liquid penetrant testing lecture 2

7/21/2019 Liquid Penetrant Testing lecture 2

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Nondestructive valuation of Structures

MEC 4867

Dr. Meftah Hrairi

Lecture 8

Liquid Penetrant Testing

Applications



Dr. M. Hrairi MEC 4867 - Nondestructive Evaluation of Structures 3

utline

Processing cycles

Applications

Advantages and limitations

Project discussion



Dr. M. Hrairi 4

Processing Cycles

1. Pre-Cleaning

2. Penetrant Application

3. Excess Penetrant Removal

4. Developer Application

5. Inspection/Evaluatation6. Post-cleaning

MEC 4867 - Nondestructive Evaluation of Structures



Dr. M. Hrairi 5

Step 1 – Pre-cleaning

Parts must be free of dirt ,

rust , scale , oil , grease , etc.to perform a reliableinspection.

The cleaning process mustremove contaminants fromthe surfaces of the partand defects, and must notplug any of the defects.

Pre-cleaning is the

most important step

in the PT process!!!




Dr. M. Hrairi 6


Some metal-finishing processes can obscure surface

defects, and should be removed prior to penetrant testing: Oxidising

phosphating

anodising

chromating

electrodeposition

metallising

Penetrant methods should not be used after these metal-finishing procedures.



http://slidepdf.com/reader/full/liquid-penetrant-testing-lecture-2 7/37Dr. M. Hrairi 7

Caution about Metal Smearing

Some machining, surface finishing and cleaningoperations can cause a thin layer of metal tosmear on the surface and prevent penetrant fromentering any flaws that may be present.

Etching of the surface prior to inspection issometimes required.





The first stage of precleaning is usually the removal of

scale with appropriate descaling or rust-removing liquids.

These generally contain inhibitors , but may be stronglyacid or alkaline and residual acid or alkali can adversely

affect the performance of penetrants.

After descaling, the liquid must be thoroughly removed.





The second stage of precleaning is degreasing followed bya detergent wash and subsequent drying in hot air.

In general, chemical methods of contaminant removal arepreferred to physical methods.

As the latter essentially only remove material from thesurface, and may partially seal the surface discontinuities

which are being sought. If any contaminants, or the liquids used for cleaning, or

any deposits produced by these liquids, fill the cracks, theflaws will not be detected.

The ideal surface preparation therefore is one which leavesthe surface and the flaw in a clean, dry condition.

Moisture must be removed, as well as any chemical orphysical residues.





In the aerospace industry, a high proportion of aluminum alloycomponents have a high-resistance paint coating, and adhesion isimproved by the application of a chromating or anodized layer.

If this paint has to be removed during overhaul , methods should beused which leave the surface suitable for subsequent penetrant testing.

Usually, blasting methods are used, nowadays with lignocellulosespowder, or with materials derived from fruit stones (plum, walnut,almond, hazel-nut, etc.).

It has been found that both the precise nature of the material and the blasting pressure are critical.

Loss of flaw sensitivity is probable if the blasting pressure is greater

than 1. 7 bar on aluminum alloy components of hardness 160 VPN orless, and a light chemical etch is recommended to render the surfacereceptive to penetrant inspection.




Step 2 – Penetrant Application

Many methods

of applicationare possible

such as:

– Brushing

– Spraying

– Dipping/Immersing

– Flow-on

– And more




Dr. M. Hrairi 12

Dwell Time

The penetrant solution

must be allowed to“dwell” on the surface of

the part to allow the

penetrant time to fill any

defects present.

The dwell time vary

according to:

Penetrant type Temperature

material type

Surface finish.




Dr. M. Hrairi 13

Dwell Time




Dr. M. Hrairi 14

Step 3 – Excess Penetrant Removal

The removal technique depends upon the type of

penetrant used, as stated earlier… – Water Washable

– Solvent Removable

– Post Emulsifiable




Dr. M. Hrairi 15


Water Washable

A coarse water sprayis used to remove the

excess penetrant.

The procedure used as

a guideline for the

inspection will specify:

- water temperature (typically 10-38°C)

- pressure (typically not more than 275 kPa),

- etc.




Dr. M. Hrairi 16


Solvent Removable

The part is wiped witha clean dry cloth toremove the bulk of theexcess penetrant.

Then, a cloth lightlydampened with solvent

is used to remove anyremaining penetrant on

the surface.

Any time a solvent is used in the penetrantinspection process, a suitable flash time isrequired to allow excess solvent toevaporate.




Dr. M. Hrairi 17


Post Emulsifiable When there is concern about removing much of the penetrant from the

defect, a post emulsifiable system is used.

This involves an additional step

in which an emulsifier is applied

to the surface of the part after

the penetrant dwell time.

The emulsifier is given just

enough time to react with

the penetrant on the surfaceto render it water washable

but not enough time to diffuse

into the penetrant trapped in

the defects.




Dr. M. Hrairi 18

Step 4 – Developer Application

The method of developer application is dependent on the

type of developer used. Dwell time for developer should be similar to the

penetrant one. It needs to be determined in each case(minimum time is 10 min).




Dr. M. Hrairi 19


Dry Powder Developer

Prior to applying a dry

powder developer, the

component must be

thoroughly dried. Drying

is usually accomplished

in a hot air circulating

oven.

The developer is then

applied by immersing the

part in the powder or by

dusting of the part withthe powder.

The part can also be

placed in a developer dust

cloud chamber .




Dr. M. Hrairi 20


Wet Developer (water-suspended and water-soluble)

Wet developers are applied byimmersing or spraying the

part while it is still wet from

the penetrant removal process.

