This paper investigates a strain-based structural health monitoring (SHM) method for damage detection and localization in composite sandwich structures. As case example, an idealized aircraft spoiler of a large civil aircraft is considered. Critical failure modes of such sandwich structures composed of fiber reinforced polymer (FRP) face layers and a honeycomb core are, e.g., face layer delamination and debonding from the core. The latter challenges today’s non-destructive testing methods, and thus, shall be addressed in the present work. The presented research compromises an idealized spoiler model of the real spoiler structure in the scale 1:2. The model is composed of glass fiber reinforce polymer (GFRP) face layers and a honeycomb core. The damage identification is investigated based on static strain measurements along so-called zero-strain trajectories (ZST) at the loaded structure. A zero-strain direction exists for every plane strain state with major strain directions with opposite signs (tensile and compression). Connecting zero-strain direction vectors at various points of a structure yields a ZST. Strains along ZST are most sensitive to changes of the load path due to, e.g., damage. This work investigates the use of strain measurements along ZST for damage detection at the edge of the considered sandwich structure by means of numerical and experimental analysis. A real wind-load condition of a large civil aircraft spoiler is considered as load case. An in-depth numerical and experimental investigation is performed to calculate the ZST for the pristine structure. The experimental validation of the Finite element (FE) model is realized by deformation measurements via a digital image correlation system (DIC). Moreover, strain measurements via strain gauge (SG) rosettes were performed to evaluate the correct calculation of the zero-strain directions. Finally, the application of strain measurements along a ZST for debonding detection and localization is demonstrated and its potential and issues for real online monitoring is discussed.

本文研究了基于应变的结构健康监测（SHM）方法，用于复合材料夹层结构中的损伤检测和定位。作为示例，考虑了大型民用飞机的理想飞机扰流板。这种由纤维增强聚合物（FRP）面层和蜂窝状芯构成的夹层结构的关键破坏模式是，例如，面层从芯上脱层和脱粘。后者挑战了当今的非破坏性测试方法，因此应在当前工作中解决。提出的研究折衷了比例为1：2的实际扰流器结构的理想扰流器模型。该模型由玻璃纤维增​​强聚合物（GFRP）面层和蜂窝芯组成。基于沿加载结构处的所谓零应变轨迹（ZST）的静态应变测量来研究损伤识别。对于每个平面应变状态，零应变方向存在，主要应变方向具有相反的符号（拉伸和压缩）。在结构的各个点处连接零应变方向向量会产生ZST。由于例如损坏，沿ZST的应变对负载路径的变化最敏感。这项工作通过数值和实验分析研究了沿ZST的应变测量在所考虑的夹心结构边缘的损伤检测的用途。大型民用飞机扰流板的实际风荷载条件被认为是荷载情况。进行了深入的数值和实验研究，以计算原始结构的ZST。有限元（FE）模型的实验验证是通过数字图像相关系统（DIC）进行变形测量来实现的。此外，通过应变仪（SG）的花环进行了应变测量，以评估零应变方向的正确计算。最后，展示了沿ZST进行应变测量以进行脱胶检测和定位的应用，并讨论了其在实际在线监测中的潜力和问题。