In this short introduction of non-destructive testing, you will get an overview of what non-destructive testing means, its purpose, and the most common types of non-destructive testing methods used within the industry.
What is non-destructive testing?
Just like the name indicates, it is evaluating the test object for its characteristics without causing any damage or destruction. From an engineering view, we can define NDT as the process of inspecting, testing or evaluating material or components for differences or discontinuities without interfering with the usability of the product. Other commonly used terms are non-destructive examination, non-destructive inspection, or non-destructive evaluation.
Why non-destructive testing?
The main objective of NDT is to test the presence or absence of discontinuity, or differences in characteristics of the material like size, dimension, configuration, hardness, alloy content etc. It is routinely applied in industries to detect the failure of the component at different stages such as manufacturing, fabrication, finished goods and in-service inspection, thereby giving an excellent balance between quality control and effectiveness.
Application of NDT:
NDT methods are used in engineering disciplines like mechanical engineering, petroleum engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, and also medical procedures like radiography, ultrasonic testing, visual testing, phased array ultrasonic testing, endoscopy etc. It is widely used in the oil and gas industry to detect the welding or structural flaws in pressure vessels, storage tanks, piping, and hoisting equipment.
Common NDT methods:
There are many NDT methods and techniques available, from simple visual to advanced radiography or ultrasonic. Therefore, personnel performing these techniques should be well-trained, experienced and certified, they must choose the right technique with a sound understanding of the purpose, applications, and requirements.
Visual Testing- VT- This is the oldest and commonly used method to identify corrosion and erosion in metals, weld defects, cracks, fractures or leakage. Visual testing uses human senses and non-specialized inspection equipment like a torch, magnifying glass, or gage. It is useful in identifying surface defects that are visible to the human eye, but this method is inadequate to analyse defects that are small or internal.
Dye Penetrant Testing- It is a method limited to detect the surface breaking discontinuities like cracks, cold shut, porosity, seams, laps etc. in non-porous materials.The area to be inspected is cleaned to make it free from lint, paint, or grease. After cleaning and degreasing the material a penetrant is applied. The capillary action principle is used here, that is, if there is a crack or surface-breaking penetrant will sip into the crack, and when you apply developer penetrant will become visible. Penetrant is allowed to rest for a few minutes and wiped off with a clean cloth. A developer shall be applied few minutes before the inspection. The indication and amount of bleeding give an estimate of the intensity of defects.
Magnetic Particle Testing- MPT or MPI (Magnetic Particle Inspection) is a method to check the surface and subsurface discontinuities in a ferromagnetic material, by inducing a magnetic flux into the material.MT has a broad discipline and we can use a/c or d/c yokes to induce magnetic fields. Calibration of yokes is checked with lifting power and a test is done on a flux indicator to check the overall performance of the machine. A contrast enhancer is applied to the test area that is clean and free from grease. The enhancer will give better visibility of defects. Dry or wet iron particles are then applied and electromagnetic field is induced. The field will cause the particles to move towards the defects, giving an indication of flaws. It is important to examine each area with different field directions kept perpendicular to each other. The size and amount of the iron particles will give an estimate of the discontinuities on the tested part.
Radiographic Testing-The testing material is placed between short-wavelength electromagnetic radiations like X-rays or gamma rays and a recording medium. Due to their high intensity, these rays will easily penetrate the object and falls on the radiographic film. The region on the film that is exposed to the rays will get dark. Any volumetric flaws or defects on the testing material thus create a density variation causing more exposure and allowing more radiation. Hence it creates a darker image in that area.
Radiographic testing is also known as volumetric testing because the entire depth of the test object can be checked. Safety precaution is a must while performing this to prevent overexposure to these harmful radiations.
Ultrasonic Testing- UT is done using high-frequency sound waves. The principle behind this method is the propagation and reflection of sound waves. An ultrasonic testing probe made of piezoelectric material will be used to transmit and receive ultrasonic pulses. The probe is moved over the testing material, and a monitor connected to the probe will display the transmission pulse and the back wall echo. For material with defects, holes, cuts or cracks, the pulse is reflected back from that position showing defect echo on the monitor. By collecting and interpreting these defect echoes, we can analyze the depth of these irregularities.
Eddy Current Testing: Eddy Current Testing performs better with a conductive material. It uses electromagnetic induction to detect corrosion, flaws or coating thickness measurements. Many industries rely on this NDT method due to its high accuracy and speed. Its applications are wide enough to be deployed across oil and gas, railway, transportation, automotive, aerospace, power generation and manufacturing.While performing eddy current testing, current is induced to the probes that test the material. Within the area of defects, eddy current won’t flow freely, hence it will interact less with the testing object causing a noticeable drop in the current. By noticing these irregularities in the displayed result, we can conclude the intensity of defects.
Other popular NDT methods are Thermal Infrared Testing, Accoustic Emission Testing, Leak Testing. Each method is better suitable for a certain application, using the right method and tool, Global Pageant Inspection team will do it perfectly well for you.