PE-HD/PC/POE-g-MAH复合材料断裂性能的研究

采用基本断裂功(EWF)法评价了高密度聚乙烯(PE-HD)/聚碳酸酯(PC)/聚烯烃弹性体接枝马来酸酐(POE-g-MAH)复合材料处于平面应力与平面应变过渡状态下的断裂韧性和塑性功。研究不同PC含量对复合材料断裂行为的影响,并把宏观断裂参数的变化与复合材料的微观结构联系起来,从物质结构上寻求断裂功参数变化的原因。结果表明,随PC含量的增加,复合材料的比基本断裂功增加,比塑性功降低;复合材料的断裂韧性主要取决于屈服后材料抵抗裂纹扩展的能力,复合材料的塑性变形能力也更依赖于屈服后的行为;复合材料的缺口冲击强度随PC含量的增加而降低,缺口冲击强度高的材料比基本断裂功较小。     

用J积分法表征聚碳酸酯／弹性体合金的韧性

采用Ｊ积分法表征聚碳酸酯（ＰＣ）／弹性体共混体系的韧性。结果表明，当加入弹性体后，体系断裂韧性ＪＩＣ明显提高，提高了约８０％。在弹性体含量为５－１５％时，断裂韧性ＪＩＣ变化不大，撕裂量Ｔｍ随着弹性含量的增加而增大，在弹性体含量为１０％时增加了１２．５倍。弹性体增韧ＰＣ的原因主要在于提高其抗 扩展能量。采用ＳＥＭ观察材料中的裂纹在平面应变下的发展过程发现，起裂前裂尖出现钝化和靴状超钝化现象，从微观上     

发动机密封用弹性体的连续压缩应力松驰试验

采用压缩应力松驰(CSR)试验对汽车工业用密封件和密封垫圈弹性体进行筛选的方法已经逐渐被接受。应力松驰现象是橡胶样品在一定应变下(拉伸或压缩模式)产生的，保持应变所必须的力是不定的，随时间而下降。压缩模式下固定应变，密封接触力随时间下降。橡胶基体中发生的这种变化本质上是物理和化学过程。     

基于SEM与数字图像相关方法的HTPB推进剂裂尖扩展过程分析

为了从细观角度获得端羟基聚丁二烯(HTPB)推进剂裂纹的扩展特性并分析裂纹的细观破坏机理,通过原位扫描电镜(SEM)对HTPB推进剂三点弯试验裂纹尖端损伤及扩展过程进行了观察,获得了不同变形阶段的裂纹扩展变形形貌,并采用数字图像相关方法分析了图片序列,获得了推进剂裂纹尖端变形场。结果表明,随着推进剂裂纹的不断张开,当挤压位移达到1mm时,裂尖附近应变极值为0.3474,固体颗粒出现脱湿现象,颗粒周边基体受到了较大的应变作用;当挤压位移为2.5mm时,应变极值达0.4168,颗粒和基体界面产生的微裂纹与主裂纹汇聚导致裂纹的扩展。数字图像相关方法和扫描电镜相结合,可用于推进剂在细观尺度下的变形场测量与裂尖扩展过程的破坏机理分析。     

PBX平面应变断裂韧度随温度的变化规律

采用三点弯曲试验测试了JO、JOB和JB 3种RBX在不同温度下的平面应变断裂韧度（KⅠc）.结果表明,随着温度的增加,PBX的平面应变断裂韧度降低;在45℃以后JOB的平面应变断裂韧度降低最快,60℃时已很差,比JO的低,可以认为JOB在高温下抗裂纹扩展能力最差;在不同温度下,JB的平面应变断裂韧度比JOB和JO的均高,表现出相对较强的抗裂纹扩展能力.提出了PBX中高聚物成分及其含量是造成其KⅠc随温度变化规律不同的主要原因.     

