Age- and Strain-Hardening Effects on Internal Thread: Tensile, Hardness and Pull-Out Tests

The aim of this paper is to compare the effects of age and strain-hardening on the mechanical behavior of aluminum internal thread obtained through specific test with that identified from tensile and hardness standards tests. An actual threaded assembly associating aluminum tapped hole and steel screw was selected. AA2017 and AA1050 aluminum alloys were chosen as materials for the tapped hole. Both T3 and H14 conditions were investigated to practice age- and strain-hardening, respectively and compared with annealing (O condition). Pull-out test was selected to develop an actual stress state in the threaded assembly. The observed failure mode was the internal thread stripping. A qualitative analysis was focused on the load-displacement relationship and a quantitative one was conducted for investigating the yield and ultimate shear loads of internal thread. Tensile and hardness tests were used to check the quality of different aluminum states based on their properties such as elongation, hardness, yield and tensile strengths. The effects of age- and strain-hardening were quantified by calculating the ratio of mechanical characteristics of hardened to annealed states for all tests. The comparison between obtained results showed that tensile and hardness tests give a good information about the trend of strength improvement of internal thread under pull-out test. However, an accurate result for the internal thread requires performing an actual test on the entire assembly.