轮盘应变疲劳寿命可靠性分析方法

为较准确评估应变疲劳寿命可靠性,通过引入两个相关随机变量对Coffin-Manson公式随机化,发展了一种轮盘应变疲劳寿命可靠性分析概率模型。对线性回归残差重新定义,使得到的回归参数不随坐标系的选取发生变化。采用响应面法和蒙特卡洛模拟相结合的方法获得轮盘危险点应变幅的分布,最后通过多次解非线性方程数值模拟得到轮盘危险点应变疲劳寿命分布及其可靠寿命。

Low cycle fatigue behavior of T4-treated Al-Zn-Mg-Cu alloys prepared by squeeze casting and gravity die casting

Gravity die casting(GC) and squeeze casting(SC) T4-treated Al-7.0Zn-2.5Mg-2.1Cu alloys were employed to investigate the microstructures,mechanical properties and low cycle fatigue(LCF) behavior.The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing(SDAS).Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy.GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes(less than 0.4%) and cyclic hardening at higher total strain amplitudes.The degree of cyclic hardening of SC samples is greater than that of GC samples.Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them,while the crack initiation sites for SC samples are slip bands,eutectic phases and inclusions at or near the free surface.

Effect of nano-sized Al2O3 reinforcing particles on uniaxial and high cycle fatigue behaviors of hot-forged AZ31B magnesium alloy

The effect of hot-forging process was investigated on microstructural and mechanical properties of AZ31 B alloy and AZ31 B/1.5 vol.%Al2 O3 nanocomposite under static and cycling loading. The as-cast alloy and composite were firstly subjected to a homogenization heat treatment at 450 ℃ and then an open-die forging at 450 ℃. The results indicated that the presence of reinforcing particles led to grain refinement and improvement of dynamic recrystallization. The forging process was more effective to eliminate the porosity in the cast alloy workpiece. Microhardness of the forged composite was increased by up to 80% and 16%, in comparison with those of the cast and forged alloy samples, respectively. Ultimate tensile strength and maximum tensile strain of the composite were improved by up to 45% and 23%, compared with those of the forged alloy in similar regions. These enhancements were respectively 50% and 37% in the compression test. The composite exhibited a fatigue life improvement in the region with low applied strain;however, a degradation was observed in the high applied strain region. Unlike AZ31 B samples, tensile, compressive and high cycle fatigue behaviors of the composite showed less sensitivity to the applied strain, which can be attributed to the amount of porosity in the samples before and after the hot-forging.

Fatigue reliability analysis of fixed offshore structures:A first passage problem approach

This paper describes a methodology for computation of reliability of members of fixed offshore platform structures, with respect to fatigue. Failure criteria were formulated using fracture mechanics principle. The problem is coined as a “first passage problem”. The method was illustrated through application to a typical plane frame structure. The fatigue reliability degradation curve established can be used for planning in-service inspection of offshore platforms. A very limited parametric study was carried out to obtain insight into the effect of important variables on the fatigue reliability.

Effect of post weld heat treatment on properties of variable polarity TIG welded AA2219 aluminium alloy joints

AA2219 aluminium alloy joints were fabricated by variable polarity tungsten inert gas （VPTIG） welding process and the effects of post weld heat treatment （PWHT） on the tensile properties, microstructure and fatigue behaviour of the welded joints were investigated. The VPTIG welding process was adopted because it could meet the need of cathode cleaning and meanwhile it could reduce the deterioration of tungsten electrode furthest. The welded samples were divided into as-welded （AW） sample and PWHT sample. The PWHT method used on the samples was solution treatment （535 ℃, 30 rain）, water quenching and artificial aging （175 ℃, 12 h）. The experimental results show that, compared with the AW samples, the microstructure characteristics and mechanical properties of the AA2219 joints after PWHT were significantly improved. The improvement of yield strength, ultimate tensile strength, and fatigue strength are 42.6%, 43.1% and 18.4%, respectively.

Tensile behaviors of fatigued AZ31 magnesium alloy

The relationship between microstructure and tensile behaviors of fatigued AZ31 magnesium alloy was investigated. Axial fatigue tests were performed on PLG-100 fatigue machine at stresses of 50 and 90 MPa. Tensile samples were cut from the fatigued samples, named as L-sample and H-sample respectively, and the O-sample was cut from original rolled AZ31 alloy. The EBSD and TEM were used to characterize the microstructure. It is found that the twinning-detwinning was the main deformation mechanism in high stress fatigue test, while dislocation slipping was dominant in low stress fatigue test. After fatigue tests, the average grain size of the L-sample and H-sample decreased to 4.71 and 5.33 μm, and the tensile and yield strength of the L-sample and H-sample increased slightly. By analyzing SEM images, the ultimate fracture region of the L-sample consisted of dimples, while there were many microvoids in the ultimate fracture region of the H-sample. Consequently, the tensile behaviors of fatigued magnesium have a close relationship with microstructure.

