Three-point Bending Fatigue Research Articles

Low Temperature Crack Resistance and Fatigue Properties of Asphalt Mixture with Pre-buried Heat Conduction Tube

In order to study the effect of geothermal heat transfer tube on the asphalt pavement performance, a heat transfer tube with a diameter of 8 mm was used, which was placed in an AC-16 hot mix asphalt mixture and crushed into a ruthenium plate test piece, using MTS-810 hydraulic servo test. The system evaluates the low temperature crack resistance and fatigue performance, and compares the performance with the asphalt mixture test piece without the heat conduction tube. The results show that after the heat transfer tube is buried, the failure strain of the asphalt mixture is reduced by 40.9%, and the repeated load of the three-point bending fatigue test is reduced by 53.9%. The low temperature crack resistance and fatigue performance are both larger. The degree is reduced.

High fatigue endurance limit of a Ti-based metallic glass

In this work, we investigated the three-point bending fatigue behavior of a brittle Ti32.8Zr30.2Ni5.3Cu9Be22.7 MG (ZT3) combined with damage morphology observation using scanning electron microscope. It is found that the brittle ZT3 MG possesses the excellent fatigue resistance with the fatigue endurance limit of ~527 MPa based on stress amplitude, and the fatigue ratio of ~0.29 termed as fatigue endurance limit divided by ultimate tensile strength, respectively. The superior fatigue property of ZT3 MG is mainly clarified based on shear-banding mechanism. The embrittlement suppresses shear band formation before fatigue crack initiation, probably delaying the fatigue crack initiation; the profuse shear bands as well as the staircase-like crack path during fatigue crack propagation further increases the resistance to fatigue damage and fracture. Besides, the fatigue behavior of ZT3 MG shows a lessened sensitivity to the defect, as evidenced by the slightly decreased fatigue lives and the low scatter of fatigue data in stress-life (S–N) curve. The ZT3 MG with excellent fatigue performance as well as high specific strength is expected to be a prime candidate material for structural applications in future.

The Effect of Microstructure and Axial Tension on Three-Point Bending Fatigue Behavior of TC4 in High Cycle and Very High Cycle Regimes.

The effects of microstructure and axial tension on the fatigue behavior of TC4 titanium alloy in high cycle (HCF) and very high cycle (VHCF) regimes are discussed in this paper. Ultrasonic three-point bending fatigue tests at 20 kHz were done on a fatigue life range among 105–109 cycles of the alloys with equiaxed, bimodal and Widmanstatten microstructures. Experimental results without axial tension show that three typical shapes of S-N curves clearly present themselves for the three different microstructures. Moreover, the crack initiation sites abruptly shifted from surface to subsurface of the specimen in the very high cycle fatigue regime for equiaxed and bimodal microstructures. But for the Widmanstatten microstructure, both surface and subsurface crack initiation appeared in the high cycle fatigue regime, and the multi-points crack initiation was found in the bimodal microstructure. The subsurface fatigue crack originated from the αp grains in equiaxed and bimodal microstructures. However, it originated from the coarse grain boundary α in the Widmanstatten microstructure. Additionally, the S-N curve shape, fatigue life and fatigue crack initiation mechanism with axial tension are similar to that without axial tension. However, the crack origin point shifts inward with axial tension.

Fatigue Performance of Tunnel Invert in Newly Designed Heavy Haul Railway Tunnel

The Haoji railway in China is the longest heavy haul railway in the world, including 235 tunnels located along the 1837 km railway. With the increasing axle load of the new line and the basal deterioration of the existing heavy haul railway in China, studying the fatigue performance of the newly designed tunnel structure is essential. To study the coupling effect of the surrounding rock pressure and 30 t axle load train, in this study, we combined three-dimensional numerical simulation and three-point bending fatigue tests to investigate the fatigue performance of the basal structures. The results of numerical simulation indicate that the center of the inverted arch secondary lining is the position vulnerable to fatigue in the lower tunnel structures; the surrounding rock pressure performance exerts a stronger influence on the stress state of the vulnerable position than the dynamic train loads. The S–N formula obtained from the experiment showed that the fatigue life of tunnel bottom structures decreases with increasing surrounding rock pressure and dynamic load. In typical grade V surrounding rock and 30 t axle loads, fatigue failure will not occur in the newly designed tunnel bottom structures within 100 years if bedrock defects are lacking and pressure of surrounding rock is not excessive.

