PA6相对黏度对PA6/POE-g-MAH共混物缺口冲击性能的影响

研究了尼龙(PA)6相时黏度对PA6/马来酸酐接枝乙烯-辛烯共聚物(POE-g-MAH)共混物缺口冲击性能的影响.结果表明,PA6的相对黏度对PA6/POE-g-MAH共混物的缺口冲击断裂韧性有明显影响;低黏度PA6/POE-g-MAH共混物冲击断裂过程中的裂纹扩展功明显高于中、高黏度PA6/POE-g-MAH共混物;...     

温度和测试速率对PA 6/ABS共混物韧性的影响

采用熔融共混方法制备苯乙烯-马来酸酐共聚物(SMA)增容的PA 6/ABS共混物,考察温度和测试速率对PA 6/ABS共混物韧性的影响.结果表明:共混物的缺口冲击强度随着温度升高而提高,达到脆-韧转变温度后冲击强度急剧增加,超过脆-韧转变温度后,共混物的冲击强度基本不变;随着夹头移动速率的增加,PA 6/ABS共混物的裂纹引发功先减少后增加,裂纹扩展功随夹头移动速率增加而先增加后减少.     

核-壳结构有机硅/丙烯酸酯聚合物增韧PA6

采用核-壳结构的有机硅-丙烯酸丁酯/甲基丙烯酸甲酯-甲基丙烯酸缩水甘油酯聚合物(Si BMG)增韧尼龙6 (PA6),以双酚A型环氧树脂(DGEBA)扩链,通过熔融共混法制备了PA6/Si BMG/DGEBA共混物,探究了Si BMG、DGEBA添加量对共混物力学性能、加工性能和结晶性能的影响。研究表明,Si BMG对PA6有一定的增韧作用,加入DGEBA后,PA6/Si BMG共混物常温、低温韧性进一步提高,拉伸、弯曲强度则略微下降。与PA6相比,添加了10. 0%SiBMG和2. 0%DGEBA的PA6/Si BMG-10/DGEBA-2. 0,其常温、低温缺口冲击强度分别提高了301%、168%,韧性明显增加。缺口冲击断面的扫描电镜(SEM)照片显示共混物断面粗糙层次明显,呈现显著的韧性断裂特征。动态热机械分析(DMA)图谱显示各PA6/Si BMG-10/DGEBA中PA6对应的玻璃化转变温度(T_g)高于PA6/Si BMG-10,可能是由DGEBA的扩链作用使PA6分子链刚性增大引起。     

POE-g-MAH/OMMT对PA6的增韧机理研究

制备了马来酸酐接枝(乙烯/辛烯)共聚物(POE-g-MAH)/有机蒙脱土(OMMT)增韧粒子,并以其增韧尼龙(PA)6.研究表明,随着增韧粒子中OMMT含量的增加,PA6/POE-g-MAH/OMMT共混物的缺口冲击强度呈现先增加后降低的变化趋势.这一性能变化与增韧粒子中OMMT质量分数对增韧粒子分散粒径的影响密切相关.在增韧粒子质量分数相同的情况下,较小的分散粒径和基体韧带厚度,有利于基体韧带从平面应变状态到平面应力状态的转变,有利于基体塑性变形的发生,从而消耗较多能量,导致缺口冲击强度增加.     

ABS噁唑啉化对PA6／ABS共混物的增容作用研究

采用反应挤出接枝改性技术,制备了嗯唑啉官能化的苯乙烯-丁二烯-丙烯腈共聚物(ABS)相容剂AB-Sm,并制备了PA6/ABS/ABSm共混物.共混物的力学性能研究结果显示,适量添加ABSm可以提高共混物的缺口冲击强度;在ABSm质量分数为4%时,共混物的冲击强度出现最大值,而后略有降低;而ABSm的加入对共混物的拉伸强度和弯曲强度的影响不大.共混物的形态观察表明,ABSm加入后明显地改善了PA6与ABS的相容性,使ABS更均匀地分散于PA6中;而对样品断面的形貌分析表明,加入相容剂后共混物从脆性断裂转变为韧性断裂,断面呈现大量塑性变形.     

