Fatigue Life Curves Research Articles

Proposed Material Properties, Allowable Stresses, and Design Curves of Diffusion Bonded Alloy 800H for the ASME Code Section III Division 5

AbstractIncreased interest in compact heat exchangers (CHXs) to serve as intermediate heat exchangers of very high temperature reactors resulted in significant research and development on their design, analysis, and construction. Printed circuit heat exchangers are a type of CHXs with high thermal efficiency and compactness achieved through diffusion bonding a stack of etched plates with millimeter scaled channels. The diffusion bonding process changes the microstructural and mechanical properties of the wrought metal plates. The current nonnuclear design code ASME section VIII, division 1 captures the material property change through a “joint efficiency factor.” However, the current nuclear design code ASME section III, division 5 does not address or support the diffusion bonded material properties. Hence, there is a need to develop allowable stresses, isochronous curves, and fatigue life curves for various diffusion bonded alloys. In this study, Alloy 800H material was selected to establish the diffusion bonded material properties under tension, creep, fatigue, and creep-fatigue loads at elevated temperatures in the range 550–760 °C. A set of tests on diffusion bonded Alloy 800H (DB 800H) were performed and the acquired data are used in developing allowable stresses Sy, Su, Sr, Sm, St, Smt, So, isochronous curves and fatigue life curves according to the ASME section III, division 5 requirements. This paper also presents detailed procedures used in developing the ASME code section III division 5 design provisions for diffusion bonded Alloy 800H.

Fatigue performance of corrosion-damaged beams strengthened with FRP grid-reinforced ECC matrix composites

An investigation on the flexural fatigue behavior of corrosion-damaged reinforced concrete (RC) beams that were strengthened with a fiber-reinforced polymer (FRP) grid-reinforced engineered cementitious composite (ECC) matrix is presented in this paper. The concrete cover, which was damaged by the corrosion of the tensile steel bars, was chipped away and reinstated with an ECC layer that was embedded with an FRP grid. A total of 12 beams were constructed, and 11 of them were subjected to cyclic fatigue loading. The test results indicated that the final failure mode of the strengthened beams was still dominated by the brittle fracture of the tensile steel reinforcements. The fatigue life of the beams increased significantly with increasing strengthening amounts of the FRP grid but decreased with the corrosion ratio and the upper limit fatigue load level. At a sufficient strengthening amount, the fatigue life of strengthened beams could be restored to that of the original uncorroded beams. By regression analysis of the stress range–fatigue life (S–N) curve, a prediction formula for fatigue life was derived with a satisfactory correlation coefficient. In addition, the tested midspan deflections and the results that were calculated using available empirical models or theoretical calculation approaches were compared and evaluated.

Experimental and computational investigation of weathering steel Q450NQR1 under high cycle fatigue loading via crystal plasticity finite element method

The fatigue properties of the Q450NQR1 steel were experimentally and computationally investigated. The experimental results show a large scatter of the fatigue lifetime under the high cycle fatigue loading. Based on the Voronoi diagram and crystal plastic model, a polycrystalline representative volume element (RVE) model with a pore or inclusion was constructed to analyze the effect of local stress–strain on the high cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) life. Computational results show that the large deformation mostly occurs at the grain with a large Schmid factor. The increment of the mean accumulated plastic strain of the critical grain was proposed as the fatigue indicator parameter (FIP). A fatigue model was suggested based on the FIP, and the fatigue model can describe the influence of the material microstructures on fatigue life. The fatigue life curve with different sizes of defects was plotted and it is found that inclusions are the major factor in fatigue life dispersion.

Prediction of actual fatigue test temperature and isothermal fatigue life curves for non-crystallising rubber under fully relaxing uni-axial loading conditions

The time to complete fatigue tests can be rather long, especially with high cycle fatigue. Increasing frequency in fatigue tests decreases the time needed to complete material characterisation. For rubber materials, considerable internal heat generation and relatively low thermal conductivity causes temperature rises that can greatly reduce fatigue lifetime. Based on both experimental tests under fully relaxing uni-axial loading conditions on a non-crystallising rubber compound and numerical simulations, a method for estimating test temperature and isothermal fatigue life curves is presented. The results of the comparison between measured and estimated surface temperature of the specimen show that the maximum estimation error is below 10 %.

