Fatigue Life Prediction under Service Load Considering Strengthening Effect of Loads below Fatigue Limit

Lightweight design requires an accurate life prediction for structures and components under service loading histories. However, predicted life with the existing methods seems too conservative in some cases, leading to a heavy structure. Because these methods are established on the basis that load cycles would only cause fatigue damage, ignore the strengthening effect of loads. Based on Palmgren-Miner Rule （PMR）, this paper introduces a new method for fatigue life prediction under service loadings by taking into account the strengthening effect of loads below the fatigue limit. In this method, the service loadings are classified into three categories： damaging load, strengthening load and none-effect load, and the process for fatigue life prediction is divided into two stages： stage I and stage II, according to the best strengthening number of cycles. During stage I, fatigue damage is calculated considering both the strengthening and damaging effect of load cycles. While during stage II, only the damaging effect is considered. To validate this method, fatigue lives of automobile half shaft and torsion beam rear axle are calculated based on the new method and traditional methods, such as PMR and Modified Miner Rule （MMR）, and fatigue tests of the two components are conducted under service loading histories. The tests results show that the percentage errors of the predicted life with the new method to mean life of tests for the two components are –3.78% and –1.76% separately, much lesser than that with PMR and MMR. By considering the strengthening effect of loads below the fatigue limit, the new method can significantly improve the accuracy for fatigue life prediction. Thus lightweight design can be fully realized in the design stage.

Segregation of alloying atoms at a tilt symmetric grain boundary in tungsten and their strengthening and embrittling effects

We investigate the segregation behavior of alloying atoms （Sr, Th, In, Cd, Ag, Sc, Au, Zn, Cu, Mn, Cr, and Ti） near Z3 （ 111 ） [1]-0] tilt symmetric grain boundary （GB） in tungsten and their effects on the intergranular embrittlement by performing first-principles calculations. The calculated segregation energies suggest that Ag, Au, Cd, In, Sc, Sr, Th, and Ti prefer to occupy the site in the mirror plane of the GB, while Cu, Cr, Mn, and Zn intend to locate at the first layer nearby the GB core. The calculated strengthening energies predict Sr, Th, In, Cd, Ag, Sc, Au, Ti, and Zn act as embrittlers while Cu, Cr, and Mn act as cohesion enhancers. The correlation of the alloying atom＇s metal radius with strengthening energy is strong enough to predict the strengthening and embrittling behavior of alloying atoms; that is, the alloying atom with larger metal radius than W acts as an embrittler and the one with smaller metal radius acts as a cohesion enhancer.

Fabrication of quaternary Mg-Zn-Y-Ce quasi-crystal alloys and their strengthening effects on AZ91 magnesium alloys

Quaternary Mg-Zn-Y-Ce quasi-crystal-containing alloys were fabricated using a metal mold casting route. The microstructures of Mg-Zn-Y-Ce alloys and the distribution of the major elements were analyzed. The differences in morphology and micro-hardness between quaternary Mg-Zn-Y-Ce quasi-crystals and ternary Mg- Zn-Y quasi-crystals were discussed. The result showed that the micro-hardness of Mg-Zn-Y-Ce quasi-crystals reached peak value when the content of Ce was 0.8at.%. Subsequently, various contents of Mg-Zn-Y-0.8Ce alloys were used to strengthen AZ91 magnesium alloys. The microstructural modification and mechanical properties of reinforced AZ91 alloys with and without solution, as well as their aging treatments, were also studied. Further research showed that aging-treated AZ91-10wt.%(Mg-Zn-Y-0.8at.%Ce) alloys exhibited excellent mechanical properties: their Brinell hardness, tensile strength, and elongation were HV91, 252 MPa, and 4.1%, respectively. Through this study, the range of application of AZ91 alloys can further be extended.

Influence of bismuth on microstructure,thermal properties,mechanical performance,and interfacial behavior of SAC305−xBiCu solder joints

This research sought to improve the properties of SAC305 solder joints by the addition of 1 and 2 wt.%Bi.The effects of bismuth doping on the microstructure,thermal properties,and mechanical performance of the SAC305−xBiCu solder joints were investigated.Bi-doping modified the microstructure of the solder joints by refining the primaryβ-Sn and eutectic phases.Bi-doping below 2 wt.%dissolved in theβ-Sn matrix and formed a solid solution,whereas Bi additions equal to or greater than 2 wt.%formed Bi precipitates in theβ-Sn matrix.Solid solution strengthening and precipitation strengthening mechanisms in theβ-Sn matrix increased the ultimate tensile strength and microhardness of the alloy from 35.7 MPa and 12.6 HV to 55.3 MPa and 20.8 HV,respectively,but elongation decreased from 24.6%to 16.1%.The fracture surface of a solder joint containing 2 wt.%Bi was typical of a brittle failure rather than a ductile failure.The interfacial layer of all solder joints comprised two parallel IMC layers:a layer of Cu6Sn5 and a layer of Cu3Sn.The interfacial layer was thinner and the shear strength was greater in SAC305−xBiCu joints than in SAC305Cu solder joints.Therefore,small addition of Bi refined microstructure,reduced melting temperature and improved the mechanical performance of SAC305Cu solder joints.

