Extracting, Protecting, Cleaning, and Preserving Fracture Surfaces

1. Extracting, Protecting, Cleaning, and Preserving Fracture Surfaces David M. Christie Senior Failure Analyst IMR Test Labs

2. Fractures come in various forms • Fully separated • Possible loss of one “side” of the fracture • Greater possibility of post fracture damage • Partially separated • Implies ductile overload for at least the last load cycle • Cracked • Often difficult to detect, challenging to photodocument • After documentation of as-received condition, will require extraction of fracture faces

3. Fracture Surface Extraction • Frequently we are examining cracks, not completely fractured components, or secondary cracks exist which we want to examine. • The task is to separate the entire crack surface with zero damage….. Why? • We can only presume the locations of the origin of cracks. • If the crack fracture surface is oxidized or corroded, the best information may be obtained at the crack tip, not at the crack mouth. • This often can be difficult and will involve some assumptions.

4. Steps in Opening a Crack • Identify the length of the crack. • Does the crack extend through the wall of the component? If so, on which side is the crack longer? What does this tell us? • Initial cuts may be necessary in order to remove the cracked section of the part – these should be made well away from the perceived crack ends. • When possible, dry cut the part. • When this is not possible either: • Mask the fracture surface with tape or spray anti-corrosion fluid over and into crack. • Or, after wet cutting, rinse the part with water, then ethanol, blow dry, and immediately work to open the crack – retained cutting fluid will corrode the fracture.

5. Steps in Opening a Crack • Under stereo microscope, examine the crack closely, and identify the ends of the crack. If the crack is open, measure the width of the crack mouth (this may later be useful in determining deformation during cracking). • Mark the ends using a suitable marker/paint stick, etc. • Mark your planned saw cut, leaving a small distance between your cut and the observed end of the crack. Frequently the crack will extend a bit farther than you can perceive under the stereo microscope.

6. Steps in Opening a Crack

7. Steps in Opening a Crack • From what you can observe about the crack length on each side of the component, try to imagine the crack shape. • This can be difficult if the crack is only visible on one side of the part wall. • Crack Aspect Ratio is Highly Variable • Sometimes the aspect ratio can be deduced by examining the crack closely.

8. Steps in Opening a Crack • If the crack has numerous small “steps” (is jagged), one can be fairly certain that multiple fatigue origins are present. • If so, the crack is likely to be shallow, relative to its length. • If not, the crack may be deeper, relative to its length. • The desire is to make cuts that end close to the crack tip, so that when the crack is opened small ribbons of laboratory-induced overload fracture are between the crack tip and the saw cut

9. Steps in Opening a Crack A “Jagged” or “Stepped” crack implies multiple origins, and is likely shallow relative to length

10. Steps in Opening a Crack

11. Steps in Opening a Crack Large steel extruder bore with crack on OD, centered on counter-bored thermocouple hole

12. Steps in Opening a Crack Crack is visible on the ID

13. Steps in Opening a Crack Cuts were made at angles, based on OD and ID crack lengths. The entire crack surface was exposed for examination.

14. Steps in Opening a Crack • When opening a crack, put the origin in tension, never in compression or shear. • Origin is typically in the center of a crack • If the crack is visible on both sides, the origin is likely on the side of greatest crack length. • Pre-chill ferrous materials to make them easier to part. • Use a vise, tensile tester, or hammer blows to part. You might have to get creative here…...

15. Steps in Opening a Crack Crack Cut Notch Vise jaws put compression on cut notch, producing tension at the crack mouth, where the crack origin is located.

16. Cleaning Fractures: Documentation • Keep a record of treatments and procedures used during fracture cleaning and examination: • For example: • 5 minutes ultrasonic cleaning in ethanol • 10 minutes ultrasonic cleaning in Alconox • 60 minutes ultrasonic cleaning in stabilized acid • Two hours re-writing my resume’ after the part dissolved in the acid……..

17. Initial Condition of Fracture Surfaces • From “Pristine” to “Heavily Corroded” to “Abused”, to “loose parts in a box, some missing” to “carefully protected, well documented part life cycle”. • First examine for obvious, ongoing corrosion issues • If present: • Retain samples of the corrosion oxides, by-products, contaminants for later analysis • Remove by replica tape, scraping a non-critical surface, or retaining the mating fracture face “as received”. • Document the overall part, as well as fracture surfaces and other relevant surfaces ASAP • Immediately clean the part so that no further corrosion damage occurs.

18. Preserving Fractures During the Analysis • Failure may result in litigation: preservation is essential. • Wrap with cloth or cotton covering – Beware of fibers, as they can be devilish to remove from rough fracture surfaces. • Chemical damage is harder to prevent. • Oils/greases/contaminants may destroy evidence. • Corrosion-resistant paper, acrylic sprays, etc. • Corrosion inhibitors: Cortex VpCl-368D, or Tectyl 506 • Some alloys are easily oxidized in typical office/laboratory atmospheres (Eg. Magnesium, some steels, cast iron…..)

19. Preserving Fractures During the Analysis • Use a desiccator and/or desiccants. • Steel fractures of small size can be immersed in alcohol between cleaning and examination. • A light oil or corrosion inhibitor can be sprayed on the surface, and removed with solvents/detergents before visual or SEM examination. • Softened acetate tape or acrylic spray can also be used. • Avoid contact of the fracture surface with any hard objects: • Mating surface, tools used to separate or clean surfaces, etc. • Even soft bristle brushes can damage soft materials.

