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Penetrant testing is one of the most used non destructive testing methods due to its versatility and relatively cheap costs compared to other methods.

This coupled with the ability to process very large quantities of components at once in a short space of time is what makes it such a popular method.

There are however limitations and constraints as there are within all methods.

There are 3 main constraints which can slow down the penetrant testing process which are listed as follows:

1. Pre and post cleaning and subsequent drying – before any part can be processed through penetrant testing they must first be thoroughly pre cleaned, this is to ensure that the surface of the part doesn’t contain any contaminants which would interfere with the test and potentially mask any surface breaking defects. Furthermore, to the cleaning operation the parts must then be thoroughly dried, many specifications mandate a minimum 45 minute drying operation following an alkaline cleaning process due to the ability for alkaline substances to “quench” the fluorescence of the dye penetrant potentially making surface breaking defects undetectable.

 

2. Drying following excess penetrant removal – in the penetrant testing process many of the stages within the process are relatively fast and only require times such as a 10 minute dwell period or a wash off operation only lasting a matter of minutes. Drying of components however can cause a serious constrain on the penetrant line. Parts which thicker wall thicknesses or complex geometry can cause pooling and greater difficulty in drying. This can then lead to a bottleneck in the department.

 

3. Developing following drying of components – similarly to the drying oven issue the developer cabinet can cause a bottleneck as parts have a tendency to remain In this area for a prolonged period.

 

ATH NDT as an equipment manufacturer have found innovative ways of removing these bottlenecks from the NDT department.

Our oven & developer combination units are a great addition to any existing or new dye penetrant line system and can dramatically increase the overall throughput of your penetrant line.

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.

 

Process needs?

• 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.

 

Materials?

• 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.

 

Equipment?

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.

 

 

 

 

The purpose of this advisory is to give an Insight to new technicians, trainees or managers or process owners of non-destructive testing who may only have administrative control. One thing to remember here, even though a method or technique maybe perceived as easy in application and “just a bit of cleaning”, without due care and attention it’s also very easy to get wrong, Also If it’s worth doing it’s worth doing right?

For the technician or manager of a process, there’s a lot of aspects to consider when selecting an appropriate cleaning technique, both for in-process examinations and for pre and post cleaning operations for a given NDT method, but the one method where this section is most critical has to be Liquid penetrant testing.

Essentially a visual surface examination based method, the removal of contaminants is one of the keys to the process and essential to help deliver liquid penetrants to the test area with enough dwell time to enter surface breaking flaws or potential crack openings, but without an operator’s experience and awareness the examination could lead to unreliable outcomes and even failure to locate critical defects.

 

Below are key points for consideration, and discussed in further detail:

• what stage of manufacture or service are we examining and what are we searching for?
• What material alloy or material group are we cleaning
• What manufacturing stage(s) precedes the NDT examination, and could these be detrimental to the outcome of the examination itself? What is the condition of supply?
• Do we select a detergent cleaner or chemical etchant solution which will remove up to 0.00025” from the surface of the material?
• And is the cleaner correctly utilised with the required steps during applications?

………………………………………..

 

• What stage of manufacture or service are we examining and what are we searching for?

In service or manufacture, we are searching for widely different defects. In-service products may require removal of barrier coatings prior to Liquid testing, but not all. Manufacturers can                  also create more efforts where etchants are required for reworked surfaces, polished, burnished, or smeared surfaces. Most common material groups for this application are soft wrought                    aluminium alloys, and fully machined examples.

 

• What material alloy or material group are we cleaning? The chose chemical or process selected should be trialled before use in production for suitability, effectiveness, and the outcome. For harsh processes this may involve further examination magnification and measurement of chemical attack and end grain pitting caused. Reactive materials of aluminium and magnesium are typical examples.

 

• What manufacturing stage(s) precedes the NDT examination, and could these be detrimental to the outcome of the examination itself? What is the condition of supply? That is a question that should be asked by contract review…. even if you’re a sub-tear manufacturer. The choices made at this point can significantly affect the outcomes of the NDT examination and the cleaning processes used pre and post testing.

 

• Do we select a detergent cleaner or chemical etchant solution which will remove upto 0.00025” from the surface of the material? Etchants are not recommended for In-service components, where fatigue cracks are sought, most cleaning can be achieved by solvent swabbing or aqueously with the use of detergent cleaners.

 

• And is the cleaner correctly utilised with the required steps during applications? Ensure your aqueous cleaning process has a rinse stage or swill to remove residues, and a water break check to ensure the cleaning has been effective. Or if utilising solvent cleaners allow a clean tissue wipe after completion and allow a sufficient time for residues to evaporate before the application of Liquid penetrant testing materials.

