Introduction to Non Destructive Testing (NDT)
NDT is a non invasive technique to determine the integrity of a material, component or structure.
Or quantitatively measure some characteristic of an object.
i.e. Inspect or measure without doing harm.
There are six most common NDT methods,
- Visual Testing (VT)
- Liquid Penetrant Testing (PT
Visual Testing (VT). Most basic and common inspection method. Tools include fiberscopes, borescopes, magnifying glasses and mirrors. Portable video inspection unit with zoom allows inspection of large tanks and vessels, railroad tank cars, sewer lines.
Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors, pipelines.
Liquid Penetrant Testing (PT). A liquid with high surface wetting characteristics is applied to the surface of the part and allowed time to seep into surface breaking defects.The excess liquid is removed from the surface of the part. A developer (powder) is applied to pull the trapped penetrant out the defect and spread it on the surface where it can be seen. Visual inspection is the final step in the process. The penetrant used is often loaded with a fluorescent dye and the inspection is done under UV light to increase test sensitivity.
Magnetic Particle Testing (MT). The part is magnetized. Finely milled iron particles coated with a dye pigment are then applied to the specimen. These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.
Radiographic Testing (RT). Uses penetrating electromagnetic radiation to produce a shadow image of the internal structure of an object. The radiation used in radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light. The radiation can come from an X-ray generator or a radioactive source. The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and denser area will stop more of the radiation. The film darkness (density) will vary with the amount of radiation reaching the film through the test object.
Ultrasonic Testing (UT). Uses high frequency sound beams to locate, size & characterise flaws inside or at the surface of a solid object. High frequency sound waves are introduced into a material and they are reflected back from surfaces or flaws. Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depth of features that reflect sound.
Eddy Current Testing (ET). Eddy current testing is particularly well suited for detecting surface cracks but can also be used to make electrical conductivity and coating thickness measurements. Here a small surface probe is scanned over the part surface in an attempt to detect a crack.
NDT is used for;
- Flaw detection and evaluation of welds
- Leak detection of vessels and tanks
- Dimensional measurements
- Structure and microstructure characterization
- Estimation of mechanical and physical properties
- Stress (strain) and dynamic response measurements
- Material sorting and chemical composition determination
NDT is used at almost any stage in the production or life cycle of a component;
- To assist in product development
- To screen or sort incoming materials
- To monitor, improve or control manufacturing processes
- To verify proper processing such as heat treating
- To verify proper assembly
- To inspect for in-service damage
Inspection of Following Raw Products
- Forgings
- Castings
- Extrusions
- etc.
Inspection Following Secondary Processing
- Machining
- Welding
- Grinding
- Heat treating
- Plating
- etc.
Inspection for In-Service Damage
- Cracking
- Corrosion
- Erosion/Wear
- Heat damage
- etc.
Power Plant Inspection
Periodically, power plants are shutdown for inspection.
Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage.
Pressure Vessel Inspection
The failure of a pressure vessel can result in the rapid release of a large amount of energy.
To protect against this dangerous event, the tanks are inspected using radiography and ultrasonic testing.
Storage Tank Inspection
Robotic crawlers use ultrasound to inspect the walls of large above ground tanks for signs of thinning due to corrosion.
Cameras on long articulating arms are used to inspect underground storage tanks for damage.
Pipeline Inspection
NDT is used to inspect pipelines to prevent leaks that could damage the environment.
Visual inspection, radiography and electromagnetic testing are some of the NDT methods used.
Bridge Inspection
Corrosion, cracking and other damage can all affect a bridge’s performance.
The collapse of the Silver Bridge in 1967 resulted in loss of 47 lives.
Bridges get a visual inspection about every 2 years.
Some bridges are fitted with acoustic emission sensors that “listen” for sounds of cracks growing.
Rail Inspection
Special cars equipped with UT flaw detectors are used to inspect thousands of miles of rail to find cracks that could lead to a derailment.
Aircraft And Jet Engine Inspection
NDT is used extensively during the manufacturing of aircraft, to find cracks and corrosion damage during operation of the aircraft.
Aircraft engines are overhauled after being in service for a period of time and are completely disassembled, cleaned, inspected and then reassembled.
Wire Rope Inspection
Electromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope that is used in chairlifts, cranes and other lifting devices.
