Liquid Penetrant Testing Lecture

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Nondestructive Evaluation of Structures

MEC 4867

Dr. Meftah Hrairi

Lecture 7

Liquid Penetrant Testing Fundamentals

Dr. M. Hrairi MEC4867 - Nondestructive Evaluation of Structures 3

Outline

Introduction

Principles

Materials

Project discussion


Introduction

Penetrant testing (PT) is one of the most widely used nondestructive testing methods for the detection of surface discontinuities (such as cracks, laps and folds) in nonporous solid materials: most commonly used surface NDT method today because it can be applied

to virtually any magnetic or nonmagnetic material (complementary to MT).

provides industry with a wide range of sensitivities and techniques that make it especially adaptable to a broad range of sizes and shapes.

useful for examinations that are conducted in remote field locations, since it is extremely portable.

appropriate in a production type environment where many smaller parts can be processed in a relatively short period of time.

builds on the principle of Visual Inspection.

increases the “seeability” of small discontinuities that the human eye might not be able to detect alone.


How Does PT Work ?

In penetrant testing, a liquid with high surface wetting characteristics is applied to the surface of a component under test.

The penetrant “penetrates” into surface breaking discontinuities via capillary action and other mechanisms.

Excess penetrant is removed from the surface and a developer is applied to pull trapped penetrant back to the surface.

With good inspection technique, visual

indications of any discontinuities

present become apparent.


Basic Process of PT

1) Clean & Dry Component

2) Apply Penetrant 4) Apply Developer

3) Remove Excess 5) Visual Inspection

6) Post Clean Component


What Makes PT Work ?

Every step of the penetrant process is done to promote capillary action.

This is the phenomenon of a liquid rising or climbing when confined to small openings due to surface wetting properties of the liquid.

Some examples: − Plants and trees draw water up

from the ground to their branches

and leaves to supply their

nourishment.

− The human body has miles of

capillaries that carry life sustaining

blood to our entire body.


What Types of Discontinuities can be Detected via PT ?

All defects that are open to the surface.

– Rolled products-- cracks,

seams, laminations.

– Castings-- cold shuts, hot

tears, porosity, blow holes,

shrinkage.

– Forgings-- cracks, laps,

external bursts.

– Welds-- cracks, porosity,

undercut, overlap, lack of

fusion, lack of penetration.


What can be Inspected via PT ?

Almost any material that

has a relatively smooth,

nonporous surface

on which discontinuities

or defects are suspected.

PT can be used on any

material which has a

non-absorbent surface.

PT is generally found to be more sensitive than X-radiography or ultrasonics for fine surface cracks.


What can NOT be Inspected via PT ?

Components with rough surfaces,

such as sand castings, that trap

and hold penetrant.

Porous ceramics

Wood and other fibrous materials.

Plastic parts that absorb or react

with the penetrant materials.

Components with coating

that prevent penetrants

from entering defects.

Defect indications become less distinguishable as the background “noise” level increases.


Choices of Penetrant Materials

Penetrant Type

I - Fluorescent

II - Visible

Remover Removal Method

A Water Washable

B Post-emulsifiable, Lipophilic

C Solvent Removable

D Post-emulsifiable, Hydrophilic

Developer Form

Dry Powder

Wet, Water Soluble

Wet, Water Suspendable

Wet, Non-Aqueous


Penetrant Material

Penetrants are formulated to possess a number of important characteristics. To perform well, a penetrant must:

− Spread easily over the surface being inspected.

− Be drawn into surface breaking defects by capillary action or other

mechanisms.

− Remain in the defect but remove easily from the surface of the part.

− Remain fluid through the drying and developing steps so it can be drawn

back to the surface.

− Be highly visible or fluoresce brightly to produce easy to see indications.

− Not be harmful to the inspector or to the material being tested.


Penetrant Materials

The characteristics of a good penetrant liquid are therefore related to its surface tension, density and wetting properties rather than viscosity.

Penetrants are solutions of colored or fluorescent dyes in oil-based liquids.

Penetrants can be classified as: Fluorescent

visible dye


Penetration of a Liquid into a Crack

Penetrant materials are often pulled into surface breaking defects by capillary action.

The capillary force F driving the penetrant into the crack is a function of the surface tension of the liquid-gas interface, the contact angle, and the size of the defect opening:

F = 2 π r σLV cosθ

r = radius of the crack opening (2 π r is the line of contact between the liquid and the solid tubular surface.)

σLV = liquid-gas surface tension

θ = contact angle

The capillary force balances

the weight of the liquid so that

h = 2 σLV cosθ / r g ρ

σLV

σSV σSL Solid

Vapor

2r

h


Sensitivity Levels

Penetrants are also formulated to produce a variety of sensitivity levels. The higher the sensitivity level, the smaller the defect that the penetrant system is capable of detecting.

