Low-cycle Regime Research Articles

J0310103 Critical distanceを用いた疲労寿命予測法の接触端応力集中部への適用性

In the previous report, it was found, from fatigue tests using notched round bar specimens, that critical distance approach is available for fatigue life prediction at stress concentration part at low cycle regime. In this study, in order to confirm the capability of the life assessment method for stress concentration part at contact edge such as dovetail structure in gas turbine disks, fatigue tests using the dovetail structured specimens with contact configuration of Ni-based single crystal superalloy were conducted at high temperature. Critical distance, which means the region of stress field related to the fatigue process zone, was obtained from fatigue tests and analyses using notched round bar specimens. The average stresses within the critical distance at the contact edge, where fatigue crack initiates, for dovetail structured specimens were obtained. It was found that the fatigue life of dovetail structured specimen can be predicted within the almost factor of 2 scatter bands by life assessment method using the average stresses.

High-temperature mechanical properties of thermal barrier coated SiC/SiC composites by PIP process with a new precursor polymer

Three dimensional (3D) braided silicon carbide (SiC) fiber reinforced SiC matrix (SiC/SiC) composites were fabricated by precursor impregnation and pyrolysis (PIP) process. Liquid polyvinylcarbosilane (LPVCS) with active SiH and –CHCH2 groups was adopted as the SiC matrix precursor. SiC/SiC composites were coated with environmental barrier coatings (EBCs) to enhance their durability and air oxidation resistance at high temperature. The fabricated SiC/SiC composites exhibited superior density and high-temperature mechanical properties. Bulk density and porosity of the composites were 2.16g/cm3 and 6.7%, respectively. The tensile strength of the composites was 339.7MPa at 1300°C. Tension–tension fatigue tests were conducted at frequency of 1Hz ranging from 40 to 120MPa at 1300°C in air. Fatigue life could be divided into four regimes, low cycle regime, stable-state regime, high cycle regime and run-out regime. The microstructure as well as damage and failure mechanisms of the composites were investigated.

Cyclic behavior of 9–12% Cr steel under different control modes in low cycle regime: A comparative study

Cyclic behavior of 9–12% Cr steel under both stress and strain control modes was investigated at 873K. Significant asymmetric deformation and cyclic softening were observed for both modes. Under the strain-controlled fatigue, a strain level-independent softening factor (SF) was observed. The SF was dependent on applied stress under the stress-controlled fatigue. For the strain-controlled fatigue, the magnitude of cyclic asymmetry decreases with the increase of strain amplitude, while under the stress-controlled fatigue, the asymmetry increases with the increase of stress amplitude. The anomalous ratcheting strain resulted from the asymmetry under the stress-controlled mode has a detrimental effect on fatigue life.

Fatigue damage evaluation of austenitic stainless steel using nonlinear ultrasonic waves in low cycle regime

The interrupted low cycle fatigue test of austenitic stainless steel was conducted and the dislocation structure and fatigue damage was evaluated subsequently by using both transmission electron microscope and nonlinear ultrasonic wave techniques. A “mountain shape” correlation between the nonlinear acoustic parameter and the fatigue life fraction was achieved. This was ascribed to the generation and evolution of planar dislocation structure and nonplanar dislocation structure such as veins, walls, and cells. The “mountain shape” correlation was interpreted successfully by the combined contribution of dislocation monopole and dipole with an internal-stress dependent term of acoustic nonlinearity.

A structural strain method for low-cycle fatigue evaluation of welded components

In this paper, a new structural strain method is presented to extend the early structural stress based master S–N curve method to low cycle fatigue regime in which plastic deformation can be significant while an elastic core is still present. The method is formulated by taking advantage of elastically calculated mesh-insensitive structural stresses based on nodal forces available from finite element solutions. The structural strain definition is consistent with classical plate and shell theory in which a linear through-thickness deformation field is assumed a priori in both elastic or elastic–plastic regimes. With considerations of both yield and equilibrium conditions, the resulting structural strains are analytically solved if assuming elastic and perfectly plastic material behavior. The formulation can be readily extended to strain-hardening materials for which structural strains can be numerically calculated with ease. The method is shown effective in correlating low-cycle fatigue test data of various sources documented in the literature into a single narrow scatter band which is remarkable consistent with the scatter band of the existing master S–N curve adopted ASME B&PV Code since 2007.With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure in 2007 ASME Div 2 Code can now be eliminated, such as its use of Neuber's rule in approximating structural strain beyond yield. More importantly, both low cycle and high cycle fatigue behaviors can now be treated in a unified manner. The earlier mesh-insensitive structural stress based master S–N curve method can now be viewed as an application of the structural strain method in high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke's law. In low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. The use of a pseudo-elastic structural stress definition is not fundamental, but merely a means to put low-cycle and high-cycle fatigue test data in a conventional stress-based S–N data representation which is typically preferred in engineering practice, than a strain-based representation.

