Combined Fatigue Tests Research Articles

Realization of Tensile-Bending Mechanical-Thermal Coupling Fatigue Based on a Uniaxial Tensile-Fatigue Testing Device

Lack of advanced testing technology and instrument restricted development and application of material subjected to multiaxial mechanical-thermal coupling fatigue load. This paper developed the mechanical-thermal coupling tensile-bending combined fatigue testing instrument by modifying the existing uniaxial tensile testing device and the shape of specimens. The thermal loading unit, thermal insulation unit, and temperature monitoring unit were integrated into the tensile testing device to obtain fatigue properties of different temperatures. A single v-notch used to adjust the ratio of bending component to tensile component was prepared on the symmetric specimen to establish the tensile-bending coupling stress state of the minimum cross-section of specimens. The v-notch specimens and conventional symmetrical specimens were used on the modified instrument to test at room temperature to 600 °C. The performance of the modified instrument was verified through the stability of load response, the consistency of fatigue fracture process and displacement response, and the softening trend of material with the increase of temperature. The tensile-bending stress state caused by v-notch was proved through finite element analysis and the stress analysis based on the crack growth morphology. A deep neural network model was established connecting the testing parameters and the tensile-bending combined stress state, and the feasibility of the model was verified by small enough training error and testing error.

Improving the durability of titanium alloy products through a combined hardening technology

Based on the experimental data, the combined method of surface hardening of titanium compressor blades is evaluated. Metallographic studies, structural characteristics, and the results of the combined fatigue test are presented. The studies found that the combined hardening technology increases rigidity, but at the same time retains its strength, reducing elastic – plastic deformation under the influence of applied stresses, and thereby reducing the propagation of cracks in the material. It has been established that the technology of hardening compressor blades by the method of combined hardening reduces the number of fracture sites, improves the quality of the surface layer and provides an increase in fatigue strength by 18–20% compared to ultrasonic hardening.

Fatigue-caused damage in trabecular bone from clinical, morphological and mechanical perspectives

Bone quantity and quality are considered the main predictors of bone mechanical properties (i.e., strength and fracture resistance). These factors deal with the morphology and chemical composition of bone and can be assessed by non-invasive techniques such as dual-energy x-ray absorptiometry (DXA), providing the bone mineral density (BMD) and the trabecular bone score (TBS). These parameters, and in particular BMD, are currently used as clinical predictors of fracture risk but do not provide information regarding the fatigue life. Bone is continuously subjected to fatigue loading and fatigue-induced damage can be crucial in fragility fractures. To probe the effect of fatigue-induced damage on bone microarchitecture and elucidate the effect of such damage on the bone clinical parameters, we combined fatigue testing on ex-vivo porcine trabecular bone samples with DXA measurements and μCT imaging. In addition, we performed interrupted cyclic tests at different load levels and measured fatigue-induced damage accumulation in the form of stiffness degradation. We also highlighted the change of clinical and microstructural parameters during the accumulation of fatigue-induced damage in interrupted fatigue tests. Our results suggest that the parameters obtained from the current non-invasive diagnostic protocols (i.e. μCT and DXA) are not able to assess the amount of fatigue-induced damage. This can be due to the fact that such techniques provide global parameters, whereas fatigue-induced damage is a local phenomenon, closely connected to the microarchitecture.

High-Temperature Combined Fatigue Tests on Full-Scale Turbine Blades

Compared with standard specimens, fatigue tests on full-scale turbine blades can take factors such as geometry and manufacturing process into consideration of life assessment. However, for combined fatigue tests of full-scale turbine blades, there exist two challenges. The first one is that it is difficult to apply combined loads of centrifugal force (low cycle fatigue, LCF) and vibration force (high cycle fatigue, HCF) properly because of the interaction between these loads. The second one is that it is hard to determine the range of HCF load/stress which the blade experiences at service conditions. To address these two challenges, firstly, a set of two-path fixture is designed to apply combined loads on the test blade, which can transfer LCF and HCF load separately by different paths. And secondly, two methods, i.e. the inverse method and the contrast method are proposed to estimate the HCF stress level for turbine blades at service conditions. The inverse method infers the HCF stress level by comparing blade failure data between field (in service) and bench tests conditions, while the contrast method obtains HCF stress level by comparing blade failure data between new and used blades under bench tests conditions. Detailed procedures of high temperature combined fatigue tests on full-scale blade are presented, and experimental life data is also included and analyzed.

Research on the Fatigue Performance of TC6 Compressor Blade under the CCF Effect

This paper studied the influence of high and low combined fatigue (CCF) on compressor blade fatigue performance. We investigated the coupling between low cycle fatigue (LCF) loading from centrifugal force with high cycle fatigue (HCF) loading from vibration and determined the blade disc vibration frequency using static analysis at maximum rotational speed. We designed and constructed a combined fatigue test rig, and CCF tests were performed on a TC6 compressor blade to analyze fatigue life characteristics. Results showed that CCF could significantly shorten blade life compared with pure LCF and that larger HCF caused more significant fatigue life reduction. Fatigue source characteristics and CCF fracture appearance were observed and analyzed using a scanning electron microscope (SEM).

The fatigue of impellers and blades

Abstract Impellers and blades are the key components of turbomachines, and fatigue is the main failure mode of impellers and blades. The status and progress of fatigue research on impellers and blades were systematically introduced through reviewing fatigue failures, numerical simulations of fatigue, and fatigue tests of impellers and blades in several typical turbomachines. High cycle fatigue caused by vibration was the main failure mechanism, and fatigue cracks usually initiated from the location of stress concentration. The resonance caused by the aerodynamic load was the main cause of fatigue failure of impellers and blades in a steady operating condition. The coupling of the meshless method and the finite element method and the combined fatigue test of actual impellers and blades are the developmental direction and research focus of the future.

Investigation on experimental load spectrum for high and low cycle combined fatigue test

As the outfield load spectrum is so complicated that it cannot be used directly for test study in laboratory. This paper presents a method to determine load spectrum for high and low cycle combined fatigue of turbine mortise at elevated temperature through experimental and numerical method. First of all, the low cycle load spectrum with duration time is determined through cumulative damage rule. The rain flow counting method is applied to obtain main cycles and sub cycles, and then the stress cycle is converted into pulsation cycle (stress ratio=0) based on the S-N curve and the Goodman curve; Secondly, three groups of different amplitudes tests are established to determine the high cycle amplitude. Finally, another three groups of the high-low combined cycle fatigue (HLCCF) tests for turbine mortise of a certain type engine are carried out. The results show that the macro and micro failure modes are identical with outfield's, which verifies the accuracy of the conversion method.

Fatigue Life under Random Loading Condition (5 th Report)

In the previous paper, a method for estimating fatigue strength under random loading condition was proposed, a method that combined fatigue tests with calculations. The loading condition is limited to the case where the mean stress level, or the stress ratio of the minimum stress to the maximum stress, is constant. However, in the actual service loading condition, both mean stress and stress amplitude generally vary from cycle to cycle. Therefore, a random fatigue test with a constant mean stress or stress ratio is not enough to clarify fatigue behavior under actual service loading conditions.In this paper, a random test procedure similar to the method shown in the previous paper is proposed. However, the new-procedure both mean stress and stress ratio are able to change according to the service loading condition.To check the applicability and accuracy of this new procedure, axial load controlled random loading tests, with an exponential distribution of the peak to peak-stress, were carried out for plane mild steel specimens. Fatigue behavior patterns are then examined.