The five sensitivity levels are: − Level ½ - Ultra Low Sensitivity

− Level 1 - Low Sensitivity

− Level 2 - Medium Sensitivity

− Level 3 - High Sensitivity

− Level 4 - Ultra-High Sensitivity

As the sensitivity level increases, so does the number of nonrelevent indications. Therefore, a penetrant needs to be selected that will find the defects of interest but not produce too many nonrelevent indications.


Visible vs Fluorescent PT

Inspection can be performed using

visible (or red dye) or fluorescent

penetrant materials.

Visible PT is performed under white light. All are in the level 1 sensitivity range.

Fluorescent PT must be performed using an ultraviolet light in a darkened area. Fluorescent PT is more sensitive than visible PT because the eye is more sensitive to a bright indication on a dark background. Sensitivity ranges from 1 to 4.

Dr. M. Hrairi MEC4867- Nondestructive Evaluation of Structures 17

Penetrant Removal Method

The excess penetrant can be removed in four different ways

1. by water only

2. by emulsifier which is oil soluble (lipophilic), followed by water.

3. by a liquid solvent

4. by water, followed by a penetrant removal solution (detergent) which is

water soluble (hydrophilic), followed by water.


Penetrant Removal Method

Penetrants are also classified by the method of removing the excess penetrant. Water Washable penetrants are removed with a

course spray of water. They are the easiest to

employ and most cost effective when inspecting

large areas.

Solvent Removable penetrants are removed by

wiping with a cloth dampened with solvent. They

are supplied in aerosol cans for portability and are

primarily used for spot checks.

Post-Emulsifiable penetrants are water washable

only after they have reacted with an

emulsifier solution. A post-emulsifiable system is

used when washing the penetrant out of the defect

is a concern. The emulsifier is given time to react

with the penetrant on the surface but not the

penetrant trapped in the flaw.


Developers

The role of the developer is to pull trapped

penetrant out of defects and to spread it out

on the surface so that it can be seen.

Also provides a light background to increase

contrast when visible penetrant is used.

Developer materials are available in several different forms • Dry Powder is a mix of light fluffy powder that clumps together where penetrant

bleeds back to the surface to produces very defined indications.

• Wet, Water Suspendable is a powder that is suspended in a water that covers the surface with a relatively uniform layer of developer when the water is evaporated. The solution is somewhat difficult to maintain as the powder settles out over time.

• Wet, Water Soluble is a crystalline powder that forms a clear solution when mixed with water. The solution recrystallizes on the surface when the water is driven off. Indications sometimes lack definition and look milky. Not recommended for use with water-washable penetrants.

• Wet, Non-Aqueous - is supplied in a spray can and is the most sensitive developer for inspecting small areas. It is too costly and difficult to apply to large areas.


Penetrant Inspection System

Penetrant systems can therefore be classified as:

1. water-washable

2. post-emulsifiable (i.e. the excess penetrant is emulsified for removal)

3. solvent-removable

The appropriate process to be used on any specific application is based on: 1 The flaw sensitivity required.

2 The surface finish of the component.

3 The compatibility of the materials with the component.

4 The size, shape and accessibility of the region to be inspected.

5 The ultimate use of the component.



Water-removal processes are more suitable for routine high-volume production inspection.

For maximum sensitivity, post-emulsifier fluorescent penetrants with a hydrophilic solution remover are generally preferred.

Often, the final choice of a penetrant system is made in terms of the most suitable penetrant remover related to surface finish, compatibility, flash point, etc.

Some aerospace companies have compared penetrant materials and systems by producing a series of similarly cracked specimens, such as ex-service turbine blades with leading-edge cracks.

The results of comparisons of different processes on these are valuable at the time of the test, but rapidly become out-of-date as newer penetrant materials are marketed.



Equipment available can be divided into three types, as follows:

1. Portable kits are used for carrying out inspection of small areas, for use

on site; these often contain the materials to be used in aerosol form.

2. Fixed installations are used for testing components on a continuous basis,

with a series of processing stations in sequential order to form a flow line.

Increasingly, these are being automated to provide automatic component

handling and timing.

3. Self-contained processing booths are used for testing large components

which cannot be moved during testing.

Portable Penetrant System Stationary Penetrant System


Verification of Penetrant System Performance

Since penetrant testing

involves multiple

processing steps, the

performance of the

materials and the

processes should be

routinely checked using

performance verification

tools, which include:

− TAM Panels

− Crack Sensitivity Panels

− Run Check Panels


Penetrant Technique Classification Summary


Project Discussion

Next Topics

Liquid penetrant

Further Readings Handouts:

– Liquid penetrant

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../Future trends in automotive body materials.pdf

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Liquid Penetrant Testing Lecture

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