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Strain rate sensitivity will change the cyclic mechanical response of materials under multiaxial thermomechanical fatigue loading; thus, it will lead to errors in fatigue life prediction if strain rate sensitivity is not considered in constitutive simulation. In order to more accurately express the time/rate dependence of viscoplastic behavior, a strain rate sensitivity factor is proposed to modify the viscoplastic function of Chaboche model first. On this basis, a viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading is proposed, which can describe the varying influence of strain rate sensitivity on cyclic mechanical behavior. Meanwhile, the exponential isotropic hardening rule with the linear term is adopted to characterize the initial rapid softening and subsequent stable softening of the material under fatigue loading at elevated temperature. Moreover, the material‐dependent nonproportional hardening coefficient and the loading path‐dependent rotation factor are used in the kinematic hardening rule to consider the effect of nonproportional additional hardening on the cyclic mechanical behavior of materials. Finally, the proposed method is verified by the stress–strain data of titanium alloy TC4 and Ni‐based superalloy GH4169 under uniaxial and multiaxial thermomechanical fatigue loadings, and a good agreement is obtained. HighlightsThe time/rate dependence of viscoplastic behavior is expressed more accurately.Initial rapid cyclic softening and subsequent stable cyclic softening are described.The effect of nonproportional hardening on cyclic mechanical behavior is considered.The proposed model is verified by two materials with different damage types. [ABSTRACT FROM AUTHOR]

In this paper, a notch stress correction method based on the structural yield surface is proposed, in which the actual stress increment at the notch root can be obtained by multiplying two quantities, that is, the pseudo stress increment and the ratio of the slopes of the material and structural stress-strain curves. In the proposed method, the influence of temperature on the notch stress correction can be taken into account, and no iteration is required during the calculation process. Moreover, based on the proposed notch correction method and a unified viscoplastic constitutive model at high temperature condition, a notch stress-strain estimation method is developed under multiaxial thermo-mechanical cyclic loading. In order to assess the prediction accuracy of the proposed method, the calculated results are correlated to those from thermal-structural non-linear finite element analysis under various proportional and non-proportional mechanical loadings at non-isothermal high temperature condition. The comparison between the calculated and analyzed results shows that the proposed method can precisely estimate the notch stress and strain under multiaxial thermo-mechanical cyclic loading. [ABSTRACT FROM AUTHOR]

A fatigue–oxidation–creep damage model that can take into account the effect of multiaxial cyclic feature on the damage mechanism is proposed under axial-torsional thermo-mechanical fatigue loading. In the proposed model, the effects of non-proportional additional hardening on fatigue, oxidation, and creep damages are considered, and the variation of oxidation damage under different high temperature loading conditions is also described. Moreover, the intergranular creep damage needs to be equivalent to the transgranular damage before accumulating with the fatigue and oxidation damages. The fatigue, oxidation, and creep damages can be expressed as the fractions of fatigue life, critical crack length, and creep rupture time, respectively, which allows the linear accumulation of different types of damages on the basis of life fraction rule. In addition, the proposed model is validated by various fatigue experimental results, including uniaxial thermo-mechanical fatigue, axial-torsional thermo-mechanical fatigue, and isothermal axial-torsional fatigue under proportional and non-proportional loadings. The results showed that the errors are within a factor of 2. [ABSTRACT FROM AUTHOR]

Highlights • A constitutive model was proposed under multiaxial thermo-mechanical cyclic loading. • The proposed model can consider the effect of non-proportional hardening. • The dynamic strain aging can induce a cyclic hardening. • The effect of dynamic strain aging is considered in the proposed model. Abstract A unified viscoplastic constitutive model, which can consider the influences of non-proportional hardening and dynamic strain aging on the responded stress, was proposed under axial-torsional thermo-mechanical cyclic loading. In the proposed model, the rotation factor and the non-proportional hardening coefficient were considered in the kinematic hardening rule. And the dynamic strain aging influence factor was also proposed to further modify the kinematic hardening rule, which can consider the induced dynamic strain aging under special strain rate and temperature conditions. Moreover, the simulation using the proposed model can achieve a better fit with the measured data under isothermal and non-isothermal axial-torsional cyclic loadings. Graphical abstracts Image, graphical abstract [ABSTRACT FROM AUTHOR]

