Materials and Processes in Manufacturing
E. Paul Degarmo, J T. Black, Ronald A. Kohser
Chapter 11 Nondestructive Evaluation
Brittle Coatings for Nondestructive Evaluation
An Overview of the Use of Brittle Coatings
If the coating applied to a surface is very thin compared to the substrate thickness, the coating does not affect part behavior. The strain in the coating is then the same as the strain at the substrate surface. If the strain in the coating is known then the test surface strain is known and the surface stress can be calculated. Note: only a particular stress/strain state is available from the test, i.e., the strain in the coating is known only for the conditions at which coating fracture occurs.
The procedure for measuring the strain in the part is
The procedure for calculating the stress in the part is
- The coating is formulated to be brittle
- Coating fracture strain is tailored to be less than the yield strain of the part material
- The fracture strain of the coating is measured in coating calibration tests
- A thin coating is applied to the part surface
- Loading is applied to the part
- At locations of coating fracture the strain at the part surface is equal to the coating fracture strain
- For this introduction we ignore biaxial stress/strain effect on coating fracture
- At the location of a crack the strain is the known fracture strain of the coating, efc
- The strain in the part ep is equal to efc
- The stress in the substrate is sp = Ep ep
with Ep the modulus of elasticity of the part
- so sp = Ep efc
A Physical Example: A commonly used example of the requirement that describing brittle fracture must include both stress magnitude and direction is the breaking of chalk. Take a piece of chalk that is used to write on blackboards and try to break it by pulling on it along its length. Usually the chalk cannot be broken. If it can be broken, the orientation of the fracture surface is approximately perpendicular to the applied load direction.
Now if the ends of the chalk are twisted it is easy to break the chalk and the fracture is at an angle Ø of about 30° - 50° to the axis of the chalk. If the brittle chalk breaks due the action of the maximum stress acting, the implication is that the twisting action must have caused a maximum stress at the observed fracture angle Ø.
The calculation of the stress in a part using the known fracture strain of a brittle coating is provided
© 2001 by Barney E. Klamecki. All rights reserved