Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures

In this chapter, a general introduction and background information pertinent to the manufacturing of fiber-reinforced composite materials is provided outlining the general analysis and properties of these types of materials to be used as the starting point to understand the thematic topic of the book. In this context, this chapter provides basic information outlining the main types of composite materials with varying types of matrix and reinforcement, as well as some general hints on the fabrication processes that help to obtain high-quality composites. A section is also dedicated to the description of the main types of defects/flaws that may arise in composites, either during the processing of the raw materials, the manufacturing of the composite itself as well as during its service. Particularly, we believe that such a description can help our readers to understand the main weaknesses of fiber-reinforced composite materials and provide a viable source of information to the engineers on what they should expect when using these types of materials in their designs. The chapter ends with a look at the environmental impact and the future research trend of the existing composite material, which appears to be dominated by nanotechnology, and the apparent move towards the development of environment-friendly and sustainable fiber-reinforced composite materials.

This chapter discussed the different approaches used for the evaluation of the health status of composite materials using nondestructive testing and evaluation (NDT&E) techniques. The discussion started with the terminology, proceeding to some general requirements of NDT in terms of performance, portability, and safety concerns. A description of the perception of defects and flaws vis-à-vis the different NDT techniques is also provided. A section was also devoted to personnel qualifications and certification as well as a brief introduction on the semantics of the most important formulations used in the NDT of composite materials. In the context of this book, NDT denotes a descriptive term used to indicate the examination of fiber-reinforced composite materials and components without changing or destroying their original attributes. Although the terms NDT and NDE are often used interchangeably in most of the literature (including some references in this chapter), these two terms are different and should be treated as such by the readers of this book consistent with the definitions outlined in the first section. Typical examples of the application of NDT are found in a number of fiber-reinforced composite structures including aircraft, spacecraft (i.e., space shuttle), motor vehicles, pipelines, bridge decks, refineries, buildings, and oil platforms which are all inspected using NDT. When used correctly, NDT can also serve as a Quality Assurance Management tool that can give impressive results, but this requires an understanding of the various methods available, their capabilities and limitations, as well as the knowledge of the relevant standards and specifications for performing the tests. In general, NDT can easily help identify materials, products, and equipment that fail to achieve their design requirements or projected life due to undetected defects and help schedule repair, maintenance, or replacement activities. NDT also helps engineers to mitigate unsafe conditions or catastrophic failure of composite structures, as well as the loss of revenue by providing an optimum window for timely planned repair and maintenance activities.

In general, visual testing (VT) of fiber-reinforced composite structures (FRCS) is very important not only as an NDT technique in itself but also as a supplementary tool to any other instrumented NDT for the inspection of FRCS. The technique is mostly applicable to the examination of all types of surfaces and is most effective when the test surface is cleaned prior to the examination. The technique is primarily performed at optical wavelengths suitable for the human eye’s vision whereby the main features or damage of interest include the surface porosity, resin-rich areas or resin pockets, wrinkles, dry ply, mark-off, bow waves, cracks, scabs, and voids. In most cases, damage/flaws on the surface of the composites appear to be in the form of indentations or cracks and less experienced inspectors may confuse these different types of flaws and may be unable to evaluate their sizes. To overcome all these difficulties and ensure visual tests are conducted correctly, visual inspectors should be adequately trained to be able to use this technique effectively and accurately interpret the test results. Although there are many types of limitations associated with visual testing, access to the test structure and adequate illumination of the test surface are the most commonly observed challenges when using VT to examine the structural integrity of fiber-reinforced composite structures. Although VT has changed very little over the years, there is a constantly growing development of new mechanical and optical visual aids and applications of VT are expected to grow as its reliability continues to increase to more acceptable levels. It is believed that this technique will continue to be an important asset for the NDT community as it remains the very first technique that any NDT inspector uses to identify areas of the composite structures that require advanced NDT techniques for accurate evaluation. Also, as computer-related technologies continue to advance and designers continue to make state-of-the-art image-gathering packages that are smaller and easily accessible by the general NDT practitioners, the limitations of access, information storage, and complication of use will be further reduced thereby boosting the acceptability of VT in many NDT applications involving the examination of FRCS. In military applications, in particular, VT is dominated by the use of drones, robotic devices, and automated visual inspection techniques which should continue to bring innovations to the technology of remote VT.