The axial-torsional thermo-mechanical fatigue behavior for Ni-based superalloy GH4169 was investigated experimentally in the temperature interval from 360 °C to 650 °C. The experimental results showed that, in the same von Mises equivalent mechanical strain amplitude condition, the fatigue life can be seriously decreased when the axial mechanical strain and the temperature are in-phase, and the non-proportional loading of mechanical strains can result in the fatigue life to further decrease. The analysis results showed that the fatigue lives depend on the summation of fatigue, creep and oxidation damages. The interaction between dislocations and precipitations can induce the additional hardening under mechanical non-proportional loading, which can obviously increase the fatigue damage. The tensile mean stress and the non-proportional additional hardening can further increase the oxidation damage existed in all loading conditions, which result in the increase of inelastic strain at high temperature. Creep damage behavior on grain boundaries can be shown in the case of the high tensile stress and the high temperature acting simultaneously, and increased with shear stress at the same time. In addition, the tensile stress has an obvious effect on the nucleation of creep cavities contrasted to the shear stress at high temperature, and more creep damage can be caused by the non-proportional additional hardening. [ABSTRACT FROM AUTHOR]

• A mechanism based multiaxial thermo-mechanical fatigue damage model is proposed. • The value of the scatter band of life prediction result is less than ± 2. • The mean relative error between predicted and experimental strain amplitudes is 6.54%. In this paper, a life prediction method based on damage mechanism for titanium alloy TC4 under multiaxial thermo-mechanical fatigue loading is proposed. Temperature and time dependent influence factors are proposed and introduced into the strain amplitude-life curve method to consider the temperature and time dependence of damage, respectively. The increase in fatigue and oxidation damages caused by tensile mean stress and non-proportional additional hardening is expressed by the mean stress in Macaulay brackets and by using a multiaxial damage parameter, respectively. A fast cracking influence factor is proposed to consider the influence of fast cracking of materials caused by the combined action of high temperature, tensile stress and shear stress on damage behavior. The material constants are identified by test data under uniaxial and proportional loadings, and are further used for prediction under non-proportional loading. The failure life results of uniaxial isothermal fatigue tests with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue tests are used to verify the proposed method. The value of the scatter band of life prediction result is less than ± 2, as well as the maximum and mean relative errors between predicted and experimental strain amplitudes are 18.49% and 6.54%, respectively. [ABSTRACT FROM AUTHOR]

• Multiaxial thermo-mechanical fatigue behavior of titanium alloy is understood. • Effects of tensile and compressive dwell time on fatigue behavior are found out. • Fast cracking mode may be activated by multiaxial thermo-mechanical loading. • Failure life of material is reduced by non-proportional additional hardening. • Fatigue and oxidation damages are increased by tensile mean stress. In this paper, the experimental investigation was carried out on the TC4 titanium alloy to understand the cyclic deformation behavior and fatigue damage mechanism of materials under multiaxial thermo-mechanical loading. The results showed that the fast cracking mode of the material may be activated by the combined action of high temperature, tensile stress and shear stress, inducing a fast degradation in the axial mechanical properties of the material, which results in a drastic decrease in the failure life of the material. Moreover, it is also found that the non-proportional additional hardening and the tensile mean stress can increase the fatigue and oxidation damages of the material, and the material damage behavior is temperature dependent and time dependent. It should be pointed out that the damage mechanisms identified in this paper can reasonably explain the life law under uniaxial isothermal fatigue loading with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue loadings. [ABSTRACT FROM AUTHOR]

