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

Simultaneous spectral and spatial information of any object investigated through electromagnetic radiation leads to the concept of spectral imaging technique (SIT): in terahertz (THz) range, becaus...

A Review of the Concept, Applications and Implementation Issues of Terahertz Spectral Imaging Technique

Characterization of image sequences of a defect using pulsed eddy current signals

Layered adhesive structures are widely used in the aerospace field, and thus their defect detection is a critical part of quality control. Hence, this paper proposes a method to identify bonding defects relying on dynamic time warping (DTW) and simulation analysis. Based on the transfer matrix method, the propagation simulation model of the layered structure is established, and the simulation signal of the normal area and the characteristic interval of the defects are obtained. DTW is adopted to regularize the simulated and the real signals, and then the Pearson correlation coefficient is employed to calculate the regularized signals’ similarity. On this basis, the proposed scheme is considered a similarity imaging method, attaining an appealing identification error for the three- and the five-layered adhesive structure cases examined involving various defect thicknesses and glue layers. Compared to current imaging methods, the proposed one demonstrates a better and more effective imaging effect appropriate for bonding defect detection. 

Defect identification of layered adhesive structures based on dynamic time warping and simulation analysis

Pores are inevitably produced during the production of glass fiber reinforced polymers (GFRP). Quantitative characterization of porosity is a most important aspect of performance evaluation of GFRP. Herein, we present a novel strategy for porosity detection of GFRP based on the interaction mechanism between terahertz (THz) wave and porous GFRP (porosity: 0.29%–4.01%; pore size: about 20–600 μm). By using the transmission and absorption spectra of GFRP, a porosity prediction model is established by combining the supervised learning approach of support vector regression (SVR) and ensemble methods, which can predict the porosity of unknown test samples with a coefficient of determination R2 = 0.976 and root mean square error RMSE = 0.174%. Conversely, THz transmission and absorption spectra for porous GFRP are successfully reconstructed by SVR and the ensemble methods. The results indicate that THz spectroscopy in combination with SVR and the ensemble methods is robust and accurate in porosity analysis and could play a significant role in industrial applications where nondestructive on-line detection of porosity in polymer composites is desirable. 

Xiong Weihua

Papers

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