Experimental Results Of Fatigue Tests Research Articles

Carbonitriding Effect on Fatigue Behaviour and Mechanical Properties of Steel Beam

This work deals with the influence of carbonitriding on the fatigue behavior of notched beam (v-notch) with a depth of 1mm and an angle of 45° is studded. The experimental work include mechanical tests, heat treatments and fatigue test. The heat treatment is done by using carbonitriding at a constant temperature of 800 °C, and soaking time variation 30, 60, and 90 minutes, then followed after treatment by quenching of water and tempering process. The experimental fatigue test results of this work affected by soaking time. The fatigue test has been performed on a cantilever rotating-bending samples. The result shows that the fatigue strength and lifetime increased with increasing soaking time. It is found that The Fatigue Life Improvement Factor (FLIF) at time 30, 60 and 90 minutes for notched beam (v-notch) was increasing by 11.3%, 18.6% and 20.2% respectively with respect to un-treat specimens.

Misalignment factors to affect the fatigue of welded load-carrying joints

To assess the effects of misalignment on the weld fatigue, we present experimental fatigue test results of load- carrying cruciform joints which are subjected to axial variable amplitude loading. The welds were produced from steel AH36 grade and fatigue strength improved by high-frequently mechanical impact treatment. We identify several misalignment factors affecting the fatigue by considering the real and reference welded joint geometries. Our methodology includes proposals for the empirical calibration functions and local stress analyses by using the Structural Hot Spot Stress, Effective Notch Stress and Peak Stress Method. Additionally, we apply our methodology to other sixteen data sets which are extracted from the literature for load-carrying cruciform and butt welded joints subjected to constant amplitude loading. Finally, we present the resultant calibration functions with respect to corresponding fatigue test data.

Fatigue strength assessment of riveted shear splices

AbstractOld‐steel riveted bridges are part of the heritage asset in many countries. Assessing the remaining life and fatigue strength of their structural riveted details is crucial in managing these structures. According to JRC‐ECCS document Assessment of Existing Steel Structures: Recommendations for Estimation of Remaining Fatigue Life, fatigue class 71 is recommended to estimate the fatigue strength of riveted details. However, the predicted fatigue behaviour can lead to excessively conservative estimates. Hence the research community is still focusing on the suitability of current recommendations. This work performs a numerical study to evaluate the fatigue strength of riveted double‐shear splices. Experimental fatigue test results are collected from the literature, and some are used to develop a finite element model in ABAQUS and Fe‐Safe. The adopted modelling fatigue approach is applied to investigate the fatigue behaviour of double‐shear riveted connections with different geometrical properties. The numerical results are compared with all collected data, showing the procedure's effectiveness in predicting the fatigue strength of double‐shear riveted splices. Finally, the numerical and experimental results are compared with the recommended S‐N curves.

Comparative Study on the Fatigue Behavior of Jute-Wool Felt/Epoxy Hybrid and Glass Fiber/Epoxy Composite

<div>Currently, there is a growing tendency to incorporate natural fibers in composites due to their affordability, lightweight nature, and eco-friendliness. Researchers are continuously exploring new materials that offer improved mechanical properties for a broader range of applications. In this work, an experimental investigation on tensile and fatigue behavior of jute-wool felt-reinforced epoxy hybrid laminate is carried, in addition to an E-glass fiber-reinforced epoxy laminate that helps in comparison. Constant amplitude tensile fatigue test is conducted for 80%, 70%, and 60% of the ultimate load of respective composites at a stress ratio of 0.1 and frequency of 7 Hz for both laminates. The jute-wool felt composite showed good fatigue resistance. Though glass fiber composite showed higher tensile strength, jute-wool felt composite exhibits higher fatigue performance than glass fiber composites at higher stress levels. However, at lower stress levels, glass fiber composite shows better fatigue resistance than jute-wool felt composite. A prediction model that uses a few set of experimental results of fatigue test was able to validate the fatigue behavior of composites. The optical microscope images of failed samples were analyzed to detect the damage in composites.</div>

