In a composite bar, two materials with different coefficients of thermal expansion (CTE) are rigidly joined together, which forces both materials to undergo the same change in length — even though each material naturally "wants" to expand or contract by a different amount.

·    •    On cooling, the part with the higher CTE wants to shrink (contract) more than the part with the lower CTE.

·    •   Since both parts are joined together, the high-CTE part is restrained from shrinking as much as it wants by the low-CTE part.

·      •   This restraint effectively stretches it relative to its natural (free) contracted length, which induces tensile stress in the high-CTE part.

·      •   Conversely, the low-CTE part is pulled inward more than it naturally would contract, inducing compressive stress in it.

So, on cooling: the high CTE part experiences tensile stress, and the low CTE part experiences compressive stress.