• Multiaxial fatigue behavior at high temperature is experimentally characterized. • Fatigue-oxidation-creep damage behaviors are comprehensively captured. • The effect of non-proportional hardening on fatigue behavior is considered. • The notch stress–strain history is determined by the elastic-viscous-plastic analysis. • A new method is proposed to evaluate the fatigue life and applied to a notched bar. In this paper, a new life assessment framework is proposed based on the elastic-viscoplastic modeling and the damage behavior for the structural component under multiaxial non-proportional loading at high temperature. The viscoplastic constitutive model with the ability to capture the non-proportional hardening effect is used to obtain the stress–strain fields, in which a new method based on the rotation of strain axis is proposed to evaluate the notch rotation factor. The damage model, which can comprehensively capture the multiaxial fatigue-oxidation-creep behaviors, is used to assess the failure life. In addition, the proposed framework is evaluated by the life results under proportional and non-proportional fatigue loadings at 650 °C, and the errors are found to be within a factor of 2. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fatigue behavior under thermo-mechanical fatigue loading of titanium alloy is understood. • Effects of strengthening particles debonding on fatigue behavior is identified. • Fatigue damage of material is increased by tensile mean stress. • Failure life of material is reduced by non-proportional additional hardening. The fatigue experiments for titanium alloy Ti60 under different uniaxial/multiaxial thermo-mechanical loading modes found that the combined action of high temperature and tensile stress can cause the debonding of the second phase strengthening particles between grain boundary, reducing the ability to resist deformation of Ti60, which leads to a decrease in the fatigue life of the material. In addition, mean tensile stress increases the ability of cracks to break through intergranular barriers and the non-proportional additional hardening caused by multiaxial loading exacerbates the formation of microcracks. Both will increase the fatigue damage of the material. The fatigue damage mechanism identified in this investigation can reasonably explain the fatigue life law under multiaxial loading at high temperature, uniaxial and multiaxial thermo-mechanical fatigue loadings. [ABSTRACT FROM AUTHOR]

• Physically-based modeling of cyclic softening damage is implemented. • Microstructural degradation of the material under cyclic loading can be captured. • The decrease of dislocation density can be quantitatively described. • The coarsening of martensitic lath width can be reasonably calculated. A unified viscoplastic constitutive model coupled with a physically-based damage variable, is proposed to capture the cyclic mechanical behavior and microstructural evolution of the material at elevated temperature. The mechanical strength can be reduced by the decrease in the dislocation density, the coarsening of the martensitic lath and the loss of the martensitic structure under cyclic loading. The proposed physically-based damage variable is driven by the evolutions of dislocation density and martensitic lath width. The good comparisons with test results mean that the proposed model can reasonably model the cyclic elastic-viscoplastic constitutive behavior of the material at high temperature. [ABSTRACT FROM AUTHOR]

• Online fatigue-oxidation-creep damage evaluation is implemented. • Influence of temperature on the damage accumulation is considered. • Effect of non-proportional additional hardening on all damages is considered. • Changed oxidation damage is quantitatively calculated. A real-time multiaxial thermo-mechanical damage evaluation method was proposed under variable amplitude loading. In order to achieve real-time damage evaluation, a real-time multiaxial cycle counting method that can take into account temperature loading history was proposed. The proposed multiaxial cycle counting method can obtain the reversals one by one in order with the reading of the load-time history data, which can realize the damage calculation immediately as a single reversal is counted out. At the same time, to consider the effect of temperature loading on the damage accumulation, an equivalent temperature was determined for the time period of the counted reversal. A fatigue-oxidation-creep damage model that can consider the influence of the non-proportional additional hardening on the fatigue, oxidation and creep damages was used to predict the lives in cases of isothermal axial-torsional fatigue, uniaxial thermo-mechanical fatigue and axial-torsional thermo-mechanical fatigue under constant and variable amplitude loadings. The results showed that the prediction errors were within a factor of 2. [ABSTRACT FROM AUTHOR]

