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.     

SMA含量对PA6/ABS共混体系结构和韧-脆转变温度的影响

研究了苯乙烯-马来酸酐共聚物(SMA)含量(0～4.5phr)对PA6/ABS(75/25)共混体系结构和冲击断裂的影响。结果表明,PA6/ABS共混体系的橡胶相粒径和基体层厚度都随着SMA含量的增加而减小。PA6/ABS共混体系冲击断裂存在脆韧转变现象,其脆韧转变温度随SMA含量的增加先减小后增加,并且在对应SMA含量为0.9phr和1.5phr时取得极小值。同时,脆韧转变温度随着基体层厚度的增加而逐渐升高,即升高温度与降低基体层厚度都可以获得脆-韧转变。     

相容性对PA66/POE共混材料形态、微结构及力学性能的影响

采用SEM、TEM、DSC及材料力学性能实验方法研究了马来酸酐 ( MAH ) 接枝乙烯-辛烯共聚物弹性体 ( POE ) 对PA66/POE共混材料形态、微结构及力学性能的影响。结果表明:热引发官能化POE产物 ( POE-g-MAH ) 可显著改善PA66/POE共混材料的相容性,使材料分散相尺寸减小,分布均匀,且材料缺口冲击强度显著增大。实验发现,PA66/POE-g-MAH共混材料分散相的弹性体颗粒内部存在较多份量的有序结构,材料中的分散相颗粒具有明显促进结晶的作用,此作用引起PA66基体结晶温度增加,结晶度增大,并在分散相质量分数为15% 的脆韧转变条件下,达到极大值。试样熔体的冷却速率越快,则此种促进结晶的作用就越明显。      

PA6/ABS共混物的脆-韧转变研究

采用熔融共混方法制备了苯乙烯-马来酸酐共聚物(SMA)增容的PA6/ARS共混物,结合吴守恒的临界基体层厚度(IDc)理论,考察了基体层厚度与界面粘接对PA6/ABS共混物脆一韧转变的影响.结果表明,温度低于8℃,当ID减小时,冲击强度先缓慢增加,当ID＜ID.时,共混物缺口冲击强度急剧增加;测试温度处于13℃～23℃...     

HDPE/E-TMB共混物和HDPE/弹性体共混物的脆韧转变和断面形态

用材料力学性能测试方法和扫描电镜研究了HDPE/E-TMB共混物和HDPE/弹性体共混物的脆韧转变行为和冲击断面形态。结果表明,HDPE/E-TMB共混物的脆韧转变可在弹性体含量较低的情况下发生;弹性体含量相同时,HDPE/E-TMB共混物有更高的冲击强度和拉伸屈服应力;冲击强度相同时,HDPE/E-TMB共混物有更高的拉伸屈服应力和弯曲弹性模量。弹性体含量为8%的两种共混物的冲击断面形态属于不同的断裂机理,这种机理的不同揭示了HDPE/E-TMB共混物具有特殊的结构特征。     

尼龙11/POE共混物的形态与力学性能

采用动态力学分析(DMTA)和扫描电镜(SEM)等方法研究了相容剂乙烯辛烯共聚物接枝马来酸酐聚合物(POE-g-MAH)对尼龙11(PA11)共混体系力学性能及共混物形态的影响。结果表明,混合弹性体中m(POE-g-MAH)/m(POE)=2∶1时,PA11共混物的缺口冲击强度达到81.2 kJ/m2;共混物缺口冲击强度对MAH接枝率敏感;加入混合弹性体,共混体系分子间的作用力增大,损耗峰向高温方向移动,Tg升高;PA11与POE各个共混体系的β松弛峰高显著低于纯PA11的,加入POE和POE-g-MAH可有效降低PA11的吸水性;POE-g-MAH可显著改善PA11/POE共混材料的相容性,使分散相尺寸减小,分布均匀,且材料缺口冲击强度显著增大。     

