Low-cycle Fatigue Strength Research Articles

Assessment of low cycle fatigue strength of notched components

Abstract In this paper, an attempt is made to assess low cycle fatigue (LCF) strength of notched components criteria, which are used in the practical design procedures for the fatigue life prediction. The Von Mises stress, as a proven in design practice parameter was used. The assessment based on both the finite element method analysis and on numerous experimental data obtained by LCF testing of two alloyed steels and two titanium alloys. Specimens of various shapes (cylinder, plate) with theoretical stress concentration factors ranging from 1.39 to 7.8 as well as smooth ones were used. The investigation shows that the stress concentration factor K t predetermines stress or strain cycling in the concentration zone, and the cyclic life may be predicted by use of the Von Mises stress or strain, correspondingly.

Corrosion-Mechanical Strength of Structural Materials in Contact with Liquid Lead

Corrosion-mechanical tests of 10Kh15N9S3B and 10Kh9NSMFB steel samples in liquid lead at 360–550°C have shown that the long-time strength in lead flow decreases at short times to fracture and converges to the results of tests performed in air at about 10000 h. No liquid-metal embrittlement of 10Kh15N9C3B steel is observed. 10Kh9NSMFB steel becomes brittle in the hardened state. Embrittlement intensifies in notched samples. The low-cycle fatigue strength is correlated with a change in ductility with short-time fracture. No decrease of fracture toughness of 10Kh9NSMFB steel with a fatigue crack was observed in tests in liquid lead.

Low cycle fatigue strength of butt-welded steel joint by means of new testing system with image technique

In this study, we investigated the low cycle fatigue strength of welded joints using a new fatigue testing system in which the image analysis technique was used to measure the strains in specimens. After checking the validity of the testing system, we used this new system to conduct fatigue tests on welded joint specimens. The test results indicate that the fatigue strength of welded material (weld deposit and heat affected zone) is much lower than that of the plain material.

Low cycle fatigue strength assessment of butt and fillet weld connections

The paper presents the analysis of the low cycle fatigue strength of axially tensioned welded connections between I- or H-profile and gusset plate, in which butt and fillet welds cooperate in transmitting the load. The presented approach is based on the local strain state at the notch directly. The method of stress analysis was presented, which was divided into global finite element analysis at the geometrical stress level, coupled with a local finite analysis of the notch area. Local elastic–plastic stress–strain behaviour at the notch root was evaluated from Neuber’s rule and load–life curves were established for such connections from equation based on Langer’s formula. Although in steel structures the applied nominal load does not vary in fully cyclic type, material anisotropy developed during first plastic strain in notch area can lead after unloading to fully reversed plastic stress. In some cases, the first crack can occur after a small number of load cycles.

Low-cycle fatigue strength under step loading of a Si3N4 + SiC nanocomposite at 1350°C

The low-cycle fatigue behaviour of a hot pressed silicon nitride/silicon carbide nanocomposite and a reference monolithic Si3N4 have been investigated in 4-point bending at 1350°C in air using stepwise loading. The nanocomposite was prepared using 20% of SiCN amorphous powder as an additive, together with 5% yttria, to crystalline α-silicon nitride powder. Two types of specimen have been tested, with and without a sharp notch (notch tip radius ∼10 μm) at applied loads from 50 N with steps of 25 N and from 50 N with steps of 50 N, respectively. Five cycles have been performed at all applied load levels with an amplitude of 50 N for both types of specimen. The deflection of the specimens has been recorded up to specimen failure. The failure load of the unnotched nanocomposite was significantly higher than that of the monolithic material whereas for the notched specimens only a small difference has been found between the failure loads of the monolithic and the composite. Notched specimens of both materials exhibited a similar size of the slow crack growth area at catastrophic fracture, whereas for unnotched specimens the size of the slow crack growth area was significantly larger for the monolithic ceramic. The nanocomposite exhibits higher fatigue strength due to its higher resistance against stress corrosion damage and stress corrosion crack growth.

