损伤条件下聚氯乙烯涂层膜结构材料拉伸蠕变特性

为进一步研究膜结构材料的力学特性,以机织聚氯乙烯涂层膜结构材料为研究对象,对其无损及单边切口试样的短期拉伸蠕变性能进行测试,分析切口尺寸及蠕变应力对蠕变性能的影响,并对其拉伸蠕变特性进行研究。实验结果表明：无损及切口试样的蠕变曲线均体现了初始蠕变和等速蠕变阶段的特征。在同等净切口蠕变应力条件下,切口试样的蠕变量小于无损试样,且随着切口尺寸的增加而降低。相对于Findley 模型,4 元件Burger 模型较适合描述聚氯乙烯涂层膜结构材料的短期蠕变行为。同时,基于4元件Burger 模型拟合系数,分析了蠕变应力及切口尺寸对初始蠕变应变、缓弹性变形和黏性变形的影响。     

基于分数阶模型的纱线蠕变性能模拟与预测

针对现有纱线拉伸力学模型描述不准确问题,以分数阶微积分模型对纱线拉伸行为进行分析。首先,引入分数阶微积分理论改进的黏壶模型建立了分数阶纱线蠕变模型,使用XL-2型纱线强伸度仪对不同线密度及不同张力纱线进行蠕变实验,获得纱线蠕变的全过程曲线。通过对不同线密度下的试验曲线进行回归分析得到了模型参数以及蠕变模型参数与施加张力大小的变化关系。最后,利用不同模型对不同张力的纱线蠕变曲线进行拟合和预测。结果表明：分数阶纱线蠕变模型,相比于三元件模型、整数阶模型和Burgers模型,具有结构简单、参数少的特点,并且对纱线蠕变的拟合和预测也具备较高的精度。     

水工沥青混凝土粘弹性能的实验研究

进行水工沥青混凝土在不同加载应力和不同实验温度下的单轴蠕变实验,将四种常用粘弹性模型编入非线性拟合程序对蠕变曲线进行拟合,得到拟合曲线、模型参数值及相关系数,对比拟合曲线与实验曲线,分析四种本构模型的优势和不足,选出最优模型,并研究其参数在不同实验条件下的变化规律,结果表明Burgers模型能够较好地反映蠕变的第一、第二阶段特征,可用来描述水工沥青混凝土粘弹性能。     

方平组织PVC涂层膜材料的塑性变形性能

为了解塑性变形同作用应力、作用时间的相互关系,对方平组织为基布的PVC涂层膜材料进行经纬2个方向的单轴向拉伸蠕变试验,测量卸载1 a后试样的塑性变形量.利用塑性应变为作用应力差的幂函数与作用时间幂函数的乘积所构成的多元函数,对试验数据进行了多元回归分析.结果表明所采用的模型具有较好的拟合效果.同时,对塑性变形中的瞬时变...     

应力回复对PVC涂层膜材料蠕变性能的影响

为深入研究建筑膜材料的蠕变性能,对PVC涂层膜材料在应力回复条件下的蠕变性能进行了测试和分析,阐述了非寻常蠕变性能的产生机理.结果 表明,PVC涂层膜材料的蠕变性能与应力回复阶段的最大应力、蠕变应力及加、卸载速率密切相关.无应力回复和应力回复量较高时,PVC涂层膜材分别表现出寻常蠕变与逆蠕变行为特征,加、卸载速率仅影响...     

涂层过程中织物拉伸对膜材料拉伸性能的影响

针对机织基布涂层前后结构相变化所导致的拉伸力学性能的变化,建立了膜材料拉伸力学的空间桁架模型,旨在讨论涂层过程中织物经向拉伸对膜材料拉伸性能的影响。建模过程中分别考虑了以经纬纱屈曲转换为主及纱线拉伸伸长为主的变形机制,用最小二乘法分段线性拟合来描述织物拉伸性能的非线性,用PVC/PES膜材料的拉伸试验验证模型的可靠性。结果表明:通过控制织物在涂层过程中的应变量,可以由涂层前织物的拉伸行为预测涂层后膜材料的拉伸性能。     