Incone l625合金波纹管失效分析

用化学分析、透射电子显微镜(TEM)、X射线光电子能谱仪(XPS)和扫描电子显微镜(SEM)等微观测试分析技术对失效波纹管的化学成分、力学性能、断口腐蚀产物及断口微观形貌进行综合分析。结果表明,波纹管的失效原因是材料处于腐蚀环境,在局部高应力处发生高温硫化及氧化,诱发疲劳裂纹源,在交变应力的作用下发生腐蚀疲劳开裂。     

SBR/HIPS TPV的制备及性能研究

采用动态硫化法制备SBR/高抗冲聚苯乙烯(HIPS)热塑性硫化胶(TPV),并对其性能进行研究.结果表明,当SBR/HIPS共混比大于60/40时,SBR/HIPS TPV呈现出典型的弹性体应力-应变行为;当SBR/HIPS共混比为60/40～70/30时,SBR/HIPS TPV的硬度适中,综合物理性能良好.扫描电子显微镜分析表明,SBR以带状形貌均匀分散在HPS基体中,两相界面结合良好.     

炭黑填充丁苯橡胶在疲劳载荷条件下的断裂分析

为了研究炭黑填充的丁苯橡胶(CB-SBR)在完全松弛载荷条件下的疲劳裂纹扩展,并弄清裂纹增大的大致情况,利用以前针对天然橡胶(NR)的"微切削"技术,借助扫描电子显微镜(SEM)观察与疲劳裂纹相关的微观现象。结果表明:裂纹尖端会沿着带状撕裂线而扩展——这就解释了可结晶橡胶与非结晶橡胶在裂纹增大期间其疲劳响应的差异。因此,与NR之类可结晶弹性体相反,SBR在裂纹尖端处及块状材料中的微观结构是相似的,裂纹尖端并不会阻挡裂纹的蔓延。不仅如此,断裂表面的形态结构仅取决于疲劳裂纹蔓延过程中所遇到的粒子。     

橡胶疲劳寿命研究概况

橡胶制品通常是在周期性应力状态下使用的 ,硫化胶的疲劳寿命往往决定橡胶制品的使用寿命。1 .影响橡胶疲劳寿命的因素弹性体性质。影响疲劳寿命最重要的因素是弹性体性质 :在低应变疲劳条件下 ,橡胶的玻璃化转变温度愈高 ,耐疲劳破坏性能就愈好 ;在高应变疲劳条件下 ,具有拉伸结晶性的橡胶 ,耐疲劳破坏性能较好。疲劳裂纹增长也与橡胶种类有关。应变周期。对天然橡胶硫化胶 ,当最小应变增加时 (尽管能量输入降低 ) ,其疲劳寿命却增加了 ,这表明应变周期在很大程度上影响橡胶制品的性能。操作温度。操作温度对橡胶疲劳寿命的影响相当复杂 ,…     

PP-R材料在准静态加载下的断口形貌研究

利用扫描电子显微镜(SEM)及宏观断口形貌法观察无规共聚聚丙烯(PP-R)试件在高周期疲劳以及准静态载荷作用下的Ⅰ型试件断面与复合型试件断面的显微组织形貌与宏观断口组织,对PP-R材料的断裂机理进行研究。结果表明:PP-R材料在准静态Ⅰ型与复合型两种准静态加载方式下,裂纹在稳定扩展区域属于韧性断裂;裂纹在高速扩展区域属于脆性断裂;疲劳裂纹预制阶段属于韧性断裂。     

玻璃在平面双向和单向应力状态下慢裂纹扩展特性

采用热弹性力学方法对含裂纹的玻璃薄片在双向和单向平面拉和下的亚临界裂纹扩展进行了观测和研究。该实验方法使得玻璃、陶瓷等危性材料的双向拉伸,直通裂纹预制和裂纹扩展过程记录这３个难度大的实验得以完成。对６０多个玻璃试件的断裂阻力随载荷速度的变化进行了观测和分析,从而较精确地了解了玻璃在平面和状态下的阻力曲线和断裂特性。     

Fracture and impact properties of polycarbonates and MBS elastomer-modified polycarbonates

