Biaxial Fatigue Tests Research Articles

Simulation of the scenario of the biaxial wheel fatigue test

This paper presents a new approach to the virtualization of the scenario of the biaxial wheel fatigue test. This test is the state-of-art and the standard requirement for the validation of vehicle wheels. During this test, tire and wheel specimens run inside an inner drum while standardized vertical and horizontal loads are applied. Thus, the scenario of this test can be modeled in three levels: the multibody dynamics of the test facility, the wheel/tire/inner drum contact, and the analysis of the flexible wheel. In the proposed virtualization, the multibody dynamics of the test facility was implemented in MSC.ADAMS. The wheel/tire/inner drum contact was simulated by means of CDTire; as it works parallel to MSC.ADAMS, one single co-simulation could perform the tire dynamics and the test facility dynamics. Finally, the wheel strains were calculated by an ABAQUS simulation, which received the tire/wheel load data from the simulation in MSC.ADAMS and CDTire. A physical test facility and a physical wheel specimen were instrumented, allowing the comparison between acquired and simulated data. The use of this specialized software is a novelty in the virtualization of the scenario of this test; furthermore a high detailed simulation is required for the further development of such already well established test procedure.

Multiaxial fatigue: From materials testing to life prediction

Fatigue damage has special relevance on the life span of mechanical components and structures, as it takes responsibility for a large majority of the registered structural failures in service. The continuous need for the weight reduction and improved design optimization of components and a growing need for greater lifespans of equipment, forced the understanding of the fatigue behavior of materials either under multiaxial loading cycles or under increased number of loading cycles. Fatigue tests were always performed under uniaxial fatigue loads. However, it is generally recognized that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. In the following, a wide range of multiaxial fatigue failure criteria have been developed that may be applied to different metallic materials. These failure criteria may be classified as strain based for high strain loading conditions or stress based for higher lifespans in the elastic loading regime. These criteria are nowadays used in design optimization also contributing to the weight reduction and improved lifespans of modern equipment. Present manuscript is a review of the main specimens and equipment where research can be carried out for performing multiaxial fatigue tests under bi-axial stress states. Being a personal view, it will be based on the author experience on the performance of multiaxial fatigue tests under axial/torsion loading using mechanical actuated or servo-hydraulic fatigue testing machines or biaxial fatigue tests using electromagnetic actuators in cruciform specimens. Further research is needed and is being carried out concerning multiaxial very high cycle fatigue in order to achieve better improvements in weight reduction of components and structures and also in complex variable amplitude loading with adequate damage accumulation models.

Comparison of multiaxial fatigue parameters using biaxial tests of Waspaloy

In laboratories most fatigue testing is uniaxial as many aeronautical components such as wing spars are uniaxially loaded. However important components, such as aero engine disks, operate under multiaxial loading conditions. Hence multiaxial data is desirable to safely design them. This paper describes biaxial fatigue tests carried out under a range of loading conditions with cruciform specimens made of Waspaloy, a nickel based superalloy widely used in the fabrication of aero engine disks. Finite Element modeling of the specimen was performed to set the test conditions and to predict stress-strain fields and failure location. In order to investigate the effect of strain biaxiality, different biaxiality ratios with the same maximum principal stress were considered. Results demonstrate a variation in total life to failure and failure location, confirming the FE predictions. Then the experimental results obtained were used with three groups of multiaxial criteria. Initially, von Mises and elastic strain energy density yield theories were extended to the multiaxial fatigue context. Secondly, the stress invariant based criterion proposed by Crossland was considered, where the maximum shear stress amplitude was calculated through the Minimum Circumscribed Circle and the Maximum Prismatic Hull methods. Lastly, the critical plane approach was investigated by considering the propositions made by Findley and Matake. In general all the criteria probed here gave good predictions. As for the first group, better correlation was obtained with the energy parameter. For the last two groups, Crosslands formulation performed the best, with close agreement with experimental observations. The critical plane criteria gave conservative predictions for uniaxial stress cases and non-conservative for the other assessed conditions. In addition, both criteria misbehaved outside their calibration region, i.e. outside the tension-torsion region, and were unable to properly describe equi-biaxial conditions.

