Residual stresses in the dangerous section of toothed wheels made of materials and according to practices accepted in aviation engine construction have been determined using a mechanical method. The influence of the technology of manufacturing of toothed wheels, thermo-chemical treatment, hardening and coating on the residual stresses has been examined. Current technologies of manufacturing toothed wheels lead to significant dispersion of residual stresses, especially after thermo-chemical treatment. Therefore, the results have been statistically processed for each batch of parts and the average values are shown on the residual stress diagrams. We demonstrate the possibility of determining residual stress distribution in the dangerous section of a wheel tooth by the initial deformations of the reference specimen, which makes it possible to keep the part under examination unimpaired. A plate made of the same material as the toothed wheel and hardened simultaneously with the part under examination was used as a reference specimen. The influence of residual stresses on the fatigue resistance of toothed wheels by the average integral residual stresses criterion that takes into account both the value of residual stresses and the nature of their distribution in the dangerous section of toothed wheels was examined.
Самарский государственный аэрокосмический университет имени академика С.П. Королёва (национальный исследовательский университет)Изучена возможность использования образцов-свидетелей, обрабатываемых одновременно с дета-лями, для оценки влияния гидродробеструйной обработки на предел выносливости деталей с концентра-торами напряжений на примере цилиндрических образцов с отверстием из стали 40Х с круговым надре-зом полукруглого профиля радиуса 0,3 мм. Исследование показало, что расхождение между расчётными и экспериментальными значениями остаточных напряжений в гладких образцах не превышает 8%, в об-разцах с надрезом -6%. При оценке влияния гидродробеструйной обработки на предел выносливости при изгибе в случае симметричного цикла полых цилиндрических образцов из стали 40Х с надрезом ра-диуса 0,3 мм установлено, что использование рассчитанного по первоначальным деформациям образца-свидетеля распределения остаточных напряжений позволяет прогнозировать приращение предела вы-носливости образцов в условиях концентрации напряжений с точностью до 8%. Показано, что при про-гнозировании приращения предела выносливости упрочнённых гидродробеструйной обработкой образ-цов из стали 40Х с концентраторами напряжений целесообразно использовать критерий среднеинте-гральных остаточных напряжений, вычисленный по толщине поверхностного слоя опасного сечения, равной критической глубине нераспространяющейся трещины усталости.Поверхностное упрочнение, образец-свидетель, остаточные напряжения, предел выносливости.
It is shown that, growth of the layer with compressive residual stresses at advanced surface plastic deformation of smooth samples leads to an increase in the endurance limit of the notched specimens due to an increase in the compressive residual stresses in their dangerous section. The study was conducted on the specimens with outer diameterDequal to 10 and 25 mm and on a 25 mm specimen with inner diameterd= 15 mm which were subjected to air shot blasting and roll-in. Residual stresses in smooth specimens were determined by the mechanical method. Circular incisions (R= 0.3 and 0.5 mm) were applied on hardened smooth samples with a shaped cutter. Residual stresses in notched specimens were determined by a numerical method calculating the redistribution of the residual stresses upon the incision. As the diameter of the samples increases with the same strengthening surface treatment,the depth of the layer with the compressive residual stresses increases due to an increase in the rigidity of the samples. The effect of surface hardening on the endurance limit of notched specimens was evaluated according to the criterion of the average integral residual stresses, which takes into account both the magnitude and completeness of the residual stresses. This criterion was calculated from the depth of the non-propagating fatigue crack that occurs in the surface layer in the dangerous section of the specimen during the fatigue test at the limit endurance. Fatigue tests are carried out in symmetrical cycle, the test base being 3 × 106cycles of loading. Estimation of the increment of the limit endurance of surface-hardened cylindrical specimens with semicircular notches gave the results acceptable for practice. The use of the proposed method can significantly reduce long-term and expensive fatigue tests.
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