Digital image correlation-based investigation of self-healing properties of ferrite-filled open-graded friction course asphalt mixture

Asphalt-based composites can self-heal, and this ability can be enhanced with induction heating and microwave heating. However, few methods are available to quantitatively analyze and monitor crack propagation in a composite before and after healing to evaluate the efficiency of self-healing. Digital image correlation (DIC) techniques, which are used herein, are widely applied for this purpose because they can directly measure the displacement field and strain field of an object under load. We conducted a three-point bending fatigue test and a three-point bending fracture test to study the influences of microwave heating time and healing time on the self-healing of asphalt-based composites with 5% ferrite and without ferrite. The influence of ferrite on the strain development process of the specimens was discussed based on the indexes of crack initiation time, crack propagation rate, and horizontal strain. The addition of ferrite can significantly delay secondary cracking and decrease the crack propagation rate of an open-graded friction course (OGFC) asphalt mixture. Based on DIC observations, the optimal microwave heating time for the OGFC asphalt mixture with 5% ferrite is 80 s. Meanwhile, the longer the healing time, the stronger is the healing effect, but the influence of healing time is weaker than that of heating time. Moreover, based on the change in the horizontal strain during crack development, it can be concluded that the addition of ferrite significantly improves the healing effect, in addition to improving the mixture toughness.

Microstructure and Three-Point Bending Fatigue Behavior of Al-Cu-Mg-Ag Alloys with Various Mg Contents

This paper presents an investigation on the influence of Mg content on the microstructures, tensile properties and three-point bending fatigue behavior of Al-Cu-Mg-Ag alloys. The results indicate that the alloy containing 0.70 wt.% Mg possesses lower tensile properties and higher fatigue performance compared to the one with 1.15 wt.% Mg. Increasing Mg content in Al-Cu-Mg-Ag alloys can enhance the nucleation and precipitation of Ω phase, as well as increase the volume fraction and number density of constituent particles. The investigation of the fatigue crack initiation and growth behavior of Al-Cu-Mg-Ag alloy using interrupted fatigue tests suggests that micro-cracks mainly nucleate on Al2Cu constituent particles and the fatigue life is mainly associated with the density of constituent particles.

Fatigue Behaviour and Crack Initiation in CoCrFeNiMn High-Entropy Alloy Processed by Powder Metallurgy

Single-phase equiatomic five-element high entropy alloy CoCrFeMnNi was prepared by powder metallurgy. Two materials with ultra-fine-grained microstructure were prepared by spark plasma sintering (SPS) of ball-milled powder at two sintering times (5 and 10 min), assigned as HEA 5 and HEA 10, respectively. Basic microstructural and mechanical properties were evaluated. The median grain size of the microstructures was determined to be 0.4 and 0.6 μm for HEA 5 and HEA 10, respectively. The differences in the microstructure led to a significant change in strength and deformation characteristics evaluated at room temperature. The effect of cyclic loading was monitored by three-point bending fatigue test. The results show that even relatively small change in the microstructure causes a significant effect on fatigue life. The fatigue endurance limit was measured to be 1100 MPa and 1000 MPa for HEA 5 and HEA 10, respectively. The detailed fractographic analysis revealed that abnormally large grains, localised in the microstructure on the tensile loaded surface, were a typical fatigue initiation site. The formation of (nano) twins together with dislocation slips caused the crack nucleation because of the cyclic loading.