PA6／POE-g-MAH体系的结构与性能研究

以动态硫化及非动态硫化方法制备了马来酸酐接枝乙烯-辛烯共聚物(POE-g-MAH)增韧的聚酰胺6(PA6) 共混物,研究了PA6／POE-g-MAH体系的力学性能以及POE-g-MAH在PA6中的分散状况。结果表明动态硫化 PA6／POE-g-MAH共混物的屈服强度、冲击强度、断裂伸长率及弯曲模量均高于非动态硫化方法制备的PA6／POE-g- MAH共混物,动态硫化的PA6／POE-g-MAH共混物可以在材料的刚性与韧性之间达到良好的平衡。扫描电子显微镜图像的统计分析表明,非动态硫化PA6／POE-g-MAH共混物中分散的橡胶相数均颗粒尺寸随组成中POE-g-MAH 的含量增大而不断增大,动态硫化的PA6／POE-g-MAH共混物中分散的橡胶相数均颗粒尺寸随POE-g-MAH含量的增大变化不大。     

PA6/POE-g-MAH/纳米BaSO4三元复合体系增韧研究

采用纳米BaSO4及马来酸酐接枝乙烯-辛烯共聚物(POE-g-MAH)对聚酰胺(PA)6进行增韧改性,研究了PA6/POE-g-MAH共混体系的脆韧转变,讨论了纳米BaSO4的用量对PA6/POE-g-MAH脆韧转变体系的增韧作用.结果显示,当POE-g-MAH质量分数为10%～20%时,PA6/POE-g-MAH共混...     

共混工艺对PA6/POE-g-MAH/CaCO3三元复合体系力学性能的影响

研究了4种不同共混工艺对PA6/POE-g-MAH/CaCO3三元复合体系韧性的影响。通过不同的加料顺序和挤出次数对PA6、POE-g-MAH、CaCO3进行熔融共混后挤出注塑,并对其进行力学性能测试和SEM观察。研究结果表明:形成"核壳"结构界面的材料的韧性最高。不同制备工艺下,POE-g-MAH/CaCO3先挤出,再与PA6挤出,注塑得到的共混物的冲击强度最高但弹性模量最低;PA6/CaCO3挤出,再与POE-g-MAH挤出,注塑得到的共混物韧性次之,但弹性模量最高;PA6/POE-g-MAH/CaCO3PP一起挤出,注塑得到的共混物韧性再次;PA6与POE-g-MAH挤出,再与CaCO3挤出,注塑得到的共混物韧性最差。     

短切玻纤增强尼龙6的缺口拉伸性能

研究了短切玻纤增强尼龙6（PA6／SGF）的缺口拉伸性能。结果表明,在实验速度范围（0．1～500mm／min）内,PA6／SGF的拉伸断裂功主要消耗在裂纹萌生过程。一旦裂纹源尺寸达到临界值,裂纹瞬间扩展。拉伸位移随着拉伸速度的增加而降低,最大力值随着拉伸速度的增加先提高后降低。当拉伸速度达到300mm／min后,最大力值和拉伸位移急剧下降导致拉伸断裂功大幅度下降。PA6／SGF的缺口拉伸断面主要分为裂纹萌生区及裂纹扩展区,基体的塑性变形主要集中在裂纹萌生区,剧烈的塑性变形可使基体上出现明显的孔洞。在裂纹扩展区,裂纹的快速扩展导致基体断面平坦,且由于SGF的阻碍作用,基体具有断裂分层现象。     

PA6／POE／POE-g-MAH形态与性能研究

本文以POE和POE-g-MAH并用，通过改变其比例来改变MAH含量以控制弹性体在PA6中的粒径分布。研究了POE弹性体粒径大小PA6／POE共混力学性能的影响。结果表明：弹性体分散相粒径的大小与分布是影响共混物韧性的主要因素，弹性体分散相粒径小于0．2μm时，对PA6增韧效果不佳。但弹性体粒径大小和分布并不影响共混物的屈服强度和模量。75％PA6／25％POE缺口冲击强度比PA6提高不到一倍；对于弹性体分散相粒径的大部分在0．2μm以下，最小的小于50nm的80％PA6／20％POE-g-MAH，其缺口冲击强度仅比PA6提高了三倍多；弹性体粒子在0．2～0．5μm间的80％PA6／13％POE／7％POE-g-MAH缺口冲击强度比PA6提高了近八倍。     

Fracture Toughness and High-Temperature Slow Crack Growth inSiC

The subcritical growth of macroscopic cracks in hot-pressed and in sintered SiC was examined at 900° to 1100°C in oxygen partial pressures of 10⁻⁴ and 10⁻⁸ atm. The K ₁-V data for the hot-pressed SiC were generated using the double torsion specimen in the incremental displacement rate mode. These data, in addition to earlier results, included three distinct regions in the plot of log V vs log K ₁,: a low slope region, N∼20, of substantial slow crack growth which was insensitive to testing temperature and environment; a plateau region which shifted to higher velocities with increasing temperature; and a region of very high slope with K values near K _IC The results may be described qualitatively by viscous flow of the gram-boundary phase. No slow crack growth could be detected in the sintered SiC when tested hi the environments described.     

Fracture Toughness Predictions for Crack Bowing in Brittle Particulate Composites

The bowing of cracks between obstacles has been proposed as a mechanism for increasing the .fracture toughness of brittle materials. An analytical expression is presented based on previous theoretical studies that can be used to predict the increase in fracture toughness. The approach incorporates the effect of obstacle failure and the assumptions needed to compare the predictions with experimental data.     

