# Development of Aircraft Spoiler Demonstrators for Cost-Efficient Investigations of SHM Technologies under Quasi-Realistic Loading Conditions

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

- 1.
**Coupon and element level**(plate or beam-like structures)—development of sensors and SHM methods;- 2.
**Component level**(e.g., aircraft spoilers)—application of SHM to real structural components;- 3.
**Full-scale airframe level**(complete aircraft)—application of SHM systems to real aircraft structures.

## 2. Materials and Methods

#### 2.1. Structural Definition

#### 2.2. Loading Definition and Optimization

^{®}[50]. The algorithm was designed to identify the optimal locations and amplitudes of three concentrated loads acting on a symmetrical half model of the spoiler demonstrator based on parametric simulations by incorporating a simple finite element (FE) shell model that was solved in Abaqus/Standard [51].

#### 2.2.1. Simple FE Shell Model

#### 2.2.2. Optimal Locations and Amplitudes of Concentrated Loads

^{®}. Herein, the out-of-plane deformation in a defined operating condition of the real aircraft spoiler acts as the target function; see Figure 1. The main objective is to minimize the difference between the numerically calculated out-of-plane deformation of the real aircraft spoiler and that of the spoiler demonstrator FE shell model. The structure of the optimization algorithm is depicted in Figure 4.

^{®}function

`fminsearchbnd`[53], where ${\zeta}_{i,j}^{D}$, ${\zeta}_{i,k}^{D}$, and ${\zeta}_{i,l}^{D}$ are calculated compliances of node i for each defined unit load at nodes $j\in \mathcal{J}$, $k\in \mathcal{K}$, and $l\in \mathcal{L}$. The inner minimization of Equation (2) represents a least-squares search with the superposition of compliances ${\zeta}_{i,j}^{D}$, ${\zeta}_{i,k}^{D}$, and ${\zeta}_{i,l}^{D}$ multiplied by unknown ${F}_{j}$, ${F}_{k}$, and ${F}_{l}$ (which yields the displacements of the demonstrator) and subtracted by the target displacements of the civil aircraft spoiler ${w}_{i}^{S}$ for all relevant nodes n. The additional constraint of $0\le {F}_{j},{F}_{k},{F}_{l}\le 5000$ ensures forces in the negative z-direction (see Figure 5), as well as a limitation to a maximum load amplitude of 5000 N. Subsequently, the sum of the squared differences between the target deformations and the deformations of the demonstrator is weighted by the sum of the loads (${F}_{j}$, ${F}_{k}$, and ${F}_{l}$) found. This energy-type expression is used to find an optimal solution that balances the deformation accuracy and required load sizes.

