Application of the covariance on the critical plane for determination of fatigue life under cyclic loading

AbstractThe paper presents application of the covariance extremum between normal end shear stresses for determination of the critical plane. The fatigue stresses criterion was formulated as a linear combination of normal and shear stresses on the defined critical plane. The weight coefficients occurring in this criterion were determined from fatigue tests in the layer of pure bending and pure torsion. The proposed model was verified while fatigue tests under cyclic proportional and non-proportional bending with torsion (specimens made of aluminium alloy PA6-T4 were tested)

Comparison of fatigue characteristics for some selected structural materials under bending and torsion

The paper presents a review of the multiaxial fatigue criteria including a ratio of normal and shear stresses. The paper also contains fatigue characteristics for bending and torsion of some selected constructional materials. The ratio of normal stresses to shear stresses was determined for the defined number of cycles Nf in the range 5⋅10⁴÷2⋅10⁶. Moreover, from the performed analysis of fatigue equations and the relative difference R it appears that materials can be divided into groups for which it is possible or it is not possible to apply the constant value of the considered ratio in the criteria including this ratio.Подано огляд критеріїв багатовісної втоми, які охоплюють співвідношення нормальних та зсувних напружень. Розглянуто втомні характеристики деяких вибраних конструкційних матеріалів за згину та закруту. Визначено співвідношення нормальних до зсувних напружень для певної кількості циклів Nf у діапазоні 5⋅10⁴÷2⋅10⁶. Із результатів аналізу втомних характеристик та відносної зміни R виявлено, що матеріали можна поділити на дві групи, для яких доцільно чи недоцільно застосувати стале значення розглянутого в критерії співвідношення.Дан обзор критериев многоосной усталости, которые включают соотношения нормальных и напряжений сдвига. Рассмотрены усталостные характеристики некоторых избранных конструкционных материалов при изгибе и кручении. Определено соотношение нормальных и напряжений сдвига для определенного числа циклов Nf в диапазоне 5⋅10⁴÷2⋅10⁶. Из результатов анализа усталостных характеристик и относительной разницы R установлено, что материалы можно разделить на две группы, для которых возможно или невозможно применить постоянное значение рассмотренного в критерии соотношения

Determination of stress and strain concentrations in the elastic-plastic materials under bending and torsion

The analysis of stress and strain concentrations of constructional materials subjected to bending and torsion is presented. The known methods of stress determination for any elastic-plastic material assume the linear strain concentration under bending and torsion. Determination of stress and strain concentrations and their presentation as graphs were the main aims of this paper. Five materials were tested. They had different cyclic strain curves, and were loaded by the torsional and bending moments. The results proved a linear character of the strain distribution for any material in the all applied loading range.Визначено розклад напружень і деформацій в перетині проаналізованого елемента вальців та подано графічно за допомогою комп’ютерної моделі. Аналіз п’яти матеріалів та їх кривих циклічного деформування з різними характеристиками за умов кручення та згину показав лінійний характер розкладу деформацій для кожного з них за всіх прикладених навантажень.Определено распределение напряжений и деформаций в сечении проанализированного элемента валков и представлено графически с помощью компьютерной модели. Анализ пяти материалов и их кривых циклического деформирования с разными характеристиками при кручении и изгибе показал линейный характер распределения деформаций для каждого из них при всех приложенных нагрузках

The use of spectral method for fatigue life assessment for non-gaussian random loads

Abstract The well-known problem with the fatigue lifetime assessment of non-Gaussian loading signals with the use of spectral method has been presented in the paper. A correction factors that transform the non-Gaussian signal into an equivalent Gaussian signal proposed by Bracessi et al. (2009) has been used for the purpose of lifetime calculations together with Palmgren-Miner Hypothesis. The calculations have been performed for the 10HNAP steel under random non-Gaussian load with four dominating frequencies. The signal has been generated on the test stand SHM250 for random tension-compression tests. The results with zero and non-zero mean stresses have been used to calculate the fatigue life with the frequency domain method based on Dirlik's model and with a time domain method with the use of the rainflow cycle counting algorithm. The obtained calculation results have been compared with experimental results

On creep-fatigue endurance of TIG-dressed weldments using the linear matching method

This paper is devoted to parametric study on creep-fatigue endurance of the steel type 316N(L) weldments at 550◦C identified as type 3 according to R5 Vol. 2/3 procedure classification. The study is implemented using a direct method known as the Linear Matching Method (LMM) and based upon the creep-fatigue evaluation procedure considering time fraction rule for creep-damage assessment. Seven configurations of the weldment, characterised by particular values of a geometrical parameter ρ, are proposed. Parameter ρ, which represents different grades of TIG dressing, is a ratio between the radius of the fillet of the remelted metal on a weld toe and the thickness of welded plates. For each configuration, the total number of cycles to failure N⋆ in creep-fatigue conditions is assessed numerically for different loading cases defined by normalised bending moment ˜M and dwell period t. The obtained set of N⋆ is extrapolated by the analytic function dependent on ˜M, t and parameter ρ. Proposed function for N⋆ shows good agreement with numerical results obtained by the LMM. Therefore, it is used for the identification of Fatigue Strength Reduction Factors (FSRFs) effected by creep, which are intended for design purposes, and dependent on t and ρ

Characteristics of CuCrTiAl alloy after plastic deformation

The development of the automotive industry leads to the search for newer construction materials, but also those used in the production process. Although many types of joints are used in vehicles, resistance welds remain the dominant one. The commonly used alloy for welding caps (CuCrZr) is already well known and therefore newer alloys and technologies for their production are sought to increase service life, and thus reduce production costs. Precipitation hardening CuCrTiAl alloy was analysed in this article. After casting, CuCrTiAl alloy was subjected to cold and hot deformation and then tested for its usability. The obtained results confirmed the potential of this alloy to make electrodes for resistance welding

Influence of the speed of downward semi-continuous casting on the crystal size and mechanical properties of recycled copper

In this study, the influence of the speed of downward semi-continuous casting on the crystal size and mechanical properties of recycled copper after casting and cold deformation was investigated. It was shown that the increase in the casting speed from 0,09 m / min to 0,22 m / min resulted in refinement of the macrostructure and reduction of the crystal cross-sectional area from 3,221 mm2 to 1,2 mm2, which resulted in an increase in microhardness after casting by 8 %, while in the state after cold working, an increase in microhardness by 3 % and ultimate tensile strength by 2,5 %

Influence of the speed of downward semi-continuous casting on the crystal size and mechanical properties of recycled copper

In this study, the influence of the speed of downward semi-continuous casting on the crystal size and mechanical properties of recycled copper after casting and cold deformation was investigated. It was shown that the increase in the casting speed from 0,09 m / min to 0,22 m / min resulted in refinement of the macrostructure and reduction of the crystal cross-sectional area from 3,221 mm2 to 1,2 mm2, which resulted in an increase in microhardness after casting by 8 %, while in the state after cold working, an increase in microhardness by 3 % and ultimate tensile strength by 2,5 %