Rotary bending fatigue behavior of A356 –T6 aluminum alloys by vacuum pressurizing casting

Vacuum pressurizing casting technique, providing better mould filling and inter-dendritic feeding, can reduce the porosity greatly in cast aluminum alloys, and improve the fatigue properties. The rotary bending fatigue properties of A356-T6 alloys prepared by vacuum pressurizing casting were investigated. The S-N curve and limit strength 90 MPa under fatigue life of 107 cycles were obtained. The analyses on the fatigue fractography and microstructure of specimens showed that the fatigue fracture mainly occurs at the positions with casting defects in the subsurface, especially at porosities regions, which attributed to the crack propagation during the fatigue fracture process. Using the empirical crack propagation law of Pairs-Erdogon, the quantitative relationship among the initial crack size, fatigue life and applied stress was established. The fatigue life decreases with an increase in initial crack size. Two constants in the Pairs-Erdogon equation of aluminum alloy A356-T6 were calculated using the experimental data.

Experimental investigation of the relation between the surface integrity and bending fatigue strength of carburized gears

Bending fatigue is an essential parameter that needs to be considered in the improvement process of the power density and reliability of gear drives. Quantitative relations among the manufacturing parameters, surface integrities, and fatigue performance are not clear, which seriously limits the effectiveness of an anti-fatigue design. For this work, tooth-bending fatigue tests of carburized gears with different surface integrities were performed using a pulsator. The effects of the manufacturing parameters and surface integrities on the gear fatigue, such as surface hardness and residual stress, were investigated. The experimental results revealed that due to the improvement of surface integrities after shot peening, the nominal bending stress number(fatigue limit) increased by 6.3%–31.1%, with an amplitude range of 39–143 MPa. A supervised learning algorithm of a random forest was implemented to determine the contribution of the surface hardness and surface residual stress to the nominal stress number. An empirical formula was proposed to predict the nominal stress number considering the surface integrities. The prediction error was less than 7.53%, as verified by several gear-bending fatigue tests. This provided theoretical support for the modern, anti-fatigue design of the gears.

Bending fatigue behavior of thin Zr_(61)Ti_(2)Cu_(25)Al_(12)bulk metallic glass beams for compliant mechanisms application

In this work,bending fatigue behavior of thinner Zr_(61)Ti_(2)Cu_(25)Al_(12)(ZT1)BMG beams with thicknesses of 500μm and 100μm were investigated with three-point bending(3PB),to evaluate the reliability and safety of this potentially material used in compliant mechanisms under cyclic bending load.Fatigue endurance limits(FELs)of ZT1 beams with thicknesses of 500μm and 100μm were determined to be 470 MPa,and~0.3 of its tensile strength.Fatigue life and FEL of ZT1 beams in bending are substantially independent of the beam thickness,in the scale from 500μm down to 100μm.Fatigue cracks of all ZT1 beams initiated at the extrinsic microvoids,either indirectly derived from surface scratches during mechanical polishing or originated from as-cast process.However,fatigue crack propagation behavior is highly related to the magnitude of cyclic stress level.By interrupted fatigue tests,larger proportion about 60-70%of fatigue life was spent on crack propagation under high stress level,while crack initiation occupied larger proportion about 80-85%under low stress level.The medium stress level of about 590 MPa almost shared the crack-initiation life and crack-propagation life.In addition,there are two crack deflection mechanisms during cracks steady propagation stage.One mechanism is crack deflection along the interacted shear bands ahead of crack-tip,the other one is crack"jump"from one shear band to another shear band at a certain angle.These two mechanisms alternate dominate the crack propagation process,generating typical staircase-like crack propagation trajectory.Fatigue endurance limits of ZT1 beams in the form of either stress amplitude or strain amplitude are superior to traditional candidate materials in the field of compliant mechanisms,showing that the better reliability and larger deflection deformation can be achieved for flexible members made by ZT1 BMG even under cyclic loading.

Performance of steel bridge deck pavement structure with ultra high performance concrete based on resin bonding

This research investigated a pavement system on steel bridge decks that use epoxy resin(EP)bonded ultra-high performance concrete(UHPC).Through FEM analysis and static and dynamic bending fatigue tests of the composite structure,the influences of the interface of the pavement layer,reinforcement,and different paving materials on the structural performance were compared and analyzed.The results show that the resin bonded UHPC pavement structure can reduce the weld strain in the steel plate by about 32%and the relative deflection between ribs by about 52%under standard axial load conditions compared to traditional pavements.The EP bonding layer can nearly double the drawing strength of the pavement interface from 1.3 MPa,and improve the bending resistance of the UHPC structure on steel bridge decks by about 50%;the bending resistance of reinforced UHPC structures is twice that of unreinforced UHPC structure,and the dynamic deflection of the UHPC pavement structure increases exponentially with increasing fatigue load.The fatigue life is about 1.2×10^(7) cycles under a fixed force of 9 kN and a dynamic deflection of 0.35 mm,which meets the requirements for fatigue performance of pavements on steel bridge decks under traffic conditions of large flow and heavy load.