Triglycerides as Novel Phase-Change Materials: A Review and Assessment of Their Thermal Properties

Latent Heat Storage (LHS) with Phase-Change Materials (PCMs) represents a high energy density storage technology which could be applied in a variety of applications such as waste heat recovery and integration of renewable energy technologies in energy systems. To increase the sustainability of these storage solutions, PCMs have to be developed with particular regard to bio-origin and biodegradability. Triglycerides represent an interesting class of esters as the main constituents of animal and vegetable fats, with attractive thermal properties. In order to be used as PCMs, the thermal behaviour of triglycerides has to be fully understood, as in some cases they have been reported to show polymorphism and supercooling. This study assesses the suitability of triglycerides as PCMs by reviewing the literature published so far on their behaviour and properties. In particular, melting points, enthalpies of fusion, polymorphism, thermal conductivities, heat capacities and thermal cycling stabilities are considered, with a focus on LHS and thermal energy storage applications. In addition, the efforts conducted regarding modelling and the prediction of melting points and enthalpies based on chemical structures are summarized and assessed.     

基于变量修正分布的混凝土结构长期变形预测Prediction of long-term deformation for concrete structure based on revised distribution of variables

由于随机因素的影响，混凝土结构长期变形通常表现出很强的离散性，Bayesian理论提供了改善这种离散性的方法。基于Bayesian理论的混凝土结构长期变形预测方法的核心是引入短期变形构造似然函数，通过修正先验概率得到长期变形的后验概率分布。但是对于实际结构而言，在施工之前短期变形及其标准差无法获取，这就使得这种方法在开展时机和实际应用方面存在一定的限制性。为了改善这种限制性，在随机变量修正分布的基础上，结合拉丁超立方抽样技术，采用CEB‐FIP（MC90）模型建立了钢筋混凝土梁长期变形的随机分析模型。采用该模型进行混凝土梁长期变形随机分析，得到基于变量修正分布的混凝土梁长期变形预测结果，并分别与先验预测结果和Bayesian预测结果进行比较。研究结果表明，基于变量修正分布的预测结果与Bayesian预测结果十分接近，比先验预测结果不确定性降低50％左右，与试验结果吻合良好。     

基于实测数据的扁平钢箱温度梯度疲劳应力谱分析方法

为研究扁平钢箱梁温度疲劳应力谱,以南溪长江大桥悬索桥主梁为研究对象,基于温度传感器长期实测数据,筛选实测数据的日温度极值,运用广义极值模型描述季节极值概率分布并采用极值外推方法得到设计基准期极值模型。引入拉丁超立方抽样(LHS)法对极值模型进行抽样,得到日温度极值样本。结合日温度极值样本和正弦函数模型,构建服役期内关注点的温度时程曲线。基于有限元ANSYS软件平台,分析不同温度梯度下关注点的应力效应,回归温度梯度与疲劳应力的线性关系式,依据温度梯度时程曲线与线性关系式模型,采用雨流计数法得到钢箱梁温度梯度疲劳应力谱。研究表明,模拟抽样生成的温度样本数据符合温度场的季节变化特征,样本概率模型与实测数据概率模型相对吻合。关注点温度梯度疲劳应力谱能够为扁平钢箱梁疲劳寿命设计提供参考。     

Influence of harmonically varying normal load on steady-state behavior of a 2dof torsional system with dry friction

A nonlinear dry friction problem with harmonically varying normal load is formulated, in the context of a two-degree of freedom torsional system, since virtually all of the prior literature focuses on the topic of time-invariant normal load. First, pure stick, pure slip and stick-slip motions are computationally and analytically determined when excited by a sinusoidal torque, in the presence of harmonically varying saturation torque; mean terms are included in both. These analyses yield both transient and steady-state time histories under various conditions. Second, the effects of time-varying normal load on steady-state responses have been investigated and nonlinear spectral maps (including super-harmonics) are developed. Results show that the actuation system parameters could affect steady-state stick-slip motions in different ways over the lower and higher frequency regimes, as a result of time-delay in slip motions with respect to the torque excitation. In particular, the negative slope characteristics in the friction law exaggerate the stick-slip vibration problems, and it is the major cause of bifurcations and quasi-periodic or chaotic motions. Around the super-harmonic peak frequencies, the nonlinear system tends to lose stability as abrupt jumps in the spectral maps take place. An equivalent viscous damping model is considered to analytically investigate the instability mechanism. Further, the periodicity of the system response under harmonically varying actuation is conceptually by employing the harmonic balance method. Finally, steady-state behavior is examined for the nonlinear, time-varying dry friction problem.     

基于LHS-Kriging法的正交异性钢桥面板疲劳可靠度分析

为解决随机车载下正交异性钢桥面板疲劳应力谱有限元求解耗时问题，采用拉丁超立方抽样（LHS）与Kriging方法，建立了快速获取随机车流作用下细节疲劳应力谱的LHS-Kriging有限元替代模型，并将此模型应用于南溪长江大桥正交异性钢桥面板疲劳可靠度计算。结果表明，基于LHS-Kriging方法的有限元替代模型， 不需要经过大量车辆荷载的有限元加载，可直接快速获取细节疲劳应力谱；与传统的响应面法（RSM）相比，Kriging法预测的细节等效疲劳应力更符合有限元计算结果；随着交通量增长率的增大，桥梁的疲劳可靠度显著减少；100年后，当交通量增长率为3%和5%时，正交异性桥面板与纵肋焊接处的细节疲劳可靠度小于2。