The strain-rate, temperature and pressure dependence of yield of isotropic poly(methylmethacrylate) and poly(ethylene terephthalate) (original) (raw)
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Phenomenological description of strain rate and temperature-dependent yield stress of PMMA
Journal of Applied Polymer Science, 1995
A constitutive equation to describe the yield behavior of poly(methyl methacrylate (PMMA) is useful not only from the technological point of view, but also for the comprehension of the nonlinear mechanisms acting in the material. In both compression and tension, the yield stress is usually represented as a function of the strain rate at different temperatures. In PMMA and other glassy polymers these curves are related by scaling, that is, they can be matched to form a master curve. Particularly in PMMA the temperature and strain rate dependence of the master curve has been characterized by two different models. The first involves two thermally activated rate processes, one acting only at high strain rates. The second model interprets the yield process as a cooperative movement of several independent structural units, all with the same activation energy. In this article it is demonstrated that only the second phenomenological model is correct because it provides a good fit to the master curve of PMMA both in compression and tension, and verifies the properties of a set of curves related by scaling. Moreover, it is pointed out that the first model leads to severe inconsistencies because it does not consider the nonlinear behavior of PMMA. Finally, the physical parameters obtained (internal stress, activation volume, and enthalpy) are compared with those given in the literature. © 1995 John Wiley & Sons, Inc.
Uniaxial compression stress–strain tests were carried out on three commercial amorphous polymers: polycarbonate (PC), polymethylmethacrylate (PMMA), and polyamideimide (PAI). The experiments were conducted under a wide range of temperatures (À40 °C to 180 °C) and strain rates (0.0001 s À1 up to 5000 s À1). A modified split-Hopkinson pressure bar was used for high strain rate tests. Temperature and strain rate greatly influence the mechanical response of the three polymers. In particular, the yield stress is found to increase with decreasing temperature and with increasing strain rate. The experimental data for the compressive yield stress were modeled for a wide range of strain rates and temperatures according to a new formulation of the cooperative model based on a strain rate/temperature superposition principle. The modeling results of the cooperative model provide evidence on the secondary transition by linking the yield behavior to the energy associated to the b mechanical loss peak. The effect of hydrostatic pressure is also addressed from a modeling perspective.
2012
The goal of this paper is to investigate the strain-rate and temperature sensitivity of PMMA (polymethylmethacrylate) polymer through uniaxial compression tests under quasi-static and dynamic loading at room temperature. The experiments were conducted with an universal testing machine coupled to a climatic chamber for a range of strain rates and temperatures varying respectively from of 0.001 s À1 to 0.1 s À1 and 213-378 K. Moreover, dynamic behaviour of the material at high strain rate from 700 to 4000 s À1 was investigated using a Split Hopkinson Pressure Bar (SHPB) technique. At strain rate equal or higher than 0.1 s À1 the thermal softening effect due to adiabatic heating cannot be neglected and a correction procedure was used to take it into account. According to quasi-static and dynamic experimental results, the compressive behaviour of PMMA is greatly influenced by the applied strain rate and initial temperature; indeed, an elastic brittle or quasi-brittle response is observed at high strain rates or low temperatures whereas a ductile behaviour is obtained otherwise. The secondary relaxation temperature corresponding to the rotation of the lateral ester group was used to explain this brittle-ductile transition. Finally, the obtained experimental results were compared to the analytical predictions provided by the cooperative model proposed in the literature. Based on experimental results a new constitutive model was also proposed to describe the strain rate and temperature sensitivity and the strain hardening or softening behaviour observed during experiments.
The macroscopic yield behaviour of polymers
Journal of Materials Science, 1973
A yield criterion, not previously compared with the actual macroscopic behaviour of polymers, is herein compared with the pressure-modified octahedral shear stress criterion earlier suggested by others.This new relation, which is a version of the von Mises criterion, accommodates differences in tensile and compressive yield strengths and accounts for any dependence of yielding on the hydrostatic component of the applied stress state. With the use of thin-wall tubes accounting for the majority of experimental points, the yield behaviour of polycarbonate and polyvinylchloride was investigated. Besides these findings, results previously reported by others have also been utilized in this paper. Since these various studies employed quite different polymers, the excellent overall correlation of experiment with prediction should merit the serious attention of persons interested in the macroscopic yield behaviour of polymers. Comparisons between this new criterion and the modified octahedral shear stress are also made in regard to the effect of pressure on subsequent yield behaviour. Although not fully verified, it is suggested that the predictions which result using this new criterion, seem a little more reasonable.
Pressure dependent yield criteria for polymers
Materials Science and Engineering, 1974
It is difficult to distinguish among them using the type of experiments that produce data used in two-dimensional plots of yield loci. This is due to the fact that the maximum range of values of mean normal stress is relatively small in such experiments. Marked differences between these criteria do occur however as the hydrostatic pressure or mean stress is altered substantially. Experiments that show the effect of applied pressure on tensile and/or compressive yield strength provide one means for describing such differences. This paper considers two forms of a pressure modified yon Mises criterion and shows a comparison with available experimental information.
1983
Investigation of polymer properties under a hydrostatic loading assumes major significance, since pressure alters the character and mechanisms of the deformation processes, gives rise to transformations, and affects the melting point and other material characteristics [1]. Scientifically based utilization of these effects has expanded the sphere of technical applications of polymers and contributed to the solution of a number of technological problems, for example, to the production of high-strength materials by methods of hydroextrusion .
