Complex Cyclic Loading Research Articles

Complex High‐Cyclic Loading in an Accumulation Model for Sand

ABSTRACTExperimental evidence indicates that multidimensional cyclic loading of soils causes larger accumulation of deformations than equivalent one‐dimensional loading. The response of sand to high‐cyclic loading with 10,000 cycles and up to four‐dimensional stress paths (i.e., four independent oscillating components) is examined in 120 triaxial and hollow cylinder tests in this work to extend these findings. With increasing number of oscillating stress components, the accumulation of permanent strains tends to increase. It is demonstrated that the definition of the multidimensional strain amplitude incorporated in the high‐cycle accumulation (HCA) model can account for this. The validation of the HCA model for complex cyclic loading is complemented by the simulation of model tests on monopile foundations of offshore wind turbines subjected to multidirectional cyclic loading, for which the consideration of spatially variable cyclic loading with nonconstant load amplitudes in the HCA model is discussed. For this purpose, an extension of the HCA model considering multiple strain amplitudes is presented.

Threshold strain for the accumulation of excess pore-water pressure in saturated coral sands under complex cyclic loading patterns

The liquefaction of coral sands caused by the accumulation of excess pore-water pressure is a major factor contributing to catastrophic events on coral reefs, and accurately estimating this excess pore-water pressure accumulation holds significant importance. High-quality laboratory test results are essential for analytical or numerical calculations. In this study, a new test method is employed to conduct a series of undrained, multi-staged, stress-controlled multidirectional hollow cylinder tests on saturated coral sand under complex loading conditions. The concept of threshold strain (γt) and the method for determining γt of saturated coral sand specimens under complex loading conditions are proposed. The test results demonstrate that γt of saturated coral sand remains insensitive to cyclic loading conditions (including frequency, stress path, and mode) but increases with increasing relative density. The range of volumetric threshold strain, degradation threshold strain, and flow threshold for saturated coral sand under different initial states and cyclic loading conditions are 0.0183%–0.0341%, 0.0242%–0.0454%, and 1.006%–1.614%, respectively. This research provides a novel approach for accurately determining input parameters required for resolving and implementing coupled models in numerical modeling.

Addressing the inherent stress non-uniformity in dynamic hollow cylinder testing

The Dynamic Hollow Cylinder Apparatus (DHCA) is renowned for its ability to subject soil samples to various cyclic stress paths, allowing for the investigation of the dynamic behavior of soils under complex cyclic loading conditions. This note explores the errors arising from the stress non-uniformity along the height of DHCA samples and examines their impact on the measured soil dynamic properties. After discussing the two globally used DHCA types, this study presents the stiffness and damping of sand derived from a set of Dynamic Hollow Cylinder experiments covering a wide range of dynamic stress paths and shear strain amplitudes. It is highlighted that the deviation of the results from established degradation models is primarily attributed to the errors associated with the stress non-uniformity, leading to up to a tenfold underestimation within the medium strain range. A simple correction to the shear stress amplitude calculation is proposed to minimize the impact of stress non-uniformity and improve the accuracy of the test results.

A refined deviatoric hardening plastic model for sand

The classical deviatoric hardening models are capable of characterizing the mechanical response of granular materials for a broad range of degrees of compaction. This work finds that it has limitations in accurately predicting the volumetric deformation characteristics under a wide range of confining/consolidation pressures. The issue stems from the pressure independent hardening law in the classical deviatoric hardening model. To overcome this problem, we propose a refined deviatoric hardening model in which a pressure-dependent hardening law is developed based on experimental observations. Comparisons between numerical results and laboratory triaxial tests indicate that the improved model succeeds in capturing the volumetric deformation behavior under various confining/consolidation pressure conditions for both dense and loose sands. Furthermore, to examine the importance of the improved deviatoric hardening model, it is combined with the bounding surface plasticity theory to investigate the mechanical response of loose sand under complex cyclic loadings and different initial consolidation pressures. It is proved that the proposed pressure-dependent deviatoric hardening law is capable of predicting the volumetric deformation characteristics to a satisfactory degree and plays an important role in the simulation of complex deformations for granular geomaterials.

