Comparison of the effects of submerged laser peening,cavitation peening and shot peening on the improvement of the fatigue strength of magnesium alloy AZ31

To investigate the improvement in the fatigue strength of magnesium alloy by peening methods,magnesium alloy AZ31 was treated by submerged laser peening(SLP),cavitation peening(CP),and shot peening(SP),and the fatigue properties were evaluated by a plane bending fatigue test.In the case of SLP,both the impact induced by laser ablation(LA)and that caused by laser cavitation(LC),which developed after LA,were used.In the present study,the fatigue life at a constant bending stress was examined to determine the suitable coverage.It was found that the fatigue strengths at N=10^(7)for the SLP,CP,and SP specimens treated by each optimum condition were 56%,18%,and 16%higher,respectively,than that of the non-peened(NP)specimen,which was 97 MPa.The key factors in the improvement of fatigue strength by peening methods were work hardening and the introduction of compressive residual stress.

RELIABILITY OF AEROCRAFT GEAR FATIGUE STRENGTH

With the method of group test, fourty pairs of carburization-quenching gears made from 16NCD13 steel for aerocraft were tested to research the contacting fatigue strength on tooth flank. As a result, the samples of fatigue life at the moments when the pitting appears and reaches failure criterion were obtained at four stressing levels respectively. The distribution rule of fatigue life were distinguished, and the distribution parameters were estimated by statistical analysis. Based on that, the R-S-N curves with confidence 95% of contacting fatigue on gear tooth flank were evaluated. Therefore, the basic data were provided for the reliability design of the gears and prediction of their life.

New Developed Welding Electrode for Improving the Fatigue Strength of Welded Joints

A new welding electrode, low transformation temperature electrode (LTTE), was introduced in this paper. It was described in design principle, mechanics, chemical compositions of their deposited metal and manufacturing methods. It was proved that the best transformation starting temperature from austenite to martensite of the deposited metal of LTTE was at about 191℃ and it was obtained by adding alloying elements such as Cr, Ni, Mn and Mo. The microstructure of the weld metal of the LTTE was low carbon martensite and residual austenite. The compressive residual stress was induced around the weld of the LTTE and the -145 MPa in compression could be obtained in middle of weld metal. The fatigue tests showed that the fatigue strength of the longitudinal welded joints welded with the LTTE at 2×106 cycles was improved by 59% compared with that of the same type of welded joints welded with conventional E5015 and the fatigue life was increased by 47 times at 162 MPa. It is a very valuable method to improve the fatigue performance of welded joints.

CALCULATION OF FUZZY RELIABILITY IN THE CASE OF RANDOM STRESS AND FUZZY FATIGUE STRENGTH

The fuzzy sets theory is introduced into the fatigue reliability analysis. The concepts of maximizing set and minimizing set are developed to decide the ordering value of each fuzzy number, and these values can be used to determine the order of the fuzzy numbers. On the basis of the works mentioned above, the membership function defining the fuzzy safety event can be calculated, and then the fuzzy reliability in the case of stress and fuzzy fatigue strength is deduced. An example is given to illustrate the method.

EFFECT OF SUBSTRUCTURE AND RESIDUAL STRESS IN STRENGTHENED LAYER ON FATIGUE STRENGTH OF STAINLESS OR LOW CARBON STEEL

In the strengthened layer of stainless steel after shot peening,there are a great amount of deformation microtwins which may act as structural strengthening factor and prevent the gradual relaxation of surface residual stress during fatigue,so as to keep its rather high level of bending fatigue strength.However,in the strengthened surface layer of low carbon steel, dislocation cell structure is so unstalbe during fatigue that its surface residual stress relaxation cannot be retarded.Therefore,the bending fatigue strength of the low carbon steel can not be improred by shot peening.

Fatigue strength evaluation of self-piercing riveted joints of AZ31 Mg alloy and cold-rolled steel sheets

The application of magnesium alloys to automobiles is increasing due to their superior specific strength and specific stiffness.In this study,an upper sheet of AZ31 magnesium alloy and a lower sheet of cold-rolled steel were joined by self-piercing riveting(SPR),a method commonly used to join automotive panels.A cross-shaped specimen was fabricated with a punching force of 35 kN,which exhibited the best joint strength for the SPR joint specimen geometry.Monotonic and fatigue strengths were evaluated using cross-shaped specimens at loading angles of 0°,45°,and 90°.The load amplitude corresponding to the fatigue endurance limit was assumed to be at 106 cycles,and the fatigue ratios(=fatigue endurance limit/static strength)at the loading angles of 0°,45°,and 90°are 22%,13%,and 9%,respectively.For all three loading angle specimens,fatigue cracks initiated at the triple point where the rivet shank,the upper sheet and the lower sheet are in contact with each other,with the cracks propagating through the thickness of the upper sheet and ultimately leading to fracture.The fatigue lifetimes were evaluated through the von-Mises stress,maximum principal stress,and equivalent stress intensity factor.It was found that the fatigue lifetimes could be evaluated most appropriately through the maximum principal stress.

