HDPE与PPR全切痕拉伸断裂过程银纹性能测试

通过对高密度聚乙烯(HDPE)和无规共聚聚丙烯(PPR)进行不同拉伸速率的全切痕拉伸测试,考察了两种材料在近似于平面应变条件的拉伸断裂性能。对其全切痕拉伸过程应力-位移曲线、位移-屈服应力曲线分析:HDPE和PPR的屈服应力均随着拉伸速率的增加而增大;在较高拉伸速率时,HDPE全切痕拉伸断裂可认为是由常临界位移控制的银纹断裂,PPR的全切痕拉伸断裂过程几乎无恒定的常拉伸位移。通过显微镜显示的断裂形貌观察揭示了沿拉伸方向上HDPE有微纤空穴产生;在全切痕拉伸断裂过程中,PPR的银纹化易于发生在非晶区的片晶间,银纹微纤可产生在任意方向上。     

ABS树脂拉伸性能及形变机理的研究

研究了丙烯腈丁二烯苯乙烯共聚物（ABS）树脂在不同温度和不同拉伸速率时的拉伸行为以及物理老化对其拉伸行为的影响。结果表明,屈服强度随测试温度的升高而下降,断裂伸长率并不随着测试温度的升高而提高,直到测试温度升高到接近ABS树脂塑料相的玻璃化转变温度时,断裂伸长率才显著提高;断裂伸长率随拉伸速率的增加而降低,在不同的拉伸速率下,ABS的形变区内均可观察到银纹现象;在较高的拉伸速率下,形成的银纹数量较多,但银纹较短,银纹的扩展得到了有效抑制;ABS树脂经物理老化后断裂伸长率明显降低,银纹数量增加并出现了空洞成串现象。     

由应力-应变曲线研究的MgO-C砖的机械性能

在实验室、高温非氧化气氛下,研究了一系列树脂和沥青结合、含金属添加物的MgO-C砖的机械性能。在室温、600℃、1 000℃和1 400℃流动的N2气下获得了压力下的应力-应变曲线。带电容式应变计的液压伺服机适用于对高温下轴向应变的测定。以0.1mm·min-1恒定位移速率直至试样断裂。由应力-应变曲线计算几个参数:断裂应力、断裂应变、屈服应力和正割杨氏模量。此外又进行了综合性能试验。在全部试验温度范围,树脂结合砖显示出高的机械强度值和高的杨氏模量值。直至1 000℃,机械性能由结合剂的种类控制,而在1 400℃气孔率变化。树脂和沥青结合砖的主要区别是由金属添加剂的反应引起的。     

高聚物的开裂

开裂的概念和理论开裂的极端类型是脆性开裂和韧性开裂。Griffith理论认为:断裂功转化为断裂表面的表面能,脆性材料的强度由裂缝决定。断裂力学认为材料都是有缺陷的,因而引发裂缝,导致开裂。微观研究发现了有些脆性高聚物在应力作用下产生银纹,银纹当中有一些高度取向的大分子链束,这些大分子链断裂以后就发展成裂缝。韧性开裂往往跟屈服和冷拉伸相联系。当局部应力达到屈服应力时,这一局部迅速形变,而应力反而下降或几乎不变。以后就在     

汽车内饰件模外装饰用PMMA的力学性能

为研究汽车内饰件模外装饰材料聚甲基丙烯酸甲酯(PMMA)的力学性能,对PMMA膜材分别在2.5 mm/min(0.001 667 s-1),5 mm/min(0.003 333 s-1)和10 mm/min(0.006 667 s-1)不同拉伸速率下进行高温[90℃(363K),105℃(378 K)和115℃(388 K)]单向拉伸实验,得到了不同条件下的应力–应变曲线。实验结果表明,当加热温度相同时,随着应变速率的增大,PMMA的应力、延伸率和屈服应力也逐渐提升。当应变速率一致时,其屈服强度与应力随着温度的升高而逐渐变小。     

聚丙烯树脂在高拉伸应变速率下的拉伸行为

研究了拉伸应变速率特别是高应变速率对聚丙烯树脂应力应变曲线的影响,比较了不同滑石粉添加比例和不同温度下聚丙烯树脂高速度拉伸破坏行为的差异。结果表明:聚丙烯树脂的拉伸强度随着应变速率对数的增大而线性增加;不同分子链结构的聚丙烯树脂在同一高拉伸应变速率下有不同的拉伸强度和拉伸应变;应变速率从6 s~(-1)增加到125 s~(-1)时,聚丙烯树脂拉伸强度对应变速率变化的敏感性相差不大。高应变速率下,滑石粉含量越高,拉伸强度越低;但当应变速率增加到100 s~(-1)时,不同滑石粉含量试样的拉伸断裂应变相差不大。而且,聚丙烯树脂的拉伸强度随温度升高而明显降低,断裂应变随温度的升高则有所增加;温度越低,试样的应力发白区域则逐渐变小。     

