天然橡胶/顺丁橡胶共混胶体系裂纹生长行为的形态演变机理

研究了不同共混比例的顺丁橡胶(BR)与天然橡胶(NR)组成的共混胶的裂纹尖端生长行为。采用裂纹扩展在线研究平台(DMA-Crack Growth)、超高速摄像仪、扫描电子显微镜和扫描探针显微镜对裂纹尖端进行实时追踪观测,拍摄不同共混比例的橡胶裂纹尖端的裂纹扩展动态图像。结果表明,共混比例达到m(BR)/m(NR)=50/50时,韧带均匀分布在裂纹尖端,韧带多而细且均匀,可以有效减缓橡胶裂纹扩展速度;橡胶开裂的断面随着BR共混比例的增加,表面形貌由类三角形向扇形形貌过渡。最后讨论了裂纹增长的机理。     

聚乙烯管材抗裂纹扩展性能的评价

本文介绍了聚乙烯管材抗慢速裂纹增长和抗快速裂纹扩展评价方法的研究进展。简述了正在完善的标准方法的实验原理和实验装置，对我国在此领域的研究工作提出了建议。     

用疲劳方法预测聚乙烯管材抗慢速裂纹扩展能力

本文用疲劳断裂力学方法对聚乙烯管材进行了抗疲劳裂纹扩展能力的研究,从而给出了一种快速预测这些材料长期使用性能的新方法。     

Si3N4/TiC纳米复合陶瓷材料R曲线行为

在微米Si3N4基体中加入亚微米Si3N4及纳米TiC颗粒,热压烧结制备出力学性能良好的Si3N4/TiC纳米复合陶瓷材料。采用压痕-弯曲强度法测定了复合材料的裂纹扩展阻力曲线(R曲线)。结果表明:材料呈现出上升的阻力曲线特性,显示出增强的抗裂纹扩展能力。其中,加入质量分数为10%亚微米Si3N4颗粒和15%纳米TiC颗粒的复合材料显示出较为优越的抗裂纹扩展能力,其阻力曲线上升最陡,上升幅度最大。分析表明:弥散的TiC粒子同基体之间弹性模量和热膨胀失配以及Si3N4类晶须拔出与桥联补强协同增韧,有助于纳米复合材料抑制主裂纹失稳扩展,导致复合材料的阻力曲线行为。     

GH33A合金在蠕变-疲劳交互作用下的裂纹扩展特性

通过对 GH33A 合金在蠕变与疲劳复合加载条件下的系列试验,发现拉伸保时使蠕变与疲劳发生了交互作用,加快了疲劳裂纹扩展速率,加速裂纹早期进入失稳扩展,大大降低了疲劳寿命。GH33A 合金具有良好的抗蠕变裂纹扩展能力,但疲劳裂纹扩展阻力较低。由此讨论了拉伸保时对裂纹扩展的影响,并对在蠕变-疲劳交互作用下的裂纹扩展模型作了探讨。     

非对称嵌段共聚物PI-b-PB对IR/BR并用胶相形态与性能的影响

主要研究非对称嵌段共聚物PI-b-PB(液体橡胶LIR-390)对异戊橡胶/顺丁橡胶(IR/BR)共混体系相形态、加工性能和力学性能的影响。采用原子力显微镜(AFM)表征IR/BR并用胶的相形态,结果显示,LIR-390能够细化分散相(BR)的尺寸,且使分散相分布更加均匀;但加入过量LIR-390(7份)则会减弱其增容作用,使BR的相尺寸增大。通过哈克旋转流变仪测试IR/BR未硫化胶的流变特征,发现LIR-390能够降低未硫化胶的储能模量(G′),得出在橡胶共混过程中加入LIR-390可以减少加工能耗。利用裂纹生长测试平台分析并用胶的抗疲劳裂纹生长性能,结果表明,LIR-390的增容作用有助于减小分散相的尺寸,从而降低裂纹生长速率;而加入过量LIR-390则会致使开裂加快。     