Fatigue crack growth in the re-entrant auxetic structure

The investigation of fatigue behaviour of the re-entrant and rotated re-entrant auxetic specimens made of Al-alloy 7075-T651 is presented in this study. In the proposed work, the influence of the orientation of the unit cell on the crack path and fatigue life were studied by using both experimental and computational approaches. In the experimental and computational approach, two geometric layouts of the base unit cell were analysed (re-entrant and rotated re-entrant structure). The Low Cycle Fatigue (LCF) tests were performed at load control with the load ratio 0.1 in tensile. In the LCF tests, five loading levels were selected and at least two tests were performed at each loading level. The same loading conditions were then applied in the computational model in the framework of the ANSYS software package. For the fatigue life calculation, the strain life approach was used, based on the Coffin-Manson model with a Morrow mean stress correction. The comparison between computational and experimental results regarding Fa − N fatigue-life curves and observed fatigue failure path showed a reasonable agreement.

Tensile fatigue behaviour and life distribution model of the pultruded fibre reinforced composites

Pultruded fibre reinforced composite (PFRC) can be used in oil drilling because of the corrosion resistance and energy conservation. The studies of fatigue behaviour and life distribution model are essential to the design and application of PFRC. In this paper, the tensile fatigue behaviour of PFRC was studied by the experiment combining with the acoustic emission technology, and the fatigue life distribution model is established to explore the distribution characteristics of fatigue life. The results show that, the fatigue load level has little effect on the failure modes of PFRC, and the matrix damage occurred firstly followed by the appearance of the delamination and the fibre damage. The proposed fatigue life distribution model can accurately describe the three stages of the fatigue life curve, and can well predict the fatigue life under different survival rates.

Simulation of Stress Concentrations in Notches

The fatigue life curves of materials are very sensitive to the magnitude of the stress amplitude. A small change or inaccuracy in the determination of the stress value causes large changes or inaccuracies in the calculated fatigue life estimate. Therefore, the use of computer simulations for fatigue life estimation requires a proper model development methodology. The paper is devoted to the problem of the modeling of components in notches using FEM. The modeling parameters significantly influencing the obtained stress results have been defined. Exact analytical solutions served as a benchmark for comparing the accuracy of the stress values obtained using FEM models. For the selected 2D and 3D notched components, diagrams were created for sensitivity analysis of the influence of the mesh element density at the root of the notch in correlation with the exact analytical solution. The findings from model building were applied to model the stress concentration at the root of a V-weld joint in a gas pipeline.

Fatigue life prediction of hypervapotron for CFETR divertor

The engineering design of China Fusion Engineering Testing Reactor (CFETR) will be finished in the end of this year. Compared with Experimental Advanced Superconducting Tokamak (EAST), this is a large development which means closer to commercial fusion reactors. The design objective of CFETR divertor requires a steady-state thermal load of 10–20MW/m2 exhaust. There are several design options for the CFETR divertor. Hypervapotron with its superior performance has been researched for several decades as one important candidate for future fusion device divertor. In this paper, a new design of hypervapotron plasma facing unit (PFU) was developed. Horizontal and longitudinal ribs were adopted to generate more vaporized cores to enhance heat transfer. Some optimizations were performed based on the allowable max heat flux (MAHF) to get the final design. Rensselaer Polytechnic Institute (RPI) boiling model of Eulerian Multiphase fluid and k-epsilon model of Viscous were chosen to deal with sub-cooled boiling. 3Sm rule and Bree-diagram was used to confirm the rationality of the structure after thermal-mechanical analysis. Numerical simulation results suggested that CuCrZr alloy will experience cyclic plastic strain. Based on this, fatigue life prediction of the hypervapotron PFU at a steady-state heat flux of 10MW/m2 were carried out through local stress-strain method, neuber curve and design fatigue life curve. Eventually, a fatigue life of approximately 1.2489 year was verified for CFETR divertor. This paper provides a systematic method for fatigue life prediction and reliability analysis of hypervapotron.

Very low cycle fatigue life evaluation models and validation using notched compact tension test data

Strain-based and energy-based model are proposed for the evaluation of very-low cycle fatigue life considering multi-axial stress states. In the strain-based model, based on void growth and shrinkage, the void shrinkage ratio is proposed as a function of the plastic strain range. The energy-based model is developed by considering stress triaxiality in the relationship between fatigue life and fracture strain energy. For validation, two models are determined using smooth and notched bar tensile test and best-fit fatigue life curves of SA508 Gr.3 and TP316L. Fatigue lives predicted by both models are within 85% of experimental values in all cases.