Multiaxial fatigue investigation of titanium alloy annular discs by a vibration-based fatigue test

A novel vibration-based fatigue experimental system has been established to study the fatigue properties of titanium alloy annular discs in near resonance conditions. The excitation frequency-response curves are experimentally obtained in order to clarify the large amplitude nonlinear vibration characteristics of the discs. Then a vibration-based fatigue test of the discs and the corresponding 10° segment specimens are conducted respectively to obtain the S-N curves and the fatigue limit stresses. A multiaxial fatigue criterion is subsequently proposed to predict the fatigue properties of the discs, which is compared with the present experimental results finally. It is demonstrated that the high-cycle fatigue properties of the discs can be obtained by the present vibration-based fatigue test, which is an in-phase biaxial fatigue test. The fatigue limit stress of the discs can be predicted well by the proposed elliptic multiaxial fatigue criterion together with the fatigue testing results of the corresponding segment specimens.

Equi-biaxial loading effect on austenitic stainless steel fatigue life

Fatigue lifetime assessment is essential in the design of structures. Under-estimated predictions may result in unnecessary in service inspections. Conversely, over-estimated predictions may have serious consequences on the integrity of structures. In some nuclear power plant components, the fatigue loading may be equibiaxial because of thermal fatigue. So the potential impact of multiaxial loading on the fatigue life of components is a major concern. Meanwhile, few experimental data are available on austenitic stainless steels. It is essential to improve the fatigue assessment methodologies to take into account the potential equi-biaxial fatigue damage. Hence this requires obtaining experimental data on the considered material with a strain tensor in equibiaxial tension. Two calibration tests (with strain gauges and image correlation) were used to obtain the relationship between the imposed deflection and the radial strain on the FABIME2 specimen. A numerical study has confirmed this relationship. Biaxial fatigue tests are carried out on two austenitic stainless steels for different values of the maximum deflection, and with a load ratio equal to -1. The interpretation of the experimental results requires the use of an appropriate definition of strain equivalent. In nuclear industry, two kinds of definition are used: von Mises and TRESCA strain equivalent. These results have permitted to estimate the impact of the equibiaxiality on the fatigue life of components.

A novel method of strain - bending moment calibration for blade testing

A new method of interpreting strain data in full scale static and fatigue tests has been implemented as part of the Offshore Renewable Energy Catapult's ongoing development of biaxial fatigue testing of wind turbine blades. During bi-axial fatigue tests, it is necessary to be able to distinguish strains arising from the flapwise motion of the blade from strains arising from the edgewise motion. The method exploits the beam-like structure of blades and is derived using the equations of beam theory. It offers several advantages over the current state of the art method of calibrating strain gauges.

Numerical study of in-plane biaxial fatigue crack growth with different phase shift angle loadings on optimal specimen geometries

Nowadays for real world applications, mechanical components in the automotive, aerospace, aeronautical and other industries are subjected to multiaxial loading conditions. Although the fatigue behavior of materials like steel alloys, aluminum alloys or even magnesium alloys is fairly well established for uniaxial loading, one should not use this knowledge for biaxial loading. Developing new testing machines and new specimen geometries have been the previous goal of several authors. A new generation of smaller and more efficient biaxial fatigue testing machines has arrived on the market. Using electrical motors these machines are not able to achieve the higher loads as their hydraulic counterparts can, and therefore the cruciform specimen needs to be optimized. The authors have previously optimized the cruciform geometry for biaxial fatigue initiation, using a revolved spline to reduce the specimen center thickness. For crack initiation experimental results have proven that the obtained design detail is effective, but there are no studies about the behavior of these specimens for crack propagation. In this paper the authors firstly set out to determine the conditions for crack initiation using traditional criteria like Findley, Brown-Miller, Fatemi-Socie, Smith, Watson and Topper (SWT), Liu I and Chu, as a function of different biaxial loading with phase differences. On a second stage the authors compared the biaxial fatigue crack propagation on the optimal specimens, with the behavior of notched specimens, using the stress intensity factors for mode I and mode II. Several crack and loading parameters were studied, including the starting crack length and angle, and different loading paths. Different biaxial loadings were applied to the model, including 30°, 45°, 60°, 90° and 180° out-of-phase angles. Very similar small crack propagation parameters were obtained for both specimens, although as the crack growths the stress intensity factor for the optimal specimen do not behave as expected. Therefore limiting the use of this specimen for crack propagation.

