Composite materials are new materials created through advanced material preparation techniques that combine components with different properties in an optimized manner. In the 1940s, due to the needs of the aviation industry, fiberglass-reinforced plastics (commonly known as fiberglass) were developed, marking the beginning of the term "composite materials." After the 1950s, high-strength and high-modulus fibers such as carbon fiber, graphite fiber, and boron fiber were successively developed. By the 1970s, aramid fibers and silicon carbide fibers also emerged. With the increasing application of composite materials across various fields—particularly in aerospace, automotive, construction, electronics, and new energy sectors—the global composite materials industry has shown a continuous growth trend.

As more composite materials and structures are used in different industries, understanding how to inspect them for damage has become an important topic. This article reviews several common non-destructive testing (NDT) methods for composite materials.

Non-destructive testing, or NDT, is a standard testing method in the composite materials industry that does not require cutting or altering the material in other ways. It is also essential to producing high-quality products. There are several NDT methods to choose from, and sometimes multiple methods need to be used simultaneously to fully understand the types, sizes, locations, and depths of defects in composite laminates.

(2) Impact testing is another fundamental inspection method. This involves gently tapping the surface of the part with a hammer or a coin. A bright metallic sound indicates that the structure is sound, while a dull "thud" suggests the presence of defects, such as delamination or debonding. Digital hammers can measure the impact response and display the time it takes for the laminate to respond in milliseconds. A shorter response time indicates that the structure absorbs less impact, suggesting that it is solid. Conversely, when defects are present, the response time is longer, resulting in higher readings on the display.

This method is more effective for thin laminates with a thickness of less than 3.05 mm, while it may not yield reliable results for very thick laminates. Another disadvantage is that it can sometimes provide false readings, especially when the back of the laminate is bonded to another structure.

(3) Ultrasonic Testing (UT) is currently the most widely used non-destructive testing method. The principle of ultrasonic testing involves sending high-frequency energy waves (ranging from 0.5 to 25 MHz) into a laminate, capturing and quantifying the amplitude and time-of-flight of these waveforms to analyze material properties and structural changes. The main methods used in ultrasonic testing are:

Pulse-echo ultrasonic testing: This method can be performed with a single-sided ultrasonic probe that functions as both a transmitter and receiver. It operates using high-pressure pulse excitation, where each electrical pulse activates the transducer element. This element converts electrical energy into mechanical energy in the form of ultrasonic waves. The wave energy enters the test part through a Teflon® or methacrylate contact tip. Waveforms are generated within the test part and are picked up by the transducer element. Any changes in the amplitude of the received signal or the time it takes for the echo to return to the transducer indicate the presence of defects. Pulse-echo testing is used to detect delaminations, cracks, voids, water, and debonding of adhesive components, but it is more challenging to identify delaminations or defects between the core and the skin of sandwich structures.

Through-transmission ultrasonic testing: This method utilizes two transducers, one on each side of the area being inspected. The ultrasonic signal is transmitted from one transducer to the other. The intensity loss of the signal is then measured using instrumentation, which represents this loss as a percentage of the original signal strength or in decibels. Areas where the signal loss exceeds reference standards are identified as defect areas.

Low-Frequency and High-Frequency Bonding Testers: These bonding testers use inspection probes equipped with one or two transducers. High-frequency bonding testers are designed to detect delaminations and voids, capable of identifying defects as small as 0.5 inches in diameter. However, they cannot detect debonding or voids from the surface to the honeycomb core. Low-frequency bonding testers utilize two sensors to detect delaminations, voids, and the peeling of honeycomb cores, but they cannot determine which side of the part is damaged and are unable to detect defects smaller than 1.0 inch.

Phased Array Ultrasonic Testing: Phased array testing is one of the latest ultrasonic methods for detecting defects in composite structures. It operates on the same principle as pulse-echo methods but employs dozens or even more sensors simultaneously, significantly speeding up the inspection process.

