Average Fatigue Strength Research Articles

Competition between microstructure and defect in multiaxial high cycle fatigue

This study aims at providing a better understanding of the effects of both microstructure and defect on the high cycle fatigue behavior of metallic alloys using finite element simulations of polycrystalline aggregates. It is well known that the microstructure strongly affects the average fatigue strength and when the cyclic stress level is close to the fatigue limit, it is often seen as the main source of the huge scatter generally observed in this fatigue regime. The presence of geometrical defects in a material can also strongly alter the fatigue behavior. Nonetheless, when the defect size is small enough, i.e. under a critical value, the fatigue strength is no more affected by the defect. The so-called Kitagawa effect can be interpreted as a competition between the crack initiation mechanisms governed either by the microstructure or by the defect. Surprisingly, only few studies have been done to date to explain the Kitagawa effect from the point of view of this competition, even though this effect has been extensively investigated in the literature. The primary focus of this paper is hence on the use of both FE simulations and explicit descriptions of the microstructure to get insight into how the competition between defect and microstructure operates in HCF. In order to account for the variability of the microstructure in the predictions of the macroscopic fatigue limits, several configurations of crystalline orientations, crystal aggregates and defects are studied. The results of each individual FE simulation are used to assess the response at the macroscopic scale thanks to a probabilistic fatigue criterion proposed by the authors in previous works. The ability of this criterion to predict the influence of defects on the average and the scatter of macroscopic fatigue limits is evaluated. In this paper, particular emphasis is also placed on the effect of different loading modes (pure tension, pure torsion and combined tension and torsion) on the experimental and predicted fatigue strength of a 316 stainless steel containing artificial defect.

Additively manufactured porous tantalum implants

The medical device industry’s interest in open porous, metallic biomaterials has increased in response to additive manufacturing techniques enabling the production of complex shapes that cannot be produced with conventional techniques. Tantalum is an important metal for medical devices because of its good biocompatibility. In this study selective laser melting technology was used for the first time to manufacture highly porous pure tantalum implants with fully interconnected open pores. The architecture of the porous structure in combination with the material properties of tantalum result in mechanical properties close to those of human bone and allow for bone ingrowth. The bone regeneration performance of the porous tantalum was evaluated in vivo using an orthotopic load-bearing bone defect model in the rat femur. After 12weeks, substantial bone ingrowth, good quality of the regenerated bone and a strong, functional implant–bone interface connection were observed. Compared to identical porous Ti–6Al–4V structures, laser-melted tantalum shows excellent osteoconductive properties, has a higher normalized fatigue strength and allows for more plastic deformation due to its high ductility. It is therefore concluded that this is a first step towards a new generation of open porous tantalum implants manufactured using selective laser melting.

Effect of the Rheocasting Process and of the SLS Layer on the Fatigue Behavior of 357 Aluminum Alloy

Two objectives were targeted: 1) compare the high cycle fatigue behavior of rheocast aluminum alloy 357 prepared by the swirl enthalpy equilibration device (SEED) and by the Council for Scientific and Industrial Research (CSIR) process, and 2) study the effect of surface liquid segregation (SLS) on the fatigue behavior of the CSIR material. Rectangular hourglass specimens machined from rheocast plates were tested at four stress amplitudes in axial fatigue with a stress ratio of R = -1 and a frequency of 20 Hz. Results obtained for SLS free specimens show that the SEED and the CSIR processes produce rheocast materials with comparable high cycle fatigue properties, 115 MPa at 107 cycles. In order to study the influence of surface liquid segregation, slightly polished specimens with a remaining SLS of nearly 750 microns thick were also tested. According to the results, the SLS reduces the average fatigue strength by approximately 5% (110 MPa vs. 115 MPa at 107 cycles). For SLS free specimens, the fatigue crack initiated at shrinkage cavities, oxide films or in the alpha globules. On the other hand, for specimens with SLS, no crack initiation in the alpha globules was observed. The main crack initiation mechanism was identified to be a deformation incompatibility between regions characterized by higher silicon content compared to nominal eutectic regions. The originality of the work is provided by the rigorous comparative analysis of the fatigue performance of components produced in two different rheocasting facilities, but tested in a single laboratory. It is also the first fundamental research published on the mechanical effect of surface liquid segregation. It confirms that SLS should be removed in critical areas in order to optimize the fatigue resistance of rheocast components.