马来酸酐共聚方式对PA6/ABS共混物性能的影响

分别制备了马来酸酐与苯乙烯-丙烯腈无规共聚物(SAM)增容的尼龙6(PA6)/ABS/SAM共混物、马来酸酐接枝共聚的丙烯腈-丁二烯-苯乙烯共聚物(ABS-g-MA)增容增韧的PA6/ABS-g-MA共混物。结果表明,两个体系中ABS都可以均匀分散;冲击测试发现样条厚度为6.35 mm时,PA6/ABS-g-MA共混物出现明显的脆韧转变,PA6/ABS/SAM共混物为脆性断裂;样条厚度为3.18 mm时,两个体系都有明显脆韧转变;Vu-Khanh方程表明,PA6/ABS-g-MA共混物具有更高的裂纹扩展能(Gi)和撕裂模量(Ta),性能更好。     

PTFE/PA6和PTFE/PA66共混物的吸水性及流变行为

通过浸水实验、缺口冲击断裂实验和动态流变测试,考察了PTFE含量对PTFE/PA6和PTFE/PA66共混物的吸水率、冲击断裂强度及熔体黏度的影响以及熔体黏度随温度和频率的变化规律。结果表明,PTFE/PA6和PTFE/PA66共混物的吸水率均随着PTFE含量的增加而减小,即PTFE的加入抑制了共混物的吸水性。两种共混物的冲击强度比纯PA明显降低,但是吸收水对两种共混物冲击强度的影响不显著。随着PTFE含量的增加,共混物熔体的黏度先减小后增加,说明适量的PTFE可以改善共混物的成型加工特性。共混物熔体的黏度随加载频率的增大而降低,符合假塑性流体流动规律。有趣的是,对于PTFE/PA6共混物的黏度随着温度的升高而减小,而PTFE/PA66共混物黏度随着温度升高近似成指数规律增大。     

增韧PA1010的力学性能及熔融与结晶行为

采用熔融共混法制备了PA1010/纳米橡胶/POE-g-MAH共混物,测试了共混材料的力学性能,用DSC法研究了熔融和结晶行为.结果表明,共混物的冲击强度显著提高,在纳米橡胶质量含量为10%时达到极大值.纳米橡胶并未降低PA1010的拉伸强度,加入POE-g-MAH后略有降低.共混物的熔融过程呈现双峰,纳米橡胶的加入提高了高温峰的熔融焓.纳米橡胶促进了共混物结晶峰向高温区移动.     

γ射线辐照对PA6/PTFE合金吸水与力学性能的影响

为了探讨辐照对尼龙6(PA6)基共混材料吸水性能和力学性能的影响,制备了PA6和聚四氟乙烯(PTFE)共混合金并在室温下进行不同剂量的60Coγ射线辐照.通过吸水率测试、准静态拉伸和弯曲实验以及缺口冲击测试,研究了辐照对PA6/PTFE共混体系吸水性能以及吸收水和辐照剂量对共混合金动静态力学性能的影响.结果表明,辐照过程引发交联反应,共混合金的拉伸强度、拉伸模量和弯曲模量均随辐照剂量的增大而增大,而缺口冲击强度和吸水率则随之减小.吸收水对共混合金缺口冲击强度的影响与合金组分有关.PTFE含量较低时,吸收水抑制PTFE对合金冲击强度的减弱作用;当PTFE的质量分数大于7%时,吸收水反而加剧PTFE对材料冲击强度的减弱效应.此外,PA6/PTFE合金的吸水率随PTFE含量的增加而降低,而弯曲模量则先增后减.     

Deformation Analysis of Notched Rubber Specimens

Abstract:  The influence of the initial crack tip radius on the strain distribution in the vicinity of the blunted crack tip was determined experimentally by an optical full-field strain analysis method. These experimental results were compared with the calculations of the commercial finite element code ABAQUS 6.4-1. For the simulation a suitable hyperelastic material law was chosen and fitted to experimental data of three different tests (uniaxial tensile, planar tensile, biaxial tensile). Two different elastomer grades (SBR and EPDM) were selected for the experimental work. The utilized optical full-field strain analysis method based on the image correlation technique was found to be an effective tool to determine strains, strain distributions and gradients near to the crack tip for elastomeric materials. Different material behaviour was observed for the two rubber types investigated. While the crack tip was regularly blunted (half circle shape) for EPDM and the strain gradient was low (less steep), the crack tip was sharp (less blunted) with a higher strain gradient for SBR.     

