BMFB 4283 NDT & FAILURE ANALYSIS

1. BMFB 4283NDT & FAILURE ANALYSIS Lectures for Week 2 Prof. Qumrul Ahsan, PhD Department of Engineering Materials Faculty of Manufacturing Engineering

2. Issues to address 2.0 Liquid Penetrant 2.1 Introduction 2.2 Fundamentals 2.3 Techniques 2.4 Applications

3. Introduction • This lecture is intended to provide an introduction to the NDT method of penetrant testing. • Penetrant Testing, or PT, is a nondestructive testing method that builds on the principle of Visual Inspection. • PT increases the “seeability” of small discontinuities that the human eye might not be able to detect alone.

4. PENETRANT TESTING (PT) • PT effectively requires: • Discontinuities open to the surface of the part (subsurface discontinuities or surface discontinuities not open to the surface aren’t detected) • Special cleaning of parts • Good eyesight

5. What Can Be Inspected Via PT? Almost any material that has a relatively smooth, non-porous surface on which discontinuities or defects are suspected. SURFACE BREAKING DEFECTS

6. 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.

7. Principles of Penetrant Testing • 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 (blotter) is applied topull trapped penetrantback the surface. • Developer provides a contrasting background for visual indications of any discontinuities present become apparent.

8. Basic Process of PT 1) Clean & Dry Component 4) Apply Developer 2) Apply Penetrant 5) Visual Inspection 3) Remove Excess 6) Post Clean Component

9. What Makes PT Work? • Surface Tension : An elastic force that acts tangential to the fluid surface to reduce the area is called surface tension The surface tension for a droplet of liquid is  = ½ (po - pi ) r, where (po - pi ) is the difference between the bubble’s exterior and interior pressure and r is the radius of curvature • The surface tension at interface of two mediums can be expressed as lg • Wetting is the ability of a liquid to maintain contact with a solidsurface, resulting from intermolecular interactions when the two are brought together. • The degree of wetting (wettability) is determined by a force balance between adhesive and cohesiveforces. • Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface. • Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface At the liquid-solid surface interface, if the molecules of the liquid have a stronger attraction to the molecules of the solid surface than to each other (the adhesive forces are stronger than the cohesive forces), wetting of the surface occurs. Alternately, if the liquid molecules are more strongly attracted to each other than the molecules of the solid surface (the cohesive forces are stronger than the adhesive forces), the liquid beads-up and does not wet the surface of the part. Figure 2: Wetting of different fluids. A shows a fluid with very little wetting, while C shows a fluid with more wetting.

10. What Makes PT Work? • The contact angle is the angle formed by the solid/liquid interface and the liquid/vapor interface measured from the side of the liquid Contact angle of a liquid droplet wetted to a rigid solid surface • For a penetrant material to be effective, the contact angle should be as small as possible • Typical penetrant materials have contact angles on the order of 10o

11. What Makes PT Work? • In liquid penetrant testing, two conditions must be met. • First, the surface energy of the solid-gas interface must be greater than the combined surface energies of the liquid-gas and the solid-liquid interfaces. • Second, the surface energy of the solid-gas interface must exceed the surface energy of the solid-liquid interface. • Capillary action : If a tube is sufficiently narrow and the liquid adhesion to its walls is sufficiently strong, surface tension can draw liquid up the tube. The height the column is lifted to is given by: where h is the height the liquid is lifted,  is the liquid-air surface tension,  is the density of the liquid, r is the radius of the capillary, g is the acceleration due to gravity, θ is the angle of contact described above. Illustration of capillary rise and fall. Red=contact angle less than 90°; blue=contact angle greater than 90°

12. 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. • Surface tension of liquid vs tube surface wetting • Surface tension – cohesive force • Tube surface wetting – adhesive force • Cohesive force> Adhesive force : Convex surface -> liq. fall below • Cohesive force< Adhesive force : Concave surface -> liq. rise up Finer Tube, greater liquid rise finer defect (hairline) -> greater indication

13. Penetrability • For cylindrical volume capillary pressure P = 2glcosθ/r = 2Scosθ/r • Where S is the surface tension, r is the radius of the crack and θ is the contact angle. • Influenced by variables: surface condition, and type of test object, type of penetrant, temperature of test object and contamination. • Fluid penetration into a real crack will generally be different from the above estimation • Crack width is not a constant ( a crack typically narrows with depth) • Portions of the crack may be closed • Trapped gas or contaminants within the crack limits fluid penetrationion A liquid penetrant will continue to fill the void until an opposing force balances the capillary pressure. This force is usually the pressure of trapped gas in a void, as most flaws are open only at the surface of the part. Since the gas originally in a flaw volume cannot escape through the layer of penetrant, the gas is compressed near the closed end of a void.