Flexural fatigue strength of steel fibrous concrete containing mixed steel fibres

This paper reports investigation conducted to study the fatigue performance of steel fibre reinforced concrete (SFRC) containing fibres of mixed aspect ratio. An extensive experimental program was conducted in which 90 flexural fatigue tests were carried out at different stress levels on size 500 mm×100 mm×100 mm SFRC specimens respectively containing 1.0%, 1.5% and 2.0% volume fraction of fibres. About 36 static flexural tests were also conducted to determine the static flexural strength prior to fatigue testing. Each volume fraction of fibres incorporated corrugated mixed steel fibres of size 0.6 mm×2.0 mm×25 mm and 0.6 mm×2.0 mm×50 mm in ratio 50:50 by weight. The results are presented both as S-N relationships, with the maximum fatigue stress expressed as a percentage of the strength under static loading, and as relationships between actually applied fatigue stress and number of loading cycles to failure. Two-million-cycle fatigue strengths of SFRC containing different volume fractions of mixed fibres were obtained and compared with plain concrete.

Assessment of Fatigue Strength of An Offshore Floating Production and Storage Unit

The procedure of assessment of structural fatigue strength of an offshore floating production and storage and offloading unit (FPSO) in this paper. The emphasis is placed on the long-term prediction of wave induced loading, the refined finite element model for hot spot stress calculation, the combination of stress components, and fatigue damage assessment based on S-N curve.

FATIGUE STRENGTH DETERMINATION OF BUTT WELDED JOINTS BY FEM

The influence of welding parameters of butt joints, made by three processes (MMA, MAG and T.I.M.E), on their geometric characteristics has been evaluated, which in turn caused a change of the fatigue strength. That fore the statistic analysis and FEM were used and a method developed in the Institute of Welding, which consist in a direct connection of parameters used during welding with the fatigue strength of joints welded by various processes. It has been proven, among others, which by using more advanced welding processes (T.I.M.E) the fatigue strength of welded joints can be increased in comparison to joints welded by MMA and MAG.

The Influence of the Inclusion on the Fatigue Strength of Ceramic Bush

In traditional plain bearing, bushes are made of metal. In order to improve the properties of the bush, it is necessary to conduct an experiment in which the purpose is to replace metal with ceramic. Because of its high hardness, long fatigue wear life, excellent corrosion resistance, good self-lubricating and low density, ceramic bush is suitable for plain bearing. In this paper the influence of the inclusion on the fatigue wear strength of the bush are analysed under the action of a external load with the finite element method. All these work may provide proofs for analysing the wearing mechanism and for controlling the effect of the inclusion. When the normal outer load is 6 N/mm, namely P=6 N/mm. The compressive main stress will be formed inside the basic part of the bush, which gathers in the center touching area between the journal and the the bush around the inclusion, most of the units have their own maximum alternate stress σ max, which is smaller than the fatigue limit of the material of bushes σ 0 (σ 0=60 MPa). If there is P=30 N/mm, some units will have σ max approaching the fatigue limit, stress concentration will occur around the inclusion, so will the local plastic deformation. Some special units exceed the fatigue limit of the material. When P=225 N/mm,most of the units will have the σ max, about 50～300 MPa, which surpasses the fatigue limit. At this time micro pitting and cracking on the bush surface can be observed with SEM. When P=350 MPa, the σ max is about 300～900 MPa, the center touching area on the surface of the sample and the units around the inclusion will exceed the fatigue limit generally, the surface of the bush, the serious wear corrosion and big fatigue cracking can be observed.

FINITE ELEMENT ANALYSIS TO THE FATIGUE STRENGTH OF SYNCHRONOUS BELT TEETH

By means of finite element analysis methods calculated the stress, strainand displacement of the belt teeth and analysisied the factors such as load distribution,specific property coefficient of material and temperature variation, which influence the fatigue strength of the belt teeth..

EFFECT OF DYNAMIC STRAIN AGING ON PURE BENDING FATIGUE STRENGTH OF AUSTENITIC STAINLESS STEEL

The effect of various dynamic strain aging(DSA)pre-treatment processes on pure bending fatigue strength of an 18-8 austenitic stainless steel was investigated.The results show that DSA pre-treatment processes increase remarkably the fatigue strength and the strengthening effects increase with increasing pre-strain temperature and pre-strain.The fatigue limit of specimens pre-treated by DSA is 87% higher than that by solution treatment,and 20% high- er than that by cold-working.TEM observations show that the uniform and stable dislocation networks with high density formed after DSA pre-treatment,which suppress effectively the initiation and propugation of fatigue cracks and increase the fatigue strength of materials.

STUDY ON THE METHOD OF FATIGUE STRENGTH DESIGN FOR DYNAMICALLY LOADED JOURNAL BEARING

In this paper, a novel design method, which is different from the traditional and empirical one (i. e., taking p and pv as the basic checking parameters) is presented for the fatigue strength design of dynamically loaded journal bearings. The method makes it possible that dynamically loaded bearings can be desed as same as other machine elements by stress-strength criterion. The practical design results show that the method has high accuracy and reliability, and may open a new visa in bearing fatigue designs.

