Fatigue lifetime of ADI from ultimate tensile strength to permanent fatigue limit

S — N curve of austempered ductile iron was obtained in the range of lifetime including low cycle fatigue domain and high cycle fatigue domain up to 108 cycles. Ultimate tensile strength is used as a limiting value of the curve. Symmetric push-pull fatigue and tensile tests were performed at room temperature on isothermally treated nodular cast iron alloyed with copper and nickel having positive impact on mechanical, technological and fatigue properties of austempered ductile iron. Suitable functions for the fit of experimentally determined points were tested and their parameters were calculated. The best results were obtained using the Palmgren function and the function introduced by Kohout and Vechet. Since the loading frequency in high-cycle region is two orders higher than in low-cycle region, the effect of loading cycle frequency on fatigue behavior of the studied material is also studied. A possibility of discontinuity of experimental data between low-cycle and high-cycle regions is discussed.Получена кривая усталости аустенитно-отпущенного ковкого чугуна (ADI) в диапазоне долговечности, включая области малоцикловой и многоцикловой усталости до 108 циклов. В качестве предельного значения указанной кривой используется предел прочности при растяжении. Проведены испытания на усталость при симметричном растяжении-сжатии и на растяжение при комнатной температуре изотермически термообработанного чугуна с шаровидным графитом, с добавками меди и никеля, которые оказывают положительное влияние на механические, технологические и усталостные свойства ADI. Осуществлена проверка соответствующих функций для описания экспериментально определенных точек; выполнен расчет параметров указанных функций. Наилучшие результаты получены при использовании функции Пальмгрена и функции, предложенной Когутом и Вечетом. Исследовано также влияние частоты циклов нагружения на усталостное поведение рассматриваемого материала с учетом того, что частота нагружения в многоцикловой области на два порядка превышает частоту нагружения в малоцикловой области. Рассмотрена также возможность отсутствия непрерывности в экспериментальных данных на участке между малоцикловой и многоцикловой областями

Reakcje genotypow pszenicy ozimej na porazenie przez Mycosphaerella graminicola

Reaction of six winter wheat culti vars and lines (Vlasta, Šárka, Charger, 00ST022, SG-U8044C a SG-U2113B) to Mycosphaerella graminicola isolate BR-331 and UH-05 on leaf segments of the detached second seedling leaf of cultivars placed on water agar with bezimidazole in clear plastic box were tested. The isolate BR-331 produced high occurrence of the disease in the cultivar Šárka, middle occurrence (the percentage covered by lesions bearing pycnidia) in the cultivar Vlasta and SG-U8044C and low occurrence in the line 00ST0022. The isolate UH-105 produced high occurrence in the cultivar 00ST022 and middle occurrence in the cultivar Vlasta. The cultivar Charger was resistant. The cultivar Šárka was attacked at least. Results show on different virulence of M. graminicola isolates to wheat cultivars.Badano reakcje 6. odmian i linii pszenicy ozimej (Vlasta, Šárka, Charger, 00ST022, SG-US8044C, i SG-U2113B) na porażenie przez izolaty RB - 331 i UH - 105 grzyba Mycosphaerella graminicola. Doświadczenia przeprowadzono na fragmentach drugich liści siewek pszenicy umieszczonych w czystych, plastikowych pudelkach z wodnym agarem zawierającym benzimidazol. Stopień porażenia izolatem BR - 331 był najwyższy na liściach odmiany Šárka, średni na odmianie Vlasta i linii SG - U8044C oraz niski na roślinach linii 00ST022. W przypadku izolatu UH -105 najsilniejsze objawy odnotowano na liściach linii OOST022, średnie porażenie stwierdzono na roślinach Vlasta. Na liściach odmiany Charger brak było objawów choroby. Wyniki doświadczenia wskazują na zróżnicowaną wirulencję izolatów Mycosphaerella graminicola względem testowanych genotypów pszenicy

The Relation between Microstructure Composition and Fatigue Properties of ADI

The presented work deals with a high-strength variant of nodular cast iron, so-called ADI (austempered ductile iron). This material ranks among prospective structural materials and is more and more applied in all important branches of machine industry. Recently it has even been applied to castings for cyclically loaded components, i.e. gears and traversing wheels, crankshafts of motor-cars, vans and trucks, swivel pins, rail brakes, pressure pipes in oil industry etc. The microstructure composition of an ADI matrix consists especially of bainitic ferrite and stabilized austenite. However, in dependence on transformation dwell some amount of martensite can appear as well.In this work the relation between mechanical properties and microstructure composition of the matrix mixture was studied in detail for the case of ADI transformed at 380 °C, with emphasis on fatigue properties

Comparison of a New Equation Describing Fatigue Crack Growth Curves with the Nasgro Equation

The so-called NASGRO equation allows a very good description of fatigue crack growth curves, i.e. the dependence of crack length increase per one fatigue cycle da/dN on the stress intensity factor range K. In 1999 the first author of this paper published quite similar equation, which written for given loading cycle asymmetry with positive stress ratio R contains the same parameters having similar meaning as in the NASGRO equation. In most cases studied by the authors this equation leads to a better fit than the NASGRO equation, above all when the experimental curve contains relatively long Paris straight line and/or relatively sharp bend from this line to the threshold stress intensity factor range. The generalization of the NASGRO equation for various values of stress ratio R was made in a quite complicated way. The new equation was generalized using the Walker model based on the relation K(R) K(0)(1 R)m which is valid also for threshold value Kth but not for critical stress intensity Kc being a constant independent of R. Then the shift of the growth curves with the change of R ratio is described only by one parameter m (0 m 1) both for positive and for negative values of R. It means that the crack closure models are very important for explanation and deep study of fatigue crack growth mainly in negative R region but play no crucial role in a phenomenological description of the growth in the case of short-term tests in non-aggressive media

Fatigue Lifetime of 7075 Aluminium Alloy from Ultimate Tensile Strength to Permanent Fatigue Limit

The aim of this paper is to study fatigue behaviour of 7075 aluminium alloy at room temperature for both important directions (longitudinal as well as transversal) with respect to the direction of forming. Smooth test bars were subjected to symmetrical push-pull loading at stresses which cover the whole range of lifetime from UTS to permanent fatigue limit. The results are presented above all as S-N curves fitted using suitable regression functions which lead to precise and reliable determination of fatigue limits

Fatigue Behaviour of Nodular Cast Iron at Various Loading Cycle Asymmetry

Fatigue behaviour of nodular cast irons is described in specialized literature quite well but the majority of fatigue tests in high cycle region has been made at symmetrical bending or at symmetrical tension-compression loading. The role of loading cycle asymmetry is very important because the fatigue limit of nodular cast irons is substantially influenced by mean static stress. Unfortunately, the number of papers describing the influence of mean static stress is quite low. The aim of the presented paper consists in presenting the results of long-term research of nodular cast irons at high cycle region at different loading cycle asymmetry. Several heats with various heat treatments leading to various structures of matrix were used for fatigue tests performed mainly at symmetrical tension-compression loading and at repeating tensile loading. From S-N curves the fatigue limits for 107 cycles were determined and then used for the calculation and construction of the Haigh and the Smith diagrams. Nodular cast irons seem to be more sensitive to mean static stress than structural steels. It was found that, in contrast to many papers, the dependence of upper stress on mean stress is not linear but parabolic with the exponent lower than 1 whose value is linear function of UTS