Four-point Bending Fatigue Tests Research Articles

Effect of bimodal grain size distribution on fatigue properties of Ti-6Al-4V alloy with harmonic structure under four-point bending

Titanium alloy (Ti-6Al-4V) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network structure of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. The microstructure of the sintered compacts was characterized using electron backscattered diffraction (EBSD). The areal fraction of the fine-grained structure in the harmonic structure tended to increase with the milling time. Tensile tests and four-point bending fatigue tests at a stress ratio of 0.1 were performed in air at room temperature. The tensile strength, 0.2% proof stress and fatigue limit of Ti-6Al-4V alloy with harmonic structure tended to increase as the areal fraction of the fine-grained structure increased. In contrast, elongation decreased due to the formation of a high areal fraction of the fine-grained structure (79.0%), which resulted in a reduction of the fatigue life with a low cycle regime. Thus, titanium alloy with high strength, ductility and fatigue resistance can be formed by optimization of the milling conditions. Furthermore, the mechanism for fatigue fracture of the Ti-6Al-4V alloy with a harmonic structure is discussed with respect to fractography and crystallography. A fatigue crack was initiated from the α-facet of the coarse-grained structure in the harmonic structure.

Surface integrity and fatigue behaviour of electric discharged machined and milled austenitic stainless steel

Machining of austenitic stainless steels can result in different surface integrities and different machining process parameters will have a great impact on the component fatigue life. Understanding how machining processes affect the cyclic behaviour and microstructure are of outmost importance in order to improve existing and new life estimation models. Milling and electrical discharge machining (EDM) have been used to manufacture rectangular four-point bend fatigue test samples; subjected to high cycle fatigue. Before fatigue testing, surface integrity characterisation of the two surface conditions was conducted using scanning electron microscopy, surface roughness, residual stress profiles, and hardness profiles. Differences in cyclic behaviour were observed between the two surface conditions by the fatigue testing. The milled samples exhibited a fatigue limit. EDM samples did not show the same behaviour due to ratcheting. Recrystallized nano sized grains were identified at the severely plastically deformed surface of the milled samples. Large amounts of bent mechanical twins were observed ~5μm below the surface. Grain shearing and subsequent grain rotation from milling bent the mechanical twins. EDM samples showed much less plastic deformation at the surface. Surface tensile residual stresses of ~500MPa and ~200MPa for the milled and EDM samples respectively were measured.

Effect of temperature on fatigue tests parameters for conventional and asphalt rubber mixes

Fatigue cracking is a major form of distress in asphalt pavements all over the world. Traditionally, fatigue has been analysed using a phenomenological approach, which relates the number of cycles to failure with the imposed strain or stress level. More recently, advanced concepts of continuum damage theory have been applied to investigate damage evolution in asphalt mixes. Another approach relates the damage evolution with the slope of the stiffness reduction curve during uniaxial fatigue tests. This approach is revised in this study to analyse four-point bending (4PB) fatigue tests and to consider non-linear effects over a wider range of imposed strain amplitudes. In this paper, the three approaches are applied to analyse the effect of temperature on the results of 4PB tests performed on hot mix asphalts using different aggregate gradation (dense, open and gap graded) and different asphalt binders (conventional and asphalt rubber). All results clearly showed the dependence of the fatigue parameters on temperature, in that faster damage evolution was observed for mixes at lower temperatures. The results also showed that the asphalt rubber mixtures had lower susceptibility to temperature variations compared to the conventional mixtures.

Compatibility of repair materials with substrate low-modulus cement and asphalt mortar (CA mortar)

The compatibility of the mechanical properties of low-modulus cement and asphalt mortar (CAM) and repair materials was studied using the four-point bending test method. According to the bearing characteristics of CAMs, a four-point bending fatigue test was further developed to evaluate the compatibility of this material with the repair material. With the help of a finite element model, the effect of the difference in the modulus of the substrate mortar and the repair mortar on the stress and strain distribution of a composite sample was studied. The results indicated that the CAM compatibility criteria with repair materials were different from those of the concrete repair system. The difference in the moduli of the substrate mortar and repair mortar determined the loading transfer efficiency and failure mode, which mainly influenced the compatibility of the mechanical properties. The repair materials were compatible with substrate CA mortar when its modulus was approximately 0.75–1.28 times the substrate mortar modulus according to the experimental results. The four-point fatigue test met the requirements of the bearing characteristics of the CA mortar and the present quantitative experimental results, which provide supplementary information for compatibility evaluation.