TIG与FSW焊接工艺下2219铝合金疲劳裂纹扩展性能研究

目的 研究钨极惰性气体保护焊(TIG)和搅拌摩擦焊(FSW)对2219铝合金焊接接头疲劳性能的影响,并探究这2种不同焊接技术条件下焊接接头疲劳裂纹的产生与裂纹扩展原理,了解2种焊接接头的抗裂纹扩展能力,为工程实践应用提供数据参考。方法 采用疲劳裂纹扩展试验方法,测试上述2种焊接工艺条件下焊缝金属和热影响区组织的疲劳裂纹扩展速率da/dN和阈值,使用光学显微镜和扫描电子显微镜观察并分析金相组织和疲劳断口形貌特征。结果 疲劳裂纹倾向于沿裂纹处萌生,裂纹的存在成为主要的裂纹扩展源头,有利于加速裂纹向前延伸。热影响区由于组织结构不均匀,不同位置的晶粒尺寸存在明显差异,疲劳裂纹扩展路径倾向于沿靠近焊缝一侧向靠近母材区域扩展。TIG焊接工艺下焊缝金属和热影响区的裂纹扩展速率明显低于FSW焊接工艺下的焊缝金属和热影响区,与此同时,TIG焊接接头表现出优良的抗疲劳裂纹扩展性能。结论 通过此研究,建议2219铝合金焊接接头采用TIG焊接工艺,抗疲劳裂纹扩展效果更佳。     

X60钢在硫化氢溶液中的应力腐蚀裂纹扩展研究

输送石油和天然气的X60钢管存在应力腐蚀裂纹扩展的危险.为此,将X60螺旋埋弧焊管试样置于100,500,1000 mg/L的H2S水溶液中,采用改进的楔形张开加载(WOL)试样和恒位移的加载方法进行试验,测定了X60焊接接头在不同环境下的应力腐蚀裂纹扩展速率.结果表明:在低浓度(100 mg/L)下没有观察到裂纹扩展的情况,说明X60材料具有一定的抗硫化氢应力腐蚀开裂能力;随硫化氢浓度的提高,裂纹扩展速率增大,所以在高浓度硫化氢溶液中,这种材料的硫化氢应力腐蚀开裂情况仍需特别注意.     

非对称嵌段共聚物PI-b-PB对IR/BR并用胶相形态与性能的影响

主要研究非对称嵌段共聚物PI-b-PB(液体橡胶LIR-390)对异戊橡胶/顺丁橡胶(IR/BR)共混体系相形态、加工性能和力学性能的影响.采用原子力显微镜(AFM)表征IR/BR并用胶的相形态,结果显示,LIR-390能够细化分散相(BR)的尺寸,且使分散相分布更加均匀;但加入过量LIR-390(7份)则会减弱其增容作用,使BR的相尺寸增大.通过哈克旋转流变仪测试IR/BR未硫化胶的流变特征,发现LIR-390能够降低未硫化胶的储能模量(G7),得出在橡胶共混过程中加入LIR-390可以减少加工能耗.利用裂纹生长测试平台分析并用胶的抗疲劳裂纹生长性能,结果表明,LIR-390的增容作用有助于减小分散相的尺寸,从而降低裂纹生长速率;而加入过量LIR-390则会致使开裂加快.     

结构钢S355N及S355N-Z25的低温CTOD断裂韧性及其da/dN研究

分别采用三点弯曲SE(B)与紧凑拉伸C(T)试样,通过-20℃的低温裂纹尖端张开位移CTOD断裂韧性试验及疲劳裂纹扩展速率da/dN试验,研究了S355N及S355N-Z25结构钢的裂纹启裂与扩展的抗力.研究结果表明:S355N钢抵抗裂纹启裂及前期扩展的能力比S355N-Z25钢强,但前者抵抗裂纹后期扩展的能力比后者弱...     

高密度聚乙烯/顺丁橡胶共混物的耐环境应力开裂性的研究

用定应变方法和定拉力方法研究了高密度聚乙烯(HDPE)及其共混物的耐环境应力开裂性(ESCR)。结果表明,丁基橡胶(IIR)和顺丁橡胶(BR)都可以提高HDPE的ESCR。对HDPE/橡胶二元共混物,BR的效果不如IIR。在改性剂DX-2或DX-3存在下,HDPE/BR共混体系的ESCR有大幅度提高,三元共混物在初始应力强度因子K_0为0.2MPam~1/2时,断裂时间可达到HDPE断裂时间的4倍左右,断面形貌的扫描电镜照片呈网孔状,表明断裂过程具有韧性特点。     

Investigation of fatigue crack growth characteristics of NR/BR blend based tyre tread compounds

Tyre tread directly comes in contact with various road surfaces and is prone to damage due to cuts from sharp objects during service. As tyres undergo millions of fatigue cycles, these cuts propagate continuously and lead to catastrophic failure. Therefore fatigue crack growth (FCG) characteristics should be an essential criterion for tread compound selection. The present study investigates FCG behavior of blends comprising of Natural Rubber (NR) and Polybutadiene Rubber (BR) over a wide range of tearing energy. Pure shear specimens with a notch on both edges were tested in a Tear Analyser. Rapid increase in FCG rate after a certain strain level was observed. This transition point appeared in a strain range of 20–35 %, depending on the blend composition. The higher BR containing compounds exhibited better FCG characteristics below the transition point but reversal of ranking was seen above this point. The influence of temperature, R ratio, waveforms and cure system on FCG characteristics was also investigated in NR and 60–40 NR/BR blend compounds. Higher FCG rate was achieved under pulse loading compared to the sine waveform. The relaxation time between pulse cycles played an important role. With an increase in relaxation time, FCG rate decreases significantly. The higher sensitivity towards R ratio was observed in NR compound. The 60–40 NR/BR blend showed higher FCG rate with increase in temperature compared to the NR compound. The NR compound with high Sulfur/Accelerator (S/Ac) ratio showed better FCG characteristics whereas for 60–40 NR/BR blend with low S/Ac ratio achieved superior FCG characteristics.     

