Nonlocalized and localized fatigue damage in metals and alloys

Abstract

Проаналізовано основні закономірності розсіяного і локалізованого втомного пошкодження металів і сплавів. Запропоновано та обґрунтовано методи визначення умов переходу від розсіяного до локалізованого втомного пошкодження, засновані на аналізі закономірностей розповсюдження «коротких» і «довгих» тріщин. Із використанням розроблених методів визначено розміри тріщин і кількість циклів навантаження, що відповідають переходу від розсіяного до локалізованого втомного пошкодження вуглецевих, легованих і аустенітних сталей та алюмінієвого сплаву з урахуванням рівня напружень і властивостей досліджених матеріалів.The main regularities of nonlocalized and localized damage in metals and alloys are analyzed. Based on the analysis of the regularities of «short» and «long» crack propagation, the methods for determining the conditions for the transition from nonlocalized to localized fatigue damage are proposed and justified. By the «conditions for the transition» are mainly meant the crack sizes corresponding to the main crack initiation and the number of load cycles to this crack initiation. The first of the methods is based on the analysis of the kinetics of the growth of short fatigue cracks evaluated from either the crack-size or the crack growth rate dependence on the number of load cycles. In the transition from nonlocalized to localized fatigue damage at stresses above the fatigue limit, the form and parameters of the equations describing the crack size and the crack growth rate dependences on the number of load cycles are changed. The second method is based on the analysis of the kinetics of variation in the short crack propagation rate depending on the stress intensity factor. The stress intensity factor range and its corresponding crack size at which the SIF begins to change at an increasing rate is taken as the condition for the transition from nonlocalized to localized fatigue damage. Using the developed methods, the crack sizes and the numbers of load cycles corresponding to the transition from nonlocalized to localized fatigue damage in carbon, alloyed and austenitic steels and in an aluminum alloy are determined by considering the stress level and the properties of the materials under investigation. It is shown that the fatigue crack sizes corresponding to the transition from nonlocalized to localized fatigue damage at stresses above the fatigue limit and found with both the first and the second methods, decrease with increasing stresses, always remaining lower than the crack size at the fatigue limit. For the metals and alloys under investigation, the fatigue crack values corresponding to the transition from nonlocalized to localized fatigue damage in the region of high-cycle fatigue (105–107 cycles) vary in the range from 0,01 to 0,74 mm. In this case, the fatigue crack value corresponding to the transition from nonlocalized to localized fatigue damage is 0,07 to 0,02 mm for carbon and alloy steels, 0,2 to 0,74 mm for ductile austenitic steels, and 0,155 mm for an aluminum alloy. The ratio of the number of cycles to main crack initiation to the number of cycles to fracture increases with increasing number of cycles to fracture. The value of this ratio for the materials under investigation in the region of high-cycle fatigue varies within the range of 0,3 to 0,85