Strain measurements on specimens subjected to biaxial ultrasonic fatigue testing

The very high cycle region, up to 10E10 cycles of the S-N fatigue curve has been the subject of intensive research on the last decades, with special focus on axial, bending, torsional and fretting fatigue tests. This can be achieved using ultrasonic exciters which allow for frequency testing of up to 30kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a special designed cylindrical specimen is tested at 20kHz on a bi-axial loading condition, with axial and torsional components on the specimen throat, using a new biaxial ultrasonic fatigue testing machine. The characterization of the specimen loading is measured by rotational and axial laser transducers at the bottom of the specimen and additionally by strain gauges applied at its throat. Strain gage data from very high frequency excitation is compared to strain gage data obtained from equivalent specimen geometry at 0.5Hz in a biaxial servohidraulic machine. Results indicate good correlation between strain measurements for both fatigue tests at low and ultrasonic frequency tests. This data will help to extend very high cycle fatigue curves for biaxial loading conditions and serve as groundwork for future research on this field.

Characterization of the biaxial fatigue behaviour on medium carbon steel using the strain-life approach

This study aims to investigate the fatigue behaviour and determine the fatigue life prediction under biaxial loading. Fatigue tests were performed according to ASTM 2207-02 and the values of tensile stresses are selected from the ultimate tensile strength which is 0.5 Su, 0.6 Su, 0.7 Su, 0.8 Su and 0.9 Su, while the torsion angle representing the shear stresses acting was set at 15 degrees. The biaxial fatigue test was conducted using a combination of two types of stresses acting on the same frequency, namely 1 Hz, on smooth specimens made from medium carbon steel. The biaxial fatigue lives of the specimens are recorded when the specimen has completely fractured. The results indicate that the observed fatigue lives are in good agreement with the predicted lives by using the Coffin–Manson, Morrow, and Smith–Watson–Topper strain-based models. Mohr’s circle approach was used to determine the maximum shear stress and principal normal stress. The maximum shear stress increased from 457 MPa to 486 MPa with the increment of principal normal stress from 612 MPa to 767 MPa. The principal stresses, maximum shear stresses, and energy dissipated were used to explain and describe the behaviour of biaxial fatigue. Both stresses are inversely proportional to the fatigue life. Meanwhile, the energy is linearly proportional to the stress applied, where the values increase in the range 500 kJ/m3 to 605 kJ/m3 . Thus, the basic understanding of the material behaviour may be used in the processes of declaring component service lives and the fatigue life prediction of a particular automotive component. Therefore, the cost incurred can be reduced for the development process in material engineering.

Development of a Very High Cycle Fatigue (VHCF) multiaxial testing device

The very high cycle region of the S-N fatigue curve has been the subject of intensive research on the last years, with special focus on axial, bending, torsional and fretting fatigue tests. Very high cycle fatigue can be achieved using ultrasonic exciters which allow for frequency testing of up to 30 kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a device designed to produce biaxial fatigue testing using a single piezoelectric axial exciter is presented, as well as the preliminary testing of this device. The device is comprised of a horn and a specimen, which are both attached to the piezoelectric exciter. The steps taken towards the final geometry of the device are presented. Preliminary experimental testing of the developed device is made using thermographic imaging, strain measurements and vibration speeds and indicates good behaviour of the tested specimen.

Biaxial fatigue tests of notched specimens for AISI 304L stainless steel

High cycle fatigue tests were conducted for stainless steel AISI 304L. The geometry was a thin walled tube with a passing through hole. The tests were axial, torsional and in-phase axial-torsional, all of them under load control with R = ?1. The S-N curves were constructed following the ASTM E739 standard and the fatigues limits were calculated following the method of maximum likelihood proposed by Bettinelli. The crack direction along the surface was analysed, with especial attention to the crack initiation zones. The notch fatigue limits for different hole diameters were compared with the predictions done with a microstructural fracture mechanics model.

Fatigue crack growth analysis of T junction under biaxial compressive–compressive loads

The objective of the present research is to analyze systematically the effect of biaxial compressive–compressive stresses on crack growth behavior of T junctions. Pressure structures have many components which are designed to be compressively stressed. This paper describes and summarizes biaxial compressive fatigue crack initiation and propagation characteristics through several T junctions under biaxial fatigue tests. Unlike uniaxial compressive fatigue, multiaxial compressive–compressive fatigue has the intricate characteristics of multi-point crack initiating, short crack group growth behavior and long crack propagating behavior. Moreover, the effective stress intensity factor range Keff and the crack opening threshold Kop,max are used to explain the crack growth under biaxial loads instead of the Paris law. Predictions of fatigue crack growth behavior based on the extended McEvily formula agree well with experimental observations.

