Fracture Mechanics & Failure Analysis: Lecture Preservation Technique

Preservation Techniques of Fractured Specimen:
 FRACTURE SURFACES are fragile and subject to
mechanical and environmental damage that can destroy
microstructural features. Therefore, fracture specimens must
be carefully handled during all stages of analysis.
 Unless a fracture is evaluated, immediately after it is
produced, it should be preserved as soon as possible to
prevent attack from the environment.
 The best way to preserve a fracture is to dry it with a gentle
stream of dry compressed air, then store it in a desiccator, a
vacuum storage vessel, or a sealed plastic bag containing a
desiccant.
 However, such isolation of the fracture is often not practical.
 Therefore, corrosion-preventive surface coatings must be
used to inhibit oxidation and corrosion of the fracture surface.
 The main requirements for a surface coating are as follows:
1. It should not react chemically with the base metal
2. It should prevent chemical attack of the fracture from the
environment
3. It must be completely and easily removable without
damaging the fracture features.

 Fractures may be coated with fresh oil or grease.
 Clear acrylic lacquers or plastic coatings are sometimes
sprayed on the fracture surfaces. These clear sprays are
transparent to the fracture surface and can be removed with
organic solvents.
 However, on rough fracture surfaces, it can be difficult to
achieve complete coverage and to remove the coating
completely.
 Another type of plastic coating that has been successfully
used to protect most fracture surfaces is cellulose acetate
replicating tape. The tape is softened in acetone and
applied to the fracture surface with finger pressure. As the
tape dries, it adheres tightly to the fracture surface.
 The main advantage of using replicating tape is that it is
available in various thicknesses.
 Rough fracture surfaces can be coated with relatively thick
replicating tape to ensure complete coverage.
 The principal limitation of using replicating tape is that on
rough fracture surfaces it is difficult to remove the tape
completely.

Fracture-Cleaning Techniques:
Before any cleaning procedures begin, the fracture surface
should be surveyed with a low-power stereo binocular
microscope, and the results should be documented with
appropriate sketches or photographs.
The most common techniques for cleaning fracture surfaces are:
1. Dry air blast or soft organic-fiber brush cleaning
2. Replica stripping
3. Organic-solvent cleaning
4. Water-base detergent cleaning
5. Cathodic cleaning
6. Chemical-etch cleaning

1. Air Blast or Brush Cleaning:
Loosely adhering particles and debris can be removed from
the fracture surface with either a dry air blast or a soft
organic-fiber brush, such as an artist's brush, should be used
on the fracture surface because a hard-fiber brush or a metal
wire brush will mechanically damage the fine details.
2. The replica-stripping cleaning technique:
 It is very similar to that described in the section
"Preservation Techniques". However, instead of leaving
the replica on the fracture surface to protect it from the
environment, it is stripped off of the fracture surface,
removing debris and deposits. Successive replicas are
stripped until all the surface contaminants are removed.
 The one disadvantage of using plastic replicas to clean a
fracture surface is that on rough surfaces it is very difficult
to remove the replicating material completely.
 However, if the fracture surface is ultrasonically cleaned
in acetone after each successive replica is stripped from
the fracture surface, removal of the residual replicating
material is possible.
 Ultrasonic cleaning in acetone or the appropriate solvent
should be mandatory when using the replica-stripping
cleaning technique.

3. Organic solvents:
 Organic solvents such as xylene, naphtha, toluene, ketones,
and alcohols, are primarily used to remove grease, oil,
protective surface coatings, and crack-detecting fluids from
the fracture surface.
 The sample to be cleaned is usually immersed in a solvent
bath where jets from a pump introduce fresh solvent to the
fracture surface, or placed in a beaker containing the solvent
and ultrasonically cleaned for a few minutes.
4. Water-base detergent cleaning:
 Water-base detergent cleaning assisted by ultrasonic
agitation is effective in removing debris and deposits from
the fracture surface and, if proper solution concentrations
and times are used, does not damage the surface.
 E.g.; a detergent Alconox, has proved effective in cleaning
ferrous and aluminum materials.
The cleaning solution is prepared by dissolving 15 g of
Alconox powder in a beaker containing 350 mL of water. The
beaker is placed in an ultrasonic cleaner preheated to about
95 °C (205 °F). The fracture sample is then immersed in the
solution for about 30 min, cleaned in water then alcohol, and
air dried.

5. Cathodic cleaning:
General Requirement:
a. Cathode = Fracture Sample
b.Anode = carbon/Platinum
c. Electrolyte = any current conducting solvent
d.vibrate the electrolyte ultrasonically or to rotate the specimen
(cathode) with a small motor
 Cathodic cleaning is an electrolytic cleaning process in which
the sample to be cleaned is made the cathode (negative),
and hydrogen bubbles generated at the sample cause
primarily mechanical removal of surface debris and deposits.
An inert anode, such as carbon or platinum, is normally used
to avoid contamination by plating upon the cathode. During
cathodic cleaning, it is common practice to vibrate the
electrolyte ultrasonically or to rotate the specimen (cathode)
with a small motor.
 The electrolytes commonly used to clean ferrous fractures
are sodium cyanide, sodium carbonate, sodium hydroxide
solutions, and inhibited sulfuric acid.
 Because cathodic cleaning occurs primarily by the
mechanical removal of deposits due to hydrogen liberation,
the fracture surface should not be chemically damaged after
elimination of the deposits.

6. Chemical Etching:
 If the above techniques are attempted and prove ineffective,
the chemical-etch cleaning technique, which involves treating
the surface with mild acids or alkaline solutions, should be
implemented.
 This technique should be used only as a last resort because
it involves possible chemical attack of the fracture surface. In
chemical-etch cleaning, the specimen is placed in a beaker
containing the cleaning solution and is vibrated ultrasonically.
 It is sometimes necessary to heat the cleaning solution.
Acetic acid, phosphoric acid, sodium hydroxide, ammonium
citrate, ammonium oxalate solutions, and commercial
solutions have been used to clean ferrous alloys.
.

Fracture Mechanics & Failure Analysis: Lecture Preservation Technique

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Fracture Mechanics & Failure Analysis: Lecture Preservation Technique