Low temperature degradation and reliability of one-piece ceramic oral implants with a porous surface

Low temperature degradation of zirconia (3Y-TZP) oral implants and its effect on fatigue reliability is poorly documented. ObjectiveThe aim of this investigation was to follow the aging process occurring at the surface of implants exhibiting a porous coating and to assess its influence on their mechanical (fatigue) properties. MethodsTetragonal to monoclinic transformation (t–m) was evaluated during accelerated aging tests up to 100h in autoclave (134°C, 2 bars) by X-ray diffraction (XRD) and focused ion beam (FIB). A series of implants were steam-aged for 20h before fatigue testing. Such temperature–time conditions would correspond roughly to 40 years in vivo. The aged specimens and a non-aged control group were step-stress fatigued until failure or survival. ResultsThe evolution of XRD surface monoclinic content was slow, i.e. 16% and 35% for 20 and 100h respectively. However, FIB revealed a significant transformation, initiated at the interface between the porous layer and the bulk, preferentially growing towards the bulk. FIB is therefore better indicated than XRD to follow aging in such implants. Higher average fatigue strength (aged 1235N versus non-aged 826N) and reliability levels were observed for the 20h aged group. SignificanceAfter aging for durations compatible with clinical use, 3Y-TZP with porous surface presented higher fatigue performance. This is in contrast to previous studies where loss of strength due to aging was often reported. Generalizations must therefore be avoided when considering aging of zirconia dental products and every new material/process combination should be tested before drawing conclusions.

Influence of Surface Integrity of 15-5PH on the Fatigue Life

The effect of surface integrity parameters on fatigue life has been investigated. Various surfaces have been manufactured with a controlled surface integrity in order to achieve fatigue test and to compare the influence of each parameters independently (residual stresses, surface roughness, and microstructure). The difficulty to reproduce the same surface integrity on each sample had been pointed out. It is necessary to characterize the surface of all the samples before testing and take it into account in the fatigue test procedure. Staircase tests have been carried out to determine the average fatigue strength at 2.106 cycles for each case of surface integrity. First results show that very good surface finish conditions combined with compressive residual stresses are beneficial.

Propagation of short fatigue cracks in permanent and semi-solid mold 357 aluminum alloy

The high cycle fatigue strengths of aluminum–silicon–magnesium 357 alloy prepared by permanent molding and semi-solid molding are measured. Results show that precipitation hardened permanent mold (PM) and semi-solid mold (SSM) materials have fatigue strengths at 10 7 cycles of 82 MPa and 113 MPa respectively at R = −1, and of 47 MPa and 78 MPa respectively at R = 0.1. For all tested specimens, the propagation of short cracks occupies a significant portion of the fatigue life. Observation of many short cracks using the replication technique indicates that grain boundaries are important microstructural barriers to short crack propagation in these materials. It is proposed that the fatigue strength at 10 7 cycles of the studied materials is the stress amplitude under which a short crack will be stopped by the first grain boundary encountered. In the absence of defects bigger than the grain size, the average fatigue strength is inversely proportional to the square root of the average grain size.

Tension–Tension Fatigue Behavior of NextelTM720/Alumina under Combustion Environment

Fatigue behavior of Nextel720™/alumina was investigated under combustion environment, which was then compared with that in laboratory environment. Fatigue strength for 90,000 cycles was equal to 150 MPa under combustion condition, while 170 MPa in laboratory environment. However, normalized fatigue strength (ratio of fatigue strength and ultimate tensile strength) was equal in combustion and laboratory environments. The difference in fatigue strength was due to thermal stresses from impingement of combustion flame on one side of specimen. Fractographic analysis showed that combustion had little effect on fiber/matrix bonding in only flame impingement region. Combustion environment was not thus damaging to Nextel720™/alumina.

Fatigue properties and micromechanism of fracture of an alsimg0.6 cast alloy used in diesel engine cylinder head

The increase usage of casting aluminum–silicon alloys in the automotive industry is due to reduce weight, fuel consumption, and emission levels. It includes the aluminum-silicon cast alloy EN AlSiMg0.6 (ASTM A357.0) which is used to make diesel engine cylinder head. It is important to know the impact on the integrity and reliability of this component in the presence of intrinsical defects of conventional casting parts produced on permanent mold process. Such defects, as porosity and oxide film, when located on the surface or subsurface of casting parts, can be fatigue crack initiators. In this paper, the fatigue strength and micromechanisms of fracture is analyzed using 7×14×60 mm specimens machined from cylinder head drew from production assembly line, and submitted to fatigue three point bending tests. Fracture surface of the fatigue specimens were analysed by SEM to characterize the micromechanism and the initiation fracture local. The average fatigue strength, based on 106 cycles, was about 140MPa. It was observed on the fracture surface of fatigue test specimens, a clear contrast between the micromechanism of fatigue zone (striations) and the final fracture zone (dimples), and fatigue crack initiation occurs at the porosities near the surface.

