Penetrant testing all seems so simplistic for those with only a casual interest. but its well worth reminding that its just as easy to get wrong, with the poor selection choices having big effects upon productivity, cost and even the outcome of the test examination itself. This guide is provided to aid target selections of process needs, materials and equipment.
Liquid penetrant testing is predominantly a visual examination method, searching for surface breaking discontinuities and defects as well as cracks. A test method with a long history that has moved on since its inception of the oil and whitening techniques of rail and early transport uses, through the post war aviation boom and widely used in today’s Petro-chemical and nuclear manufacturing sectors, the versatility of the method continues to develop through specialist chemical manufacturers such as “Magnaflux” (other chemical manufacturers are available).
In the early 1960’s the US-Air force developed many of the known technique applications we recognise today but there can still be quite a few things to consider and choices to make, these are typically flowed to the end user or operator as choices through Industry construction codes, standards and specifications.
Examination stage is the first question we should ask ourselves, and what are we searching for is the question lots of people forget to ask.
In-service examinations are typically searching for fatigue cracking and service effects of corrosion, with many of the near surface discontinuities being sifted out by NDT examinations during final inspections in manufacturing. But another point to consider would be can the engineered item or component be inspected in a workshop environment or do we need to examine in-situe?
Manufacture examinations are dependent upon the construction code and safety requirements surrounding the product or Installation, and we utilise Liquid penetrant testing to aid visual examinations, searching for surface breaking defects which could be time consuming and difficult to find without the use of an NDT technique. Welding, heat treatment, grinding and forming techniques where component products are strained all cause stresses in materials, where if not controlled can lead to rupture or failure of a product. Our aim is to locate and rectify such instances before final Inspection and delivery to the end-user.
- For manufacture we should consider the construction code, specification, and acceptance criteria’s resulting from the Purchase order. What are we required to locate? This can vary from defect sizes of 5/16” to as low as 0.72mm, are we searching for Inclusions, porosity or cracking. All these questions lead to Sensitivity and material selections later on.
- In-situe inspection or workshop based examination? Do we need to go to the product, or can the component be brought to me?
- What stage of manufacture or service? Pre-cleaning could be an issue and should not be over-looked. Some surface coatings may require removal for in-service examinations, but not all. (please read the “cleaning blog” for specification detail)
- Costs? Are we inspecting a few items or a lot? This can have a huge outcome on what materials and equipment we select, and how productive we choose the examination to be for larger volumes of testing.
- Thixotropic (gel Based) penetrants have been developed for in-service examinations where inspection locations can be awkward for low viscosity materials.
- Sensitivity selection is driven from our earlier question of what are we searching for? If the defect requirement is large (5/16”) we may as well, select a colour contrast material. But if we are searching for fatigue cracks in a high integrity component, we may select a high sensitivity Fluorescent material.
- Sensitivity shouldn’t be a flippant choice as colour contrast penetrants do not require a sensitivity grade, fluorescent materials range from 2 to 4 (4 being High, 2 being low), considered to be more detectable than colour contrast but the higher the sensitivity the more difficult the removal and post cleaning requirements can become. Another point to remember is that Aerospace applications mandate the use of fluorescent material products for examinations.
This can be a simple as three 400ml aerosol cans to complete, or the selection of an electrostatic processing installation…all depending upon the production volume and productivity we choose to achieve.
It is worth noting that even with the three aerosol cans, providing we select the correct material and product family of chemical the is no loss of sensitivity or detectability, equipment choices here are predominantly based on cost and efficiencies.
This is where ATH-NDT Ltd’s experience and expertise can help with turn-key solutions for Liquid penetrant Installations, helping to make the right choices first time. We also offer support and chemical consumable supply helping make your choices sustainable and affordable…. Please view the website for more Information.