温度对胎面材料拉伸力学性能的影响研究

研究温度和拉伸速率对胎面材料拉伸力学性能的影响。结果表明:胎面材料的拉伸力学性能对拉伸速率具有一定的敏感性;在0~90℃温度范围内及大应变时,胎面材料的拉伸模量随温度升高而减小;在-30℃时,应力-应变曲线表现出"玻璃态"的特性,拉伸模量明显提高。Mooney-Rivlin模型模拟结果与试验结果相一致。     

PE-LLD/乙烯基笼形倍半硅氧烷共混熔体的流变行为与力学性能

采用毛细管流变仪研究了线形低密度聚乙烯( PE-LLD)与乙烯基笼形倍半硅氧烷(E-POSS)共混物熔体的流变行为;讨论了共混物的组成、剪切应力、切变速率及温度对熔体流变性、非牛顿指数和挤出膨胀比的影响;测定了共混物的屈服应力、断裂应力和断裂伸长率对E-PUSS含量的依赖性。结果表明,E-PUSS加人量在J%以内和实验温度低于160℃熔体的流动性随切应力增大而变好,假塑性增强,超过3%和高于160℃假塑性降低。PE-LLD/E-PUSS共混物的强度在E-PUSS含量为3%时达到最大。     

退火对PA6结晶和拉伸性能的影响

通过调节退火时间和退火温度,研究了退火工艺对尼龙(PA)6结晶性能以及PA6在不同拉伸速率下的拉伸性能的影响。结果表明,在100℃恒温退火时,随着退火时间的增加,PA6初始熔融温度逐渐升高,熔限减小,γ晶的晶粒尺寸增大,但结晶度变化不大;在5 h退火时间下,随着退火温度的增加,PA6结晶度呈现先增加后降低的趋势,而α晶晶粒尺寸逐渐增大。总体上看,不同拉伸速率下退火时间和退火温度对拉伸强度的影响较小,相对来看,退火温度为100℃时拉伸强度较高,而当退火温度达到200℃时,拉伸强度显著下降;当拉伸速率较高或较低时,不同退火时间下的PA6断裂伸长率相差不大,而当拉伸速率为20,50 mm/min时,随退火时间的增加,断裂伸长率逐渐降低;退火温度为100,130℃的PA6断裂伸长率保持在相对较高值,且拉伸速率越低,其值越高,但随着拉伸速率增加,其下降趋势也最快;高拉伸速率下,退火温度对PA6断裂伸长率影响较小。以上结果表明,当对PA6进行退火处理时,在拉伸性能方面需要考虑拉伸速率的影响。     

高性能己烯-1共聚缠绕膜的开发

抚顺石化公司烯烃厂45万t/a线性低密度聚乙烯气相法装置利用淤桨催化剂开发了一种高己烯-1含量的高性能拉伸缠绕膜料。测试结果表明:产品具有优良的拉伸和抗穿刺性能。拉伸屈服应力达到11 MPa,拉伸断裂应力达到25 MPa,拉伸断裂标称应变≥810%。     

Fracture prediction in tough polyethylene pipes using measured craze strength

In this study, an empirical model is developed that predicts the time to failure for PE pipes under combined pressure and deflection loads. The time‐dependent craze strength of different PE materials is measured using the circumferentially deep‐notched tensile (CDNT) test. In agreement with previous research, results indicate that bimodal materials with comonomer side‐chain densities biased toward high‐molecular‐weight PE molecules exhibit significantly higher long‐term craze strengths. A comparison of currently available PE materials with CDNT samples taken from a PE pipe that failed by slow crack growth in service clearly indicates the superior performance of new‐generation materials. Using measured craze strength data from the CDNT test, a simplified model for predicting failure in buried PE pipes is developed. Extending previous research, the reference stress concept is used to calculate an equivalent craze stress for a pipe subjected to combined internal pressure and deflection loads. Good agreement is obtained between the model predictions and observed failure times in an experimental test‐bed study of pipes under in‐service loading conditions. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

The continuous measurement of the damping and fatigue properties of high-impact polystyrene with a controlled-amplitude torsion pendulum

Torsional damping and fatigue studies were made on high-impact polystyrene using a controlled-amplitude, continuously recording torsion pendulum. The damping behavior was measured as a function of the angular displacement, axial tensile stress, and the number of repeated cycles. At a critical axial tensile stress, the damping behavior changed from amplitude-independent to amplitude-dependent behavior. Damping measurements on high-impact polystyrene specimens that had been previously crazed in tension showed that both the damping and the tensile stress necessary to produce amplitude-dependent damping varied with the orientation of the craze. Repeated cycling of HIPS specimens produced crazing and amplitude-dependent damping at a tensile stress below that expected from simple static tensile loading. Scanning electron micrographs of fracture surfaces from these specimens revealed that the rubber particles could be clearly distinguished from the surrounding matrix.     

