High temperature experiments were performed using the same load frame and a temperature-controlled chamber (Instron Model 3119-405). Contact surfaces between the specimens and the tungsten carbide platens were lubricated with MoS 2 grease to minimize resistance to radial expansion during compression. The cylindrical specimens for these experiments were nominally 5 mm in diameter and length. Load was measured with an Instron 311 G-135 load cell, and specimen strain was inferred from a linear variable differential transformer (LVDT) measurement of crosshead displacement, using a correction for machine compliance. Low rate tests were performed with an Instron model 1331 servo-hydraulic load frame. The test was performed at 10 ☌/min to 200 ☌ to determine the degree of crystallinity and melting temperature (T m) of the material.Ĭompression tests were conducted across a range of strain rates from quasi-static to dynamic. The LDPE material was characterized using differential scanning calorimetry (DSC) using a TA Instruments DSC Q2000 on approximately 15 mg of material. The majority of the samples were machined in the through-thickness direction, with a few experiments conducted in the two orthogonal directions. The density of the material was measured as 924.2 kg/m 3. The LDPE was obtained in plate form from Allied Resinous Products, Inc. The experimental results on LDPE are presented in this paper and discussed in the context of similar PE material conformations. This study complements the work by Brown et al. The degree of crystallinity in the LDPE was determined so that the data could be compared with similar materials in the literature. In this study, LDPE was characterized in compression across a range of strain rates and temperatures, using quasi-static loading, split Hopkinson pressure bar loading, and Taylor impact experiments. However, the simple linear relation is also reported by Nakai and Yokoyama, who illustrated the dramatic bilinear dependence on log strain rate in many other polymers. observed a non-linear increase in stress with log strain rate albeit based on data at a limited number of strain rates. observed a single linear relationship between flow stress and log strain rate and a linear relationship with temperature from room temperature to −100 ☌. for HDPE with 60.99 % crystallinity for a given temperature and strain rate. with 80.9 % crystallinity exhibited flow stress two to three times higher than reported by Omar et al. However, the actual strength values differed greatly between the two sources, probably due to the almost 20 % difference in crystallinity between the two HDPE materials the HDPE investigated by Brown et al. studied LDPE, HDPE, and linear low density polyethylene (LLDPE), in which HDPE exhibited the highest strength in agreement with Brown et al. The same materials have also been studied under a range of loading conditions including shock loading and dynamic tensile extrusion. studied the effects of conformation on HDPE, UHMWPE, and PEX across a range of strain rates and temperatures and found that UHMWPE and PEX had very similar behavior that differed noticeably from HDPE. There have been a few studies in the literature which have investigated the high rate mechanical response of varying PE conformations. These materials can be thought of as molecular networks consisting of an amorphous phase containing entangled chains with the randomly oriented crystallite phase acting as physical cross-links. In semi-crystalline materials, like polyethylene and polytetrafluoroethylene, the response of the material depends on molecular conformation and volume fraction of crystallinity, in addition to temperature and strain rate. The high rate properties of polymers, including time–temperature superposition in these materials, was recently reviewed by Siviour and Jordan. Taylor impact experiments were conducted showing a double deformation zone and yield strength measurements in agreement with compression experiments. based on time–temperature superposition using a single mapping parameter indicating that there are no phase transitions over the rates and temperatures investigated. The temperature and strain rate data were mapped using the method developed by Siviour et al. A single linear dependence was observed for flow stress on temperature and log strain rate over the full range of conditions investigated. The mechanical response was found to be temperature and strain rate dependent, showing an increase in stress with increasing strain rate or decreasing temperature. In this paper, the compressive response of low density polyethylene (LDPE) was investigated across a range of strain rates and temperatures. The mechanical properties of polymers, particularly as a function of temperature and strain rate, are key for implementation of these materials in design.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |