Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber-Reinforced Ceramic Composite

Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of ∼1 to ∼0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: ( i ) reductions of ∼20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, ( ii ) almost complete recovery of the longitudinal diffusivity following unloading, and ( iii ) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity.     

About the influence of temperature and environmental relative humidity on the longitudinal and transverse mechanical properties of elementary alfa fibers

Composites reinforced with plant-based fibers present a high potential for valorization in new industrial applications due to their good specific mechanical characteristics, renewability, and recyclability. In order to accelerate their wide industry adoption, it is critical to assess their behavior and durability in heat and humid environments. This article aims at investigating the effects of temperature and relative humidity (RH) on the longitudinal and transverse mechanical properties of the lignocellulosic fibers extracted from alfa plant (Stipa tenacissima L). For this purpose, tensile and nanoindentation tests were performed on elementary alfa fibers subjected to a thermal cycle of 200°C. The fibers were held at various periods of 15, 30, 60 and 120 min. The test results showed that the longitudinal and transverse Young's moduli are moderately affected by short thermal cycles having duration of 15–30 min. However, for longer thermal cycle (i.e., 2 hr), a degradation of 21% for the transverse modulus was recorded. This degradation doubled for the longitudinal modulus (43 vs. 21%). A similar trend was observed for the breaking strength. This study also showed that the RH strongly affects the mechanical performances of alfa fibers.     

THE CONFORMATIONAL MECHANISM OF THERMOELASTICITY OF ORIENTED POLYETHYLENE

Temperature variations of oriented polyethylene (PE) with a draw ratio of ～ 450% were measured in longitudinal adiabatic tension and compression (along the orientation axis) and transverse compression. Stress-strain curves (for tension and compression) were recorded. Linear thermal expansion coefficients in the longitudinal and transverse direction were measured. Thermoelastic characteristics and also thermal properties of oriented PE were quantitatively analyzed. It has been concluded that conformational dynamics has a dominating influence on these characteristics.     

Thermal Shock Behavior of Silicon Nitride Flexure Beam Specimens with Indentation Cracks

The experimental results of thermal shock testing of silicon nitride flexure beam specimens containing indentation cracks are presented. The thermal stress induced by water quenching is much greater in the transverse direction than in the longitudinal direction, resulting in an insensitivity of residual bend strength to temperature differences up to 580°C. This result indicates that a flexure beam configuration is not an appropriate geometry for thermal shock testing when thermal shock behavior is to be evaluated from residual bend strength data.     

Thermal diffusivity of 3D C/SiC composites from room temperature to 1400 °C

A 3D C/SiC composite and a bulk CVD SiC material were prepared. The effects of the CVD SiC coating and the heat treatment on the longitudinal and transverse thermal diffusivity of the C/SiC composites were investigated. The thermal diffusivity of the C/SiC composites could be well fitted by a multinomial function from room temperature to 1400 °C which includes a power term, an exponential term and a constant term. The exponential term affected the thermal diffusivity and led to its increase above 1200 °C with activation energy of 77 kcal/mol. The microstructure change in the composites was the reason that the thermal diffusivity was increased above 1200 °C. The longitudinal thermal diffusivity of the composite was twice or more than the transverse one and increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it. The heat treatment could increase the thermal diffusivity and make the exponential term disappeared in the functions. The functional curve before the treatment intersected that after the treatment at the treatment temperature.     

PBAT/PLA全生物降解薄膜的紫外光老化行为

采用挤出共混造粒、吹塑成型工艺制备了聚对苯二甲酸-己二酸丁二酯(PBAT)/聚乳酸(PLA)全生物降解薄膜。研究了薄膜在紫外光老化过程中热性能和横纵向力学性能随老化时间的变化,并利用红外光谱仪、X射线衍射仪、扫描电子显微镜对薄膜老化前后进行了表征。结果表明:在紫外光老化过程中,PBAT/PLA薄膜的横向拉伸强度和断裂拉伸应变均随着老化时间的延长不断减小,薄膜纵向拉伸强度和断裂标称应变随着老化时间的延长却先减小后增大,纵向的取向结晶结构有助于紫外光辐照交联的发生;紫外光老化更易发生在材料的无定型区,复合材料结晶度整体上不断增大。     

