Low Fatigue Strength Research Articles

Effect of re-filling probe hole on tensile failure and fatigue behaviour of friction stir spot welded joints in Al–Mg–Si alloy

In this study, the fatigue behaviour of Al–Mg–Si aluminium alloy joints joined by friction stir spot welding (FSSW) was investigated. FSSW was performed using a specially designed double-acting tool consisting of outer flat shoulder and inner retractable probe, which could re-fill probe hole of the joints. Lap-shear specimens with probe hole and re-filled one were fabricated. The tensile strength of the joint with re-filled hole was higher than that of the joint with probe hole, where shear fracture through the nugget occurred in both joints. The re-filling process increased effective cross sectional area of the nugget, resulting in higher tensile strength. Fatigue tests were conducted using lap-shear specimens at a stress ratio of R = 0.1. The fatigue strength of the joint with re-filled probe hole was nearly the same as, but lower than, at low and high applied loads, respectively, that of the joint with probe hole. The pull out of the nugget, i.e., plug type fracture, was seen in the joint with re-filled probe hole, while shear fracture through the nugget was observed in the joint with prove hole. The observation of fatigue crack growth path revealed that the plug type fracture through the upper sheet was responsible for the observed lower fatigue strength of the joint with re-filled prove hole at high applied loads.

High-temperature fatigue properties of austenitic superalloys 718, A286 and 304L

High-temperature fatigue properties of austenitic superalloys 718, A286, and 304L were investigated in region between 10 2 and 10 7 cycles. The alloys 718 and A286 are precipitation-hardening type austenitic superalloys, and 304L is a solid solution-hardening type austenitic steel. The fatigue strengths of alloys 718 and A286 in high-cycle region over 10 4 cycles showed a grain size effect. The initiation sites of the fracture were the crystallographic facets corresponding to the austenitic grain size. Therefore the coarse-grain alloys showed the lower fatigue strength than the fine-grain alloys, because the fatigue strength is generally inversely proportionate to the initiation defect areas for the fatigue cracks. In low-cycle region under 10 4 cycles, on the other, the initiation sites of the fracture were the surface of the specimens. This type fracture mode is common for fatigue fracture of smoothed specimens. The ratio of the fatigue limit at 10 7 cycles to the tensile strength is extremely low for the coarse-grain alloys compared with the common case of 0.5. From these results fatigue fracture map is proposed.

浸炭材の低サイクル衝撃疲労特性に及ぼす表層炭素濃度と硫黄含有量の影響

Effect of surface carbon and sulfur on the low cycle impact fatigue properties in carburized steels were studied by using gear impact testing machine and strain gauge. The results obtained are summarized as follows: The low cycle impact fatigue strength was especially improved by decreasing content of the surface carbon and content of sulfur. It was found that increasing the crack initiation life by decreasing content of the surface carbon, and the initial crack length shorten. Furthermore, It was explained that the crack propagation speed decreased with decreasing sulfide. The method for estimating the crack initiation life from tensile properties of carburized layer is proposed by modifying Manson's Universal Slope Method.

1059 SNCM439の内部起点型疲労特性に及ぼす焼もどし温度の影響(J12-5 金属材料の疲労特性と破壊機構(5) 低サイクル疲労/超高サイクル疲労,ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Uniaxial tension-compression fatigue tests of Ni-Cr-Mo steel "SNCM439" tempered at different temperatures were conducted in air environment. Effects of tempering temperature on fatigue properties were investigated. As a result, the following conclusions were obtained; (1) Fatigue limit of surface fracture was proportional to Vickers hardness, and the relation was in good agreement with the expression σ≈ 1.6HV. (2) In very high cycle region, the fatigue strength of the material tempered at 433K was lower than that tempered at 573K. In other words, the harder material showed lower fatigue strength. (3)The kinds of inclusions, which can be interior fracture origins, probably change between different tempering temperature.

