Nondestructive Testing (NDT) is the name given to the processes designed to verify the integrity of a structure in such a way that the performance of that component is not impaired by the test. It has applications in many diverse fields from the industrial such as medical, construction and automotive to the entertaining such as amusement park rides. In all cases, the role of NDT is now recognized as vital to ensure the reliability of the product. This vital role is never more important than it is in the aerospace industry.
In its infancy, NDT was quite a crude technology. Going back hundreds of years, metalworkers such as blacksmiths and bell makers would tap their work to listen to the ringing noise this produced. This would indicate the strength of the material. As recently as the early twentieth century, this methodology was still in use: people were employed as “wheel tappers.” When locomotives were stationary, these individuals would test the wheels by tapping them to find cracks, which were typically caused by fatigue. This test is still performed - albeit by instrumentation and it is now referred to as the acoustic impact technique. In testing of honeycomb structures in aircraft a backup used to be a “coin tap” to verify a dis-bond.
Many different NDT methods have been developed through the years and for different reasons. For example, although X-rays were discovered in 1895 and used for diagnostic purposes by the medical field, it was not until the early 1920s that X-ray was shown to be effective when used on inanimate material. However, although radiographic testing was able to effectively indicate some material defects, it was expensive and had serious health concerns, so industry developed other tests as well.
Functional fluorescent penetrant system
Modern nondestructive tests are used by manufacturers for several reasons, ranging from product reliability verification to improved product design or manufacturing process control and cost reduction. Another factor that has influenced the application of NDT in today’s world is the increasing pace of life: the modern world demands everything to be more efficient and more convenient and this places stress on equipment as well as people. Products are lighter and smaller than they used to be while still bearing the same load, or more, as before. Aerospace is indeed one of the many areas in which NDT methods are used to indicate material defects. For obvious reasons, it is vital to detect material flaws without impairing the use of the component through the testing process. Individual component reliability contributes to the ultimate final product safety; i.e., the aeroplane structure or engine.
Essentially, if a component has a one in 1,000 chance of failing and it is assembled together with 99 other parts, which all have the same failure probability rating, that final product will have a failure probability rating of about 10%, meaning that one in 10 may fail. This is obviously an unacceptable risk to the user and further explains the use of testing methods such as NDT in order to anticipate and minimize the potential failure opportunity.
However, for the aerospace field, it is not enough to perform nondestructive tests: it is also vital to monitor those involved in the administering of these tests to ensure that this function is being performed, reported and, where applicable, issues are being resolved appropriately. This role has been allocated by the aerospace industry to the Performance Review Institute, which audits and accredits NDT suppliers to the aerospace industry through Nadcap.
A crack in a weld found with fluorescent penetrant
Nadcap is a leading worldwide cooperative program of major companies designed to manage a cost-effective consensus approach to special processes and products and provide continual improvement within the aerospace & automotive industries. NDT is just one of the special processes that it includes in its scope of accreditation services.
Major aerospace organizations all over the world such as Airbus, The Boeing Company, GE Aviation and Rolls-Royce plc work together with industry suppliers to agree to the standard that a supplier must attain in order to become and remain Nadcap approved. Increasingly, aerospace companies will only sub-contract with Nadcap approved suppliers, as this is a globally recognized industry-wide standard of work ensuring that a part purchased from Beijing is of similar quality to a part originating from New York.
Nadcap NDT covers the following NDT methods:
Fluorescent Penetrant Inspection (FPI)
Described by the British Institute of NDT as a “simple low-cost method of detecting surface-breaking flaws,” cracks as small as 150 nanometers can be identified using FPI. However, the restriction of this method is that the flaw must reach the surface of the material in order to be detected.
A cracked engine mount found with magnetic particle inspection
There are several steps to this process: first, the surface of the test piece must be very clean and free from all contaminates. Then the penetrant is applied; this may be done via dipping, spray or brush etc.; after a specified time, the excess penetrant is removed from the surface. At this point a developer is applied to facilitate the surface inspection and pull the penetrant trapped in the flaw back to the surface. In the past this was done visually although in recent years, some organizations have begun to automate the inspection process via television or camera for robotic pattern recognition. Finally, a post-inspection cleaning to protect against corrosion should be applied.
