Repair and Strengthening Assessment of Existing Bridges Based on Time-Dependent Reliability Analysis

The strategy on repair and strengthening of existing bridges based on time-dependent reliability was analyzed with the maximum expected benefit as the objective function. A sample of risk-ranking decision was illustrated based on updated inspection information with 35 survival age. The effect of improvement of live loads and difference of repair methods on time-dependent reliability of existing bridges are considered. The results show that the decision method can be used in real project, with the cost of failure consequence and the risk of failure considered.

Time-dependent viscoelastic behavior of an LDPE melt

Two differential constitutive equations, i.e. Giesekus model and Johnson-Segalman model were employed here to predict the time-dependent viscoelastic behavior of an LDPE melt in thixotropy-loop experiments and step shear rate experiment. Multiple relaxation modes were adopted, and the parameters used to describe the nonlinear viscoelasticity in the two models were obtained by fitting the shear-thinning viscosity. The predictions on those transient shear characteristics by the two models are found in qualitative agreement with our previous experiments. JohnsonSegalman model predicts oscillation behavior in the thixotropy-loop and step shear rate experiments, whereas Giesekus model does not. Both models predict higher shear stresses than the experimental data in the case of long time shearing, implying that both models are not able to completely characterize the time-dependent shear stress of the melt at high shear rate.

Reliability analysis of laminated composite under compression and shear loads

Carrying on a series of compression and shear tests by a large number of specimens, reliabilities of T300/QY8911 laminated composite were studied when dispersibility models were described. The results show that the stress is linearly dependent on the strain and the damage modes of specimens are brittle fracture for both kinds of tests. Dispersibility models of compression and shear strength are expressed as Re-N（415.39, 6 586.36） and Rs-ln（5.071 8, 0.155 3）, respectively. When normal and lognormal distributions were used to describe the dispersibility models of compression and shear strength, and the compression or shear load follows the normal distribution, the almost same failure probability can be obtained from different reliability analysis methods.

Operation safety risk analysis method of hydropower project considering time-dependent effect

In order to consider the time-dependent characteristic of risk factors of hydropower project,the method of stochastic process simulating structure resistance and load effect is adopted.On the basis of analyzing the structure characteristics and mode of operation,the operation safety risk rate assessment model of hydropower project is established on the comprehensive application of the improved analytic hierarchy process,the time-dependent reliability theory and the risk rate threshold.A scheme to demonstrate the time-dependent risk rate assessment method for an example of the earth-rock dam is particularly implemented by the proposed approach.The example shows that operation safety risk rate is closely related to both the service period and design standard;considering the effect of time-dependent,the risk rate increases with time and the intersection of them reflects the technical service life of structures.It could provide scientific basis for the operation safety and risk decision of the hydropower project by predicting the trend of risk rate via this model.

考虑套筒灌浆随机缺陷的装配式剪力墙结构可靠度评估

在装配整体式剪力墙结构中,由于套筒灌浆连接的质量具有一定的随机性,势必影响结构的竖向连接性能和结构抗震性能。根据不同缺陷程度的套筒灌浆拉拔试验,建立了一套等效套筒灌浆缺陷连接承载力模型,并基于某实际工程结构,建立了装配整体式剪力墙结构有限元模型。通过考虑灌浆缺陷的随机性,赋予连接接头相应缺陷程度的力学连接性能,来反映套筒灌浆中可能存在的缺陷。通过非线性有限元分析并结合概率密度演化方法(probability density evolution method,PDEM)进行了结构随机非线性反应分析和可靠度评估。结果表明:在动力作用下,结构非线性与随机性具有明显的耦合效应;缺陷的随机性会随着时间的推移,逐渐放大对结构响应的影响;在不同的安全域内,结构的整体可靠度将存在较大的差异。

基于结构面粗糙度随机场表征的抗剪强度变异性

岩体结构面抗剪强度参数具有变异性是客观事实,而受限于室内外直剪试验成本而导致抗剪强度变异性指标获取困难是目前制约岩质边坡可靠度评价发展的主要原因之一。鉴于此,提出了一种利用少量实测结构面粗糙度曲线样本和岩块物理力学参数,结合JRC-JCS(joint roughness coefficient-joint compressive strength)剪切强度模型、随机场理论和非侵入式离散元数值试验,获取结构面抗剪强度参数变异性指标的方法。将此方法应用于某隧道进口段岩质边坡可靠度评价中,研究发现:(1)该岩质边坡主控结构面粗糙度纵坐标方差为0.012 5 cm2,相关距离为0.75 cm,生成的随机粗糙度曲线与实测样本相似;(2)结构面黏聚力和内摩擦角均值分别为0.427 MPa和54.9°,标准差分别为0.229 MPa和4.47°,变异系数分别为53.63%和8.14%;(3)此边坡稳定性系数服从正态分布,失稳概率为2.8%,可靠度为1.52。

