Elsevier

Composite Structures

Volume 256, 15 January 2021, 112951
Composite Structures

Review
Advances, limitations and prospects of nondestructive testing and evaluation of thick composites and sandwich structures: A state-of-the-art review

https://doi.org/10.1016/j.compstruct.2020.112951Get rights and content

Abstract

In recent years, composite materials have gained popularity in numerous high-tech and engineering applications, owing to their outstanding physico-mechanical properties. They were initially used as fairings/reinforcements for different structures, but their application has recently shifted from general-purpose structures to primary and secondary load-bearing structures, where structural failures would result in catastrophic safety repercussions. This increased scope of application has prompted the introduction of composite structures featuring significant thickness and complexity. As such, the application of nondestructive testing and evaluation (NDT&E) to localize and characterize flaws in these materials at their incipient initiations could save resources, eliminate unplanned breakdown, and provide a timely window for repair-maintenance activities. Therefore, this paper critically reviews the recent advances in NDT&E as applied to the inspection of thick composite parts and sandwich structures (composites with a thickness ≥15 mm) and determines possible research prospects to address the limitations of the current technologies. A brief overview of defects/damage occurring in composite structures is provided followed by the main NDT&E techniques used to detect these flaws. Since there are many NDT&E techniques available, this work limits its scope on techniques that focus on the detection, localization, and characterization of flaws in thick composites and sandwich structures.

Introduction

In recent years, composite materials (also called composition materials or shortened to composites) have increased their scope of usage in a wide range of high-tech and engineering structures, owing to their excellent physico-mechanical properties [1], [2]. They present a high-strength-to-low-mass ratio and can be designed to meet specific functions [3], which makes them great candidates for many engineering applications [4]. Recent efforts to produce these composites through novel processing mechanisms (viz. out-of-autoclave processing) have rendered them more affordable and available for use in a wide range of industries [5], [6]. In the aerospace industry, for example, carbon fiber-reinforced polymers (CFRPs) are used in wing planks, sandwich panels, and fuselages; they are also used to strengthen and repair some of the existing structures of the airplanes. Glass fiber-reinforced polymers (GFRPs) are used in thicker composites such as helicopter’s rotor structures [7] as well as piping and storage tanks in the petrochemical industry [8]. Currently, nearly all marine hulls are made of molded GFRPs or laminate skins bonded to the foams, and in some cases, wood-filled sandwich panels [8], [9]. In the construction industry, composites are used as reinforcements to concrete pillars [10] and bridge decks [11]. Hybrid polymer-matrix composites involving different types of fibers (e.g. carbon and glass fibers) are extensively used in the manufacturing of wind turbine aerofoils and hydrofoils for marine propulsors [8], [12].

Although the applications of composites have been quite successful in structural engineering, further applications are still hindered by some of their physical properties (e.g. thermal and electrical conductivity, fire resistance, etc.) [13], [14]. Indeed some of these properties can be controlled by adding some nanosized fillers such as nanoclays or carbon nanotubes (CNTs) [15], [16], [17]. However, recent studies indicate that the more complex the composite is, the more likely it is to develop damage [8]. In addition, the integrity and the lifecycle of the structure cannot be guaranteed under different loading conditions [18], [19], [20]. Given the fact that composite structures are intrinsically complex and their behaviors in fatigue loading and fracture mechanism are barely understood (i.e.; compared with their metallic predecessors), a continuous investigation into their structural integrity and interlaminar strength is of great importance [3], [8]. Hence, composites must be routinely examined for possible flaws or damage using appropriate NDT&E techniques to ensure they are performing as designed or detect and characterize flaws and defects at their incipient stages of initiation in order to timely conduct the replacement or maintenance-repair activities. According to ASTM E1316-17A [21], a flaw is defined as an imperfection or discontinuity that may be detectable by nondestructive testing (NDT) method and is not necessarily rejectable, whilst a defect is defined as one or more flaws whose aggregate size, shape, orientation, location, or properties do not meet specified acceptance criteria and are therefore rejectable. In most cases, “flaw” and “defect” are used interchangeably, including in some of the references in this paper. Damage is defined as changes to the material or geometric properties of a structural system that affect its performance.