Fatigue behavior of RC beams strengthened with CFRP laminate under hot-wet environments and vehicle random loads coupling

Fatigue behavior of main load-bearing reinforced concrete (RC) members of highway bridges is heavily influenced by service environmental attacks and vehicle random loads. To investigate the fatigue behavior of RC beams strengthened with CFRP on highway bridges served under hot-wet environments and vehicle random loads, a novel environmental fatigue test method was proposed in the current research. Three-point bending fatigue tests on RC beams strengthened by carbon fiber laminate (CFL) with five combinations of hot-wet environments (10 °C & 50%R∙H, 23 °C & 78%R∙H, 50 °C & 50%R∙H, 50 °C & 78%R∙H, 50 °C & 95%R∙H) and three sets of random loading levels (ΔPmean = 18.3 kN, 19.9 kN, 21.6 kN) were carried out. The results showed that coupling action of hot-wet environment and vehicle random load has a significant effect on the fatigue failure mode (including tensile rebar fracture and CFL debonding), fatigue stiffness and fatigue lives of the strengthened beams. Fatigue lives of the strengthened beams under vehicle random loads was lower than constant amplitude loads and ladder amplitude loads. Environmental random fatigue lives decreased with the increase of relative humidity and temperature. To facilitate and accurate environmental random fatigue life prediction, environmental random fatigue equations which were able to consider the influence of coupling effect of hot-wet environments and the vehicle random loads on fatigue lives and fatigue limits of the strengthened beams were established and verified against the test data.

Three-Point Bending Fatigue Test of TiAl6V4 Titanium Alloy at Room Temperature

A polycrystalline alpha-beta TiAl6V4 alloy in the annealed condition was used for the three-point bending fatigue test at frequencyf∼100 Hz. The static preloadFstat. = −15 kN and variable dynamic forceFdyn. = −7 kN to −13.5 kN were set as fatigue test loading parameters. The fatigue life S-N curve presented the stress amplitudeσaas a function of a number of cycles to fractureNf. A limiting number of cycles to run out of 2.0 × 107cycles were chosen for the 3-point fatigue tests of rectangular specimens. In addition, the Smith diagram was used to predict the fatigue life. The alpha lamellae width has a significant influence on fatigue life. It is assumed that the increasing width of alpha lamellae decreases fatigue life. A comparison of fatigue results with given alpha lamellae width in our material to the results of other researchers was performed. The SEM fractography was performed with an accent to reveal the initiation sites of crack at low and high load stresses and mechanism of crack propagation for the fatigue part of fracture.

Fatigue Cracking Resistance of Engineered Cementitious Composites (ECC) under Working Condition of Orthotropic Steel Bridge Decks Pavement

In order to investigate the fatigue cracking resistance of engineered cementitious composites (ECC) used in in total life pavement, the semi-circular bending (SCB) test and improved three-point bending fatigue test (ITBF) were utilized in this study. The digital image correlation (DIC) method was also utilized to track the surface strain fields of specimens during the SCB test. X-ray computed tomography (CT) and digital image processing (DIP) technologies were applied to measure the internal-crack distribution of the ITBF specimen. The results of the SCB test showed that the fatigue cracking damage process of ECC can be divided into three stages and that the cracking stable propagating stages occupied the main part, which indicates that ECC has excellent ductility and toughness and could work very well with existing cracks. The ITBF results showed that the fatigue cracking resistance of ECC was better than epoxy asphalt concrete (EAC). In addition, the internal-crack distribution along the depth direction of the ITBF specimen could be presented well by the image pixel statistical (IPS) method based on CT scanning of image slices. It could be found that multiple cracks propagate simultaneously in ECC, instead of a single crack, under the OSBD pavement working condition.

