薄层芳纶复合材料(内衬吸能垫)的平头冲击性能研究

为研究薄层芳纶复合材料的冲击机理－为轻质薄层结构设计提供基础数据和理论依据，本文采用圆柱平头型冲头（直径为２５ｍｍ）在一定的冲击速度范围内冲击薄层复合材料（厚度：１．５０－２．５０ｍｍ），利用加速度传感器分析复合材料冲击背面的应力动态变化情况，研究了影响复合材料抗平头碰撞能力的因素和复合材料的冲击损伤过程。结果表明：影响复合材料抗平头碰撞能力的主要因素是纤维织物本身特性、纤维含量和界面强度；冲击能     

碳纤维/环氧界面相准静态纳米压痕表征方法分析

界面相作为纤维与树脂之间载荷传递的桥梁,其性质对整体复合材料力学性能和耐环境性能有直接影响。本研究用准静态纳米压痕技术对碳纤维T300/环氧E51体系的界面相模量进行测试分析。研究发现,当探针压入深度大于30nm以后,试样表面粗糙度对测试结果的影响可以忽略;同时,三种测试排点方式下的纳米压痕结果表明:纤维增强效应对载荷-压深曲线影响很大,由于压深和压痕间距较大,对于界面相宽度较小的碳纤维环氧复合材料体系,采用准静态纳米压痕技术不易得到其界面相的模量。     

复合绝缘子振动蠕变特点及试验参数的确定

复合绝缘子运行过程中的振动，在由纤维束浸渍树脂拉挤固化成型的芯棒与金具交接部位，因纤维受力不均，纤维树脂层问界面出现，将存在着振动蠕变现象。复合绝缘子振动蠕变试验，将依据易振区线路上架空导线凤振动实际情况确定出合理的试验参数为：静载荷不低于复合绝缘子额定机械应力的20％～25％；振动角为其最大振动角波动范围平均值；振动次数应以其振动角大于5′的振动时间，相应振动频率以及复合绝缘子运行期限进行选取。通过对复合绝缘子进行振动蠕变试验认为，在其芯棒与金具交接部位存在着振动蠕变现象，但对组装工艺先进产品整体机械性能影响较小。     

SiC纤维增强玻璃陶瓷的结构与高温力学性能研究

通过ＳＥＭ，ＴＥＭ，ＡＥＳ，抗弯及蠕变试验考察了ＳｉＣ纤维增强的玻璃瓷的微观结构及高温力学性能。实验结果表明：除了在复合材料的基质中存在着大量的针状晶体之外，在纤维与基质的界面还存在着８０ｎｍ厚的富碳层。     

压痕SMA丝复合材料界面本构与性能分析

基于混杂形状记忆合金(Shape Memory Alloy,简称"SMA")驱动器的自适应可变形复合材料结构正常服役的前提是界面性能良好。研究表明在光圆SMA丝上引入细压痕可显著提升界面性能。进一步研究压痕SMA丝复合材料的本构是开展混杂压痕SMA丝复合材料结构设计、应用的基础。本文通过圆柱拉拔试验,基于搭建的粘结-滑移测试系统,采集到了载荷-滑移数据,结合理论分析,建立了界面的粘结-滑移本构模型,并通过分析得出了基于区域平均准则思想的界面临界失效剪应力强度值。     

超临界CO_2预处理对UHMWPE纤维电子束辐照效果的影响

采用正交试验,研究了超临界CO_2预处理工艺(压力、温度、时间)对辐敏剂三羟甲基丙烷三甲基丙烯酸酯(TMPTMA)渗入率及超高相对分子质量聚乙烯(UHMWPE)纤维凝胶含量和蠕变率的影响,并利用Minitab软件分析UHMWPE纤维凝胶含量和蠕变率与TMPTMA渗入率之间的关系,最后通过对UHMWPE纤维各项性能的测定,进一步优化超临界CO_2预处理工艺。研究结果表明:处理温度对TMPTMA的渗入率及UHMWPE纤维凝胶含量和蠕变率影响最大,其次为压力,时间的影响最小;确定了最佳工艺为处理压力30 MPa、温度80℃、时间50 min;TMPTMA渗入率是引起各个影响因素不同水平之间凝胶含量和蠕变率出现差别的重要因素;超临界CO_2预处理对UHMWPE纤维的辐照交联起到了重要的促进作用,使其抗蠕变性能得到很大程度的改善。     