模具流道环流协同LDPE/HDPE吹塑薄膜结构性能研究

通过新型的旋转芯棒薄膜吹塑设备制备了低密度聚乙烯(LDPE)/高密度聚乙烯(HDPE)复合薄膜,并研究了该设备模具流道环流协同作用对吹塑聚乙烯薄膜结构和性能的影响。结果表明,制备的聚乙烯薄膜呈现一种互锁片晶结构;芯棒转速的提高对聚乙烯薄膜样品的结晶度、片晶厚度也有一定增益作用,使得结晶更完善;聚乙烯薄膜的拉伸性能和撕裂性能随芯棒转速提高都有所提升,尤其是横向力学性能,横向拉伸性能最大提升幅度为25.75%,横向撕裂性能最大提升幅度为27.64%;同时,该技术实现了在不影响聚乙烯薄膜的纵向热收缩率的情况下,大幅提高其横向热收缩率,提升幅度可达128.41%。     

Role of Interfacial Gaseous Heat Transfer in the Transverse Thermal Conductivity of a Uniaxial Carbon Fiber-Reinforced Aluminoborosilicate Glass

The transverse thermal conductivity of an aluminoborosilicate glass uniaxially reinforced with carbon fibers was found to be lower under near-vacuum than in nitrogen, whereas no such difference was found for the longitudinal thermal conductivity. This effect was attributed to the existence of an interfacial gap resulting from the thermal expansion mismatch between the matrix and fibers. The presence of this gap permits the gaseous environment access to the fiber-matrix interface and thereby contributes to the interfacial heat transfer. Its presence does not affect the longitudinal thermal conductivity, however, because the gap is aligned parallel to the fibers and, therefore, the direction of heat flow. Analysis of the experimental data indicates that, in nitrogen at atmospheric pressure, the gaseous conductance constitutes about one-third of the total interfacial conductance.     

Structure-thermal expansivity relation for isotropic polyoxymethylene sheets

A detailed parametric mapping analysis of the thermal expansion coefficient for isotropic polyoxymethylene is presented. The molecular composite model has been computerized to investigate the variation of the thermal expansivity of isotropic polyoxymethylene sheets with some typical morphological and structural parameters. The results are reported and analyzed in terms of three-dimensional structure-property maps. The thermal expansion coefficient is found to be significantly dependent on the crystallite content, in the first place, followed by the transverse aspect ratio of the crystallites and the amorphous phase stiffness. It is only at relatively low levels of crystallinity that the thermal expansivity is sensitive to the variation of the amorphous phase tensile modulus. The longitudinal aspect ratio is slightly effective in altering the expansion coefficient as long as it is kept below 25; otherwise it is ineffective.     

Mechanical and thermal properties of environment-friendly “green” composites made from pineapple leaf fibers and poly(hydroxybutyrate-co-valerate) resin

This paper presents the mechanical and thermal properties of unidirectional, degradable, environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) (PHBV) resin. Tensile and flexural properties of the “green” composites with different fiber contents were measured in both longitudinal and transverse directions. Compared to those of virgin resin, the tensile and flexural strengths of “green” composites are significantly higher in the longitudinal direction while they are lower in the transverse direction. However, the mechanical properties are lower than those predicted by simple models. Scanning electron microscope (SEM) photomicrographs of the tensile fracture surfaces demonstrate fibers being pulled out from the matrix, the interfacial failure, fiber fibrillation, and the nonunidirectional nature of the “green” composites. The thermal behavior of the “green” composites, studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), showed that the presence of pineapple fibers does not affect the nonisothermal crystallization kinetics, crystallinity, and thermal decomposition of PHBV resin.     