灰色Verhulst幂函数模型在沉降预测中的应用

分析指出不同生长曲线模型在沉降预测应用中的不足,通过对灰色Verhulst模型的建模数据进行幂函数变换,利用梯形积分公式和辛普森积分公式建立新的灰微分方程以及利用模式搜索法对参数进行二次优化,建立灰色Verhulst幂函数模型,新模型不仅适用于不同类型沉降曲线预测,而且不受制于数据类型的限制。工程实例应用结果显示灰色Verhulst幂函数模型具有较高的预测精度。     

PVC 压延柔性复合材料应力松弛的力学模型

根据黏弹性材料力学模型的假设,选取线性黏弹性力学模型中四元件模型的等效模型和非线性黏弹性力学模型中的欧林模型,基于材料相同但增强织物组织结构不同的3 种试样进行的应力松弛试验,通过MatLab 软件计算,拟合出四元件模型的等效模型及欧林模型的方程参数,并采用MatLab 软件计算机画图显示实测点和拟合曲线,从而进一步比较、分析得出最适合压延类柔性复合材料松弛性能的力学模型,证明PVC 压延类柔性复合材料具有非线性黏弹性的力学特征,属于非线性黏弹性的范畴,且增强织物的组织结构对这一属性影响不明显。     

涂层工艺对PVC膜结构复合材料力学性能的影响

采用涂层法制备高经密机织物增强体PVC膜结构柔性复合材料,以实验为基础分析了涂层前后拉伸性能变化的原因,同时研究了涂层工艺(焙烘温度、涂层厚度、增塑剂份数)对PVC膜材的力学性能的影响。结果表明:涂层厚度对力学性能的影响最明显,增加增塑剂份数可导致膜材的力学性能下降;综合考虑各涂层因素对膜材力学性能的影响,选择焙烘温度160℃、涂层厚度0.685 mm、增塑剂份数50份时,可获得力学性能较佳的膜结构复合材料。     

鲜猪肉中沙门氏菌生长预测模型的建立

以新鲜猪肉滤汁为培养液,接种沙门氏菌,绘制沙门氏菌在17、25、30、35、37℃下的生长曲线.一级模型分别使用Logistic和Baranyi模型拟合.二级模型以Ratkowsky方程为基础,代入Baranyi模型拟合数据,描述了温度对最大生长速率的影响.二级模型通过残差图、准确因子、偏差因子以及预测值与实测值对比图进行了检验,结果显示残差在±0.05之间,准确因子为1.0464,偏差因子为0.9933,预测值与实测值比较接近.     

循环加载处理对聚氯乙烯涂层膜材料蠕变性能的影响

为深入研究膜材料在受力过程中的蠕变作用机制,以聚氯乙烯涂层膜材料为研究对象,对其历经循环加载过程后的蠕变性能进行测试,分析了循环加载次数、循环峰值应力与循环加载速率对蠕变性能及初始变形保持能力的影响规律。研究结果表明:经不同循环加载过程后的涂层膜材料蠕变机制基本一致,蠕变性能及对初始变形的保持能力不仅受循环加载次数、循环峰值应力的影响,同时还与循环加载速率密切相关;对于聚氯乙烯涂层膜材料,增加循环加载次数、提高循环应力峰值、降低循环加载速率,均有助于提高蠕变过程对初始变形的保持能力,减少膜结构建筑后期维护工作量,但会延长循环加载阶段的耗时,增加施工过程预加张力阶段的工作量。     

Modelling of rheological characteristics of various spaghetti types

Spaghetti enriched with resistant starch was produced to increase the dietary fibre intake of consumers. In the product base it was compared to bran and control spaghetti regarding to viscoelastic properties. Viscoelastic properties were tested by relaxation and creep tests by using TA-XT2i texture analyser. The viscoelastic properties gave structural information. Relaxation and creep data were analysed by generalized Maxwell (two- and three-termed), Peleg & Normand, Burgers and Peleg models, respectively. For relaxation three-termed Maxwell and for creep behaviour Burgers model represented viscoelastic behaviour satisfactorily. In general, bran spaghetti was found to have the lowest elasticity. All the samples started to loose their elasticity as cooking time proceeded and it became easy to deform them. Instrumentally, spaghetti with resistant starch was found to be better than bran enriched spaghetti.     