The recent years have been characterized by significant breakthroughs in the development and utilization of ultrasonic-based NDT techniques for the detection and characterization of various types of flaws in a wide range of fiber-reinforced composite materials. However, ultrasonic-based NDT techniques have also been the subject of intense criticisms due to the limitations that they inflict on the plants and the cost of the maintenance downtime while inspecting in-service composites structures and/or components. Attempting to address these challenges, major modifications have been implemented including the improvement of the methods of sending ultrasonic waves into the test structure (e.g., by contact between ultrasonic probes and test structure, immersion or ‘water delay line’ testing, air-coupled ultrasonic, laser ultrasonic testing, electromagnetic acoustic transducers, etc.). Each of these approaches presents its own advantages and disadvantages depending on the application, size of the test structure, and accessibility (i.e., access to one side or both sides of the test structure). Typically, localized or point testing can be achieved using water, gel, or grease coupling but achieving and maintaining constant coupling via these substances does add considerable time to the successful completion. Although an air-coupled ultrasonic system can be applied while the test structure remains in use and does not require additional acoustic couplants, overcoming the high impedance mismatch between the transducer/air and subsequent air/component interfaces is not a trivial success, and requires several modifications. Apart from the methodologies of sending ultrasonic waves into the test structures, additional improvements include the speed of the scan, improved spatial resolution, and advanced flaw detection capabilities of ultrasonic systems that are constantly being achieved, which prompted the design of fully portable systems that can be used in the field even in structures with complex geometries where accessibility is perceived to be challenging. Perhaps one of the most interesting advances is the recent development of the FlawInspecta system, which features high-speed ultrasonic array imaging fixtures that use a relatively low-cost ultrasonic array driver platform for operation. The FlawInspecta system also allows for the simultaneous scanning of a large area and includes additional fixtures to help users to overcome the problems of coupling. In general, ultrasonic NDT systems remain excellent tools for use in assembly lines during the manufacturing process of composite structures to ensure high-quality structures are manufactured and guaranteed their predicted life span while in-service. Nevertheless, a few challenges such as the difficulty of set up makes the system difficult to operate by the general NDT practitioners and requires a high level of skills and/or training for operators to be able to accurately scan the composite and interpret the test results. Although the capabilities of ultrasonic-based inspection techniques have been demonstrated at certain frequencies, studies employing appropriately-sized deeply buried flaws are still warranted to determine the reliability of use in thick fiber-reinforced composite structures. To date, significant advancements in ultrasonic inspection are still limited by the lack of sufficient lower-frequency, higher power evaluation systems which are likely to be overcome soon with the constantly observed cost reduction of phased array ultrasonic probes.

Infrared thermography (IRT) aims at the detection of surface or subsurface features of composite materials (e.g., fiber misalignments, voids, slag inclusions, etc.), based on temperature differences on the test surface during the monitoring by an IR camera even when only one side of the test structure is accessible. IRT technique is viewed as one of the most valuable NDT tools for online control and structural health monitoring of the materials and structures operating in environments with different levels of mechanical stress levels, thanks to its ability to provide a quick online appraisal of the health status of the test structures, thus avoiding the waste of time that would otherwise be spent when conducting the back-and-forth testing to investigate the performance of newly installed structures or designed materials under impact loading (i.e., reliability test). In addition, IRT is also capable of detecting several types of damage and/or material degradation effects (e.g., impact damage, delamination, disbonds, holes, corner splits, etc.) that occur during the material’s service life. IRT instruments are generally easy to operate and the fact that they can provide useful information for the material characterization that can be evaluated through the visualization of impact-induced thermal signals, specifically when analyzing the initiation and propagation of the impact damage, is another added advantage. Additionally, IRT techniques present several advantages, which include greater inspection speed, higher resolution/sensitivity, as well as the accurate and fast detection capabilities of the material or test structure inner defects/damage due to heat conduction and require no couplants. In this context, IRT can be used to test nearly all kinds of fiber-reinforced composite material and structural systems without fear of contamination by the test systems. However, there is little expectation of its successful application to thick sections, other than for sandwich panels enclosing significantly high levels of water content, or large voids, and the probable need of an active through-the-thickness heat source. In addition, the technological progress with the continued release of new and more sophisticated IR thermographic devices, more ergonomic, lighter, and user-friendly would pave the way to possible new applications, which equally requires a continued upgrading of procedures and data analysis methods.

Terahertz (THz) systems constitute an effective tool for the NDT&E community for the testing and characterization of fiber-reinforced composite materials. However, their systems are still very complicated and expensive to commercialize. Also, establishing the inspection limits for the vast majority of fiber-reinforced composite structures is still not achieved because this technique is relatively new in the area of material testing and evaluation. Nevertheless, this technique presents several advantages including the fact that it can see “through” the defects in thin composites and examine the underlying fabric of the material, overcoming the shadowing effect that is commonly observed with other NDT techniques such as ultrasonic testing and most of the radiographic testing techniques. Although the technology had been deferred for many years because of the inadequacy of its emission and detection devices, the so-called “THz gap”, this problem has recently been addressed thanks to the development of highly performing semiconductors and ultrafast electronics. To date, extremely short pulses required for the energy frequency of the THz waves can be achieved, suggesting that spatial resolution of the inspection levels higher than those of the normal microwave-based NDT techniques can be reached using THz systems. A lot has been done but much still needs to be done, particularly because there are no reported studies on the inspection of moisture uptake in fiber-reinforced composite structures nor are there any studies that confidently inspect conductive materials using THz waves. Indeed, this would be a highly valued milestone to the literature if it was achieved. In applications involving the inspection of thick composites and sandwich structures, THz systems do not, unfortunately, provide reliable inspection results owing to the attenuation and/or the scattering effects of the THz waves in thick sections.