Fatigue behaviors and damage mechanism of a Cr-Mn-N austenitic steel

Four-point bending fatigue tests were conducted on a CrMnN austenitic steel at room temperature, at frequency of 20 Hz and the stress ratio of R = 0.1, in air. The fatigue strength of this CrMnN austenitic steel was measured to be 503 MPa in the maximum stress from the SN curve obtained. It was found that multi-site crack nucleation took place on the surface of the steel during fatigue, and that the crack population (i.e., fatigue weak-links) was found to be a Weibull function of the applied stress. Usually only one or two of the initiated cracks could lead to the final failure of the samples. Most of the cracks were initiated at the {111}<110> primary slip bands, especially within coarse grains. The cracks were deflected at grain boundaries, which effectively resisted short crack growth and arrested most of the short cracks in the alloy. It can be anticipated that grain refinement could further enhance the fatigue properties of the alloy.

Influence of Oxygen on the Fatigue Behaviour of Ti-6Al-7Nb Alloy

One of the challenges in PM Ti alloys is to control the impurities level. Oxygen affects the microstructure and the mechanical properties of titanium alloys. Ti-6Al-7Nb is a promising alloy to use in PM due to its outstanding biocompatibility and mechanical properties required for load bearing medical implants. In this work, the influence of the impurities content on the ductility, fatigue resistance and microstructure of Ti-6Al-7Nb alloy processed by metal injection moulding was examined. Tensile and fatigue specimens were manufactured using Ti-6Al-7Nb gas atomized powder. Depending on the thermal treatment time, various oxygen contents were introduced into the specimens. The resulting oxygen content was determined by melt extraction technique. Tensile tests and high cycle four-point bending fatigue tests at room temperature were performed. First studies about the effect of oxygen content on crack initiation and propagation were done by the observation of microstructures and fractured surfaces using light and electron microscopy (SEM).

Biomechanical Study of the Fixation Strength of Anteromedial Plating for Humeral Shaft Fractures.

Background:Open reduction and internal fixation with plate and screws are the gold standard for the surgical treatment of humeral shaft fractures, this study was to compare the mechanical properties of anteromedial, anterolateral, and posterior plating for humeral shaft fractures.Methods:A distal third humeral shaft fracture model was constructed using fourth-generation sawbones (#3404, composite bone). A total of 24 sawbones with a distal third humeral shaft fracture was randomly divided into three Groups: A, B, and C (n = 8 in each group) for anteromedial, anterolateral, and posterior plating, respectively. All sawbones were subjected to horizontal torsional fatigue tests, horizontal torsional and axial compressive fatigue tests, four-point bending fatigue tests in anteroposterior (AP) and mediolateral (ML) directions and horizontal torsional destructive tests.Results:In the horizontal torsional fatigue tests, the mean torsional angle amplitude in Groups A, B, and C were 6.12°, 6.53°, and 6.81°. In horizontal torsional and axial compressive fatigue tests, the mean torsional angle amplitude in Groups A, B, and C were 5.66°, 5.67°, and 6.36°. The mean plate displacement amplitude was 0.05 mm, 0.08 mm, and 0.10 mm. Group A was smaller than Group C (P < 0.05). In AP four-point bending fatigue tests, the mean plate displacement amplitude was 0.16 mm, 0.13 mm, and 0.20 mm. Group B was smaller than Group C (P < 0.05). In ML four-point bending fatigue tests, the mean plate displacement amplitude were 0.16 mm, 0.19 mm, and 0.17 mm. In horizontal torsional destructive tests, the mean torsional rigidity in Groups A, B, and C was 0.82, 0.75, and 0.76 N·m/deg. The yielding torsional angle was 24.50°, 25.70°, and 23.86°. The mean yielding torque was 18.46, 18.05, and 16.83 N·m, respectively.Conclusions:Anteromedial plating was superior to anterolateral or posterior plating in all mechanical tests except in AP four-point bending fatigue tests compared to the anterolateral plating group. We can suggest that anteromedial plating is a clinically safe and effective way for humeral shaft fractures.

Tensile and fatigue behaviour of Ni–Ni3Si eutectic in situ composites

A Ni–Ni3Si composite was fabricated via a eutectic reaction (Ni–Ni3Si) using a rapidly cooled directional solidification technique at a solidification rate of 40 μm s−1. The composite consisted of approximately 62.2% Ni–Si solid solution and 37.8% Ni–Ni3Si eutectic phase in volume. Four-point bend fatigue tests were carried out on the composite. The fatigue strength of the alloy was measured to be 520 MPa (maximum cyclic stress). It was found that the fatigue cracks were preferably initiated in the Ni–Ni3Si eutectic phase, and that the Ni matrix was fractured in a cleavage fashion. It was probably attributed to the high level of supersaturated Si in the Ni matrix, which led to inducing the embrittlement of the Ni matrix.