 

Typical Cleaning considerations for:

Manufacture:

Solvent swabbing: Ideal for local area examinations and the removal of light soils or contaminants. Carried out with pre-dampened towelettes, or aerosol applications with tissue or cotton rags to remove.

Remember to wipe across the surface after cleaning with a clean cloth to ensure a contaminant free surface, if we’re still removing soils and contamination, we’ll have to re-clean the area before testing. Also allow sufficient time for solvents to evaporate from the surface before applying Liquid penetrant oils and testing materials.

 

Aqueous immersion: Rapidly becoming the choice option following the on-going demise of Hot solvent techniques in recent years. And ideal for medium to high volume applications but will require some maintenance to monitor the solution strength and contaminant build ups before total replacement is required.

Achieved with the use of a low foaming detergent or mild alkaline chemical. Immersing component parts by basseting or jigged and suspended into solution for a sufficient time necessary to remove contaminants.

Surface rinsing is required following cleaning and the visual verification of a water break free surface. If the water film breaks during viewing, the surface may still be contaminated and require more time to clean before progressing.

 

Chemical etchants: A controlled chemical surface treatment used to evenly remove a surface layer of material from the test component. Utilised wherever burnishing, smearing of soft alloy surfaces (aluminium’s etc) or reworking or a component surface by polishing or dressing has caused an un-desirable surface for NDT examination.

well suited to complex geometries but be mindful of possible entrapment in bores, passageways, and assembled items. Ensure you can rinse away the etchant by rinsing and clean water immersion following the process

 

In-service: In many cases dependent upon the industry sector, components or assemblies for NDT are protective treated and do not require removal….and there are a myriad of treatments to contend with but most common is the use of primer and enamel paint finishes as a barrier coating against corrosion.

Most suitably stripped by dry media blasting with the use of plastic or crushed walnut shells, sufficiently low impact enough to leave any Anodise finishes intact when working with aluminium alloys. Chemical removals are available but will likely remove the anodise treatment and will require trials prior to widespread application to prevent corrosive attack to the substrate material.

 

 

 

Penetrant testing is the testing of a part to look for the presence of surface breaking discontinuities, this can be achieved using both colour contrast and fluorescent types of penetrants with a variety of application and removal options.

Due to the nature of the process one of the most critical steps within the process is the pre cleaning / surface preparation step. The pre cleaning operation can be carried out using a variety of methods including solvent cleaning, aqueous alkaline cleaning and vapour degreasing.

One of the most important factors to consider when it comes to the selection of a pre cleaning method is the type of contamination you are looking to remove along with the surface condition of the part you are cleaning.

Vapour degreasing is one of the most utilised pre cleaning methods due to its ability to remove a large variety of contaminants specifically those such as cutting fluids and oil often used in CNC manufacturing of metallic components, the process does however have its limitations. Vapour degreasing using a product such as perchloroethylene is not capable of removing debris such as rust or scale and it cannot be used on all material types such as titanium which cannot be vapour degreased.

Another common method used for pre cleaning is the alkaline aqueous cleaning method, this method utilises an alkaline based cleaning solution such as Turco 4215NCLT or Adrox 6333A diluted with water. The cleaning solution is the operated in either an immersion tank where components are immersed and left for a time period of via a spray application in a purpose-built cleaning machine.

A final important pre cleaning step often used is pre penetrant etching. Many manufacturing methods used on softer materials especially aluminium can cause peening or smearing of the surface of the material. This can lead to a surface breaking discontinuity being partially of fully covered by the peened metal meaning that during the penetrant inspection the dye would not be able to penetrate the opening therefore the defect would likely be missed during the inspection. 

To rectify this issue many specifications call for the etching of the surface prior to penetrant inspection, this involves removing a minimum of 0.0002 inches from the surface of the part using a chemical etchant.

The etching solutions are often controlled via a customer specification and can include things such as 6/16 deoxidiser or HN03 with the time of the operation determined by prior calculations using a representative test piece.

Water washable penetrants are all classified into sensitivity levels based upon their performance.

The sensitivity levels as per ASTM 1417 and AMS264 are classified as:

• Sensitivity Level ½ – Very Low
• Sensitivity Level 1 – Low
• Sensitivity Level 2 – Medium
• Sensitivity Level 3 – High
• Sensitivity Level 4 – Very High

The reason for the various sensitivity levels of penetrant is mainly attributed to the need to have various levels of sensitivity to allow for the testing of multiple types of components, to which different levels are suited to the task at hand.

