## 2 Materials and Methods
### Case Example  
The monitored structure in this case is a demonstrator model of an Airbus A340 break spoiler (Airbus SE, Toulouse, France). It was developed by [PERSON] et al. [23] for the purpose of closely replicating the strain state of the real spoiler under an aerodynamic loading scenario in a cost efficient laboratory setting, to specifically support the development of SHM methods. The demonstrator includes several simplifications compared to the real spoiler. Its overall size was scaled down by a factor of 1:2 and the original asymmetric shape was simplified to a rectangular shape. Additionally, the wedge-shape sandwich structured composite was approximated by a standard sandwich plate with a constant thickness. More specifically, it is a 15 mm thick sandwich plate with a Nomex honeycomb core and glass fiber-reinforced plastic (GFRP) layers on its top and bottom surface. Each GFRP layer is 0.5 mm thick and consists of four prepreg fabric piles with fiber orientations [0, 45, \(-\)45, 0]. Lastly, the hinge components of the real spoiler were simplified by less complex aluminum hinge brackets [21,23]. The overall dimensions of the demonstrator, as well as the outlined simplifications, are illustrated in Figure 1.  
The spoiler demonstrator is designed to reproduce the spatial strain states, which the upper surface layer of the real spoiler experiences when it is deformed by the aerodynamic pressure caused by a 35\({}^{\circ}\) extension immediately after touchdown of the aircraft. This scenario is referred to as the design load case. In order to reproduce the deformation state of the real spoiler, the demonstrator is deformed by a whiffle tree. This mechanism enables a defined force distribution across multiple introduction points via mechanical linkages [23]. A grid of strain gauge rosettes HBM RY93-6/120 (HBM GmbH, Darmstadt, Germany) is applied to the surface of the demonstrator, close to its trailing edge, where all sensors are spaced 55 mm apart. Each strain gauge rosette measures strain in 0\({}^{\circ}\), 45\({}^{\circ}\) and 90\({}^{\circ}\) directions [20]. Figure 2 shows the relevant dimensions and location of the sensor grid on the spoiler demonstrator. Figure 1: Comparison of the real spoiler and the idealized spoiler demonstrator. Reprint from [23] with the author’s permission. Parallel to the mechanical build of the demonstrator, [PERSON] et al. [23] created a detailed numerical 3D model in the FE software _Abaqus/Standard 2019_[24]. The symmetric forces (with respect to the \(yz\)-plane) introduced by the whiffle tree are represented in the FE model by nodal forces, the magnitude and location of which are controllable parameters. The sandwich core was modeled with continuum elements C3D8R (average mesh size: \(10\,\mathrm{mm}\times 10\,\mathrm{mm}\times 7.5\,\mathrm{mm}\)). The GFRP layers on the top and bottom surface of the sandwich core were modeled using the composite layup feature provided by the property module in _Abaqus_. The surface layers are discretized by conventional shell elements S4R and an average mesh size of \(10\,\mathrm{mm}\times 10\,\mathrm{mm}\). In the course of prior studies utilizing the demonstrator, two damages in the form of different sized holes were introduced. First, strain data due to five unique load cases was acquired from the pristine spoiler. In a following step, a \(12.5\,\mathrm{mm}\) diameter hole was drilled just below sensor 8 and strain measurements were again conducted for the same load cases as used for the pristine case. Lastly, the same procedure was repeated after enlarging the existing hole to a diameter of \(19\,\mathrm{mm}\). This data represents the available experimental test data; thus, there are 15 samples with strains of three different health states (pristine and damaged by hole with diameters \(12.5\,\mathrm{mm}\) and \(19\,\mathrm{mm}\)) [20]. The corresponding experimental setup is depicted in Figure 3.  
Figure 3: Experimental setup of the spoiler demonstrator. Reprint from [21] with the author’s permission.