• The competition between protective and harmful effects of oxidation is revealed. • A high-temperature environmental damage model is proposed. • A mechanism based multiaxial thermo-mechanical fatigue life model is proposed. • The normal mean stress and cycle period are used to express competition mechanism. • The temperature dependence of material damage is quantitatively described. In this paper, the competition mechanism between protectiveness and harmfulness of oxidation, as well as the effects of cycle period, mean stress and temperature on the damage behavior of TA15 titanium alloy are revealed under multiaxial thermo-mechanical fatigue loading. Based on the damage mechanism, a high-temperature environmental damage model is proposed under multiaxial thermo-mechanical fatigue loading first, then a life prediction method is developed by combining a multiaxial fatigue damage model. The experimental verification results under uniaxial and multiaxial thermo-mechanical fatigue loadings showed that almost all of the prediction errors are within a factor of 2. [ABSTRACT FROM AUTHOR]

A new creep–fatigue damage cumulative model is proposed under multiaxial thermomechanical random loading, in which the damage at high temperature can be divided into the pure fatigue damage and the equivalent fatigue damage from creep. During the damage accumulation process, the elementary percentage of the equivalent fatigue damage increment is proportional to that of the creep damage increment, and the creep damage is converted to the equivalent fatigue damage. Moreover, combined with a multiaxial cyclic counting method, a life prediction method is developed based on the proposed creep–fatigue damage cumulative model. In the developed life prediction method, the effects of nonproportional hardening on the fatigue and creep damages are considered, and the influence of mean stress on damage is also taken into account. The thermomechanical fatigue experimental data for thin‐walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed model. The results showed that the proposed method can obtain satisfactory life prediction results. [ABSTRACT FROM AUTHOR]

[Display omitted] • Path-dependent line integral multiaxial cycle counting method was proposed. • The proposed method can be reduced to a uniaxial rain-flow counting form. • A recursive algorithm is provided to implement the proposed method. • The proposed method achieved excellent results in fatigue life prediction. A multiaxial cycle counting method was proposed by path-dependent line integral (PDLI). In the proposed PDLI multiaxial cycle counting method, the length increment of loading path is assigned a positive or negative sign in the strain space, and the interval from zero to the maximum value of PDLI is defined as a half-cycle. A very concise recursive algorithm is provided to implement the proposed multiaxial cycle counting method. The experimental verification showed that the proposed multiaxal cycle counting method was used to count multiaxial cycle, which can obtain satisfactory life prediction results under multiaxial variable amplitude loading. In addition, the PDLI multiaxial cycle counting method can be reduced to a uniaxial rainflow counting form. [ABSTRACT FROM AUTHOR]

• A multiaxial total life prediction method based on short crack growth is proposed. • The additional damage from non-proportional loading path is characterized. • Effect of shear stress on crack growth under tensile/compressive stress is considered. • The proposed method is verified by two materials under multiaxial loading. A total life prediction method based on short crack propagation considering additional damage from non-proportional loading is proposed under multiaxial fatigue loading. Firstly, a new calculation method of equivalent stress range is proposed to consider the effect of shear stress on crack propagation under tensile or compressive stress, then it is further modified to consider the additional damage. Combined with Paris formula and crack closure correction, a total life prediction method is proposed. The life results were used to verify the proposed method for two materials, which showed that most errors are within a factor of 3. [ABSTRACT FROM AUTHOR]

