Experimental study on the mechanical properties of screws

This article presents study on mechanical properties of screws, which are widely used in our lives and industries. Over one hundred screws, M6 5.8 bought in a store, were experimentally tested. Ultimate stress Rm, ultimate force Fm and hardness HRB were studied. The obtained results were compared against data presented in ISO 898-1 standard. The obtained results show, that the examined screws do not meet ISO 898-1 standard. Moreover, the Shapiro–Wilk test was employed to test distributions of ultimate stress and ultimate force. The distributions are not normal. Next, parameters of a few distributions models were identified from experimental results, then these models were verified with Kolmogorov–Smirnov test, against experimental results. Good agreement was obtained for 3-parametric Weibull distribution, beta distribution and a 7-parametric mixture model. Chemical composition of steel was tested too, the obtained results show, that the compositions are heterogeneous. This is one of the reasons of large dispersion of results. Moreover, an equation dedicated to calculate ultimate force was proposed. This equation shows excellent agreement with experimental results.