Clay hypoplasticity coupled with small-strain approaches for complex cyclic loading

Constitutive models that are able to accurately predict cyclic soil behaviour are crucial for finite element design of offshore foundation or railway embankments. Basic hypoplastic models introduce the history of loading in state variables such as the stress and void ratio and are therefore incapable of describing small-strain stiffness and cyclic loading. In this work, clay hypoplasticity is extended with a modified intergranular strain proposed by Duque et al. [3]. The new model is compared to the one coupled previously with ISA based on unconventional as well as complex cyclic loading paths. Abilities and limitations of the models are addressed: (i) showing that both models predict a reduction in strain accumulation with an increasing number of cycles. (ii) For both models pronounced over- and undershooting effects can occur for certain cyclic loading paths and certain parameters. Despite the consensus in the literature, the results show that a yield surface in the (intergranular) strain space is not sufficient to ban these effects. Furthermore, the models’ predictive capabilities are verified with simulations of monotonic and cyclic tests of Lower Rhine clay.

Research on a Method for Online Damage Evaluation of Turbine Blades in a Gas Turbine Based on Operating Conditions

Performing online damage evaluation of blades subjected to complex cyclic loads based on the operating state of a gas turbine enables real-time reflection of a blade’s damage condition. This, in turn, facilitates the achievement of predictive maintenance objectives, enhancing the economic and operational stability of gas turbine operations. This study establishes a hybrid model for online damage evaluation of gas turbine blades based on their operational state. The model comprises a gas turbine performance model based on thermodynamic simulation, a component load calculation model based on a surrogate model, an updated cycle counting method based on four-point rainflow, and an improved damage mechanism evaluation model. In the new model, the use of a surrogate model for the estimation of blade loading information based on gas turbine operating parameters replaces the conventional physical modeling methods. This substitution enhances the accuracy of blade loading calculations while ensuring real-time performance. Additionally, the new model introduces an updated cycle counting method based on four-point rainflow and an improved damage mechanism evaluation model. In the temperature counting part, a characteristic stress that represents the stress information during the cyclic process is proposed. This inclusion allows for the consideration of the impact of stress fluctuations on creep damage, thereby enhancing the accuracy of the fatigue damage assessment. In the stress counting part, the model incorporates time information associated with each cycle. This concept is subsequently applied in determining the identified cyclic strain information, thereby improving the accuracy of the fatigue damage evaluation. Finally, this study applies the new model to an online damage evaluation of a turbine stationary blade using actual operating data from a micro gas turbine. The results obtained from the new model are compared with the EOH recommended by the OEM, validating the accuracy and applicability of the new model.

An Online Fatigue Damage Evaluation Method for Gas Turbine Hot Components

The failure of gas turbines’ hot components due to fatigue significantly affects their efficient and stable operation. Conducting online damage assessment of components subjected to complex cyclic loads based on the working conditions of gas turbines can provide real-time reflection of component fatigue damage and achieve the purpose of predictive maintenance. In this study, we propose an online cycle counting method that considers temperature fluctuations during the cycle process. Our method is based on the four-point online rainflow counting method by coupling the counting variable with time, introducing the concept of the duration time for full cycles and half cycles, and incorporating a characteristic temperature that better represents the temperature information during the cycle process. With reference to the characteristic temperature, our proposed method comprehensively considers the form and parameters of subsequent life assessment models. This paper provides a detailed explanation of the proposed method and applies it to the fatigue damage assessment of turbine vanes in a micro gas turbine, thereby verifying its accuracy and applicability.

Modeling and Simulation of the Hysteretic Behavior of Concrete under Cyclic Tension-Compression Using the Smeared Crack Approach.