#### 2.3. Stress and Strain Analysis with a Detailed 3D FE Model

## 3. Experimental Validation of the Developed Idealized Spoiler Demonstrator

#### 3.1. Assembly of the Idealized Spoiler Demonstrator

#### 3.2. Experimental Setup

## 4. Results and Discussion

#### 4.1. Out-of-Plane Displacements

#### 4.2. Principal In-Plane Strains

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

SHM | structural health monitoring |

FEM | finite element method |

EIT | electrical impedance tomography |

FOS | fiber optical sensor |

FRP | fiber-reinforced polymer |

GFRP | glass-fiber-reinforced polymer |

NDT | non-destructive testing |

SNR | signal-to-noise ratio |

FE | finite element |

DOF | degree of freedom |

CHB | center hinge bracket |

DS | displacement sensor |

DIC | digital image correlation |

3D | three dimensional |

LDL | load distribution lip |

Al | aluminum alloy |

St | steel |

Ad | adhesive |

cam | camera of the DIC system |

## References

- Niu, M.C.Y. Airframe Structural Design: Practical Design Information and Data on Aircraft Structures, 2nd ed.; Hong Kong Conmilit Press Ltd.: Hong Kong, China; Lockheed Aeronautical Systems Company: Burbank, CA, USA, 1999. [Google Scholar]
- Hermanutz, A.; Hornung, M. Aeroelastic Wing Planform Design Optimization of a Flutter UAV Demonstrator. Aerospace
**2020**, 7, 45. [Google Scholar] [CrossRef] - Rozov, V.; Volmering, A.; Hermanutz, A.; Hornung, M.; Breitsamter, C. CFD-Based Aeroelastic Sensitivity Study of a Low-Speed Flutter Demonstrator. Aerospace
**2019**, 6, 30. [Google Scholar] [CrossRef] [Green Version] - Jouannet, C.; Lundstrom, D.; Amadori, K.; Berry, P. Design of a Very Light Jet and a Dynamically Scaled Demonstrator. In Proceedings of the 46th AIAA Aerospace Sciences Meeting and Exhibit, AIAA 2008-137, Reno, NV, USA, 7–10 January 2008. [Google Scholar] [CrossRef]
- Jordan, T.; Langford, W.; Hill, J. Airborne Subscale Transport Aircraft Research Testbed—Aircraft Model Development. In Proceedings of the AIAA Guidance, Navigation, and Control Conference and Exhibit, AIAA 2005-6432, San Francisco, CA, USA, 15–18 August 2005. [Google Scholar] [CrossRef] [Green Version]
- Bierig, A.; Nikodem, F.; Gallun, P.; Greiner-Perth, C. Design of the general systems for the SAGITTA demonstrator UAV. In Proceedings of the 2017 International Conference on Unmanned Aircraft Systems (ICUAS), Miami, FL, USA, 13–16 June 2017; pp. 1767–1777. [Google Scholar] [CrossRef]
- Bergmann, D.P.; Denzel, J.; Baden, A.; Kugler, L.; Strohmayer, A. Innovative Scaled Test Platform e-Genius-Mod—Scaling Methods and Systems Design. Aerospace
**2019**, 6, 20. [Google Scholar] [CrossRef] [Green Version] - Balaram, B.; Canham, T.; Duncan, C.; Grip, H.F.; Johnson, W.; Maki, J.; Quon, A.; Stern, R.; Zhu, D. Mars Helicopter Technology Demonstrator. In Proceedings of the 2018 AIAA Atmospheric Flight Mechanics Conference, AIAA 2018-0023, Kissimmee, FL, USA, 8–12 January 2018. [Google Scholar] [CrossRef] [Green Version]
- Sepe, R.; Citarella, R.; De Luca, A.; Armentani, E. Numerical and Experimental Investigation on the Structural Behaviour of a Horizontal Stabilizer under Critical Aerodynamic Loading Conditions. Adv. Mater. Sci. Eng.
