What is NDT ? The most valuable source of industry testing.
The simplest and most accurate way of testing materials and components is often to test them to destruction. Destructive testing is used in aerospace to determine the physical properties of materials, components and assemblies. It can reveal useful information about characteristics of materials including ductility, tensile strength and fracture toughness.
However, destructive testing is not always possible or desirable when testing parts and materials destined for, or already in use on, aircraft mainly because of their high value.
Instead engineers and technicians use non-destructive testing techniques, equipment and analysis to examine materials and components before they enter service and while they are in service, without destroying them. NDT can also detect flaws and differences in materials that would otherwise be very difficult to detect using destructive methods.
How is NDT used ?
The vital aspect of NDT is that the part or the material being tested remains whole and serviceable after the testing.
NDT can generally be split into two areas: surface techniques and sub-surface techniques. Within these two areas is a range of established and trusted tools, such as tap tests conducted with hammers to more recent and advanced technologies, such as 3D computed tomography.
Visual inspections are the oldest and simplest method of non-destructive testing. It’s often said the human eye is the most powerful NDT tool. Visual inspections of aircraft structures and components for damage such as cracks, corrosion and misalignment will often be the first sign of a problem.
Various equipment is used for visual inspections, from magnifying glasses and mirrors to video borescopes for viewing hard to reach places, charge-coupled devices and remote viewing systems.
Liquid penetrant testing is another simple and quick method widely used in aviation to detect surface defects and structural damage in non-porous materials.
Test objects are coated with a highly viscous dye. Once the dye has settled into any cracks or flaws, the object is cleaned, leaving just the dye which has penetrated the cracks. Some of this remaining dye will flow back out, revealing an indication of the cracks and flaws.
Fluorescent penetrants are often used for sensitive materials and parts. Liquid penetrant testing is a flexible technique and can be carried out in-situ or in a workshop or hangar. It is also often part of cleaning and servicing parts, where the surfaces of objects are inspected after cleaning.
Leak testing uses four main techniques: bubble, pressure change, halogen diode and mass spectrometer testing, and involves pressurizing and immersing the test object in liquid to trace and record leaks. Aircraft and engines use large amounts of liquids and gas – leak testing is therefore an important part of manufacturing and maintenance.
Radiography in aerospace can use both x-rays for thin materials and gamma rays for thicker materials. Traditionally film has been used to capture the image, but is being superseded by digital methods.
Magnetic particle testing involves inducing a magnetic field in the test object, applying magnetic particles to it either in dry form or suspended in a liquid, which could be colored or fluorescent, and then examining the object using suitable lighting.
The technique is used to detect discontinuities in ferromagnetic materials. Several different pieces of equipment can be used for magnetic particle testing depending on the environment, including yokes, prods, coils and heads.
Eddy-current testing is a sub-surface technique that induces an electromagnetic field in a conductive test object and measures the secondary magnetic field generated around the electric current to determine where flaws are. Eddy current testing is widely used in aircraft maintenance to detect cracks caused by fatigue or corrosion.
NDT standards and training
Although looking for cracks and flaws in aircraft and materials is an arduous task it can mean the difference between life and death. The need to ensure high standards of quality and safety during manufacturing, assembly and while in-service has led to the development of NDT standards and specifications for both civil and military aviation.
There are several national and international organizations that develop standards for NDT methods, equipment and training. These include the American Society for Nondestructive Testing (ASNT), the British Institute of Non-Destructive Testing, the International Committee for Non-Destructive Testing and the European Federation for Non-Destructive Testing. International standards are also overseen by the ISO and the ASTM International (American Society for Testing and Materials).
Standard exists for both civil and military tests and training. Common certifications for staff using non-destructive testing include ASNT’s Central Certification Program (ACCP), while the SNT-TC-1A Personnel Qualification and Certification in Nondestructive Testing provides guidelines and a framework for in-house NDT certification programs. NDT has three different levels of qualification, each with more duties and responsibilities than the last.
