# 11.1: Introduction

- Page ID
- 7850

This TLP covers some basic mechanics of multi-layered systems. It relates primarily to a 2-layer system, such as a coating on a substrate, although it should be fairly clear how the concepts involved can be extended to multi-layer systems. The treatments presented are for the general case, although the behaviour expected for the special case of one layer being much thinner than the other (eg a thin coating on a massive substrate) is also described.

The TLP is focused on the distributions of (in-plane) stress and strain that can arise within the two constituents under various imposed conditions. These could include applying well-defined external forces, such as an in-plane load or a bending moment. In those cases, the response of the system (in-plane extensions or out-of-plane curvatures) can be obtained from well-known expressions related to the mechanics of composites or of beam bending. However, in layered systems, such as coatings on substrates, the imposed conditions are often more complex than this. For example, heating or cooling of the system will lead to differential thermal expansion or contraction, generating a* misfit strain* - that’s to say, a difference between the stress-free (in-plane) dimensions of the two constituents. Misfit strains, which can also arise in other ways (such as plastic deformation of one of the layers), are important in the mechanics of layered systems. They can give rise to relatively complex through-thickness distributions of stress and strain, and also to both in-plane length changes and out-of-plane curvature.

It may also be noted that misfit strains can arise simultaneously in more than one in-plane direction. In fact, it is common for a given misfit strain to be generated in all in-plane directions - this would normally be the case for differential thermal contraction, assuming in-plane isotropy of the thermal expansion coefficients. As a consequence of Poisson effects, this effectively raises the in-plane stiffness (ratio of stress to strain in any given direction), which can be handled by simply using a *biaxial stiffness* in place of the conventional one.