Nondestructive evaluation of forced delamination in glass fiber-reinforced composites by terahertz and ultrasonic waves

Abstract

Glass fiber-reinforced composite laminates in polyetherimide resin have been studied via terahertz imaging and ultrasonic C-scans. The forced delamination is created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Using reflective pulsed terahertz imaging, we find high-resolution, low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in three dimensions. Furthermore, terahertz imaging enables us to determine the thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic C-scan images are also successfully obtained; however, in our samples with small thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers. Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.

Introduction

Fiber-reinforced composites provide an alternative to conventional structural materials such as concrete, steel, aluminum, and wood. Used for structural purposes, fiber-reinforced composites have the advantage of combining a number of properties not usually found together in a single material. In particular they combine high strength and low weight, while at the same time they may be resistant to corrosion and have thermal and electrical insulation properties. As a result, the wide applicability of fiber-reinforced composites has created the need for correspondingly advanced nondestructive evaluation (NDE) techniques for inspection and failure detection during manufacturing and maintenance.

Various NDE techniques capable of characterizing damage and defects in fiber-reinforced composites have been developed. Among them, ultrasonic testing is the most well-known tool to characterize fiber composites, including ultrasonic C-scans [1], [2], [3], ultrasonic polar scans [4], [5], nonlinear ultrasonics [6], and guided-wave inspection [7], [8]. However, until now, only the ultrasonic C-scan technique has found widespread implementation in industry, because of simplicity of analysis and its effectiveness in geometrically locating damage and defects. In ultrasonic C-scans, the ultrasonic waves show specific transmission and reflection features depending on the spatial variation of acoustic impedance within the fiber-reinforced composite. Ultrasonic C-scans can provide a good trade-off between material penetration and measurement resolution, and ultrasonic C-scans in pulse-echo mode can also provide qualitative information in depth for thick fiber composite samples [9].

Because of certain limitations associated with ultrasonic C-scans (see below), there is growing interest in alternative imaging modalities and NDE techniques, such as shearography, IR thermography, and X-ray radiography, to name a few. Considerable work has been carried out to compare such approaches with ultrasonic C-scans, which stands as the reference standard for NDE in these materials [10], [11], [12], [13], [14], [15]. These comparisons highlight some of the difficulties associated with the ultrasonic C-scan technique in these materials: (1) negligible quantitative information in depth can be obtained in thin samples with small thickness-to-wavelength ratio due to the relatively large time duration of ultrasonic pulse signal; (2) because of the attenuation of ultrasonic waves in fiber-reinforced composites (especially in glass fiber-reinforced composites), the operating frequency cannot be sufficiently high (usually less than 10 MHz) [16], thus limiting the resolution; and (3) liquid coupling may be required. Although contactless ultrasonic techniques using laser [17], [18] and air-coupled transducers [19] have been proposed, problems (1) and (2) remain. Therefore, alternative nondestructive, noncontact, and nonionizing (to minimize health risks) techniques with relatively high resolution are still needed for inspection of fiber-reinforced composites.

As an alternative to ultrasonic waves, we here investigate the use of terahertz-frequency electromagnetic waves. The terahertz (THz) portion of the electromagnetic spectrum extends from approximately 100 GHz to 10 THz, and lies between the microwaves and infrared; the wavelength range in this region is 3 mm down to 3 μm. THz waves can penetrate numerous nonmetallic materials that may be opaque in the range of visible and infrared light [20]. Moreover, as nonionizing radiation, THz waves present minimal known health risks [21]. Due to these remarkable properties of THz wave, THz imaging was firstly introduced to NDE of fiber-reinforced composites in 2006 [22], and has already become a new promising tool nowadays [23]. For carbon fiber-reinforced composites, due to the conductivity of carbon fiber [24], THz waves have to date only been demonstrated to detect the defects near the surface of the material or in the coating material on a carbon fiber substrate [25], [26]. For glass fiber-reinforced composites, THz waves can penetrate further to detect buried and underlying defects, including voids, delamination, and intrusions [27], [28], [29], [30], [31]. THz waves can also been used to detect the fiber content and orientation inside glass fiber-reinforced composites [32], [33].

In the present study, we carry out both THz imaging and ultrasonic C-scans of glass fiber-reinforced composite laminates in polyetherimide resin with delaminations. The defects are created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Based on the results, we conclude that THz imaging can provide a nondestructive, noncontact, and nonionizing method to evaluate glass fiber-reinforced composites with higher spatial resolution, and can be regarded as an alternative or complementary to ultrasonic C-scans. We point out, moreover, the most important merit of THz imaging is the ability of providing quantitative information in depth and three-dimensional imaging of the samples.