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Penetrant Colour and Fluorescence

The colour of the penetrant material is of obvious importance in a visible dye penetrant inspection, as the dye must provide good contrast against the developer or part being inspected. Remember from the earlier discussion of contrast sensitivity that generally the higher the contrast, the easier objects are to see. The dye used in visible dye penetrant is usually vibrant red but other colours can be purchased for special applications.

When fluorescent materials are involved, the effect of colour and fluorescence is not so straightforward. LPI materials fluoresce because they contain one or more dyes that absorb electromagnetic radiation over a particular wavelength and the absorption of photons leads to changes in the electronic configuration of the molecules. Since the molecules are not stable at this higher energy state, they almost immediately re-emit the energy. There is some energy loss in the process, and this causes photons to be re-emitted at a slightly longer wavelength that is in the visible range. The radiation absorption and emission could take place a number of times until the desired colour and brightness is achieved. Two different fluorescent colours can be mixed to interact by a mechanism called cascading. The emission of visible light by this process involves one dye absorbing ultraviolet radiation to emit a band of radiation that makes a second dye glow. Since the human eye is the most used sensing device, most penetrants are designed to fluoresce as close as possible to the eyes’ peak response.

Penetrant Brightness

Fluorescent brightness was erroneously once thought to be the controlling factor with respect to flaw detection sensitivity. Measurements have been made to evaluate the intrinsic brightness of virtually all commercially available penetrants and they all have about the same brightness. Intrinsic brightness values are determined for thick liquid films, but the dimensional threshold of fluorescence is a more important property. The measurement of fluorescent brightness is detailed in ASTM E-1135, “Standard Test Method for Comparing the Brightness of Fluorescent Penetrants.”

The penetrant materials used today are much more sophisticated than the kerosene and whiting first used by railroad inspectors near the turn of the 20th century.  Today’s penetrants are carefully formulated to produce the level of sensitivity desired by the inspector.  To perform well, a penetrant must possess several important characteristics. A penetrant must:

• Spread easily over the surface of the material being inspected to provide complete and even coverage.
• Be drawn into surface breaking defects by capillary action.
• Remain in the defect but remove easily from the surface of the part.
• Remain fluid so it can be drawn back to the surface of the part through the drying and developing steps.
• Be highly visible or fluoresce brightly to produce easy to see indications.
• Not be harmful to the material being tested or the inspector.

All penetrant materials do not perform the same and are not designed to perform the same. Penetrant manufactures have developed different formulations to address a variety of inspection applications.   Some applications call for the detection of the smallest defects possible and have smooth surfaces where the penetrant is easy to remove.  In other applications, the rejectable defect size may be larger, and a penetrant formulated to find larger flaws can be used.  The penetrants that are used to detect the smallest defect will also produce the largest number of irrelevant indications.

Penetrant materials are classified in the various industry and government specifications by their physical characteristics and their performance. Aerospace Material Specification (AMS) 2644, Inspection Material, Penetrant, is now the primary specification used in the USA to control penetrant materials.  Historically, Military Standard 25135, Inspection Materials, Penetrants, has been the primary document for specifying penetrants but this document is slowly being phased out and replaced by AMS 2644.  Other specifications such as ASTM 1417, Standard Practice for Liquid Penetrant Examinations, may also contain information on the classification of penetrant materials but they are generally referred back to MIL-I-25135 or AMS 2644.

Penetrant materials come in two basic types. These types are listed below:

• Type 1 – Fluorescent Penetrants
• Type 2 – Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation.  Visible penetrants contain a red dye that provides high contrast against the white developer background. Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication.  However, visible penetrants do not require a darkened area and an ultraviolet light to make an inspection. Visible penetrants are also less vulnerable to contamination from things such as cleaning fluid that can significantly reduce the strength of a fluorescent indication.

Penetrants are then classified by the method used to remove the excess penetrant from the part.  The four methods are listed below:

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

Water washable (Method A) penetrants can be removed from the part by rinsing with water alone.  These penetrants contain an emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone.  Water washable penetrants are sometimes referred to as self-emulsifying systems.   Post-Emulsifiable penetrants come in two varieties, lipophilic and hydrophilic.  In post-emulsifiers, lipophilic systems (Method B), the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible.  Post-Emulsifiable, hydrophilic systems (Method D), use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash.  Solvent removable penetrants require the use of a solvent to remove the penetrant from the part.

Penetrants are then classified based on the strength or detectability of the indication that is produced for several very small and tight fatigue cracks. The five sensitivity levels are shown below:

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

The major US government and industry specifications currently rely on the US Air Force Materials Laboratory at Wright-Patterson Air Force Base to classify penetrants into one of the five sensitivity levels.  This procedure uses titanium and Inconel specimens with small surface cracks produced in low cycle fatigue bending to classify penetrant systems.  The brightness of the indication produced is measured using a photometer. The sensitivity levels and the test procedure used can be found in Military Specification MIL-I-25135 and Aerospace Material Specification 2644, Penetrant Inspection Materials.