Fatigue properties analysis of cracked rock based on fracture evolution process

Fracture evolution process (initiation, propagation and coalescence) of cracked rock was observed and the force- displacement curves of cracked rock were measured under uniaxial cyclic loading. The tested specimens made of sandstone-like modeling material contained three pre-existing intermittent cracks with different geometrical distributions. The experimental results indicate that the fatigue deformation limit corresponding to the maximal cyclic load is equal to that of post-peak locus of static complete force?displacement curve; the fatigue deformation process can be divided into three stages: initial deformation, constant deformation rate and accelerative deformation; the time of fracture initiation, propagation and coalescence corresponds to the change of irreversible deformation.

Effects of pre-rolling on mechanical properties and fatigue crack growth rate of 2195 Al-Li alloy

While pre-deformation is often conducted before aging treatment to increase the strength and microhardness of 2195 Al-Li alloy, it often increases the fatigue crack growth(FCG) rate and thus reduces the fatigue life of the alloy.To determine the effects and causes of pre-deformation and heat treatment on the mechanical properties and FCG rate of2195 Al-Li alloy, and to provide a suitable calculation model for the FCG rate under different pre-deformation conditions, 2195 Al-Li alloy specimens with different degrees of pre-rolling(0, 3%, 6%, and 9%) were investigated. The experimental results indicate that with the increase of pre-rolling, the density of the T1phase and the uniformity of the S′distribution and the microhardness, tensile strength, and yield strength of the alloy increase and at the same time the FCG rate increases, and thus the fatigue life is reduced. It was also found that the normalized stress intensity factor of elastic modulus(E) can be applied to correlate the FCG rate of pre-rolled 2195 Al-Li alloy with constant C and K parameters.

英国劳氏船级社“CM”标志在37600吨级双壳散货船设计阶段的应用

介绍了英国劳氏船级社建造监测入级符号“CM”对满足船舶结构疲劳性能要求的重要性,并介绍了“CM”标志在37 600吨级双壳散货船的实际应用。在充分理解船级社规范要求的前提下,通过一定的技术手段,将结构建造过程中的绝对对中转化为理论线对齐,既可满足船级社规范要求,又能最大程度地方便施工建造。

Effects of Si addition on microstructure, mechanical and thermal fatigue properties of Zn-38Al-2.5Cu alloys

The influence of Si addition on microstructure, mechanical properties and thermal fatigue behavior of Zn-38Al-2.5Cu alloys was investigated. The results show that constitutional supercooling of ZA38 alloys is formed because of the Si addition. Zn-38Al-2.5Cu-0.55Si alloy shows the dramatically refined microstructure and the best mechanical properties. When the Si addition exceeds 0.55%,αdendrites develop and Si phases become larger and aggregate along the dendrites boundaries, decreasing the mechanical properties. Oxides and pits formed by the plastic deformation are the main factors of cracks initiation. During the early stage of crack propagation, the cracks grow at a high speed well described by Paris law because of the porous and loose oxide, and mainly propagate along the dendrites boundaries. During the slow-growth stage, secondary cracks share the energy of crack growth, delaying the propagation of cracks, and the cracks propagate and fracture by the mixture of intergranular and transgranular modes.

Influence of Carbonation on Mechanical Properties of Concrete

As one of the most important factors that determine the lifespan of a reinforced concrete structure, carbonation not only corrodes the reinforcing steel, but also changes the mechanical properties of concrete. For better understanding the performance of carbonated concrete structure, it is necessary to study the mechanical properties of carbonated concrete. The strees strain relationship of carbonated concrete was analyzed on the basis of experiments. The specimens were made by means of accelerated carbonation and then compressed on the testing machine. Some very important characteristics of carbonated concrete were revealed by the testing results. In addition, a useful constitutive model of carbonated concrete, which proved to be suitable for analyzing carbonated concrete members, was established in this research.

Study of vehicle accident reconstruction based on the information of the tire marks

The tire mark is the important legacy information left at the accident scene. Based on the vehicle collision dynamics model, this study provided an optimized algorithm with vehicle final location and other related information for the tire marks. When the tire marks simulation results fit the real one well, the state of vehicle can be understood as the real state in the accident. Based on above, the vehicle velocity and direction are decided. According to the velocity and direction of the vehicle, the complete accident process can be simulated. With the help of the Pc-Crash software, the method has been applied in typical collision accident cases analysis. The reconstruction results will provide the scientific and numerical references for vehicle collision accidents analyzing and appraising.