Blends of polycarbonate with unmodified and maleic anhydride grafted ABS: fracture mechanics

Physical blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene terpolymer (ABS) at two different weight fractions were made (PC35/ABS65 and P75/ABS25). Reactive blended similar compositions of PC with maleic anhydride grafted ABS (MABS) were also made at the same compositions. The crack resistance behaviour of these two types of blends and feedstocks (PC and ABS) were studied. The generalized locus method was used to investigate the invariance of crack resistance from any set of characteristic points. PC and PC/ABS blends failed immediately after crack initiation. The modified blends (PC/MABS) exhibited failure through crack propagation after crack initiation started. The resistance to crack initiation is determined in terms of critical J-integral value (J_c). The resistance to crack propagation at maximum load point is also determined from the locus of maximum load point on the load-displacement curves. The resistance to steady state crack growth (R_p) during extensive crack propagation is determined from the total essential energy for a complete fracture. The crack resistance values for modified blends are much high compared to unmodified blends and even feedstocks.     

Fracture behaviour of liquid crystal epoxy resin systems based on diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene: Part II Effect due to blending with TACTIX 556 epoxy resin and phenolic monomers

The mechanical behaviours of unoriented, poured resin castings based on formulated blends containing the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene monomer are studied. It is found that the mechanical and fracture behaviours of these liquid crystalline epoxy (LCE) blends vary significantly. In general, the LCE blends possess much higher fracture toughness and fatigue crack resistance than conventional epoxy resins. At low temperatures (−40°C), the KIC values of the LCE blends are slightly higher than those measured at room temperature. The common fracture mechanisms observed in the ductile LCE blends are crack segmentation, crack branching, crack bridging and crack blunting. The fracture surfaces of the tougher LCE blends only exhibit limited ductile drawing (furrow pattern) at the slow crack growth region; no signs of shear lips on the edges of the starter crack region are observed. The optical microscopy and transmission electron microscopy work suggests that orientation and/or transformation toughening may be the source for such high fracture toughness of the LCE blends. The possible cause(s) of the unusual fracture behaviour of the LCEs is discussed. Approaches for making high performance LCE blends are also addressed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of expanded graphite and modified graphite flakes on the physical and thermo-mechanical properties of styrene butadiene rubber/polybutadiene rubber (SBR/BR) blends

The aim of the present research work is to develop expanded graphite (EG) and isocyanate modified graphite nanoplatelets (i-MG) filled SBR/BR blends, which can substitute natural rubber (NR) in some application areas. The present study investigated the effect of i-MG on the physical, mechanical and thermo-mechanical properties of polybutadiene rubber (BR), styrene butadiene rubber (SBR) and SBR/BR blends in the presence of carbon black (CB). Graphite sheets were modified to enhance its dispersion in the rubber matrices, which resulting in an improvement in the overall physical and mechanical properties of the rubber vulcanizates. Compounds based on 50:50 of BR and SBR with ∼3 wt% nanofillers with CB were fabricated by melt mixing. The morphology of the filled rubber blends was investigated by wide angle X-ray diffraction (WAXD) and high resolution transmission electron microscopic (HR-TEM) analyses. The intercalated and delaminated structures of the nanofiller loaded rubber blends were observed. Scanning electron microscopic (SEM) analysis of the cryo-fractured surfaces of the rubber compounds showed more rough and tortuous pathway of the fractured surfaces compared to the fractured surfaces of the only CB loaded rubber composites. Filled rubber compounds exhibit increase in the ΔS (torque difference) value, reduced scorch and cure time compared to their respective controls. Dynamic mechanical thermal analysis (DMTA) of the filled rubber compounds shows an increase in the storage modulus compared to the controls. Isocyanate modified graphite nanoplatelets (i-MG) containing rubber compounds in the presence of CB showed an increase in the mechanical, dynamic mechanical, hardness, abrasion resistance and thermal properties compared to the alone CB filled rubber vulcanizates.     

