TO THE CALCULATION OF MACHINE PARTS FOR FATIGUE RESISTANCE

Abstract

Purpose. The aim of the work was to summarize the reference and experimental data and to provide recommendations for the design of machine parts for fatigue resistance under conditions of cycle asymmetry.
 Research methods. To study the problem of resistance of structural materials to fatigue under conditions of cycle asymmetry, an installation for testing samples at any coefficients of cycle asymmetry, both at tensile and compressive average stresses, created at the National University “Zaporizhzhya Polytechnic”. On this installation, tests were carried out on flat smooth specimens with a cross section of 5×20 mm and specimens with a cross section of 5×24 mm, with a form-hole concentrator with a diameter of 5 mm, made of 09Г2С steel in accordance with DSTU 8541. The smooth specimens were tested at average cycle stresses of 0, 100, 200, 300, -100, -150, -200 MPa up to a service life of 5×106 cycles. The reference data of steel with a tensile strength of σв=520 MPa for a service life of 2×106 cycles and the effective stress concentration factor were processed after determining the ultimate amplitudes and the corresponding average cycle stresses at different cycle asymmetry factors.
 Results. The obtained results of studying the work of structural materials under cyclic loads with different cycle asymmetries allow us to better understand the physical processes of fatigue materials of machine parts, as well as to reveal the dependence of the effective stress concentration factor not only on the amplitude, but also on the value of the average cycle stress. The work shows that the effect of the stress concentrator depends on the value of the average cycle stress.
 Scientific novelty. A formula for obtaining a complete diagram of ultimate amplitudes that takes into account durability is proposed.
 Practical value. The work can be useful for designers to more effectively use the service properties of structural materials, as well as for researchers to save reimbursements in fatigue resistance tests.