Neue Beobachtungen über das Phänomen der Wechseldehnungs-Versprödung an einer Rißfront anhand von Sprödbruchversuchen

New Observations about the Phenomenon of Cyclic Strain Embrittlement at the Fatigue Crack Tip by Means of Brittle Fracture Tests . Fracture tests were performed on flat samples notched on both sides (net section 120 × 30 mm²) of material 19 Mn 5 to determine the influence of a local embrittlement on the nominal fracture stresses. The local embrittlement in a very small area could be produced by pulsating tensile tests due to cyclic strain could be produced by pulsating tensile tests due to cyclic strain embrittlement at 350°C gradually different in the base of the notch respectively at the tip of the growing fatigue crack. Fractures due to “low” nominal stress tend to occur in those cases where the rate of crack growth in the pulsating tensile test is approaching zero or is equal to zero.     

On the thermal blunting of crack tips in polymers

The analysis of crack tip blunting in the impact test is reviewed and the results for polymethylmethacrylate at room temperature are presented, together with more recent data for which loading times have been measured. A successful comparison is drawn between the results of impact tests at various strain rates and those of static single edge notched tests where the crack tip region is heated. Such a correlation confirms the suspected suppression of crack tip crazing owing to thermal effects.     

The brittle-ductile transition in silicon: The influence of pre-existing dislocation arrangements

It has been shown that the value of the brittle-ductile transition temperature in specimens of silicon containing pre-cracks at the surface is strongly dependent on the pre-existing dislocation arrangement close to the crack tip. In particular, removing dislocations from the vicinity of the crack tip has the effect of raising the transition temperature, while introducing more surface dislocations by grinding reduces the transition temperature. In both cases the transition temperature remains sharp, implying that the crack tip sources have to be nucleated in the test before effective shielding occurs. The differences in the transition temperatures reflect the differences in the distances of the dislocations from the crack tips, and in the source lengths of the pre-existing sources which send dislocations to the crack tip.Specimens which are pre-stressed at the brittle-ductile transition to a K value at which crack tip sources are expected to be nucleated exhibit a gradual “soft” transition over a wide range of temperature. This result is in agreement with the predictions of the computer modelling of dislocation emission from crack tips, developed by Hirsch et.al [3,7], and together with the results on the dependence of T_c on dislocation arrangements, provides strong support for their theory of the brittle-ductile transition.     

Simulation of the shear-tensile mode transition on dynamic crack propagations

We propose an approach to the simulation of the shear-tensile transition in dynamic crack growth based on two points: a new crack propagation criterion which is suitable for shear, and an algorithm which is capable of handling the transition from shear mode to tensile mode and back in the same simulation. The new crack propagation criterion for brittle crack growth is based on the maximum shear stress rather than the maximum hoop stress. The shear stress direction becomes the new crack??s direction in which propagation is initiated for shear-type failure. The stress state at the crack??s tip is obtained through a local approach which can be used even in the case of extensive plasticity. Additionally, we propose to control the transition from shear mode to tensile mode during the simulation of crack propagation using an equivalent strain estimated at the crack??s tip. Depending on a threshold strain, the propagation direction is predicted using the maximum shear stress (in the shear case) or the maximum hoop stress (in the tensile case).     

Cleavag Fracture Criterion of Low Alloy Steal and Weld Metal in Notched Specimens

The cleavage fracture criterion of low alloy steel and weld metal in notched specimens is investigated in detail based on a great number of experimental data. It has been found that the most cleavage fractures initiate at a distance shorter (left side) than that of the peak stress location below a notch root, and the cleavage fracture in notched specimens must satisfy a dual criterion, i.e., a critical plastic strain (ε_p ≥ ε_pc) for initiating a crack nucleus, and a critical tensile stress (σ_yy ≥ σ_f) for its propagation. According to the dual criterion model, the great number of experimental data of 4PB (four-point bending) tests for the low alloy steel and weld metal and their statistical distribution are explained. The effects of temperature , the local fracture stress σ_f and the critical plastic strain ε_pc on the locations of cleavage initiation sites and the controlling steps of cleavage fracture process are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.