高密度聚乙烯管耐氯氧化试验系统设计

为了研究高密度聚乙烯（PE?HD）管的耐氯氧化性能，基于ASTM F2263标准，设计了一种封闭循环试验系统，可在流动状态下通过改变温度、压力和有效氯含量加速试样的氧化失效。结果表明，该系统可独立测试24根DN 20的PE?HD管状试样，可编程逻辑控制器（PLC）作为控制主机可以控制管路的压力、温度和流量，再加上耦合各传感器，可实现可视化的人机界面，内置的报警系统可保证试验的安全可靠。本试验系统可用于对PE?HD输水管的耐氯氧化性能进行评价。     

热熔焊接工艺参数对聚乙烯管道力学性能的影响

聚乙烯（PE）管道在天然气运输中应用广泛，热熔焊接是连接PE管道的主要方式之一，焊接工艺参数影响热熔接头以及管道整体的性能。文章以焊接温度、加热时间和焊接压力等工艺参数作为主要影响因素，设计正交试验，制备不同的热熔接头拉伸试样，对比分析其抗拉性能。通过数值模拟得到带卷边热熔接头拉伸试样的本构模型。结果表明：焊接温度、加热时间和焊接压力使PE管道的抗拉性能先增高后降低，焊接温度和加热时间对热熔接头的抗拉性能影响显著。PE管道热熔接头的抗拉性能比基材好。载荷-位移曲线与实验曲线较吻合。基于有限元分析和Kwon聚合物本构模型可得到PE管道热熔焊接接头准确的本构关系。     

轨道交通橡胶材料多通道疲劳试验机的研制及应用

针对轨道交通橡胶材料疲劳性能试验，设计了一种新型带有高低温环境箱18通道单轴拉伸疲劳试验机，并与现有橡胶材料疲劳试验方案进行了分析比对，重点应用新型试验机对2种试样进行了3种不同应力水平的拉伸疲劳试验，获取了橡胶材料载荷与疲劳次数曲线。结果表明，新型试验方案结构设计合理，各项功能均满足橡胶材料疲劳寿命预测试验要求；疲劳载荷增大，橡胶材料疲劳次数减小，尤其在高应力水平区变化最为明显，试样2#疲劳性能优于试样1#。     

聚乙烯管材热熔对接接头性能的研究

通过使用两种不同聚乙烯(PE)原料制成的管材热熔对接接头试样进行(5±1)mm/min、(50±1)mm/min恒定速度拉伸试验,阐述了PE管材热熔对接接头的性能低于管材本体;利用对PE管材热熔对接接头试样加热的方法,以及对热熔对接接头试样焊缝四周切口后进行的(5±1)mm/min恒定速度拉伸试验,判定PE管材热熔对接接头的性能,这种方法与其它破坏性试验的统计结果相一致;并介绍了国外对PE管材热熔对接接头质量的评价方法。     

钢塑转换连接结构的研究

对不同结构形式的钢塑转换管件进行了拉伸荷载后密封测试试验。结果表明,钢塑结合处高密度聚乙烯（PE?HD）塑端的厚度对拉伸荷载后密封性能有影响,当结合处PE?HD塑端壁厚减薄越大时,拉伸测试时,越容易在该处发生屈服、拔脱或断裂泄漏;同时钢管端低高度齿形的过盈结构,对于较大口径管件更容易发生拔脱泄漏;采用注塑PE-HD塑端或提高一等级SDR值的加厚PE?HD管材塑端制成的钢塑转换管件优于普通PE-HD管材塑端制成的钢塑转换,前二者的安全裕度更高,此类管件在低温、PE?HD发生蠕变等极限条件下稳定性相对更好。     

高密度聚乙烯管热熔焊接接头本构模型研究

采用实验方法研究高密度聚乙烯（PE?HD）热熔焊接接头区域的材料性能与管材本体的差异。按照ISO 21307标准方法形成热熔焊接接头后,去除内外卷边,设计接头取样方式并改进拉伸试样。通过不同应变率下管材和焊接接头的单轴拉伸试验,使用一种新的本构分析方法与Suleiman方法研究其本构模型差异,得到了模型参数随应变率变化的非线性关系。对比模型预测结果和试验结果,验证了非线性拟合本构模型的有效性和合理性,同时该模型可预测低应变率下（准静态）的应力应变曲线。结果表明,PE?HD焊接区域的弹性模量和屈服应力优于管材,但差别不明显。     