**2017**, 2017, e1092701. [Google Scholar] [CrossRef] [Green Version] - Caputo, F.; Lamanna, G.; Perfetto, D.; Chiariello, A.; Di Caprio, F.; Di Palma, L. Experimental and Numerical Crashworthiness Study of a Full-Scale Composite Fuselage Section. AIAA J.
**2021**, 59, 700–718. [Google Scholar] [CrossRef] - Rossow, C.C.; Wolf, K.; Horst, P. (Eds.) Handbuch der Luftfahrzeugtechnik; Carl Hanser Verlag München: München, Germany, 2014. [Google Scholar]
- Berger, U.; Hayo, T. Onboard-SHM System Using Fibre Optical Sensor and LAMB Wave Technology for Life Time Prediction and Damage Detection on Aircraft Structure. In Proceedings of the EWSHM—7th European Workshop on Structural Health Monitoring, IFFSTTAR, Inria, Nantes, France, 8–11 July 2014; Université de Nantes: Nantes, France, 2014. [Google Scholar]
- Nyikayaramba, G.; Murmann, B. S-Parameter-Based Defect Localization for Ultrasonic Guided Wave SHM. Aerospace
**2020**, 7, 33. [Google Scholar] [CrossRef] [Green Version] - Barman, S.K.; Maiti, D.K.; Maity, D. Vibration-Based Delamination Detection in Composite Structures Employing Mixed Unified Particle Swarm Optimization. AIAA J.
**2020**, 59, 386–399. [Google Scholar] [CrossRef] - Philibert, M.; Soutis, C.; Gresil, M.; Yao, K. Damage Detection in a Composite T-Joint Using Guided Lamb Waves. Aerospace
**2018**, 5, 40. [Google Scholar] [CrossRef] [Green Version] - Alzahrani, M.; Choi, S.K.; Choi, H.J. Structural Health Monitoring of Damaged Beams Using an Improved Variational Vibration Model. AIAA J.
**2018**, 56, 4595–4603. [Google Scholar] [CrossRef] - Winklberger, M.; Kralovec, C.; Humer, C.; Heftberger, P.; Schagerl, M. Crack Identification in Necked Double Shear Lugs by Means of the Electro-Mechanical Impedance Method. Sensors
**2021**, 21, 44. [Google Scholar] [CrossRef] [PubMed] - Song, F.; Huang, G.L.; Hu, G.K. Online Guided Wave-Based Debonding Detection in Honeycomb Sandwich Structures. AIAA J.
**2012**, 50, 284–293. [Google Scholar] [CrossRef] [Green Version] - Gómez González, A.; Zugasti, E.; Anduaga, J. Damage Identification in a Laboratory Offshore Wind Turbine Demonstrator. Key Eng. Mater.
**2013**, 569–570, 555–562. [Google Scholar] [CrossRef] - Scholz, M.; Rediske, S.; Nuber, A.; Friedmann, H.; Moll, J.; Arnold, P.; Krozer, V.; Kraemer, P.; Salman, R.; Pozdniakov, D. Structural Health Monitoring of Wind Turbine Blades using Radar Technology: First Experiments from a Laboratory Study. In Proceedings of the 8th European Workshop on Structural Health Monitoring, Bilbao, Spain, 5–8 July 2016; p. 10. [Google Scholar]
- Martins, B.L.; Kosmatka, J.B. Health Monitoring of Aerospace Structures via Dynamic Strain Measurements: An Experimental Demonstration. In Proceedings of the AIAA Scitech 2020 Forum, AIAA 2020-0701, Orlando, FL, USA, 6–10 January 2020. [Google Scholar] [CrossRef]
- Giurgiutiu, V.; Santoni-Bottai, G. Structural Health Monitoring of Composite Structures with Piezoelectric-Wafer Active Sensors. AIAA J.
**2011**, 49, 565–581. [Google Scholar] [CrossRef] [Green Version] - Dong, T.; Kim, N.H. Cost-Effectiveness of Structural Health Monitoring in Fuselage Maintenance of the Civil Aviation Industry. Aerospace
**2018**, 5, 87. [Google Scholar] [CrossRef] [Green Version] - Gschoßmann, S.; Humer, C.; Schagerl, M. Lamb wave excitation and detection with piezoelectric elements: Essential aspects for a reliable numerical simulation. In Proceedings of the 8th European Workshop on Structural Health Monitoring, Bilbao, Spain, 5–8 July 2016; p. 10. [Google Scholar]
- Humer, C.; Kralovec, C.; Schagerl, M. Application of the Scattering Analysis Method for Guided Waves Measured by Laser Scanning Vibrometry. In Proceedings of the 12th International Workshop on Structural Health Monitoring, Stanford, CA, USA, 10–12 September 2019. [Google Scholar] [CrossRef]
- Yeasin Bhuiyan, M.; Shen, Y.; Giurgiutiu, V. Interaction of Lamb waves with rivet hole cracks from multiple directions. Proc. Inst. Mech. Eng. Part C J. Mech. Eng. Sci.