Concrete structures under wind and earthquake loads will experience tensile and compressive stress reversals. It is very important to accurately reproduce the hysteretic behavior and energy dissipation of concrete materials under cyclic tension-compression for the safety evaluation of concrete structures. A hysteretic model for concrete under cyclic tension-compression is proposed in the framework of smeared crack theory. Based on the crack surface opening-closing mechanism, the relationship between crack surface stress and cracking strain is constructed in a local coordinate system. Linear loading-unloading paths are used and the partial unloading-reloading condition is considered. The hysteretic curves in the model are controlled by two parameters: the initial closing stress and the complete closing stress, which can be determined by the test results. Comparison with several experimental results shows that the model is capable of simulating the cracking process and hysteretic behavior of concrete. In addition, the model is proven to be able to reproduce the damage evolution, energy dissipation, and stiffness recovery caused by crack closure during the cyclic tension-compression. The proposed model can be applied to the nonlinear analysis of real concrete structures under complex cyclic loads.

Plasticity analysis and constitutive model of salt rock under different loading speeds

Salt rock is recognized as an ideal source for oil, natural gas and compressed air due to its good deformation and low permeability. The injection–production frequency/rate under different working conditions greatly influences various mechanical behaviours, such as the deformation and convergence of the reservoir. Therefore, it is crucial to analyse the rate correlation of the mechanical characteristics of salt rock and to establish the corresponding constitutive model; this model accurately evaluates reservoir safety and reasonably designs reservoir operating pressure. According to the experimental deformation characteristics of salt rock under different loading and unloading rates, the time correlation, stage characteristics and evolution law of salt rock deformation were analysed, and a viscoplastic constitutive model of salt rock in the whole process was constructed based on loading plasticity and rate-related plasticity. The prediction effect of the model was verified by graded creep tests and different rate cyclic loading and unloading tests, revealing the good adaptability of the model for salt rock under complex loading paths. The results showed that the plastic (unrecoverable) deformation of salt rock could be divided into loading plasticity caused by stress state change and creep plasticity generated over a long period by stress accumulation. The creep deformation characteristics of salt rock could be better described using hardened damping elements, classical Norton elements and fracture damage variables; the relationship between loading plasticity and stress path changes was linear. The model had a high accuracy in predicting the deformation of salt rock under complex cyclic loading paths. For radial strain and volume strain, the accelerating point that protended the rapid growth of microcracks was used to distinguish the accelerated creep stage from the steady state creep stage. The number of events with an average frequency in the range of 0–50 kHz after the accelerating point suddenly increased significantly, which could be regarded as a precursor to the primary fracture of the rock.

Thermomechanical fatigue of additively manufactured 316L stainless steel

An important issue in energy conversion is the performance of materials under complex cyclic loading in a variable temperature field. The present study addresses a new field of research – thermomechanical fatigue of additively manufactured metallic materials, which is crucial for understanding the behaviour of this promising material class under real operating conditions. The material of interest – 316L austenitic stainless steel, commonly used for heat exchangers – was manufactured to bars using laser powder bed fusion. Cylindrical specimens with characteristic hierarchical, non-equilibrium cellular microstructure were machined out of the bars. Two orientations corresponding to the inclination of the building direction to the specimen axis were considered: 0° and 90°. The specimens were subjected to thermomechanical fatigue loading under in-phase (maximum tension coincides with maximum temperature) and out-of-phase (maximum compression coincides with maximum temperature) conditions. The cellular dislocation microstructure showed good stability despite gradual coarsening under the combined effect of thermal loading up to 750 °C and severe plastic deformation. Systematic electron microscopy observations further revealed that basic damage mechanisms – either creep or stress-assisted oxide cracking, the prevalence of which depends on thermomechanical loading conditions – correspond to the behaviour of conventional metallic materials. Under in-phase loading, intergranular creep damage is dominant, hence a key factor affecting the lifetime is the number of grain boundaries in the loading direction. Under out-of-phase loading, fatigue damage is dominant, and the lifetime is determined by transgranular propagation of a principal crack. Comparing the two orientations, the inherent microstructural texture was found to be a crucial factor, also determining the number of grain boundaries and cell walls in the loading direction. Hence, tailoring the microstructure for the service relevant loading conditions via additive manufacturing techniques enables to enhance the component performance in the important field of energy conversion.