On the conjoint influence of broaching and heat treatment on bending fatigue behavior of Inconel 718

In this study, the conjoint effect of a broaching operation, similar to that used for machining fir-tree slots on turbine discs, and subsequent heat treatments at 550°C and 650°C on the fatigue performance and corresponding crack initiation behavior of forged Inconel 718 has been investigated. Four-point bending fatigue tests were conducted under load control on specimens of two groups, i.e. a polished group and a broached group, with totally six different surface conditions. Compared to the as-polished specimens, a beneficial effect of the broaching operation was found on the fatigue life due to the high compressive residual stresses on the broached surface which transfer the fatigue crack initiation from surface to sub-surface regions. Introducing a heat treatment generally deteriorated the fatigue performance of the alloy because of the oxidation assisted crack initiation, while the reduction in fatigue life was found to be more remarkable for the broached specimens, in particular when heat treated at 650°C, as the thermal impact also led to a great relaxation of the compressive residual stresses; the combined effect, together with the substantial anomalies created by broaching on the surface, such as cracked carbides and machining grooves, caused an increased propensity to surface cracking in fatigue and consequently a loss of the lifetime. Furthermore, it was found that the occurrence of surface recrystallization at elevated temperatures in machined Inconel 718 could lead to intergranular oxidation, creating micro-notches as preferable sites for the fatigue crack initiation.

Biomechanical Analysis of Locking Reconstruction Plate Using Mono- or Bicortical Screws

This experimental in vitro study evaluated the influence of screw length on the mechanical properties of a locking reconstruction plate designed with locking rings inserted into plate holes. Synthetic bone cylinders with 10 mm fracture gap and seven-hole locking reconstruction plates were used. Two groups of bone-plate constructs were assembled: Group 1 – three monocortical screws on each fracture side, Group 2 - three bicortical screws on each fracture side. In each group nine bone-plate constructs were tested until failure, three each in bending, compression and torsion. In each group, 21 bone-plate constructs were tested for failure in fatigue testing, seven each for bending, compression and torsion. In all static testing no significant differences were found between G1 and G2, except ultimate moment in torsion test (G2>G1; P=0.008). Statistical analysis revealed significant differences between groups in axial compression fatigue testing (G1>G2; P<0.05) and four-point bending fatigue testing (G1<G2; P<0.05) in maximum load, minimum load, maximum moment, and minimum moment. In conclusion, screw length can affect the mechanical properties of locking reconstruction plate. Compared to bicortical screws, monocortical screws were less resistant to bending than axial compression. This must be considered when choosing implant, particularly in fractures under high axial loads.

Influence of Surface Conditions on the Fatigue Behavior of A357 Aluminum Alloy

The influence of surface conditions on the fatigue behavior A357 aluminum alloy was studied in this paper. Four-point bending fatigue tests method were performed to obtain the fatigue strength of A357 cast aluminum alloy specimens with different surface conditions. A joint bearing rig was designed to minimize the experimental error caused by the misalignment associated with a four-point bend test. The results showed that the fatigue strength of specimens with as-cast surfaces was higher those with machined surface roughness of Ra=1.6μm and 3.2μm, while lower than that with Ra=0.4μm. Optical microscope (OM) and scanning electron microscope (SEM) observations, indicating that the fatigue cracks initiate from machined grooves for specimens with roughness of Ra=1.6μm and 3.2μm, while subsurface crack initiates from cast defects inside the specimen with roughness of Ra=0.4. For specimens with as-cast surface, the fatigue cracks initiate from the surface irregularities or cast defects near the surface.

Fatigue of Ti6Al4V Structural Health Monitoring Systems Produced by Selective Laser Melting.