20. A Word to the Wise: • Deposits/oxide layers can tell a lot about the failure. • Chronology, aggressive species, contamination, etc - examine the fracture surface before cleaning. • Map out and document any differences in coloration, topography and unusual features.

21. Cleaning Fracture Surfaces • Employ the Hippocratic Oath: Do no Harm! • Photo-document As-received condition. • Obtain samples of the oxides/contaminants from the fracture BEFORE cleaning. (for EDS, etc.) • Clean starting with least aggressive method. • Use step-wise approach and examine at each step. • It is often not necessary to remove all oxides or contamination from the fracture and attempting to do so may damage the surface. • If in doubt, submit a polished metallographic mount of your material to the proposed cleaning method, examine for etching or other damage, before exposing your fracture to the method.

22. Cleaning Fracture Surfaces • From least aggressive to most aggressive: • Gentle dry compressed air to blow surface clean • Gentle use of soft organic fiber brush • Acetate replica stripping • Ultrasonic using organic solvents • Ultrasonic using water-based detergents • Cathodic cleaning • Stabilized acid cleaning • Weak acid or base cleaning

23. Cleaning Fracture Surfaces • Acetate replica stripping • Acetone is flooded on the surface. • Acetate film is pressed into the surface. • After the Acetone evaporates, the film is stripped off. • Oxides and corrosive species removed can then be analyzed. • Many repetitions are often required, with ultrasonic cleaning in acetone between each stripping. • Rough Fractures can retain the acetate film, requiring additional solvent cleaning. • Post-stripping cleaning in an organic solvent is usually required before viewing the fracture. • Personally, I rarely use this technique.

24. Cleaning Fracture Surfaces • Ultrasonic using organic solvents • Avoid chlorinated organics such as carbon tetrachloride and trichloroethylene, as they are carcinogenic. • Use of xylene, naphtha, toluene, ketones, or alcohol is permitted. • Will only remove organic contaminants (grease, oil, other lubricants) • is not useful for oxides, in general.

25. Cleaning Fracture Surfaces • Water-based detergents: • The alkaline labware cleaner “Alconox” is useful • Mix 160 g to one gallon of DI water (15 g per 350 ml). • Can be used at room temperature or heated to 100 F • Ultrasonic for up to 15 – 30 minutes in five minute increments, with examinations after each five minutes. • Alconox can damage soft metals, such as aluminum • Alconox can remove lead from leaded materials. • “Citranox” is an acidic detergent, which is more aggressive than Alconox • Use caution, corrosion is more possible • Rinse thoroughly from surfaces • Try alternating with Alconox to remove stubborn high temperature oxides. • Any water-based detergent will damage the fracture surface if it is used for more than 30 minutes.

26. Cleaning Fracture Surfaces – Before Alconox

27. Cleaning Fracture Surfaces – After Alconox

28. Cleaning Fracture Surfaces • Cathodic Cleaning • The sample is made the cathode, and a platinum or carbon anode is used. • The action of hydrogen bubbles generated at the fracture surface mechanically removes deposits. • Performed in an ultrasonic cleaner. • Electrolytes: sodium cyanide, sodium carbonate, sodium hydroxide, or inhibited sulfuric acid. • Commercially available: Endox 214 • Beware: high strength steels may develop hydrogen induced cracks during cathodic cleaning.

29. Cleaning Fracture Surfaces • Inhibited acid cleaning • Acid will attack the base metal, so careful monitoring is required. • Both Ferrous and Non-Ferrous can be cleaned using: • HCl (1.190 sp. gr.) 3 ml. • 2-Butyne-1,4-diol (35% aqueous) 4 ml. • Distilled water 50 ml. • Weak Acid/Base cleaning • Last resort • Ferrous: Acetic, phosphoric, or sodium hydroxide • Titanium: nitric • Aluminum: orthophosphoric, chromic, distilled water mix • Generally, these methods will limit what can be gained by SEM examination.

30. Removal of Stubborn Oxides • Remember – the oxide formed FROM the fracture surface! • Examine the patterns on the oxidized surface first. • Do not attempt to remove all of the oxides from 100% of the fracture face.

31. Removal of Stubborn Oxides When oxides form on a fracture they are rarely uniform in thickness, both on a macroscale and on a microscale. The original surface is represented by the black line. The oxide formation consumes some of the metal, producing a layer of oxides. The patterns revealed IN THE OXIDES can sometimes be your best information – document before cleaning!

32. Removal of Stubborn Oxides Where are the fine fracture features most likely to be located in the drawing below? (hint: what is the oxide composed of ?)

33. Removal of Stubborn Oxides • When do you quit? • On severely oxidized surfaces, removal of oxidation damage becomes moot. • Consumption of the fracture surface as oxidation proceeds removes all of the fine features. • Often the primary fracture surface is cleaned to reveal macroscopic features, with the realization that microscopic features are already damaged to the point of being entirely removed.

34. Removal of Stubborn Oxides • Partially cleaned fracture, viewed using the SEM. • Use Backscatter imaging mode to determine what regions are “clean” and what regions are still covered with oxides. • Switch to Secondary imaging to go to high magnification for examination and documentation of the cleaner regions of the fracture.

35. SEM backscatter view of a partially cleaned fracture The light regions are clean metal. The darker regions are still covered with oxides.

36. Another Example of partially cleaned surface, viewed using BEI mode