**2017**, 231, 2974–2987. [Google Scholar] [CrossRef] - Zhao, Y.; Viechtbauer, C.; Loh, K.J.; Schagerl, M. Enhancing the Strain Sensitivity of Carbon Nanotube-Polymer Thin Films For Damage Detection and Structural Monitoring. In Proceedings of the 11th International Workshop on Advanced Smart Materials and Smart Structures Technology, Champaign, IL, USA, 1–2 August 2015; University of Illinois: Champaign, IL, USA, 2015; p. 8. [Google Scholar]
- Zhao, Y.; Schagerl, M.; Gschossmann, S.; Kralovec, C. In situ spatial strain monitoring of a single-lap joint using inkjet-printed carbon nanotube embedded thin films. Struct. Health Monit.
**2019**, 18, 1479–1490. [Google Scholar] [CrossRef] - Nonn, S.; Schagerl, M.; Zhao, Y.; Gschossmann, S.; Kralovec, C. Application of electrical impedance tomography to an anisotropic carbon fiber-reinforced polymer composite laminate for damage localization. Compos. Sci. Technol.
**2018**, 160, 231–236. [Google Scholar] [CrossRef] - Milanoski, D.P.; Loutas, T.H. Strain-based health indicators for the structural health monitoring of stiffened composite panels. J. Intell. Mater. Syst. Struct.
**2021**, 32, 255–266. [Google Scholar] [CrossRef] - Grassia, L.; Iannone, M.; Califano, A.; D’Amore, A. Strain based method for monitoring the health state of composite structures. Compos. Part Eng.
**2019**, 176, 107253. [Google Scholar] [CrossRef] - Ohanian, O.J.; Davis, M.A.; Valania, J.; Sorensen, B.; Dixon, M.; Morgan, M.; Litteken, D. Embedded Fiber Optic SHM Sensors for Inflatable Space Habitats. In Proceedings of the ASCEND 2020, Virtual Event, AIAA 2020-4049, online, 16–18 November 2020. [Google Scholar] [CrossRef]
- Kressel, I.; Shapira, O.; Ben-Simon, U.; Bergman, A.; Shoham, S.; Glam, B.; Tur, M. Airworthiness Monitoring of theWings of a UAV Fleet Using Fiber Optic Distributed Sensing. In Proceedings of the 29th ICAF Symposium, Nagoya, Japan, 7–9 June 2017; p. 5. [Google Scholar]
- Guo, S.; Li, D.; Liu, Y. Multi-objective optimization of a composite wing subject to strength and aeroelastic constraints. Proc. Inst. Mech. Eng. Part G J. Aerosp. Eng.
**2012**, 226, 1095–1106. [Google Scholar] [CrossRef] - Narayana Naik, G.; Gopalakrishnan, S.; Ganguli, R. Design optimization of composites using genetic algorithms and failure mechanism based failure criterion. Compos. Struct.
**2008**, 83, 354–367. [Google Scholar] [CrossRef] - Zhao, Y.; Wang, C. Shape Optimization of Labyrinth Seals to Improve Sealing Performance. Aerospace
**2021**, 8, 92. [Google Scholar] [CrossRef] - Song, W.; Keane, A.J. Surrogate-Based Aerodynamic Shape Optimization of a Civil Aircraft Engine Nacelle. AIAA J.
**2007**, 45, 2565–2574. [Google Scholar] [CrossRef] - Gomes, G.F.; de Almeida, F.A.; da Silva Lopes Alexandrino, P.; da Cunha, S.S.; de Sousa, B.S.; Ancelotti, A.C. A multiobjective sensor placement optimization for SHM systems considering Fisher information matrix and mode shape interpolation. Eng. Comput.
**2019**, 35, 519–535. [Google Scholar] [CrossRef] - Ostachowicz, W.; Soman, R.; Malinowski, P. Optimization of sensor placement for structural health monitoring: A review. Struct. Health Monit.
**2019**, 18, 963–988. [Google Scholar] [CrossRef] - Bergmayr, T.; Kralovec, C.; Schagerl, M. Vibration-Based Thermal Health Monitoring for Face Layer Debonding Detection in Aerospace Sandwich Structures. Appl. Sci.
**2020**, 11, 211. [Google Scholar] [CrossRef] - Bergmayr, T.; Winklberger, M.; Kralovec, C.; Schagerl, M. Structural health monitoring of aerospace sandwich structures via strain measurements along zero-strain trajectories. Eng. Fail. Anal.