Selective laser melting (SLM) is an additive manufacturing (AM) process which is used for producing metallic components. Currently, the integrity of components produced by SLM is in need of improvement due to residual stresses and unknown fracture behavior. Titanium alloys produced by AM are capable candidates for applications in aerospace and industrial fields due to their fracture resistance, fatigue behavior and corrosion resistance. On the other hand, structural health monitoring (SHM) system technologies are promising and requested from the industry. SHM systems can monitor the integrity of a structure and during the last decades the research has primarily been influenced by bionic engineering. In that aspect a new philosophy for SHM has been developed: the so-called effective structural health monitoring (eSHM) system. The current system uses the design freedom provided by AM. The working principle of the system is based on crack detection by means of a network of capillaries that are integrated in a structure. The main objective of this research is to evaluate the functionality of Ti6Al4V produced by the SLM process in the novel SHM system and to confirm that the eSHM system can successfully detect cracks in SLM components. In this study four-point bending fatigue tests on Ti6Al4V SLM specimens with an integrated SHM system were conducted. Fractographic analysis was performed after the final failure, while finite element simulations were used in order to determine the stress distribution in the capillary region and on the component. It was proven that the SHM system does not influence the crack initiation behavior during fatigue. The results highlight the effectiveness of the eSHM on SLM components, which can potentially be used by industrial and aerospace applications.

Characterization of the hydroxyapatite layer formed by fine hydroxyapatite particle peening and its effect on the fatigue properties of commercially pure titanium under four-point bending

Abstract Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles was introduced to form the HAp surface layer and improve the fatigue properties of commercially pure (CP) titanium. The surface microstructure of the FPP-treated specimens was characterized using a micro-Vickers hardness tester, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), and non-contact scanning white light interferometry. FPP could create a HAp layer on the surface of CP titanium within a relatively short time (1 s) by shot particle transfer. In addition, FPP increased the surface hardness and generated compressive residual stress at the treated surface. Four-point bending fatigue tests were performed at a stress ratio of 0.1 in air at room temperature to examine the effect of FPP using HAp shot particles on the fatigue properties of CP titanium. It was found that the fatigue limit for the FPP-treated specimen was higher than that for the unpeened specimen. The fatigue fracture mechanism for the CP titanium treated with FPP was discussed from the viewpoint of fractography. The HAp layer remained on the surface without delamination after the fatigue tests.

Formation of nitrided layer using atmospheric-controlled IH-FPP and its effect on the fatigue properties of Ti-6Al-4V alloy under four-point bending

The purpose of this study is to develop a rapid nitriding using a surface modification which is a combination of atmospheric-controlled induction-heating fine particle peening (AIH-FPP) and nitrogen gas blow. AIH-FPP was performed for titanium alloy (Ti-6Al-4V ELI grade) at 1173 K for 180 s in a controlled nitrogen atmosphere with an oxygen concentration less than 10 ppm without supplying particles. A nitrogen compound (TiN) was formed on the AIH-FPP treated surface, which results in increasing the surface hardness of Ti-6Al-4V alloy. To examine the effect of AIH-FPP on the fatigue properties of Ti-6Al-4V alloy, four-point bending fatigue tests were performed at a stress ratio of 0.1 in air at room temperature. It was found that the AIH-FPP treated specimens showed low fatigue limit due to the formation of a brittle compound layer and coarse microstructure.

微粒子ピーニングを利用した生体用低ヤング率型チタン合金表面へのハイドロキシアパタイト層の創製

Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles was introduced to form the HAp surface layer and improve both the fatigue properties and osteoconductivity of biomedical titanium alloy (Ti-29Nb-13Ta-4.6Zr: TNTZ) with low Young’s modulus. The surface microstructure of the FPP-treated specimens was characterized using a micro-Vickers hardness tester, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). FPP could form a HAp layer on the surface of TNTZ alloy within a relatively short time by HAp shot particle transfer. Four-point bending fatigue tests were performed at a stress ratio of 0.1 in air at room temperature to examine the effect of FPP using HAp shot particles on the fatigue properties of TNTZ alloy. It was found that the fatigue limit and fatigue life for the FPP-treated specimen were higher than that for the un-peened specimen. This was because FPP increased the hardness of TNTZ alloy at the treated surface.

Glass fiber reinforced asphalt membrane for interlayer bonding between asphalt overlay and concrete pavement

The aim of this study is to evaluate the shear and fatigue behavior of asphalt concrete (AC) overlay on Portland cement concrete (PCC) pavement with different interlayer bonding materials, including modified emulsified asphalt (MEA) membranes, modified asphalt (MA) membranes, glass fiber reinforced modified emulsified asphalt (GFMEA) membranes and glass fiber reinforced modified asphalt (GFMA) membranes. An in-house shearing device was used to evaluate the shear behavior between asphalt layer and PCC layer. An orthogonal experimental design was performed to determine the impact of each influential factor on the interlayer shear behavior, including the testing temperature, the bonding material, and the confinement pressure applied during the tests. The results indicate that AC overlay on PCC pavement can reach its optimal performance in terms of shear toughness when the GFMA is used at the interlayer. Four-point bending fatigue tests were conducted on the AC beam with interlayers using the Material Test System (MTS). The fractal dimension (FD) of fatigue cracks in AC overlay was analyzed using image measurements. The results show that the AC beam with the GFMA has a longer fatigue life. Therefore, it is concluded that the GFMA can strengthen bonding and mitigate reflection cracking of AC overlay on PCC pavements.

