ADDITIONAL SOURCES OF CHANGING SHRINKAGE RATES DURING KILN DRYING

This paper emphasizes the established need for a stress based kiln control system. It refers to I) the work which verified that the initial shrinkage rate changes within a board are the result of reduced stress levels and 2) the new statistical method short/long-term slope comparison method used to detect slope changes. The paper continues by giving the explanation for the additional shrinkage slope changes as the equilibrium moisture content changes during fan reversal and kiln schedule advancements. The kiln schedule advancements can cause both shrinkage and swelling. Reduced drying rate resulting in reduced shrinkage rate explains other changes. A stress based automated kiln control system is conceptually possible with 1) shrinkage rate changes understood and 2) a production useful statistical method to detect these changes.     

ADDITIONAL SOURCES OF CHANGING SHRINKAGE RATES DURING KILN DRYING

ABSTRACT

This paper emphasizes the established need for a stress based kiln control system. It refers to I) the work which verified that the initial shrinkage rate changes within a board are the result of reduced stress levels and 2) the new statistical method short/long-term slope comparison method used to detect slope changes. The paper continues by giving the explanation for the additional shrinkage slope changes as the equilibrium moisture content changes during fan reversal and kiln schedule advancements. The kiln schedule advancements can cause both shrinkage and swelling. Reduced drying rate resulting in reduced shrinkage rate explains other changes. A stress based automated kiln control system is conceptually possible with 1) shrinkage rate changes understood and 2) a production useful statistical method to detect these changes.     

Drying Shrinkage Mechanisms in Portland Cement Paste

The shrinkage mechanisms of portland cement paste were investigated from shrinkage, weight loss, and pore structure measurements using nitrogen sorption and mercury intrusion porosimetry (MIP). Thin samples (2.3 mm) of well-hydrated (165 d) pastes of 0.4 and 0.6 water-to-cement (W/C) ratios were dried directly from saturated surface dry state to 75%, 50%, 11%, and 0% relative humidity (rh). From equilibrium shrinkage vs calculated increase in surface free energy curves two active stress mechanisms were identified. The Gibbs-Bangham (surface free energy) effect is the major stress mechanism, which is active in the entire rh range investigated, whereas the capillary stress effect is active above 25% rh. From elastic modulus calculations it can be concluded that true Gibbs-Bangham shrinkage accounts for only 33% of total first drying shrinkage. Thus nearly 67% of first drying shrinkage may be due to a decrease in interlayer spacing caused by Gibbs-Bangham and capillary induced stresses. Further, nitrogen measures the true external surface area, and total external pore volume can be obtained from combined measurements using nitrogen sorption and MIP.     

Modeling and On-line Measurement of Drying Stress of Pinus massoniana Board

A drying stress model was established by considering that the total shrinkage of wood is the sum of free shrinkage, instantaneous strain, viscoelastic strain, and mechanosorptive strain. From the stress model, the stress can be calculated once the actual wood shrinkage and moisture content gradient are known. Based on this theory, on-line measurement of the drying stress has been realized by measuring the moisture content (MC) gradient between the surface and the core layers, and the actual shrinkage of the board for Pinus massoniana.

A sensor for measuring wood shrinkage was developed based on electric resistance and strain relationship in a selected element material within the sensor. A resistance type of MC sensor was used for the MC gradient measurement. These sensors are reliable and can meet the requirement of the measurements in practical drying. The technique reported in this article for detecting drying stress from the on-line measurements of board shrinkage and MC gradient can be applied to develop optimized drying schedule in commercial drying.     

一维固化收缩应力的研究

本文利用自制的测试固化应力的仪器测试研究了树脂固化时一维固化收缩应力的产生情况,然后通过低温固化低轮廓添加剂控制收缩,进一步测试研究其产生情况,观察了低收缩对固化应力的影响.     