Programming metastable transition sequences in digital mechanical materials

The realization of unconventional computing methods in soft matter has inspired the creation of mechanological systems capable of processing information. These systems often utilize bistable mechanisms that serve as mechanically abstracted bits to perform digital logic operations. Yet, the input of such operations often requires manual control of complex cyclic loading to reach a desired configuration. This research presents a method to design digital mechanical materials that can enter a programmable sequence of metastable configurations through simple displacement-controlled inputs. Interactions between serially connected bistable units enable the transition and reset behavior of mechanical bits. An analytical model using the principle of minimum total potential energy of a single bit is presented to articulate systematic ways to tune the critical force thresholds and displacements of metastable transition sequences. This work elucidates how the application of a prescribed displacement allows for novel resetting behavior that enables simultaneous transitions of multiple bits. By combining the principles of deterministic transition sequences and mechanical computing, a mechanical analog to digital conversion (ADC) material system is introduced that digitizes a continuous force stimulus into binary configurations that perform computations within the mechanical material based on the applied load. The mechanical ADC establishes a foundation for the integration of environmental sensing, information processing, and response in a soft material system.

Analysis of multiaxial fatigue models based on the results of biaxial tests for aluminum alloys

During operation, modern structures and their parts are subjected to complex cyclic loads. In this case, a complex stress-strain state can arise in products, which can lead to a significant decrease in their fatigue life. In this regard, the development of methods for predicting fatigue fracture of critical structures becomes urgent. The purpose of this work is to test a simple approach for describing multiaxial fatigue datasets presented in the literature and compare it with known multiaxial fatigue models. Within the framework of the study, actual experimental data was considered, containing experimental data on the multiaxial fatigue of aluminum alloys 2024-T3 and 2024-T4. The alloys are actively used in the designs of aircraft, ships and spacecraft. As a result, the Sines model and its modernized version were considered (taking into account the first invariant of the stress tensor with respect to amplitude values). At the same time, an approach was developed to determine the constants of the models based on the results of two variants of training experiments. The paper presents the results in the form of relationships of the predicted and experimental fatigue life, obtained using the Sines model and its modification for two types of training tests. In the final part of the work, a comparative analysis of the Sines model and its modification, Smith-Watson-Topper, Fatemi-Socie and its modification is carried out. The analysis showed that the modified Sines model significantly improves the predictive ability, the proposed model better described the experimental data than the traditional Sines model and the Smith-Watson-Topper approach. At the same time, the descriptive ability of the model is comparable to the Fatemi-Socie one.

Shear modulus and damping ratio of saturated coral sand under generalised cyclic loadings

The influence of complex cyclic loadings associated with earthquakes on the shear modulus and damping properties of saturated coral sands has yet to be established. This paper presents comprehensive data sets from multi-staged strain-controlled (denoted Mγ), constant-amplitude strain-controlled (Cγ) and variable-amplitude strain-controlled (Vγ) undrained cyclic triaxial (UCTX) tests on a coral sand. The strain-dependent shear modulus and damping ratio from the Mγ UCTX tests and the cyclic stiffness degradation behaviour in the Cγ and Vγ UCTX tests are investigated. A correlation-function-based method is proposed to calculate the secant shear modulus and damping ratio at different strain levels, which shows higher precision at both small and large strain levels compared with the conventional methods. An important finding of the study is that the shear modulus reduction rate of the coral sand is significantly lower than that of siliceous sands, while the damping ratio for shear strain amplitude between 0·01% and 0·1% is well below that of siliceous sands. Two quantities: one is referred to as the strain ratio (ΓN) and used for characterising the normalised cycle-dependent strain amplitude, and the other referred to as the damage energy measure (δN) and used for characterising the cumulative normalised elastic strain energy absorption, are proposed such that a unique formula can be established for predicting the cyclic stiffness degradation in the UCTX tests with different loading patterns. This energy-strain-dependent formula is further validated using independent data from stress- and strain-controlled UCTX tests involving four patterns of cyclic loadings, and a satisfactory performance is demonstrated.