Mechanical properties of polycarbonates (PCs) and elastomer-modified polycarbonates with various molecular weights (MW) are investigated. Higher MW PCs show slightly lower density, yield stress, and modulus. The ductile–brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of PC MW and with the increase of elastomer content. The elastomer-modified PC has higher impact strength than does the unmodified counterpart if the failure is in the brittle mode, but has lower impact strength if the failure is in the ductile mode. The critical strain energy release rate (G_c) measured at ?30°C decreases with the decrease of PC MW. The extrapolated zero fracture energy was found at M_n = 6800 or MFR = 135. The G_c of the elastomer-modified PC (MFR = 15, 5% elastomer) is about twice that of thee unmodified one. The presence of elastomer in the PC matrix promotes the plane–strain localized shear yielding to greater extents and thus increases the impact strength and G_c in a typically brittle fracture. Two separate modes, localized and mass shear yielding, work simultaneously in the elastomer-toughening mechanism. The plane–strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane–stress mass shear yielding dominates at higher temperatures and ductile failure. For the elastomer-modified PC (10% elastomer), the estimated extension ratio of the yielding zone of the fractured surface is 2 for the ductile failure and 5 for the brittle crack. A criterion for shifting from brittle to ductile failure based on precrack critical plastic-zone volume is proposed.     

Temperature scanning stress relaxation behavior of water responsive and mechanically adaptive elastomer nanocomposites

The decrease of stress at constant strain, that is, the stress relaxation process as a function of temperature, is a central mechanical characteristics of elastomer nanocomposites for their potential applications. However, in the conventional stress relaxation test, the relaxation behavior is usually determined as a function of time at constant temperature. The present work reports the temperature scanning stress relaxation (TSSR) characteristics of a new kind of mechanically adaptive elastomer nanocomposite by monitoring the nonisothermal relaxation behavior as a function of temperature. This kind of adaptive elastomer nanocomposite was prepared by introducing calcium sulfate (CaSO₄), as the water-responsive phase into the hydrophilic elastomer matrix. The influence of water-induced structural changes on TSSR behavior was investigated. Water treatment had a strong effect on the shape of the relaxation spectrum of the nanocomposite. It was revealed that the in situ development of hydrated nano-rod crystal structures of CaSO₄ in the elastomer matrix was responsible for the changes in the mechanical relaxation behavior of the composites. Atomic force microscopy was used to verify this nano-rod crystal morphology in the elastomer matrix. The mechanism of water-induced mechanical reinforcement of the composite was explored from dynamic mechanical analysis of the material and correlated with its stress relaxation behavior. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48344.     

Influence of thickness and processing history on fatigue fracture of nylon 66. Part I: Crack propagation measurements

The influence of sample thickness on fatigue crack propagation rates in injection molded nylon 66 was determined by preparing 12.7 mm thick plaques along with more conventional 3.0 mm thick samples. Initial results suggested a large effect of thickness as the crack propagation rates were accelerated in the thicker samples and the stress dependence was also increased. Since the calculated thickness for a plane stress to plane strain transition in nylon 66 is 9.0 mm, it was thought that these results were related to the stress state at the crack tip. However, a more thorough study of the thicker plaques has now demonstrated that neither the magnitude nor the stress dependence of the fatigue crack growth rates is necessarily changed under plane strain conditions as similar results can be obtained for thick and thin plaques. It is suggested that the earlier results were confounded by a previously unrecognized processing history effect which does accelerate fatigue fracture. The latter effect is shown by thermal analysis and optical microscopy to be related to a rearrangement of the polymer network during melt processing.     