Effect of Powder Size on Fatigue Behaviour in Mg<sub>2</sub>Si-Dispersed Magnesium Alloys Produced by Solid-State Synthesis

The fatigue behaviour of newly developed Mg2Si-dispersed magnesium (Mg) alloys produced by solid-state synthesis was studied. Rotary bending fatigue tests have been performed using smooth specimens of materials produced with fine and coarse AZ31 alloy powders. Both Mg2Si-dispersed Mg alloys exhibited lower fatigue strength than a conventional extruded AZ31 alloy and the powder size dependence of fatigue strength was clearly recognized, where the material produced with fine alloy powder showed considerably higher fatigue strength than the counterpart. Fatigue cracks invariably initiated at large Mg2Si particles immediately after cyclic loading was applied and subsequent small crack growth was faster than the extruded AZ31 alloy. It was concluded that the lower fatigue strength of Mg2Si-dispersed Mg alloys was attributed to premature crack initiation at Mg2Si particles and faster small crack growth, and the observed powder size dependence of fatigue strength was due to difference in the size of the particle from which the crack initiated.

Possibilities and extension of XRD material response analysis in failure research for the advanced evaluation of the damage level of Hertzian loaded components

AbstractDue to clearly distinguishable damage symptoms, it is differentiated between the surface and sub‐surface failure mode of rolling bearings. Material states red out by X‐ray diffraction (XRD) residual stress measurements point to a variety of loading conditions especially at raceway surfaces that are associated with several competing failure mechanisms. The corresponding lifetime reduction can range from the lower fatigue strength region to material ratcheting in extreme cases. Relevant position of the microstructural changes and nature of the failure mechanisms are characterized. The time alteration of the XRD material parameters measured at or near the surface and at the depth of the maximum equivalent stress correlates, in a different manner, with the statistical parameter of the 10 % bearing life. Both failure modes are illustrated by concrete examples. Contaminated lubricant and boundary lubrication, which represent practically important surface‐induced failures, are discussed in more detail. Gray staining, i.e. shallow pitting, often occurs without distinct indication of global material aging by means of XRD characteristics. Here, scanning electron microscopy observations and electron microprobe analyses point to corrosion fatigue as acting surface failure mechanism. The interaction between material and lubricant under complex loading regimes particularly of mixed friction and corrosion opens further failure research areas in the field of tribology.

Effect of coating microstructure on the fatigue properties of steel thermally sprayed with Ni-based self-fluxing alloy

In order to investigate the factors that most strongly influence fatigue properties of steel thermally sprayed with Ni-based self-fluxing alloy, we prepared three types of thermally sprayed specimens which differed in heating time in the fusing process. Rotational bending fatigue tests were carried out, and coating microstructures were observed. The results show that fatigue properties of steel thermally sprayed with Ni-based self-fluxing alloy were affected by two factors. The first factor is the size and population of porosities in the coating. This factor was changed by fusing time with longer fusing times producing larger and more porosities. Because the fatigue crack initiates at the porosities, specimens fused for longer time exhibit lower fatigue strength. The second factor is lowered hardness of Ni matrix accompanying chromium segregation. The sprayed specimens fused for longer time exhibit lower fatigue strength, because the longer fusing process induces segregation of the chromium compound in the coated microstructures. Therefore, performing the fusion for a shorter time is effective for producing sprayed materials of enhanced fatigue properties.

2311 摩擦攪拌により組織改質した鋳造アルミニウム合金AC4CHの疲労挙動(S07-2 非鉄材料の疲労特性,S07 構造材料の疲労強度とき裂進展問題)

The fatigue behaviour of friction stir processed (FSPed) cast aluminium alloy, AC4CH-T6 was investigated. Fully reversed axial fatigue tests have been performed using as-cast, FSPed and post heat treated (PHTed) specimens. The FSPed specimens exhibited lower fatigue strength in high stress region, but higher fatigue limit than the as-cast specimens. Casting defects were eliminated by FSP, resulting in higher fatigue limit. However, the crack growth rates of the FSPed specimens were faster than the as-cast ones due softening of the material by heat input during FSP. This led to the lower fatigue strength of the FSPed specimens in high stress region. Post heat treatment could recover the hardness, but it was detrimental to crack initiation resistance. The effects of FSP and PHT were discussed based on the microstructural consideration.

