The nomenclature for matrix materials and the classification of reinforced composite materials are used in conjunction, with priority given to the name of the matrix material when emphasis is placed on the matrix, and to the name of the reinforcing phase when emphasis is placed on the reinforcement.
The classifications of composite materials are:
(1) Classified by matrix material:
Polymer matrix composites, metal matrix composites, ceramic matrix composites, cement-based composites, carbon-based composites;
(2) Classified into the following three categories based on the form of the reinforcing material:
1. Fiber-reinforced composites: continuous-fiber composites and discontinuous-fiber composites;
2. Particle-reinforced composites: including micron-sized and nano-sized particles;
3. Plate-like reinforcements and woven composites: These are formed by combining planar two-dimensional or three-dimensional reinforcement materials with a matrix.
4. Laminated composite materials.
(3) Classified into two types according to the function of the material
1. Functional composite materials: These utilize the non-mechanical properties of materials, such as optical, electrical, magnetic, thermal, and acoustic characteristics.
2. Structural composite materials: Mechanical properties of the materials used.
II. Challenges in the Testing of Composite Materials
Composite materials are increasingly being employed in the development of both military and civil aircraft due to their ability to effectively enhance aircraft performance and deliver significant weight savings; moreover, with advances in science and technology, a wide range of new composite materials are now being utilized more effectively.
During the development of a certain aircraft model, the design team adopted a novel composite structure to ensure optimal part geometry and weight reduction. This structure features a carbon-fiber epoxy prepreg face sheet with a polymethylmethacrylate foam core.
Foam-core sandwich materials are porous and highly porous, and their unique properties not only pose significant challenges to manufacturing processes but also make it extremely difficult to assess the quality of internal bonding.
The main challenges in its detection are:
(1) The non-compactness of the foam core results in substantial ultrasonic attenuation, making it difficult to obtain reflected echoes from the foam core in carbon-fiber-reinforced foam sandwich structures. This poses significant challenges for detecting defects at the interface between the adhesive film and the foam core.
(2) Components of a certain aircraft model manufactured with a carbon-fiber-reinforced foam sandwich structure feature complex geometries and numerous structural transition zones, resulting in a high likelihood of defect formation at multiple locations and the occurrence of various defect types. Consequently, it is essential to accurately quantify and qualitatively characterize these defects.
(3) Designers impose stringent acceptance criteria on components manufactured using this structure, requiring that the smallest detectable defect be no larger than 3 μm. Abroad, laser speckle technology is commonly used for inspecting similar material structures; however, the minimum detectable defect size with this method is at least 25.4 μm.
This paper summarizes an ultrasonic testing method for carbon-fiber cellular sandwich structures through process trials and verifies the accuracy of this ultrasonic testing approach.
III. Characteristics of Ultrasonic Waves
(1) Ultrasonic beams can be focused in a specific direction, propagate in straight lines through a medium, and exhibit excellent directivity. During propagation in a medium, ultrasonic waves undergo attenuation and scattering. At the interface between dissimilar media, ultrasonic waves give rise to reflection, refraction, and mode conversion.
By exploiting these characteristics, it is possible to detect defects by receiving the reflected ultrasonic waves that bounce back from defect interfaces. Ultrasonic waves carry much more energy than ordinary sound waves. They experience very little attenuation when propagating through solids, allowing for deep penetration. Moreover, ultrasonic waves undergo reflection, refraction, and other phenomena at interfaces between different materials; notably, they cannot pass through gas–solid interfaces. When ultrasonic waves encounter a metal–defect interface—such as one containing pores, cracks, delaminations (which often trap gas), or inclusions—they will be wholly or partially reflected.
(2) The advantages of ultrasonic flaw detection include large detection depth, high sensitivity, fast inspection speed, low cost, harmlessness to humans, and the ability to locate and quantify defects.
Ultrasonic flaw detection provides an indirect indication of defects, involves significant technical challenges, is susceptible to both subjective and objective factors, and yields results that are difficult to archive. Moreover, ultrasonic testing requires a smooth test surface and relies on highly experienced inspectors to accurately identify defect types; it is particularly suited for inspecting thick components, which in turn imposes limitations on its applicability.
There are many types of ultrasonic flaw detectors, but the pulse-echo ultrasonic flaw detector is the most widely used.
In general, the presence of defects in a homogeneous material leads to material discontinuities, which often result in variations in acoustic impedance. According to the law of reflection, ultrasonic waves are reflected at the interface between two media with different acoustic impedances.
The magnitude of the reflected energy is dependent on the difference in acoustic impedance between the two media at the interface, as well as the orientation and size of the interface. Pulse-echo ultrasonic flaw detectors are designed based on this principle.
(3) Most pulse-echo ultrasonic flaw detectors are A-scan type. In an A-scan display, the horizontal axis represents the propagation time or travel distance of the ultrasonic wave in the test material, while the vertical axis indicates the amplitude of the reflected ultrasonic wave.
For example, if a defect is present in a workpiece, the presence of the defect creates an interface between two different media—the defect and the surrounding material. Because the acoustic impedances at this interface differ, when the emitted ultrasonic wave encounters the interface, it undergoes reflection. The reflected energy is then received by the transducer and displayed on the instrument’s screen as a reflected-wave waveform at a specific position along the horizontal axis. This position on the horizontal axis corresponds to the depth of the defect within the inspected material.
The amplitude and waveform of this reflected wave vary depending on the type of defect, thereby reflecting the nature of the defect.
