The Definition of Non-Destructive Testing (NDT)

Definition of Non-Destructive Testing (NDT)

Non-destructive testing (NDT), also called non-destructive examination (NDE) or non-destructive inspection, is a group of methods used to find cracks, corrosion, thinning and other defects in materials and equipment without damaging them. Because the part is not cut, broken or altered, it can stay in service after the test if no problem is found.

NDT is essential to workplace safety wherever equipment failure could hurt people. It is used to check welds, pressure vessels, boilers, storage tanks, piping, crane booms and hooks, lifting equipment, wire rope, aircraft, bridges, pipelines and structural steel. Finding a crack or wall loss early allows repairs before a component ruptures, collapses or drops a load.

NDT is a core part of mechanical integrity programs. In the United States, OSHA's Process Safety Management standard, 29 CFR 1910.119(j), requires inspection and testing of process equipment according to recognized and generally accepted good engineering practices, which often specify NDT methods and intervals. Codes from organizations such as ASME and API also set NDT requirements for pressure equipment and tanks.

Common NDT Methods

• Visual testing (VT): the most basic method, using the eye, magnifiers, borescopes or cameras to look for surface defects, corrosion and damage.
• Liquid or dye penetrant testing (PT): a dye is applied to the surface and drawn into cracks, then a developer makes the cracks visible. Used for surface-breaking flaws on non-porous materials.
• Magnetic particle testing (MT): the part is magnetized and iron particles collect at surface and near-surface cracks. Used on ferromagnetic steels, such as crane hooks and welds.
• Ultrasonic testing (UT): high-frequency sound waves detect internal flaws and measure wall thickness, making it widely used for corrosion monitoring on tanks and piping.
• Radiographic testing (RT): X-rays or gamma rays create an image of the inside of a part, often used for weld inspection.
• Eddy current testing (ET): electromagnetic induction detects surface flaws and is common for heat exchanger tubes and aircraft parts.
• Other methods: acoustic emission, infrared thermography and leak testing.

Managing NDT Safely and Effectively

Qualified personnel

NDT results are only as good as the technician. Many employers certify NDT personnel to Level I, II or III under the American Society for Nondestructive Testing guideline SNT-TC-1A or the international standard ISO 9712. Inspection results for lifting equipment and pressure systems should be reviewed by a competent or qualified person.

Planning inspections

• Use risk-based inspection to focus NDT on equipment with the highest likelihood and consequence of failure.
• Set inspection intervals based on codes, manufacturer guidance and past results.
• Record results so trends such as corrosion rates can be tracked over time.
• Act on findings through repair, derating or removal from service.

Hazards of NDT work

Some NDT methods create their own hazards. Industrial radiography uses ionizing radiation and requires licensing, dosimetry and controlled exclusion zones. NDT work often involves confined spaces, work at height, chemicals in penetrant kits and access to energized equipment, so it should be covered by permits and job hazard analysis.

NDT complements, but does not replace, routine workplace inspections and pre-use equipment checks.

Example of Non-Destructive Testing

During the annual inspection of a mobile crane at a steel fabrication yard, the inspector performed magnetic particle testing on the hook and the boom's main weld points. The test revealed a small crack at the throat of the hook that could not be seen with the naked eye.

The hook was removed from service immediately and replaced. The company reviewed its lifting records and found the crane had regularly been used near its rated capacity on heavy steel lifts. It added more frequent NDT for hooks and critical welds on its high-use cranes and reinforced lift planning for loads above 75 percent of capacity.

Frequently Asked Questions: Non-Destructive Testing

What is the difference between NDT and destructive testing?

NDT checks a part without damaging it, so it can stay in service. Destructive testing, such as tensile or bend tests, breaks samples to measure their properties and is usually done on test pieces rather than equipment in use.

Which NDT method is best?

It depends on the material, the type of defect and access. Surface cracks are often found with penetrant or magnetic particle testing, while internal flaws and wall thickness are measured with ultrasonic or radiographic testing.

Who can perform NDT?

NDT should be performed by trained and certified technicians, commonly qualified under SNT-TC-1A or ISO 9712, with higher-level technicians interpreting and approving results.