Modeling and On-line Measurement of Drying Stress of Pinus massoniana Board

A drying stress model was established by considering that the total shrinkage of wood is the sum of free shrinkage, instantaneous strain, viscoelastic strain, and mechanosorptive strain. From the stress model, the stress can be calculated once the actual wood shrinkage and moisture content gradient are known. Based on this theory, on-line measurement of the drying stress has been realized by measuring the moisture content (MC) gradient between the surface and the core layers, and the actual shrinkage of the board for Pinus massoniana.

A sensor for measuring wood shrinkage was developed based on electric resistance and strain relationship in a selected element material within the sensor. A resistance type of MC sensor was used for the MC gradient measurement. These sensors are reliable and can meet the requirement of the measurements in practical drying. The technique reported in this article for detecting drying stress from the on-line measurements of board shrinkage and MC gradient can be applied to develop optimized drying schedule in commercial drying.     

Drying mechanisms and stress development in aqueous alumina tape casting

Mass loss, shrinkage, Young’s modulus evolution and stress development of aqueous (alumina + latex) tape cast suspensions were observed during drying. Mass loss showed a constant drying rate period, followed by a falling rate period. Concurrently a linear shrinkage rate has been observed in the thickness direction, up to a drying point after which shrinkage abruptly stops. End of constant drying rate and end of shrinkage were not necessarily concomitant and depend on the latex/alumina ratio in the tape. A Young’s modulus value appears in the tape cast suspensions at the transition from liquid to solid like state. Then an increase of Young’s modulus is observed corresponding to latex film formation. The stresses generated by drying in the tape exhibited a first period of increase due to capillary pressure in the pores, a small relaxation immediately followed by a second stress increase due to latex film formation, and a stress plateau at the end of latex coalescence. Alumina powder granulometry and surface tension of the liquid had a preponderant influence on the first stress maximum whereas the properties of the latex and the drying conditions dominated the second stress maximum. By increasing latex proportion up to 25 wt.% on alumina basis, it was possible to make the first and the second stress rise concomitant.     

注塑工艺参数对制品残余应力和收缩的影响

注塑成型工艺参数对制品的最终残余应力和收缩有着直接的影响。基于线性黏弹性模型模拟计算了注塑成型过程中由温度和压力引起的残余应力和收缩。以无定型材料PS和ABS为例,系统地研究了不同成型工艺条件下平板制件的最终残余应力和收缩,并和实验结果进行对比验证。结果表明：在流动方向上无定型材料的收缩基本保持不变,残余应力沿壁厚分布的形状也基本相同,但流动末端处的应力值稍大于流动入口处;保压压力是影响制品收缩的关键因素,提高保压压力和注射温度可以降低制品的最终收缩,而模具温度对收缩的影响较小。     

Constrained shrinkage of highly oriented poly(methyl methacrylate) fibers

Constrained shrinkage of fibers is the primary method used to examine orientation in amorphous materials. During the test, fibers are constrained, heated, and the stress that develops is measured as a function of time and temperature. This article describes an apparatus developed to measure that stress and a series of experiments for melt‐spun poly(methyl methacrylate) fibers fabricated under three conditions: (1) constant viscosity, (2) increasing temperature, and (3) increasing draw velocity. Results show that both the rates of rise and the decay of the fiber shrinkage stress have an Arrhenius relationship with temperature. Fibers fabricated at a constant viscosity have the same maximum shrinkage stress and rate of stress decay. As the processing temperature decreases or as draw velocity increases, for other parameters held constant, the maximum shrinkage stress increases. The rate of stress rise increases with decreasing processing temperature or increasing draw velocity. Maximum shrinkage stress also increases with increasing molecular orientation when measured by a different test, free heat‐induced shrinkage of the fibers. However, it was not possible to correlate both of these results to rubber elasticity theory attributed to the high degree of orientation present in the fibers and high polydispersity in the starting material. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 4047–4056, 2004     

