DIC Test - Quantitative Image Analysis of Bird Strike Test - In Collaboration with Saint-Gobain

Author:N. Cordero, F. Fremy, M.Beneck for Saint-Gobain, F. Mathieu for EikoSim

Keywords: digital image correlation, bird strike test,non-contact strain measurement, composite material performance testing,

The aircraft radome produced by Saint-Gobain is made of composite materials. In addition to the usual design requirements (brightness, aerodynamic shape, impact resistance) that these structures must comply with, they also need to have radio frequency transparency so that the systems they shield can operate normally. This last point greatly limits the number of applicable materials and imposes specific designs on the composites.

To ensure satisfactory mechanical performance under these constraints, optimizing the radome structure and assembly design is crucial. Furthermore, the rapid development of embedded communication systems requires frequent design updates based on reliable and efficient analytical tools. Material property testing is very important and needs to be conducted at the actual loading rates expected in application. Assemblies with the aircraft fuselage also need to be characterized and validated through mechanical testing. The goal of this work is to optimize this critical area to meet qualification standards and ensure optimal structural performance in flight.

To study impact resistance, impact tests are typically conducted at speeds close to those of aircraft landing or takeoff (approximately 150 m/s): these bird strike tests are well established in the industry and can be conducted using birds or gel-like substitutes. Due to the relatively high impact speeds, many testing centers have employed high-speed cameras (with capture rates ranging from 100 frames per second to several thousand), which are important diagnostic tools for establishing structural damage and fracture patterns. These images, along with more traditional sensors (lasers, strain gauges), serve as qualitative supplementary information, which Saint-Gobain engineers rely on for quantitative analysis.

Figure 1 – Test setup of a bird strike test on a composite material (approximate size: 80 x 50cm)

The drawback of these traditional sensors is that they only produce localized, dispersed information, making it difficult to correlate with numerical simulations. If the sensors are slightly misaligned, or if the impact does not occur exactly where it should, the test results can vary significantly.

Using2D digital image correlation (DIC)的测试仪器在圣戈班已经建立，因此，借助立体DIC将这种方法扩展到平面外运动是他们之前工作的自然结果。摄像机定位是专门为鸟击试验进行的，摄像机输出是同步的。在本例中，对玻璃/环氧树脂材料进行撞击，以测试该方法，并确定该简单荷载路径的材料响应。为了使仪器具可以使用DIC，在冲击对面的样品表面上沉积了油漆斑点图案（见图1）。

对于该应用，使用EikoTwin DIC软件直接在模拟中使用的FE网格上执行立体DIC。这种测量可以直接将测量结果与数值结果进行比较。使用图像信息可以获得应变下结构变形的更广泛估计，甚至比通常的传感器覆盖更长的时间。事实上，由于冲击过程中产生的能量，样品中会出现较大的应变和损坏，测试过程中仪表接线经常会被扯断。因此，通过DIC对这些试验进行后处理，可以了解结构在一段时间内的行为，以及损伤模式对部件周围固定件（见图2和图3）机械阻力的影响。图3显示了加载是不对称的，尤其是在图像左侧附近，其中应变主要在前几毫秒内传播。使用传统传感器无法证明这种行为，而这一结果本身就为研究人员更好地理解实际组件行为。

Figure 2 – Images of the structure taken between t=0.001s et t=0.0013s, and displacement profiles measured at these instants. Image processing provides a displacement profile that could hardly have been anticipated by direct observation.

Figure 3 – Displacement profiles over time along the horizontal central line, as measured by digital image correlation

该试验的下一步是与DIGITAL TWIN进行试验模拟比较，以确定复合材料的本构参数。DIC提供的全场测量使我们有机会验证边界条件，这是这些模拟中的一个关键点。直接在FE网格上进行测量的事实将允许直接进行比较。这一基本步骤的最终目标是在接近工业应用的试验条件下，对本构关系进行稳健的识别，并对其进行模拟。