Prediction of strength and fatigue life for 2D plain-woven high-alumina fiber reinforced alumina matrix composites under a complex in-plane stress state

This paper presents a strength criterion and fatigue life prediction method for 2D braided alumina matrix composites under a complex in-plane stress state. The static strength of the material was obtained by in-plane tensile, compression, and pure shear tests. Considering the difference between tensile and compressive properties of materials and the influence mechanism of in-plane tensile and shear coupling on material strength, a revised Hoffman strength theory was proposed. The predicted off-axis tensile strength is consistent with the test results, and the deviation is not more than 10%. Tensile fatigue tests were carried out with the off-axis angle θ=0°, 15°, 30°, 45°, the stress ratio R=0.1, and frequency f=10 Hz. The test results show that the fatigue life decreases with the increase of off-axis angle. Due to the in-plane shear stress component, the fatigue failure is gradually changed from fiber-dominated to fiber-matrix dominated mode. Based on a combination of the uniaxial tensile fatigue life curve, the Broutman-Sahu residual strength model, which is used to characterize the variation of the residual strength with the fatigue cycles, and the modified Hoffman strength theory, the paper proposes a fatigue life prediction model under complex in-plane loading conditions. The fatigue shear damage factor is defined to characterize the effect of the normal and shear stress interaction on fatigue life. The fatigue life prediction model is used to predict the fatigue life of specimens in the off-axis tensile fatigue tests. The predicted result agrees with the test result, and the deviation is within the 1-time life span. The results indicate that the proposed fatigue life prediction model can be used to predict the fatigue life of 2D braided alumina matrix composites under the complex in-plane stress condition with the given stress ratio, temperature, and fatigue load frequency.

Low cycle fatigue performance of DP900 steel under cyclic shear paths

AbstractIn this study, a sheet fatigue shear test device is designed and applied to the low‐cycle fatigue testing of DP900 with varying strain amplitudes within the range of 0.5%–6.0%. The microstructure is analyzed by using electron backscatter diffraction, and fracture surfaces are examined via scanning electron microscopy. Results indicate that the material exhibits cyclic softening behavior after the first two cycles of hardening, with a stable softening rate and a high damage evolution rate related to loading amplitude. The variation of the hysteresis curve in the cyclic process is shown. Total plastic strain energy absorbed increases as loading amplitude decreases and it reaches the peak at approximately 1%. The life prediction model based on plastic strain energy density and strain amplitude is verified to be suitable for the cyclic shear path. The influence of microinhomogeneity on the distribution of stress and strain, especially the deformation of martensite, is closely related to the bi‐linear region of fatigue life curve.

A time-varying mechanical structure reliability analysis method based on performance degradation

In mechanical engineering, most mechanical structural systems are composed of many parts, so their structures are complex. When a mechanical structure fails, it is generally caused by various failure modes. When designers want to improve the quality of mechanical products in the design of mechanical products, reliability analysis can play an important role. It is indispensable in the design and manufacture of mechanical products, engineering use, repair and maintenance. Research on time-varying reliability of mechanical structures is of great significance in improving the quality of mechanical products and bringing more economic benefits to engineering. It has received extensive attention in both academic fields and practical engineering applications. This paper takes the time-varying reliability analysis of mechanical structures based on performance degradation as the research topic, focusing on the reliability analysis methods and techniques of mechanical structures such as Gamma degradation process, material P-S-N curve, mixed Copula function, and nested Copula function. The main research content is divided into three parts. (1) The Gamma degradation process and the P-S-N material probabilistic fatigue life curve are used when establishing the strength degradation random model of the mechanical system. It provides the theoretical basis of strength degradation level for time-varying reliability analysis of mechanical structure based on performance degradation. (2) Combine the mixed Copula function and the nested Copula function to establish a comprehensive failure correlation analysis model of the mechanical system. It provides a theoretical basis for failure correlation for time-varying reliability analysis of mechanical structures based on performance degradation. (3) The time-varying reliability analysis of the one-stage gear transmission system in the engineering example is carried out. Researched and analyzed the time-varying reliability of the large and small gears in the one-stage gear transmission system and the mechanical structure of the system itself under the correlation of strength degradation and failure.

Correlation of load drop and crack initiation criteria in fatigue life experiments of metallic materials

The simulation and assessment of fatigue damage of metallic materials are strongly dependent on the definition of critical damage initiation, i.e. the formation of a crack after a certain number of cycles under fatigue loading. To get this described in an appropriate way classical fatigue analysis needs to be combined with fracture mechanics to obtain a realistic size of an initiating macro crack in a component in the end. Ultimately, this size depends on the procedure of how fatigue life curves are generated from the fatigue tests performed. In this paper strain-controlled fatigue tests and fractographic results have been combined with a computational assessment to better understand the relationship between a traditional load drop criterion characterizing the size of a crack being initiated. It is stated that the location of an initiating crack versus the location of an extensometer applied to the specimen can have an influence on the crack size being initiated and thus the fatigue life for a load drop criterion being fixed. A relationship is proposed to link a crack size dependent on the fatigue test evaluation scheme with the fatigue life, which can be used in engineering assessments. The relevance of practical application cases is discussed.