阻燃PA66复合材料翘曲性能研究

对比研究了无玻璃纤维、普通圆形玻璃纤维、扁平玻璃纤维对溴系阻燃聚酰胺66（PA66）复合材料的翘曲性能影响,分别从力学性能、结晶性能、收缩率和横向/纵向收缩率比等因素阐述复合材料翘曲性能。结果表明,相同阻燃剂含量条件下,不加玻璃纤维复合材料的结晶度最高,横向收缩率与纵向收缩率最大,但横向/纵向收缩率比最小,复合材料翘曲度最小;相同玻璃纤维含量条件下,扁平玻璃纤维复合材料翘曲度明显优于普通圆形玻璃纤维,其横向/纵向收缩率比明显小于普通圆形玻璃纤维;力学性能方面,扁平玻璃纤维复合材料的拉伸强度和弯曲强要低于圆形玻璃纤维体系;不同形态玻璃纤维对阻燃增强PA66复合材料结晶性能影响小,其结晶度和结晶峰温度非常接近;相同阻燃剂含量条件下,随着扁平玻璃纤维含量增加,复合材料横向收缩率与纵向收缩率均降低,但横向/纵向收缩率比呈增大趋势,导致复合材料翘曲度随玻璃纤维含量增加而变大。     

全消光异形异收缩涤纶FDY的制备与性能

以全消光聚酯切片为原料,通过异形纺丝和异拉伸工艺路线,制得具有纵向竹节效果、横向波浪一字形截面的涤纶全拉伸丝(FDY)。采用全自动拉伸强力仪、扫描电镜、双折射仪、差示扫描量热仪等对纤维的力学性能、沸水收缩率、外观形貌、光学性能、热性能等进行表征。结果表明:纤维具有良好的波浪一字形、异形横截面及纵向粗细竹节效果;纤维粗节处比细节处具有较低的断裂强度、双折射率和结晶度,较高的断裂伸长率和沸水收缩率。     

PS/PMMA共混物中超声波传播特性研究

利用自行设计的装置,采用改变入射角的方法将超声波纵波和横波导入不相容的聚苯乙烯/聚甲基丙烯酸甲酯(PS/PMMA)共混物体系中。研究了两种波的声速和声衰减随共混体系共混比变化的情况,并结合扫描电子显微镜(SEM)检测得到的相畴尺寸进行了分析。结果表明:共混物中的相畴尺寸与其散射衰减之间存在对应关系;由于横波波长更为接近共混体系中的相畴尺寸,横波的散射衰减明显大于纵波。     

Nonlinear response and thermomechanochemical degradation of a urethane elastomer

We have studied the failure of a urethane elastomer due to cyclic compressive loading, using loading frequencies and specimen sizes for which internal heat generation is an important factor. The eventual failures were generally manifest by internal cracks growing transverse to the loading direction. A variety of experimental analyses indicate that this failure is primarily thermal, in that the temperature rise due to viscous dissipation eventually leads to a melting of the hard segment domains which act to reinforce the material. No strong indication of thermal or thermomechanical bond scission was obtained, although a progressive reduction in the rubbery modulus was noted.     

Transverse and longitudinal coefficient of thermal expansion of carbon fibers at high temperatures (300-2500 K)

The transverse and longitudinal coefficients of thermal expansion (CTE) of carbon fibers are important parameters in C/C composites behaviour. In this study, the measurements of CTE are presented at very high temperatures (up to 2500 K). Experimental validations are first realised on isotropic metallic (tungsten) fiber. Then, measurements are performed on four carbon fibers (rayon-based, PAN-based and pitch-based). The results show that the mean transverse CTE varies from 5 × 10⁻⁶ K⁻¹ to 10 × 10⁻⁶ K⁻¹ and the longitudinal one from 1.6 × 10⁻⁶ K⁻¹ to 2.1 × 10⁻⁶ K⁻¹ depending on the fiber structural properties. For that, the relationships between the structural properties and the heat treatment temperature of the carbon fibers are discussed in relation to thermal expansion behaviour. A calculation of the ratio between the transverse and the longitudinal measured CTE as a function of the temperature allows us to classify fibers from the most isotropic to the most anisotropic in relation with the structural properties measured at room temperature.     