Numerical study of fatigue crack initiation and propagation on optimally designed cruciform specimens

A new generation of smaller and more efficient biaxial fatigue testing machines has arrived on the market. Using electrical motors these machines are not able to achieve the higher loads their hydraulic counterparts can, and therefore the cruciform specimen needs to be optimized. Following the authors previous work, several different optimal specimens’ configurations were produced, using the base material sheet thickness as the main design variable. Every design variable was optimized in order to produce the highest stress level on the specimen center, while the stress distribution is still uniform on a 1 mm radius of the specimen center. Also it was guaranteed that the stress level on the specimen arms was always considerably lower, in order to achieve failure at the specimen center. In this paper traditional criteria like Findley, Brown-Miller, Fatemi-Socie, Smith, Watson e Topper (SWT), Liu I and Chu were considered to determine crack initiation direction for several loads in this biaxial in-plane specimens. In order to understand the fatigue propagation behavior, the stress intensity factors for mode I and mode II was determined for different cracks introduced on the geometry. Several crack and loading parameters were studied, including the starting crack length and angle, and different loading paths. Several biaxial loads were applied to the model, including 30°, 45°, 60°, 90° and 180° out-of-phase angles.

Simulation of biaxial wheel test and fatigue life estimation considering the influence of tire and wheel camber

The traditional fatigue test of wheel comprising the radial and cornering fatigue tests cannot simulate the real stress state of wheel well. Biaxial wheel fatigue test combining these two traditional tests has become an internationally recognized method that can reproduce the real loading condition of the wheel in service. Since the test is time- and cost-consuming, developing the simulation method on biaxial wheel fatigue test is urgently necessary. In this paper, a new method is proposed to evaluate the fatigue life of commercial vehicle wheel, in which the finite element model of biaxial wheel fatigue test rig is established based on the standards of EUWA ES 3.23 and SAE J2562, and the simulation of biaxial wheel test and fatigue life estimation considering the effects of tire and wheel camber is performed by applying the whole load spectrum specified in ES 3.23 to the wheel. The radial and cornering fatigue tests are also simulated, and the results are compared with ones of the biaxial fatigue test. The research shows that the proposed method provides an efficient tool for predicting the fatigue life of the wheel in the biaxial fatigue test.

Optimization of cruciform specimens for biaxial fatigue loading with direct multi search

In order to correctly assess the biaxial fatigue material properties one must experimentally test different load conditions and stress levels. With the rise of new in-plane biaxial fatigue testing machines, using smaller and more efficient electrical motors, instead of the conventional hydraulic machines, it is necessary to reduce the specimen size and to ensure that the specimen geometry is appropriate for the load capacity installed. At the present time there are no standard specimen’s geometries and the indications on literature how to design an efficient test specimen are insufficient. The main goal of this paper is to present the methodology on how to obtain an optimal cruciform specimen geometry, with thickness reduction in the gauge area, appropriate for fatigue crack initiation, as a function of the base material sheet thickness used to build the specimen. The geometry is optimized for maximum stress using several parameters, ensuring that in the gauge area the stress distributions on the loading directions are uniform and maximum with two limit phase shift loading conditions (δ=0° and δ=180°). Therefore the fatigue damage will always initiate on the center of the specimen, avoiding failure outside this region. Using the Renard Series of preferred numbers for the base material sheet thickness as a reference, the reaming geometry parameters are optimized using a derivative-free methodology, called direct multi search (DMS) method. The final optimal geometry as a function of the base material sheet thickness is proposed, as a guide line for cruciform specimens design, and as a possible contribution for a future standard on in-plane biaxial fatigue tests.

Biaxial Fatigue Crack Growth Behavior in Aluminum Alloy 5083-H116 Under Ambient Laboratory and Saltwater Environments

Crack growth of aluminum alloy 5083 was investigated when subjected to the in-plane biaxial tension-tension fatigue with stress ratio of 0.5 under ambient laboratory and saltwater environments. Cruciform specimens with a center hole, containing a notch and precrack at 45° to the specimen’s arms, were tested in a biaxial fatigue test machine. Two biaxiality ratios, λ = 1 and λ = 1.5, were studied. For λ = 1, crack propagated along a straight line collinearly with the precrack, while for λ = 1.5 case, the crack path was curved and non-collinear with the precrack. Uniaxial fatigue tests were also conducted. Crack growth rates were faster under the biaxiality fatigue in comparison to uniaxial fatigue at a given crack driving force (ΔK I or ΔG) in both environments. Further, an increase in biaxiality ratio increased the crack growth rate, i.e., faster for λ = 1.5 case than λ = 1 case. Both biaxial fatigue and saltwater environment showed detrimental effects on the fatigue crack growth resistance of 5083, and its combination is highly detrimental when compared to uniaxial fatigue.