Multiaxial low cycle fatigue of notched 10CrNi3MoV steel and its undermatched welds

It is a fundamental step to quantify the multiaxial fatigue damage considering the complex service conditions for notched components or welded structures. In our study, multiaxial Low Cycle Fatigue (LCF) tests of notched specimens were conducted on 10CrNi3MoV high strength steel and related undermatched welds under proportional and non-proportional cyclic loadings. The characteristic states of normal/shear stress, strain and energy around notch roots are analyzed considering the elasto-plastic material behavior under low cyclic loadings by the cyclic plasticity model. The predicted fatigue life curves of the test data were constructed by five strain-based and energy-based parameters combining with the critical plane approach and notch Theory of Critical Distance (TCD). According to Smith-Watson-Topper (SWT) damage parameters definitions, the paper proposed a simplified SWT model incorporating the effect of non-proportional loading, notch energy concentration characteristics and cyclic hardening behaviors for the LCF assessment of notch specimens. The proposed model shows good agreements between predictions and test data by comparing different multiaxial fatigue damage models for notch specimens.

Torsional Fatigue Life Prediction of 30CrMnSiNi2A Based on Meso-Inhomogeneous Deformation.

In this paper, torsional fatigue failure of 30CrMnSiNi2A steel which exhibited non-Masing behavior was studied under different constant shear strain amplitudes, using thin-walled tubular specimens. The relationship between shear fatigue and the evolution of meso-deformation inhomogeneity and the prediction method of the torsional fatigue life curve were investigated. Shear fatigue of the material under constant amplitude was researched by numerical simulation with reference to tests, by using crystal plasticity of polycrystalline representative volume element (RVE) as the material model. Considering the non-Masing behavior of material, when determining the parameter values of the crystal plasticity model the correlation between these parameters and strain amplitude was taken into account. The meso-deformation inhomogeneity with increments in the number of cycles was characterized by using the statistical shear strain standard deviation of RVE as the basic parameter. Considering the effect of strain amplitude on fatigue damage, ratio cycle peak stress/yield stress was taken as the weight to measure the torsional fatigue damage and an improved fatigue indicator parameter (FIP) to measure the inhomogeneous deformation of the material was proposed. The torsional fatigue life curve of 30CrMnSiNi2A steel was predicted by the critical value of the FIP and then the result was confirmed.

Fatigue behavior of Ti6Al4V alloy components manufactured by selective laser melting subjected to hot isostatic pressing and residual stress relief

AbstractFatigue behavior (Rε = −1) of HIPed and stress relieved Ti6Al4V alloy specimens produced by selective laser melting (SLM) was analyzed and compared resulting that the hot isostatic pressing (HIP) process caused a microstructural transformation decreasing the hardness and monotonic properties leading to cyclic softening that not allowed fatigue strength to increase. A bilinear behavior in the elastic strain–fatigue life curve was observed due to the decrease of the Young's modulus during the cyclic elastoplastic tests, consequence of subgrains formation. The Smith–Watson–Topper and total strain energy density models adjusted by the bilinear behavior showed a good concordance between predicted and experimental fatigue lives in notched samples.

Investigation of Deformation Inhomogeneity and Low-Cycle Fatigue of a Polycrystalline Material

Considering the relationship between inhomogeneous plastic deformation and fatigue damage, deformation inhomogeneity evolution and fatigue failure of superalloy GH4169 under temperature 500 °C and macro tension compression cyclic loading are studied, by using crystal plasticity calculation associated with polycrystalline representative Voronoi volume element (RVE). Different statistical standard deviation and differential entropy of meso strain are used to measure the inhomogeneity of deformation, and the relationship between the inhomogeneity and strain cycle is explored by cyclic numerical simulation. It is found from the research that the standard deviations of each component of the strain tensor at the cyclic peak increase monotonically with the cyclic loading, and they are similar to each other. The differential entropy of each component of the strain tensor also increases with the number of cycles, and the law is similar. On this basis, the critical values determined by statistical standard deviations of the strain components and the equivalent strain, and that by differential entropy of strain components, are, respectively, used as fatigue criteria, then predict the fatigue–life curves of the material. The predictions are verified with reference to the measured results, and their deviations are proved to be in a reasonable range.