Effects of particulate volume fraction on cyclic stress response and fatigue life of AZ91D magnesium alloy metal matrix composites

In this work, both AZ91D monolithic and AZ91D/SiCp composites prepared by squeeze casting followed by hot extrusion are tested under fully reversed strain-controlled loading conditions. The effects of SiCp volume fraction (i.e. Vf=0%, 5%, 15%, 25%, and 30%) and the imposed strain amplitude on the strain-controlled fatigue life and properties are examined. Increasing the SiCp content degrades the strain-controlled fatigue properties in the low cycle regime, with a potential benefit in the high cycle regime. The relationship between elastic strain amplitude, plastic strain amplitude and reversals to failure are well described by the Basquin and Coffin–Manson equations, respectively. A comparison made between the monotonic and cyclic stress–strain curves reveals that the reinforcement amount as well as the imposed strain amplitude significantly affects the characteristics of the evolving hysteresis loops.

Microstructure-Sensitive Fatigue Modeling of AISI 4140 Steel

A microstructure-based fatigue model is employed to predict fatigue damage in 4140 steel. Fully reversed, strain control fatigue tests were conducted at various strain amplitudes and scanning electron microscopy was employed to establish structure-property relations between the microstructure and cyclic damage. Fatigue cracks were found to initiate from particles near the free surface of the specimens. In addition, fatigue striations were found to originate from these particles and grew radially outward. The fatigue model used in this study captured the microstructural effects and mechanics of nucleation and growth observed in this ferrous metal. Good correlation of the number of cycles to failure between the experimental results and the model were achieved. Based on analysis of the mechanical testing, fractography and modeling, the fatigue life of the 4140 steel is estimated to comprise mainly of small crack growth in the low cycle regime and crack incubation in the high cycle fatigue regime.

Experimental Evaluation of the Hysteretic Damping of Elastomeric Systems at Low-Cyclic Harmonic Kinematic Displacements

The paper presents the experimental results regarding the hysteretic behavior of elastomeric systems made in conformity with the European Standards SR EN 1337-3 and SR EN 15129. It is also mentioned that the hysteretic damping was determined by experimentally raising the hysteresis loop in a low-cycle harmonic regime, with a kinematic exterior excitation defined by a harmonic displacement law. In this context, it is mentioned that the area of the hysteretic loops was determined instrumentally (in analogical representation of the signals) as well as under digital form, through a sampling of the physical signal, which allows a high precision of the determination.

Low Tension-Tension Cycle Fatigue Properties of 301 Stainless Steel Thin Sheets

By using of a micro mechanical fatigue testing system, low tension-tension cycle fatigue properties of 301 stainless steel thin sheets with a thickness of 0.1 mm were studied. The effects of loading frequency and stress ratio were considered in the tests. The results show the S-N curves descend continuously in the low cycle regime. Cyclic σ-N curve was obtained according to the traditional fatigue theory. It agrees well with the experimental data, showing that the traditional fatigue research methods are also suitable to describe thin sheets in a certain extent. With the increase of loading stress ratio, the fatigue strength of thin sheets is increased. There is an evident effect of frequency on the fatigue behavior of the thin sheets.

Grain-level statistical plasticity analysis on strain cycle fatigue of a FCC metal

The micromechanical strain cycle fatigue-life is systematically investigated by the micro-level numerical simulation, compared with symmetrical strain cycle experiments of copper, focusing on the characteristics of polycrystalline aggregation and the mechanism of microscale plastic deformation. A methodology to predict the low-cycle fatigue life by micro-level simulation along with statistical analysis is proposed through the following steps: (1) A crystal plasticity model is developed based on the nonlinear kinematic hardening mechanism of crystal slipping system. This model is applied to the calculations of crystal grain interior stresses and plastic strains. (2) A statistical representative volume element (SRVE) is constructed for a pure copper as a material model which features a polycrystalline Voronoi aggregation consisting of a number of crystal grains. This SRVE can be used for statistical analysis of the material inhomogeneous stresses and strains during cycle loading. (3) The simulations are performed to model the experimental cycle evolution of strain fatigue by using the SRVE under the symmetrical tensile–compressive loading. (4) Statistical and micromechanical analyses are carried out for the inhomogeneous interior stresses and strains of the SRVE of the polycrystalline copper in the low cycle regime. The resulting analysis can render the microscale interpretation and numerical simulation for the low-cycle fatigue evolution accordingly.