Fatigue property of stationary shoulder friction stir welded additive and non-additive T joints

Additive and non-additive T joints were successfully fabricated by stationary shoulder friction stir welding using various welding parameters. No interior defects were observed in all joints by microstructural examination. The fatigue property of both additive and non-additive joints was revealed by four-point bending fatigue test. The microstructure and the stress concentration were investigated at the fatigue crack initiation sites of various T joints. Stress concentration determines the fracture location, and the surface microstructure influences the fatigue property. The stress concentration at the internal corner was evidently lower for additive joint than non-additive joint. The surface of stationary shoulder friction welded joint was characterised by ultrafine grains, which evidently enhanced the fatigue property.

Feasibility study on integrated structural health monitoring system produced by metal three-dimensional printing

Numerous structural health monitoring systems have been investigated extensively in order to enhance safety level and reduce direct operational costs. This work demonstrates the feasibility study of a new concept, the effective structural health monitoring system. The effective structural health monitoring system detects cracks using a system of capillaries incorporated into a structure. The structure with the integrated capillaries is produced by additive manufacturing, a process of adding material layer by layer. The first objective of this study is to prove that the developed system has reached technological readiness level 3. In order to prove that, four-point bending specimens with the integrated effective structural health monitoring system were tested after being produced by additive manufacturing, more specifically by laser metal deposition. The second objective of the study is to indicate that during four-point bending fatigue tests, the integrated structural health monitoring system has no influence on the crack initiation behavior. To do so, the specimens were subjected to the so-called step method. We demonstrate that the effective structural health monitoring has reached technological readiness level 3 and that the presence of effective structural health monitoring did not negatively influence the fatigue initiation process. As higher technology readiness levels are required, further investigations are still in progress.

Experimental Investigations of the Bending Fatigue Performance of TRC-Strengthened RC Beams in Conventional and Aggressive Chlorate Environments

AbstractTo study the fatigue performance of a reinforced-concrete (RC) beam strengthened with textile-reinforced concrete (TRC), a four-point bending fatigue test was conducted, in which the strengthening forms, reinforcement ratio, textile ratio, static load damage, and sustained load corrosion were considered. The results of this test show that TRC can modify the fatigue failure mode and significantly improve the fatigue life of RC beams. The strengthened beams exhibited better performance than the unstrengthened beams with regard to controlling cracks. TRC can increase the number of cracks and decrease the crack width in RC beams, and the strengthened beams exhibit an increase in midspan deflection that rapidly progresses, while the unstrengthened beams do not exhibit this progression. In terms of improving the fatigue life of beams, the single-sided strengthening form is shown to perform better than the three-sided form, and a beam with a high reinforcement ratio is superior to that with a low ratio. ...

Application of the partial healing model on laboratory fatigue results of asphalt mixture

Fatigue characterization of an asphalt mixture is commonly estimated by laboratory fatigue tests. Based on the classical fatigue analysis, fatigue lives obtained from different test methods are not comparable even though the test conditions are the same. One of the main reasons causing the difference in fatigue results is the difference in stress–strain distribution of the different specimens. The stiffness measured from the inhomogeneous test is not a material property but a specimen property, which depends on the geometry and dimension of the specimen. With regard to the homogenous tests, the stress–strain field within the specimen is uniform in theory. The measured stiffness corresponds to a material property and is not influenced by the specimen dimension. The objective of this research is to find a correlation between the different fatigue test results by means of the partial healing (PH) model.In this paper, a homogenous test, the uniaxial tension and compression (UT/C) fatigue test, and a inhomogeneous test, the four-point bending (4PB) fatigue test were conducted. For each type of test, specimens with different sizes were tested to explore the size effect on the fatigue results. It is found that for the standard specimen size, the model parameters deduced from the UT/C and 4PB are comparable. It indicates that the PH model is able to describe the fatigue process in the 4PB test based on the UT/C fatigue results.