Abstract
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.
Highlights
Fiber reinforced plastics (FRP) are well established in the aerospace industry due to their high strength-to-weight and stiffness-toweight ratio
This approach makes it feasible to define a threshold value that indicates an imminent critical repair failure. Another strain-based structural health monitoring (SHM) strategy by fiber Bragg grating (FBG) senors was presented by Fernandez-Lopez et al [26], which uses the differential strain of sensors that are far from a damage and maintain a linear relationship to a reference strain, while sensors that are closest to a damage exhibit nonlinear behavior
This paper presents a study of the capability of strain measurements along zerostrain trajectories (ZST) as a potential SHM method on an idealized aircraft spoiler for face layer debonding detection and localization in composite sandwich structures as typically used in aerospace applica tions
Summary
Fiber reinforced plastics (FRP) are well established in the aerospace industry due to their high strength-to-weight and stiffness-toweight ratio. There are only a few studies, for example, Li. et al [25] introduced a simple differential strain approach, where two FBG sensors are strategically positioned so that their strain difference increased with respect to the debonding size in composite bonded patches This approach makes it feasible to define a threshold value that indicates an imminent critical repair failure. To monitor the damage a health indicator, which combines the normalized sensor strain of the right and left side positioned sensors, is introduced and evaluated based on numerical results. The presented approach evaluates the correlations between strain sensors, e.g. strain gauge or FBG senors, at different locations with artificial neuronal networks for the conclusion on the health state of a mechanical structure Significant deviations between this correlation can be classified as potential damage
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