The plastic deformation of drawn polyethylene was extensively investigated in the 1960s by Peterlin et al. [1, 2] and their results were obtained mainly from a microscopic point of view. It was the microscopic aspects of the polyethyleneÐfor example, the lamellar structure and the formation of micro®brilsÐthat were probed, which excluded the in-situ observation of the drawing process. In recent years, the mesoscopic views were successfully introduced to analyze or simulate the structure±property relationship in metallic physics [3, 4]. When compared to the microscopic structure, the mesoscopic structure can more directly re ect the material's mechanical property, but, to our knowledge, the mesoscopic analysis has still not been used in the ®eld of polymer physics. As in the case of drawn polyethylene, with existing technology such as the television±optics measuring system complex (TOMSC) used in our work, the drawing process can be directly observed at a mesoscopic level. With systematic analysis, it will be possible to forecast the material's performance during drawing. There exist both crystalline and amorphous domains in partially crystalline polyethylene. Strong elastic interaction between amorphous and crystalline phases is expected in the loaded state. Such interaction will lead to the movement of crystalline phase as a whole, which corresponds to the deformation at a mesoscopic level. Therefore, analysis of such material's response to mechanical loading could be performed from the standpoint of physical meso-mechanics of structurally non-uniform materials [5]. A partially crystalline polyethylene, which is used as cable isolation material at Shenyang Cable Plant (People's Republic of China), was investigated to increase the practical value of this work. This material initially contains about 50% of crystalline phase with an orthorhombic lattice, the parameters of which are a 0:728 nm, b 0:502 nm and c 0.252 nm. The working part of the specimens for tensile tests was 20 mm in length, 4 mm in width and 1.5 mm in height. Tensile experiments were conducted on a WD-(10, 50, 100)E apparatus with deformation rates of 10, 15, 20 and 25 mm miny1, respectively. To investigate the in uence of deformation on the materials' structure, specimens at different deformation stages were ®xed at a deformed state with a special frame. X-ray analysis was then performed using a Rigaku-20 device. The crystalline degree and the orientation degree of the partially crystalline polyethylene were then computed from the X-ray pro®les. Some specimens ®xed at deformed stages were cooled in liquid nitrogen and then subjected to impact testing. The specimens were fractured along or across their axis. Fractographic investigations of the fracture surfaces, both parallel to and perpendicular to the axis of specimen, were carried out on an X-650 scanning electron microscope (SEM). To investigate the structural changes after irradiation, specimens were irradiated with a high-energy electron beam. For this purpose, a 20 kW IAU6MCE2 electron accelerator was used. The energy of the electrons was in the range of 1.0±2.5 MeV. The radiation dose varied from 5 to 40 MRad. An IMASH-2078 testing machine equipped with TOMSC was used to investigate the deformation mechanisms at the mesoscopic level. TOMSC can obtain images of displacement vector ®elds on the specimen surface by recording the movements of individual points within a small area [6]. The results of mechanical testing are presented in Fig. 1 as the stress±strain curves under different deformation rates. All these curves look like the stress±strain curves for metallic materials with blocked dislocations, for example, low carbon steel. Each curve can be divided into three parts that correspond to different deformation stages. The ®rst part corresponds to the stage of elastic deformation. The second one is practically at. This part of the curves corresponds to the stage of plastic deformation. The plastic deformation begins locally (necking) and then propagates sequentially all over the working length of the specimen (like a Luders band in metallic materials, for example, in low carbon steel). Some crystalline phase grows from the
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Aug 7, 2002
Alexander Epishin + 3
Open Access