PA6/POE-g-MAH共混体系缺口冲击断裂机理研究

研究了PA6/POE-g-MAH共混体系缺口冲击断裂的裂纹萌生、裂纹扩展、冲击断面形貌及紧邻断面下方截面上的相形态.结果显示:随着POE-g-MAH质量分数增加,PA6/POE-g-MAH共混体系缺口冲击断裂发生脆韧转变,裂纹源由"中心辐射型"转变为"线型",裂纹扩展形貌由岩石状脆性断裂特征转变为具有横纹形貌的韧性断裂特征.在紧邻韧性断面下方的截面上可观察到网状形貌,可能是由被刻蚀掉的POE-g-MAH和冲击时基体微观破坏共同组成.在脆性断裂的截面上则无明显的网状形貌.PA6/POE-g-MAH共混体系的断面及截面形貌变化与其脆韧转变的逾渗行为完全对应.DSC分析认为POE-g-MAH提高POE-g-MAH/PA6共混物的缺口冲击韧性与其对PA6晶体结构的改变没有关系.     

Fracture toughness and fracture mechanisms of polybutylene-terephthalate/polycarbonate/ impact-modifier blends

A series of polybutylene-terephthalate/polycarbonate (PBT/PC) blends with different compositions were prepared using a twin-screw extruder. The morphologies of the blends were revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that a 50/50 PBT/PC blend possessed a bicontinuous structure and the other blends had a dispersed phase of either PBT or PC depending on which was the minor component. A relatively strong interface was observed in the blends with 20%, 40% and 50% PBT; but poor interfacial adhesion was found in the blends with 60% and 80% PBT. The strength of the interfacial boundary was believed to depend on the composition and blending conditions of the individual blend. Fracture experiments showed that the sharp-notch fracture toughness of PC could be significantly increased by mixing with up to 50% PBT without losing its modulus and yield stress. The toughening mechanisms involved in the fracture processes of the blends were studied using both SEM and TEM together with single-edge-double-notched-bend (SEDNB) specimens. It was found that in the toughened blends the growing crazes initiated by the triaxial stress in front of the crack tip were stabilized by the PC domains. The debonding-cavitation mechanism occurred at the PBT/PC interface, which relieved the plane-strain constraint and promoted shear deformation in both PBT and PC. This plastic deformation absorbed a tremendous amount of energy. Crack-interface bridging by the PC domains was clearly verified by the TEM study. Thus, the PC domains not only stabilized the growing crazes they also bridged crack surfaces after the crack has passed by. This effect definitely caused a large plastic-damage zone and hence a high crack resistance. Poor crack resistances of the blends rich in PBT was caused by the poor interfacial adhesion between PBT and PC. In these polymer blends, the growing crazes easily developed into cracks, which subsequently passed through the weak interface of PBT/PC and finally produced fast unstable fracture.     

Toughening effect of maleic anhydride grafted linear low density polyethylene on linear low density polyethylene

The blend of linear low density polyethylene (LLDPE) and maleic-anhydride grafted LLDPE with the grafting degree of 1.3% and the gel content of 27.0% (designated as LLDPE/MA-PE) was melt-compounded. Their thermal, rheological, and mechanical properties were studied. The crystallization temperature and crystallization rate of LLDPE/MA-PE blends increase due to the nucleation of MA-PE, their crystallinity is between those of LLDPE and MA-PE due to the balance between the nucleation of MA-PE and simultaneously produced more defects. The addition of MA-PE increases the apparent viscosity of blend melts, but the shear-sensitivity of blends provides them with melting processing. Interestingly, the lamellar crystallites induced by MA-PE decrease the tensile yielding strength of LLDPE/MA-PE blends. During the impact fracture, the formation of oriented crystalline lamellae parallel to the crack front and perpendicular to the crack flank, leads to the deformation and microstriations in LLDPE/MA-PE blends. Subsequently, toughness of LLDPE/MA-PE blends is improved.     

混炼工艺对SBR/BR/CB共混硫化胶性能的影响

为了研究混炼工艺对最终硫化胶性能的影响,利用HAAKE转矩流变仪对不同配比的丁苯橡胶/顺丁橡胶/炭黑进行混炼,混炼采用两种不同的混炼顺序——一种是炭黑首先与丁苯橡胶(SBR)混炼,然后加入顺丁橡胶(BR)混炼;另一种是BR与炭黑混炼之后再加入SBR进行混炼。对制得共混物进行结合胶含量、拉伸以及动态力学性能测试。结果显示,在相同配比下,炭黑首先跟份数多的富相橡胶混炼可以得到较多的结合胶,有利于体系的补强;动态力学测试结果表明硫化胶的粘弹特性受第一段混炼所用橡胶的影响较大,炭黑与柔顺性较好的BR先混炼,共混体系工艺相容性较好。