Effect of Washer Placement on Performance of Direct Tension Indicators with Curved Protrusions

Abstract

A series of tests was performed to evaluate the effect of a hardened washer placed between the turned element and a direct tension indicator (DTI) with curved protrusions. Configurations with 3/4 in., 7/8 in. and 1.0 in. diameter bolts with and without hardened washers were evaluated. Tests were also performed with ¾ in. and 7/8 in. bolts using a new type of DTI, where the DTI is staked to a nut. The purpose of these tests was to compare the performance of the various configurations, as measured by the number of gaps open at the specified pretension level, the load required to close at least half of the gaps, and the tensile load on the bolts when all or all but one of the gaps in the DTI are closed. When an ASTM A563 grade DH nut was used for a given bolt diameter, some differences were observed to be statistically significant. However, no consistent trends were observed in these differences, and the actual differences were of the same order of magnitude as the load increments used in testing. Therefore, it was concluded that there are no practical differences between the various configurations considered when grade DH nuts are used. However, the DTI did not perform well without a secondary hardened washer when an ASTM A563 Grade C nut was used.