Despite being a major cause of failure of polymer products under service, environmental stress cracking (ESC) is still a phenomenon poorly investigated and, as consequence, not fully understood. This article aims to contribute to this field in a study of the stress cracking behavior of polybutylene terephthalate (PBT) exposed to ethanol (a common fluid, used in several daily situations), by means of two types of mechanical tests (tensile and stress relaxation). During mechanical testing the technique of acoustic emission (AE) was applied to identify certain mechanisms of internal failure. As a non-destructive technique, AE is widely used with metals to monitor the integrity of components but it is very scarcely applied to polymers. The article, therefore, innovates by using AE as a tool to better understand the ESC behavior of an engineering polymer produced under different conditions. The PBT samples were prepared by injection molding, using different mold temperatures (20°C, 40°C, and 80°C), resulting in different crystallinities. The results showed that the tensile properties were highly dependent on the crosshead speed and that ethanol affected the mechanical performance of the materials, anticipating the necking, but its evaporation during the experiment reduced the actual temperature on the sample bar. This latter effect caused an unexpected behavior, with an increase in tensile strength and reduction of stress relaxation rate, the opposite trends that would be expected from a stress cracking agent. The crystallinity of PBT had some influence on the results, with the more crystalline samples being more resistant to ESC, as observed by both stress relaxation and AE analyses. The technique of AE showed to be a very useful tool to understand the mechanical behavior and to differentiate the various testing conditions, with the detection and quantification of hits and released energy, related to the several stages of deformation and failure, including the formation of shear bands, nucleation and growth of crazes and cracks, and so forth. The fracture surface analyses by scanning electron microscopy were very consistent with the AE experiments, in which rough topography were related to high acoustic activities.

中文翻译：

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声发射法研究PBT的应力开裂行为

尽管环境应力开裂 (ESC) 是聚合物产品在使用中失效的主要原因，但它仍然是一种研究不足的现象，因此尚未完全了解。本文旨在通过两种类型的机械测试（拉伸和应力松弛）研究聚对苯二甲酸丁二醇酯 (PBT) 暴露于乙醇（一种常见流体，用于多种日常情况）的应力开裂行为，从而为该领域做出贡献。 ）。在机械测试期间，应用声发射 (AE) 技术来识别某些内部故障机制。作为一种非破坏性技术，AE 广泛用于金属以监测组件的完整性，但很少应用于聚合物。因此，文章 通过使用 AE 作为工具进行创新，以更好地了解在不同条件下生产的工程聚合物的 ESC 行为。PBT 样品是通过注塑成型制备的，使用不同的模具温度（20°C、40°C 和 80°C），从而产生不同的结晶度。结果表明，拉伸性能高度依赖于十字头速度，乙醇影响材料的机械性能，预计会出现颈缩，但在实验过程中其蒸发降低了样品棒上的实际温度。后一种效应引起了意想不到的行为，拉伸强度增加，应力松弛率降低，这与应力开裂剂所预期的趋势相反。PBT的结晶度对结果有一定影响，正如应力松弛和 AE 分析所观察到的那样，结晶越多的样品对 ESC 的抵抗力越强。AE 技术被证明是一种非常有用的工具，可以了解机械行为并区分各种测试条件，检测和量化与变形和失效的几个阶段相关的碰撞和释放的能量，包括剪切的形成带、裂纹和裂纹的成核和生长等。扫描电子显微镜的断口分析与 AE 实验非常一致，其中粗糙的地形与高声学活动有关。AE 技术被证明是一种非常有用的工具，可以了解机械行为并区分各种测试条件，检测和量化与变形和失效的几个阶段相关的撞击和释放的能量，包括剪切的形成带、裂纹和裂纹的成核和生长等。扫描电子显微镜的断口分析与 AE 实验非常一致，其中粗糙的地形与高声学活动有关。AE 技术被证明是一种非常有用的工具，可以了解机械行为并区分各种测试条件，检测和量化与变形和失效的几个阶段相关的碰撞和释放的能量，包括剪切的形成带、裂纹和裂纹的成核和生长等。扫描电子显微镜的断口分析与 AE 实验非常一致，其中粗糙的地形与高声学活动有关。