Although the application of acoustic emission (AE) to the inspection of fiber-reinforced composite materials is highly beneficial due to its effectiveness in providing information about the incipient initiation of the damage, there are limited published studies outlining the quantitative limitations of damage identification at any specific depth in composites using AE. Also, there is rare reports on the thickness limitation of the material structure being tested vis-à-vis the applicability of the AE. It is observed that most of the AE-based inspection practices reported in the literature have been devoted to the standard tensile test configurations specifically for testing in lateral (in-plane) directions on loaded pressure vessels. One of the key improvements that have been suggested to this technique involves the sourcing of elastic disturbance from within the structure as opposed to conventional ultrasound sources. However, for disturbances to be detected by the receiving transducers, they must move along the surface of the composite suggesting that good results are dependent on the adequate propagation of these disturbances. The acousto-ultrasonic technique, also known as the acoustic-emission-simulation technique or the stress-wave factor method, is an advanced complementary form of AE signal processing. The technique uses active, offset pitch-catch transducers, as opposed to a reliance on the energy released from within the structure. This improved control over the transmitted energy and the modes of obtaining the excitation waves allow for greater in-depth diffusion, and hence, the technique becomes more suitable for the examination of thick fiber-reinforced composites. Nevertheless, when it comes to complex composites such as woven fiber-based composites, the above-mentioned modifications have not always provided any noticeable benefits over conventional AE. To this end, published reports have been restricted to the examination of thin-skinned composites.

To respond to the constantly growing utilization and complexity of fiber-reinforced composite materials, robust and reliable NDT techniques are constantly being developed. However, there have been no clear inspection guidelines set by NDT practitioners to facilitate the choice of appropriate techniques to be used especially when considering some special features such as the thickness of the structures and the size of the defect/damage among others. To this end, the detection targets are always set based on the available resources or the effects of defects/damage on the composite structures being considered for inspection, partly because setting robust and quantitative NDT inspection requirements necessitates comprehensive reliability studies to generate probability-of-defect curves and establish the minimum unfailingly detectable defects sizes which is costly and time consuming process. In this context, several NDT techniques including vibration testing, strain monitoring, electrical testing, ground-penetrating radar, microwave, millimeter waves, optical interferometric techniques, as well as radiography and tomography are often used to evaluate the structural integrity of fiber-reinforced not because they are the most suitable NDT techniques but because they are the only options available to the NDT practitioners. In this chapter, these NDT techniques are discussed to provide NDT practitioners with insightful information regarding their characteristics, advantages, limitations and prospects in the testing and evaluation of fiber-reinforced composite materials. Indeed, some of these methods may or may not find their direct applications in the testing and evaluation of fiber-reinforced composite materials depending on their sizes, shape as well as their physico-mechanical and electrical properties. As such, the present chapter also provides useful information to help users to determine the suitability of these NDT techniques to testing and evaluation of the different types of fiber-reinforced composites. For simplicity and convenience, the aforementioned techniques have been grouped in approximate order of increasing rate of application in the detection, localization, and characterization of defects in composite structures.

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. In general, all fiber-reinforced composite structures are manufactured for specific purposes (e.g., carry loads, operate over a long time, etc.), and irrespective of their usage, the manufacturers must always ensure that the manufactured structures are respectively in compliance with the required load-bearing capacity or can operate over a long time without losing their structural integrity to comply with both the customer’s requirements and national/international standards. All these requirements along with the increased scope of application of fiber-reinforced composite structures have simultaneously prompted the introduction of composite structures with specific features such as significant thickness and complexity. As such, the application of nondestructive testing and evaluation (NDT&E) techniques to localize and characterize flaws in these materials at their incipient initiations is inevitable for safety reasons and could also save resources, eliminate unplanned breakdown, and provide a timely window for repair-maintenance activities. In this spirit, the present chapter summarizes the main findings pertinent to the in-depth information provided in the different chapters regarding the most common NDT techniques used for the detection, characterization, and evaluation of the different types of flaws observed in fiber-reinforced composite materials during their manufacturing and/or in-service stages. This chapter also discusses the current developments in the NDT of fiber-reinforced composites and outlines possible research prospects to address the limitations of the current NDT technologies.