Energy characteristics of sandstones with different crack angles under true triaxial cyclic loading and unloading

AbstractThe effect of crack angle on energy evolution during sandstone failure subjected to true triaxial cyclic loading and unloading is of great concern. However, the energy consumed in the crack initiation and compaction process is generally referred to the dissipated energy, which is inaccurate. To reveal the energy law in the breaking process of sandstone, a complex cyclic loading and unloading test is carried out on precast angled sandstone samples by employing the triaxial test system. The area integral methodology is exploited to evaluate the evolution of total energy density, elastic energy density, dissipated energy density, and plastic energy density of the rock samples of different crack angles. The variations of these four energy density levels in terms of the number of loading and unloading cycles and the upper limit of stress are explained and discussed. The obtained results reveal that during the entire cycle of loading and unloading, the variations of the above‐mentioned factors of the rock sample fairly do not rely on the crack angle. The energy density increases with the number of cycles, and the total energy density and elastic energy density significantly increase faster than the consumption mode. The growth rate of the dissipative energy density is greater than that of the plastic energy density. Additionally, the aforementioned four energy quantities grow in a quadratic manner as a function of the upper limit of the loading stress. The releasable elastic energy gradually magnifies by increasing the upper limit of the loading stress. The plasticity density first decreases and then increases, reflecting the change law of the irreversible plastic deformation caused by newly developed cracks in the rock sample. The elastic energy density, the fitting coefficient of the dissipated energy density, and the total energy density are employed to define the characteristic coefficients of the rock energy storage and energy dissipation. The results indicate that the larger the rock crack angle, the stronger the energy storage energy and the weaker the energy dissipation capacity. It also explains that how the strength of the rock sample becomes higher as the crack angle increases.

Shakedown analysis of bounded kinematic hardening engineering structures under complex cyclic loads: Theoretical aspects and a direct approach

There exist two basic formulations of static shakedown theorem for bounded kinematic hardening (KH) model in literature. Whether the two formulations are equivalent, whether the conditions of loading-path-independence are satisfied, and how to employ the shakedown theorem to obtain the safety margin of realistic engineering structures under cyclic loading are some key issues to be solved and addressed. This paper presents theoretical aspects of the static shakedown theorem of KH and proposes a direct approach for shakedown analysis of KH structures. The presented direct approach allows for both the two shakedown formulations and considers the simultaneous influences of multiple load vertices. The two formulations are suitable for plastic models with different hardening laws and determine different shakedown load boundaries in the region of incremental collapse. Instead of dealing with a complicated problem of mathematical programming, the presented approach performs several linear elastic finite element iterative calculations, which ensures its high computational efficiency. Furthermore, the direct approach is implemented into Abaqus software platform so that it becomes a general tool to handle large-scale shakedown problems of engineering structures with complex geometry and loading conditions. An illustrative sample with analytical solution and three numerical examples from power plant engineering are adopted to validate these theoretical aspects and to present the potential of the direct approach.

The Three-Level Elastoviscoplastic Model and Its Application to Describing Complex Cyclic Loading of Materials with Different Stacking Fault Energies.

