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
Inspection type versus NDT Method
For a particular type of inspection depending on the structure, materials, or damage characteristic have been identified to work the best. (Source)
A basic and useful part of inspection on 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 NDT that many instances use because it can save both time and money by reducing the number of other testings, 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.
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. 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, its orientation, and other characteristics could be achieved. There are three basic types, A-Scan, C-Scan, and ANDSCAN.
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.