Journal of Materials Research and Technology, 2012
Conventional yield criteria for ductile materials, such as Tresca and von Mises, predict that yielding is independent on the hydrostatic stress state (pressure), which means that tensile and compressive stress-strain behaviors are considered equal and are equally treated. This approach is reasonable for ductile metallic materials but sometimes inaccurate for polymers, which commonly present larger compressive yield strength, therefore being characterized as uneven. Some pressure dependent theories are available, but there is no consensus concerning the choice of the most appropriate criterion, its use and bene ts. As a step in the direction of improving structural integrity practices taking advantage of unevenness, this work performs three key-activities: i) rst, a critical review about existing theories and its accuracy; ii) second, a series of experiments under tension and compression including four selected polymers (PA-66, PA-6, PP, and HDPE) to assess real unevenness levels; iii) third, a numerical evaluation of the potential bene ts of using modi ed criteria. Stress states, safety, and stiffness were evaluated for a typical application to illustrate the proposals. Mass reductions up to 39% could be achieved even with simple geometric changes, while keeping original safety and stiffness levels.
The effect of hydrostatic pressure on the shear yield behaviour of polymers
Journal of materials Science, 1970
The torsional stress-strain behaviour of isotropic poly(methylmethacrylate) (P M M A), poly(ethylene terephthalate) (P E T) and polyethylene has been studied under hydrostatic pressures up to 7 kbar.~ In P M MA the following important features were observed. First, there is a monotonic increase in the initial slope of the stress-strain curve with increasing pressure. Secondly, there is a substantial increase in the yield stress and the strain to yield as pressure is raised. Thirdly, there is a transition in the mode of failure at elevated pressure, the specimens fracturing in the high pressure region before a drop in stress occurs. Finally, in the high pressure region the fracture stress increases with increasing pressure but the strain at fracture decreases. The observed yield behaviour can be represented formally in a number of ways, and the results will therefore be discussed accordingly, in an attempt to give a general yield criterion for P M M A. The fracture behaviour has been analysed in terms of the Grimth ideas for fracture of glassy materials, and this will also be discussed. The results for poly(ethylene terephthalate) (Arnite) differ significantly from those for P M M A. Specimens of Arnite as received from the manufacturers were ductile in torsion at atmospheric pressure, and the torsional yield stress rose monotonically with increasing hydrostatic pressure. Annealing the specimens produced embrittlement at atmospheric pressure, but on testing under conditions where there is no tensile component of stress (i.e. at very low hydrostatic pressures) ductile behaviour was observed. The contrast between P M M A and Arnite suggests that in the former case there are surface flaws which are penetrated by the hydraulic fluid at high pressures, whereas in the latter case internalflaws are produced by annealing. Polyethylene remained ductile over the complete pressure range, with a pressure dependence of the tensile yield stress which was similar to that shown by polyethylene terephthalate.
Key Engineering Materials, 2010
The effect of strain rate on the mechanical behavior of thermoplastic polymers (Polymethyl methacrylate, Polycarbonate and Polyamide 66) has been studied. Deformation tests in tension were conducted over the range of strain rate varying between 2.6 10 -4 s -1 to 1.3 10 -1 s -1 . The Young's Modulus E and Yield stress σ ST evolutions have been identified and modelled as a function of the strain rate. It has been established that, in the range of the considered strain rates, the yielding behavior of PMMA and PC is well described by the Eyring theory while for PA66 the Ree-Eyring theory is successfully used to illustrate the yielding behavior. During tensile tests the specimen surface temperatures were monitored using an infrared camera. Results reveal a significant temperature rise at large deformations for PA66 and PC. As the strain rate increases the temperature is steadily increased with deformation due to plastic work. Hence, for PC and PA66, a significant thermal softening is observed after yielding which affects the stress-strain behavior. Thermomechanical coupling during polymer deformation can be considered in the modeling of the mechanical behavior of polymers. No self-heating has been detected for PMMA.
Journal of Polymer Research, 2014
The mechanical behavior of polycarbonate (PC) polymer was investigated under the effect of various temperatures and strain rates. Characterization of polymer was carried out through uniaxial compression tests and split Hopkinson pressure bar (SHPB) dynamic tests for low and high strain rates respectively. The experiments were performed for strain rates varying from 10 −3 to 10 3 and temperature range of 213 to 393 K. By conducting these experiments, the true stressstrain (SS) curves were obtained at different temperatures and strain rates. The results from experiments reveal that the stress-strain behavior of polycarbonates is different at lower and higher strain rates. At higher strain rate, the polymer yields at higher yield stress compared to that at low strain rate. At lower strain rate, the yield stress of the polymer increases with the increase in strain rate while it decreases significantly with the increase of temperature. Likewise, initial elastic modulus, yield and flow stress increase with the increase in strain rate while decreases with the increase in temperature. The yield stress increases significantly for low temperature and higher strain rates. On the basis of experimental findings, a phenomenological constitutive model was employed to capture the mechanical behavior of polymer under temperature and loading rate variations. The model predicted the yield stress of polymer at varying strain rate and temperature also it successfully predicted the compressive behavior of polymer under entire range of deformation.