• A novel method of unconstraint ultrasonic vibration used to heal fatigue damage is proposed. • The fatigue crack initiation life of the polycrystalline copper is increased by 3.0 times. • Microstructure after the unconstraint ultrasonic vibration treatment was almost restored to the original state. • Fatigue resistance of the material was recovered due to the unconstraint ultrasonic vibration treatment. The fatigue performance and microstructure of polycrystalline copper treated by one (UUV1), two (UUV2), three (UUV3) passes of unconstraint ultrasonic vibration (UUV) treatment were investigated in this paper. The surface roughness, fatigue life, microstructure, and plastic strain energy before and after UUV treatment were compared. The results revealed that the surface integrity of the sample for UUV treatment higher than that of the original sample, and the microscope observation found that the grain orientation, the dislocation density, the low angle grain boundaries, the plastic strain energy of the damaged sample restored to the original state to a great extent after UUV treatment. The fatigue crack initiation life increased by 3 times after UUV3 treatment. [ABSTRACT FROM AUTHOR]

• A modified incremental multiaxial Neuber notch correction method considering temperature change was proposed. • The proposed approximate calculation method can accurately and quickly obtain the notch-root local stress-strain history. • A fast life prediction method is developed under variable amplitude multiaxial thermo-mechanical loading. The modified incremental multiaxial Neuber rule by considering the temperature change was firstly proposed, which combined with the Chaboche viscoplastic constitutive model to approximately compute the notch local stress and strain under multiaxial thermo-mechanical loading. Based on the proposed method, a life prediction method that can take into account fatigue, oxidation, and creep damage was developed, which can quickly predict notch fatigue failure lifetime under variable amplitude multiaxial thermo-mechanical loading. The proposed method was verified by the coupled thermal-structural nonlinear FEA and the experimental life results for the tenon joint structure specimens under eight thermo-mechanical loading paths. The comparison of computed and analyzed stress-strain history showed that a good agreement can be obtained. Comparing with experimental life data, all predicted lives fall in a 2-factor scatter band. [ABSTRACT FROM AUTHOR]

• An equivalent energy-based critical plane damage parameter is proposed. • No other additional material constants are needed for the proposed method. • Non-proportional cycle hardening and mean stress can be reflected more perfectly. • The method shows great potential under multiaxial random fatigue loading. Combining the concept of energy on the critical plane, a multiaxial fatigue damage parameter without any weight coefficient was proposed and the effects of non-proportional additional hardening and mean stress on fatigue damage can be reflected more perfectly using an equivalent form of shear strain energy. Furthermore, two sets of experimental data are employed to verify the proposed method, and the sensitiveness to fatigue life of the proposed equivalent stress correction factor is discussed as well. The results show that satisfactory and conservative assessments can be obtained by the proposed method, especially under multiaxial irregular loading with great randomness. [ABSTRACT FROM AUTHOR]

A new calculation approach is suggested to the fatigue life evaluation of notched specimens under multiaxial variable amplitude loading. Within this suggested approach, if the computed uniaxial fatigue damage by the pure torsional loading path is larger than that by the axial tension–compression loading path, a shear strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage; otherwise, an axial strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage. Furthermore, the presented method employs shear strain‐based and axial strain‐based multiaxial fatigue damage parameters in substitution of equivalent strain amplitude to consider the influence of nonproportional additional hardening. The experimental data of GH4169 superalloy and 7050‐T7451 aluminium alloy notched components are used to illustrate the presented multiaxial fatigue lifetime estimation approach for notched components, and the results reveal that estimations are accurate. [ABSTRACT FROM AUTHOR]

• A multiaxial cycle counting algorithm was proposed for thermomechanical random loading. • An interaction modulus B Ti varying with temperature was proposed. • A prediction model was proposed under multiaxial thermomechanical variable amplitude loading. Based on the Wang-Brown method, a time-dependent multiaxial cycle counting algorithm was proposed under multiaxial thermomechanical variable amplitude loading. By using the proposed multiaxial cycle counting algorithm, a new thermo-mechanical fatigue life prediction method was proposed, in which the pure fatigue damage, creep damage, and the interaction damage taking into account the interaction modulus varying with temperature were calculated for each reversal counted by the cycle counting algorithm. Three types of damage from all the counted reversals for the whole strain-time history were accumulated, and the thermo-mechanical multiaxial fatigue life can be predicted. The thermo-mechanical fatigue experimental data for thin-walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed method. The results demonstrated that the proposed method can provide very satisfactory prediction results. [ABSTRACT FROM AUTHOR]