EFFECT OF FRICTIONAL FORCE ON GEARING CONTACT FATIGUE STRENGTH

The model for computing frictional coefficient between two teeth faces at the state of mixed elastohydrodynamic lubrication is established. And then more than 80 sets of numerical calculations and six sets of disc fatigue tests are completed. The results show that when the film thickness ratio λ 〈1.6, frictional coefficient μ is drastically decreased as λ. rises; Thereafter it decreases smoothly until λ=4.5. When λ〉4.5, however, it goes up again with λ, which indicates that the excessive film thickness ratio will deteriorate gearing contact fatigue strength. At the end, the formulae for determining the frictional coefficients are formed.

A Proposal of Deciding Fatigue Strength in Bending Fatigue Test

Generally, the fatigue crack is initiated and then it is propagated toward the welding direction and the thickness direction. Finally, the joints lose the resistance to the external force. At present, as there is no deciding method of the fatigue strength （fatigue life）, this paper proposed it from the result obtained by bending test for fillet welded joints. Judging initiation of the fatigue crack from the measured value of strain gages, there was a possibility that the fatigue crack occurred at both sides of fillet welded joints. However, this was a different result from that of macrograph of cross section. On the other hand, the results obtained by FSM （field signature method） coincided with the result of macrograph of cross section. For the initial state, potential difference obtained by the electrostatic analysis based on FEM （finite element method） and that by FSM was accurately coincided. After confirming validity of the crack model for analysis, the crack model was specified by reproducing the propagating process of crack accurately through trial and error. It was concluded that the state which could not resist to the external force was regarded as fatigue strength based on equivalent stress obtained by elastic stress analysis for specified crack model. From the experimental result, it was proposed that 90% of repetition number corresponding to the state which could not resist to the external force （at the finish of the test） was regarded as fatigue strength （fatigue life） in consideration of safety and as the first approximation.

Fatigue Strength of Laser Welded Joints of PP and PC Components

The paper describes research of fatigue properties of laser welded joints of PP and PC reinforced composites. Welded specimens were tested on dynamic testing machine to obtain data for fatigue strength and fatigue life evaluation. Materials used for testing were PP and PC plastics filled with various amounts of glass fibers. Nine combinations of PP specimens and 6 combinations of PC specimens were welded and loaded on testing machine by fluctuating stress with constant stress ratio. Presented results include fatigue strength of welded joints and also the S-N curves based on experimental data and theory are derived.

Low Fatigue Strength of Clinch Joints

Clinching can be used for effective joining of different metallic materials, e.g., different grades of carbon and alloy steels, aluminum and its alloys and others. This is an alternative joining method to traditional methods involving screws, rivets or welding. Clinching is applied in many fields of manufacturing industry. Future applications of clinch joining require advances in quality and among others things, combination with other processes, e.g., hybrid joining by clinching and adhesive bonding. Although the static strength of clinched joints is lower than that of the others joints （e.g., pressure welded joints）, the fatigue strength is comparable. But there is a lack of information about fatigue strength of clinched joints. The static and low cyclic fatigue strengths of mechanically clinched joints were determined experimentally. The load-displacement curves of static strength were compared with low cyclic fatigue strength curves. It was observed the effect of strain hardening for clinched joint of low carbon steels （DC4） under cyclic loading.

Improving fatigue strength of bainite/martensite dual-phase steels in very high cycle fatigue regime by refining microstructures

Very high cycle fatigue behaviors of two bainite/martensite dual-phase steels were investigated.One of the steels was cyclic rapid heat treated and its microstructures were refined. Fatigue strength of the steel is 225 MPa higher than that without refining.Observation of fracture surfaces show that the fatigue cracks initiate at bainites for non-refined steel and at non-metallic inclusions for the refined steel.The size of inclusions is much smaller than that of bainites which results in the improvement of fatigue strength.

High-temperature fatigue strength of grain boundaries with different misorientations in nickel-based superalloy bicrystals

Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably during their manufacture, which are often characterized by grain boundary misorientation (GBM) and will weaken the mechanical properties of superalloys. However, the relationship between GBM and the fatigue properties of superalloys at elevated temperatures has seldom been investigated due to the difficulty in the sample preparation and experiment process. Based on six kinds of bicrystals with different tilt LAGBs made by a second-generation single-crystal superalloy, the effects of misorientation on the grain boundary microstructure and fatigue properties (980 °C) of superalloys were studied systematically in this work. It is found that, with the increase of GBM, the GB precipitates combined with the cast micropores increase monotonically, accordingly both the fatigue life and fatigue strength decrease successively. Fatigue fracture observations show that the cracks of all the bicrystals initiated from the cast micropores at GBs, and then propagated along the GBs. Therefore, the coupling effect of cast micropores and GBM on the fatigue damage mechanisms of the bicrystals are evaluated according to their hindering degrees on the piled-up dislocations. Combining with a hysteresis energy model, a quantitative fatigue strength prediction model of superalloys is established and is well verified by abundant experimental data. This study could provide guidance for fatigue performance prediction and structural design of superalloys.