1116 Sn-Ag-Bi-Cu系Pbフリーはんだの機械的性質と低サイクル疲労強度(OS-1 鉛フリ-はんだ)

1116 Sn-Ag-Bi-Cu系Pbフリーはんだの機械的性質と低サイクル疲労強度(OS-1 鉛フリ-はんだ)

Low cycle fatigue strength of diffusion bonded joints of alumina dispersion-strengthened copper to stainless steel

It is proposed that the first wall and divertor components of ITER employ alumina dispersion-strengthened copper (DS Cu) joined to austenitic stainless steel. In this work, low cycle fatigue tests were performed on a direct diffusion bonded joint, a diffusion bonded joint with a Au interlayer, stainless steel and DS Cu in order to investigate their fatigue strength and fracture behavior. For the direct diffusion bonded joint, the fatigue strength in the small strain range was considerably lower than that of the DS Cu, while in the large strain range the fatigue strength was similar to that of the DS Cu. The low cycle fatigue strength of the Au interlayer joint increased compared with the direct diffusion bonded joint, and was the same as that of the DS Cu. The strain distribution in joint specimens was not uniform, because the deformation stress was different between the 316 stainless steel and the DS Cu. The fracture locations for the joint specimens varied depending on their strain distribution.

ANALYSIS OF CYCLIC FRACTURE OF THE THREADED JOINTS/SRIEGINIŲ JUNGČIŲ CIKLINIO IRIMO SĄLYGŲ ANALIZĖ

ANALYSIS OF CYCLIC FRACTURE OF THE THREADED JOINTS/SRIEGINIŲ JUNGČIŲ CIKLINIO IRIMO SĄLYGŲ ANALIZĖ

335 Ni 基超合金コーティング材の界面強度と低サイクル疲労寿命

335 Ni 基超合金コーティング材の界面強度と低サイクル疲労寿命

Very low cycle fatigue properties of butt welded joints containing weld defects. Study of acceptable size of defects in girth welds of gas pipelines

Summary This paper describes an investigation of the very low cycle fatigue strength of HT570 steel welded joints containing weld defects to determine the acceptable size of weld defects in girth welds of underground gas pipelines subjected to cyclic ground displacements due to earthquakes. Butt welded joints containing incomplete penetration (IP), blowholes (BH), lack of fusion in theintermedi‐ate pass (LF), and cracks in the penetration bead (CR) were prepared and tested under strain‐controlled conditions. All specimens tested in the present research study satisfy the fatigue design curve for girth welds of underground gas pipelines. The fatigue strength of specimens containing weld defects generally decreases with an increasing equivalent defect size. The shape of a crack initiated from a defect is affected by the reinforcement, with the surface crack propagating rapidly along the weld toe of the penetration weld over the width of the specimen. The relationship between the J‐integral range and the crack propagation rate under a very low cycle fatigue load is virtually the same as the extension of the relationship in the low J‐integral range. A crack propagation analysis based on the defect being regarded as a crack is performed to determine the relationship between the defect size and the number of cycles over the plate thickness. The critical size of defects in welds of gas pipelines under cyclic ground displacements is proposed by the analysis. The critical crack size of surface defects is applicable to buried defects, because surface defects give a more conservative evaluation than buried defects. The analytical results provide more conservative estimations than the experimental ones.

1002 Ti-6Al-4V合金の高温低サイクル疲労強度に及ぼすひずみ波形の影響(O.S.6-1 高温・腐食環境強度)(オーガナイズドセッション6 : 先進材料の機能と強度特性)

1002 Ti-6Al-4V合金の高温低サイクル疲労強度に及ぼすひずみ波形の影響(O.S.6-1 高温・腐食環境強度)(オーガナイズドセッション6 : 先進材料の機能と強度特性)

Influence of Testing Temperature on Low-Cycle Fatigue Strength of Thermal Barrier-Coated Ni-Base Superalloy.

Thermal barrier coating process has been established in gas turbine and was used for some parts operating at elevated temperature. Thermal barrier coating was constructed with ceramics layer and bond layer as MCrAlY alloy (where M is Ni, Co or Fe). The mechanical properties such as ductility of CoNiCrAlY alloy change drastically with temperature. Then it is important to clarify the influence of temperature on low-cycle fatigue property of thermal barrier coated material. Low-cycle fatigue tests of thermal-barrier coated Ni-base superalloy were performed at room temperature, 773 K, 973 K and 1 088 K. From the experimental results, decrease in fatigue life of the thermal-barrier coated Ni-base superalloy was observed at room temperature as compared with that of the uncoated one. On the other hand, fatigue life of the coated one was increased as compared with that of the uncoated one at 1 088K. Difference in fatigue life between coated and uncoated material was explained by a crack initiation behavior, which was obtained by A. C. potential method and observation by optical microscope. The number of cycles to crack initiation of coated specimen is smaller than that of uncoated one at room temperature. On the other hand, crack initiation life of coated specimen is larger than that of uncoated one. If was discussed from the experimental results that crack initiation and fatigue life were affected by the ductility of the bond layer.

