This thesis presents a study of the dependency of loosening parameters on secondary locking features of threaded inserts subjected to dynamic shear loads. Secondary locking is used to assist and/or provide redundancy to the primary locking feature (threads) in preventing preload loss in almost any mechanical applications. Two different secondary locking features are studied: the Locking Heli-Coil insert and the Loctite Threadlocker (R) applied before assembly to a Standard Heli-Coil insert. Five parameters are studied in this thesis: percentage loss of initial preload, initial rate of preload loss, secondary rate of preload loss, steady-state value, and the final preload value. Statistical analysis was used to quantify the dependencies between locking levels. Results show that the loss of initial preload is dependent on secondary locking features, the initial and secondary rate of preload loss are dependent on secondary locking features, the steady-state value and the final preload value are dependent on secondary locking features. Also, due to secondary locking features, 83% of the "Locking Heli-Coil with Braycote" tests reached steady-state while only 16% of the "Standard Heli-Coil with Loctite" tests reached steady-state even though the final preload value were higher for "Standard Heli-Coil with Loctite."

The motivation of this research is to explore the viability of a method to directly verify whether or not an anaerobic adhesive within a threaded fastener assembly has cured sufficiently to provide secondary locking. Direct verification was implemented via the application of a test torque in the loosening direction of a fastener assembly with Loctite (given a 24 hour cure time). A three phase test plan was developed with the intent of identifying and utilizing this verification torque value which is unique to a given fastener assembly. It was proved that the direct verification method, as outlined in the test plan, was in fact a valid method of verification in some cases. Results were dependent on the materials and coatings of the fastener assemblies. The curing properties of the liquid locking compound (LLC) with plain steel specimens resulted in a verification test that could accurately predict sufficient locking and cure (using distributions in torque measurements). Tests with zinc coated and stainless steel specimens, however, did not produce the same level of predictability in cure as the plain steel specimens. The direct verification method as defined herein is not suited to certain materials and coatings. The less predictable curing properties of the stainless steel specimens caused complications in determining a verification torque that could reliably determine cure, resulting in dropping stainless steel from static and dynamic testing. The zinc coated specimen data was more consistent, but the adhesive did not add sufficient breakaway strength to the fastener assembly needed to define a usable verification value. These results led to the conclusion that the direct verification method as presented in this work is limited to more active material selections. One possibility to improve the secondary locking of less active materials is the use of a higher strength adhesive. Testing to observe the effect of application of verification torque on the secondary locking was also performed. Multiple verifications were found to be destructive to the point that just over a third of samples failed that might have otherwise passed a single verification test. The single verification testing, on the other hand, caused substantially less locking mechanism degradation, leading to the use of a single verification torque in further testing.

The purpose of this research is to determine if anaerobic adhesive can be used as a prevailing torque locking feature. Maintaining preload in critical joints is the usual standard that anaerobic adhesives are held to in aerospace and other industry. To test if anaerobic adhesive can be used as a prevailing torque locking feature a test procedure was developed and implemented to measure the removal torque of threaded fasteners after an allotted cure time. In total, 191 threaded fasteners of different material and coatings were tested in the unseated and seated states with various strengths and varieties of anaerobic adhesive. A series of three tests were conducted: initial use, reuse with no added anaerobic adhesive, and a third test with added product to the bolt and nut to see how removal torque would behave in these conditions. It was found that using anaerobic adhesive as a prevailing torque locking feature is viable in many cases. No published work to date analyzes anaerobic adhesive at the standard of a prevailing torque locking feature.