Fractography of injection molded polycarbonate acrylonitrile-butadiene-styrene terpolymer blends

Fractography has been used in the post-failure analysis of single edge notched specimens of injection molded blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene terpolymer (ABS). The mode of ductile tensile fracture of single edge notched specimens depended on comosition. Plane stress shear tearing was observed in the composition range PC/ABS 90/10 to 70/30 by weight where PC was the continuous phase. Intermediate compositions, PC/ABS 60/40 to 40/60, had a co-continuous or almot co-continuous phase morphology; these blends fractured by mixed mode pop-in, where a tunneling center crack relieved the triaxiality and permitted plane stress shear lips to form near the edges. Herringbone fracture, a plane strain mode characterized by discontinuous crack growth, was observed when ABS was the continuous phase, PC/ABS 30/70 to 10/90. An S-shaped relationship was observed between the ductile-to-brittle transition temperature and the composition. Addition of ABS to PC increased ductility up to PC/ABS 70/30 and 60/40, which were the most ductile compositions. Further addition of ABS decreased the ductility, and the least ductile compositions were PC/ABS 30/70 and 10/90.     

Analytical estimation of fracture toughness for circumferential cracks in thin hard films on ductile substrates

In this study, the fracture toughness of circumferential crack caused by indentation effect of a rigid indenter on a thin and elastic coating deposited on the elastic substrate was calculated. In the coating and substrate, the analytical solution of displacement and stress field was used. The complete adhesion was considered for the coating on the substrate. The location of maximum circumferential stress was investigated using the analytical calculation of the stress and it was found that this place was located at a distance away from the center of the indenter. Then, the stress intensity factor and energy release rate for plane strain state was determined, and consequently, the energy release rate for a channel crack was calculated. Finally, the fracture toughness was calculated with energy release rate curves for plane strain crack and crack channeling. This method was used to calculate the fracture toughness of TiN/TiCrN ceramic multilayer coating which was deposited on the GTD450 substrate using the Cathodic Arc PVD method. To validate the results, the analytically calculated crack radius was compared with the experimental crack radius in the fracture load and the difference between the radiuses was in the acceptable range.     

Mechanism of cumulative damage to short fiber type C/SiC under tension

Stress–strain relations at different degrees of peak stress were investigated using loading–unloading tests to elucidate cumulative damage mechanisms of short fiber type C/SiC under tension. Damage observations revealed their crack length, number, and angle characteristics. Furthermore, stress–strain relations were estimated by expanding Basista’s equations and by substituting measured damage characteristics into them, which revealed a nonlinear stress–strain relation. Cracks propagated in transverse fiber bundles without fiber fracture, connecting other cracks that had 75 ° – 90 ° orientation to the tensile axis. Stress–strain relations estimated qualitatively and quantitatively suggest that mixed mode I and mode II crack opening in transverse fiber bundles in the through-thickness plane caused the stress–strain nonlinear relations.     

Effect of molecular weight on craze shape and fracture toughness in polycarbonate

The shape of the craze at the tip of a crack has been studied using optical microscopy on polycarbonates of various molecular weights at ?30°C. For all molecular weights studied the craze shape was well approximated by the Dugdale plastic zone model and this model was used to calculate the craze stress and the release rate in plane strain. It was found that the craze dimensions, the craze stress and the strain energy release rate in plane strain all increased with increasing molecular weight. Fracture of macroscopic specimens showed a mixed mode fracture in all molecular weights. By studying the effect of thickness the strain energy release rate in plane strain was calculated for various molecular weights. Agreement was found between these values and those determined from the craze shape measurements. The overall strain energy release rate, the strain energy release rate in plane strain and the contributions from the plane stress mode increased with increasing molecular weight.     

Temperature dependence of craze shape and fracture in polycarbonate

The shape of the craze at the tip of a loaded crack has been determined by optical microscopy for polycarbonate. The effect of temperature was examined, and measurements were made on samples of different molecular weight. In all cases the craze shape can be described to a good approximation by the Dugdale model for the plastic zone at a crack tip. The crack opening displacement depended on sample molecular weight, but was independent of temperature. Fracture toughness values deduced from the craze shape were in good agreement with plane strain fracture toughness obtained from direct cleavage fracture measurements, on the assumption that failure occurs by combined plane strain and plane stress fracture modes.