试验频率和温度对橡胶/纤维帘线复合材料动态粘合性能的影响

研究试验频率和温度对橡胶/纤维帘线复合材料动态粘合性能的影响,并通过扫描电镜分析粘合失效后纤维帘线的表面形貌。结果表明,降低试验频率和提高试验温度会加速橡胶/纤维帘线复合材料的粘合失效,这主要是由橡胶基体的破坏以及橡胶与纤维帘线的界面脱粘引起的。     

短纤维增强SiC／Al复合材料在高温下粘弹性拉伸性能的细观力学分析

本文采用细观力学模型和有限元法研究短纤维增强ＳｉＣ／Ａｌ复合材料在高温下的粘弹性行为，着重讨论了纤维体分比和长径比对复合材料总体蠕变性能的影响。结果表明，随着纤维体分比和长径比的增加，纤维能显著抑制复合材料沿轴向蠕变行为。     

碳化硅纤维热处理对单向碳化硅纤维增强磷酸铝基复合材料性能的影响

用不同条件热处理的碳化硅纤维制备了单向连续碳化硅纤维增强磷酸铝基复合材料.研究了碳化硅纤维热处理的温度、时间及热处理方法对制成的复合材料性能的影响.测试了复合材料的断裂强度,相对介电常数和介电损耗.通过扫描电镜分析复合材料的微观形貌,并使用电子探针对碳化硅纤维/磷酸铝基体界面进行了微区元素分析.结果表明:碳化硅纤维热处理降低了复合材料的介电常数和介电损耗;纤维/基体界面之间未发生任何化学反应.由于热处理使纤维/基体形成了强结合界面,大大降低了复合材料的力学性能.快速热处理方式直接降低纤维的自身强度.     

连续纤维复合材料环形试样蠕变行为研究

对连续玻璃纤维复合材料进行了拉伸蠕变试验研究，为了模拟复合材料在压力容器中的受力状态并减少夹具加持力对试样的影响，采用环形复合材料试样拉伸蠕变试验方法。对复合材料环形试样的拉伸强度及不同应力等级下的拉伸蠕变性能进行了研究，并基于时间?应力等效原理，通过双对数法拟合出压力容器50年使用寿命时复合材料的最大蠕变应力，为复合材料压力容器的设计提供支持。并基于时间?应力等效原理，通过双对数法拟合出压力容器50年使用寿命时复合材料的最大蠕变应力应低于其拉伸强度的44.4 %。     

Improved Analysis for Flexural Creep with Application to Sialon Ceramics

By using a statistical least-squares method to minimize the differences between predicted and measured load-point displacement rates from four-point bend specimens, power-law creep parameters for tension and compression were estimated. An alternative but simpler method of estimating power-law creep parameters from flexural creep data is also proposed. This method entails the direct measurements of steady-state creep strain rates at two stress levels by an indentation technique. Based on a closed-form solution, the power-law creep parameters could then be estimated from both the measured neutral axis locations and curvature rates. The results from these two methods compare favorably with one another, and with the simple compressive creep data. Both methods yield a high stress exponent of about 14 for tension and a stress exponent of about unity for compression. Cavitation-enhanced creep in tension and diffusional creep in compression are responsible for this asymmetric behavior.     

Strain Rate Dependence of the Hardness of Glass and Meyer's Law

Depth-sensing Vickers hardness test on glass for various loading regimes were analyzed by applying a flow law for material deformation behavior used by Han and Tomozawa, who investigated the creep behavior of glassy materials during indentation experiments. Load, indentation depth, and time were measured simultaneously at deformation rates covering 6 orders of magnitude. In all cases, the results yielded the well-known Meyer hardness law, by which the parameters of flow law are unequivocally determined using experimentally estimated Meyer parameters. Experiments conducted with various dependencies on commercial sheet glass all gave material parameters in good agreement. Differences originating from different loading regimes must be resolved in future works.     

