Brittle Coating Method

Definition

The process of brittle-coating method under experimental stress analysis procedure consists of applying a brittle coating on the surface of the part to be tested. Cracks in the coating that appear due to loading the part can be analyzed for direction and magnitude of the surface strains.

Principle

 The principle of stress analysis involves the adherence of a thin coating (resin- or ceramic-based) brittle in nature on the surface of a specimen. When the specimen is subjected to external loads, the thin brittle coating cracks under tensile stresses. Strains produced in specimen are transmitted to the coating resulting in coating cracks. From the threshold strain of coating, i.e. minimum strain required to cause the coating to crack, determined through calibration of coating, the stresses in specimen are determined. The behavior of the coating is quite complicated as it depends on the number of parameters influencing the behavior of the coating, such as coating thickness, coating temperature

Brittle Coating Methods:

The equipment necessary for stress analysis through brittle coating is listed below:

1. A wide range of brittle coatings.

2. Aluminium under coat paint.

3. Red dye etchant.

4. Etchant emulsifier.

5. Two spray guns—one for aluminium under coat and another for coating.

6. A small portable air compressor.

7. Respirator for operator.

8. Focused light for visual inspection.

9. About a dozen calibrating beams.

10. A beam bending device.

11. A strain scale with beam bending device.

12. Temperature and humidity measuring instruments.

13. A storage cabinet.

The specimen with brittle coating is loaded, then load is maintained for 15 seconds on the specimen and then load is released. After unloading the entire surface of the coating is examined for coating cracks. Now the load on the specimen is increased and the entire process is repeated. The crack patterns located after each loading cycle are encircled with line (isoentatic line) and marked with a number corresponding to load on the specimen who produced the strain. Isoentatic line is loci of points of approximately constant principal stress.

If the test is performed on a cylindrical pressure vessel then first family of cracks is perpendicular to σc (hoop stress) and second family of cracks is perpendicular to σa (axial stress).

Use Brittle Coating

Coating Selection:
Brittle coating is designed to fracture at a specified strain, usually 500 microstrain. Since fracture strain depends on application conditions, the particular brittle coating to be used should be chosen for the test conditions to be used. The most important factors that determine the coating fracture strain for well-applied coatings are temperature and relative humidity. Coating manufacturers produce coatings for application and use at a variety of temperatures and humidity’s. The coating chosen should be appropriate for the test conditions.

Surface Preparation:
Surfaces must be clean of all dirt, grease, loose scale and any paint that is softened by the coating thinner. Plastic surfaces that are softened by the thinner can be protected by the brittle coating undercoat.

Previously used brittle coating can be removed by scraping, wire brushing or sandblasting followed by using an appropriate cleaner.

Undercoating:
The undercoat is used to provide an easy-to-see surface under the brittle coating and to eliminate directional reflectance characteristics of the test surface. The undercoat is composed of aluminium particles in a carrier.

Apply several thin coats of undercoat. Spray from about 15 cm (6 in) from the surface. The individual thin undercoats dry in three to five minutes. Allow at least 30 minutes drying time for the entire undercoat before applying the brittle coating.

Coating Application:
The brittle coating must be built up slowly by applying several light coats. The final coating thickness should be 0.06 mm – 0.11 mm (0.0025 in – 0.0045 in). Coating thickness can be measured from before and after measurements of the calibration specimen thickness, and sometimes on the actual test surface depending on the actual test part. For the brittle coating used in the lab a coating of about 0.09 mm (0.0035 in) thickness has a pale green colour.

Each coat should be applied in one spray pass. Spray passes should be quick and steady from a distance of about 15 cm (6 in). Coats should not be applied so wet/thick that they run nor so dry that they appear dusty. Excessive coating thickness causes sagging, running and trapping of large air bubbles. The first coat may not cover the surface evenly, but subsequent coats should even out the coating. A good coating while it is still wet will appear glossy pale yellow. A light dust may occur on drying and this is acceptable. Heavy dust is not acceptable and can be dissolved by rapid spraying of a 50/50 mixture of coating and thinner.