A Study on Delamination Behavior of Coating Thermally Sprayed with Ni-Based Self-Fluxing Alloy under Fatigue Loadings

In order to examine the mechanisms of delamination between coating layer and substrate under fatigue loading in steel thermally sprayed with Ni-based self-fluxing alloy, three types of specimens with different coating thickness were prepared. The fusing was performed by using an induction heating system in order to change the stress distribution. Rotating bending fatigue tests and observations of fracture surfaces were carried out. With all fracture surfaces, the delamination between the coating and the substrate occurred. However, the coating thickness strongly affected the fatigue properties ; the thicker the coating, the lower the fatigue strength. In order to clarify the mechanism of delamination, the differences of strain on the interface between the coating layer and the substrate were performed by using FEM analysis. The differences of strain strongly affected the delamination between the coating and substrate ; the thinner the coating, the smaller the differences of strain. Considering these results and observing the interface during the cyclic loading in detail, it is considered that the greater the differences of strain on the interface between the coating and the substrate, the earlier the delamination occurred and propagated, and a lower fatigue strength resulted.

435 Fatigue behavior of die-cast AM60 alloy under different environmental conditions

Four-point bend (Inner span: 10mm, Outer span: 30mm) fatigue tests were carried out with two-groups of AM60 alloy; one group was as-cast and another group was shot-blasted. The as-cast specimens were tested in three different environmental conditions; low humidity (20℃, 55%RH), high humidity (50℃, 80%RH) and 5wt% NaCl corrosive environment while shot blasted specimens were tested in high humidity and 5wt% NaCl environment. In each case, frequency was kept at 20Hz, load ratio at 0.1 together with sinusoidal waveform. S-N curves show that shot blasted specimens present better fatigue life and fatigue strength as compared to those of as-cast specimens under any environmental condition. The lowest fatigue strength was shown by as-cast specimens under NaCl environment. However, fractographic observations did not reveal presence of any corrosion pit which is generally found by other researchers. On the contrary, corrosion induced crack nucleating sites were observed for both groups of alloys and assumed to be the main cause of degrading fatigue strength under aggressive environment. As for shot-blasted specimens compressive residual stress might have been induced during shot blasting which could have delayed initiation of fatigue cracks. Hence they showed better fatigue performance as compared to those of as-cast specimens under different environments.

525 1050および5083アルミニウム合金摩擦攪拌接合継手の疲労挙動(GS.B 接着・接合)

Fully reversed axial fatigue tests have been performed in order to investigate fatigue behaviour in friction stir welded (FSW) joints of 1050 and 5083 aluminium alloys. In 5083 FSW joints, fatigue failure occurred in the parent metal away from the weld centre, leading to similar fatigue strength to the parent metal. In 1050 FSW joints, however, fatigue crack initiated near the boundary between thermo-mechanically affected zone and heat affected zone, which resulted in lower fatigue strength of the joint than the parent metal. This was attributed to the local plastic deformation due to the discontinuity of hardness at the boundary.

Ultrasonic Fatigue Testing with Dumbbell-type Specimens Having a Straight Part

Dumbbell-type specimens having a straight part was used in ultrasonic fatigue testing for high-strength steel in order to enlarge risk volumes. In this case, the stress distribution in the straight part was almost uniform in the axial direction. The steel used was low-temperature-tempered SCM440 low-alloy steel and the steel showed fish-eye fracture originating mostly from an Al2O3 inclusion. The ultrasonic fatigue testing results were compared with the conventional servo-hydraulic fatigue testing. The two kinds of fatigue testing results showed good agreements in case of fish-eye fracture. This agreements supported the validity of the ultrasonic fatigue testing with specimens having straight part in the gigacycle fatigue tests ending in fish-eye fracture. Moreover, the ultrasonic fatigue testing were conducted also with specimens without a straight part to confirm the effects of the risk volumes. In comparing those ultrasonic fatigue testing results, the effects of the risk volume were cleary observed, i.e. the result with larger risk volumes showed lower fatigue strengths due to presence of a larger inclusion at the fish-eye fracture origin.

2031 平行部付き試験片を用いた超音波疲労試験の検討(S11-2 軸荷重下の超高サイクル疲労特性,S11 金属材料の超高サイクル疲労特性の解明)

Specimens having a straight part were used in ultrasonic fatigue testing for low-temperature-tempered SCM440 low-alloy steel. The steel used showed fish-eye fracture originating mostly from an Al_2O_3 inclusion. The ultrasonic fatigue testing results were compared with the conventional servo-hydraulic fatigue testing. The two kinds of fatigue testing results showed a good agreement in case of fish-eye fracture. This agreement supported the validity of the ultrasonic fatigue testing with specimens having a straight part. Moreover, the ultrasonic fatigue testing was conducted also with specimens without a straight part to confirm the effects of the risk volumes. As the result, the effects of the risk volumes were clearly observed i.e. the result with larger risk volumes showed lower fatigue strengths due to presence of a larger inclusion at the fish-eye fracture origin.

