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With more and more composite structures getting used in various industries, knowing how to inspect them for damage is an important topic. Not only are there more composite parts, but in a case such as the planes, essentially the entire body structure is made from advanced composite materials. This article will take a look at some different inspection techniques to consider when working with composites.
Non-destructive Testing (NDT)
There are varieties of methods to evaluate materials or components and non-destructive methods are an important category of them with many applications. The field of Non-Destructive Evaluation (NDE) or Non-Destructive Testing (NDT) involves the identification and characterization of damages on the surface and interior of materials. without cutting apart or otherwise altering the material. The basic types of NDT methods include contact and non-contact methods and both of them have their specific applications in testing and evaluating the composites. Categorized NDT methods to contact methods and non-contact methods are shown in the table below
Figure 1. Contact methods and non-contact methods in non-destructive testing (NDT) methods
Inspection type versus NDT Method
For a particular type of inspection depending on the structure, materials, or damage characteristics have been identified to work the best. (Source)
Figure 2. Inspection Types and Non-destructive Testing (NDT) Methods
A basic and useful part of the inspection of composite structures is a visual inspection. The inspector looks for visible signs of damage to the structure like burns, debonds, and delaminations. Of course. This should be the most basic type of Non-destructive Testing (NDT)
that many instances use because it can save both time and money by reducing the number of other tests, or in some cases reducing the need for other types of testing altogether. The most important advantage of visual inspection is its quick process. The other advantage of visual inspection is the relative affordability of the process. The visual inspection needs no equipment but this method has its intrinsic disadvantage.
Figure 3. Tap Testing Method (Source: Flight Mechanic)
This is another basic method of inspection. You tap a coin along the part's surface. A bright, metallic sound indicates good structure. A dull "thud" would indicate a flaw such as delamination or debond. It is more effective on thinner structures and loses its effectiveness on very thick laminates. Another disadvantage is that it won't go through the core. It can also sometimes give false readings, say if another structure is attached on the backside.
Ultrasonic inspection is another type of non-destructive testing that may be considered for the inspection of composite structures. The ultrasonic evaluation system consists of a transmitter and receiver circuit, transducer tool, and display devices. Based on the information carried by the signal, crack location, flaw size, orientation, and other characteristics could be achieved. There are three basic types, A-Scan, C-Scan, and ANDSCAN.
Video 1. Non-destructive Testing (NDT) - Ultrasonic Testing (Source: Applus RTD)
A-Scan ultrasonic equipment uses a pulse-echo system to locate flaws. A probe, or transducer, is placed on the surface to be inspected using a gel couplant to ensure good sound conductivity. The probe then transmits ultrasonic pulses through the part. It measures the time it takes to bounce from the back surface of the part. Flaws in the material like debonds and water intrusion would result in a different time of flight. The user needs to know the material velocity (the speed at which sound travels through a material) of the material being tested and input that into the tester.
C-Scan also uses ultrasonic sound to inspect parts. But unlike A-Scan, it measures the amount of time that the sound passes through the thickness of the part. For this reason, you need access to both sides of the part. In addition, it needs to be submerged in a liquid medium to ensure good sound transmission. This can cause water intrusion in some parts. But C-Scan does perform better than A-Scan on composites. It provides a good overall scan image of the part that the inspector can use to locate damage.
This product from Agfa NDT takes A-Scan to the next level. A mechanical arm is attached to the transducer, which is connected to a computer. The arm provides the computer with x, y, and z coordinates in space for the transducer. This enables the computer to generate a C-Scan-like image of the inspection area. The computer also stores the information by tail number allowing for trend analysis.
Thermography can be used to inspect composite structures. In its most basic form, heat is applied to a part and then the part is viewed through an infrared camera as it cools. This can give indications of a flaw to a trained eye.
Radiographic Testing (RT) is the most commonly used testing method. The most common type of damage to composites is delamination resulting in an air pocket; delamination can only be seen in RT if its orientation is not perpendicular to the x-ray beam. There are many types of radiography and each has specific applications. Conventional radiography is the most useful when the parts are neither too thick nor too thin. For thin parts, 1 to 5 mm, low voltage radiography is used and γ-rays radiography is good for thick parts. These types of radiography are useful in detecting large voids, inclusions, trans-laminar cracks, non-uniform fiber distribution, and fiber misorientation such as fiber wrinkles or weld lines.
X-ray inspections of composites are performed like those on metal structures. Images are based on material density. It can also see some delaminations if set up at an angle. Water and inclusions can also be seen
Acoustic Emission (AE) is an effective method of imperfection analysis. This mechanical vibration is generated by material defects such as matrix micro-cracking, fiber-matrix debonding, localized delamination, or fiber pullout and breakage. The stress waves that result from these types of defects spread out concentrically from their origin and are detected by an array of highly sensitive piezoelectrics.
Video 2. Acousto-Ultrasonic Imaging of delaminations in CFRP hat profile (Source: CREO Dynamics AB)
Acousto-Ultrasonic is a combination method of acoustic and ultrasonic testing that is used specifically to determine the severity of internal imperfections and inhomogeneity in a composite. In non-destructive testing, the acoustic/ultrasonic class of testing has great potential based on optimal economy, flexibility, and sensitivity. However, no available method is sensitive or reliable enough to effectively detect. It is a useful method because it allows non-critical flaws to see and assess. The second advantage is that it is a good indicator of accumulated damage in a structure due to fatigue loading or impact damage.
Video 3. NDT of a composite panel using Shearography (Source: Vic S.)
Shearography Testing is a laser optical method. The failure of composites usually happens by stress concentrations and the criticality of defects will easily deduct by the degree of strain concentrations around a particular defect, this is an advantage of shearography. The second advantage of shearography is that it is less susceptible to noise than many other types of nondestructive testing. This is good because it allows less skilled users to be able to inspect and determine the usability of a part without extensive training. A major disadvantage of shearography is that characterization of defect types other than delamination is extremely difficult. Therefore it is sometimes paired with other types of non-destructive evaluation techniques that can help to identify certain defects.
In the end, it all comes down to training. Many companies offer training on composite inspection and repair. In addition, training is available on the different types of testing equipment mentioned. The more knowledgeable you are on inspection of composite structures and how to use the various inspection tools, the more comfortable you can be that the aircraft or component you are inspecting is airworthy.
Not all the NDT techniques are covered in this blog. NDT techniques vary from very basic to very advanced image/signal processing. Given the application or given instrument/resources the NDT approach can be chosen. The goal is to identify defects and plan a repair or discard the composite part.
Addcomposites is the provider of the Automated Fiber Placement (AFP) system. The AFP system can be rented on a monthly basis to work with thermosets, thermoplastic, dry fiber placement, or in combination with 3d printers.