The concept of combining two or more constituent materials to produce an end material with improved and unique properties dates back to 3400 B.C. when Mesopotamians glued wood strips at different angles to create plywood for structural applications. Later, in the 12th century, the Mongols used composite materials to craft the first composite bow made of a combination of wood, bamboo, bone, cattle tendons, horns, silk, and pine resins. The modern era of composites did not begin until the development of plastics in the early 1900s . Since then, manufacturers, engineers, and researchers have continued to develop composites consisting of a wide variety of materials for a broad spectrum of applications. The two constituents of a composite are the matrix and reinforcement, whereby the matrix surrounds and binds the reinforcement. Composites combine the best properties of the constituent materials, which remain separate and distinct within the final composite structure as they do not blend or dissolve each other . Composite materials are often preferred to conventional materials, such as metals and ceramics, due to their high specific strength, lightweight, ease of fabrication, design flexibility, resistance to fatigue and corrosion, and low cost [3–7]. Today, composites are commonly applied in the aerospace, military, automotive, and construction industries, where they have largely replaced conventional materials.
As with all materials used to create objects, defects and damage can occur during manufacture or in-service. In service, composites are subjected to static, fatigue, and impact loads and extreme conditions (e.g., high temperatures and moisture), affecting their performance markedly . Therefore, it is of utmost importance to ensure the structural integrity, strength, and performance of composites throughout their service life by robust and reliable inspection techniques, especially in safety-critical industries such as aerospace. However, compared with conventional materials, which comprise only one type of uniform, isotropic material with known and predictable properties, composite materials are inhomogeneous and anisotropic with varying and less predictable properties . Thus, several non-destructive testing techniques have been developed for composite inspection purposes. This introduction will briefly describe some composite types included in the following digest articles and the most important techniques to inspect and monitor composite materials.
Particulate reinforced metal matrix composites (PRMMCs):
PRMMCs consist of a metal matrix reinforced by ceramic or organic particles and exhibit better mechanical properties (e.g., higher stiffness and strength) than the corresponding unreinforced matrix . PRMMCs are most commonly manufactured by powder metallurgical and casting processes, with the latter being performed in the liquid or liquid-solid state [11,12]. The mechanical properties of this composite type depend mainly on the load-carrying capacity and morphology of the ceramic reinforcement particles. Aluminum has a low density, high ductility, and good corrosion resistance; therefore, it is widely used in PRMMCs. Particle-reinforced aluminum composites are popular because many inexpensive reinforcements and working processes to shape aluminum are available . Moreover, aluminum-based composites are in great demand in the aerospace and automotive industry because of their high strength-to-weight ratio . In the digest article “Wettability of Low Weight Borides by Commercial Aluminum Alloys − A Basis for Metal Matrix Composite Fabrication”, the wetting behavior of different boron compounds by aluminum alloys was investigated to identify promising boride reinforcements for the fabrication of aluminum-based composites.
Piezoelectric materials are commonly used as actuator and sensor materials because of their ability to translate a mechanical impulse into an electric response [15–17]. Ceramic/polymer composites in which a ferromagnetic ceramic filler is embedded in a polymer matrix combine the piezoelectric properties of the filler with the flexibility of the matrix. The properties of a composite depend on the number of phases, the volumetric fraction of each phase, the properties of each phase, and the connectivity of the phases . The connectivity describes the configuration; that is, how the phases are interconnected in the composite. Each phase can connect up to three directions, whereby the first configuration number indicates the connectivity of the dispersed filler and the second one that of the matrix. The connectivity of the filler controls the electric flow distribution in the composite . Composites with 0–3 connectivity consist of homogeneously dispersed particles in the matrix and are most commonly produced because of their simple fabrication. However, composites with 1–3 connectivity exhibit higher piezoelectric activity because their particles align along the preferred direction in the composite [20,21]. Piezoelectric composite-based sensors have various applications, especially in the automotive industry, where they are used to assess the acceleration and damping force in semi-active suspension [22,23]. The digest article „Micro‐Structuration of Piezoelectric Composites Using Dielectrophoresis: Toward Application in Condition Monitoring of Bearings“ describes the fabrication of a piezoelectric composite-based sensor in 1–3 configuration, consisting of a lead zirconate titanate filler and polydimethylsiloxane matrix, for the condition monitoring of aircraft ball bearings.