树脂固化时体积收缩内应力的本质及消除途径

树脂固化收缩应力的主要原因是单体之间的分子间距离缩短变为共价距离。工艺上可通过降低官能团浓度,加入高分子增韧剂和无机粉状填料,改进固化工艺等方法来减少体积收缩率,进而减少内应力。利用膨胀单体共聚有可能根本消除收缩应力。     

Assessment of residual stresses during cure and cooling of epoxy resins

A new stress monitoring technique, a stress-tracking device, is described here. It has been used to study some important properties of epoxy resin. Residual stresses, including a curing shrinkage stress and a cooling shrinkage stress, were measured automatically and continuously during curing and cooling. Simultaneously, information such as an apparent gelation time and glass transition temperature were obtained directly during the experiment. These epoxy resin properties were related to the extent of cure. Varying cure temperature produced changes of cure behavior, which resulted in different residual stresses.     

A study of the drawing behavior of poly(ethylene terephthalate)

Cold-drawn and hot-drawn samples of poly(ethylene terephthalate) were studied by means of measurements of shrinkage stress, birefringence and differential scanning calorimetry (DSC). The values of shrinkage stress were comparable for both types of sample, implying that the deformation of a molecular network is important for both cold drawing and hot drawing. The DSC results indicate that substantial crystallization occurs in hot drawing for other than the lowest draw ratios, and this crystallization gives rise to an additional peak in the shrinkage stress measurements. In addition to temperature, strain rate is also an important variable, and changes in strain rate caused significant changes in both hot-drawn and cold-drawn samples.     

减缩抗裂型混凝土超塑化剂的性能及其作用机理

开发了一种新型的减缩抗裂型超塑化剂(shrinkage-reducing and anti-cracking polycarboxylate-based superplasticizer,SRPCA),掺量为水泥质量的0.2%时,减水率可达22.3%,且增强效果明显.和掺传统萘系减水剂的混凝土相比,掺SRPCA的混凝土90d的自收缩率和干燥收缩率分别降低了52.1%和44.9%.平板和圆环开裂实验证实:SRPCA大大提高了混凝土的抗裂能力.SRPCA的减缩抗裂效果基本接近减缩剂掺量为胶凝材料总质量2%的减缩抗裂效果,但SRPCA的掺量不到复掺萘系减水剂和减缩剂的1/10.     

In-mold shrinkage and stress prediction in injection molding

In-mold shrinkage may occur for product parts that solidify under low holding pressure and are not restricted by ribs or flanges. It not only affects the final product dimensions but in addition may have a large effect on the residual stress distribution. A simple elastic model is used to study the effect of in-mold shrinkage on final product dimensions and residual stress distributions. Friction between polymer surface and mold wall as well as deformation of the mold cavity are taken into account. The model uses local values for temperature, pressure, and crystallization, which belong to the standard output of most simulation codes.     

Surface tension-pressure superposition principle for anisotropic shrinkage of an ellipsoidal pore in viscous sintering

The shrinkage of a spherical pore is a classical model of the final stage of sintering; however, closed pores observed in real sintering have irregular shapes, which can be approximated as ellipsoids in many cases. In this study, we used the finite element simulation to evaluate the anisotropic shrinkage of a non-spherical pore. The shape evolution was expressed as the superposition of the deformation of a pore driven by surface tension and the deformation of a void driven by remote pressure in the absence of surface tension. By using this superposition principle, we can predict the shape evolution of any spheroidal pore, which is affected by the initial aspect ratio, the remote pressure, and the gas pressure inside the pore. We analyzed the role of sintering stress tensor on the anisotropic shrinkage also.     