Relaxation of stresses in crazes at crack tips and rate of craze extension

The Barenblatt theory of cohesive stresses at crack tips is used to investigate the effect of the relaxation of craze stresses at crack tips on the rate of craze extension. The craze stresses are equated to the cohesive stresses of the Barenblatt theory. The cancellation by the cohesive/craze stress of the singularity that would exist at the crack tip in their absence is assumed to hold for an extending craze. With this assumption, relaxation of the craze stresses produces craze extension, an effect which has been called ‘relaxation controlled growth’ by Williams and Marshall. A general equation relating the rate of change of craze length to the rate of change of stress intensity factor (K₁) and the rate of change of the craze stress is derived. It is argued from this equation that uniform crack growth with a constant craze length can occur only at constant K₁. Using plausibility arguments for the behaviour of the craze stress with time and position in the craze, and assuming a generalized Dugdale model, differential equations for the rate of craze extension with no crack growth are derived for the constant load and constant K₁ cases. These equations relate the rate of change of craze length to the craze stress at the tip of the crack. Assuming a specific form for the time dependence of this stress, the equation for the constant K₁ case is solved to yield an expression for the craze length as a function of time.     

The effects of stress cracking on the fracture toughness of polycarbonate

The growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in k_IC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.     

Interpreting instrumented three point bend fracture tests on craze forming polymers

Abstract

Tests on three point bend loaded specimens, containing a sharp initial notch of measured length, are the basis of a standard method (ISO/CD 17 281) for measuring the toughness of a plastic under either quasi-static or impact loading. In some polymers the fracture surface reveals that a long, stable, coplanar craze has extended from the notch tip during loading. Doubts about how to treat this notch extension have sometimes confused interpretation of the test. Using a quasi-static Dugdale–Barenblatt cohesive zone model, this paper presents a simple correction to the standard linear elastic analysis for tests in which a craze length can be measured. The corrected toughness results are higher and linearity restrictions on their validity can be significantly relaxed. Results are presented from Charpy type impact fracture tests on polyethylene. The computed craze stress reveals a craze size dependence which is thought to reflect a two stage process of craze fibril extension.     

Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

The true stress-true strain behavior of polyoxymethylene, n(-CH₂O), as an example of a bulk semi-crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure. In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure-induced increase in the β-transition temperature which points to the potential usefulness of the concept of pressure-temperature super-position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb-Navier criterion, are not valid for general deformation (non-plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi-crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.     

Experimental Investigation on High Strain Rate Tensile Behaviors of HTPB Propellant at Low Temperatures

To study the high strain rate tensile behaviors of hydroxyl‐terminated polybutadiene (HTPB) propellant at low temperatures, uniaxial tensile tests were conducted at different strain rates (0.4–42.86 s⁻¹) and temperatures (233–298 K) using an INSTRON testing machine. Scanning electron microscopy (SEM) was employed to observe the tensile fracture surfaces. Experimental results indicate that strain rate, temperature and test environment remarkably influence the tensile behaviors of HTPB propellant. The stress‐strain curves exhibit three different shapes. The elastic modulus and maximum tensile stress increase with decreasing temperature and increasing strain rate. However, the strain at maximum tensile stress decreases with increasing strain rate at low temperatures and there is a maximal value at 298 K and 14.29 s⁻¹. The effects of strain rate, temperature and test environment on the tensile behaviors are closely related to the changes of properties and fracture mechanisms of HTPB propellant. The dominating fracture mechanism depends on not only temperature but also strain rate, and it changes from the dewetting and matrix tearing at room temperature and lower strain rate to the particle brittle fracture at low temperatures. Based on the time‐temperature superposition principle (TTSP), the master curves of mechanical parameters for HTPB propellant were obtained.     

Craze growth and void coalescence in PMMA round notched bars

A study of the kinetics, failure mechanism, and fracture surfaces of PMMA/methanol crazes has been made on notched circular bar specimens subjected to constant tensile loads. Failures were by void growth and coalescence inside the craze away from the notch tip and near to the specimen center. Two distinct features of void coalescence were observed; the first a cluster of very small voids (each of the order of 3.4 × 10³ Å) and the second, separate larger voids which usually caused final failure. An analysis of craze growth, based on fracture mechanics concepts in conjunction with simple flow analysis, suggests that the growth is controlled mainly by the ability of the environment to flow through the voided structure of the craze. Good agreement has been obtained between predicted behavior and experimental data.