On the influence of molecular weight and crystallisation condition on the development of defect in highly drawn polyethylene

Longitudinal morphology of highly drawn high density polyethylenes has been investigated. Three grades of HDPEs with different molecular weights were used. Two different initial morphologies were obtained by quenching and slow cooling from the melt. The samples were drawn at 75 °C close to their breaking points and their longitudinal morphologies examined under SEM after etching. Two types of defects were observed. These are the structure containing a large number of parallel longitudinal etched pockets laid in the regions between almost parallel continuous transverse bands or ‘Pisa’ structure recently reported and transverse cracks. The appearance of these defects was found to correlate with molecular weight and thermal history of the samples. For all quenched samples, both Pisa structure and transverse cracks were observed. The number of transverse cracks was found to increase with increasing molecular weight. Apparently, the transverse cracks, which thought to limit the drawing to high draw ratios, can be retarded by drawing at higher temperature (100 °C). This suggests that the formation of transverse cracks relates to chain mobility and drawability of the materials. The Pisa structure was found to disappear from slowly cooled low molecular weight samples. For slowly cooled high molecular weight sample, the Pisa structure became less salient. It is suggested that the formation of Pisa structure is determined by molecular entanglement. Analysis of the band separation of the Pisa structure shows that there seems to be a characteristic value which depends on drawing temperature.     

Effect of a net on the limiting current distribution in a parallel plate electrochemical reactor: part I. Individual effects

In some aqueous-metal batteries or electrochemical parallel plate reactors, the spacing between the electrodes is controlled by a porous net. This net affects the limiting current distribution because it disrupts the parabolic laminar flow velocity distribution. Here, computational fluid dynamics (CFD) is used to solve the Navier-Stokes equations surrounding the inert net and the effect of the net geometry on the limiting current density is studied. The location, spacing, and number of the longitudinal and transverse ribs of the net are shown to affect the local and average current density distributions on each of the two electrodes. The effect of transverse ribs on the current distribution was found to be much higher than the longitudinal ribs. The results show that the longitudinal ribs decrease the local current density at the electrode which is not in contact and increase the current density the space between two adjacent longitudinal ribs at the electrode in contact. The transverse ribs on the other hand, increase the local current density to very high values at the electrode that is not in contact. The current density, however, falls along the flow direction as it exits the transverse-ribs region. These effects were observed to be mainly due to the changes in flow field distribution. A deviation of −40% was observed for a system of 4 longitudinal ribs and no transverse ribs at the non-dimensional axial position 0.06. For 2 transverse ribs, the deviation at the same axial position was approximately 250% of the local current density. All the results are for a net with a spacing of 0.94 × 10⁻³ m.     

Evaluating unidirectional composite thermal conductivities through engineered interphase

ABSTRACT

Effect of fibre/matrix interphase parameters, including thickness and material properties on the equivalent thermal conductivities of unidirectional fibre-reinforced polymer composites. A unit cell-based micromechanical method is proposed to evaluate the thermal conductivities of unidirectional multi-phase composites. The longitudinal thermal conductivity of unidirectional fibre-reinforced polymer matrix composites is seen to be independent of interphase region. When the thermal conductivity of interphase is higher than that of matrix, the increase of interphase thickness leads to an improvement in transverse thermal conductivity of fibre-reinforced polymer composites. The influences of fibre volume fraction, orientation angle and shape of cross-section as well as temperature on the thermal conducting behaviour are widely examined. The model predictions are in good agreement with the experimental data reported in the literature.     

轮胎有限元分析软件计算精度分析

采用轮胎经典力学推导出周向和横向2个普适力学平衡方程.根据周向和横向力学平衡方程及最大势能原理,以11.00R20全钢载重子午线轮胎为例,采用轮胎行业使用较为普遍的有限元分析软件ABAQUS进行分析计算.结果表明,ABAQUS轮胎有限元分析软件的计算精度较高.     

风电叶片复合材料压缩损伤破坏声发射监测

研究了风电叶片单向复合材料的压缩力学特性及其声发射响应特征.结果表明,复合材料的横向和纵向压缩力学性能及其声发射响应特性明显不同,纵向压缩强度、模量高,失效应变小,对应的声发射相对能量、幅度高,但撞击累积总数少.复合材料具有脆性破坏的特点,横向压缩以45°剪切失效为主,纵向压缩以层间劈裂为主.风电叶片复合材料压缩损伤破...