The development of new technologies for thethermomechanical processing of metals and the improvement of the existing ones would be unattainable without the use of mathematical models. The physical and mechanical properties of alloys and the performance characteristics of the products made of these alloys are generally determined by the microstructure of materials. In real manufacturing processes, the deformation of metals and alloys occurs when they undergo complex (non-proportional) loading. Under these conditions, the formation of defect substructures, which do not happen at simple (proportional) loading, can take place. This is due to the occurrence of a great number of slip systems activated under loading along complex strain paths, which leads, for instance, to the more intense formation of barriers of different types, including barriers on split dislocations. In these processes, the formation and annihilation of dislocations proceed actively. In this paper, we present a three-level mathematical model that is based on an explicit description of the evolution dislocations density and the formation of dislocations barriers. The model is intended for the description of arbitrary complex loads with an emphasis on complex cyclic deformation.The model is composed of macrolevel (a representative macrovolume of the material that can be considered as an integration point in the finite-element modeling of real constructions), and mesolevel-1 (description of the mechanical response of a crystallite) and mesolevel-2 (description of the defect structure evolution in a crystallite) submodels. Using the model, we have performed a series of numerical experiments on simple and complex, monotonic and cyclic deformations of materials with different stacking fault energies, analyzed the evolution of defect densities, and analyzed the challenges of a relationship between the complexity of loading processes at a macrolevel and the activation of slip systems at low scale levels.

The cyclic strain evolution and the fatigue prediction in non-proportional multiaxial loadings of NiTi SMAs

NiTi shape memory alloy is a kind of smart material which present unique properties of superelasticity, shape memory and excellent biocompatibility. The applications of such alloys are generally the connectors and fasteners, vibration-dampers, and endovascular stents, etc.. The cyclic deformation and fatigue failure of NiTi SMAs are key issues that need to be investigated since they usually endure complex cyclic loadings during the using process. In this paper, the evolutions of the peak/valley strains in non-proportional multiaxial fatigue loadings of NiTi SMAs are considered, and the fatigue failure mechanism is investigated. The results show that the martensite transformation and martensite re-orientation process greatly influence the transformation ratcheting and fatigue life of NiTi SMAs, and a life-prediction model that considers these deformation mechanisms particular for NiTi SMAs is also proposed to give reasonable predictions.

ЧИСЛЕННОЕ МОДЕЛИРОВАНИЕ УСТАЛОСТНОЙ ДОЛГОВЕЧНОСТИ ДЕТАЛЕЙ АВИАЦИОННЫХ ГТД ПРИ ЦИКЛИЧЕСКОМ ДВУХЧАСТОТНОМ НАГРУЖЕНИИ

The problem of assessing the resource characteristics of responsible engineering facilities, taking into account the peculiarities of operational loading modes, is discussed. The processes of fatigue durability of polycrystalline structural alloys under cyclic two-frequency loading (with the combined action of mechanisms of low and multi-cycle fatigue) are considered. A mathematical model describing the processes of cyclic elastoplastic deformation and accumulation of fatigue damage in structural alloys under multiaxial disproportionate modes of combined thermomechanical loading has been developed from the modern positions of damaged medium mechanics and fracture mechanics. The model consists of three interrelated parts: relations determining the cyclic elastic-plastic behavior of the material, taking into account the dependence on the destruction process; evolutionary equations describing the kinetics of fatigue damage accumulation; criteria for the strength of the damaged material. The variant of the defining elastic-plasticity relations is based on the idea of the yield surface and the principle of gradiency of the velocity vector of plastic deformations to the yield surface at the loading point. This version of the equations of state reflects the main effects of cyclic elastic-plastic deformation of the material for arbitrary folded loading trajectories. The variant of kinetic equations of fatigue damage accumulation is based on the introduction of a scalar damage parameter, is based on energy principles and takes into account the main effects of formation, growth and fusion of microdefects under arbitrary complex cyclic loading regimes. A joint form of the evolutionary equation of fatigue damage accumulation in the areas of low- and multi-cycle fatigue is proposed. As a criterion of the strength of the damaged material, the condition of reaching the critical value of the damage value is used. The results of numerical simulation of fatigue durability of a compact sample (model of the disk deflector of the high pressure turbine of an aviation gas turbine engine) are presented with cyclic two-frequency loading. The results of the comparison of the calculated and experimental data showed that the proposed model qualitatively and with the accuracy necessary for practical calculations quantitatively describes the fatigue life of structural elements under cyclic two-frequency loading.