Study on very-high-cycle fatigue property of 2A60 aluminum alloy

During the rotation, aero-engine impeller blades are subjected to the alternating load of high frequency, low stress and small strain, so it is effortless for the impeller blades to enter the stage of very-high-cycle fatigue (VHCF). The research on the VHCF performance of 2A60 aluminum alloy is of great engineering significance to verify the reliability of the impeller in service. In this paper, the VHCF performance of 2A60 aluminum alloy is studied by fatigue life curve and fracture morphology. The results show that the fatigue properties of 2A60 aluminum alloy under different cooling conditions are different, and the VHCF properties under ethanol cooling conditions are better than those under water cooling conditions. However, under either water-cooled or ethanol-cooled conditions, the fatigue fracture mode of 2A60 aluminum alloy is single, showing a surface crack initiation mode, and the S-N curve shows a continuous downward trend without the intermediate platform. The VHCF crack originated from the stress concentration on the sample surface and the cleavage fracture occurred in the grains near the crack source in the early stage of crack initiation. The VHCF process of the sample is also the relaxation process of its residual stress.

Cutting parameters influence analysis on fatigue behaviour of dry turned UNS A97075 alloy after corrosion

Aluminum alloys, particularly series 2000 (Al-Cu) and 7000 (Al-Zn), are widely used for structural elements in aircraft. Machining processes are frequently applied in manufacturing these parts. The actual trend in the machining of these components is to reduce or eliminate the use of lubricants, due to environmental reasons. Among the different techniques, dry machining is commonly used for aluminum alloys. However, dry machining generates higher severe cutting conditions, which may negatively affect the surface integrity and the mechanical properties of the machined parts. In this regard, fatigue life is one of the most important mechanical properties to take into account. Micro-cracks generation and nucleation strongly depend on the surface conditions of the machined parts. Additionally, the addition of Zn to pure aluminum (7000 series) reduces the corrosion resistance. Despite its importance, there is a lack of research devoted to analyze the cutting parameters influence on fatigue behaviour, before and after corrosion, in dry machining of these alloys. Therefore, in this work, the cutting speed and feed rate influence on fatigue behaviour of the UNS A97075 (Al-Zn) alloy has been analyzed. Several rotating bar bending fatigue tests have been carried out, studying and comparing the fatigue life curves obtained as a function of the cutting parameters, before and after an immersion corrosion process. The experimental results have revealed that an increment in the cutting speed and feed rate gives rise to fatigue life reduction.

The effects of service temperature on the fatigue behavior of a polyurethane adhesive joint

In order to study the influence of service temperature on the fatigue performance of the adhesive structure, Sikaflex®-265 polyurethane adhesive was used to make thick-adherend shear joints (TASJs) with aluminum alloy. According to the service environment of the vehicle, the temperature points of −40 °C, −10 °C, 20 °C, 50 °C and 80 °C were selected to carry out quasi-static tensile test on the joints, and the change law of failure load and failure displacement with time was analyzed. The variation law of the fatigue frequency and load on the internal temperature of the adhesive layer was analyzed by using the environment-fatigue coupling loading device. The fatigue load levels (60%, 50%, 40%, 30% and 20%) were selected to carry out fatigue tests, the fatigue life curves at different temperatures were analyzed, and the functional relationship of nominal stress-temperature-fatigue fracture cycles was obtained by fitting. The fatigue failure section of the joints at different temperature was analyzed by macrography and scanning electron microscopy (SEM). The results show that the temperature has obvious influence on the quasi-static and fatigue performance of the joints. Since the glass transition temperature (Tg) of the adhesive is −66 °C, the quasi-static failure load and failure displacement of the joints gradually decrease with the increase of temperature. Compared with −40 °C, the failure load and failure displacement decrease by 76.88% and 57.36% respectively at 80 °C. The temperature inside the adhesive layer increases with the increase of the fatigue frequency. With the increase of temperature, the fatigue performance of the joint decreases gradually. The closer the temperature is to the Tg of the adhesive, the larger the fatigue performance decreases. The temperature has obvious effect on the fatigue failure section, and the fatigue failure mechanism may change with the change of temperature. Interface failure is easy to occur at low temperature, and the ligament area of the failure section changes significantly. Cohesive failure is easy to occur at high temperature, and the obvious cracks and convex particles are easy to appear on the failure section.