Determination of Fracture Toughness at High Temperatures after Subcritical Crack Extension

As a consequence of R -curve behavior, ceramic materials may exhibit increased fracture toughness ( K _Ic) following slow crack extention. In this investigation, the effect of crack propagation on fracture toughness is studied in static bending tests. For the calculation of stress intensity factors ( K _I) the stress distribution must be known at the moment of fracture. As a consequence of creep, this stress distribution must deviate from the linear distribution. The corresponding stress intensity factors are computed using the fracture mechanical weight function. Experimental results for fracture toughness are communicated for a 2.5%-MgO-doped hot-pressed Si₃N₄ at 1300°.     

Indentation Fracture Toughness of Sintered Silicon Carbide in the Palmqvist Crack Regime

The fracture toughness of a sintered dense α-SiC was estimated by the Vickers indentation microfracture method in the low-load Palmqvist crack regime. It was observed that the use of simultaneously obtained Vickers hardnesses does not yield reliable fracture toughness values, nor does application of the median-crack-derived equations. It is necessary to utilize a load-independent, crack-free hardness value with this toughness estimation method. Although several of the curvefitting equations yield similar toughnesses, it is concluded for the Palmqvist crack system in this α-SiC that the Niihara-Morena-Hasselman equation is the only one which yields fracture toughness values in agreement with conventional measurement techniques.     

Measurement of Crack Tip Toughness in Alumina as a Function of Grain Size

Crack profile measurements near the crack tip in the SEM were used to measure crack tip toughness of alumina as a function of grain size (average grain size 0.9–16 μm). For comparative tests, two crack configurations were included in the present study: straight cracks (CT specimen) loaded with an in situ device; and radial indentation cracks. The measured crack tip toughness values were independent of crack geometry, and no grain size dependence could be discerned. A mean crack tip toughness of 2.3 MPam^1/2 was evaluated. The crack tip toughness determined from crack profile measurements is significantly lower than the toughness evaluated with conventional indentation techniques (e.g., indentation strength bending).     

不同泊松比复合材料的层板断裂韧性和裂纹尖端应力场

比较了两组具有不同平面泊松比的玻璃纤维／环氧树脂复合材料层板的断裂韧性和缺口断裂强度，发现低泊松比复合材料层板具有较高的断裂韧性和缺口断裂强度；用复变函数一变分方法计算了它们的裂纹尖端应力场；研究了主应力方向与纤维夹角的关系；结果表明：低泊松比材料独特的裂纹尖端应力场有利于提高缺口断裂强度。     

Crack Stability and Its Effect on Fracture Toughness of Hot-Pressed Silicon Nitride Beam Specimens

The effect of stable crack extension on fracture toughness test results was determined using single-edge precracked beam specimens. Crack growth stability was examined theoretically for bars loaded in three-point bending under displacement control. The calculations took into account the stiffness of both the specimen and the loading system. The results indicated that the stiffness of the testing system played a major role in crack growth stability. Accordingly, a test system and specimen dimensions were selected which would result in unstable or stable crack extension during the fracture toughness test, depending on the exact test conditions. Hot-pressed silicon nitride bend bars (NC132) were prepared with precracks of different lengths, resulting in specimens with different stiffnesses. The specimens with the shorter precracks and thus higher stiffness broke without stable crack extension, while those with longer cracks, and lower stiffness, broke after some stable crack extension. The fracture toughness values from the unstable tests were 10% higher than those from the stable tests. This difference, albeit small, is systematic and is not considered to be due to material or specimen-to-specimen variation. It is concluded that instability due to the stiffness of test system and specimen must be minimized to ensure some stable crack extension in a fracture toughness test of brittle materials in order to avoid inflated fracture toughness values.     

Effect of Subcritical Crack Growth on Fracture Toughness and Work-of-Fracture Tests Using Chevron-Notched Specimens

A correlation between the plane strain stress intensity factor K_I , load, and crack extension has been analyzed for constant displacement and constant loading rate experiments, using chevron-notched, four-point-bend specimens. It is assumed that at the beginning of the experiment the chevron triangle tip is not ideally sharp. As loading continues, the crack initially moves with velocity v_t at K_I equal to a threshold value K_t . Maximum crack velocity is reached at K_I= K_IC , the fracture toughness. Depending on the type of material tested, a specific displacement or loading rate must be used to correlate the maximum load with K_Ic . An error in K_IC calculation is estimated if different displacement rates are applied. Repeated loading-unloading work-of-fracture (WOF) experiments generate values related to the resistance of the material to fracture initiation, K_t , only when the crack length approaches 100% of the specimen width. Values related to material's fracture toughness, K_IC are not generated in WOF tests.