Autogenous shrinkage of concrete containing granulated blast-furnace slag

This paper presents the experimental results and prediction model for the autogenous shrinkage of concrete made with various water-to-cementitious materials ratios (w / cm) ranging from 0.27 to 0.42 and granulated blast-furnace slag (BFS) in the range of 0% to 50% by mass of the total cementitious materials. Test results showed that BFS concrete exhibited greater autogenous shrinkage than ordinary concrete with no BFS with the same w / cm, and that the higher the BFS content, the greater the autogenous shrinkage. At the same content of BFS, the increasing rate of autogenous shrinkage is affected by the w / cm; the lower the w / cm, the smaller the increasing rate of autogenous shrinkage. Based on the test results, a prediction model for autogenous shrinkage was proposed. In particular, an effective autogenous shrinkage that is a realistic shrinkage strain responsible for stress development was introduced in the model. It was determined by taking into account the characteristics of ultrasonic pulse velocity evolution in concrete. This prediction method for autogenous shrinkage may be effectively used to estimate the stress induced by autogenous shrinkage.     

超长钢筋混凝土水池无缝结构设计方法

分析现浇超长钢筋混凝土水池温度缝设置存在问题,提出不设置温度缝的设计方法,施工阶段采用补偿收缩混凝土解决水化热等效温差、干缩等效温差应力的措施,确保混凝土在浇筑和凝固过程中不出现拉应力,实现浇筑时不出现干缩裂缝;长期使用阶段采用预应力筋产生的预应力,抵抗因季节负温差产生的温度拉应力,避免出现结构裂缝。     

Thermoforming shrinkage prediction

Most thermoforming product development processes rely on costly and time‐consuming forming trials to determine the adequacy of the mold and process. In this paper, an analytical method is developed for shrinkage predictions on the basis of a visco‐elastic constitutive material model with initial conditions from a commercial thermoforming simulation. The theoretical analysis has been developed to accommodate different sets of materials, process conditions, and mold geometry. The shrinkage model consists of a transient thermal analysis for temperature solution; a stretching phase analysis for Inflation‐induced stress estimation; a post‐contact analysis for thermal stress and relaxation; and a post‐molding strain analysis based on stress solution. The shrinkage prediction analysis has been developed and validated with a complex geometry thermoforming application. The results indicate that the shrinkage estimates provided by the analysis were within the objective tolerances of 0.1%, as measured in terms of absolute prediction error of part dimensions.     

高强混凝土收缩与栓钉内约束相互作用的影响研究

根据当前工程中钢板-混凝土组合墙结构普遍开裂工程现象和栓钉约束下混凝土收缩变形研究较少的现实情况,对栓钉约束下高强混凝土收缩应力大小进行相对应的研究分析.采用钢筋内约束改进试验方法,研究栓钉内约束下高强混凝土收缩情况,分析不同类型栓钉内约束下高强混凝土收缩情况,通过设计正交试验,讨论出不同组合因素下,内约束强度最优组合.与此同时,通过建立有限元模型,分析出栓钉与高强混凝土之间相互作用,得出应力集中位置,较好地提供了栓钉约束下早期混凝土开裂的分布情况.     

The role of molecular networks and thermally activated processes in the deformation behavior of polymers

The deformation behavior of three polymers, polyethylene terephthalate (PET), polymethylmethacrylate (PMMA), and linear polyethylene (LPE) is considered in terms of two key factors, the stretching of a molecular network and the influence of thermally activated processes. In PET the observation of a natural draw ratio leads to studies of shrinkage, shrinkage force, and optical birefringence to define the nature of the network. The network is further exemplified by measurements of the molecular reorientation in deformation bands, spectroscopic studies of molecular orientation in drawing, and the concept of a true stress-strain curve. Yield and plastic deformation are also to be considered as thermally activated processes, but it appears that a major part of the flow stress is associated with the stretching of the molecular network. In PMMA the concept of a true stress strain curve also appears to be valuable, but the possibility of network breakdown during deformation has to be admitted as an extra complexity. In LPE the concept of a molecular network embracing both crystalline and non-crystalline material is helpful in understanding the drawing behavior. There is also direct evidence for the existence of a network from measurements of shrinkage and shrinkage force, and the existence of a true-stress strain curve. However, the dominant contribution to the flow stress now appears to come from thermally activated processes, with a key contribution from a small activation volume process which is tentatively associated with slip in the crystalline regions.