Non-destructive testing : Liquid Penetration testing

Hello all ,

Today i am going to discuss about my favourite Subject "Non-Destructive testing" . Check my previous Post

Let's start ,

What is NDT ?

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NDT stands for Non-Destructive testing , It is a way that is used to find defects & problems in a object without damaging it or taking it apart .

• It also place an important role not only in the quality control of the finished product but also during various stages of manufacturing .

There are several Methods available for evaluation of components & structures i.e., Liquid penetration , Magnetic particle ,Eddy current , Ultrasonic , Radiography .

Liquid penetration :

Liquid penetrant testing method is a surface testing method that can be used on all the materials & common test materials we use on Carbon steels ,stainless steel & glass etc .,

This method is used for checking cracks mainly in welding joints which are not visible to naked eye .

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step 1 : cleaning

One of the most important steps in the penetrant inspection procedure is the initial cleaning of the surface area to be inspected. The defect that is being sought must be open to the surface for the penetrant to enter. scale,Flakes,Paint,Dirt,Grease and other chemicals that are not cleaned from the surface will tend to accumulate the penetrant . This leads to making of real indications or creation of defect indications where none exists .

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Cleaned surface should be dried before the application of the penetrant since presence of any excess cleaning fluid would dilute the penetrant .

Step 2 : Penetrant application

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Then next application of penetrant fluid to the cleaned surface . This fluid should spread freely and evenly over the surface & move into the crack . The Dwell time , which is the amount of time required for the penetrant to move into the crack ( vary depends on crack size) .

Step 3 : Removal of Excess penetrant

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This is the most important step in the entire process . The desired result is that the surface is completely clear of penetrant . Yet, the crack retains all of the penetrant that entered into it . Excess cleaning may remove the penetrant from the upper region of the defect with the result that the developer doesn't reach the penetrant & no defect is indicated.

Insufficient cleaning will leave a background of penetrant on surface.

Step 4 : Application of Developer

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After removing the excess penetrant, a thin coating of developer is applied over the surface to draw the penetrant out of crack & increase its visibility . Another important function of the developer is that covers the surface with a colour that provides good Visual contrast to the penetrant. This increases the visibility of the defect .

Step 5 : Inspection & Evaluation

The last step in this process is the scanning of the surface for indications . The scanning may be carried out under Visible Light Conditions or ULTRA VIOLET & the defect recognition may be made with the human eye .

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Each indication that appears should be evaluated.It may actually be false, unacceptable ,it may be real but non relevant .A real indication is caused by an unacceptable flaw such as crack.

After the Inspection stage, acceptance / rejection of the component is made based on the applicable specifications & standards .

Penetrant testing methods :

• Water washable method
• post-emulsifiable method
• solvent removable method

Applications & limitations :-

• The method is capable of detecting Discontinuties open to the surface of the material under test .

• These are usually cracks,laps,porosity etc.,in products like pressure vessels , pipes , weld joints

• Penetrant method is very reliable in the detection of Fatigue cracks which occur during the service life of a material .

• It cannot be applied to porous materials.

Until my next post Happy steem :)