Prediction Efficiency of the Ultimate Load Capacity of Nut Thread with Insufficient Engagement Length

Abstract

The objective of this work is to present an evaluation of the ultimate load prediction of nut thread with insufficient engagement length. It is focused on the case of nut with material’s stiffness and strength lower than that of the screw. Seven available predictions of ultimate load were considered from literature. A pull-out test was carried out on a threaded connection between a nut thread and a steel screw grade 8.8. The A1050-H14 aluminum alloy sheet with 2 mm thickness was chosen. During the test, the screw was animated with a progressive axial displacement until the complete destruction of the screw-nut assembly. The pull-out of the nut thread is generated by transverse shear stress without interfacial shear stress. The load-displacement relationship was investigated. The tapped hole morphology after the complete destruction of the screw-nut assembly was also inspected. The available predictions and the average experimental value of the ultimate load capacity of threaded connection were compared. This comparison supported the need for theoretical development that must be specific to the studied assembly i.e. a screw that is fastened to a nut having lower stiffness and strength as well as an insufficient engagement length.