疲労強度分布に注目したＳ‐Ｎ線図の統計的決定法に関する研究

Since the experimental results of fatigue test show large scatter, the S-N curve should be determined after the evaluation of them. In the previous paper, we have proposed a decision method for the S-N curve based on the fatigue strength distribution. In the proposed method, we have applied bilinear regression model for the data in the stress level around the fatigue limit. However, the S-N curves for some materials don't show obvious knee point. In this case, curved regression models such as hyperbola regression model are available. In this paper, we improve the proposed method to be applicable for such types of materials. In the developed method, 8 types of regression models, which are proposed by the committees on fatigue and reliability engineering in JSMS, are available, In addition, we can also decide the best-fitted model using the correlation for the plots of normalized fatigue strength on the probability paper, which are used for the evaluation of the fatigue strength distribution The proposed method has been applied to the fatigue test data for some materials. As a result, it is recognized that the reasonable S-N curves are obtained.

耐久限度を有するＳ‐Ｎ線図の統計的決定法

The experimental results of fatigue test near the stress levels around fatigue limit have large scatter. In this paper, we propose the statistical method for decision of the S-N curve considering an effect of them.The procedure of proposed method is as follows. Firstly, all fatigue test data are separated into an inclined part and a horizontal part with an arbitrary number of cycles as a boundary. Secondly, the distributions of fatigue strength are estimated with computational probability paper, and agreement of the distribution of both parts is searched by changing the boundary number of cycles. Finally, the optimum separation is the one for which the agreement of distribution reaches its maximum. Because each fatigue data is transformed into normalized fatigue strength, this method has the advantage point that the distribution of fatigue strength and the fatigue limit can be found by relatively small number of samples.As an example, the proposed approach has been applied to the fatigue test data for some materials. The well accuracy of the proposed S-N curves was proved by comparison of the obtained fatigue limit and calculational result by Probit analysis. Furthermore, the approach can be also applied to the materials which doesn't show obvious fatigue limit, such as CFRP, because non-existence of fatigue limit can be judged.

A statistical model of fatigue damage evolution in particulate‐reinforced metal‐matrix‐composites

A statistical model of fatigue damage evolution has been developed for particulate‐reinforced metal‐matrix‐composites (MMCs) by taking into considerations both the initial damage distribution and the effect of particulate reinforcement on fatigue damage development. The growth of microscopically fatigue‐damaged regions in particulate‐reinforced MMCs is considered as a stochastic process, and both the non‐equilibrium statistical method and minimum strength principle are used to establish the evolution equation of fatigue damage. The fatigue damage evolution equation developed in the present study characterizes not only the kinetic process of fatigue damage evolution but also sets up the relationship between the mechanism of fatigue damage growth of the microscopically damaged regions and the result of fatigue damage, i.e. degradation of mechanical properties of particulate‐reinforced MMCs. A new expression for calculating the cumulative fatigue damage and a new formula for predicting the average fatigue strength of the particulate‐reinforced MMCs are derived. Experimental data of 2080Al/SiCp composites are analysed and compared with results obtained with the present model. It is shown that the experimental results can be described well by the calculations.

Effects of conventional machining on high cycle fatigue behavior of the intermetallic alloy Ti47Al2Nb2Cr (at.%)

Abstract The TiAl based alloy, Ti47Al2Nb2Cr (at.%), was used as a model system to explore effects of machining-induced surface deformation on fatigue behavior of an intermetallic alloy with limited ductility. Conventional machining processes, such as grinding and turning, harden TiAl to depths ranging from 40 to 180 μm. Turning doubled the hardness to approximately 500 VHN in the outer 20 μm, and hardness increased at least 50 VHN to a depth of 180 μm. The deformed layer formed during machining recrystallized after 1 h at 760°C. Axial fatigue tests were performed by step loading every 10 6 cycles through a fixed set of stress levels until failure. At room temperature, mean fatigue strength was not affected by surface condition. The outer 20 μm of the electropolished surface hardened to the same level as the turned samples during the fatigue test. After hardening, these two surfaces would have similar crack initiation resistance and, therefore, similar fatigue strengths. At 760°C, turning improved the average fatigue strength by 5%, and the average life at the final stress level by about 1.5 orders of magnitude. Fatigue resistance of the turned samples was improved by formation of a continuous, crack initiation resistant, recrystallized layer in the outer 30–50 μm during the test.

