Surface preparation for the LDH test

Abstract

The present work is aimed at developing a surface preparation technique that reduces or eliminates random variations often found in the limiting dome height (LDH) test without diminishing the ability of the test to discriminate between different batches of material. Degreasing with a nonpolar solvent (mineral spirits) followed by the application of a low viscosity mineral oil (mineral seal oil) gave the best results and was adopted as a standard. With this preparation, the LDH was as low as with sheets degreased with chlorinated solvents, while scatter was reduced. Surfaces prepared by wiping with a clean towel gave higher LDH values, corresponding to a surface coated with SAE 30 oil. None of the preparation methods could completely eliminate or mask residual films left by oils compounded with boundary or EP additives. Day-to-day variations were evident with all preparation techniques employed over a period of three months. However, when all LDH means were plotted as a function of sheet location in the coil, the means defined a curve which—within the limited coil length represented by the total sample quantity available—had a period of about 15 m (45 ft). Thus, random variations observed in the laboratory and press shop can be attributed to small variations in mechanical properties, revealed by the great sensitivity of the LDH test and not necessarily detectable by tensile tests. Surface preparation may contribute to random variations but such contribution can be eliminated by the standard preparation technique.