Precipitation Behavior and Its Strengthening Effect of X100 Pipeline Steel

Using TEM （transmisson electron microscopy）, electron diffraction, EDX （energy dispersive X-ray） analysis and physicochemical phase analysis, the morphology, crystal structure, size distribution and chemical composition of precipitates in the microstructure of high strength Nb-microalloyed Xl00 pipeline steel were investigated, and the strengthening effect of precipitation was quantitatively calculated with Ashhy-Orowan correction model. The precipitates obtained in X100 pipeline steel can be divided into two kinds： ＂complex＂ and ＂single＂ particles by morphology. The EDX analysis of ＂single＂ precipitates reveals that the chemical composition matches well with particle dimensions, especially the Nb to Ti ratio regularly decreases with the increase of particle size. The yield strength increments in the way of precipitation strengthening of X100 pipeline steel reached about 52 MPa, suggesting that the precipitation strengthening is not the dominative strengthening mechanism for X100 pipeline steel.

A nonlinear explicit dynamic GBT formulation for modeling impact response of thin-walled steel members

A nonlinear explicit dynamic finite element formulation based on the generalized beam theory(GBT)is proposed and developed to simulate the dynamic responses of prismatic thin-walled steel members under transverse impulsive loads.Considering the rate strengthening and thermal softening effects on member impact behavior,a modified Cowper-Symonds model for constructional steels is utilized.The element displacement field is built upon the superposition of GBT cross-section deformation modes,so arbitrary deformations such as cross-section distortions,local buckling and warping shear can all be involved by the proposed model.The amplitude function of each cross-section deformation mode is approximated by the cubic non-uniform B-spline basis functions.The Kirchhoff s thin-plate assumption is utilized in the construction of the bending related displacements.The Green-Lagrange strain tensor and the second Piola-Kirchhoff(PK2)stress tensor are employed to measure deformations and stresses at any material point,where stresses are assumed to be in plane-stress state.In order to verify the effectiveness of the proposed GBT model,three numerical cases involving impulsive loading of the thin-walled parts are given.The GBT results are compared with those of the Ls-Dyna shell finite element.It is shown that the proposed model and the shell finite element analysis has equivalent accuracy in displacement and stress.Moreover,the proposed model is much more computationally efficient and structurally clearer than the shell finite elements.

Contribution of T_1 Precipitates to Strength in Two Kinds

The behavior of interaction between dislocation and T 1 plate was observed and analyzed by means of TEM technique in Al Li alloys of 2090 and 2090+Ce. The observation results show that the interaction between dislocation and T 1 plate is a mix type of shearing and Orowan looping, and the physical models are established by theory of precipitates strengthening. The results of calculation illustrate that the contribution of T 1 plate is up 30%～60% to the overall critical resolved shear stress (CRSS). The role of T 1 plate is enhanced with aging time in the material aged 3～48 h at 190℃, but this enhancing effect becomes gentle on overaged condition. The strengthening effects of T 1 plate in 2090 and 2090+Ce alloys are little different on under aged condition, but the strengthening effects of T 1 precipitates in alloy 2090+Ce is little bigger than that of 2090 on overaged condition, especially the cutting mechanism is only considered. The difference between strengthening effects of matrix in two alloys is about 3.2 MPa on underaged condition and 8.7 MPa or so on overaged condition.

Interface regulation strategy of Al-CNTs composite induced by Al-Si eutectic reaction and its strengthening mechanism

Carbon nanotubes(CNTs)reinforced aluminum matrix composites(AMCs)show broad application prospects in the fields of aerospace and transportation because of their lightweight,high specific strength and specific modulus.A new alloying strategy in this study is proposed to regulate its weak intrinsic interface and uncontrollable interfacial reaction.Our results show that Si element could hinder the dissolution and diffusion of carbon atoms in Al matrix,inhibiting Al-CNTs interface reaction.Grain refinement and dislocation increment induced by the reinforcements promote the contributions to the strength of AMCs.The wettability and interface bonding of Al-CNTs could be effectively improved by Al/Si/CNTs composite interface,the formed discontinuous strong interface is advantageous to coordinate the plastic deformation of AMCs.Si particles can also firmly fix CNTs through pinning effect during deformation,which leads to a better load transfer.Therefore,the tensile strength of Al-5Si-0.5CNTs composite can be enhanced up to 391 MPa while maintaining an acceptable plasticity of 7.5%.It brings the strengthening effect of CNTs into full play in AMCs and achieves the strengthening effect of 1+1>2 under the comprehensive effect of grain refinement strengthening,dislocation strengthening,dispersion strengthening of Si particles and load transfer to CNTs。