The influences of extrusion and equal channel angular pressing (ECAP) processes on the fatigue behavior of AM60 magnesium alloy

The fatigue behavior of AM60 magnesium alloy produced by equal channel angular pressing (ECAP) process was investigated by means of fatigue life (S–N) and fatigue crack propagation rate tests. The grain structure of the material was refined from 19.2μm to 2.3μm after 6 passes of ECAP at 220°C. The yield stress (YS) and ultimate tensile strength (UTS) of the alloy increased to their peak values after two passes but decreased with any further pressing, although the grain size became finer with increasing the number of passes. This is owing to the fact that the softening effect caused by texture anisotropy overwhelmed the strengthening effect due to grain refinement. The texture together with the plasticity-induced crack closure were also the main factors influencing the crack growth rate of the ECAPed Mg alloys. A large enhancement in the fatigue lifetime was also achieved after the 2nd pass of ECAP. This result was remarkable because the improvement was obtained for both the low and high cycle regimes. The present research work has shown that two passed AM60Mg alloy was better than the multi-passed material in view of the fatigue performance.

Enhanced fatigue durability of Al–6 Mg alloy by applying ultrasonic impact peening: Effects of surface hardening and reinforcement with AlCuFe quasicrystalline particles

Composite layer reinforced with quasicrystalline (QC) Al63Cu25Fe12 particles was fabricated on the surface of Al–6Mg alloy specimens by ultrasonic impact peening (UIP). Stress-controlled fatigue response of the specimens was studied and compared with those for the annealed and UIP-treated specimens. The notch effect of the UIP induced surface roughness calculated in terms of the stress concentration factor does not exceed∼10%. XRD, OM and SEM analyses were used to characterize formed surface layers and fatigue fracture surfaces. Surface composite layer of 40–50μm thick contains the homogeneously dispersed QC particles (the volume fraction Vf∼0.15) trapped by high compressive residual stresses. The layer demonstrates almost triple increase in microhardness comparing to that for the annealed alloy and twice exceeding of that for the UIP-treated specimen. Superior fatigue endurance of Al–6Mg alloy after the UIP process and the UIP-induced formation of the composite layer is explained by sub-surface fatigue cracks’ initiation promoted with high compressive residual stresses and tight interfacial bonding of QC reinforcement and the matrix alloy. The improved fatigue behavior of the UIP-treated specimens in both the low cycle and high cycle regimes can be ensured by combination of the following favorable characteristics: (i) sufficiently high ductility and resistance to fatigue damage and crack growth in the core parent material along with (ii) superior fatigue strength supported by high microhardness and compressive stresses in the surface layer, which contains fine grained matrix and/or uniformly distributed and tightly bonded QC reinforcements.

Fatigue Properties of 316L Stainless Steel

The present paper deals with experimental studies on the tension-tension fatigue properties of 316L stainless steel by using a servo-valve controlled electro-hydraulic testing machine at room temperature. The low cycle fatigue properties of 316L stainless steel were studied and the initiation mechanisms of fatigue cracks were investigated and analyzed with scanning electron microscopy (SEM). Preliminary results indicate that the S-N curve of 316L stainless steel descends linearly in the low cycle regime and fatigue failure initiates from inclusions/defects on the specimen surface.

Enhancement of fatigue life of polyurethane composites containing carbon nanotubes

The effects of carbon nanotube (CNT) inclusion on cyclic fatigue behavior and the tensile properties of polyurethane (PU) composites have been studied. Tension–tension cyclic fatigue tests were conducted at various load levels (30–50MPa) to establish the relationship between stress and the number of cycles to failure (S–N curves). The tensile energy to break PU composites was enhanced up to 38% by using a low amount of CNTs. In addition, the incorporation of CNTs increased the fatigue life of PU in the high-stress amplitude, low-cycle regime by up to 248%. Micrographs show highly dispersed CNTs and indicate the key mechanisms for enhancement in fatigue life such as CNT crack-bridging and pull-out. The tensile-tensile fatigue properties obtained in this work show that PU systems can outperform epoxy systems widely used in structural applications.

