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

Abstract

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.

Introduction

A composite material is defined as a combination of two or more materials to produce a material with superior properties [1]. The resultant composite material is more robust and stiffer with reduced weight. Due to these excellent advantages and unique properties, composite materials have been widely adopted in many industries, with the global market of composites expected to reach $113.2 billion by 2022 [2]. In particular, fiber-reinforced polymer (FRP) composites are used for aerospace, automotive, military, marine, and construction applications [3], and today, half of the Boeing 787 material contents are composites [2]. Furthermore, Airbus estimates an increase in its carbon fiber demand to around 20,000 tons in 2020 [4].

Despite the significant advantages in composite application, FRP composites are susceptible to complex defects during the manufacturing and assembly processes, or in-service wear and tear [5,6]. In service defects are mainly due to LVIs that cause BVIDs, such as cracks, debonding, delamination, and breaking of fibers [7,8]. These defects can develop and lead to structural integrity failure. Hence, it is vital to identify these defects and determine the tolerance limits to prevent such failures [9,10]. The importance of damage tolerance limit led to its embodiment in the aircraft certification guidelines issued by the US Federal Aviation Administration [11].

Damage development and the consequent effects can be mitigated through effective inspection and monitoring with appropriate testing procedures, such as non-destructive testing and evaluation (NDT&E) and in-situ structural health monitoring (SHM) techniques. NDT&E allows detecting, identifying, and locating flaws, despite being done over a scheduled interval. As such, NDT&E is not able to provide detailed information on damage initiation or growth between the inspection intervals. Alternatively, damage detection and evaluation can also be achieved with the aid of in-situ SHM system. SHM system integrates advanced sensing technologies with post-processing algorithms to continuously diagnose and monitor the health of structures [[12], [13], [14], [15]]. Despite the advantage of SHM, the main challenges that restrict its adoption are the added weight and cost of the SHM systems. For instance, the inclusion of SHM system in an aircraft will contribute to the maximum landing weight and this scenario will lead to the reduction of operational payload. The payload reduction limits the maximum number of passengers and hence impacting the revenue [16]. In ensuring the practical application of SHM techniques, light-weight sensors or sensors with large detection range attributes are highly desirable. Although NDT and SHM are fundamentally different methods, the advances in sensing technologies and post-processing methods have resulted in significant technological advancement in both NDT and SHM fields, making these distinctions less neat. Therefore, to ensure seamless integration of NDT and SHM for practical composite material application, it is necessary to understand the recent development of both NDT&E and in-situ SHM methods for damage characterization.

This present study reviews recent developments in the NDT&E techniques, particularly for FRP composites inspection and evaluation. Furthermore, a broad overview of the post-processing techniques used, the advantages and limitations of each reviewed NDT technique are also outlined. Besides, the suitability of an NDT technique for a specific application depends on several factors, and in this study, AHP is applied to compare and rank the eight NDT techniques based on four criteria: test object size, test time, test cost, and test. These criteria were chosen based on what is learned from the reviewed studies and due to the to the availability of tangible information to properly evaluate the alternatives.

Furthermore, emerging technologies such as guided-waves SHM systems and nano-based self-sensing FRP composites, and optical fiber sensors are presented. These SHM systems have a promising potential to overcome the added weight and costs challenges. Besides, AI tools have revolutionized both NDT and SHM systems, and thus, this study provides the recent advances in this filed. Lastly, the growing power requirement of SHM systems is discussed. In addressing the growing power requirement of these systems, in addition to the conventional power optimization techniques, a feasible solution can be achieved through microelectromechanical systems (MEMS) harvester and nanogenerators, enabling self-powered guided-waves SHM system.

The paper is organized as follows; Section 2 presents in detail the NDT techniques. Section 3 presents the evaluation and ranking of NDT methods using AHP. Section 4 presents the hybrid/combined NDT techniques. Section 5 discusses the in-situ SHM techniques. Section 6 presents the approaches for solving the power consumption problem associated with guided-waves SHM systems. The conclusion of the review and some recommendations for future work in this field are presented in Section 7.