耐久限度を考慮した疲労設計基準決定システムの構築

The experimental results of fatigue at low stress levels around the fatigue limit have large scatter. For the design of structures and machines, it is important to evaluate the scatter and to decide the design margin. In this paper, the statistical method to decide the fatigue limit is proposed. In the proposed method, the fatigue limit is decided with considering the distribution of fatigue strength. Because each fatigue data is transformed into normalized fatigue strength, this method has the advantage point that the distribution of fatigue strength and the fatigue limit can be found by relatively small samples. Using the proposed method, the computer system to decide the design margin for fatigue is developed. In this system, the S-N curve with fatigue limit and the distribution of fatigue strength are obtained at first. Then the design margin with a given probability of failure and a confidence level is calculated.The developed system is applied to some experimental results for fatigue test. As a result, the S-N curve with the design margin has obtained easily. It is recognized that the obtained S-N curve is decided after consideration of scatter.

Fatigue Behavior of a Plain-Woven Glass Fabric Laminate under Tension/Torsion Biaxial Loading

Fatigue strength and stress-strain response of a plain-woven glass fabric laminate subjected to pulsating tension/pulsating torsion biaxial loading were investigated. Biaxial loads were proportionally applied. S-N curves at different biaxial stress ratios were obtained. For the S-N diagram, the normalized fatigue strength defined by the ratio between the cyclic biaxial stress and the static strength, was used. All data roughly locate on a slightly wide band on the normalized S-N diagram, although the tendency that the slope of the S-N curve becomes low with an increase of the shear stress component is distinguishable. Modulus decays both in tension and shear with respect to loading cycles under biaxial loading are almost the same as those under uniaxial tension and pure torsion loadings, respectively. The direction of matrix cracks is dependent on the principal stress direction under biaxial loading although it does not coincide with the direction of the principal plane.

Fatigue Behavior of Plain Woven Glass-Fabric Laminates under Tension/Torsion Combined Loading. Effect of Shear Stress and Cyclic Condition on Fatigue Failure.

Fatigue strength and stress-strain response of plain woven glass fabric laminates subjected to combined loadings with pulasating tension and alternate torsion were investigated in this study. To distinguish the difference of fatigue failure behavior between that under pulsating torsion loading and that under alternate torsion loading, the alternate torsion test was also conducted. Fatigue strength and stress-strain response under alternate torsion loading were compared with that under pulsating torsion loading. From the experiment, it is found that the normalized fatigue strength (which is cyclic biaxial stress divided by the static strength) under pulsating tension/alternate torsion combined loading is dependent on the biaxial stress ratio. The fatigue strength under alternate torsion loading is much lower than that under pulsating torsion loading. The stress-strain response also shows nonlinear behavior more strongly than that under pulsating torsion loading.

Fatigue Behavior of Plain Woven Glass Fiber Laminates under Pulsating Tension/Pulsating Torsion Combined Loading.

Fatigue Strength and stress-strain response of plain woven glass fibric laminates subjected to pulsating tension/pulsating torsion combined loading were studied in this paper. In order to evaluate the biaxial fatigue failure strength, the normalized fatigue strength that a cyclic biaxial stress is divided by the static strength was used in S-N curves. According to the test results, it is found that all data locate in a slightly wide band on the S-N plot in spite of different biaxial stress ratios, but the tendency that the slope of the S-N curve becomes low in case of a high shear stress component, can be distingushable. The tendency of modulus decays in both tension and shear even under combined loading is almost the same as those under uniaxial tension and pure torsion loadings, respectively. The direction of matrix cracking seems dependent on the principle stress direction under pulsating tension/pulsating torsion combined loading although it is also strongly influenced by fiber orientation.

Fatigue Strength of Deteriorated Steel Highway Bridges

The effect of long-term environmental exposure and an extended history of highway loading was quantitatively determined for steel bridge beams that had been in service more than 50 years. Fatigue tests were conducted on specimens removed from the tension flange of truss stringers salvaged from four bridge replacement projects. Based on the fatigue strength reduction factor \IK\df\N determined on a basis of mean fatigue life for all specimens in one bridge, the fatigue sensitivities encountered were less than that associated with an AASHTO Category C welded connection detail. For individual specimens, the maximum \IK\df\N value determined was 3.0. At 36-ksi (248-MPa) stress range, average specimen lives exceeded 300,000 cycles for all bridges, for both center-span (maximum prior stress history) and near-support (insignificant prior stress history) specimens. The corrosion deterioration experienced by the plain (nonwelded) simple-span steel bridge beams in this study caused the average fatigue strength to be reduced by approximately one AASHTO fatigue category.