ME5247: Brittle Coating

Calibration and Beam Bending

This lab experiment in run in conjunction with another experiment - Strain Gage Application. Carefully read the report requirements so that all lab work can be performed efficiently and all information needed is obtained..

There are three goals for this lab exercise
- to introduce the use of brittle coating,
- to emphasize that the accuracy of many experimental stress analyses depends on measurement system calibration,
- to show the advantages of different types of strain measurement techniques.

Often the overall strain/stress pattern on a part or structure is not known. Before a detailed stress analysis can be planned the regions of high stress and high stress gradients have to be determined. Brittle coatings allow the use of the actual structure in determining strain fields while photoelasticity analysis requires either a model or the application of a photoelastic coating to the structure. In addition to making the strain field visible, brittle coatings can also be used to obtain quantitative measures of strain and so stress.

Perform a brittle coating calibration. Using brittle coating and electrical resistance strain gage techniques measure the stress acting in a cantilever beam under various loads.

Note - General Aspects of Applying and Using Brittle Coatings:
The effective use of brittle coatings requires coating of a uniform thickness and constant characteristics. This means that some practice with coating application should be expected before useful coating can be consistently produced.

The properties of brittle coatings depend on the application and curing of the coatings. So, coating and drying of the calibration beam and test specimen should be identical.

Use Brittle Coating

Coating Selection:
Brittle coating are 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 humidities. 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.

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 aluminum 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 color.

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.

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.

Two tests will be performed.
i) - a brittle coating calibration,
ii) - a brittle coating and strain gage measurement.

i) Procedure: Brittle Coating Calibration
1. Read the following descriptions of the procedure, testing and results required. Look at the test setup. Make sure you understand the procedure and goal for producing brittle coatings.
2. Since the strain on the calibration strip surface depends on strip thickness, no abrasive method should be used for cleaning the strip. The brittle coating thinner should be used to clean the calibration strip. Clean and coat one side of the calibration strips with undercoat and brittle coating. An area of from 25 mm to 40 mm from one end should not be coated. At the same time and in the same way, coat one side of the entire test beam that has a strain gage already mounted on it. The calibration bars and test surface should be sprayed and cured in the same way.
3. After the coating has completely dried
- mount the calibration strip in the calibration fixture,
- set the strain range by turning the cam. The 300 µe - 1500 µe range will be used most often.
- place your thumb directly over the stop (not at the end of the strip) and deflect the strip until it touches the stop,
- the first full crack on the strip near the low end of the strain scale is the threshold strain for the coating. Cracks are most easily seen while under load and with a light source behind the observer.

ii) Procedure: Strain and Stress Measurements Using Brittle Coating and Strain Gage
1. After viewing the video on strain gage application and receiving instruction form the teaching assistant, mount enough practice patterns so that real gages can be mounted consistently well.
2. Based on the results required, determine the location along the test beam length at which to mount the strain gage.
3. Mount a strain gage on the test beam with the gage longitudinal axis transverse to the beam length.
4. Using a three-wire lead configuration and quarter bridge circuit
- obtain strain gage readings with the load weights available,
- when any cracks appear in the brittle coating record their location.

Report: minimum content
I. Brittle Coating Section
1. Show a table with the brittle coating calibration data in it. Assign a value to the fracture strain of the brittle coating. Provide a quantitative discussion of the calibration data, a suggestion for the number of calibration tests that should be done when using brittle coatings and a justification for the number of calibration tests suggested.

2. If there were problems with making strain measurements using the brittle coaing technique, list them and suggest causes of them.

II. Strain Gage Installation Section
Include the Strain Gage Installation lab report here.

III. Questions

1 The beam bending model

stress = {(moment)(distance from neutral axis) / moment of inertia}
was probably used in this exercise. What are the restrictions placed on the use of this model by the mechanistic development of the model? That is, under what conditions of structure geometry and loading is this equation for stress valid?

2. Define plane stress and plane strain states. Explain whether the location in the beam where the strain gage was used is in one or the other or neither of these states.

3. By how much does the moment of inertia change when the strain gage is mounted on the beam?

4. If the brittle coating-undercoat system has time dependent behavior (creep) under load such that the coating fracture strain increases by 10% while under the test load for one minute, what should be the maximum test loading time if measured stress error is to be held to 3%? A typical way to characterize material creep is to assign a creep modulus to the material which is a modified value of the modulus of elasticity. If the 10% increase in observed fracture strain is due to a change in modulus of elasticity with fracture strain constant, by how much does the modulus of elasticity change in one minute?

IV. Extensions of the Lab Work and/or Analysis

Remember, extensions of the experimental work and/or use and discussion of results can lead to a higher report grade.

© 2002 by Barney E. Klamecki. All rights reserved.