Теоретичні та експериментальні методи визначення характеристик міцності лопаток турбін при термомеханічному навантаженні

The subject of this article is the methods of research and evaluation of the properties of turbine blades of a cooled structure under thermomechanical loading. The purpose of the article is to review the world achievements of leading enterprises and research institutions in the issue of fatigue tests of turbine blades under complex loading (cyclic temperature exposure, dynamic and static loading), as well as an overview of the state of this topic at SE "Ivchenko-Progress" and suggestions for its further studying. As a result of the analysis of publications and scientific articles, it can be concluded that specialized research institutes and leading aircraft engine-building enterprises from the end of the twentieth century are studying the properties of turbine blades in the conditions of their operation as part of an engine. In world practice, there are calculated and experimental methods for thermomechanical testing of turbine blades. These tests are aimed at determining the most damaging loads, establishing the flight cycle modes at which these loads are recorded. As a result, it was found that the greatest threat to the strength of the turbine blades is carried by transient modes of engine operation, which are short in time (measured in seconds), but at which there is a change in the parameters of the temperature field, loads from axial and centrifugal forces. And it is the cycling of these parameters that leads to a decrease in the cyclic durability of the turbine blades, especially of the cooled structure (the presence of perforations, internal cooling channels, and other structural elements leads to a complication of the volumetric stress state of the blades). The article analyzes various crystallographic structures of blades and their relationship with the volumetric stress state; examples of studies that were carried out at SE "Ivchenko-Progress" and their results are given, which emphasize the need for further experiments in the field of assessing strength characteristics under complex cyclic loading. An example of an installation for testing blade joints and samples of gears is considered, which can be adapted for testing blades with three-component loading (temperature, dynamic loads, and imitation of the effect of centrifugal forces). It is concluded that when using exclusively computational methods, it is impossible to reliably estimate the level of stresses and their distribution since the calculations are limited by the boundary conditions, which are set according to the capabilities of a particular computational model. Summing up, it can be noted that it is advisable to start assessing the strength of blades under thermomechanical loading with several series of tests of samples of blade material to study the effect of temperature and power cycles of loads, the effect of the orientation of the load vector concerning the crystallographic orientation of the blade. It is noted that tests of full-scale blades under thermomechanical loading are also important since the features of the volumetric stress state of the material during real operation of the blades as part of an engine are not reproduced during testing of samples. The above entails the development of methods and specialized installations for thermomechanical testing.

Enhanced high-cycle fatigue strength of Al–12Si–4Cu-1.2Mn-T6 cast aluminum alloy at room temperature and 350 C

As a promising material for application in lightweight automobile, Al–Si alloys are subjected to complex cyclic thermo-mechanical loading when employed in engines. This study examined the high-cycle fatigue behavior of Al–12Si–4Cu-1.2Mn-T6 cast aluminum alloy at room temperature (RT) and 350 °C and evaluated the effect of different second phases by microstructure observation and nano-indentation characterization. The high-cycle fatigue strength of the studied alloy at RT and 350 °C is 125.0 MPa and 47.5 MPa, respectively, exhibiting excellent fatigue resistance compared with other heat-resistant Al alloys. Fatigue cracks originate from casting defects at RT, but nucleate from primary Al15Mn3Si2 phase on the specimen surface at 350 °C. The modulus difference between α-Al15Mn3Si2 and α-Al phases is higher at 350 °C than at RT, allowing cracks to initiate and propagate more likely along the phase interface at 350 °C. The hardness and reduced modulus of α-Al15Mn3Si2 phase are greatly higher at 350 °C than at RT, both in the suspended specimen and the fractured specimen. The significant difference in residual stress between α-Al15Mn3Si2 and α-Al phases indicates that α-Al15Mn3Si2 phase bears more stress loading than α-Al phase during fatigue process, especially at 350 °C. The findings presented in this study may shed some light on the design of high-performance heat-resistant aluminum alloys.