Micromechanical Investigation of the Fatigue Crack Propagation and Damage Development in Al/Al2O3/SiC Hybrid Metal Matrix Composite

In this study, the micro-mechanisms involved in fatigue crack propagation are investigated qualitatively in a Al/Al2O3/SiC hybrid metal matrix composite (MMC) and the results are compared with Al2O3 fibre reinforced MMC and monolithic Al alloy. The three-point bending fatigue test was carried out in a rectangular notched specimen and crack propagation was monitored until the fracture of the specimen. The crack profile on the surface of the specimen was analyzed via optical microscope. The fracture surface and the crack-path profile in the fracture surface were analyzed by scanning electron microscopy (SEM) and three dimensional (3D) surface analysis respectively. The hybrid MMC shows higher crack propagation resistance than that of fibre reinforced MMC and Al alloy in the low ∆K region. In the threshold region, the crack in hybrid MMC is directed by the reinforcement–matrix debonding, followed by void nucleation in the Al alloy. Additionally, the crack propagation in the stable-crack-growth region is controlled by reinforcement-matrix interface debonding caused by the cycle-by-cycle crack growth along the interface, as well as by the transgranular fracture of particles and fibres. The presence of large volumes of inclusions and the microstructural inhomogeneity reduces the area of striation in hybrid MMC, leading to unstable fracture.

Fatigue behavior of the 3D orthogonal carbon/glass fibers hybrid composite under three-point bending load

Hybrid composites containing more than one type of high performance fibers show better mechanical properties than traditional composites reinforced with one kind of fibers. This paper investigated the static and fatigue flexural properties of a newly designed 3D orthogonal carbon/glass fibers hybrid composite. The flexural properties of the 3D orthogonal carbon fibers composites and the laminated carbon/glass fibers hybrid composite were discussed to reveal the advantages of the static flexural properties of the 3D orthogonal carbon/glass fibers hybrid composite. 50%, 55%, and 60% of the maximum stress of the 3D orthogonal and laminated carbon/glass fibers hybrid composites were respectively selected for the three-point bending fatigue test. The failure propagation of the composites and their fracture morphologies were captured in detail using the 3D microscope. The results found that the 3D integrated structure and hybrid effects resulted in the 3D orthogonal carbon/glass fibers hybrid composite showing the best static flexural strength. The fatigue performance of the 3D orthogonal carbon/glass fibers hybrid composite was superior to the laminated carbon/glass fibers hybrid composite since Z-direction yarns were beneficial to resist the interface failure. The stress levels contributed to the variations of the failure modes of the composites under three-point bending fatigue loading.

The Effect of Laser Peening without Coating on the Fatigue of a 6082-T6 Aluminum Alloy with a Curved Notch

Laser shock peening has established itself as an effective surface treatment to enhance the fatigue properties of metallic materials. Although a number of works have dealt with the formation of residual stresses, and their consequent effects on the fatigue behavior, the influence of material geometry on the peening process has not been widely addressed. In this paper, Laser Peening without Coating (LPwC) is applied at the surface of a notch in specimens made of a 6082-T6 aluminum alloy. The treated specimens are tested by three-point bending fatigue tests, and their fatigue life is compared to that of untreated samples with an identical geometry. The fatigue life of the treated specimens is found to be 1.7 to 3.3 times longer. Brinell hardness measurements evidence an increase in the surface hardness of about 50%, following the treatment. The material response to peening is modelled by a finite element model, and the compressive residual stresses are computed accordingly. Stresses as high as −210 MPa are present at the notch. The ratio between the notch curvature and the laser spot radius is proposed as a parameter to evaluate the influence of the notch.

