Almost Each & Everyone of us benefits from the Applications of NDT, in one way or another, Yet Hardly Anyone Knows what is NDT, what are the benefits of NDT, where NDT is used & what are the prospects in the field of NDT.
NDT, as its full form goes, “Non Destructive Testing”, is a wide group of Methods OR Analysis Techniques OR Process of inspecting, testing, or evaluating materials, parts, components or assemblies for detecting discontinuities, or evaluating material properties, strength, integrity or differences in characteristics without impairing its future usefulness OR destroying the serviceability of the part or system.
In contrast to NDT, other tests are destructive in nature and are therefore done on a limited number of samples (“lot sampling”), rather than on the materials, components or assemblies actually being put into service.
NDT is a testing system evolved to overcome problems of the olden days methods of Inspection. (i.e. Visual Testing & destructive Testing)
1) Visual Testing – Only surface Inspection was possible &
2) Destructive Testing – Inspection of only few samples was possible, as the test are destructive.
But With NDT; You can Inspect or measure 100% Volume of 100% samples !
1) Not only surface inspection, but inspection of 100% Volume below surface is also possible &
2) Inspection of all the 100% Samples as the testing is Nondestructive !
Nondestructive tests are used in;
Manufacturing, Fabrication and In-Service Inspections;
- To ensure product integrity and reliability,
- To control manufacturing processes,
- To lower production costs and
- To maintain a uniform quality level.
During construction, NDT is used to ensure the quality of materials and joining processes,
During the fabrication and erection phases, and
In-Service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public.
The terms Nondestructive examination (NDE), Nondestructive inspection (NDI), and Nondestructive evaluation (NDE) are also commonly used to describe this technology.
Since the 1920s, nondestructive testing has developed from a laboratory curiosity to an indispensable tool of production, as, it can be used for inspection of the entire 100% lot, which was not possible with Destructive Testing (DT) OR Visual examinations, which were the only methods used in the earlier days.
Because NDT does not permanently alter the article being inspected & facilitates 100% inspection easily, it is a highly valuable & much in demand Quality Control technique as it saves both money and time in product evaluation, troubleshooting, and research, and is making our lives & property more & more safer, with continuous technological developments, coming in the same.
In today’s world where new materials are being developed, older materials and bonding methods are being subjected to higher pressures and loads, NDT ensures that materials can continue to operate to their highest capacity with the assurance that they will not fail within predetermined time limits.
NDT can be used to ensure the quality right from raw material stage through fabrication and processing to pre-service and in-service inspection. Apart from ensuring the structural integrity, quality and reliability of components and plants, today NDT finds extensive applications for condition monitoring, residual life assessment, energy audit, etc.
Non Destructive test, is in use in great variety worldwide, for some of the following applications;
1) To detect internal, external flaws or other physical discontinuities,
2) To measure geometric characteristics,
3) To determine material structure,
4) To determine understand material composition,
5) To measure or detect some of the material’s properties,
6) For in-service inspection,
7) For condition monitoring of operating plant,
8) For measurement of components and spacing’s,
9) For measurement of physical properties such as hardness and internal stress,
10) For detecting variations in structure,
11) For detecting minute changes in surface finish,
12) To measure the thickness of materials and coatings,
13) To determine other characteristics of industrial products.
The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion.
NDT Technicians/Inspectors/Engineers/Third Party Inspectors & Managers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of less visible, but equally troubling events.
Because NDT allows 100% inspection without interfering with a product’s final use, NDT provides an excellent balance between quality control and cost-effectiveness, is highly valuable & much in demand Quality Control technology that saves both money and time in product evaluation, troubleshooting, and research, and is making our lives & property more & more safer, with continuous technological developments, coming in the same.
All sorts of industries use this type of inspection to help insure that processes run smoothly and safely and products are manufactured with a high degree of integrity. Some of the industries that profit from NDT are the Oil and Gas Industry, Power Generation, Aviation and Aeronautics, the Auto Industry including Racing, Maritime Construction, Research & Development all branches of the Military and Construction of all types.
NDT is used in a variety of settings that covers a wide range of industrial activity, with new NDT methods and applications, being continuously developed. NDT services  are not only integrated with Asset Integrity Management (AIM) solutions, but also with Material Testing laboratories and seamlessly fit into Supply Chain services.
There are many NDT techniques/methods used, depending on four main criteria:
- Material Type
- Defect Type
- Defect Size
- Defect Location
Volumetric Examination Methods
1) Ultrasonic Testing – (UT) : Ultrasonic inspection uses high frequency sound waves to detect imperfections or changes in properties within the materials. Sound is introduced into the object being examined and reflections from internal imperfections, areas of acoustic impedance, or varying geometrical surfaces are returned to a receiver. It can also be used to measure the thickness of a wide range of metallic and non-metallic materials where access from one side only is available.
