NDT or non-destructive testingis the process of analyzing and testing the properties of a structure, component and material looking at characteristic differences, discontinuities, and defects without doing any damage to it. It is also called NDE or NDI, the former being Non-destructive Evaluation and the latter Non-destructive Inspection.
A few NDT testing options
There are a number of options for NDT so here are just a few.
GPR – Ground-penetrating radar – This is a geophysical NDT where radar pulses are sent through the material’s surface or through the subsurface such as through soil, water, ice or rock. The waves are refracted or reflected back when they come across a boundary or buried object that has electromagnetic properties that are different.
AE – Acoustic Emission Testing – This is a passive technique that looks at the ultrasound emitted in short bursts by cracks that are active and under a heavy load. This is detected with sensors placed over the structure’s surface. It is also a method used for monitoring structural heath in things like bridges. Corrosion and leaks are also detectable.
MFL – Magnetic flux leakage – Magnetic fields are created with a strong magnet saturating steel things like tanks and pipelines. A sensor is also used to look for changes in the flux density indicating corrosion, pitting or erosion.
ET – Electromagnetic testing – This uses a magnetic or electric current passed through a conductive part of the material. There are three kinds, remote field, eddy current and alternating current field measurement.
LM – Laser testing methods – There are 3 categories of laser testing non destructive testing methods. Laser profilometry, holographic testing and laser shearography.
- Holographic testing is using a laser to check changes in the material’s surface after it has been through stress like vibration, pressure or heat.
- Laser profilometry uses a rotating laser moving at high speed with tiny optics to look for changes such as cracks, erosion, corrosion or pitting.
- Laser shearography uses laser light to create images before the surface goes through stress when a new image is then created. These two images are compared.
MT – Microwave testing – Only to use on dielectric materials it uses microwave frequencies to detect changes in the properties.
LT – Leak testing – With this non destructive testing there are four methods of leak testing, pressure change, bubble leak, mass spectrometer and halogen diode.
- Pressure change testing is used on closed systems only using a vacuum or a loss of pressure over a set time to see if there is a leak.
- Bubble leak tests use liquid in a tank or a soap solution to look for a gas leak, most often air that causes bubbles.
- Mass spectrometers are used to detect changes in the air using helium or a mix of helium and air in a test chamber. It might also use a vacuum chamber to look for ionised helium which indicates a leak.
- Halogen diode testing uses pressure to locate leaks using a halogen-based tracer gas and air.