New Approach for Predicting the Ultimate Load Capacity of AA2017 Aluminum Tapped Thread

The paper discusses the prediction of the ultimate load capacity of tapped thread with insufficient engagement length. Six formulas available in the literature were considered to predict the evolution of the ultimate load capacity versus the thread engagement length. Threaded assembly associating the AA2017-T3 aluminum tapped thread and steel screw of grade 8.8 was considered. To vary the thread engagement length, the tapped hole was made on washers with different thicknesses. Pull-out test was conducted on tension/compression machine to acquire the load-displacement relationship which characterizes the response of tapped thread. Experimental ultimate load capacity of tapped thread was determined by considering the maximum shear load reached during tapped thread stripping. Great disagreements were noted by comparing between all predicted evolutions of the ultimate load capacity versus the thread engagement length. Great disagreement was also observed when comparing experimental evolution to the obtained predictions. Accordingly, a new formula was proposed for the specific geometry of tapped thread with insufficient engagement length. It was founded on the basis of an approach which associates an empirical correction coefficient for the incomplete thread portion and another for the complete thread portion instead of a unique multiplicative correction coefficient like in the literature.