Summary of Different Non-Destructive Testing Methods for Composite Materials

1. Non-Destructive Testing (NDT)

There are various methods for evaluating materials or components, and among them, non-destructive testing has become an important category due to its wide application. Non-Destructive Evaluation (NDE) or Non-Destructive Testing (NDT) involves identifying and characterizing surface and internal damage in materials without cutting or altering them. The basic methods of NDT include contact and non-contact techniques, both of which have specific applications in testing and evaluating composite materials.

Inspection Types and Non-Destructive Testing Methods
For specific types of inspections, the optimal working method has been identified depending on the structure, material, or damage characteristics.

2. Inspection Methods

1. Visual Inspection:
Visual inspection is a fundamental and useful part of inspecting composite structures. Inspectors look for visible signs of damage, such as burns, delaminations, and debonding. This is the most basic type of non-destructive testing (NDT).

Many instances utilize visual inspection because it can save time and money by reducing the number of other tests or, in some cases, eliminating the need for other types of testing altogether. The greatest advantage of visual inspection is the speed of the process. Another advantage is the relatively low cost of the process. Visual inspection requires no equipment, but this method also has inherent drawbacks.

2. Tap Testing:
This is another basic inspection method. You can tap the surface of the component with a coin. A bright, metallic sound indicates a good structure. A dull "thud" sound may indicate defects such as delamination or debonding. This method is more effective on thinner structures, but its effectiveness diminishes on very thick laminates. Another drawback is that it cannot penetrate the core. Sometimes, false readings can occur if other structures are attached to the back.

3. Ultrasonic Inspection:
Ultrasonic inspection is another non-destructive testing method that can be used for composite structures. An ultrasonic evaluation system consists of a transmitter and receiver circuit, transducer tool, and display device. Based on the information carried by the signal, the location of cracks, size of defects, orientation, and other characteristics can be determined. There are three basic types of ultrasonic inspection: A-Scan, C-Scan, and ANDSCAN.

Figure 1: Ultrasonic Non-destructive Testing

A-Scan:

A-Scan ultrasonic equipment uses a pulse-echo system to locate defects. A probe (or transducer) is placed on the surface to be inspected and uses a gel coupling agent to ensure good acoustic conductivity. The probe then sends ultrasonic pulses through the component and measures the time it takes for the pulses to bounce back from the rear surface of the component. Defects in the material, such as delamination and moisture ingress, cause different flight times. The user needs to know the speed of sound in the material being tested and input it into the tester.

C-Scan:

C-Scan also uses ultrasound to inspect components, but unlike A-Scan, it measures the time it takes for the sound to pass through the thickness of the component. Therefore, you need to be able to access both sides of the component. Additionally, it requires submersion in a liquid medium to ensure good sound transmission, which may lead to moisture ingress in some components. However, C-Scan performs better on composites than A-Scan. It provides an overall scan image of the component that the inspector can use to locate damage.

ANDSCAN:

This is a product from Agfa NDT that takes A-Scan to the next level. A mechanical arm is attached to the transducer, which is then connected to a computer. The mechanical arm provides the x, y, z coordinates of the transducer in space to the computer. This allows the computer to generate a C-Scan-like image of the inspection area. The computer also stores the information for trend analysis through tail numbers.

4. Thermography:

Thermography can be used to inspect composite structures. In its most basic form, heat is applied to the component, and the cooling process is observed through an infrared camera. This process can indicate defects to a trained eye.

Figure 2: Thermography Non-destructive Testing

5. Radiographic Testing:

Radiographic testing (RT) is one of the most commonly used testing methods. The most common type of damage in composites is delamination, which leads to the formation of air pockets. Delamination can only be seen in RT if it is not perpendicular to the X-ray beam. There are many types of radiographic testing, each with its specific application. Conventional radiographic testing is most useful when components are neither too thick nor too thin. Low-voltage radiographic testing is used for thin components ranging from 1 to 5 millimeters, while gamma-ray testing is suitable for thick components. These types of radiographic testing are useful in detecting large voids, inclusions, interlaminar cracks, uneven fiber distribution, and fiber misalignment, such as fiber wrinkles or weld lines.

6. X-ray Inspection:

X-ray inspection of composites is similar to X-ray inspection of metal structures. The image is based on the material's density. If the X-ray is set at a certain angle, it can also reveal some delaminations. Additionally, water and inclusions can be detected.

Figure 3: Application of X-ray Non-destructive Testing Instruments in the Casting Industry

7. Acoustic Emission:

Acoustic emission (AE) is an effective method for defect analysis. This mechanical vibration is generated by material defects such as matrix microcracks, fiber-matrix debonding, localized delamination, or fiber pull-out and breakage. The stress waves generated by these defects propagate concentrically outward from their origin and are detected by a series of highly sensitive piezoelectric elements.

Figure 4: Acoustic Emission System

8. Acousto-Ultrasonic:

Acousto-ultrasonic is a combined method of acoustic and ultrasonic testing, specifically used to determine the severity of internal defects and inhomogeneities in composite materials. Among NDT methods, acoustic/ultrasonic testing has great potential for its economic efficiency, flexibility, and sensitivity. However, there is currently no sufficiently sensitive or reliable method for effective testing. Acousto-ultrasonic is a useful method because it allows the observation and evaluation of non-critical defects. A second advantage is that it is a good indicator of accumulated damage in the structure due to fatigue loading or impact damage.

9. Shearography:

Shearography testing is an optical laser method. Failures in composites are often caused by stress concentrations, and the criticality of defects can be inferred by the degree of strain concentration around specific defects, which is an advantage of shearography. A second advantage of shearography is that it is less affected by noise than many other types of NDT. This is beneficial because it allows less skilled users to inspect and determine the usability of a component without extensive training. A major drawback of shearography is that it is extremely difficult to characterize defect types other than delamination. Therefore, it is sometimes combined with other types of non-destructive evaluation techniques to help identify certain defects.

3. Conclusion

This article does not cover all NDT techniques. NDT methods range from very basic to highly advanced image/signal processing. Depending on the application or the available instruments/resources, different NDT methods can be chosen. The goal is to identify defects and plan for repair or disposal of composite material components.