Non Destructive Testing for cracks
Non destructive testing and evaluation is used in a wide variety of industries such as power production, construction, oil and gas industries, nuclear power and more. Non destructive testing companies employ certified, expert staff who are trained in highly specialised methods of testing the composition and integrity of materials. Many industries are required by law to conduct regular non destructive testing to ensure equipment functions efficiently and safely and complies with certification regulations. In many cases, for example transporting oil and gas or nuclear power, the presence of cracks or other weaknesses in storage containers or pipelines could have catastrophic consequences. A crack in any material is a point of weakness and any stresses put on the material could lead to the crack widening and leading to shearing and splitting. Cracks can occur during manufacture, as a result of poor welding or due to stresses placed on equipment during work
The main advantages of non destructive testing are that it can often be carried out on site or while equipment is working, reducing the need for shutdowns or delays in work. Non destructive testing equipment, while highly sensitive, is usually easily portable. There are four main non destructive testing methods used to detect cracks, depending on the materials being inspected.
Methods of testing
- Visual Inspection – Visual inspection is a means of testing that can expose a variety of physical flaws that could compromise structural integrity in working machinery or components being manufactured. Although this can be as basic as a trained inspector looking over the surface in detail, there is a variety of optical equipment that can be used to inspect the surface of the material, such as borescopes and flexible fibre optics to examine the inside of pipes or containers. These can be combined with video surveys to give evidence that can be carefully examined at a later time. This technique is usually used to test welding joins for cracks that could weaken the join but can be applied to a variety of items and materials. Due to its simplicity, this is the oldest type of non destructive testing and popular due to its low cost.
- Ultrasonic testing – This is a more technical type of inspection that can be used on all materials, metallic and non metallic but has the greatest resolution on metals. It uses very short ultrasonic pulse waves, usually from 0.1 to 15 MHz in frequency, that are transmitted into the material to be inspected from a transducer. A liquid such as oil or water (known as a couplant) is applied to the surface of the material to facilitate this. The reflected ultrasonic pulses are then measured by the transducer. The speed of the reflected pulses will be altered by any imperfections such as cracks and this will be sensed by the transducer. In another variation of this method, known as attenuation, an emitter sends ultrasonic pulses through the material and a receiver further along the surface picks them up. Any flaws in the material will affect the pulses, revealing the presence of any damage.
- Magnetic Particle testing – This type of inspection is used on ferromagnetic metals (magnetisable metals such as iron, cobalt or nickel, or their alloys). Very fine ferromagnetic particles are applied to the surface of the material, either dry or suspended in a liquid and the local area of the material is then magnetised. This is known as magnaflux testing and the magnetisation can be provided by an outside source (indirect magnetisation) or by passing an electrical current through the metal (direct magnetisation). If the metal is intact the magnetic flux will be retained within the metal, however if there is a break in the metal such as a crack, the magnetic flux will leak around the flaw and the ferromagnetic particles will be attracted and concentrate around the crack. This will show the location and size and shape of the crack. To identify smaller cracks or imperfections, an ultraviolet light can be used to more easily observe the particles. Once the magnetic particle inspection is complete, the area is demagnetised.
- Dye penetrant inspection – This is also known as liquid penetrant testing and involves applying a coloured liquid by spraying, brushing or dipping to a surface that has been thoroughly cleaned. Due to the liquid involved this method is unsuitable for porous materials. This is then given what is known as ‘dwell time’ to take effect (typically from five to thirty minutes depending on the specific liquid used). The liquid will flow into any cracks in the surface by capillary action, the smaller the crack the longer this will take. Once the dwell time is complete, excess penetrant is removed from the surface of the material being tested and a developer is applied to the surface and given time to take effect. The developer will draw penetrant from any cracks or other imperfections, known as ‘bleed out’, making them visible to the naked eye of the trained observer. A more rigorous form of dye penetrant test uses a fluorescent dye that can only be seen under ultraviolet light. Although this can be more expensive even the smallest cracks or flaws will be easily visible.
Format NDT Ltd are an independent non destructive testing company offering testing and inspection services to companies throughout the UK. Founded in 1976, we have over forty years of experience in the field and are confident we provide unrivalled service and quality to all our customers. we have a fully equipped testing facility at our base near St Helens but can provide on site testing and inspection anywhere in the country and operate 24-7. Our technicians are qualified up to level 3 in a range of examination services and compliant with ISO 9712 and SNT-TC 1-A (ASNT) and pride themselves on always putting the customer first and guaranteeing satisfaction with every job they undertake. If you have any queries about the services we provide, please contact us as soon as possible, we will be happy to help.
Telephone : 01744 816225