Non-linear creep-recovery measurements as a tool for evaluating the viscoelastic properties of wheat flour dough

Creep-recovery measurements were used to analyze the non-linear viscoelastic properties of wheat flour. First the effect of creep time, recovery time and shear stress was investigated on the non-linear viscoelastic properties of Bussard dough. The Burgers model was fitted to the creep and recovery curves. A linear increase of maximum creep compliance was observed with increasing creep time. On the other hand, maximum recovery compliance remained constant but an increase of the retardation time was observed which indicates a slower recovery. A recovery time of 10 min seemed to be sufficient to obtain most of the recovery. Maximum creep compliance increased proportionally with increasing shear stress until a plateau was reached. Maximum recovery was constant between 100 and 500 Pa but the speed of the recovery increased as shear stress increased. Finally, the optimised creep-recovery methodology was used to analyze the non-linear viscoelastic properties of 17 pure wheat cultivars. By applying principal component analysis, it was possible to identify three groups of wheat cultivars with similar rheological properties and bread volumes.     

RHEOLOGICAL BEHAVIOR AND TIME-DEPENDENT CHARACTERIZATION OF ICE CREAM MIX WITH DIFFERENT SALEP CONTENT

The effect of salep concentration on the rheological characteristics of ice cream mixes (0.5–1.5% salep content), prepared from nonfat cow's milk and sugar, was studied using a controlled stress rheometer. The flow curves and time‐dependent flow properties of the ice cream mixes were assessed at different temperatures. The ice cream mixes’ samples showed slightly thixotropic behavior, which was reduced as the salep content decreased. The forward and backward measurements of the flow curves of ice cream mixes were modeled with the power law model. The ice cream mixes showed pseudoplastic flow behavior after destruction of the thixotropic structure. In mixes that were first presheared at a high shear rate flow properties could also be described by the power law model. The second‐order structural kinetic, first‐order stress decay and Weltman models were applied to model the time‐dependent flow properties of the ice cream mixes. Among these, the first‐order stress decay model was found to fit well the experimental data.     

COMPARISON OF TWO TYPES OF MODELS FOR CONCAVE FORCE-DEFORMATION CURVES PRESENTATION1

Compressive force-deformation curves of solid foods with upward and downward concavities are described by a power law and two, two-parameter mathematical models that can be transformed into linear equations by rearrangement. The fit of the latter to experimental data was comparable to or better than that of a two-parameter power law model. These two models can simplify the procedure and shorten the time of data processing in certain applications of computer interfaced testing machines. Their main advantage over the power law model, however, is that they can facilitate the analysis of the compressive behavior of double-layered arrays of objects in general and that of finger(s)-food system in particular.     

Evaluation of models to describe ruminal degradation kinetics from in situ ruminal incubation of whole soybeans

Different mathematical models were evaluated as candidates to describe ruminal dry matter (DM) and crude protein (CP) degradation kinetics of raw and roasted whole soybeans from data obtained using the in situ polyester bag technique. Three models were used: segmented with up to 3 straight lines (model I), negative exponential (model II), and rational function or inverse polynomial (linear over linear; model III). A fourth, a generalized sigmoidal model, was also considered but the data did not exhibit sigmoidicity, so it was dropped from the analysis. Lagged and nonlagged versions of each model were fitted to the DM and CP disappearance curves of 6 different feeds (2 cultivars of raw or differently heat-processed whole soybean). The comparison between lagged and nonlagged versions of each model, based on statistical and behavior characteristics, showed for all models that the discrete lag parameter did not significantly improve the fit to ruminal DM and CP disappearance curves. The comparison between models (using nonlagged equations) showed that models I and II gave better goodness-of-fit than model III. Based on biological characteristics, models II and III underestimated the undegradable DM and CP fractions, but there was no significant difference between models for extent of degradation.