ＤＬＣ／高濃度浸炭複合表面改質ＳＣＭ４１５における高疲労信頼性・高トライボロジ特性の発現

In order to achieve highly functional materials which show high fatigue and high tribological properties, two surface modifications, high carbon carburizing and DLC (Diamond Like Carbon) coating, were applied to low carbon steel (SCM415). However, there are some reports which raise the concern that coat materials like DLC show low fatigue strength compared with substrate only due to high heating history during the coating process. So, in this study, in order to prevent the base material from losing fatigue strength, DLC was coated by using the UBMS (Unbalanced Magnetron Sputtering) method, which can coat at low temperature. Furthermore, three types of DLC-coated specimens with different coating structures were prepared. Rotational bending fatigue tests and friction-wear tests were carried out. It was clear that the friction coefficient of each specimens coated with DLC stabilized under 0.1. And, the specimen coated a 5 μm-thick DLC film shows the highest tribological properties because the DLC coating was transcribed on the opposite material during the wear process. Furthermore, no decreases in fatigue strength were observed for any of the coated specimens. It can be concluded that it is possible to achieve highly functional materials by applying DLC hybrid surface modification as suggested in this search.

The Improvement of Tribological and Fatigue Properties of Aluminum Alloy coated DLC Hybrid Surface Modification

Aluminum alloy has been used in several kinds of industries against a background of environmental problem and energy saving because aluminum alloy has a lot of advantages ; lightness, recyclability and good corrosion resistance. However, aluminum alloy has poor wear-resistance, so in order to solve this problem, DLC (Diamond Like Carbon) coating which indicates high tribological properties was performed. However, there are some reports which are concerned that coated materials like DLC indicate low fatigue strength compared with substrate only due to high heating history during the coating process. So, in this study, in order to improve wear resistance and to prevent the base material from decrease of fatigue strength, original DLC coating was suggested ; (i) coating process was performed by using UBMS (Unbalanced Magnetron Sputtering) method, and (ii) inter-layer which was doped Tungsten was formed on the substrate, and on the inter-layer (iii) two kind of DLC coatings were laminated for nano order. Wear tests and fatigue tests were carried out. Aluminum alloy coated DLC indicated very low wear rates, excellent tribological properties. Furthermore, it indicated high fatigue strength compared with substrate only.

自由鍛造におけるインコネル７１８プロセスモデリング技術の開発

INCOLEL alloy 718(IN718) has been used for important components such as gas turbine disk and jet engine shaft, and the amount of the usage are increasing. As for gas turbine disk, the demand of reliability has been getting stronger. One of the most effective methods to improve the reliability is grain refinement giving the increase of low fatigue strength and toghness. In order to refine the micro-structure of IN718, the only method is recrystarization at lower temperature in forging because of single-phase material. To obtain the forged components having fine micro-structure, forging temperature, strain and their distribution must be controled precisely. Process modelling using FE analysis is very effective to design optimal forging processes to obtain the target micro-structure and mechanical properties of the components. We have originally developed the digital engineering system named DAINUS composed of 4 prediction modules, which are (1)damage prediction module, (2)micro-structural prediction module, (3)die damege prediction module, (4)cavity consolidation prediction module. In this work, we have developed micro-structural prediction data base and system for IN718. Micro-structural prediction under multiple recrystallization and partial recrystallization condition is very difficult to obtain sufficient accuracy. Moreover the it's system in incremental free forging is also difficult, because of the rapid increase and decrease of the temperature, strong strain distribution, occurrence of additional recrystallization by repeated forging during partial recrystallization. In this paper, to solve these problems we have developed new micro-structural prediction system of IN718 by the following approaches, (1)establishment of the precise recrystallization data base in wide temperature range, (2)establishment of the incremental method corresponding to rapid temperature and strain change, (3)development of new averaging method. By these new approaches, we could obtain enough prediction accuracy even in incremental forging of gas turbine disk.

