Effects of Matrix Porosity on the Mechanical Properties of a Porous-Matrix, All-Oxide Ceramic Composite

The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregnation and pyrolysis of a ceramic precursor solution. Mechanical tests are performed to assess the role of the matrix in both matrix-dominated and fiber-dominated loading configurations. The results demonstrate a loss in damage tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are obtained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a consequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double-notched shear specimen are broadly consistent with the limited values obtained by the short beam shear method, although the former exhibit large variability. In addition, effects of precursor segregation during drying on through-thickness gradients in matrix properties and their role in composite performance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabling damage tolerance in porous-matrix composites. Deficiencies in the understanding of the mechanisms that enable damage tolerance in this class of composites are discussed.     

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.     

热空气老化对丁基橡胶交联密度和力学性能的影响

采用热空气老化试验机研究了丁基橡胶（IIR）的老化行为,获得了不同老化温度和老化时间对IIR交联密度和力学性能的影响规律。结果表明,随着老化时间的延长,IIR硫化胶的交联密度先增大后减小,并随老化温度的升高变化幅度较为明显。拉伸强度、定伸应力和硬度均出现类似变化规律,而拉断伸长率先迅速下降后逐渐趋于平缓。     

Effects of Preform Shear on Tensile Properties of a Woven C/SiC Composite

This article addresses effects of weave defects in an angle‐interlock C‐fiber preform on the tensile properties of the resulting fully processed C‐fiber/SiC‐matrix composite. For this purpose, a preform was intentionally sheared in a controlled manner after weaving. The resulting distortions were quantified by analyzing high‐resolution images of the preform surface after the first step of matrix processing, while the tows were still clearly visible. Comparisons are made of tensile test results on specimens cut from this composite panel and from a pristine panel in select loading orientations. Strain maps obtained by digital image correlation are used to identify local strain variations that are attributable to weave defects. The results are discussed in terms of: (i) the shear‐normal coupling that arises in loading orientations of present interest, and (ii) the geometric effects of tow misalignment on tow continuity along the specimen gauge length. The composite is found to perform in a robust manner, in the sense that the tensile properties are not sensitive to the presence of the defects.     

氧化镧和短玻纤填充对ABS热性能与机械性能的影响

研究通过开炼机辊压加工的方法将短玻纤(Glass-Fiber,GF)和氧化镧(La2O3)与原料ABS树脂共混,通过热性能和物理机械性能的测试表征研究了短玻纤所形成的搭接结构对ABS树脂性能的影响。测试结果表明:GF添加量升高,其50%热失重温度(T50%)较纯ABS最大会有8℃的提高,拉伸强度和冲击强度均呈逐步下降的趋势。在ABS/GF共混物中加入La2O3后其热性能和物理性能均有所下降。采用扫描电镜分析了GF在ABS中的分散情况,证明了GF的搭接方式和相容性对力学性能影响较大。     

High-Temperature Mechanical Properties of a Uniaxially Reinforced Zircon-Silicon Carbide Composite

Mechanical properties of a monolithic zircon ceramic and zircon-matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide monofilaments were measured in flexure between 25° and 1477°C. Monolithic zircon ceramics were weak and exhibited a brittle failure up to about 1300°C. An increasing amount of the plastic deformation was observed before failure above about 1300°C. In contrast, composites reinforced with either uncoated or BN-coated SiC filaments were stronger and tougher than the monolithic zircon at all test temperatures between 25° and 1477°C. The ultimate strength and work-of-fracture of composite samples decreased with increasing temperature. A transgranular matrix fracture was shown by the monolithic and composite samples tested up to about 1200°C, whereas an increasing amount of the intergranular matrix fracture was displayed above 1200°C.     

丁羟推进剂的热加速老化力学性能及寿命预估

用单轴拉伸试验和扯离试验测试了不同老化温度(55、65、75和85℃)下热加速老化后丁羟(HTPB)推进剂的力学性能及其粘接试件的扯离强度,用Berthlot方程预估了推进剂及其粘接试件的寿命。结果表明,HTPB推进剂的最大延伸率随老化时间的增加呈现降低趋势;老化温度越高,推进剂的最大延伸率降低幅度越大,85℃贮存30d时最大延伸率降幅为29.81%,而55℃贮存30d时最大延伸率降幅仅为4.34%;粘接试件的扯离强度随着老化时间的增加呈降低趋势,老化时间相同时,扯离强度随老化温度的升高而降低。预估HTPB推进剂和推进剂粘接试件的贮存寿命分别为9.4y和15.9y。     

Thermal Conductance of Delamination Cracks in a Fiber-Reinforced Ceramic Composite

The thermal conductance of delamination cracks in a unidirectionally reinforced ceramic composite is investigated. A phase-sensitive photothermal technique is used to measure the crack conductance in situ under load. Special emphasis is given to the effects of the local crack opening displacement (δ). A crack conductance model that considers the contributions from both the air and the fibers within the crack is developed and compared with the measurements. Despite considerable scatter in the experimental data, the model adequately predicts the increased conductance that is associated with fiber bridging, as well as the overall trend that is observed with δ.     