Hybrid composites comprise two or more types of reinforcements in the same matrix. They are fabricated with the aim to synergize the properties of the reinforcements, leading to materials with superior properties to those of conventional and (non-hybrid) single composites. The overall properties of hybrid composites are considered a weighted sum of the individual constituents in which the inherent advantages and disadvantages are balanced . Thus, the advantageous property of one reinforcement can counterbalance the disadvantageous property of another reinforcement. Accordingly, cost-effective hybrid composites with desired properties can be produced by proper material choice. The mechanical properties of a hybrid composite strongly depend on fiber orientation, fiber length, fiber content, fiber/matrix adhesion, and failure strain of individual fibers . A positive hybrid effect is obtained when the mechanical properties of the hybrid exceed those of the corresponding non-hybrid composites. The properties of a hybrid system consisting of two components can be determined by the rule of mixtures : PH = P1V1 + P2V2, where PH is the mechanical property of the hybrid, P1 and P2 the properties of the first and second system, respectively, and V1 and V2 the corresponding volume fractions of the systems. Hybrid composites are utilized in many engineering applications because of their lightweight, high strength, and ease of fabrication; especially, the automotive industry applies hybrid composites for various applications . In the digest article “Mechanical properties and slurry erosion resistance of a hybrid composite SiCfoam/SiC particles/EP”, a novel hybrid composite is presented consisting of an E‐51 epoxy resin (EP) as the matrix and SiCfoam/SiC particles as reinforcements. The hybrid composite outperformed the non-hybrid composite SiCfoam/EP in terms of mechanical and anti-erosive properties.
Composite inspection solutions
Conventional ultrasound testing:
Ultrasound testing is the most commonly used non-destructive inspection method for composite materials . In ultrasound testing, a transducer generates high-frequency waves, propagating through the material before being received by the same or a second transducer. As the motion of any wave is affected by the medium (e.g., composites) through which it propagates, one or more of the following parameters associated with high-frequency waves are changed: scattering, frequency, transit time, and attenuation . The changes in these parameters provide valuable information on the properties of materials, such as hardness, elastic modulus, density, and grain structure. Therefore, ultrasonic testing can evaluate composites and detect flaws, such as hidden cracks, voids, and porosity. However, conventional ultrasound testing is often unsuitable for composite inspection because of the inhomogeneous and anisotropic nature of composites . Ultrasound wave propagation in anisotropic composites is intricate and accompanied by random scattering and high attenuation of ultrasound waves, impeding defect detection [27,29].
Phased array ultrasound testing:
Phased array ultrasound testing can overcome the limitations of conventional ultrasound testing by steering ultrasound waves to create constructive interference of the wavefronts, thereby focusing the energy . In contrast to conventional ultrasound testing, in which a single-element transducer generates ultrasound waves, phased array ultrasound testing systems use multi-element transducers that can be separately pulsed in a programmed manner. Each transducer emits a spherical wave at a specific time so that the superimposed wavefront steers and shapes the final beam. In this way, a vast number of different ultrasound beam profiles can be generated from a single probe, enabling electronic scanning. Therefore, phased array ultrasound testing is a powerful, non-destructive composite inspection solution, which is rapidly growing in the composite industry.
Confocal laser scanning microscopy (CLSM):
CLSM is an optical imaging technique that combines high-resolution optical imaging with depth selectivity, allowing the collection of two-dimensional images at different depths (optical slicing) and consequently the construction of three-dimensionally resolved images of the investigated sample. CLS microscopes comprise a confocal optical system with a spatial pinhole positioned in front of the detector that eliminates out-of-focus light, leading to micrographs of high optical resolution with excellent contrast. In detail, when a laser illuminates the pinhole, the light emitted from the pinhole passes through a beam splitter and is focused to a spot on the sample, which is placed at the focal plane, by an objective lens . Light reflected from this spot on the sample’s surface travels back to the beam splitter, where it is converged on the pinhole and directed toward the detector. Light reflected from parts of the sample outside the focal plane is not detected.
Although the confocal microscope was invented in 1953 by Minky , his invention remained largely unused until the development of lasers, endowing the system with a light source of sufficient brightness and stability. Since then, CLSM has been widely used to study and inspect composites. CLSM has some key advantages over conventional widefield microscopy in the field of material science . For example, three-dimensional imaging by CLSM allows detailed analysis of the (micro-)structure of composites without the need for thin material sections. Furthermore, the horizontal and vertical resolution of CLSM is higher than that of conventional light microscopy. Thus, CLSM represents an excellent composite inspection solution. Three of the following digest articles demonstrate the application of CLSM to analyze the surface morphology of different composite materials.
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