A critical assessment of fatigue behaviour of ferrous alloys by empirical correlation of multifactor S-N curves

Corrosion and Temperature have a significant deteriorative effect on fatigue life of mechanical structures. The reliability of the fatigue damage assessment techniques depends on the quality of the S-N curve which is derived as a result of corrosion degradation and elevated temperature. In this research paper steel specimens of 9.48 mm diameter were tested under rotating bending conditions at a frequency of 6000 cpm.The influencing factors are stress concentration (K = 2.45) (common for both the cases) corrosion (1 percent NaCl solution as corrodent) and elevated temperature (150 °C). The S-N curve so obtained were compared with experimental fatigue test results. Their correlation showed good agreement between the experimental results and the adopted approaches for fatigue life assessment.

Influence of stress concentration on fatigue life of corroded specimens under uniaxial cyclic loading

Corrosion has a significant deteriorative effect on fatigue life of aged ship and offshore structures; hence reliable numerical fatigue life analysis of aged ship structures is crucial for safe operation. The reliability of the fatigue damage assessment techniques depends on the quality of the S-N curve which should account for corrosion degradation. In this paper, the different factors affecting the randomness of stress concentration and fatigue life of corroded specimens have been studied. It was demonstrated that the Stress Concentration, which results from the distinctive thickness variation of the corroded specimens, significantly depends on the mean and minimum thickness of the corroded plate. The equivalent thickness of a typical plate, which has the same fatigue strength of the corresponding corroded one under the same cyclic load, was investigated. A time-dependent S-N curve of corroded structures considering the deterioration of material mechanical properties was proposed. The corroded surfaces were modeled using random spatial distributions and the effect of corrosion degradation on the material mechanical properties was taken into account. The numerically estimated fatigue life and the proposed S-N curve were compared with experimental fatigue test results. The comparison showed good agreement between the experimental results and the adopted approaches for fatigue life assessment.

Review of Multiaxial Testing for Very High Cycle Fatigue: From ‘Conventional’ to Ultrasonic Machines

Fatigue is one of the main causes for in service failure of mechanical components and structures. With the development of new materials, such as high strength aluminium or titanium alloys with different microstructures from steels, materials no longer have a fatigue limit in the classical sense, where it was accepted that they would have ‘infinite life’ from 10 million (107) cycles. The emergence of new materials used in critical mechanical parts, including parts obtained from metal additive manufacturing (AM), the need for weight reduction and the ambition to travel greater distances in shorter periods of time, have brought many challenges to design engineers, since they demand predictability of material properties and that they are readily available. Most fatigue testing today still uses uniaxial loads. However, it is generally recognised that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. By combining both Ultrasonic Fatigue Testing with multiaxial testing through Single-Input-Multiple-Output Modal Analysis, the high costs of both equipment and time to conduct experiments have seen a massive improvement. It is presently possible to test materials under multiaxial loading conditions and for a very high number of cycles in a fraction of the time compared to non-ultrasonic fatigue testing methods (days compared to months or years). This work presents the current status of ultrasonic fatigue testing machines working at a frequency of 20 kHz to date, with emphasis on multiaxial fatigue and very high cycle fatigue. Special attention will be put into the performance of multiaxial fatigue tests of classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low cost piezoelectric transducers. Together with the description of the testing machines and associated instrumentation, some experimental results of fatigue tests are presented in order to demonstrate how ultrasonic fatigue testing can be used to determine the behaviour of a steel alloy from a railway wheel at very high cycle fatigue regime when subjected to multiaxial tension/torsion loadings.