A study on the joining strength of laser spot welding for automotive applications

Laser beam welding offers a unique combination of high speed, precision and low heat distortion, compared with the conventional welding process. This combination of advantages is especially attractive for the automotive industry. Until now automobile fabrication has widely used resistance spot welding (RSW), however, because of geometrical and structural problems, RSW requires many welding machines for which reason by substituting RSW with laser spot welding (LSW), there are savings in equipment cost. Many factors need to be considered when replacing one joining method with another. In the present case, the strength differences between laser-spot and resistance-spot welds must be ascertained before LSW can be implemented in selected applications. Therefore, the low-cycle fatigue strength and residual stress of laser spot welded specimens were measured and compared with those of RSW results in order to adapt LSW in the automobile industry. Also for strength estimation, the residual stress of the laser spot welded zone was calculated. All of the calculations were performed with the ABAQUS code on a workstation.

溶接きずを有する継手の極低サイクル疲労特性 ガス導管円周溶接部の許容きず寸法の検討

Very low cycle fatigue strength of welded joints of HT570 steel with weld defects was investigated to study the acceptable size of weld defects in girth welds of gas pipelines under cyclic displacement of land by earthquake. Butt welded joints containing incomplete penetration, porosities, lack of fusion in the intermediate pass, and crack in penetration bead were manufactured and tested under strain controlled conditions.All specimens tested in this study satisfied the fatigue design curve for girth welds of gas pipelines. The fatigue strength of specimens with weld defects generally decreases with the increase of equivalent defect size. The shape of cracks initiated from defects is affected by the reinforcement, and the surface crack propagates rapidly along the weld toe of the penetration welds and passes through the width of the specimen. The relationship between the J-integral range and the crack propagation rate under very low cycle fatigue load is almost the same as the extension of the relationship in low J-integral range. The crack propagation analysis which regards defects as cracks leads to the relationship between the size of defects and the number of cycles passing through the thickness. The critical size of defects in welds of gas pipeline under cyclic displacement is proposed by the analysis. The critical crack size of the surface defect could be applicable to the buried defect because the surface defects give a more conservative evaluation than buried defects. The analytical results give conservative estimations compared to experimental results.

改良９Ｃｒ‐１Ｍｏ鋼厚肉鍛造材の高温低サイクル疲労強度に及ぼす切欠きおよび熱時効の影響

In this study, the influence of thermal aging and notch on low-cycle fatigue strength of forged mod. 9Cr-1Mo steel was investigated at elevated temperature. Total strain-controlled low-cycle fatigue tests using the smooth specimen and load-controlled low-cycle fatigue tests using the notched specimen were carried out at 873K in air. The triangular, tension-hold and compression-hold trapezoidal waveforms were applied to the specimens in a series of the tests. From the experimental results, there is no effect of the thermal aging on strain-controlled low-cycle fatigue life using the smooth specimen with each strain waveform. However, low-cycle fatigue life of the notched specimen decreased by thermal aging under load-controlled condition with triangular and tension-hold trapezoidal waveform.

Low-cycle fatigue of austempered ductile irons at various strain ratios

The effects of mean stress on the low-cycle fatigue (LCF) properties of austempered ductile irons (ADIs) were investigated. Uniaxial LCF tests were conducted under strain-control with three strain ratios, R=−1, 0, and 0.5, on a couple of different grades of ADIs. These ADIs were respectively produced at two austempering temperatures, 300 and 360°C, from the same base ductile iron. Results indicated that LCF lives in both austempered conditions were decreased with an increase in strain ratio due to mean stress effects. As ADI austempered at 300°C is more susceptible to the mean stress effects, a greater reduction in LCF strength at higher strain ratios of R=0 and 0.5 was observed in comparison to ADI austempered at 360°C. All the LCF data obtained under various combinations of heat treatments and strain ratios were well correlated by a modified SWT approach for lifetime prediction.