The multiaxial thermomechanical fatigue properties for nickel-based superalloy GH4169 in aeroengine turbine discs are investigated in this paper. Four types of axial–torsional thermomechanical fatigue experiments were performed to identify the cyclic deformation behavior and the damage mechanism. The experimental results showed that the creep damage can be generated under thermally in-phase loading while it can be ignored under thermally out-of-phase loading, and the responded stress increasing phenomenon, that is, non-proportional hardening, can be shown under the mechanically out-of-phase strain loading. Based on the analysis of cyclic deformation behavior and damage mechanism, a life prediction method was proposed for multiaxial thermomechanical fatigue, in which the pure fatigue damage, the creep damage, and the interaction between them were simultaneously considered. The pure fatigue damage can be calculated by the isothermal fatigue parameters corresponding to the temperature without creep; the creep damage can be calculated by the principle of subdivision, and the creep–fatigue interaction can be determined by creep damage, fatigue damage, and an interaction coefficient which is used to reflect the creep–fatigue interaction strength. The predicted results showed that the proposed method is reasonable. [ABSTRACT FROM AUTHOR]

Based on the isothermal fatigue and creep damage, a life prediction approach under multiaxial thermo-mechanical loading was proposed in this investigation. In the proposed method, the multiaxial thermo-mechanical fatigue damage during one cycle period was divided into the isothermal fatigue damage and the creep damage. In order to evaluate conservatively, the isothermal fatigue damage during one cycle period was calculated by using multiaxial fatigue damage model at the maximum temperature during the whole period, and the creep damage during one cycle period was calculated by accumulating the creep damage of all portions originated from the divided time history for the axial load component and temperature history. The life prediction results by the proposed model showed a good agreement with experimental data for nickel-base alloy GH4169 and cobalt-base Haynes188 under axial-torsional thermo-mechanical loading. [ABSTRACT FROM AUTHOR]

• Positive correlation between individual modulus and fatigue strength was identified. • Fatigue life model considering individual modulus was proposed. • Fatigue life prediction method under variable amplitude loading was proposed. Needled C/SiC composites are widely used in the aerospace field, making the evaluation of fatigue damage critically important. A fatigue damage model associated with the individual modulus is proposed. Compared to the fatigue damage model by S-N curve, the proposed fatigue damage model can better characterize the fatigue damage behavior of UN-C/SiC composites. By combining the rainflow counting method, the Goodman equation, and the Palmgren-Miner damage accumulation rule, a fatigue life prediction method considering the individual modulus is developed for UN-C/SiC composites under variable amplitude loading. Compared with the experimental life, the predicted fatigue life shows a good result. [ABSTRACT FROM AUTHOR]

• Crack initiation on the oversized facet formed by α p phase cluster cleavage is found. • The size of oversized facet can be predicted by the value of stress intensity factor range. • A simplified model is proposed to describe the microstructure short crack propagation. • A full-life prediction model based on crack propagation is proposed in VHCF regime. • A deterministic life prediction model considering the effect of temperature is proposed. The very high cycle fatigue crack initiation mechanism of Ti60 titanium alloy at different temperatures was studied. The results showed that the initiation of cracks occurred on oversized facets in the subsurface. Based on this crack initiation mechanism, a full-life prediction model based on crack propagation at room temperature was first proposed, and then a very high cycle fatigue life prediction model considering temperature was proposed based on the degradation law of tensile strength at different temperatures. The experimental and predicted life results of Ti60 specimen at room temperature with 300 °C and 500 °C showed that the proposed method can effectively predict the very high cycle fatigue life, and the error is within a factor of 3. [ABSTRACT FROM AUTHOR]