常规土类剪切波速与埋深经验公式的可靠性评价

国内常规土类剪切波速与埋深的统计关系公式(刘红帅等《常规土类剪切波速与埋深的关系分析》一文中,表2—5中的幂函数和一元二次函数模型公式)作为全国性经验公式,已成为地区经验公式检验对比的重要参考,但缺乏较系统的可靠性评价。为此,选取国内典型地区(北京、鲁西、成都、天津、常州、武汉6个区域)的剪切波速与埋深的统计经验公式,检验全国性常规土类剪切波速与埋深经验公式的可靠性。结果表明:幂函数模型预测的剪切波速随深度增大而增大,符合定性认识,而一元二次模型有可能出现不合理的回弯反常现象,不应继续采用。全国性剪切波速经验公式在不同地区的预测精度差异显著,在大多数地区对于绝大数土类预测的相对误差总体在±20%以内,在少数地区对于大数土类预测的相对误差超过±20%,仅在近地面20 m内的相对误差较大,最大可达40%左右。建议优先选择适合当地的剪切波速经验公式;当缺乏本地公式,需选用全国性剪切波速幂函数型经验公式时,应先经过本地实测资料检验确认后方可使用;20 m范围的剪切波速最好以实测为准,这有助于降低全国性剪切波速经验公式带来的显著误差。

纳米银浆低温烧结工艺及可靠性分析

对纳米银浆烧结前后的微观组织形貌进行分析,研究了不同烧结温度、烧结时间和升温速率对纳米银浆烧结样件剪切强度的影响,并对比分析了Au80Sn20和纳米银浆两种不同连接材料对GaN芯片散热性能的影响。结果表明:GaN芯片通过纳米银浆烧结到管壳后,银层与金层之间存在一个明显的互扩散层,实现了芯片和壳体之间优异互连。在烧结温度200℃、时间90 min、升温速率5℃/min的烧结条件下,样件剪切强度可达47.2 MPa。纳米银浆与Au80Sn20装配的芯片温度分布基本一致,但纳米银浆散热性能优于Au80Sn20。在经历温度冲击、扫频振动、温度循环及射频老炼试验后,纳米银浆烧结的芯片热阻波动不明显,剪切力性能略微提升。

FNM method for estimating reliability of existing bridges

Combined with current specifications and stress characteristics of concrete filled steel tubular （CFST） arch bridges, the determination principle of safe-middle-failure threestage mode is given. Accordingly, damage probability and failure probability and the corresponding reliability indices are calculated; a direct relationship between reliability indices and three-stage working status is made. Based on the three-stage working mode, a combined FNM （finite element-neural network- Monte-Carlo simulation） method is put forward to estimate the reliability of existing bridges. According to time-dependent reliability theory, subsequent service time is divided into several stages; minimum samples required by the Monte-Carlo method are generated by random sampling; training samples are calculated by the finite element method, and the training samples are extended by the neural network; failure probability and damage probability are calculated by the Monte-Carlo method. Thus, time dependent reliability indices are obtained, and the working status is judged. A case study is investigated to estimate the reliability of an actual bridge by the FNM method. The bridge is a CFST arch bridge with an 83.6 m span and it has been in operation for 10 years. According to analysis results, in the tenth year, the example bridge is still in safe status. This conclusion is consistent with the facts, which proves the feasibility of the FNM method for estimating the reliability of existing bridges.

Reliability and sensitivity of bogie frame of high-speed train with strength degradation

High-speed bogie frame is a key mechanical component in a train system. The reliability analysis of the bogie is necessary to the safety of high-speed train. Reliability analysis of a bogie frame was considered. The equivalent load method was employed to account for random repeated loads in structural reliability analysis. Degradation of material strength was regarded as a Gamma process. The probabilistic perturbation method was, then, employed for response moment computation. Example of a high-speed train bogie structure under time-variant load was employed for reliability and sensitivity analyses. Monte-Carlo simulation verifies the accuracy and efficiency of the proposed method in time-variant reliability analysis. The analysis results show that the reliability calculation considering the strength degradation and repeated load is closer to the practicality than the method of considering reliability calculation only. Its decreasing velocity is faster than the traditional reliability. The reliability sensitivity value changes over time. The analysis results provide a variation trend of reliability and sensitivity to design and usage of bogie frame.

