Insulation in sheet form, such as paper, treated cloth, plastic, and pasted mica itself, must often serve as the primary dielectric in electrical equipment. Usually, such sheet insulation is either impregnated with an insulating liquid or with varnish or compound. In all but a few purely mechanical applications, sheet insulation must not develop cracks or discontinuities even where exposed to mechanical strains such as those imposed by vibration or differential thermal expansion. In low-voltage motor and generator insulation, cracks and discontinuities may not in themselves cause failure, but they may provide the means whereby moisture, dirt, and contaminants can enter to cause electrical failure. For many years, mandrel flexibility tests on the sheet insulating materials and oven aging or operational tests on coils and complete equipment have been used to evaluate sheet insulating materials as well as other insulating components.