Relaxation of ASTM A325 bolted assemblies

Results from a series of tests intended to study loss of pretension force in bolted assemblies are presented. The test matrix included ASTM A325 assemblies with various direct tension indicators (DTIs), some through-hardened and some not, as well as assemblies without direct tension indicators. Bolt length and diameter and grade of nut were also varied. Load losses were monitored for 42 days (1,000 h). However, load levels generally stabilized after 14 days. Pretension losses were linearly dependent on initial pretension and not affected by DTI or washer configuration. Bolt diameter and length did affect the magnitude of losses over time but not the general linear relationship with initial pretension.