, which exhibit excellent agreement with experimental results.

Influence of strain rate on fracture behavior of sandwich composite T-joints

Inspection of concrete structures externally reinforced with FRP composites using active infrared thermography: A review

Nondestructive testing of bonding defects in multilayered ceramic matrix composites using THz time domain spectroscopy and imaging

Automatic defect prediction in glass fiber reinforced polymer based on THz-TDS signal analysis with neural networks

Nondestructive imaging of hidden defects in aircraft sandwich composites using terahertz time-domain spectroscopy

Using differential spread laser infrared thermography to detect delamination and impact damage in CFRP

Super-Resolution (SR) techniques for image restoration have recently been gaining attention due to their excellent performance. For powerful learning abilities, Generative Adversarial Networks (GANs) have been proven to have achieved great success. In this paper, we propose an Enhanced Generative Adversarial Network (EGAN) for improving its effects for a real-time Super-Resolution task. The main content of this paper are as follows: (1) We adopted the Laplacian pyramid framework as a pre-trained module, which is beneficial for providing multiscale features for our input. (2) At each feature block, a convolutional skip-connections network, which may contain some latent information, was significant for the generative model to reconstruct a plausible-looking image. (3) Considering that the edge details usually play an important role in image generation, a perceptual loss function was defined to train and seek the optimal parameters. Quantitative and qualitative evaluations were demonstrated so that our algorithm not only took full advantage of the Convolutional Neural Networks (CNNs) to improve the image quality, but also performed better than other algorithms in speed and performance for real-time Super-Resolution tasks.

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Single Image Super-Resolution Method Based on an Improved Adversarial Generation Network

To provide a more effective auxiliary tool for maintenance and inspection in small space, a miniature camera optical system is designed. According to the real work situation, a small camera structure with 25 mm long and 16 mm of the focal length has been formed in this paper. At the same time, Zemax software is utilized to analyze the image quality of the camera lens. The results show that the field curvature is less than 0.1 in optical band and the maximum distortion is less than 0.5%.The RMS and GEO of the image formed under the selection of height condition is small. Under the condition of different temperature and pressure, the image quality can be ensured and the MTF of this system is greater than 0.35 which meets the design requirements. This structure can be a reference structure for the optical part of a maintenance support system.

The design for the optical part of a maintenance support system

The radome composing of GFRP with sandwich structure is easy to generate impact damage, core material shedding and fiber delamination during the service causing the structural strength and the performance of the waves to fall. Then it affects the normal operation of the radar antenna system. The characteristics of terahertz wave contain the low photon energy, the strong penetrating ability of dielectric material, the transient, broadband and coherence. The simulation experiment of reflective NDT (nondestructive testing) of overlapped multi-delamination defects in the skin of the radome was realized by terahertz wave in CST STUDIO SUITE software simulation platform. The energy, reflection and transmission coefficient of THz wave were analyzed in time domain and frequency domain, and a relatively ideal result had been obtained by using the principle of THz-TDS technology. The paper verified a favorable effect of the related technology of THz at NDT and the exploration of material characteristics on composites with sandwich structure.

Hongbin Zhou

Papers

Nondestructive imaging of hidden defects in aircraft sandwich composites using terahertz time-domain spectroscopy

Using differential spread laser infrared thermography to detect delamination and impact damage in CFRP

Single Image Super-Resolution Method Based on an Improved Adversarial Generation Network

The design for the optical part of a maintenance support system

Simulation Study on Nondestructive Evaluation of Glass Fiber Reinforced Polymer with Sandwich Structure of Radome by Terahertz