Progress in fatigue life calculation by implementing life-dependent material parameters in multiaxial fatigue criteria

This paper presents the concept of applying life-dependent material parameters to several multiaxial fatigue criteria. This concept reflects the transformation of damage mechanisms in relation to the applied load level, and also in relation to the varying level of plasticity. The goal of this study is to demonstrate the benefit of introducing life-dependent material parameters into stress-based multiaxial fatigue criteria for predicting the fatigue life of materials. New experimental results of fatigue tests on 2124-T851 aluminium alloy confirm the advantage of the life-dependent concept in life assessment over the concept with fixed weight parameters.

Tension/torsion ultrasonic fatigue testing on a railway wheel

Fatigue is one of the main causes for in service failure of mechanical components and structures. The need for the weight reduction of components and a growing need for greater lifespans of equipment, forced the understanding of the fatigue behaviour of materials under multiaxial loading cycles and increased number of loading cycles. It is generally recognized that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. Nevertheless, the high cost of fatigue materials testing under multiaxial loading and for very high number of cycles, was a serious drawback for the research under these testing conditions. This study is a description of the present status of fatigue testing machines working at a frequency of 20 kHz, and the research that can be carried out in order to perform multiaxial fatigue tests under bi-axial stress states and very high cycle fatigue. Special emphasis will be given to the performance of multiaxial fatigue tests under axial/torsion and in plane bi-axial testing using low cost piezo-electric transducers either in classical cylindrical or in cruciform specimens and associated instrumentation. Together with the description of the testing machines, experimental results of fatigue tests will be presented in order to compare the behaviour of a metallic material from a Railway wheel under multiaxial tension/torsion at very high cycle fatigue.

An estimation of fatigue crack initiation and growth lives of Al 2024-T3 in double lap bolted joints with interference fitted holes

The total fatigue life of double lap bolted aluminum 2024-T3 joints with interference fitted holes have been estimated. The specimens are interference fitted with two levels of 1.5% and 4.7% and bolt clamped applying two tightening torques of 2 and 4 N·m. Also, the fatigue tests were carried out at constant-amplitude, sinusoidal loading at a load frequency of 15 Hz and a load ratio of zero with load ranges between 10 and 18 kN. The fatigue life estimation is carried out in two parts of fatigue crack initiation and fatigue crack growth lives. To do so, the calculated stress (and strain) distributions using finite element simulations were used as initial data in multiaxial fatigue criteria and NASGRO equation to predict the fatigue crack initiation and growth lives respectively. Finally, the total calculated fatigue lives are compared with the available experimental fatigue test results. The results show good agreement between the numerically predicted total fatigue lives and fatigue test results especially for the combination of SWT criterion and NASGRO equation.

Strain characteristics of non-ferrous metals obtained on the basic of different loads

The paper presents the comparison of the results of experimental fatigue tests for the 6082-T6 and 2017-T4 aluminium alloys. Tests performed under two types of load conditions. That is tensilecompressive and oscillatory bending. The results are obtained by applying the Basquin's stress-life equation and strain-life equations: Kandil’s equation, Langer’s equation and the authors’ own equation. On the basis of own experimental studies and the relevant physical relations, the stress and strain amplitudes occurring in bent rods under smooth specimens, were calculated according to the elastoplastic model. The results were then used to compare both types of loads with different types of control.

Numerical fatigue life analysis of a high frequency mechanical impact treated industrial component based on damage mechanics models

AbstractAfter post weld treatment with high frequency mechanical impact (HFMI) treatment of welds, a significant increase of fatigue life (up to a factor of 10) can be achieved. During the last years numerous experimental tests of welded joints with simple geometry under constant amplitude loading have been performed to quantify the positive effect of high frequency mechanical impact treatment. Due to the lack of methods for the prediction of the high frequency mechanical impact benefits, a widespread use of this process is not the case yet. Furthermore, it is still not clear if the results of these fatigue tests can be transferred to complex geometries and complex loading conditions such as in industrial applications. Therefore, an approach to assess the fatigue life of complex welded structures under variable amplitude loading was developed. For this purpose, high frequency mechanical impact treatment and fatigue load of simple welded specimen made of S690QL steel were simulated with finite element analysis (FEA) firstly. Then, the needed damage parameters for the fatigue life correlation were evaluated from the finite element post‐processing. The calculated life time to crack initiation was in good agreement with the experimental fatigue test results. In the next step, this procedure was implemented on a welded arm of an evacuator of type EW180B of the company Volvo Construction Equipment made of S700MC. The variable amplitude load measured under real service condition was transferred to single constant amplitude load cycles using a rainflow‐counting algorithm. By simulation and damage mechanics evaluation of each load cycle the total damage sum could be calculated and compared with the experimental results from Volvo Construction Equipment.