For example when testing an as cast rough surface whilst looking to identify large open defects such as inherent casting defects, a relatively low sensitivity penetrant would be the most suited to the inspection, this Is because a higher sensitivity penetrant would first of all be extremely difficult to wash off the surface of the parts and secondly the number of non relevant indications that would be picked up coupled with the excessive amount of background could actually reduce the sensitivity of the test.

On the reverse of this argument, if you were to be carrying out testing on a highly machined turbine blade with a smooth surface finish looking for stress cracking and micro fractures, a penetrant with a high degree of sensitivity would be the best penetrant for the task. A level 3 or 4 sensitivity penetrant would allow for the detection of even the smallest of surface breaking defects.

In addition to the penetrant sensitivity classifications penetrants are also further classified into Types which include:

• Type I – Fluorescent Dye
• Type II – Visible Dye

Penetrants are also classified depending on the method of penetrant removal from the components

• Method A – Water Washable
• Method B – Post Emulsifiable Lipophilic
• Method C – Solvent Removable
• Method D – Post Emulsifiable Hydrophilic

For any further assistance on any of the services we provide, please don’t hesitate to contact us https://athndt.uk/contact/

Penetrant testing is one of the most commonly utilised non destructive testing methods used in all industries. This can largely be attributed to its relatively low cost and high adaptability meaning it can be applied in all manor of ways without dramatically affecting its sensitivity.

The one application method however which is believed to marginally increase the sensitivity of a penetrant inspection is the electrostatic penetrant application method.

When penetrant is applied via electrostatic spray application there several benefits including:

• Reduced consumption of penetrant
• Ability to test large components
• Ability to test complex geometry
• Increase in sensitivity attributed to electrostatic attraction
• Reduced in service tests on consumables

One of the easiest advantages to quantify is the reduced cost of consumables that occurs, to immersion test a large component with dimensions of 1000mm x 1000mm x 200mm would require hundreds of litres of penetrant in a dip tank to ensure adequate penetrant to cover the parts, this could cost thousands of pounds. Electrostatic penetrant application allows a small singe 25 litre barrel to be used and apply via spray which can vastly reduce initial outlay costs.

Another easy to see advantage is the reduction of in service tests that are required when using electrostatic penetrant application. When using immersion methods, the penetrant is classified as re used, as a result there are checks to be carried out including daily contamination checks, monthly water content checks and quarterly fluorescent intensity checks. As electrostatic application is classified as a spray to waste system, this negates the need to have these checks performed. Not only does this reduce the amount of work to be carried out by the operator but also removes the chance of any of these tests constituting in a failure which could subsequently lead to down time on the penetrant line.

The final and possibly most important advantage of electrostatic penetrant application is the slight increase in sensitivity when using electrostatic penetrant application. Penetrant naturally is “drawn” into surface breaking defects by capillary action, when using the electrostatic spray method this action is further aided by the electrostatic effect which further draws penetrant to the surface of the part. As such it is believed that this application method increases the overall sensitivity of the penetrant inspection.

ISO 3452-1 is the British industry standard for penetrant testing.

This specification is used in the general engineering industry for penetrant testing a wide variety of product types.

The specification is commonly used for the testing of parts in the construction, motorsport, nuclear and other industries.

The specification outlines the basic testing principles for both fluorescent and colour contrast methods as well as outlining the requirements for personnel performing inspection in accordance with this specification.

Some of the main points which are covered are as follows:

• Technique and method selection
• Pre cleaning and its importance
• Removal techniques
• Processing parameters
• Calibration requirements
• Viewing / inspection requirements

When this specification is worked to correctly be approved personnel it can assist with the detection of surface breaking discontinuities in non-porous materials

Once a discontinuity is located it can then be evaluated in accordance with a relevant acceptance criteria and an accept or rejection decision made to sentence the part.

Craven NDT & ATH NDT have extensive experience in working to this specification and can offer fluorescent water washable penetrant inspection in accordance with the requirements of BS EN ISO 3452-1.

In addition to this British standard we also have the ability to work to other internationally recognized standards such as ATH 1417 from the American standards agency.

All the personnel employed by Craven NDT & ATH NDT hold level 2 certification as a minimum to both EN4179 and ISO 9712 (PCN) certification programmes meaning that we meet and exceed the qualification requirements laid out in the previously mentioned specifications.

Following the testing and evaluation process all parts would then have a test report created to inform the customer of any findings if any and a course of action that would be recommended such as repair or scrapping of the affected components.

The 3452-1 standard however is not suitable for all applications, for instance many aerospace components require testing to more stringent requirements usually laid out in prime contractor documents, there are also then specific specifications for processing of pressure vessels, weld testing and more.

Craven NDT & ATH NDT pride themselves on possessing the knowledge to not only work to these specifications but also having the knowledge of which specification should be used.