The part is completely coated

and the excess liquid allowed

to drain to prevent pooling.

The part is then dried in a hotair circulating oven.




Dr. M. Hrairi 21


Nonaqueous Developer (aka Solvent-Suspended)

Nonaqueous developer isapplied by a aerosol spray

to a thoroughly dried and

cooled part.

A thin even coating should

be applied. The coating

should be white but still

slightly transparent when

performing a visible dye

penetrant inspection, andeven thinner when

performing a fluorescent

penetrant inspection.




Dr. M. Hrairi 22

Step 5 – Inspection/Evaluation

In this step the

inspector evaluates the penetrant indications

against specified

accept/reject criteria

and attempts to

determine the origin of

the indication.

The indications are

judged to be either

relevant, non-relevant or

false.




Dr. M. Hrairi 23


A very important

step of evaluation isto document findings

on an inspection

report form or other

record keeping form.

This may be

supported with

drawings or photos

of indications, etc.




Dr. M. Hrairi 24


Indication interpretation is dependent on inspector

False, Nonrelevant and True indications False indications can arise from poor cleaning, presence of

lint, dirt, handling, contamination, etc.

Nonrelevant indications could be due to a fluorescent

background on a rough surface repeat PT in such case. True indications are of 3 general types:

Continuous lines : cracks, cold shuts, forging laps, scratches

Broken lines : continuous lines becomes partially closed by working such

as grinding, peening, forging, machining.

Small round holes : general porosity, gas holes, pinholes, very large grains




Dr. M. Hrairi 25

Step 6 – Post-cleaning

The final step in the

penetrant inspection process is to thoroughly

clean the part that has

been tested to remove all

penetrant processing

materials.

The residual materials

could possibly affect the

performance of the part or

affect its visual appeal.




Dr. M. Hrairi 26

Viewing and Recording

In most penetrant inspection, the indications are viewed by eye.

With color contrast materials, very good lighting should be used (500lux minimum) on the specimen surface.

With UV light, it should be in the UV(A) band (315-400 nm), with aminimum of 50 lux.

Recently, attempts have been made to automate the viewing process.

The specimen is manipulated by a robotic handler, is 'seen' by a closed-circuit television (CCTV) camera, and the output signal is digitized.

Computer programs for this type of procedure are quite feasible. It isalso possible to program a computer to recognize and ignore some

types of spurious indication.




Dr. M. Hrairi 27

Viewing and Recording

Penetrant indications can obviously be photographed, orvideo-recorded with a CCTV camera.

As with magnetic indications, with specialized methods, afluorescent indication can be photographed to retain someidentifying background.

The dry indication can be lifted off the surface withtransparent adhesive tape, or a replica can be made of thesurface with replica-transfer-coating (RTC).

This is a resin in a volatile solvent, with a white pigmentand a silicone de-bonding agent.

RTC is applied instead of developer, by aerosol spray, andallowed to dry. After drying, the edges are trimmed with aknife and the replica peeled off.




Dr. M. Hrairi 28

Applications

Penetrant testing is used on a wide range of materials,

including all metals and alloys as well as certain ceramicsand plastics.

PT can also be carried out successfully on nonmetallicsurfaces: glass, plastics: teflon and nylon, rubber after

compatibility testing. PT is somewhat limited when examining very porous

objects.

PT is not applicable to subsurface cracks.

Finest cracks that can be determined using PT have beenestimated to be about 5 μm wide by 10 μm deep (dependson penetrant/developer system).




Dr. M. Hrairi 29

Applications

Penetrant inspection systems have been developed to

inspect some very large components. Large machined aluminum forgings used to support the

number three engine in the tail of a DC-10 aircraft.




Dr. M. Hrairi 30

Applications

PT can be used for crack detection in components such as

this connecting rod, and for leak detection in assembliessuch as this welded tank heater.




Dr. M. Hrairi 31

Applications

For small components, for production inspection, it is often

automated, with a series of tanks and an inspection boothwith mechanized handling, timing, solution agitation, etc.

The critical factors are the cleaning process and theavoidance of contamination of all solutions.

Penetrants are occasionally used as a form of leak detector, by applying penetrant to one side of a specimen and adeveloping agent to the other side, but the overwhelminguse is for crack detection on non-ferrous specimens.




Dr. M. Hrairi 32

Safety Precautions

Many of the materials used in penetrant flaw detection are

a potential fire hazard and some can be toxic. Suitable safety precautions need to be taken on any

inspection installation.

If ultraviolet light is used for the inspection of fluorescent

indications, this also can be a hazard.




Dr. M. Hrairi 33

Advantages of Penetrant Flaw Detection

Relative ease of use.

Can be used on a wide range of material types. Large areas or large volumes of parts/materials can be

inspected rapidly and at low cost.

Parts with complex geometries are routinely inspected.

Indications are produced directly on surface of the partproviding a visual image of the discontinuity.

Initial equipment investment is low.

Aerosol spray cans can make equipment very portable.




Dr. M. Hrairi 34

Limitations of Penetrant Flaw Detection

Only detects surface breaking defects.

Requires relatively smooth nonporous material. Pre-cleaning is critical. Contaminants can mask defects.

Requires multiple operations under controlled conditions.

Chemical handling precautions necessary (toxicity, fire,waste).

Metal smearing from machining, grinding and otheroperations inhibits detection. Materials may need to be

etched prior to inspection. Post cleaning is necessary to remove chemicals.




Dr. M. Hrairi 35

Project Discussion




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Magnetic Particle Testing



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– Liquid penetrant

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liquid penetrant testing lecture 2

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