4.4.8 Thermal Expansion

All materials expand and contract with temperature changes. However, when two joined members have different thermal expansion rates, a temperature change will cause one member to expand more than the other. In an inline six-cylinder automobile engine, differing expansions can cause as much as a 1.5 mm (~0.060 ft) total change to the cylinder heads under extreme temperature cycling. When the change in expansion and contraction is restrained, it induces thermal stress. For a properly bonded or joined assembly, adhesive strength or friction resists the stress, but when the thermal stress exceeds adhesive strength or friction, the joined assembly will begin to fail. For every temperature cycle, excess stress will lead to further separation until critical failure occurs. When assessing the assembly to be bonded, the coefficient of thermal expansion and contraction of each substrate must be known. If the coefficients of thermal expansion and contraction are significantly different, thermal cycling and hot strength testing are recommended to verify the adhesive’s performance under applicable thermal extremes.