An overview of rhenium effect in single-crystal superalloys

Nickel-based single-crystal superalloys are the key materials for the manufacturing and development of advanced aeroengines. Rhenium is a crucial alloying element in the advanced nickel-based single-crystal superalloys for its special strengthening effects. The addition of Re could effectively enhance the creep properties of the single-crystal superalloys; thus, the content of Re is considered as one of the characteristics in different-generation single-crystal superalloys. Owing to the fundamental importance of rhenium to nickel-based single-crystal superalloys, much progress has been made on understanding of the effect of rhenium in the single-crystal superalloys. While the effect of Re doping on the nickelbased superalloys is well documented, the origins of the socalled rhenium effect are still under debate. In this paper,the effect of Re doping on the single-crystal superalloys and progress in understanding the rhenium effect are reviewed. The characteristics of the d-states occupancy in the electronic structure of Re make it the slowest diffusion elements in the single-crystal superalloys, which is undoubtedly responsible for the rhenium effect, while the postulates of Re cluster and the enrichment of Re at the c/c0 interface are still under debate, and the synergistic action of Re with other alloying elements should be further studied.Additionally, the interaction of Re with interfacial dislocations seems to be a promising explanation for the rhenium effect. Finally, the addition of Ru could help suppress topologically close-packed（TCP） phase formation and strengthen the Re doping single-crystal superalloys.Understanding the mechanism of rhenium effect will be beneficial for the effective utilization of Re and the design of low-cost single-crystal superalloys.

Metal matrix nanocomposites in tribology: Manufacturing, performance, and mechanisms

Metal matrix nanocomposites(MMNCs)become irreplaceable in tribology industries,due to their supreme mechanical properties and satisfactory tribological behavior.However,due to the dual complexity of MMNC systems and tribological process,the anti-friction and anti-wear mechanisms are unclear,and the subsequent tribological performance prediction and design of MMNCs are not easily possible:A critical up-to-date review is needed for MMNCs in tribology.This review systematically summarized the fabrication,manufacturing,and processing techniques for high-quality MMNC bulk and surface coating materials in tribology.Then,important factors determining the tribological performance(mainly anti-friction evaluation by the coefficient of friction(CoF)and anti-wear assessment with wear rate)in MMNCs have been investigated thoroughly,and the correlations have been analyzed to reveal their potential coupling/synergetic roles of tuning tribological behavior of MMNCs.Most importantly,this review combined the classical metal/alloy friction and wear theories and adapted them to give a(semi-)quantitative description of the detailed mechanisms of improved anti-friction and anti-wear performance in MMNCs.To guarantee the universal applications of these mechanisms,their links with the analyzed influencing factors(e.g.,loading forces)and characteristic features like tribo-film have been clarified.This approach forms a solid basis for understanding,predicting,and engineering MMNCs’tribological behavior,instead of pure phenomenology and experimental observation.Later,the pathway to achieve a broader application for MMNCs in tribo-related fields like smart materials,biomedical devices,energy storage,and electronics has been concisely discussed,with the focus on the potential development of modeling,experimental,and theoretical techniques in MMNCs’tribological processes.In general,this review tries to elucidate the complex tribo-performances of MMNCs in a fundamentally universal yet straightforward way,and the discussion and summary in this review for the tribological performance in MMNCs could become a useful supplementary to and an insightful guidance for the current MMNC tribology study,research,and engineering innovations.

Natural overlaying behaviors push the limit of planar cyclic deformation performance in few-layer MoS_(2) nanosheets

As a typical two-dimensional(2D)transition metal dichalcogenides(TMDCs)material with nonzero band gap,MoS_(2)has a wide range of potential applications as building blocks in the field of nanoelectronics.The stability and reliability of the corresponding nanoelectronic devices depend critically on the mechanical performance and cyclic reliability of 2D MoS_(2).Although an in situ technique has been used to analyze the mechanical properties of 2D materials,the cyclic mechanical behavior,that is,fatigue,remains a major challenge in the practical application of the devices.This study was aimed at analyzing the planar cyclic performance and deformation behavior of three-layer MoS_(2)nanosheets(NSs)using an in situ transmission electron microscopy(TEM)variable-amplitude uniaxial low-frequency and cyclic loading-unloading tensile acceleration test.We also elucidated the strengthening effect of the natural overlaying affix fragments(other external NSs)or wrinkle folds(internal folds from the NS itself)on cycling performances and service life of MoS_(2)NSs by delaying the whole process of fatigue crack initiation,propagation,and fracture.The results have been confirmed by molecular dynamics(MDs)simulations.The overlaying enhancement effect effectively ensures the long-term reliability and stability of nanoelectronic devices made of few-layer 2D materials.