Local fatigue strength assessment based on threshold values obtained by linear-elastic notch stress calculation is commonly utilized due to its applicability to complex geometries with an acceptable effort. As the result of a finite element computation is affected by the element type and mesh refinement, this paper investigates the numerical influence on the notch stress based fatigue strength assessment of non-welded and welded components. Based on extensive finite element studies employing the software package Abaqus it is concluded that quadratic shape functions with a number of sixteen for non-welded parts and twelve elements over a semicircle for welded joints should be at least applied to minimize the numerical impact. (C) 2017 Elsevier Ltd. All rights reserved.

In the present study, the out-of-phase thermomechanical fatigue (OP-TMF) behavior of a cast aluminum-silicon-magnesium alloy, the A356.0 alloy which has been widely used in diesel engine cylinder heads, is compared to room-temperature and high-temperature low cycle fatigue (RT-, HT-LCF) behaviors. For this purpose, strain/temperature-controlled isothermal and non-isothermal fatigue tests were performed based on realistic loading conditions in cylinder heads. Fatigue tests results showed that the plastic strain increased during cycles under constant mechanical strain amplitude, while the specimen failed. Under LCF loadings, the cyclic hardening occurred at low temperatures for the A356.0 alloy and the cyclic softening occurred at high temperatures, as it was also observed in TMF tests. The radial and longitudinal temperature gradients during TMF tests were almost 2 and 3°C, respectively. Comparing stress-strain hysteresis loops, tensile stresses at minimum temperatures under TMF loadings were more than tensile stresses at LCF cases, due to the out-of-phase loading condition in TMF tests. In this state, maximum temperatures occurred within compressive regimes and minimum temperatures occurred at tensile loads. In general, TMF lifetimes were less than LCF ones due to severe conditions and the temperature deviation in TMF tests.

Both geometrical optimization and time efficient design of high performance structural components require improved computational lifetime calculation methods. It is important to optimise the transferability from specimen results to real components. For the fatigue behaviour prediction of real components,it is necessary to evaluate these results from different life time calculation methods. The methods for the lifetime calculation of components are based and parameterised on the results obtained from the pure specimen tests. In this work a closed loop for the lifetime evaluation of a component is presented. The cyclic material behaviour of Ti-6Al-4 V alloy ,one of the most widely used titanium materials for space technology due to its excellent properties was determined. The influences such as microstructure, stress-ratio, etc. , were investigated in separate test series. With material data from cyclic tests,on one hand a Finite-Element-simulation can be carried out and on the other hand the lifetime calculation can be obtained. To quantify the transferability of the result from standard specimen tests to real components, the simplified component "W-Link" was developed. This was designed based on a real aircraft component and represents the load and boundary condition as in a real system. The lifetime calculation method can be achieved based on numerical results from Finite-Elementanalysis. Fatigue test was performed on the component as well. Thereby, a comparison between experimental and simulation life time was realised. This revealed that the results can be affected by further influencing factors. Microscopic investigation lead to an empirical factor which allows consideration of one of the most dominant factors i. e. , fretting. For evaluating different methods, already available lifetime calculation results with the software program FEMFA T and, lifetime evaluation based on material properties defined by the Manson-Coffin-Basquin law, focussing on strain-controlled parameters, were compared with the results from the present method.

In this article, out-of-phase thermo-mechanical fatigue (TMF) behaviours of light alloys were investigated in comparison to their high temperature low cycle fatigue (LCF) behaviours. For this objective, strain based fatigue tests were performed on the A356 aluminium alloy and on the AZ91 magnesium alloy. Besides, TMF tests were carried out, where both strain and temperature changed. The fatigue lifetime comparison demonstrated that the TMF lifetime was less than that one under LCF loadings at elevated temperatures for both light alloys. The reason was due to severe conditions in TMF tests in comparison to LCF tests. The temperature varied in TMF test but it was constant under LCF loadings. As the other reason, the tensile mean stress occurred under TMF loadings, in comparison to the compressive mean stress under LCF loadings. At high temperatures, the cyclic hardening behaviour occurred in the AZ91 alloy and the A356 alloy had the cyclic softening behaviour.

Die europäischen Initiativen, die sich in den Mitteilungen der Europäischen Kommission für Rohstoffe, in den Strategischen Implementationsplänen der Europäischen Innovationspartnerschaft für Rohstoffe (EIP) und in der Auslobung des Europäischen Instituts für Innovation & Technologie (EIT) für eine Wissens- und Innovationsgemeinschaft für Rohstoffe manifestieren, führen zu vielfältigen österreichischen Maßnahmen auf Regierungsebene und auch auf Ebene der Montanuniversität Leoben. Die ESEE-Region, also Ost- und Südosteuropa, hat dabei eine besondere Rolle.