Review of the Mechanical Properties of a 3D Woven Composite and Its Applications

Owing to their complex structural characteristics, 3D woven composites display different mechanical properties and failure modes from traditional composite laminates. The relations between the weaving geometry and main mechanical properties of composites are obtained by studying the internal structures of 3D woven textile. This paper reviews the research on the mechanical behavior such as tension, compression, shear, flexural, and impact properties, and analytical models of 3D woven polymer matrix composites mainly in four major inner structures and discusses the development of mechanical properties and applications of 3D woven fabrics and suggests further studies on aero industry.     

不同硫化体系EPDM老化性能的研究

研究了DCP用量对EPDM力学性能、老化性能的影响。结果表明,采用过氧化物DCP硫化体系的力学性能优于硫磺硫化体系。DCP硫化的EPDM加入抗氧剂MB后具有较高的耐热氧老化性能,显示出MB具有较高的防护效能。     

改性橡胶木粉/HDPE复合材料结构和力学性能的研究

用硅烷偶联剂A-171和天然橡胶胶乳(MGL-30)改性碱处理后的橡胶木粉。研究了木粉粒径、改性剂和木粉质量分数对橡胶木粉填充HDPE复合材料力学性能的影响,并对复合材料的断面形貌和木粉的表面结构进行了SEM分析。结果表明:木粉粒径、改性剂及木粉质量分数对复合材料力学性能有较大的影响。改性后木粉与基体HDPE的界面粘结强度提高,从而有效地提高了复合材料的力学性能。     

Effects of Accelerated Thermo-Oxidative Aging on Properties of a Damped-Encapsulating Epoxy Adhesive

The effects of accelerated thermo-oxidative aging treatment on structure, hardness, morphology of fractured surface, and dynamic mechanical properties of a damped-encapsulating epoxy adhesive were investigated. FT-IR indicated the formation of a small quantity of amide groups. The hardness increases rapidly at earlier stages and then tends to stabilize when the thermo-oxidative aging time is over 300 hours. SEM graphs indicated a change from some toughness to rigidity. DMA analyses revealed that the Tg of the aged samples gradually shifted to higher temperatures, the maximum value of tanδ fluctuated in a narrow range, the tanδ spectra were greatly temperature dependent, and the tanδ spectrum showed a tendency of horizontal displacement with increased thermo-oxidative aging time.     

Mechanical Properties, Thermal Shock Resistance, and Thermal Stability of Zirconia-Toughened Alumina-10 vol% Silicon Carbide Whisker Ceramic Matrix Composite

Zirconia-toughened alumina (Al₂O₃–15 vol% Y-PSZ (3 mol% Y₂O₃)) reinforced with 10 vol% silicon carbide whiskers (ZTA-10SiC _w ) ceramic matrix composite has been characterized with respect to its room-temperature mechanical properties, thermal shock resistance, and thermal stability at temperatures above 1073 K. The current ceramic composite has a flexural strength of ∽550 to 610 MPa and a fracture toughness, K _IC , of ∽5.6 to 5.9 MPa·m^1/2 at room temperature. Increases in surface fracture toughness, ∽30%, of thermally shocked samples were observed because of thermal-stress-induced tetragonal-to-monoclinic phase transformation of tetragonal ZrO₂ grains dispersed in the matrix. The residual flexural strength of ZTA–10 SiC _w ceramic composite, after single thermal shock quenches from 1373–1573 to 373 K, was ∽10% higher than that of the unshocked material. The composite retained ∽80% of its original flexural strength after 10 thermal shock quenches from 1373–1573 to 373K. Surface degradation was observed after thermal shock and isothermal heat treatments as a result of SiC whisker oxidation and surface blistering and swelling due to the release of CO gas bubbles. The oxidation rate of SiC whiskers in ZTA-10SiC _w composite was found to increase with temperature, with calculated rates of ∽8.3×10⁻⁸ and ∽3.3×10⁻⁷ kg/(m²·s) at 1373 and 1573 K, respectively. It is concluded that this ZTA-10SiC _w composite is not suitable for high-temperature applications above 1300 K in oxidizing atmosphere because of severe surface degradation.     