熔体拉伸PE–HD和PE–LLD薄膜的制备及性能

以高密度聚乙烯(PE–HD)和线型低密度聚乙烯(PE–LLD)树脂为原料,采用转矩流变仪,借助熔体拉伸法制备了具有取向结构的PE–HD膜和PE–LLD膜。利用偏光显微镜、傅立叶变换红外光谱、差示扫描量热、小角激光散射及力学性能测试分析不同熔体拉伸速率下PE–HD膜和PE–LLD膜的结构与性能变化情况。结果表明,熔体拉伸速率越高,PE–HD膜和PE–LLD膜的相对取向度越高,快速拉伸PE–HD膜和PE–LLD膜的相对取向度分别为2.043和1.556;熔体拉伸速率对PE–HD膜和PE–LLD膜的结晶温度影响不大,两种膜具有显著的结晶性,PE–HD膜的结晶性更好;随熔体拉伸速率的提高,PE–HD膜和PE–LLD膜的拉伸屈服应力和拉伸弹性模量提高,断裂伸长率降低,总体上看,PE–LLD膜的断裂伸长率较PE–HD膜高,而拉伸强度较PE–HD膜低。     

超高分子量聚乙烯无纬布准静态材料力学性能研究

超高分子量聚乙烯(UHMWPE)无纬布是制作软质防弹衣的主要材料。首先通过实验研究了UHMWPE无纬布在准静态载荷下的拉伸、面内剪切和冲压式剪切特性。实验表明,UHMWPE无纬布在准静态拉伸过程中发生突然的脆性断裂,拉伸强度约为641MPa;面内剪切实验中试件的力-位移曲线呈现明显的非线性,材料在受力方向发生了塑性强化,试件的主要失效形式为基体的剪切破坏,失效后纤维夹角从90°减小到74°;设计制造了一套用于测量UHMWPE无纬布沿厚度方向剪切强度的夹具,实验得到该型无纬布的剪切强度约为168MPa。随后采用壳单元建立对应的有限元模型,对拉伸实验和冲压式剪切实验过程进行数值模拟。拉伸数值结果在最大拉力、拉伸强度和失效应变等指标上与实验结果较一致,并给出了拉伸过程中试件中的应力分布情况;冲压式剪切数值结果给出了剪切失效部位每层单元的应力变化情况。     

基于声发射信号的PE塑料失效PCA预测方法

以聚乙烯(PE)塑料为研究对象,采用声发射监测技术对不同拉伸速度下试样失效过程进行监测,采集相应的声发射信号(持续时间、幅值、事件计数、撞击次数等),通过相关性系数分析确定评价因子,利用三角形隶属函数确定各评语集的失效隶属度,建立了PE塑料失效破坏程度主成分分析(PCA)模糊预测评价模型。另取同型号PE试样进行不同速度拉伸损伤破坏试验,并对损伤过程“力–时间”曲线的弹性阶段、屈服阶段(轻微失效)、颈缩阶段(中度失效)、断裂阶段(严重失效)特征点参数进行统计,破坏过程中所选时间点对应的失效程度与PCA预测评价结果一致,验证了模糊预测评价模型的可行性。     

试样取样与加工对美标金属材料拉伸性能的影响

对几种美国标准的金属材料拉伸试验问题进行了研讨。分析了试样取样方向、取样位置、试样尺寸偏差以及样坯的切取和制备方法对拉伸试验测试数据的影响,可为拉伸试样的加工和提高拉伸试验精度提供参考。     

压扁阻断下聚乙烯管道力学响应的实验研究

采用实验方法系统研究了压扁阻断工艺参数,包括压管速度、松管速度、压缩率、环境温度和管径对聚乙烯（PE）管道力学响应的影响。结果表明,压管过程中PE管道载荷随着压管速度的增大、压缩率的增大、环境温度的降低或管径的增大而增大;卸载过程中PE管道的残余变形随松管速度的增大而增大;维持阶段PE管道的载荷衰减随着压管速度的增大、压缩率的降低、管径的减小或环境温度的降低而增大。     

Macroindentation of polymers

Macroindentation tests were performed by means of a flat cylindrical indenter on high‐density polyethylene (HDPE) and polyamide 66 (PA66). Commercial sheets were acquired for both materials, and specimens were extracted for differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermal recovery tests. In this way, the fabrication effect was correctly identified in terms of degradation, orientation, and mechanical performance. Specimens were also prepared for tensile tests, which were performed for comparison with indentation experiments. Indentation tests were carried out on the external sheet surfaces at different indenter diameters (1 and 2 mm) and test rates (from 0.1 to 20 mm/min). A very high correlation was found between the tensile test and indentation results at the different test rates. In particular, the indentation stress at fixed depth was a linear function of the tensile yield stress, depending only on the indenter diameter and not on the material type. Moreover, a threshold value for the thickness for a correct comparison among samples having different dimensions is discussed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Mechanical and open hole tensile properties of self‐reinforced PET composites with recycled PET fiber reinforcement

The tensile strength of notched composites is an important factor for composite structural design. However, no literature is available on the notch sensitivity of self‐reinforced polymer composites. In this study, self‐reinforced recycled poly (ethylene terephthalate) (srrPET) composites were produced by film stacking from fabrics composed of double covered uncommingled yarns (DCUY). Composite specimens were subjected to uniaxial tensile, flexural, and Izod impact tests and the related results compared with earlier ones achieved on srPET composites reinforced with nonrecycled technical PET fibers. Effects of open circular holes on the tensile strength of srrPETs were studied at various width‐to‐hole diameter (W/D) ratios of the specimens. In the open hole tensile (OHT) measurements bilinear (yielding followed by post‐yield hardening) stress–strain curves were recorded. The srrPET composites had extremely high yield strength retention (up to 142%) and high breaking strength retention (up to 81%) due to the superior ductile nature of the srrPETs, which induces plastic yielding near the hole thereby reducing the stress concentration effect. The results proved that srrPET composites are tough, ductile notch‐insensitive materials. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43682.     