• Subsurface failure is in the form of oversized facet formed by α p phase cluster cleavage. • The oxide shedding causes the surface failure at high temperature. • Higher temperature and stress make the oxide shedding failure mechanism dominant. The failure mechanism of Ti60 titanium alloy in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) at different temperatures was investigated. The results showed that, at different temperatures, there are two different fatigue failure mechanisms, i.e., the oversized facet and oxide shedding failure mechanisms. At room temperature, most of the fatigue cracks initiate at the oversized facet. At 300℃, the crack initiates at the oversized facet for the test with small applied stress, while the crack initiates at the oxide shedding site for the test with large applied stress. At 400℃, all the fatigue cracks initiate at the oxide shedding site. Higher temperature and stress will be conducive to the failure mechanism of oxide shedding to become dominant. [ABSTRACT FROM AUTHOR]

• A fatigue limit equation considering the high temperature effect is proposed. • The influence of high temperature on fatigue life prediction is considered. • A stress normalized method is used to reduce the effect of composites dispersion. • The life prediction is proposed for composites dominated by compression damage. Fatigue life prediction method based on stress normalization was proposed to reduce the influence of material property dispersion on life for CFRC (carbon fibre reinforced composites). In order to consider the effect of temperature on fatigue damage, a temperature influence factor was introduced into the fatigue limit equation and the life prediction method under variable amplitude loading at high temperature. Combined with the rainflow cycle counting and Miner linear damage accumulation rule, the proposed fatigue life prediction method was verified by the experimental results of composite multidirectional laminates under constant amplitude and variable amplitude loadings at various temperatures, and the prediction errors are within a factor of 5. [ABSTRACT FROM AUTHOR]

• A life prediction model is proposed under tension–compression loading at high temperature. • The effect of strength contribution of different orientation fibre layers is considered. • The proposed model can consider the degradation of matrix properties by high temperature. In this paper, the mechanical properties of carbon fibre reinforced multidirectional laminates at different temperatures were tested. Based on the consideration of the microstructure and fibre layer thickness of the multidirectional laminates, a stress normalized method is proposed. Based on the normalized static ultimate strength and fatigue stress, a life prediction model suitable for the fatigue process dominated by compression damage is proposed at high temperature. The fatigue test results at different temperatures showed that the proposed fatigue life prediction model considering the strength contribution of different orientation fibre layers and the degradation of matrix properties at high temperature can successfully predict the fatigue life of multidirectional laminates at different temperatures, and the life prediction error is within the range of three factors. [ABSTRACT FROM AUTHOR]

Background: Valvular heart disease is a significant cause of morbidity and mortality worldwide. Artificial heart valves are currently the most widely accepted biomedical material. However, the material's blood compatibility is still not sufficient. Biomaterial with a good biocompatibility has not yet appeared. Our initial study was to evaluate the probable cytotoxicity of hydroxyapatite (HA) on human umbilical vein endothelial cells (HUVEC) for mechanical heart valve prosthesis applications. Methods and Results: Cell growth was tested by MTT assay and the cell relative growth rate (RGR) was calculated. Cells were cultured in media consisting of either leaching extracts of hydroxyapatite at 37 �C or leaching extracts at 121 �C or a negative and a positive control group, respectively. The cytotoxicity was graded using generally accepted standards. The RGR of cell lines cultured in the culture media consisting of either normal or heat temperature leaching extracts of hydroxyapatite and the negative controls were higher than in the positive controls. For 24 h, 48 h, and 72 h cultures, the cytotoxicity grade of HA was 0 in the normal temperature leaching extracts of hydroxyapatite and 1 in the heat temperature leaching extracts of hydroxyapatite. Conclusions: Human umbilical cord endothelial cells grew well in the HA extracts. Our results indicate that HA possesses good in vitro bioactivity and biocompatibility with human umbilical cord endothelial cells and could be used as film on mechanical heart valve prostheses. [ABSTRACT FROM AUTHOR]