14. Penetrability High Surface Tension Low Wetting Ability • High P  High S (High S is not necessarily a good penetrant • e.g. Water has high surface tension, yet poor penetrant • Static Penetration Parameter (SPP) = Scosθ • Smaller contact angle, θ higher P • Good penetrant 5o • Width of discontinuity; D(2r) • Narrower  higher P  Longer time to rise • Finer defect  longer dwell time Low Surface Tension High Wetting Ability

15. Penetrability High Surface Tension Low Wetting Ability • Viscosity,  • Not significantly affect penetrant ability into discontinuity • Strongly affected with temperature  penetrant inspection • Kinetic Penetration Parameter, KPP = Scosθ/  • Highly viscous penetrant: longer time to enter into defect  longer dwell time • Drain more slowly and cause excessive loss of penetrant due to drag • Density • has a slight to negligible effect on the performance of a penetrant. • Increasing the specific gravity by decreasing the percent of solvent (by volume) in the solution will increase the penetration speed. • The gravitational force acting on the penetrant liquid can be working either with or against the capillary force depending on the orientation of the flaw during the dwell cycle Low Surface Tension High Wetting Ability

16. Flaw Entrapment Efficiency • Ability of penetrant to form an indication large enough to be detected • Factors influenced efficiency • Volume of defect • Length of defect • Contaminants • Penetrant dye • Processing

17. Flaw Entrapment Efficiency • Volume of defect • Size of indication reflects the volume of defect it entered • Larger discontinuity: depth or width • More penetrant it holds • More penetrant is present to form indication • Length of Defect • Affect volume of penetrant • Strongly affect the visibility • Very fine defect has insufficient width • Reduce the ability of human eye to detect indication visually • Can only be located when the defect has sufficient length

18. Flaw Entrapment Efficiency • Contaminants • Fine and clean discontinuity vs wide and contaminated dicontinuity • Affect the penetration of penetrant • Inservice inspection encounter defects contaminated with oil, water and corrosion products • Reduce the volume available for penetration • Water adversely influence contact angle • Acidic or alkaline contaminants also fading the dye visibility • Heat and prolonged exposure under UV light cause penetrant lose their sensitivity

19. Flaw Entrapment Efficiency • Penetrant dye • Type of dye • Affect the sensitivity of penetrant visibility in terms of brilliance and intensity of the dye color • Fluorescent dye is more sensitive than color contrast dye • Concentration of dye • Different concentration within a classification affect the sensitivity level • Altering dye concentration affect the ability of penetrant penetration

20. Flaw Entrapment Efficiency • Processing • Method of processing • Dip and drain allows more volatile constituents of penetrant to evaporate during dwell time • Increases the concentration of dye within remaining penetrant • Degree of penetrant removal • No background coloration (cleaned component) high degree of contrast penetrant indication • Should be interpreted carefully: over-emulsification or over washing  removes penetrant from defects • Small degree of background coloration : over-washing not occur • Brightness of indication must exceed background brightness

21. What Can NOTbe 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 coatings that prevent penetrants from entering defects. Defect indications become less distinguishable as the background “noise” level increases.

22. Choices of Penetrant Materials PenetrantType I Fluorescent II Visible Method A Water Washable B Postemulsifiable - Lipophilic C Solvent Removable D Postemulsifiable - Hydrophilic DeveloperForm Dry Powder Wet, Water Soluble Wet, Water Suspendable Wet, Non-Aqueous

24. Penetrant Materials 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.

25. Wetting ability Specific gravity Volatility Chemical activity Solubility Solvent ability Health hazard Tolerance to contaminants Flammability / flash point Electrical conductivity Availability and cost Penetrant Properties

26. Penetrant Properties • Wetting ability: • Affect penetrability and bleed-back characteristics • Contact angle and surface tension of penetrant control wetting ability • Specific gravity: • Ratio of density of penetrant with density of distilled water at 40C • Not a problem with oil base penetrant • Penetrant used in tank system must have specific gravity less than 1  to ensure water will not float on top of penetrant  prevent penetrant from covering the test object

27. Penetrant Properties • Flash point: • Temperature at which enough vapor is given off to form combustible mixture • Typical min 93C • Should not be flammable • Volatility: • Characterize by vapor pressure or boiling point • Good penetrant usually evaporate too quickly • Low volatility is desirable  so the penetrant dry from the surface, leave stained and from any discontinuity, leave precipitated dye

28. Penetrant Properties • Chemically inert: • Must be inert, non-corrosive as possible  chemically compatible with the material being tested • Penetrant is contaminant (contain sulphur, sodium, halogen)  potential reactions must be considered • To avoid possibility of embrittlement or cracking over years • Viscosity: • Affect thickness of penetration due to molecular/internal friction • Low viscosity penetrant • Solubility: • Penetrant contain dye in liquid solution • Must hold sufficient dye at ambient or high temperature • Must not come out from solution if temperature drop

29. Penetrant Properties • Solvent ability: • Solvent must be able to remove surplus penetrant from test specimen • To ensure clean, clear background • Must not dissolve the penetrant in defect • Tolerance to contaminants: • Penetrant will be contaminated after a period of time, even if a great care is taken • Must be periodically check to ensure all is well, no residue left

