Background: The strength of materials under extreme dynamic loading conditions, such as in the case of shock wave loading, is assessed from their spallation characteristics. Under laboratory conditions, flyer plate impact, or sometimes laser-induced stress waves, is employed to instigate spall in a material. These methods are often combined with velocity interferometer system for any reflector (VISAR) technique for performing transient measurements. Although the VISAR can record the velocity of extremely fast-moving surfaces, it requires a complex optical setup and a specialized data reduction technique. Objective: In this study, a simpler approach is adopted by extending laser spallation method to determine the spall strength of epoxy, while performing in situ interferometric measurements, directly on top of thick epoxy films. Methods: The glass/epoxy test samples are prepared by transferring an aluminum coating on top of epoxy layers with different thicknesses. Laser-induced stress waves transmit across the substrate/film interface and induce subsurface failure in the epoxy at sufficiently high incident laser energy. The nature and magnitude of the waves are deciphered from the out-of-plane displacement histories of the top reflective sample surfaces, which are recorded by using a Michelson interferometer. Results: The interferometric data reveal the development of two (temporally) well-separated stress waves: an ablation-induced high-amplitude short-duration longitudinal pulse, which is referred to as the primary wave, and a secondary wave, which travels at a comparatively slower speed. The complex constructive interaction of the two waves develops a high-magnitude tensile stress region in the epoxy layer. The spall strength is quantified by superimposing the two stress wave histories associated with the critical energy fluence. Conclusions: The spall depths predicted from spatiotemporal wave travel analyses are in excellent agreement with the experimental observations. The newly adopted methodology estimates the spall strength of epoxy as 260 ± 20 MPa.

中文翻译：

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通过激光散裂表征环氧树脂中的散裂特征

背景：材料在极端动态载荷条件下的强度，例如在冲击波载荷的情况下，是通过它们的散裂特性来评估的。在实验室条件下，飞板撞击，或有时是激光诱导的应力波，被用来引发材料的剥落。这些方法通常与任何反射器的速度干涉仪系统 (VISAR) 技术结合使用，以执行瞬态测量。尽管 VISAR 可以记录极快移动表面的速度，但它需要复杂的光学设置和专门的数据简化技术。目标：在本研究中，采用一种更简单的方法，通过扩展激光散裂方法来确定环氧树脂的散裂强度，同时直接在厚环氧树脂膜上进行原位干涉测量。方法：玻璃/环氧树脂测试样品是通过将铝涂层转移到不同厚度的环氧树脂层上来制备的。激光引起的应力波穿过基板/薄膜界面，并在足够高的入射激光能量下引起环氧树脂的亚表面失效。波的性质和幅度是从顶部反射样本表面的平面外位移历史中破译的，这些历史是使用迈克尔逊干涉仪记录的。结果：干涉测量数据揭示了两个（时间上）分离良好的应力波的发展：消融引起的高振幅短持续时间纵向脉冲，称为主波，和次波，其传播速度为速度相对较慢。两个波的复杂相长相互作用在环氧树脂层中形成了一个高强度的拉伸应力区域。通过叠加与临界能量注量相关的两个应力波历史来量化剥落强度。结论：从时空波传播分析预测的剥落深度与实验观察非常吻合。新采用的方法估计环氧树脂的剥落强度为 260 ± 20 MPa。