Influence of brazing conditions on the strength of brazed joints of alumina dispersion-strengthened copper to 316 stainless steel

Brazing of alumina dispersion-strengthened copper (DS Cu) to 316 stainless steel were conducted in order to investigate the influence of filler metals and brazing conditions on the joint strength. The brazing were performed with a silver-base (BAg-8) and three kinds of gold-base (BAu-2,4,11) filler metals with varying brazing joint clearance and brazing time. The filler metal had a greater effect on the joint strength than the brazing joint clearance and brazing time. The joint with BAu-2 was superior to the joint with other filler metals. The tensile strength of the joint with BAu-2 was as large as that of DS Cu, however, the Charpy and low cycle fatigue strength were lower than those of DS Cu. The DS Cu melted near the brazed zone, consequently recrystallization and agglomeration of alumina occurred in the diffusion layer for all filler metals. The grain size after the recrystallization was small in order of BAu-2, BAu-4 and BAu-11, that was in accordance with the order of the brazing temperature. The excellent fracture strength for the joint with BAu-2 was attributed to the smallest grain size.

Low cycle fatigue strength assessment of cruciform welded joints

This paper presents the results of an experimental study on the low cycle fatigue strength of cruciform welded joints. Tests were performed on 22 specimens of three different categories, namely full-penetration, partial-penetration and fillet welded cruciform joints. These tests consisted basically of imposing on each specimen a constant amplitude cyclic displacement history in the plastic range of the material behaviour. The experimental testing procedure is described and the results are presented. The unified approach for low and high cycle fatigue strength assessment proposed by Ballio and Castiglioni [1]is applied to the test results. Such approach leads to the conclusion that the same set of Wöhler ( S– N) lines [2]usually given in recommendations [3]for high cycle fatigue can be also applicable in the low cycle fatigue range. It is demonstrated that the proposed damage model allows prediction of the number of cycles to failure for the specimens, at each given cycle amplitude, with a notable accuracy.

Metallurgical investigation of laser welds in wrought Waspaloy

Wrought Waspaloy was autogenously welded by means of Nd-YAG pulsed and CO2 continuous wave lasers and the microstructures of laser welds investigated using scanning and transmission electron microscopy. The fusion zones of as welded specimens contained fine γ′ and isolated MC carbides. The hardness profile of laser welds in wrought Waspaloy peaked at the fusion zone and heat affected zone (HAZ) boundaries, owing to fine γ′ precipitation and MC carbides in the fusion zone, and γ′ and M23C6 precipitates at the HAZ boundaries. After a full post-weld heat treatment, the hardness of the bead can be recovered to around 400 IIV but the hardness peaks at the HAZ boundaries became higher relative to the fusion zone and base material after aging at 450°C for 20 h. The microstructures in the fusion zone of post-weld heat treated welds consist of M23C6, MC, and γ′ which are quite uniform in size. The low cycle fatigue (LCF) strength of laser welds decreased to 400 MPa compared with 450 MPa for the base material. Fatigue cracks are initiated at the boundaries of the fusion zone and grow into the HAZ or along the boundary of the fusion zone. Fractography of LCF specimens showed ductile transgranular fracture.

Analytical and Experimental Hybrid Study on Thermal Fatigue Strength of Electronic Solder Joints. 2nd Report. Evaluation by Isothermal Mechanical Fatigue Tests.

An isothermal mechanical fatigue test method and fatigue testing equipment were developed to investigate the low cycle fatigue strength of Sn-Pb eutectic solder joints. Based upon the results of finite element analysis, it is found that the rate of inelastic strain components (plastic strain and creep strain) in the solder joints can be accurately controlled by the displacement rate of the linearactuator of the test equipment. The isothermal mechanical fatigue tests were carried out by controlling the displacement at a very high displacement rate, and a very low displacement rate, respectively ; only plastic strain was generated in the solder joints at the high displacement rate, and only creep strain at the low rate. It is found that the low cycle fatigue life of Sn-Pb eutectic solder joints is not grealty affected by the ratio of inelastic components. Therefore, it can be said that the low cycle fatigue life can be estimated from the equivalent inelastic strain range, and the relationship of the cycles to failure and the strain range follows Coffin-Manson's law. In comparison of the mechanical fatigue strength results with the results of the cyclic thermal tests, it was found that the mechanical fatigue test method can be used as a good accelerated test method for the thermal fatigue strength of Sn-Pb eutectic solder joints.