Abstract

In bubble assisted vacuum thermoforming, measuring pressure-induced mechanical strains through the stereo-digital image correlation (stereo-DIC) technique while shaping thermoplastic sheet requires consideration of an appropriate reference state of surface deformations. However, when the stereoscopic measurements can be only performed after the heating step, the correlation problem should be well-posed otherwise the reliability of results is limited. This study focuses on stretching by bubble inflation processes following thermal warpage and sagging of initially flat sheets. For this purpose, an experimental rig is instrumented to heat high impact polystyrene (HIPS) sheets and to perform synchronized pressure and stereoscopic measurements during 1.5 s stretching. A two-step method is introduced to separate mechanical strains which are affected by the uncontrollable change of initial conditions from the global stereo-DIC strains. The first step relies on amplification of damped oscillations at the initiation of the inflation process due to sagging. Out-of-plane displacements confirm the existence of a temperature-dependent characteristic time that marks the transition from the sagged to the strained surface shapes. The second step uses these characteristic times to objectively shift the reference of image-correlation computations. To evaluate the effectiveness of the suggested method, inaccuracy levels of global strains are evaluated at a fixed pressure level under different thermal conditions. It is shown that inaccuracy levels are the highest when stereo-DIC measurements followed warpage and they decrease with amplification of sagging. The developed approach extends the use of in-situ stereo-DIC measurements when changes of initial conditions are uncontrollable and thermal strains cannot be measured.

中文翻译：

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气泡辅助真空热成型：应考虑将原位立体DIC测量的使用范围扩展到下垂的热塑性片材的拉伸

摘要

在气泡辅助真空热成型中，在成型热塑性片材时，通过立体数字图像相关（stereo-DIC）技术测量压力引起的机械应变需要考虑表面变形的适当参考状态。但是，当只能在加热步骤之后执行立体测量时，相关问题应摆在适当位置，否则结果的可靠性将受到限制。这项研究的重点是通过热变形和初始平板下垂后的气泡膨胀过程进行拉伸。为此，使用一台试验设备来加热高抗冲聚苯乙烯（HIPS）板材，并在1.5 s的拉伸过程中执行同步的压力和立体测量。引入了两步法来将受初始条件的不可控制变化影响的机械应变与全局立体DIC应变分开。第一步依赖于由于下垂而在膨胀过程开始时衰减阻尼振荡。平面外位移确认存在温度相关的特征时间，该特征时间标志着从下陷到应变表面形状的过渡。第二步使用这些特征时间来客观地移动图像相关计算的参考。为了评估所建议方法的有效性，在不同热条件下以固定压力水平评估整体应变的不准确水平。结果表明，当立体DIC测量值随翘曲而变时，不准确度最高，并且随着下垂的放大而降低。当初始条件的变化不可控制且无法测量热应变时，开发的方法扩展了原位立体声DIC测量的使用。