Indentation creep of polymers. I. Experimental

An experimental setup is proposed to perform creep indentation tests on thermoplastics. An infrared (IR) lamp is used to locally heat the sample, and a flat punch made of tungsten carbide is loaded on the sample surface. In order to show the capability of this test in evaluating the creep properties, creep indentation tests were carried out on PA66 and HDPE sheets at different temperatures and creep loads. Dynamic mechanical analyses (DMA) and uniaxial creep tests were also performed at different temperatures to have a comparison with the indentation test results. Master curves were built for all the test results, and time shift factors were extracted. In all cases, the logarithm values of the shift factor were linearly dependent on the temperature. Small differences were found in the shift factors of the DMA and uniaxial test results, whereas high differences were observed in the case of the indentation results due to the nonuniformity of the temperature in the sample. However, indentation creep compliance is strictly correlated with tensile creep compliance, and equivalent tensile creep properties can be extracted. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

High-Temperature Behavior of Indent and Creep-Nucleated Cracks in Vitreous-Bonded Alumina

Crack behavior was studied at elevated temperatures in a commercial vitreous-bonded alumina for two types of cracks: one introduced by indentation at room temperature and the other by the creep process. Indentation cracks with relatively small initial size grew progressively longer during creep before they became blunt and arrested; however, they continued to widen throughout the creep process. Larger indentation cracks under high stress condition continued to grow until failure. The evolution of creep-nucleated cracks was so fast that they were observed only in their arrested state. Once observed, their length remained essentially constant, but they did grow in width. The crack-opening displacement rates of both types of cracks were linearly related to the creep rate as predicted by fracture mechanics for stationary cracks. All but the specimens with the largest indentation crack exhibited flaw tolerance in that they failed by the coalescence of creep-nucleated cracks instead of the growth of a single crack. The results illustrate the crack behavior in the brittle-to-ductile transition regime for ceramics that deform by grain boundary sliding.     

Adhesion of diamond coatings on steel and copper with a titanium interlayer

Adherent diamond coatings on steel and copper were obtained by using a titanium interlayer. The adhesion of the coatings was evaluated by scratch tests and micro-indentation tests. The diamond coating on steel exhibited a much higher critical load than on copper, as revealed by the scratch tests. However, an observation on the back of the scratch-delaminated film and on the corresponding substrate surface showed that the detachment occurred between the diamond film and the titanium interlayer. Therefore, the difference in the critical scratch load is due mainly to a substrate effect, making it difficult to compare the adhesion of different coatings.On the other hand, Knoop indentation tests showed interesting results: a small indentation load causes round spallation in the film with no observable crack. An exponential sink-in deformation under the indentation is proposed, y=−a exp(−bx). The coating adhesion is considered to be equivalent to the deformation stress at the edge of the spallation zone. The adhesion of diamond coatings on steel and copper with a titanium interlayer is evaluated quantitatively using this model. Furthermore, a thermal quench method is proposed to estimate the coating adhesion. The results found are in agreement with the indentation model.     

界面过渡区对混凝土徐变性能的影响

为分析界面过渡区对混凝土徐变性能的影响,运用Laplace变换原理,建立了考虑界面过渡区黏弹性的混凝土徐变模型,将该模型与不考虑界面过渡区的混凝土徐变预测模型进行对比和实验验证。结果表明:考虑界面过渡区的混凝土徐变模型与实验数据吻合较好,该模型能够反映界面过渡区在混凝土徐变中的作用,加载龄期为365 d时,界面过渡区使粉煤灰掺量为0、30%、60%的混凝土徐变度分别提高了45.7%、25.1%、66.7%。根据该模型,分析了不同加载龄期时3种粉煤灰掺量的界面过渡区对混凝土徐变贡献度。掺加粉煤灰的界面过渡区在早期对混凝土徐变贡献度高于不掺的基准组,后期基准组的界面过渡区对混凝土徐变贡献度不断增加,这是由于界面过渡区在荷载作用下不断弱化导致。而掺加粉煤灰的2组,随加载龄期的增长,粉煤灰二次水化反应使界面过渡区不断增强,界面过渡区对混凝土徐变贡献度不断降低。通过分析界面过渡区参数对混凝土徐变的影响发现,当界面过渡区弹性常数0.0001itz<0.01,黏性系数0.1<η_itz<10时,可以较好地描述不同界面过渡区的混凝土徐变。     