Deep stretch of polyethylene by transverse loading: Finite element simulation to characterize the influence of indenter size and loading speed on the stress development and distribution

Deep stretching has been found to decrease the cracking resistance of polyethylene (PE) in an aggressive environment. This idea has been adopted for developing a test method that uses indentation loading to generate a deep stretch of a PE plate so that time for crack generation is shortened. Work presented is to use finite element (FE) modeling to investigate the influence of indenter size and loading speed on the efficacy of the test method. Mechanical testing was carried out using cylindrical indenters of 7 and 13 mm in diameter on a plate that is supported so that only a 15-mm diameter area can be stretched. Test results were used to calibrate material input data for the FE modeling and to establish the stress development and distribution during the deep stretch process. FE modeling considered three types of material input, one purely based on elastic–plastic deformation and the other two considering creep or damage during the deep stretch. All FE modellings suggest that the 13-mm indenter is more effective than the 7-mm indenter in introducing a monotonic total stress increase during the deep stretch, and thus the study concludes that the former should be used to introduce the deep stretch.     

Investigating tensile behaviour of toughened epoxy paste adhesives using circumferentially notched cylindrical bulk specimens

Toughened epoxy adhesives are frequently used to bond metals and polymer-matrix composite materials in many structural applications. The mechanical properties of adhesives are often characterised by testing either bulk adhesive specimens or bonded joints (i.e. in-situ form). In this paper, cylindrical bulk specimens with circumferential notches were manufactured and tested to investigate the tensile behaviour of an epoxy paste adhesive toughened with hollow glass microspheres. Bulk specimens were manufactured from the paste adhesive using injection moulding. Tensile tests were conducted for strain-rate and stress triaxiality effects by varying displacement rates and notch radii, respectively. Fracture surfaces were examined using optical and scanning electron microscopy to identify failure mechanisms. The results obtained from the toughened paste adhesive indicate that strain-rate and stress triaxiality influence its tensile fracture behaviour.     

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.     

Effect of notches and glass fiber reinforcement on fatigue behavior of polycarbonate

In order to investigate the effect of a notch on the tensile properties of polycarbonate and 30% glass fiber-reinforced polycarbonate, two types of notched specimens were prepared. These notches were a sharp 60° notch and a dull notch with rounded tip 1.5 mm in radius at the base of the 60° notch. The notches decreased the tensile strength of polycarbonate. The sharp notch reduced tensile strength more effectively than the dull notch. In 30% glass fiber-reinforced polycarbonate, even the dull notch decreased the tensile strength considerably. Unnotched polycarbonate was subjected to cyclic tensile loading of 10⁴ cycles at 10 Hz, with varying cyclic stress. It was found that the elongation at break decreased rapidly with increase in cyclic stress. The notches considerably decreased the tensile fatigue strengths of polycarbonate and glass fiber-reinforced polycarbonate in 10⁴ cycles at 10 Hz.     

The bending of polymers under tension as related to a deep drawing process

Two engineering plastics, an acrylonitrile-butadiene-styrene resin (ABS resin, Cycolac MS) and a rigid polyvinyl chloride resin (PVC resin, Dacovin 2082) were investigated to determine the effects of cold working. In particular, the “bending under tension” stress system of a deep drawing process was considered. The object was to determine the effect of the blank-holder pressure, the ratio of the punch radius to the material thickness, the speed of deformation and the temperature of deformation. The stress-strain characteristics of the deformed and undeformed material were markedly different. Upper yield points were lower in the deformed specimens, the tensile strengths were decreased by 10 to 15% and the %-elongation at break was higher—up to double the values for underformed specimens. An analysis indicated that these effects can be attributed to non-homogeneous yielding. The results also indicated that crazing plays an important role in permitting an ABS material to permanently conform to a radius with no geometric constraints.     

Study of Crack Propagation in PMMA Using Ultrasonic Technique

Attenuation of ultrasonic waves was measured in different notched specimens of Poly(methylmethacrylate) (PMMA) at different points of crack propagation path. Crack propagation takes place when specimens, with initial single edge notch produced using sharp milling cutter, are subjected to different crosshead speeds of a tensile testing machine. These measurements were performed on specimens having different thicknesses by using pulse echo technique at different frequencies. Attenuation measurements results were confirmed by microscopic images.

The present study showed that attenuation increases when crack propagation speed increases as a result of an increment of crosshead speed of the applied load. Such relation is not affected by the specimens' thickness. The resulting attenuation curves elucidated the neck zone of the crack.