Uncertainties in fatigue assessment of structures in design and in service
Authors:   IPMash RAS, Guchinsky R. V. , Petinov S. V. ,   IPMash RAS
Published: January 2022

The rules for calculating the fatigue of bridge structures, ships, transport and other structures subjected to significant variable loads during operation are accompanied by a number of uncertainties. When calculating the fatigue of structures at the stage of assessing the operational load, significant errors are introduced. Loads on structures may depend on the economic situation, for example, traffic on bridges during the designated service life, changes in passenger traffic that affect the congestion of urban transport, etc. These uncertainties accompany the design process, since loading conditions in the service life of the structure are predicted approximately, mainly taking into account the accumulated operational experience. In addition, there are uncertainties in the existing methodology for assessing structural fatigue, embedded in the specifics of the S-N criteria for fatigue and approaches to the application of criteria for assessing fatigue damage. Despite the fact that a number of uncertainties in the current procedures for assessing structural fatigue have been discussed for many years, approaches to calculation do not change significantly. Among the power approaches based on the use of S-N curves, there are three main types. Firstly, the nominal stress approach, which represents a set of S-N curves for typical welded joints and structural parts in which fatigue life depends on the nominal stress corresponding to the critical region. Secondly, the approach of stresses at the hot spot, where the selected S-N curves are given for the main material and the weld material, in this case, the stress at the hot spot is associated with geometric features of the joint and is determined using the finite element method. Thirdly, the peak stress approach, which also uses S-N curves for the base and deposited metal materials, but the local stress is calculated taking into account the shape of the part and the weld in question with detailed modeling of geometry of the critical area, the weld sole. The approaches of stress at the hot spot and peak stresses assume linear transformation of the nominal voltage, linear elastic behavior of the material in the stress concentration area. In this case, the local cyclic elastic-plastic deformation of the material is not taken into account. However, failure to take into account the cyclic plasticity of the material, even with moderate stress concentration, leads to some underestimation of the estimated fatigue damage to structures. Another source of uncertainty in evaluation of fatigue properties is discrepancy between the size and nature of fatigue damage in standard test samples for the purpose of obtaining S-N curves and in the design areas under consideration. In addition, in methodology of fatigue assessment, it is important that the nominal stress in many parts of real structures that detect fatigue damage and the early appearance of cracks practically is not changed due to redundancy of structures during the growth of cracks. The formation of a crack leads to sharp relaxation of stresses and often to redistribution of efforts to the structures surrounding the crack. This means that areas of high stress concentration are controlled more by movements than by efforts. This thesis, based on numerous experimental observations, leads to the conclusion that testing samples to determine the fatigue characteristics of the material In addition, in the methodology of fatigue assessment, it is important that the nominal stress in many parts of real structures that detect fatigue damage and the early appearance of cracks practically does not change due to the redundancy of structures during the growth of cracks. The formation of a crack leads to a sharp relaxation of stresses and often to a redistribution of efforts to the structures surrounding the crack. This means that areas of high stress concentration are controlled more by movements than by efforts. This thesis, based on numerous experimental observations, leads to the conclusion that tests of samples to determine the fatigue characteristics of the material should be carried out within constant limits of movement of the testing machine grips. Based on the results of the work, the following conclusions were made. The local stress in the concentration area in accordance with the currently accepted approaches of stresses at the hot spot and peak stresses is determined from the assumption of linear elastic behaviour of the material over the entire range of operational loads. Fatigue tests of standard samples carried out until almost complete destruction include the initial accumulation of damage in the structure of the material, the origin and growth of the crack. The condition of structural damage in finite element models based on sample tests is estimated indefinitely. It is assumed that the calculated S-N curves uniformly characterize the fatigue resistance of low-carbon and high-strength steels and the corresponding welded joints, although the fatigue resistance of the base material differs for different structural steels. The damaging effect of medium cycle stresses, including residual welding stresses, is manifested mainly in the phase of crack growth. When testing samples with a zero loading cycle, these stresses cannot be properly taken into account in the approaches used. As measures to improve fatigue calculation methods, it is proposed to test samples under conditions of control of the displacement range before the origin of a macroscopic crack (controlled, for example, by the potential drop method). It is also recommended to distinguish between the mechanical properties of materials when planning, conducting fatigue tests, analyzing the results and constructing S-N curves.