基于纳米压痕的氧化镓不同晶面的蠕变特性

使用Berkovich压头,采用恒载荷法对氧化镓的(100)及(010)晶面进行纳米压痕试验,通过数据拟合获得氧化镓晶体的蠕变应力指数。单晶氧化镓(100)晶面的蠕变应力指数范围在15.5~27.8,(010)晶面的蠕变应力指数范围在46.1~63.4。纳米压痕法能够有效测量氧化镓的蠕变变形量;氧化镓的2个晶面均呈现了初始蠕变和稳态蠕变2个阶段,其中,(100)晶面具有较明显的初始蠕变阶段和稳态蠕变阶段,但是(010)晶面的初始蠕变阶段不明显;保载载荷与氧化镓的蠕变位移、等效压痕应力呈正相关,对蠕变应力指数没有明显影响;氧化镓不同晶面的蠕变应力指数差异很大,(010)晶面较(100)晶面具有更大的蠕变应力指数,具有更强的抗蠕变性能,更适合精密机械加工。     

Creep mechanisms in quasi-plastic silicon nitride by instrumented indentation

Quasi-plastic creep behavior of the commercial, fine-grained silicon nitride grade, ST 1, was investigated using variety of techniques with the focus on the analysis of instrumented indentation. Creep deformation in this material was characterized by high creep rates at temperatures above 1300 °C and failure strains around 20%. It was accompanied by strong oxidation, cracking of the oxide layers, excessive cavitation at multigrain junctions and slight texture formation. Instrumented indentation revealed degradation of indentation moduli in the oxide layers and enhancement of oxidation and elastic moduli degradation during creep. Because of the similarities between the mass transport processes in cavitation, diffusion processes involved in oxidation and similar activation energies, both creep and oxidation occur simultaneously, however, oxidation is enhanced by external stress. Texture formation implied from disappearance of -silicon nitride and anisotropy of indentation modulus contributes insignificantly (<5%) to total tensile strain. Creep processes in the studied material can be explained within the expanded cavitation creep model of Luecke and Wiederhorn assuming that cavitation is facilitated by low viscosity residual glass and small matrix grain size. Tertiary-like creep is attributed to the gradual increase of the applied stress resulting from the reduction of the effective cross section due to the formation of cracked oxide layers. Size and pre-oxidation effects were predicted and confirmed using creep samples with different gauge size.     

Steady-State Creep Behavior of Si–SiC C-Rings

Because of the ease of experimental setup as well as economics in sample preparation, C-ring specimens are sometimes chosen for the evaluation of mechanical behavior. In this paper, the long-term creep of siliconized silicon carbide (Si–SiC) C-rings is investigated. Creep tests on a number of Si–SiC C-rings were carried out under constant compressive loads at 1300°C in air. Load-point displacements were continually monitored as a function of time, thereby establishing the steady-state regime as a function of load and ring geometry. Optical micrography on the postcrept specimens was performed to obtain damage zone sizes. A simple curved beam theory was employed to analyze the stress state developed throughout the body during steady-state creep. Loadpoint displacement rates were numerically calculated using both geometric and energy methods. Observed damage zone sizes and shapes within the specimen agreed with those predicted theoretically. Results obtained on the stress solutions are useful as local loading parameters in the study of high-temperture fracture behavior of a cracked C-ring.     

Characterisation of the transverse mechanical properties of carbon/carbon composites by spherical indentation

The mechanical characterisation of the carbon/carbon composite in the transverse direction is essential for the design of braking discs. This paper presents a technique based on spherical indentation to identify the mechanical behaviour of such materials in the transverse direction.After the presentation of the material properties as determined from static and fatigue compression tests, the indentation technique is described in detail. The characterisation technique used takes into consideration the transverse isotropy of these materials. The method used allows to identify the material behaviour in elastic and inelastic fields. Characterisation of elastic parameters is carried out after identification of the Hertz law in the unloading indentation cycles. The hardening parameters and the elastic limit are identified by expressing the law of Hertz in strain/stress form.The identified parameters are used in a simulation of an indentation test by finite element method. A good agreement is found between numerical and experimental results.