Comparative study on fatigue properties between AA2024-T4 friction stir welds and base materials

The comparative study on fatigue properties between AA2024-T4 friction stir welds and base materials have been performed and the influence of zigzag-curve defects across weld section on the fatigue properties of FSW joints were investigated. It was shown that the zigzag-curve defects were the inherent feature existed in the friction stir nugget and could make the characteristic fatigue strength decrease from 96.19 MPa for base material to 73.71 MPa for welded joints with a reduction of 23.4%. Although the lower fatigue strength than the corresponding base material, the FSW joints of AA2024-T4 Al alloy achieved higher fatigue strength compared to the traditional fusion design curves IIW FAT40 and Draft Eurocode 9 design category 55-6 for structural aluminum alloy components.

Influences of loading direction and thickness ratio on fatigue properties of tailored welded blanks

Fatigue properties of tailored welded blanks with the same material and different thickness were investigated. Fatigue tests of specimens with angles of 0°, 70° and 90° to the laser weld line were performed in order to clarify the influence of loading direction and thickness ratio. The levels of stress range affected the crack initiation areas of the 0° and 90° specimens, and simple heterogeneous mechanical models roughly expressed the macroscopic fracture behavior. The fatigue strength of the 0° specimens was almost the same as that of the base metal. In the 90° specimens, fatigue strength showed a clear decrease under relatively low stress range. Insufficient grinding of the 70° specimen edge caused crack initiation at the weld toe and low fatigue strength. An increase of thickness ratio induced a decrease of fatigue strength of the 90° specimen, although that of the 0° specimens was independent of thickness ratio. Fatigue crack propagation analyses under cyclic stress distribution revealed a close correlation with the test results. These experimental and analytical results were less sensitive to thickness ratio than estimations by elastic stress concentration factors.

Crack-Healing under Cyclic Stress and Improvement of the Resultant Fatigue Strength of Si<sub>3</sub>N<sub>4</sub>/SiC

Si3N4/SiC composite ceramics were hot-pressed in order to investigate their crack-healing behavior under cyclic stress and the resultant static fatigue strength. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under a cyclic bending stress of 210MPa in air at 900, 1000, 1100, and 1200 °C. The bending strength and static fatigue strength of the crack-healed specimens were systematically investigated at each healing temperature. The specimens which has been crack-healed and static fatigue-tested at 900 and 1000 °C showed lower static fatigue strength than those tested at 1100 and 1200 °C. Detailed investigation on the fracture surface of static fatigue-tested specimens showed that oxidation of the base material had strong effects on the static fatigue strength. It was found that when the specimens were pre-oxidized in air at 1300 °C, the surface was covered by a protective oxide layer, leading to a significant improvement of static fatigue strength at 900 and 1000 °C.

Long-term properties of carbon fibre composite tethers

The tethers of a tension leg platform (TLP) represent a design controlled by the required natural frequency of the floating TLP. In short this implies that the total stiffness of the tethers should be as high as possible but at the same time having the lowest mass possible. This requirement favors the use of carbon fiber composites; especially for deep water TLPs. Additional benefits of using carbon fiber composites in tethers are excellent resistance against corrosion and fatigue. Composite tethers are also candidates for use on hanging bridges and other infra structure applications. The tether design investigated here is based on pultruded carbon fibre composite rods assembled into strands of 31 and 85 rods. The rods in the strands are assembled in a helical fashion and likewise the strands in the tether are assembled with a helix. This allows the strands to be coilable. A main challenge when using these new applications is the demonstration of reliable long-term performance. A test program was established to evaluate the fatigue performance starting at the basic building block up to full scale testing. Initially single pultruded rods of 6 mm diameter were tested to establish the fatigue performance of the basic building block. Fatigue testing was extended to strands build of 85 rods and terminated in a special metallic end-fitting. All results are shown in SN-diagrams. As is evident from the SN-diagrams there is a lower fatigue strength associated with the strand compared to that of the single rod. The cause of that is found in the way the load is transferred from the rods to the steel termination. Finally the entire tether made of multiple strands, six 31 rods and seven 85 rods, was tested to demonstrate performance of the entire system. The testing qualified the long-term capacity of the tether system and can be used as a basis for evaluation of a specific application. The qualification was based on the testing described and a thorough evaluation of all possible failure modes.