NDT COURSE IN CHENNAI - NON DESTRUCTIVE TESTING IN CHENNAI - LIST OF NDT METHODS AND ADVANTAGES
LIQUID PENETRANT TESTING
Liquid penetrant testing (PT) is only used for detecting discontinuities open to the surface. With the correct application, it will detect discontinuities ranging in size from the large to the microscopic. The specially prepared liquids, characterized by low a viscosity, easily enter voids open to the surface when the part is dipped into or sprayed by the penetrant. Relatively simple, inexpensive and reliable.MAGNETIC PARTICLE TESTING
Magnetic particle testing (MT) is used to detect discontinuities in ferromagnetic parts – namely parts made of iron, steel, nickel, cobalt and the alloys of these materials. Ferromagnetic materials develop strong internal magnetic fields when an electric current is passed through the part. Magnetic particles are applied to the part and a discontinuity that causes a disruption in the induced magnetic field attracts the applied particles, producing an indication. MT is a highly effective inspection tool that is sensitive to the presence of cracks, laps, seams and similar types of surface and near-surface discontinuities.ULTRASONIC TESTING
Ultrasonic testing (UT) is used in the testing of nearly all solid materials, such as fine-grained aluminum, steels and alloys, composites and plastics – practically any solid material where detection of internal discontinuities or thickness measurements are of most common concern. It is also used in the detection of interlaminar separations and regions that have been improperly processed or damaged in layered composite structures. It is used in the detection and sizing of internal reflectors of ultrasonic pulses and thus is found in the testing of welds, forgings and raw materials in the form of plate, rod, pipe and similar simple geometrical shapes.RADIOGRAPHIC TESTING
Inspection techniques using radiographic testing (RT) are some of the most common approaches to visualizing the internal structures of components, materials and assemblies. The approach requires a source of electromagnetic radiation that can penetrate the item being examined during exposure (the time the item is illuminated by the radiation). Applications for RT are found throughout manufacturing and field-service environments. Raw materials are examined for the presence of internal discontinuities (castings, forgings); fabrications and assemblies are examined for induced discontinuities (welds) and misalignments or absence of internal parts; and inservice inspections use RT for detecting and assessing time-dependant degradation, such as corrosion, cracking and environmental damage.VISUAL TESTING
Visual testing is the most commonly used test method in industry. Because most test methods require that the operator look at the surface of the part being inspected, visual inspection is inherent in most of the other test methods. As the name implies, VT involves the visual observation of the surface of a test object to evaluate the presence of surface discontinuities. VT inspections may be by Direct Viewing, using line-of sight vision, or may be enhanced with the use of optical instruments such as magnifying glasses, mirrors, boroscopes, and computer-assisted viewing systems (Remote Viewing). Corrosion, misalignment of parts, physical damage and cracks are just some of the discontinuities that may be detected by visual examinations.EDDY CURRENT TESTING
Eddy current testing is particularly well suited for detecting surface cracks but can also be used to make electrical conductivity and coating thickness measurements. Periodically, power plants are shutdown for inspection. Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage. Alternating electrical current is passed through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in the material. These currents travel in closed loops and are called eddy currents. Eddy currents produce their own magnetic field that can be measured and used to find flaws and characterize conductivity, permeability, and dimensional features.Acoustic Emission Testing (AE)
Acoustic Emission Testing is performed by applying a localized external force such as an abrupt mechanical load or rapid temperature or pressure change to the part being tested. The resulting stress waves in turn generate short-lived, high frequency elastic waves in the form of small material displacements, or plastic deformation, on the part surface that are detected by sensors that have been attached to the part surface. When multiple sensors are used, the resulting data can be evaluated to locate discontinuities in the part.Leak Testing (LT)
Leak Testing, as the name implies, is used to detect through leaks using one of the four major LT techniques: Bubble, Pressure Change, Halogen Diode and Mass Spectrometer Testing. These techniques are described below.LT Techniques
Bubble Leak Testing, as the name implies, relies on the visual detection of a gas (usually air) leaking from a pressurized system. Small parts can be pressurized and immersed in a tank of liquid and larger vessels can be pressurized and inspected by spraying a soap solution that creates fine bubbles to the area being tested. For flat surfaces, the soap solution can be applied to the surface and a vacuum box can be used to create a negative pressure from the inspection side. If there are through leaks, bubbles will form, showing the location of the leak. Pressure Change Testing can be performed on closed systems only. Detection of a leak is done by either pressurizing the system or pulling a vacuum then monitoring the pressure. Loss of pressure or vacuum over a set period of time indicates that there is a leak in the system. Changes in temperature within the system can cause changes in pressure, so readings may have to be adjusted accordingly. Halogen Diode Testing is done by pressurizing a system with a mixture of air and a halogen-based tracer gas. After a set period of time, a halogen diode detection unit, or "sniffer", is used to locate leaks. Mass Spectrometer Testing can be done by pressurizing the test part with helium or a helium/air mixture within a test chamber then surveying the surfaces using a sniffer, which sends an air sample back to the spectrometer. Another technique creates a vacuum within the test chamber so that the gas within the pressurized system is drawn into the chamber through any leaks. The mass spectrometer is then used to sample the vacuum chamber and any helium present will be ionized, making very small amounts of helium readily detectable.