Design of the novel tensile creep experimental setup, characterisation and description of the long-term creep performance of polycarbonate
Design of the novel tensile creep experimental setup, characterisation and description of the long-term creep performance of polycarbonate
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DOI：
10.1016/j.polymertesting.2019.01.023
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发表时间：
2019-05-01
期刊：
影响因子：
5.1
通讯作者：
Johlitz, Michael
中科院分区：
文献类型：
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作者：
Jorik, Stefan;Lion, Alexander;Johlitz, Michael
实验研究热塑性材料复杂的长期行为对于可靠的寿命预测模型至关重要。由于材料具有明显的应力和温度依赖性，测量结果对加载条件下的不准确性非常敏感。为此，设计了一种试验机，可以在精确的温度条件下进行长期蠕变测量。在下面，我们将首先描述样品的制备，然后详细解释内部设计的试验机的结构，特别是测试条件的优化。为了在高达170℃的恒温条件下获得不同应力水平下聚碳酸酯（PC）的可靠长期蠕变数据，共构建了5个高精度测量单元，并将测量结果与TA Instruments的商用试验机Q800的测量结果进行了比较，从而验证了测量精度。用于比较的试验是在线性粘弹性范围内进行的，该范围对温度控制和测量精度的要求特别高。测量曲线之间的偏差小得可以忽略不计。接下来，研究了不同载荷条件下聚碳酸酯在40℃、60℃、80℃和100℃等恒定温度下的应力依赖蠕变行为，并对测量结果进行了时间和应变的评估，发现裂纹对蠕变有显著影响。在聚碳酸酯中可以观察到三种基本蠕变过程的应力和温度依赖性。提出并验证了基于Eyring流动理论的单一活化体积下的屈服模型。三级蠕变阶段的加速蠕变进程不表现出韧性-脆性转变，因此可以用经验各向同性损伤变量概念来描述，该概念完全符合实验数据，包括韧性断裂。将所提出的概念应用于材料模型的弹簧单元，可以准确地再现PC的三级蠕变行为。
Experimentally investigating the complex long-term behaviour of thermoplastic materials is crucial for reliable lifetime prediction models. Due to the material's pronounced stress- and temperature-dependence, the measurement results are extremely sensitive to inaccuracies in the loading conditions. For this reason a testing machine was designed that allows long-term creep measurements under exact temperature conditions.In the following, we will first describe the sample preparation and afterwards explain the construction of the testing machine designed in-house in detail, especially the optimisation of the test conditions. A total of five high-precision measuring cells were constructed in order to obtain reliable long-term creep data for polycarbonate (PC) at different stress levels under constant temperatures of up to 170 degrees C. The measurement results were then compared with those of the commercially used testing machine Q800 from TA Instruments, thus validating the measurement precision. The tests used for the comparison were conducted in the linear viscoelastic range, where the requirements for temperature control and measuring accuracy are particularly high. The deviations between the measurement curves turned out to be negligibly small.In the next step, the stress-dependent creep behaviour of polycarbonate was investigated under different load conditions and constant temperatures of 40 degrees C, 60 degrees C, 80 degrees C and 100 degrees C. The measurement results were evaluated with respect to time and strain and show a significant influence of crazing. The stress- and temperature-dependence of the three fundamental creep processes could be observed in polycarbonate. A yield model based on Eyring's flow theory with a single activation volume is proposed and validated. The accelerated creep progression in the tertiary creep stage exhibits no ductile-brittle transition and can therefore be described with an empirical isotropic damage variable concept that fits the experimental data exactly, including ductile fracture. The proposed concept, which is applied to spring element of the material model, can accurately reproduce the tertiary creep behaviour of PC.