Magnetic Particle Inspection (MPI)
Primarily used for crack detection, this method can be used for near-surface flaws (voids) as well as surface flaws. However, it can only be used on ferromagnetic materials. Either a section or the entire component is magnetized, and the presence of a flaw distorts the magnetic flux, which causes a leakage of the magnetic field on the surface of the part. When ferromagnetic particles are applied to the surface the flux leakage holds them in place, making much more visual the flaw being detected. This deformation is not limited to the location of the flaw but may extend some.
An X-ray unit setup utilizing digital capture (DDA)
Ultrasonic Testing (UT)
Using vibrations of short wavelength (1 - 10mm) and high frequency (0.1 - 20Mhz), this method can be used to detect internal (subsurface) flaws and can also determine the size of an indication, crack or other flaw. Sound waves at very high frequencies are transmitted by a transducer through a component and are received by that or another transducer. The path of that sound is interrupted by a flaw in the component and a signal is provided to the operator. While ultrasonic testing is highly sensitive and produces accurate information, it requires expensive equipment and extensive technical knowledge. A typical component tested using ultrasound would be critical turbine blades, welds or even base material that will be used to manufacture critical components.
Radiographic Testing (RT)
Used to detect subsurface, hidden flaws, X-rays or gamma-rays produce an image of the component on film. This happens when a film is placed underneath the component and radiation is emitted from a radiation source (X-Ray or Gamma), penetrating the material and striking the film on the other side. The radiation enters the film and exposes the chemicals in that film to varying degrees depending on the thickness of the areas of the component. More radiation strikes the film in thinner sections of the part and less radiation strikes the film in thicker areas of the part. This exposure of the film in differing amounts, once the film is processed, produces the image, now referred to as a radiograph. A typical usage of radiographic testing in aerospace is to check for defects in weldments or castings.
Digital Radiography (RT)
The two digital radiographic methods monitored by Nadcap are Digital Detector Arrays (DDA) and Computed Radiography (CR). These terms both identify the method that the image is recorded. A Digital Detector Array is just that, a device made up of a multitude of digital components arranged in a fashion to be used to capture the radiographic image. The difference is in how that image can be pulled off the DDA and saved into a computer. Computed radiography is very similar in its ability to be downloaded and saved in a computer file but is actually a phosphorous plate more similar to a piece of film. Both methods allow for enhancement of the image and the storage of an enormous amount of information. Not to mention no recycle costs related to film development chemicals. Digital radiography has the same typical uses as does radiographic testing using film.
An ultrasonic machine testing a large aerospace part
Eddy Current Testing (ET)
A method of inspecting materials for surface and slight sub-surface indications that do not meet the engineering requirements. A “probe” is utilized, energized by a device to create alternating magnetic fields. These alternating magnetic fields will induce a circulating electrical current in any electrically conductive material. A crack (for example) exposed to or very near the surface will interrupt the flow of the electrical field, causing an “indication” of that interruption. That resulting indication can be evaluated and interpreted to a very high degree providing a vast amount of information concerning the “flaws” in the component under test. Typical applications are aluminum or titanium machined components; bolt holes, surface areas surrounding fasteners, bearings and races.
While NDT methods have not significantly changed over time, they are certainly more sophisticated. That means that industry must continually identify more robust methods for safeguarding the reliability of NDT. Nadcap is the system that the aerospace industry has been using since 1990 to achieve this. Nadcap is an Industry Managed Program, that means, in each of the methods you see here, the audit criteria have been developed by subscribers, suppliers and other industry experts to achieve the most robust, complete and thorough process audit in the industry.
Each NDT audit goes into a thorough review of qualification/certification requirements; process procedures that address the method; process controls required; calibration and documentation of requirements being flowed down to the auditee. All these items are verified to be addressed by procedure and then are actually witnessed and verified as conducted correctly. This is then followed up by several components being inspected to specific customer criteria and witnessed by the auditor to ensure that all requirements are met. Lastly the auditor verifies that the actual inspection is conducted in accordance with requirements and that indications are properly evaluated and dispositioned. This makes for one very thorough NDT audit.