30. Penetrant Properties • Health hazard: • Must comply with or exceed the most stringent HSE requirements • Toxicity, odour, skin contact • To prevent allergies or contaminants • Availability and cost: • Dye materials are easily obtained • Low cost

31. Penetrant Properties • Electrical conductivity: • Electrostatic spraying becomes popular • uniform coverage with complicated shapes • Reduces over spraying • Requires less penetrant over all • Spray gun applies –ve charge to penetrant • Test object ground potential • Electrostatic attraction cause penetrant be strongly attracted to the part • low viscosity and easily attracted to the part • Must readily accept and hold the electrical charge

32. 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 4 - Ultra-High Sensitivity • Level 3 - High Sensitivity • Level 2 - Medium Sensitivity • Level 1 - Low Sensitivity • Level 1/2 – Ultra-Low 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.

33. Why is Visible PenetrantRed and Fluorescent PenetrantGreen? • Visible penetrant is usually red because red stands out and provides a high level of contrast against a light background • Fluorescent penetrant is green because the eye is most sensitive to the color green due to the number and arrangement of the cones (the color receptors) in the eye.

34. Photo Courtesy of Contesco Visible Vs Fluorescent PT • Inspection can be performed using visible (or red dye) or fluorescent penetrant materials. • Visible Pt is performed under white light while fluorescent PT must be performed using an ultraviolet light in a darkened area. All are all in the level 1 sensitivity range. • 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.

35. Type of UV light • Mercury vapour arc lamp • Street lamp that has filter to reduce the visible light to minimum but allow UV-A to transmit • GE or Westinghouse lamp • Has separate filter, Hg arc is drawn between electrodes in quartz tube • 400 W Hg vapour flood lamp • Used for very large component

36. Quality of fluorescent dye • Depends on how efficient dye absorb UV light and convert into visible light • Influenced by: • The intensity of UV-A light at the surface • The ability of dye to absorb UV-A • The concentration of dye • The ability of dye to produce visible light • Film thickness

37. Penetrant Removal Method Penetrants are also classified by the method of removing the excess penetrant. • 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. • 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. • Post-Emulsifiablepenetrants 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 reacts with the penetrant on the surface but not the penetrant trapped in the flaw.

38. 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 Powderis a mix of light fluffy powder that clumps together where penetrant bleeds back to the surface to produces very defined indications. • Wet, Water Suspendableis 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 Solubleis 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.

39. Basic mechanism of Developer • Developer works due to : • Capillarity • Light scattering • Solvent action

40. Capillarity • Capillary attraction of developer overcomes the opposing attraction of the discontinuity • Increase the surface area of indication • Spread the penetrant laterally on surface  widening indication • Expands the bulk dye into many thin films  enhance brightness • Too large developer particle size will result low capillary pressure • Too small will cause block any orifice

41. Light scattering • Very important when involves fluorescent penetrants • Brightness of indication is amplified per unit area • Each particle provide scattering multiple reflector  both UV-A and fluorescent radiation • This improve contrast in dark condition  improved sensitivity of penetrant system

42. Solvent action • Applies to non-aqueous method • Have no capability for drawing penetrant out of discontinuity • The developer damp the test surface • The remaining solvent will bridge the gap between the developer particles and the penetrant in the discontinuity • Important for fine defect

43. Developer properties • Good developer: • Material must be absorptive to perform blotting action • Must have fine texture • Must mask out background contours and colors • Must be easily and evenly applicable • Must form light and even coat • Must be no fluorescing of developer when fluorescent penetrant is used • Penetrant bleeding must easily wet the material • Must be high color contrast, white is the best • Must be readily removable after test • Must be in-toxic and non-irritant

44. 6 Steps of Penetrant Testing 1. Pre-Clean 2. Penetrant Application 3. Excess Penetrant Removal 4. Developer Application 5. Inspect/Evaluate 6. Post-clean

45. Pre-cleaning – Step 1 • Parts must be free of dirt, rust, scale, oil, grease, etc. to perform a reliable inspection. • The cleaning process must remove contaminants from the surfaces of the part and defects, and must not plug any of the defects. Pre-cleaning is the most important step in the PT process!!!

46. Why Pre-cleaning important? • Penetrant unable to wet the surface of the test object • due to oils, water/hydrates left after evaporation or polishing and buffing lubricants • Penetrant is unable to enter a discontinuity (blockage) • Peening or smearing of discontinuity, carbon, scale, paint/coatings, penetrant residues • Penetrant bleed out from discontinuity is restricted • Carbon, scale, rust, anodising

47. Cleaning methods • Mechanical methods: • Brushing • Blasting • Chemical methods: • Hot solvent degreasing • Vapor degreasing • Cold solvent degreasing • Alkaline degreasing • Acid pickling • Steam cleaning • Paint strippers

48. Defect Peened or Closed After abrasive cleaning Before Cleaning Physical Cleaning • Grinding • Abrasive Blasting • Wire brushing

49. Light Acid Etching Light Acid applied Thin layer of the surface dissolved

50. The defect opened again to the surface After Acid Etching Light Acid Etching