**2021**, 126, 105454. [Google Scholar] [CrossRef] - Staszewski, W.J.; Mahzan, S.; Traynor, R. Health monitoring of aerospace composite structures—Active and passive approach. Compos. Sci. Technol.
**2009**, 69, 1678–1685. [Google Scholar] [CrossRef] - Meindlhumer, M.; Horejsi, K.; Schagerl, M. Manufacturing and Costs of Current Sandwich and Future Monolithic Designs of Spoilers. J. Aircr.
**2019**, 56, 85–93. [Google Scholar] [CrossRef] - Kesavan, A.; John, S.; Herszberg, I. Strain-based Structural Health Monitoring of Complex Composite Structures. Struct. Health Monit.
**2008**, 7, 203–213. [Google Scholar] [CrossRef] - Schagerl, M.; Viechtbauer, C.; Schaberger, M. Optimal Placement of Fiber Optical Sensors along Zero-strain Trajectories to Detect Damages in Thin-walled Structures with Highest Sensitivity. In Proceedings of the 10th International Workshop on Structural Health Monitoring, Stanford, CA, USA, 1–3 September 2015. [Google Scholar] [CrossRef]
- Schaberger, M. Damage Detection in Thin-Walled Structures with Strain Measurements along Zero-Strain Trajectories. Master’s Thesis, Johannes Kepler University Linz, Linz, Austria, 2016. [Google Scholar]
- Riedl, M. Schadensbewertung Einer Beulenden Platte Anhand der Methode der Nulldehnungstrajektorie und Digitaler Bildkorrelation Messung. Master’s Thesis, Johannes Kepler University Linz, Linz, Austria, 2018. [Google Scholar]
- Kralovec, C.; Schagerl, M. Review of Structural Health Monitoring Methods Regarding a Multi-Sensor Approach for Damage Assessment of Metal and Composite Structures. Sensors
**2020**, 20, 826. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Hofer, B. Dehnungsmessung Mithilfe der Zeitbereichsreflektometrie und Erarbeitung Eines Idealisierten Störklappenlabormodells. Bachelor’s Thesis, Johannes Kepler University Linz, Linz, Austria, 2017. [Google Scholar]
- The MathWorks, Inc. Matlab. Release: R2019b (9.7.0.1190202). Available online: https://de.mathworks.com/products/new_products/release2019b.html/ (accessed on 10 October 2021).
- Dassault Systèmes Simulia Corp. Abaqus/CAE 2019. Build ID: 2018_09_24-20.41.51 157541. Available online: https://www.3ds.com/products-services/simulia/products/abaqus/abaquscae/ (accessed on 10 October 2021).
- HEXCEL COMPOSITES. Technology Manuals; Honeycomb Sandwich Design Technology, 2000. Available online: https://www.hexcel.com/Resources/Technology-Manuals (accessed on 3 October 2021).
- D’Errico, J. Fminsearchbnd, 2006. Publisher: MATLAB Central File Exchange, Version 1.4.0.0. Available online: https://www.mathworks.com/matlabcentral/fileexchange/8277-fminsearchbnd-fminsearchcon (accessed on 3 July 2017).
- HEXCEL COMPOSITES. Data Sheets; HexWeb
^{®}A1. Available online: https://www.hexcel.com/Resources/DataSheets/Honeycomb (accessed on 16 January 2018). - Nhamoinesu, S.; Overend, M. The Mechanical Performance of Adhesives for a Steel-Glass Composite Façade System. In Challenging Glass 3; IOS Press: Amsterdam, The Netherlands, 2012; pp. 293–306. [Google Scholar] [CrossRef]
- 3M™ Scotch-Weld™. Data Sheets; Two Component Epoxy Adhesives. Available online: https://www.3maustria.at/3M/de_AT/p/d/b40066473/ (accessed on 3 October 2021).
- Hottinger Brüel and Kjaer GmbH. Sensors. Available online: https://www.hbm.com/en/5501/sensors/ (accessed on 3 October 2021).
- Hottinger Brüel and Kjaer GmbH. Data Acquisition and Analysis Software. Available online: https://www.hbm.com/en/1996/software/ (accessed on 3 October 2021).
- Correlated Solutions Inc. Products; VIC-3D. Available online: https://www.correlatedsolutions.com/vic-3d/ (accessed on 3 October 2021).