Very high cycle fatigue behavior of bridge steel welded joint

Very high cycle fatigue (VHCF) behaviors of bridge steel (Q345) welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = −1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 107 cycles. The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone (HAZ) in the very high cycle regime. The fatigue fracture surface was analyzed with scanning electron microscopy (SEM), showing welding defects such as pore, micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints. The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.

Spot weld arrangement effects on the fatigue behavior of multi-spot welded joints

In the present study, the effects of spot weld arrangements in multi-spot welded joints on the fatigue behavior of the joints are studied. Three different four-spot welded joints are considered: one-row four-spot parallel to the loading direction, one-row four-spot perpendicular to the loading direction and two-row four-spot weld specimens. The experimental fatigue test results reveal that the differences between the fatigue lives of three spot welded types in the low cycle regime are more considerable than those in the high cycle regime. However, all kinds of spot weld specimens have similar fatigue strength when approaching a million cycles. A non-linear finite element analysis is performed to obtain the relative stress gradients, effective distances and notch strength reduction factors based on the volumetric approach. The work here shows that the volumetric approach does a very good job in predicting the fatigue life of the multi-spot welded joints.

Low-cycle fatigue behavior of SiC p/Al–Si composites produced by spray deposition

In this research, the low-cycle fatigue life and cyclic stress response of SiC p/Al–Si composites produced by spray deposition were investigated in comparison to the unreinforced alloy. Both the composite and matrix alloy display cyclic hardening under total-strain amplitude 0.35–0.5%. However, under low strain amplitude (0.3%), the Al–Si alloy shows cyclic stable after the initial hardening for 200 cycles. In contrast, the composite displays cyclic hardening immediately at the beginning of cyclic loading and a secondary hardening is also observed prior to fatigue fracture. Due to the higher density of pre-existing dislocations, interaction of mobile dislocations with the reinforcing SiC particles and more dislocation–dislocation interactions, a more pronounced strain-hardening behavior was observed for the composite. Moreover, both composite and matrix alloy follow the Coffin–Manson relationship. Fatigue damage parameter indicates the inferior life in the low-cycle regime for the composite, compared to the unreinforced alloy. Fractographic analysis shows that particle/matrix debonding is the main mechanism of failure in the SiC p reinforced composite.

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension–compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 10 7 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension–compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.

Low cycle lifetime assessment of Al2024 alloy

The 2024-T351 aluminum alloy is extensively used for fabricating aircraft parts. This alloy shows a relatively low ductility at room temperature and is generally heat treated in various conditions to suit particular applications. The present study experimentally and numerically analyzes the damage mechanism of an Al2024-T351 plate (short transverse direction) subjected to multi-axial stress states. The purpose of this work is to predict the cyclic lifetime of the considered alloy, based on the local approach of damage evolution using continuum damage modeling (CDM). The experimental program involves different kinds of specimens and loading conditions. Monotonic and cyclic tests have been conducted in order to measure the mechanical response and also to perform micromechanical characterization of damage and fracture processes. The cyclic plasticity behavior has been characterized by means of smooth cylindrical specimens. For analyzing the evolution of plastic deformation and damage under multi-axial stress conditions, cyclic loading tests in the low cycle regime have been conducted on different round notched bars. The predictions of the CDM were compared to the experimentally observed mechanical response and to the micromechanical characterization of damage. Emphasis was placed on the prediction of the number of cycles to failure.

1114 マグネシウム合金押出材のギガサイクル疲労特性(OS11-4 材料の疲労挙動と損傷評価-超長寿命-)

In order to investigate the high cycle fatigue properties of extruded ZK60 and AZ80 magnesium alloys, axial tension-compression (R=1) fatigue tests were carried out at room temperature. The duplex S-N characteristics containing of different S-N curves were obtained for both alloys. The fatigue strengths at 10^9 cycles of ZK60A and AZ80A magnesium alloys were 120MPa and 110MPa, respectively. The fatigue strengths were lower than that of 0.2% proof stresses for both alloys. The fatigue crack initiated at the specimen surface in low cycle regime. On the other hand, the fatigue crack initiated at the interior Al-Mn system inclusion in very high cycle regime.