Non-Destructive Testing (NDT) is used in various industries to evaluate the properties of a material, component or system without damaging the test subject. NDT is also known by the terms Non-Destructive Examination (NDE), Non-Destructive Inspection (NDI), and Non-Destructive Evaluation (NDE).
In testing, troubleshooting, and research, NDT is a successful strategy that saves both time and money. NDT plays a crucial role in everyday life and is essential for safety and reliability. Aircraft, spacecraft, motor vehicles, pipelines, bridges, railways, power plants, and oil platforms are just some examples tested with NDT.
What's the difference between destructive and non-destructive testing?
Destructive testing is simply testing the destroys or damages the subject under test. There are many material properties that can only be evaluated by applying physical force or load. Examples include tensile strength, elongation, and hardness. Destructive tests to evaluate these include tensile test, bending test, fracture test, flattening test, hardness test, shear test, and impact test.
On the other hand, NDT doesn't damage the test subject. This means that the sample can be used in the field provided the tests pass. NDT can be used to detect defects or discontinuities. It can detect degradation of properties after long use. Surface or internal cracking, poor welding, impact damages, delamination, density, porosity and pitting some problems that NDT can detect.
Why are NDT checks necessary for a material?
All materials, products, and equipment have standard design requirements and estimated life. Sometimes products get through production, fabrication, or delivery with undetected defects. These may cause catastrophic failures. Such catastrophes can be costly and can even terminate projects. NDT can catch these problems before catastrophes occur. NDT saves lives and property. It helps companies adhere to regulations and standards.
In addition to security, NDT is used to assure the effectiveness and longevity of equipment. It's useful for asset integrity management, which leads to increased productivity and profitability for businesses. For example, in the 2020 port blast in Beirut, many buildings within the blast radius are still standing but could be structurally unsafe. Rather than demolish them, NDT is being used to assess extent of damage and repair them where possible.
Which industries are using NDT checks?
- Aerospace & Defence: Airframe structures are tested for wear and tear, and operation under extreme conditions. Ultrasonic method is widely used to reveal even the smallest defects. Users include Boeing, Airbus, GE Aviation, Hindustan Aeronautics Ltd., etc.
- Oil & Gas: Internal structures are inspected for welds, cracks, voids and other structural defects. Ultrasonic and radiographic methods are commonly used. Users include Indian Oil Corporation, Bharat Petroleum, Reliance Petroleum Limited, ONGC, etc.
- Biomedical & Medical Devices: Ultrasounds and x-rays are widely used. Vendors include Medtronic plc, Johnson and Johnson, Abbott Laboratories, etc.
- Civil & Heavy Construction: Bigger structures imply bigger stresses and a greater need for NDT. NDT is applied to buildings, bridges and dams. Users include L&T Engineering & Construction Division, Tata Projects Ltd, etc.
- Metals & Mining: Metals are the foundations on which many industries function. NDT validates material properties and quality. Users include BHP Group Ltd, Jiangxi Copper Co. Ltd, etc.
How to select a suitable NDT method for a certain material?
- Visual Testing: Used for analyzing surfaces, examining the condition of mating surfaces, and checking for leaks.
- Liquid Penetrant Testing: Detects surface-breaking defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks.
- Magnetic Particle Inspection: Mostly identifies surface and near-surface faults or cracks in ferromagnetic materials. It detects seams, porosity, and small tight cracks.
- Eddy Current Testing: Uses electromagnetic induction to find defects in conductive materials. Detects electrical conductivity, permeability, cracks, seams, alloy content, heat treatment variations, wall or coating thickness.
- Ultrasonic Testing: Uses ultrasonic waves in the range 0.1-50 MHz to pick up cracks and variations in thickness. Applicable for concrete, wood, composites, metals, alloys, and welds.
- Radiographic Testing: Electromagnetic radiation penetrates materials and exposes defects on radiation-sensitive film. It mainly uses X-rays and Gamma-rays. It detects corrosion, geometry variation, density changes, and misaligned parts. It's a good test for inspecting weld interiors and picks out cracks, porosity, inclusions, voids, and lack of fusion.
What are the best practices or precautions for NDT?
NDT itself poses a risk to testing professionals who are therefore required to follow many safety precautions. Always use trained and experienced professionals. They must know what testing method to use, how to use it, and how to correctly interpret the results. Suitable NDT software can help overcome examiner fatigue and loss of concentration.
Where ultraviolet radiation, ionizing radiation, or X-rays are used, operators should wear personal protective equipment, and use suitable filters and lenses. Even when more benign techniques such as ultrasonic or eddy current testing are employed, the testing environment itself can pose a risk. For example, changing probes without shutting down the system can cause sparks and explosions. Testing environments must be clean and free of clutter. Compressed gases (sulphur hexafluoride, acetylene, and nitrous oxide) commonly found at NDT facilities must be handled properly.
For magnetic particle testing, operator should use a local exhaust or at least wear respiratory protective equipment. For penetrant inspection, avoid skin contact, and keep away from food, drinks and smoking materials. For radiography, magneto-inductive and eddy current testing, cordon off the area so that personnel don't unintentionally expose themselves to radiation.
What are some real-world catastrophes that could've been prevented with NDT?
In 2009, one of the pressure vessels at NDK Crystal in Belvidere, Illinois violently ruptured. Later investigations confirmed that the company had failed to do NDT on the inside of the vessel that had been damaged due to corrosive chemicals. The inside wall had experienced stress corrosive cracking.
The Columbia Space Shuttle disaster of 2003 killed seven astronauts onboard. It was caused by a falling piece of foam that then damaged the left wing. Following an investigation, it was deemed that more extensive NDT on the foam and wings could have prevented this disaster. Another recommendation was to develop new NDT techniques to complement destructive testing methods.
During the 1940s, the U.S. mass produced 2710 Liberty ships, some completed within five days. But 12 ships broke in half, later attributed to tiny fractures in the steel. Modern NDT techniques could have ascertained the quality of the steel.
What certifications are available for NDT professionals?
ISO 9712:2021 is the main standard for third-party qualification and certification of NDT personnel. Methods included in its scope are acoustic emission, eddy current, leak, magnetic, penetrant, radiographic, strain gauge, thermographic, ultrasonic and visual. It's an evolution of two earlier standards: EN 473 and ISO 9712. ISO 9712 has been adopted in the U.S. as ANSI/ASNT CP-106.
EN ISO 20807 establishes a system for the qualification of personnel who perform NDT applications of a limited, repetitive or automated nature. ISO TS 11774 is a performance-based qualification. It's applicable for safety critical applications where even third-party certification (ISO 9712:2021) may not suffice.
Certification under these standards includes training, work experience under supervision, and passing a written and practical examination conducted by the independent certification authority.
For a complete list of all ISO NDT standards covering requirements on testing equipment, visit ISO's 19.100: Non-destructive testing.
The Schmidt Hammer (also known as "Swiss Hammer") is invented. The instrument uses the world's first patented non-destructive testing method for concrete. Also, J. Kaiser introduces acoustic emission as an NDT method.
NDT in Aerospace Conference was established DGZfP and Fraunhofer IIS hosted the first international congress in Bavaria, Germany.
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