**Figure 1.**Location and deformation of the considered aircraft spoiler of an Airbus A340 aircraft, overview of control surfaces (cf. [43]), and out-of-plane deformation of the spoiler’s upper skin according to considered load case.

**Figure 2.**Shape and dimensions of (

**a**) the real aircraft spoiler considered in comparison to (

**b**) the developed idealized spoiler demonstrator.

**Figure 3.**FE shell model with three predefined node sets, $\mathcal{J},\mathcal{K}$, and $\mathcal{L}$. The 2D shell elements have an exact size of 20 mm × 20 mm.

**Figure 5.**Result of loading optimization on an idealized spoiler demonstrator. Schematic sketch of (

**a**) the real aircraft spoiler due to aerodynamic loads and (

**b**) the idealized spoiler demonstrator under four-point loading (half model rendered in symmetrical full view for display purposes).

**Figure 6.**Detailed 3D FE model (all dimensions in millimeters), * shell thickness, and ${}^{\u2020}$ beam cross-section rendered for display purposes.

**Figure 7.**Out-of-plane displacement contour plots of numerical FE models (dashed rectangles indicate the size of the idealized spoiler demonstrator).

**Figure 8.**Comparison of strain directions and trajectories of numerical FE models (the region around the CHB and the hinge bracket is not considered; dashed rectangles indicate the size of the idealized spoiler demonstrator).

**Figure 11.**Comparison between the experiment and simulation of (

**a**) minor and (

**b**) major principal in-plane strains at an applied load of $\mathit{F}=$ 400 N.

**Figure 12.**Comparison of the strain directions and trajectories for the experiment and simulation at an applied load of $\mathit{F}=$ 400 N.

**Table 1.**Optimized concentrated loads and their locations on the idealized spoiler demonstrator according to Equation (2).

Node | Load [N] | x [mm] | y [mm] |
---|---|---|---|

j | 1885 | 480 | 120 |

k | 2705 | 440 | 360 |

l | 0 | 0 | 120 |

**Table 2.**Material parameters of the composite sandwich panel of the idealized spoiler demonstrator [54].

${\mathit{E}}_{11}$ | ${\mathit{E}}_{22}$ | ${\mathit{E}}_{33}$ | ${\mathit{\nu}}_{12}$ | ${\mathit{\nu}}_{13}$ | ${\mathit{\nu}}_{23}$ | ${\mathit{G}}_{12}$ | ${\mathit{G}}_{13}$ | ${\mathit{G}}_{23}$ | |
---|---|---|---|---|---|---|---|---|---|

[MPa] | [MPa] | [MPa] | - | - | - | [MPa] | [MPa] | [MPa] | |

Each layer of skin ([0,45,−45,0]) | 22,550 | 20,900 | 1 | 0.15 | 0 | 0 | 4500 | 3500 | 3500 |

Core | 1 | 1 | 500 | 0 | 0 | 0 | 1 | 66 | 34 |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Winklberger, M.; Kralovec, C.; Schagerl, M.
Development of Aircraft Spoiler Demonstrators for Cost-Efficient Investigations of SHM Technologies under Quasi-Realistic Loading Conditions. *Aerospace* **2021**, *8*, 320.
https://doi.org/10.3390/aerospace8110320

**AMA Style**

Winklberger M, Kralovec C, Schagerl M.
Development of Aircraft Spoiler Demonstrators for Cost-Efficient Investigations of SHM Technologies under Quasi-Realistic Loading Conditions. *Aerospace*. 2021; 8(11):320.
https://doi.org/10.3390/aerospace8110320

**Chicago/Turabian Style**

Winklberger, Markus, Christoph Kralovec, and Martin Schagerl.
2021. "Development of Aircraft Spoiler Demonstrators for Cost-Efficient Investigations of SHM Technologies under Quasi-Realistic Loading Conditions" *Aerospace* 8, no. 11: 320.
https://doi.org/10.3390/aerospace8110320