An interesting note about the sensitivity levels is that only four levels were originally planned.  However, when some penetrants were judged to have sensitivities significantly less than most others in the level 1 category, the ½ level was created.  An excellent historical summary of the development of test specimens for evaluating the performance of penetrant materials can be found in the following reference.

The following is a non-exhaustive list of some of the defect types which can be found during the non destructive testing process.

 

CASTING DEFECTS

The following defects are present in the material before any further processing operations such as forging, or rolling have begun.  All casting defects are therefore inherent.

Porosity

This is formed by gas which is insoluble in the molten metal.  The gas is trapped within the metal when if solidifies and remains in the form of spherical or tubular cavities.

Airlocks

A cavity formed by air which has been trapped in the mould by the metal during pouring.

Blowholes

These are small holes in or on the surface of the casting.  They are caused by gas evolution from the decomposition of grease, moisture etc., but not from the mould itself.

For example, during the sand casting operation, moisture from the mould produces steam, this is normally forced through the mould due to the absorbent nature of the sand but sometimes the steam cannot get through to the outside and is forced back into the casting, blowing holes in the casting surface.

Non-metallic inclusions

Non-metallic inclusions are impurities such as slag, oxides, and sulphides, which exist in the molten metal and finally the solidified metal.

Pipe/shrinkage defects

This is a cavity in the centre of the ingot/casting caused by shrinkage during solidification.  A primary pipe is surface breaking; secondary pipes are those which exist sub-surface.  The top of an ingot casting is removed to get rid of the primary pipe.

Other shrinkage defects may occur in steel castings where there is a localized variation in section thickness.  Shrinkage defects are not normally associated with gas, but a high gas content will magnify their extent.

Interdendritic shrinkage: very small shrinkage cavities associated with dendrite solidification.

Segregation

Segregation is chemical heterogeneity, or the non-uniform distribution of the alloys or impurities.  Pure metals do not exhibit segregation.

In carbon steels, the elements which segregate are those that are either insoluble or form lower freezing point complexers, e.g., sulphur, phosphorus, carbon, manganese, and silicon.

 

 

Cold shuts

A cold shut is an area of lack of fusion which may be surface breaking or sub-surface in a casting.  Cold shuts may result from splashing, surging, interrupted pouring, or the meeting of two streams of molten metal coming from different directions.

Hot tears (cracks)

Cracks caused by non-uniform cooling resulting in stresses which rupture the surface of the metal while its temperature is still in the brittle range.  They appear as ragged lines of variable width and numerous branches.  The tears may originate where stresses are set up by the more rapid cooling of thin sections that adjoin heavier masses of metal, which are slower to cool.  Curved surfaces and corners tend to promote hot tearing

A wrought product is a worked product, primarily produced by hot working, e.g., forging or rolling, although cold working is possible in some areas.

As the nature of penetrant testing is to locate surface breaking defects this means that the preparation of the surface is one of the most critical parts of the entire penetrant testing operation.

With this in mind many specifications and end users feel the need to specify pre penetrant etching of parts which have been subject to operations which could potentially smear or otherwise peen the surface of the parts.

These operations include but are not limited to Sand, grit, vapour, plastic or other media blasting, burnishing, lapping or honing surface finishing by tumbling, vibration or grinding.

During these operations and others with similar features softer metals such as aluminium and titanium can have small parts of their surface peened which could potentially lead to a section of material being pushed over the top of the open surface of a surface breaking defect, should this occur and be left in this condition it would essentially stop dye penetrant from entering the open surface of the defect which would render the test unable to locate the defects.

Due to this the parts must be etched prior to the dye penetrant testing process.

The process of etching is a chemical operation using a suitable chemical to remove a pre determined quantity of the material from the surface of the component.

An example type of this operation would be the pre penetrant etching of aluminium for Airbus UK. The common method to carry this out is via the use of a deoxidiser such as Henkel 6/16 deoxidiser in nitric acid, in this process an etch rate is first determined by submerging a 2024 aluminium test plate in the etching solution for a pre determined length of time and performing stock loss and weigh measurements in order to calculate a removal rate of the material.

Once the removal rate has been calculated by an approved member of staff they are then ready for the etching of production parts.

For Airbus UK parts the total amount of material that shall be removed from the surface prior to the penetrant testing operation is 5 μm (0.0002 inches).

Once the parts have be submerged in the etching solution for the correct amount of time to remove this amount of material the parts are then thoroughly dried usually at an elevated temperature for a minimum of 45 minutes before being moved on to the dye penetrant testing process.

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://www.athndt.uk/contact/