THE USE OF A NEODYMIUM FATIGUE TESTING MACHINE FOR FATIGUE STRENGTH EVALUATIONS

This article proposes a method and a laboratory setup for testing the fatigue strength of materials with the use of vibration signals and a neodymium fatigue-testing machine. The machine supports non-contact fatigue testing of construction materials, and analyses of bending and torsional behaviour of magnetic and non-magnetic materials at any excitation range. The discussed machine is a non-contact bending device, and when an appropriate adapter is used, it supports analyses of torsional behaviour in determinations of material wear. A neodymium fatigue-testing machine can also be used to evaluate sample deformation by registering amplitude, RMS deviation from equilibriumcalculated values, temperature of sample notch, deflections along the length of the sample, stress at the cross-section of the notch and load applied at the terminal section of the sample. Simplified diagram of a rotating-bending fatigue testing machine, diagrams of biaxial rotating-bending fatigue testing machines, diagrams of a bending fatigue testing machine, displacement range of a material point in the sample induced by neodymium magnets on a double disc, displacement range of a material point in the sample at rotational frequency, displacement range of a material point in the sample at rotational frequency, changes in the sample's notch temperature and displacement amplitude are presented in the paper.

Biaxial fatigue tests and crack paths for AISI 304L stainless steel

AISI 304L stainless steel specimens have been tested in fatigue. The tests were axial, torsional and in-phase biaxial, all of them under load control and R=-1. The S-N curves were built following the ASTM E739 standard and the method of maximum likelihood proposed by Bettinelli. The fatigue limits of the biaxial tests were represented in axes ?-?. The elliptical quadrant, appropriate for ductile materials, and the elliptical arc, appropriate for fragile materials, were included in the graph. The experimental values were better fitted with an elliptical quadrant, despite the ratio between the pure torsion and tension fatigue limits, ?FL/?FL, is 0.91, close to 1, which is a typical value for fragile materials. The crack direction along the surface has been analyzed by using a microscope, with especial attention to the crack initiation zones. The crack direction during the Stage I has been compared with theoretical models.

Design optimization of cruciform specimens for biaxial fatigue loading

In order to correctly assess the biaxial fatigue material properties one must experimentally test different load conditions and stress levels. With the rise of new in-plane biaxial fatigue testing machines, using smaller and more efficient electrical motors, instead of the conventional hydraulic machines, it is necessary to reduce the specimen size and to ensure that the specimen geometry is appropriated for the load capacity installed. At the present time there are no standard specimen’s geometries and the indications on literature how to design an efficient test specimen are insufficient. The main goal of this paper is to present the methodology on how to obtain an optimal cruciform specimen geometry, with thickness reduction in the gauge area, appropriated for fatigue crack initiation, as a function of the base material sheet thickness used to build the specimen. The geometry is optimized for maximum stress using several parameters, ensuring that in the gauge area the stress is uniform and maximum with two limit phase shift loading conditions. Therefore the fatigue damage will always initiate on the center of the specimen, avoiding failure outside this region. Using the Renard Series of preferred numbers for the base material sheet thickness as a reference, the reaming geometry parameters are optimized using a derivative-free methodology, called direct multi search (DMS) method. The final optimal geometry as a function of the base material sheet thickness is proposed, as a guide line for cruciform specimens design, and as a possible contribution for a future standard on in-plane biaxial fatigue tests.

An Overview of the Fatigue of Polychloroprene Rubber

Some fundamental studies carried in a synthetic rubber - Chloroprene CR29 are presented in the first part of the paper. A critical analysis of test results, shows that an energy based approach permits the determination of fatigue lives in this material. This aspect is further enhanced by biaxial fatigue tests in the same material. These tests covering a life range from 10000 to 1000000 cycles show that the energy based model is very efficient to describe the fatigue behavior. Some evidence of strain induced crystallization (previously observed in natural rubber) with associated life enhancement at high load ratios is also presented. A comprehensive model based on the determination of the constitutive laws taking into account the viscoelastic behavior is developed showing excellent correlation with experimental data.