There are currently many types of NDT techniques used to establish the quality (during the manufacturing) [3], [22] track the structural integrity (while in-service) [9], [19] of composites and locate the defects or flaws and determine their features such as size, shape, and orientation within the structural system [22], [23]. NDT is the development and application of technical methods to examine materials or components in ways that do not impair future usefulness and serviceability in order to detect, locate, measure, and evaluate flaws; to assess integrity, properties, and composition; and to measure geometrical characteristics [21]. Indeed NDT is a very broad and interdisciplinary area and it plays a very important role in assuring that structural components/systems perform their function more reliably and cost-effectively. NDT engineers and technicians define and implement tests that locate and characterize the material conditions and flaws that might otherwise cause structural component or system failure (e.g. plane crash, reactor failure, the train derail, pipelines burst) and a variety of less visible, but equally troubling events. Since NDT allows inspection of the components without interfering with a product's final use, it provides an excellent balance between quality control and cost effectiveness [8], [23]. Although nondestructive evaluation (NDE) is a term that is often used interchangeably with NDT, the former is used to describe measurements that are more quantitative in nature. That is, an NDE method would not only locate a flaw, but it would also be used to measure something about that defect (e.g. size, shape, and orientation) as well as to determine material properties (e.g. fracture toughness, formability, and other physical characteristics). NDT&E is usually used to designate a combination of both NDT and NDE and the same denomination is adopted in this paper.

Although NDT is a vital tool for the inspection of composite structures [24], [25], [26], [27] the inspection of thick composites and sandwich structures using common NDT techniques remains an appealing problem and equally challenging to most NDT engineers and technicians [8], [28]. Inspection of thick composites using common NDT techniques drew a significant interest since the early 1990s and the area is still growing and evolving [29], [30]. In the 1990s, for example, only composites with a thickness between 5 and 10 mm and near-the-surface damage were considered [27], [31], [32]. At this time, the classification of composites between thick and thin was also not adequately understood among researchers and different considerations were adopted to classify composite structures between thin and thick. For example, some classified structures as “thick” based on some random minimum value of signal-to-noise ratio (SNR) vis-à-vis the NDT&E technique [8]. Assuming this is true, some structures may end up being simultaneously categorized as both “thick” and “thin”, hence, no clear decision can be made in this regard. Also, simultaneous use of advanced signal post-processing techniques and multiple inspection parameters within a single discipline precludes the use of standard SNR values such as 3:1 to any particular examination. Another definition stipulated that a thick composite does have the geometry, material constituents, lamination scheme, processing, and service loading that exhibit three-dimensional states of stress, and hence, failures that cannot be accurately predicted using 2-D finite-element models. Although this definition was meticulously accepted, the literature reports no published work outlining the inspection reliability of the different NDT&E of thick composites nor specific indication from the structural engineering community.

The present paper critically reviews the different NDT&E techniques used to test and evaluate thick composites and sandwich structures. In doing so, recent advances, limitations, and prospects of the most commonly used NDT&E techniques to inspect thick composites are presented; and the main advantages and disadvantages of each of these techniques vis-à-vis the different types of flaws are outlined. In order to provide the most up-to-date practical information for the different NDT&E techniques and their applications to the inspection of composites and sandwich structures, some of the most important standard practices for the different NDT methods discussed will be included. The main focus is directed to the materials that are frequently used to manufacture thick composites, and therefore, a particular emphasis is laid upon polymer-matrix composites (PMCs) laminates consistent with the volume of material published in this area. Other materials such as metal-matrix composites (MMCs), ceramic-matrix composites (CMCs), fiber-metal laminates (FMLs), and reinforced concrete (RC) are also considered. Although some of the studies focusing on thin composites (with a thickness <15 mm) will be mentioned, it is not the core subject of this study but rather because many of the materials published on the inspection of composites focus on the inspection of small or thin-skinned structures. Also, while some of the studies based on the finite element methods are mentioned in this work, the mathematical modeling that goes along with these methods is not indicated. Although the present study considers the effectiveness of the different NDT techniques in terms of their capabilities to detect the different types of flaws within thick composites and sandwich structures, it will also be mentioned if a particular technique can be used to determine other features such as the size, shape, and orientation of the flaw. Finally, all the theoretical studies and the subsequent signal processing techniques that go along with all the NDT&E techniques reviewed here are not covered in this work for simplicity.