(4)Thermal Imaging Testing The working principle of pulsed infrared thermal imaging involves the use of active heating techniques to automatically record surface defects in test specimens. It measures temperature differences in the matrix material caused by varying thermal properties, allowing for the identification of damage both on the surface and internally. This method is characterized by its non-contact, real-time, efficient, and intuitive nature, making it particularly suitable for detecting delaminations, porosity, peeling, layering, and area-type defects in bonded structures between composite materials and metals.

Thermal imaging is especially useful when parts or components cannot be submerged in water for ultrasonic C-scan testing, or when the surface shape of the parts makes ultrasonic inspection difficult. This technique provides a valuable alternative for assessing the integrity of complex structures.

(5) Radiographic Testing (RT), typically referring to X-ray inspection, is a valuable non-destructive testing (NDT) method as it allows an internal view of a component without disassembly. This method works by passing X-rays through the test part, capturing the variations in absorption on a film sensitive to X-rays. After developing the exposed film, inspectors analyze the differences in opacity, effectively creating a visual representation of the internal details.

While not ideal for detecting defects such as delaminations on planes perpendicular to the X-ray direction, X-ray testing excels at identifying defects parallel to the X-ray beam. Internal anomalies like corner delaminations, crushed cores, fractured cores, water in core cells, voids in foam adhesive joints, and the relative positioning of internal details are easily visible with X-ray imaging.

Since most composite materials are nearly transparent to X-rays, low-energy X-rays are required. Due to safety concerns, RT around aircraft is generally impractical, and operators must use lead shielding and maintain a safe distance from X-ray sources.

There are multiple radiographic testing techniques, each suited to specific applications:

Standard radiography is suitable for parts of moderate thickness.
Low-voltage radiography is used for thin parts (1–5 mm).
Gamma-ray radiography is applied for thick components.
Neutron radiography, a complementary method to X-ray imaging, visualizes internal features based on attenuation through different media. Neutron transmission is influenced by the neutron cross-section of atomic nuclei within the material, allowing for visualization of features like light elements (e.g., hydrogen in corrosion or water), which X-rays alone cannot reveal.

(6) Shearography Testing: Shearography is a laser-based optical method that uses a shearographic interferometer to detect and measure out-of-plane deformations in components. Initially, the part is measured under no-load conditions. Then, the test is repeated under applied loads, which may include thermal, mechanical, acoustic, pressure, vacuum, electric, magnetic, microwave, or mechanical stresses. This process allows a camera to capture strain fringe patterns on the laminate’s surface where subsurface defects are present.

Specialized computer software extrapolates the wrapped phase map images to create an unwrapped phase map, converting it into an integrated visual image for display and assessment. Notably, this technique can quickly reveal defect locations but requires further ultrasonic testing to determine defect depth.

Acoustic Emission (AE) Testing: Acoustic emission testing detects and analyzes sound emission signals produced by composite materials or structures under load, evaluating the overall quality of composite components. This technique is effective for defect analysis, reflecting the damage progression and failure patterns within composites, predicting final load-bearing strength, and identifying weak areas in component quality.

AE technology is practical and user-friendly, providing valuable insights into material deformation and damage processes during mechanical testing. AE methods mainly include:

Parameter Analysis: By recording and analyzing signal parameters such as amplitude, energy, duration, ring count, and event number, it assesses damage characteristics like severity, location, and failure mechanisms. However, a major drawback is that AE source information may be obscured by resonant sensors, leading to poor reproducibility in experimental results.

Waveform Analysis: This approach records and analyzes AE signal waveforms to obtain spectra and correlation functions. It helps identify frequency characteristics associated with damage stages and mechanisms, offering insights into the material’s damage profile.

Spectral Analysis: This technique, which includes both classical and modern spectral analysis, transforms AE signals from the time domain to the frequency domain. It enables the identification of intrinsic AE source information by studying various signal characteristics in the frequency domain. However, spectral analysis assumes the signal is a stationary, periodic signal, which limits its ability to capture localized information variations.