Fabrication and Fatigue Behavior of Aluminum Foam Sandwich Panel via Liquid Diffusion Welding Method

Aluminum Foam Sandwich panels were fabricated via liquid diffusion welding and glue adhesive methods. The Microstructure of the Aluminum Foam Sandwich joints were analyzed by Optical Microscopy, Scanning Electron Microscopy, and Energy Dispersive Spectroscopy. The metallurgical joints of Aluminum Foam Sandwich panels are compact, uniform and the chemical compositions in the diffusion transitional zone are continuous, so well metallurgy bonding between Aluminum face sheet and foam core was obtained. The joining strength of an Aluminum Foam Sandwich was evaluated by standard peel strength test and the metallurgical joint Aluminum Foam Sandwich panels had a higher peel strength. Moreover, a three-point bending fatigue test was conducted to study the flexural fatigue behavior of Aluminum Foam Sandwich panels. The metallurgical joint panels have a higher fatigue limit than the adhesive joining sandwich. Their fatigue fracture mode are completely different, the failure mode of the metallurgical joint is faced fatigue; the failure mode for the adhesive joint is debonding. Therefore, the higher joining strength leads to a longer fatigue life.

Development of fatigue testing system for in-situ observation of stainless steel 316 by HS-AFM & SEM

A miniature three-point bend fatigue stage for in-situ observation of fatigue microcrack initiation and growth behaviour by scanning electron microscopy (SEM) and contact mode high-speed atomic force microscopy (HS-AFM) has been developed. Details of this stage are provided along with finite element simulations of the stress profiles of said stage and specimen on loading. The proposed stage facilitates study of the micro mechanisms of fatigue damage evolution when used during SEM and HS-AFM scanning of the sample surface. High amplitude low cycle fatigue tests have been carried out on annealed AISI Type 316 stainless steel to demonstrate the applicability of the system. Characteristic features of surface topography and evolution of slip bands observed have been documented. Images obtained by SEM and HS-AFM are presented for comparison. Finally, to demonstrate the capability of the new facility combined with HS-AFM, the spacing between slip bands and their height at different grains at the centre of the metal sample are measured and compared.

Fatigue behavior of load-carrying cruciform joints with partial penetration fillet welds under three-point bending

This paper presents experimental data from three-point bending fatigue tests of load-carrying cruciform joints with partial penetration fillet welds that are representative of hydraulic turbine runners. The cruciform joints were made by multi-pass FCAW of martensitic stainless steel plates (AISI 415), followed by weld toe grinding and tempering. The S–N approach is used to compare the fatigue performances for three different weld penetration depths covering both weld toe and weld root failures. In case of weld root failure, the variable fatigue lives at the weld roots in a S–N plot are consolidated by introducing an equivalent notch stress intensity factor under mixed-mode. The results demonstrate that the origin of fatigue failure mode transition is size dependent, and it also depends on the nominal stress range and weld toe surface finish. E316L austenitic weld metal yields lower fatigue performance than E410NiMo martensitic weld metal but does not require preheating.

Effect of Heat Treatment on the Mechanical Behavior and Fracture of TiNi Alloy

The mechanical properties and fracture of a TiNi alloy (Ti-55.8 wt % Ni) have been investigated under vacuum heat treatment at 700–1200°C. Three-point bending and low cycle fatigue tests were conducted on heat treated wire samples under extreme loading conditions (large strains and alternating bending loads) to determine the effect of the annealing temperature on the superelastic behavior of the alloy. It was found that an increase in the heat treatment temperature leads to grain coarsening in the alloy, but the coarsening effect on its superelastic behavior is insignificant at low bending strains (4.0–4.5%). With heat treatment temperature variation from 700 to 1200°C, the shape of the alloy stress-strain curves remains almost unchanged for all bended samples, but with increasing heat treatment temperature the martensitic shear stress and residual strain slightly increase. In low cycle bending tests, the alloy ductility reduces significantly after heat treatment above 1100°C. Fractographic analysis of the tested alloy samples revealed different fracture surface structures depending on the heat treatment conditions, but the same fracture mechanism. In all cases, fracture occurs by quasi-cleavage, and the microcrack nucleus is associated with Ti2Ni/Ti4Ni2O inclusion particles or surface defects. The general results indicate the possibility of diffusion welding of TiNi alloys at a temperature of 1000–1100°C, without pronounced changes in their mechanical properties and ductility.