Numerical Simulation of the Fatigue Behaviour of a Friction Stirred Channel Aluminium Alloy

Friction stir channelling (FSC) is one of the most promising innovations based on the friction stir concepts. FSC uses a non-consumable tool, similar to that used to perform the friction stir welding (FSW) process, to produce continuous and integral channels, with any path, in monolithic aluminium alloys components in a single step. This paper presents the mechanical behaviour of friction stirred channel components in particular the fatigue resistance of the channels analysed. Finite element analyses were performed taking into account the maximum stress values applied in experimental fatigue tests of the aluminium components. The energy release rate for different crack lengths was obtained using the software ABAQUS. The fatigue crack growth curve was established according to the Paris Law, and the crack propagation direction was calculated using the maximum tangential stress (MTS) criterion. During the analysis it was observed that the critical areas are located in the vicinity of the channels corners, as it was observed in the fracture surfaces of the experimental fatigue test results.

Crack initiation life in notched Ti-6Al-4V titanium bars under uniaxial and multiaxial fatigue: synthesis based on the averaged strain energy density approach

The fatigue behaviour of circumferentially notched specimens made of titanium alloy, Ti-6Al-4V, has been analysed. To investigate the notch effect on the fatigue strength, pure bending, pure torsion and multiaxial bending-torsion fatigue tests have been carried out on specimens characterized by two different root radii, namely 0.1 and 4 mm. Crack nucleation and subsequent propagation have been accurately monitored by using the direct current potential drop (DCPD) technique. Based on the results obtained from the potential drop technique, the crack initiation life has been defined in correspondence of a relative potential drop increase ?V/?V0 equal to 1%, and it has been used as failure criterion. Doing so, the effect of extrinsic mechanisms operating during crack propagation phase, such as sliding contact, friction and meshing between fracture surfaces, is expected to be reduced. The experimental fatigue test results have been re-analysed by using the local strain energy density (SED) averaged over a structural volume having radius R0 and surrounding the notch tip. Finally, the use of the local strain energy density parameter allowed us to properly correlate the crack initiation life of Ti-6Al-4V notched specimens, despite the different notch geometries and loading conditions involved in the tests.

Assessment of root failures in tube-to-flange steel welded joints under torsional loading according to the Peak Stress Method

In the context of the local approaches to fatigue design of welded joints, the notch stress intensity factor (NSIF) approach assumes the weld toe as well as the weld root profile as sharp V-shaped notches, the V-notch opening angle being greater than zero at the toe side and equal to zero at the root side. The Peak Stress Method (PSM) is an engineering, finite element (FE)-oriented method to rapidly estimate the NSIF local stress parameters by means of linear elastic FE analyses with coarse meshes. By using the averaged Strain Energy Density (SED, which can be expressed as a function of the relevant NSIFs) as a fatigue strength criterion, a so-called equivalent peak stress can be defined and adopted to assess either weld toe and weld root fatigue failures in conjunction with an existing design fatigue curve, that was originally calibrated on experimental results relevant only to weld toe failures. In the present paper new fatigue test results relevant to weld root failures have been generated by testing tube-to-flange fillet-welded steel joints under torsional loading. Good agreement has been obtained between theoretical estimations according to the PSM and experimental fatigue test results.