Rotating bending tests have been, up to now, typically used for the determination of S/N curves. In this paper, a method is proposed that allows the determination of the fatigue limit curve and the crack growth curve by means of such rotating bending tests. For this purpose, flaws of controlled shape and size are introduced on the surface of conventional round bar specimens. The rotating bending test rig is equipped with additional hardware for optical crack length measurement. The stress at incipient crack growth gives a point in the fatigue limit (Kitagawa-Takahashi) diagram. After the crack starts to grow, photomicrographs are taken in regular intervals in order to obtain the crack growth curve. – The fatigue limit and crack growth curves obtained from the rotating bending tests are compared with results from conventional single edge notch bending specimens. Agreement is good, with the results from the rotating bending experiments lying somewhat on the conservative side. Some factors which may contribute to these differences are the different type of loading and the simplified calculation of the stress intensity factor in the rotating bending tests.

In many assemblies of moving components, contact problems under various lubrication conditions are lifetime-limiting. There, relative motion of contacting bodies, combined with high loads transmitted via the contact surface lead to fretting fatigue failure. For a reliable prediction of in service performance load type, different damage and failure mechanisms that may be activated during operation have to be known.

It is well known that the fatigue strength of welded structures is in general independent from the material strength. In case of high-strength steels, however, a significant improvement in the fatigue behaviour can be realised through post-treatment processes. This paper deals with the effect of high-frequency mechanical impact (HFMI) on the fatigue behaviour of a range of steels starting from mild construction steel (S355) to ultra high-strength steel (S960). The experiments involve fatigue tests at a stress ratio of R = 0.1 on butt welds, T-joints, and longitudinal attachments on 5 mm, thin-walled specimens. The fatigue assessment was performed in accordance to the nominal and the notch stress approach taking the HFMI condition into account. Finally, a novel method is outlined to evaluate the notch stress fatigue behaviour of HFMI-treated joints made of high-strength steel. Applicability of this new HFMI notch stress approach is shown through fatigue assessment of about 330 HFMI post-treated specimens taken from both literature and own test results. Further work focuses on the expansion of the introduced HFMI notch stress model for load spectra influence covering overloads and multiaxial fatigue.

ABSTRACTIn this paper, a new fatigue lifetime prediction model is presented for the aluminium–silicon–magnesium alloy, A356.0. This model is based on the plastic strain energy density per cycle including two correction factors in order to consider the effect of the mean stress and the maximum temperature. The thermal term considers creep and oxidation damages in A356.0 alloy. To calibrate the model, isothermal fatigue and out‐of‐phase thermo‐mechanical fatigue (TMF) tests were conducted on the A356.0 alloy. Results showed an improvement in predicting fatigue lifetimes by the present model in comparison with classical theories and also the plastic strain energy density (without any correction factors). Therefore, this model is applicable for TMF, low cycle fatigue (LCF) and both TMF/LCF lifetimes of the A356.0 alloy. Furthermore, this model can be easily used for the estimation of thermo‐mechanical conditions in components such as cylinder heads.

Two different principles of TMF-testing were investigated for the wrought aluminium alloy AlCuBiPb (2011). On the one hand the specimens are clamped in a stiff load frame in Out-ofPhase (OP) TMF loading. The local strain is measured within the parallel cross section of the specimen. On the other hand OP-TMF tests are conducted using strain control on a servohydraulic TMF testing system, which guarantees a rigid restraint condition within the parallel section of the specimen. Because creep effects have to be considered in thermo-mechanical loaded components to take into account stress relaxation phenomena and creep damage, creep tests were carried out for the wrought aluminium alloy between 150°C and 300°C for a stress range from 90 to 250MPa. Multiple step creep tests were conducted to find an appropriate parameter for modeling the creep behavior under variable amplitude loading. In addition the microstructure and fracture surfaces were investigated in more detail using optical and scanning electron microscopy.

This paper presents the fatigue lifetime of an aluminum–silicon–magnesium alloy, widely used in diesel engine cylinder heads, both with and without a thermal barrier coating (TBC) system. The coating system in this study consists of two layers including a 150 μm thick metallic bond coat and a zirconium oxide top coat 350 μm thick. These coating layers were applied on the substrate of A356.0 alloy by air plasma thermal spraying. The isothermal fatigue tests were conducted in low cycle fatigue (LCF) regime at various temperatures. Out-of-phase thermo-mechanical fatigue (OP-TMF) tests were also performed at different maximum temperatures and constraint factors. Experimental results demonstrate that at high temperature the coating increases the LCF lifetime of A356.0 alloy at higher strain amplitude and decreases it at lower strain amplitude. At LCF situation, thicker coating has more detrimental effect. However, the coating increases significantly the OP-TMF lifetime. To analyze the failure mechanisms, the fracture surfaces of specimens were investigated by scanning electron microscopy. The fractography of surfaces showed that the failure mechanism of TBC system in coated A356.0 alloy was separations of coating layers in the interface of the substrate and the bond coat layer. This was observed on both high temperature LCF and OP-TMF conditions.