Methods of Improving Mechanical Integrity of Center-Link Chains for a Trolley Conveyor System

The purpose of this study is to identify the causes of the continuous failure of center-link chains for a trolley conveyor system by analyzing their failure characteristics under static and dynamic loading conditions and to propose methods for securing their mechanical integrity. For this purpose, center-link chains were minimally processed to prepare testing specimens with their original surface property and structural dimension maintained. These specimens were used to measure the tensile strength (1172.1 MPa), elongation (8.7%), yield strength (1073.4 MPa), hardness (44.1 HRC), and surface roughness (R_{a} = 10.5 μm). A three-point bending fatigue test (R = 0.1) resulted in a fatigue limit of 746.7 MPa. The results confirmed that the rough surface of the center-link chain caused a reduction in the elongation and fatigue limit and an increase in the scattering of the surface hardness and fatigue life, which were the direct causes of frequent chain failure in the field. To improve the fatigue life of the center-link chain, two methods were reviewed in this study. The first method was to reduce its surface roughness through surface machining of the center-link chain, and the second was to modify its shape design to allow improved structural integrity, even if the current surface roughness is retained. In the case of the first method, the results of the fatigue test using the specimen with reduced surface roughness (from R_{a} = 10.5 to 0.9 μm) by surface machining demonstrated reduced scattering and increased fatigue limit (from 746.7 to 920.3 MPa). As for the second method, a stiffener was added and two design variables (slope angle and fillet radius) of the stiffener were selected to propose a new design that can reduce the maximum stress ~ 1.6 times compared to the conventional design. While both methods for improving the structural integrity of the center-link chain were effective enough to improve the fatigue life of the chain, the second method, which requires only the initial mold manufacturing cost for modifying the shape design of the chain, was finally selected, because it exhibited better economic feasibility than the first method that increases the product cost and overall process time due to addition of the surface machining process.

Fatigue Limit Improvement and Rendering Defects Harmless by Needle Peening for High Tensile Steel Welded Joint

The effects of needle peening (NP) on the bending fatigue limit of a high tensile steel (HTS) HT780 (JIS-SHY685)-welded joint containing an artificial semicircular slit on the weld toe were investigated. Three-point bending fatigue tests were conducted at a stress ratio of R = 0.05 for NP-treated welded specimens with and without a slit. The fatigue limits of all specimens increased by 9–200% due to the NP treatment. Furthermore, NP-treated specimens with slit depths of a = 1.0 mm exhibited high fatigue limits that were equal to those of NP-treated specimens without a slit. Therefore, a semicircular slit of less than a = 1.0 mm could be rendered harmless through NP treatment. This result indicates that the reliability of HTS-welded joints can be significantly improved via NP for surface defects with depths that are less than 1 mm, which are not detected through non-destructive inspection (NDI). Therefore, the problem regarding the reliability of HTS-welded joints that restricts the industrial utilization of HTS can be solved by performing both NDI and NP. The dominant factor that contributed to the improvement of the fatigue limit and increase in the acceptable defect size was the introduction of large and deep compressive residual stress with non-propagating cracks.

Development of Fatigue Testing System for in-situ Observation by AFM & SEM

A three-point bend fatigue miniature stage for in-situ observation of fatigue microcrack initiation and growth behaviour by scanning electron microscopy (SEM) and atomic force microscopy (AFM) has been manufactured. Details of the stage design with finite element analysis of the stress profiles on loading are provided. The proposed stage facilitates study of the micro mechanisms of fatigue when used during SEM and AFM scanning of the sample surface. To demonstrate the applicability of the system, fatigue tests have been performed on annealed AISI Type 316 stainless steel. Surface topography images obtained by SEM and HS-AFM (High Speed AFM) are presented for comparison. The data can be used to validate crystal plasticity models which should then directly predict multiaxial behaviour without recourse to deformation rules such as equivalent stress or strain.