Microstructure and mechanical properties of 6061 aluminum alloy laser-MIG hybrid welding joint

In this paper, 3 mm 6061 aluminum alloy sheets were welded by laser MIG hybrid welding. Based on the experiment, the best welding parameters were determined to ensure the penetration welding. The detailed microstructure,tensile and fatigue fracture morphology and surface fatigue damage of the welded joints were analyzed by optical microscope(OM), scanning electron microscope(SEM) and energy dispersive spectrometer(EDS). The results show that there are two main kinds of precipitates, one is the long Si rich precipitates at the grain boundaries, the other is the intragranular Cu rich precipitates. The tensile test results show that the tensile strength of the joint is 224 MPa, which is only 70.2% of the base metal. Through the analysis of tensile fracture, there are great differences in the formation of tensile dimple. In the tensile-tensile fatigue test with a stress rate of 0.1, the conditional fatigue limits of base metal and welded joint are 101.9 MPa and 54.4 MPa, respectively. By comparing the fatigue fracture of the welded joints under different stress amplitudes, it was found that the main factor leading to the fracture of the joint is porosity. Through further analysis of the pore defects, it was found that there are transgranular and intergranular propagation ways of microcracks in the pores, and the mixed propagation way was also found.

Finite element model for linear-elastic mixed mode loading using adaptive mesh strategy

An adaptive mesh finite element model has been developed to predict the crack propagation direction as well as to calculate the stress intensity factors （SIFs）, under linear-elastic assumption for mixed mode loading application. The finite element mesh is generated using the advancing front method. In order to suit the requirements of the fracture analysis, the generation of the background mesh and the construction of singular elements have been added to the developed program. The adaptive remeshing process is carried out based on the posteriori stress error norm scheme to obtain an optimal mesh. Previous works of the authors have proposed techniques for adaptive mesh generation of 2D cracked models. Facilitated by the singular elements, the displacement extrapolation technique is employed to calculate the SIK The fracture is modeled by the splitting node approach and the trajectory follows the successive linear extensions of each crack increment. The SlFs values for two different case studies were estimated and validated by direct comparisons with other researchers work.

Damage assessment of a continuous beam bridge based on the strain mode

Based on the method of strain mode, damage identification of continuous beam bridges by comparing the variance of several curves of strain modes difference is studied. Three cases of numerical simulation demonstrate that the proposed method is applicable to detecting many a damage in a continuous beam bridge, which accurately identifies the damaged positions of the bridge, and detects the damage severity of an element by its according peak value of the curve of strain modes difference that is found to increase with the increasing damage severity.

Correlation of the positive stress ratio effects on fatigue crack propagation rate of aluminium alloys based on a new parameter da/dS

In-situ SEM (Scanning Electron Microscope) observation of fatigue crack propagation in aluminium alloys reveals that crack growth occurs in a continuous way over the time period during the load cycle. Based on this observation, a new parameter da/dS is introduced to describe the fatigue crack propagation rate, which defines the fatigue crack propagation rate with the change of the applied stress at any moment of a stress cycle. The relationship is given between this new parameter and the conventional used parameter da/dN which describes the fatigue crack propagation rate per stress cycle. Using this new parameter, an analysis has been performed and a model has been set up to consider the effect of the applied stress ratio on the fatigue crack propagation rate. The obtained results have been used to correlate the published test data and a good correlation has been achieved. This method is very easy to use and no fatigue crack closure measurement is needed, therefore this model is significant in engineering application.

A fatigue damage model for asphalt mixtures under controlled-stress and controlled-strain modes

A fatigue damage model based on thermodynamics was deduced for asphalt mixtures under controlled-stress and controlled-strain modes. By employing modulus of resilience as the damage hardening variable, a damage variable related with dynamic modulus was extracted as the evaluation index. Then, the damage evolution law under two control modes was proposed, and it has a similar form to the Chaboche fatigue model with a nonnegative material parameter m related to its loading level. Experimental data of four loading levels were employed to calibrate the model and identify the parameter in both control modes. It is found that the parameter m shows an exponential relationship with its loading level. Besides, the difference of damage evolution under two control modes was explained by the law. The damage evolves from fast to slow under a controlled-strain mode. However, under a controlled-stress mode, the evolution rate is just the opposite. By using the damage equivalence principle to calculate the equivalent cycle numbers, the deduced model also interprets the difference of damage evolution under two control modes on the condition of multilevel loading. Under a controlled-strain mode, a loading sequence from a low level to a high level accelerates damage evolution. An inverse order under the controlled-stress mode can prolong fatigue life.

A New Fracture Criterion for a Crack under Combined Mode Loading

A new fracture criterion was proposed. The physical explanation of the criterion is that crack will propagate when the minimum strain energy density in iso hoop stress curve reach a critical strength of the material considered. The resulting curve of critical fracture of mixed mode cracks shows that the present fracture is efficient and more accurate than the previous criteria.

Micromechanics behavior of fatigue cracks in Hadfield steel railway crossing

In order to predict the mechanism of the formation and propagation of rolling contact fatigue cracks in Hadfield steel railway crossings(HSRC),the residual strain,microstructure,crystal orientation,and microhardness surrounding fatigue cracks were investigated.Results show that the formation and propagation of fatigue cracks in HSRC have no correlation with the crystal orientation and boundaries of grains.The hardness and residual strain in the field around the fatigue crack are lower than those in other regions.The compressive strain around the fatigue crack is released after crack propagates,which reduces the hardness around the fatigue crack.Deformation twins and dislocations play a key role in the work hardening of HSRC.