Kriging-based reliability analysis of the long-term stability of a deep driftconstructed in the Callovo-Oxfordian claystone

Callovo-Oxfordian(COx)claystone has been considered as a potential host rock for geological radioactive waste disposal in France(Cigéo project).During the exploitation phase(100 years),the stability of drifts(e.g.galleries/alveoli)within the disposal is assured by the liner,which includes two layers:concrete arch segment and compressible material.The latter exhibits a significant deformation capacity(about 50%)under low stress(<3 MPa).Although the response of these underground structures can be governed by complex thermo-hydro-mechanical coupling,the creep behavior of COx claystone has been considered as the main factor controlling the increase of stress state in the concrete liner and hence the long-term stability of drifts.Therefore,by focusing only on the purely mechanical behavior,this study aims at investigating the uncertainty effect of the COx claystone time-dependent properties on the stability of an alveolus of Cigéo during the exploitation period.To describe the creep behavior of COx claystone,we use Lemaitre’s viscoplastic model with three parameters whose uncertainties are identified from laboratory creep tests.For the reliability analysis,an extension of a well-known Kriging metamodeling technique is proposed to assess the exceedance probability of acceptable stress in the concrete liner of the alveolus.The open-source code Code_Aster is chosen for the direct numerical evaluations of the performance function.The Kriging-based reliability analysis elucidates the effect of the uncertainty of COx claystone on the long-term stability of the concrete liner.Moreover,the role of the compressible material layer between the concrete liner and the host rock is also highlighted.

Accurate estimation of soil shear strength parameters

The cost and safety of geotechnical engineering are highly depending on the accuracy of soil shear strength parameters.There are three methods often used to estimate soil shear strength parameters,i.e.,moment method,3-sigma rule and linear regression method.In this study,the accuracy of these three methods is compared.Traditional linear regression method(LRM)can only offer the mean of shear strength parameters.Some engineers misuse the standard error of shear strength indexes as the standard deviations.Such misuse may highly underestimate the uncertainty and induce high risk to the geotechnical design.A modified LRM is proposed to determine both the mean and variance of shear strength parameters.The moment method,three-sigma rule and LRM are used to analyze the tri-axial test data in Xiaolangdi Hydraulic Project and three numerical shear strength tests.The results demonstrate that:1)The modified LRM can offer the most accurate estimation to shear strength parameters;2)A dimensionless formula is much preferred in LRM rather than a dimensional formula.The stress ratio formula is much better than stress relation in the shear strength parameter analysis.The proposed method is applicable to shear strength parameter analysis for tri-axial test data,direct shear test and the un-drained shear strength test of stratified clay.

Effects of thermal cycling on shear strength of soldered joints of laminar ceramic resistor

The shear strength of two joints with different compositions (Sn63Pb37 and SnAg3.0Cu0.5) was compared under thermal cycling conditions, and the effects of thermal cycling times on the two different joints were studied. Results indicate that the shear force of the joints soldered with lead-free solder is higher than that of the joints soldered with Sn-Pb solder. After 120 times thermal cycling, the shear force of both the two kinds of joints declines obviously. The fractures are located at the two interfaces which is between the resistor’s side pad and the ceramic, the other is between the resistor’s bottom pad and the solder. Analysis result on the fractures by means of SEM indicates that the micrographs of fractures change obviously and the grains grow coarser after thermal cycling.

球栅阵列焊点剪切设备研制与实验设计

该文设计了一款球栅阵列(BGA)焊点剪切实验设备,并开发了配套的控制设备运动的上位机软件。基于该设备,研究两种材料(SAC305和Sn63Pb37)的BGA焊点在固定高度(60μm)、不同剪切速率(0.1、0.3、0.5、0.7 mm/s)下的剪切力学性能,以及在固定剪切速率(0.5 mm/s)、不同剪切高度(60、80、100、120μm)下的剪切力学性能。实验表明:剪切高度固定情况下,随着剪切速率增加,两种材料的焊点的剪切强度均呈现增大趋势;剪切速率固定情况下,随着剪切高度增加,两种材料的焊点的剪切强度均呈现减小趋势。将该设备应用于实验教学,有助于培养学生的科学思维、提升学生的实践能力。