Best practice is to apply the coating at about 3C – 5C (5F – 9F) above the coating rating design temperature.

A minimum of one minute drying time should be used between spraying passes to allow for solvent evaporation. If the coating is applied slightly below the design temperature or above the specified humidity more solvent release time must be used.

Drying:
The brittle coating should dry for at least 24 hours. Best practice is to hold the coating at the elevated application temperature for drying and then to slowly cool it to the test temperature. While not recommended for best results coating drying can be accelerated by drying in air for one hour then elevating the temperature to 49C (120F) for 2 hr – 4 hr followed by slow cooling.

Holography:

Holography is a technique which enables three-dimensional images to be made. It involves the use of a laser, interference and diffraction, light intensity recording and suitable illumination of the recording. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the image appear three-dimensional.

The holographic recording itself is not an image; it consists of an apparently random structure of varying intensity, density or profile.

How holography works:

Fig: Recording a hologram

Fig: Reconstructing a hologram

Fig: Close-up photograph of a hologram’s surface.

The object in the hologram is a toy van. It is no more possible to discern the subject of a hologram from this pattern than it is to identify what music has been recorded by looking at a CD surface. Note that the hologram is described by the speckle pattern, rather than the “wavy” line pattern.

Holography is a technique that enables a light field, which is generally the product of a light source scattered off objects, to be recorded and later reconstructed when the original light field is no longer present, due to the absence of the original objects. Holography can be thought of as somewhat similar to sound recording, whereby a sound field created by vibrating matter like musical instruments or vocal, is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter.

Laser:

Holograms are recorded using a flash of light that illuminates a scene and then imprints on a recording medium, much in the way a photograph is recorded. In addition, however, part of the light beam must be shone directly onto the recording medium – this second light beam is known as the reference beam. A hologram requires a laser as the sole light source. Lasers can be precisely controlled and have a fixed wavelength, unlike sunlight or light from conventional sources, which contain many different wavelengths. To prevent external light from interfering, holograms are usually taken in darkness, or in low level light of a different colour from the laser light used in making the hologram. Holography requires a specific exposure time (just like photography), which can be controlled using a shutter, or by electronically timing the laser.

Apparatus

A hologram can be made by shining part of the light beam directly onto the recording medium, and the other part onto the object in such a way that some of the scattered light falls onto the recording medium.

A more flexible arrangement for recording a hologram requires the laser beam to be aimed through a series of elements that change it in different ways. The first element is a beam splitter that divides the beam into two identical beams, each aimed in different directions:

· One beam (known as the illumination or object beam) is spread using lenses and directed onto the scene using mirrors. Some of the light scattered (reflected) from the scene then falls onto the recording medium.

· The second beam (known as the reference beam) is also spread through the use of lenses, but is directed so that it doesn’t come in contact with the scene, and instead travels directly onto the recording medium.

Several different materials can be used as the recording medium. One of the most common is a film very similar to photographic film (silver halide photographic emulsion), but with a much higher concentration of light-reactive grains, making it capable of the much higher resolution that holograms require. A layer of this recording medium (e.g. silver halide) is attached to a transparent substrate, which is commonly glass, but may also be plastic.

Process:

When the two laser beams reach the recording medium, their light waves intersect and interfere with each other. It is this interference pattern that is imprinted on the recording medium. The pattern itself is seemingly random; as it represents the way in which the scene’s light interfered with the original light source — but not the original light source itself. The interference pattern can be considered an encoded version of the scene, requiring a particular key — the original light source — in order to view its contents.

This missing key is provided later by shining a laser, identical to the one used to record the hologram, onto the developed film. When this beam illuminates the hologram, it is diffracted by the hologram’s surface pattern. This produces a light field identical to the one originally produced by the scene and scattered onto the hologram. The image this effect produces in a person’s retina is known as a virtual image.