Non-Destructive Testing
Non-destructive testing (NDT) is exactly what it sounds like. It is the testing of materials in both science and industry that is done in such a way that the material under test in undamaged. Obviously, there are many situations where you want to test something without having to damage it. This can be important for quality control in general, and also to avoid catastrophic failure of material in service. There are a number of methods that can be used.
Visual Inspection
The most obvious form of NDT is to simply look at the material in question. This is, of course, a common form of quality control in many situations. It can also be used to collect data on the material if it is done in a systematic manner. Having said that, ‘visual inspection’, in NDT, does not simply mean looking at the material. Any unaided use of the senses, or even aided by the use of general equipment, is considered visual inspection. This could include listening to, or feeling for, vibration in the material under investigation. The use of more specialised equipment separates other types of NDT from visual inspection.
Dye Penetrant Inspection (DP)
This may also be referred to as penetrant testing (PT) or liquid penetrate inspection (LPI). DP is a frequently used and reasonably inexpensive inspection method. It is commonly used to identify or locate cracks or similar breaks on the surface of ceramics, metals, or plastics, as well as other non-porous materials. DP can be used to detect a number of faults including leaks, or fatigue cracks on the surface, of material in use. This is clearly better than having to take material out of use. Likewise, it can be used to identify issues such as hairline cracks in material that has been cast, welded, or forged. As the name suggests, it is based on dye penetrating these flaws and making them visible. While DP can be used on ferrous as well as non-ferrous materials, it is more common to use magnetic-particle inspection on ferrous materials as this is better at detecting subsurface issues.
Magnetic Particle Inspection (MPI)
Much like DP, this is primarily a method for detecting surface issues, though in this case it is only used with ferromagnetic materials. It has the added advantage of being able to detect some subsurface faults as well. The process works by inducing a magnetic field in the piece to be tested. This can be done either by applying an external magnetic field, or by electrifying the piece and generating an internal magnetic field. Small ferrous fragments, in various solutions, are added to the exterior of the part being tested, and these will be attracted to any area where there is a surface or shallow subsurface disruption, allowing the issue to be identified. This is because the disruption effectively allows the magnetic field to ‘leak’ from the material and attract the particles on the surface. Like DP, it allows for reasonably simple identification of flaws.
Ultrasonic Testing (UT)
UT is a form of NDT which is most commonly used on metals. It can be used on other materials, such as wood, concrete and composites, but with reduced resolution. It is a widely used form of testing in a number of industries, such as aerospace and the transport industry in general. In fact, UT is common in many fields where metal is widely used. The technique involves the use of ultrasonic pulses around the 0.1-15 MHz range, though occasionally larger. These pulses penetrate the materials under test and can be used to locate defects, or even simply to better understand the material’s current condition. For example, UT can be used to measure the thickness of an object’s walls without having to open it, such as when there is concern about corrosion inside pipes.
Weld Verification
Not all welds are created equal. While welding is a common method for joining metal, there are a number of issues that can influence the strength of the weld. The temperature of the base metal, the cooling rate, the materials being used in the welding process, all influence the strength of the weld. A weld may look solid, but may suffer from a number of issues including cracks, porosity, and poor weld density. There may be issues with the weld not fusing correctly to the base metal. Given the stresses and possible fatigue that welds will undergo, a proper weld is essential to avoid what could be a catastrophic failure.
Proper preparation is important to prevent such issues from arising. Acoustic emission techniques, which monitor the stresses materials are subject to, can be used to plan the best way to connect two materials in a structure. If the welds are particularly critical, digital weld monitoring can be used to make certain that the particular specifications, in terms of heat, arc voltage, and so on, are being met. With the use of these techniques, the weld should be correct and meet all standards, though further NDT would be able to confirm that.
Welder Qualifications and Assessment
Of course, one of the easiest and cost-effective means of maintaining high-quality welds is to make sure that all welds are performed by well qualified and trained welders. Adequate qualification and assessment is one of the most worthwhile forms of quality control.
In-Service Inspection and Testing
In a similar sense, proper in-service testing of production equipment is an essential means of quality control. Manufacturers of equipment will almost always establish a test and inspection regime for their systems. This ranges from simple eyeball inspections to test procedures. Given that these are based on the manufacturers’ assessment of probable failure rate, these tests should be followed as closely as possible.