基于可靠度的带暗支撑双肢剪力墙抗震性能研究

为研究带暗支撑双肢剪力墙在材料参数等不确定性下的可靠度,利用有限元软件OpenSees对带暗支撑双肢剪力墙进行可靠度分析,结合单设计参数分析与正交设计组合分析,得到了设计参数的限值及较优水平组合。分析结果表明:轴压比、高宽比、连梁跨高比增大会降低结构可靠指标,使结构偏于不安全,而暗支撑含钢率、连梁配筋率增大对结构的安全性能有利。根据研究结果,建议带暗支撑双肢剪力墙结构轴压比限制在0.6范围内,高宽比<2.8为宜,连梁跨高比可取1.0~2.5,暗支撑含钢率取0.67%~1.78%为宜,连梁配筋率取0.25%~0.45%为宜;在轴压比增大时,建议严格控制高宽比,减小连梁跨高比,增加暗支撑含钢率和连梁配筋率。

芯片剪切强度试验分离模式研究

芯片剪切强度试验是半导体器件和微电子器件可靠性试验的一个重要项目,应用非常广泛,由于现有标准对芯片剪切强度分离模式描述太笼统,很容易造成同一分离现象,出现不同记录的情况。本文通过对芯片剪切强度不同粘接系统出现的不同分离现象进行大量的分析和总结,给出了不同分离模式的划分依据和记录模式,使标准规定得到了统一的认识,使剪切强度试验更准确、更科学和更具有可操作性。

功率模块纳米银烧结技术研究进展

大功率器件工作时产热量大,如果不能及时导出将严重降低互连层的性能,从而影响功率模块性能和可靠性。纳米银烧结技术可以实现低温连接、高温服役的要求,且具有优秀的导热导电性能和高温可靠性,已成为近几年功率模块封装互连材料的研究热点。归纳总结并对比分析了近年来关于纳米银烧结的研究,探讨了影响纳米银烧结接头初始剪切强度的工艺参数以及高温长期可靠性问题,评述了纳米银烧结的新技术和发展趋势。

光机材料与光纤振动传感器连接力学可靠性研究

针对不同的典型空间相机光机结构材料,研制典型空间相机用光机材料试块,能够有效代表现有相机主要光机结构组成和光纤振动传感器安装对象。设计相关试验工装,通过工装连接进行拉伸强度和剪切强度试验,获得其强度破坏参数。从而进行连接工艺强度研究,验证连接力学工艺可靠性。以确保实现传感器与空间相机结构的有效连接,完成极限力学环境试验考核,为传感器在后续型号中的应用提供参考。

实时剪切波弹性成像量化评估健康人正中神经弹性特征的信度

背景:实时剪切波弹性成像技术可无创测量组织弹性,目前较多地应用于甲状腺、乳腺等软组织疾病,而应用于周围神经及其病变的研究相对少见。目的:使用实时剪切波弹性成像技术量化评估健康人正中神经的重测信度及测试者间信度,探究同一测量位点正中神经长轴与短轴弹性模量的相关性。方法:随机招募20名健康受试者参与该试验,10名男性,10名女性。在信度试验中,测试者A和B分别使用实时剪切波弹性成像技术测量受试者利手侧、非利手侧正中神经长轴放松状态时的弹性模量;5 d后,测试者A以相同的测量方法对受试者正中神经弹性进行量化评估。此外,使用实时剪切波弹性成像技术在受试者腕横纹处测量正中神经长轴以及短轴的弹性模量完成相关试验。结果与结论:(1)在信度试验中,利手侧正中神经的测试者间信度表现为优秀:其组内相关系数为0.98,重测信度优秀(组内相关系数=0.96);非利手侧,测试者间信度和重测信度均表现为好(组内相关系数=0.78,0.86);(2)正中神经长轴与其短轴弹性模量之间无相关性(r=0.039,P=0.87);(3)提示剪切波弹性成像技术能准确量化评估出健康人利手侧以及非利手侧正中神经弹性模量特征,且可信度较高;此外通过相关性试验可推测:相较于临床上使用正中神经长轴弹性特征作为常规参考指标,其短轴弹性模量的变化数据可信度不高。

晶圆级封装用半烧结型银浆粘接工艺

选取了一种半烧结型银浆进行粘接工艺研究,通过剪切强度测试和空洞率检测确定了合适的点胶工艺参数,并进行了红外热阻测试和可靠性测试。结果表明,该半烧结型银浆的工艺操作性好,烧结后胶层空洞率低;当胶层厚度控制在30μm左右时,剪切强度达到25.73 MPa;采用半烧结型银浆+TSV转接板的方式烧结功放芯片,其导热性能满足芯片的散热要求;经过可靠性测试后,烧结芯片的剪切强度没有下降,具有较高的稳定性和可靠性,可用于晶圆级封装中功率芯片的粘接。