High-resolution air-coupled laser ultrasound imaging of microstructure and defects in braided CFRP

Highlights

• A hybrid laser-ultrasound(LU) testing system is proposed for braided carbon fiber reinforced polymer (CFRP) in-situ inspection and characterization of microstructure.

• The hybrid LU testing system retains and combines the advantages of both of the high resolution of the all-optical laser ultrasound configuration and the low cost of the air-coupled ultrasound transducers, and was experimentally validated through detection on a CFRP structure with defects.

• Inspection can be performed in a completely noncontact, nondestructive, and nonintrusive manner.

Abstract

Defect inspection of braided carbon fiber reinforced polymer (CFRP) is very difficult due to its nonhomogeneity, anisotropy, and sensitivity to coupling agents. This paper presents a hybrid system for detecting microstructure and defects in braided CFRP that combines the advantages of laser-ultrasound and air-coupled ultrasonic testing. Through the finite element method, the ultrasonic field propagating on the braided CFRP was simulated during the laser-induced ultrasound, and the influences of laser parameters and surface braided structure were analyzed on the laser ultrasonic signal. This detection method based on air-coupled laser ultrasound can provide the near-surface microstructure characterization of the resin pockets and braided fiber bundles, and also can achieve both shallow and deep defect detection of CFRP sheets which is comparable to that of the contact-type high-frequency phased array with higher contrast and less distortion. These results indicate that it has the potential to develop a non-contact, high-resolution, and low-cost method for the detection and repairment of advanced composite materials.

Introduction

Carbon fiber reinforced polymer (CFRP) composites have become invaluable structural materials due to their corrosion resistance, fatigue resistance, and high strength [1,2]. However, many defects may be introduced in the manufacturing or during the service such as matrix cracking, delamination and fiber breakage. Incipient small-sized damage can be barely judged from a CFRP structure's appearance, as the later development and accumulation will cause the ultimate destruction of structural performance [3,4]. Therefore, the accurate detection and structure characterization are of great significance to guarantee the safety of CFRP structures [5]. Various advanced non-destructive testing techniques have been developed to identify or evaluate the damage in CFRP, such as ultrasonic testing, thermography, X-ray tomography [[6], [7], [8], [9]], etc. Among them, ultrasonic testing is considered to be one of the most efficient and widespread methods in CFRP damage inspection [10]. However, contact technique or the immersion technique with water requires a contact transducer and matching medium, which significantly reduces the overall inspection efficiency and may cause contamination [[11], [12], [13], [14]]. Infrared thermography technology is a non-contact detection technology with a wide detection range, but its low resolution makes it only able to detect large defects [9,15]. Due to the low density of CFRP materials, the contrast is poor when X-ray inspection is used for defect detection, and the most common defects such as delamination and debonding cannot be distinguished well [16]. The air-coupled ultrasonic inspection technique has shown advantages for composite materials [[17], [18], [19]]. However, the acoustic attenuation is high for high frequencies, which leads to the fact that for such an application low frequencies are to be used. This may lead to a reduction in spatial resolution and a decrease in signal-to-noise ratio.

In recent years, laser ultrasound (LU) technique based on the use of all-optical inspection for ultrasound detection has been developed due to its advantages of long-range detection, high resolution, and rapid detection [[20], [21], [22], [23]]. Optical interference techniques have many advantages such as non-contact, long-range, wide-area and real-time features [24,25]. Kyung et al. used the Pitch-catch method and pulse-echo method to receive laser-induced photoacoustic signals, and realized the detection of internal delamination defects in CFRP [26]. Sun et al. studied the propagation characteristics of laser-generated Lamb waves in multi-layered fiber-reinforced composite plates [27]. Ivan et al. used laser ultrasound to evaluate the damage caused by thermal shock to the CFRP [28]. Patrycja et al. used laser-generated shear acoustic waves to detect debonding in adhesively bonded aluminum plates [21]. Fischer et al. discussed and implemented a novel method based on air-coupled detection of laser-generated ultrasound with a broadband optical microphone [29]. However, the sensitivity of above all-optical techniques are limited by environmental disturbance, the reflected light intensity, and the performance of photodetectors et al. Moreover, the strong optical absorption of carbon fiber will lead to extremely low optical reflection, and the braided structure will result in non-uniformity of light absorption. Meanwhile, their high cost limits to build large-scale applications over the last decades [30].

In this work, a hybrid LU testing system based on a custom-designed ACT is proposed for the microstructure and defects in CFRP materials, which has the properties of being non-contact, high-precision, and low-cost et al. It allows remote scanning of composite materials for inspection without direct contact and eliminates the interference stability problems associated with all-optical techniques [31]. An ultrasonic phased array was conducted to the same sample with high-frequency and low-frequency ultrasound, confirming the performance in the near-surface structure and internal defects of CFRP.