海军舰炮弹药用碳布增强环氧复合材料热老化寿命预测

研究了热老化对海军舰炮弹药用碳布增强环氧复合材料力学性能的影响,并利用回归分析方法对复合材料热老化寿命进行了预测。结果表明,碳布增强环氧复合材料分别在温度110,120,130,140℃热空气条件下老化2000 h,材料的各项力学性能总体呈下降趋势,但降幅很小,甚至在整个试验过程中会出现性能升高的情况,说明材料具有优异的力学性能。以90%作为材料失效判据点,碳布增强环氧树脂基复合材料常温贮存条件下贮存寿命为20年。     

加速紫外老化对聚乙烯薄膜力学性能的影响

介绍了聚乙烯(PE)薄膜老化机理,并对PE薄膜进行了人工加速紫外光老化试验。分析了老化时间对PE薄膜力学性能的影响。结果表明:紫外光的照射使PE的化学键发生交联和断裂反应,其中断裂反应占主导,使聚合物分子量减小,物理性能下降;PE薄膜的拉伸强度、断裂伸长率和弹性模量随老化时间的延长而降低;PE薄膜抵抗破坏的能力在老化60 h以上会急剧下降,而此时弹性模量的变化趋势变得平缓,断裂伸长率与老化时间近似成线性关系递减。     

高温老化对碳纤维增强双马来酰亚胺树脂基复合材料力学性能的影响研究

本文对国产碳纤维增强双马来酰亚胺树脂复合材料进行了高温老化力学性能测试和分析,通过扫描电子显微镜分析了高温老化对碳纤维／双马复合材料力学性能的影响。结果表明,老化1000h的力学性能未出现明显下降趋势,纤维与树脂基体粘接牢固,界面完好,该复合材料的高温老化性能优异。     

Mechanical Properties of an Extruded Wood Plastic Composite: Analytical Modeling

Abstract

The focus of this study has been to predict wood plastic composite (WPC) properties using analytical models dedicated to reinforced composites and to define their limits. Experiment tests have been realized to characterize mechanical properties of a commercial WPC. On one hand, good compressive and three points bending performances were observed in agreement with application requirements for this decking product. On the other hand, lower tensile performance and failure appearances revealed a lack of interfacial bonding between wood fiber and matrix despite the presence of a coupling agent in the composite formulation.     

硫化剂及填料对氯醚橡胶力学性能和老化性能的影响研究

研究了3种硫化剂——2,3-二巯基-喹喔啉(DNR-12A)、三巯基均三嗪(TCY)和亚乙基硫脲(NA-22)及3种填料——高耐磨炭黑(N330)、木质素-蒙脱土复合物填料(BL-MMT)和碱性木质素(lignin)对共聚型氯醚橡胶(ECO)硫化特性、交联密度、力学性能和老化性能的影响。结果表明,固定填料为N330时,用NA-22硫化的胶料的力学性能最好,但热氧老化性能最差;改变填料后NA-22硫化胶料的老化性能得到提高,固定硫化剂为NA-22时,以碱性lignin为填料的胶料的力学性能与热氧老化性能均最好,拉伸强度达15.5 MPa,断裂伸长率达394%,100%定伸应力达6.4 MPa,老化后拉伸强度保持率为103.2%。     

碳布复合材料力学性能研究

测试了两种不同经纬编织密度和不同含胶量的碳布／环氧复合材料的基本力学性能，对碳纤维复丝及碳布在复合材料中的强度利用率作了比较与分析。结果表明：适当增大含胶量有利于改善复合材料的力学性能；经纬编织密度对复合材料力学性能的影响同样不可忽视。     

纤维类型对C/SiC复合材料性能的影响

利用化学气相浸渗法制备了Cf-C/SiC复合材料,借助SEM、TEM等研究了纤维类型对Cf-C/SiC复合材料力学性能的影响.实验证明T300碳纤维增韧补强效果优于M40碳纤维,利用T300碳纤维制备出弯曲强度为459M,断裂韧性为20.0MPa*m1/2,断裂功为25170J/m2的Cf-C/SiC复合材料.2种碳纤维增韧效果的差异是由纤维的原始强度、热膨胀系数和弹性常数的不同决定的.