Experimental Investigation on the Fatigue Strength for Different Tightening Procedures and Materials in Metric Screws

The aim of this present paper is to investigate the influence of some joint design parameters on the fatigue strength of metric screws. This investigation involved different screws strength grades (8.8 and 12.9). The experimental tests were carried out on black oxidized M6 screws, coupled with steel nuts of the corresponding strength class. Both screws and nuts were tested in “as received” lubrication condition. The screws were tested untightened and tightened with the tightening torque causing yielding (100% of the yield stress). A preliminary test to evaluate the tightening torque providing the desired equivalent stress on the screw was carried out. A tightening machine bench which was able to measure the tightening load and the friction coefficients both in the underhead and in the thread was used. The tests were run controlling the tightening torque and the spindle speed. In the following tests, the screws were tightened at the desired tightening torque and then untightened by means of the aforementioned tightening machine bench. Then the screws and the nuts were assembled on an ad hoc test fixture and tested on a resonant testing machine in order to evaluate the screw fatigue limit according to the international standard ISO 3800. The experimental results were processed by means of statistical tools of two-way ANOVA and Fisher Test in order to evaluate the effect of each parameter on the fatigue response of the screws.