INSIDE VISION BY NDT- (UT)
Crack at center of Material detected by Ultrasonic Testing.
2) Radiography Testing – (RT) :
Radiography Testing uses an X-ray device or Radioactive Isotope as a source of high frequency electromagnetic radiations which passes through the job and is captured on film or digital device, after being deferentially absorbed by the material. After processing the film an image of varying density is obtained. Possible imperfections are identified by qualified Radiographer through density changes, the same way a doctor looks for fractures or other conditions within the body.
INSIDE VISION BY NDT- (RT)
Crack at center of Material detected by Radiographic Testing.
Surface Examination Methods
3) Liquid Penetrant – PT : In Liquid Penetrant the test object or material is coated with a visible or fluorescent dye solution. Once applied to the surface, the dye will effectively penetrate any surface-breaking cavity. The excess dye is removed from the surface. A developer which acts like a blotter is applied to draw out any penetrant that remains unseen inside, surface breaking cavities. With visible dyes, the colour contrast between the penetrant and the developer makes the “bleed-out” easy to see. With fluorescent dyes an ultraviolet lamp is used to make the ‘bleed out’ fluoresce brightly allowing the smallest imperfections also to be seen much more easily.
SURFACE MICRO VISION BY NDT – (PT)
Micro-Crack on RH baggage door hinge of Aircraft detected by Penetrant Testing.
4) Visual Inspection – VT : The oldest of all the methods. Components are scanned visually, sometimes with the aid of low or high power lenses, fibrescopes, cameras and video equipment, to determine surface condition. Visual Examination can be an effective way to recognize surface imperfections that could adversely affect a part or component. Visual Examiners use knowledge of how a part is manufactured, the function of the human eye, lighting requirements, and precise measuring tools to evaluate materials. Computer controlled camera systems and optical aids such as boroscopes may also be used to recognize and measure features of a component.
Surface & Sub-Surface Examination Methods
5) Magnetic Particle – MT : Magnetic Particle inspection is used to identify surface and near surface discontinuities in ferromagnetic materials such as Iron, Cobalt, Nickel and most of their alloys, some compounds of rare earth metals, and a few naturally-occurring minerals such as lodestone. Magnetic Particle Examination is accomplished by inducing a magnetic field into a ferromagnetic material and applying iron particles to the surface of the item being examined. This Method uses the principle that magnetic lines of force (flux) will be distorted by the presence of a discontinuity. Discontinuities (for example, cracks) are located from the flux distortion following the application of fine magnetic particles to the area under test. Surface and near-surface discontinuities affect the flow of the magnetic field within the part causing the applied particles to gather at locations of flux leakage, thus producing a visible indication of the irregularity on the surface of the material.
INSIDE VISION BY NDT – (MT)
Unseen crack below surface detected by magnetic particle Testing.
6) Eddy Current – ET : In eddy current testing electrical currents are generated in a conductive material by an induced magnetic field. Distortions in the flow of the electric current (eddy currents) caused by imperfections or changes in a material’s conductive properties will cause changes in the induced magnetic field. These changes, when detected, indicate the presence of the imperfection or change in the test material.
Integrity Examination Methods
7) Leak Testing – LT : Leaks can be detected by using electronic listening devices, pressure gauge measurements, liquid and gas penetrant techniques or a simple soap-bubble test. Several techniques are used to detect and locate leaks in pressure retaining components such as pressure vessels, pipelines etc.
8) Acoustic Emission Testing – AET : When a solid material is stressed, growing imperfections, if any within the material emit short bursts of acoustic energy called “emissions”. As in ultrasonic testing, acoustic emissions can be detected by special receivers. Emission sources can be evaluated through the study of their intensity, rate and other characteristics. The growing defects can be located by triangulation technique (similar to earthquake epicenter location)
Condition Monitoring Methods
9) Thermography – Infrared Testing – IR : Thermography enables the thermal profile of an item, machine or building to be presented in a graphic form which allows a working temperature assessment to be derived. From this, variations in the material or component temperature are identified, enabling working limits or corrective actions to be identified.
10) Vibration Analysis – VA : The rotary machines produce vibration noise. By monitoring the frequency, amplitude etc. of the vibration the condition of the machine can be estimated.
Special NDT methods
NDT engineers and technicians also use magnetic resonance imaging, vibration monitoring, Phased Array(PA), Time Of Flight Diffraction(TOFD), laser ultrasonics, holography, Computed Tomography(CT) as well as many other specialized methods & techniques for specialized applications