Indeed the inspection of thick composite and sandwich structures has always been a challenging task to perform. Even with the ongoing effort to research hybrid composites (typically glass and carbon fiber-reinforced composites), it has been reported that only the usual production quality-assurance ultrasonic-based testing are leading the way but without any reference to the distinct characteristics and properties of the hybrid composite [33], [34], [35], [36]. Generally speaking, the available references focusing on the examination of thick composites and sandwich structures have been limited to those cases where the inspection relates to the through-thickness problems, rather than the disbonds beneath the thin outer skins and underlying cores [37], [38], [39]. It is noted that the determination of the inspection technique to be used for different materials is not an easy exercise to perform. It involves a lengthy deliberation process taking into consideration the size and location of the damage, the criticality and accessibility to the structure (full/single side access), available resources, and skilled personnel to apply it as well as the types of the materials in the composites. In so doing, the aim is to increase the reliability and availability of equipment with the available damage detection technique at the lowest cost without compromising the predefined standards and guidelines governing the testing of the structure/material of interest. Considering all these factors, it is undeniably difficult to comprehensively review all the published work NDT&E and thick composites in a single paper. As such, this article focuses on the NDT&E techniques that are used to inspect thick composites and sandwich structures (with only composites with a thickness ≥15 mm being considered).

Importantly, the ongoing research on the study and nondestructive characterization of weak adhesive bonds is another area that is mainly not touched by this review. The main reason is that this problem has always proved the most challenging and only little experimental progress has been demonstrated under laboratory conditions and limited to the study of thin composite plates [40], [41], [42]. Also, the inspection of bonded aircraft and ship decks composite repairs have so far been limited to a thickness ≤10 mm, and where they have been successfully examined for disbonding only standard ultrasonic inspection techniques have been used [43], and are consequently not mentioned further in this present article. Also, updated guidelines pertaining to the different NDT&E techniques for thick composites are presented; most of which appear to have focused on flat composites. Given that the literature reports reasonably fewer studies on other structural designs such as single or double curve structures and hybrid reinforced composites as pertains to NDT&E, this area is also not considered. Instead, the main focus is directed to the techniques that can help as a starting point for studies aiming at detection, localization, and characterization of damage in thick composites. Although the line of thought governing the search and structure of this review is a result of objectives beyond the scope of the present paper itself, the authors believe that, once the above is fairly understood, the updated précis such as this could also be useful for other researchers in the same field.

Section snippets

A brief introduction of the types of flaws occurring in composite structures

Indeed thick composites and sandwich structures are obtained via a multi-variable and complex process and the resulting structures are prone to several types of flaws [44]–[46] ranging from tiny faults to big impact damage [8], [19], with some of them being more severe than others and with significantly different effects on the overall performance of the composite [47]. As such, understanding the nature of the different defects/damage and their detrimental effects on performance and structural

Nondestructive testing and evaluation methods for thick composites and sandwich structures

NDT&E consists of a group of non-invasive examination techniques used to evaluate the properties of the material, the component, or the entire process unit. These techniques are also utilized to detect, characterize, or measure the damage (e.g. corrosion, cracks, delamination, notches, etc.) and its progression on the surface or the interior of the material without cutting it apart or altering its original features [119]. There are currently many NDT&E techniques including but not limited to

Comparison of the different NDT&E techniques

All NDT&E techniques reviewed in this paper have proven capable, to some extent, of detecting different flaws in thick composites and sandwich structures. However, some of these techniques have higher resolution and penetration capabilities than others and a combination of two or more (i.e.; multi-techniques) may be warranted if more accurate results are sought [27]. Although the performance of NDT&E is strongly dependent on choosing the most suitable inspection technique and applying it

Conclusions and future trends

The present article reviews the recent studies published on the application of NDT&E techniques to the inspection of thick composites and sandwich structures and highlights their advances, limitations, and prospects. Although several NDE techniques have been devised to detect and characterize flaws in thick composites (with each having its advantages, disadvantages, and scope of application), there is currently no single technique that is false-negative or false-positive free. This is

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the funding they received from the National Science Foundation of China (51675103), the Fujian Provincial Science and Technology Project (2019I0004), the State Key Laboratory of Mechanical Systems, and Vibration (MSV-2018-07), and the Shanghai Natural Sciences Fund (18ZR1414200).

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