Global Fatigue Life Modelling of Steel Half-pipes Bolted Connections

A steel hybrid structural solution for onshore wind turbine towers was proposed in the European project SHOWTIME. This solution is used in the lattice structure for the lower portion of the tower. Recently, a procedure for fatigue life estimation of steel half-pipes bolted connections applied in global structural models using multiaxial Smith-Watson-Topper (SWT) criteria was proposed by Öztürk et al. In this paper a procedure for design S-N curve modelling of steel half-pipes bolted connections is proposed. This procedure is based on a local approach using multiaxial fatigue criteria together with an elastoplastic analysis using the finite element method. The materials to be used in this analysis are the S355 and S690 steels. This evaluation to be performed is calibrated with experimental results of fatigue tests of the connection under consideration.

A new damage mechanics based approach to fatigue life prediction and its engineering application

An approach based on continuum damage mechanics to fatigue life prediction for structures is proposed. A new fatigue damage evolution equation is developed, in which the parameters are obtained in a simple way with reference to the experimental results of fatigue tests on standard specimens. With the utilization of APDL language on the ANSYS platform, a finite element implementation is presented to perform coupling operation on damage evolution of material and stress redistribution. The fatigue lives of some notched specimens and a Pitch-change-link are predicted by using the above approach. The calculated results are validated with experimental data.

Fretting fatigue phenomenon in bolted high‐strength steel plate connections

AbstractFretting fatigue is a major obstacle for bolted joints used as an alternative to welding. Several studies of the subject so far have concerned aluminium or titanium joints, but due to the recently developed high‐strength steels that can be challenging to weld, the steel industry would also like to explore this field. This paper presents the experimental fatigue test results of double‐lap joints in S355 and S960QC steels. The results are compared and analysed using the SWT parameter and an FE model of the test specimen. The noticeable difference in fretting fatigue behaviour of these materials will be explained and the controversial crack initiation issue addressed.

Fatigue properties of a dental implant produced by electron beam melting® (EBM)

Fatigue properties of a dental abutment with a lattice structure were investigated. Electron Beam Melting® (EBM) was used to produce the dental abutments, made of Ti-6Al-4V. Four levels of cyclic loads including 100N, 200N, 300N, and 500N were applied at 15Hz by using a sinusoidal wave form, and the loading ratio of 10%. According to the experimental results of fatigue test, the dental abutment tolerated the five million cycles of loading at 100N. Results of fractography suggested that the fatigue crack can be initiated from the partially sintered powder particles that were attached to the truss surface. The numerical results revealed the deleterious influence of sharp corners on lowering the fatigue life of the structure. The high level of surface roughness and the lower relative density of a lattice structure could affect its strain rate sensitivity and consequently lower the endurance limit of the lattice structure. The comparison of the experimental data and numerical modeling suggested that the more conservative Soderbrg relationship for the mean stress correction could be used for numerical modeling of fatigue in lattice structures produced by EBM®. Finally, a regression equation was developed from the experimental results that can be used to predict the fatigue life of the designed dental abutment.

G0301505 冷間工具鋼の回転曲げ疲労強度に及ぼす一次炭化物の影響

SKD11 contains many coarse carbides to improve wear resistance and hardness. However fatigue strength of SKD11 decreased due to coarse carbides. Therefore new SKD11 was developed for improving adhesion to surface coating and less primary carbide than SKD11. Rotary bending fatigue test in air at room temperature using hourglass shaped specimen is conducted to clarify S-N property and effect of the carbide on fatigue strength of cold work tool steels. From the experimental results of fatigue test, it was found that the fatigue strength of improved showed high values in the range from 10^4 to 10^7 cycles than that of JIS SKD11. Fatigue strength was increased 220〜240 MPa in low cycle and 250〜260 MPa in high cycle, significant difference in fatigue strength was observed in high cycle. From the results of fracture surface observation by SEM and EDX, it was found that fracture origin of JIS SKD11 was internal coarse carbide. Fracture origin of new SKD11 was surface.