Current trends in aircraft design are to increase the economic efficiency by integrating different features in multifunctional materials. One strategy is to embed resistance heater elements between glass-fibre epoxy layers in (heated) fibre metal laminates and to use them as anti or de-icing devices in leading edges of wings. Heated glass fibre reinforced aluminium (GLARE) is an example of such a multifunctional material where heating functionality was added to the (certified) structural feature of GLARE. As heated fibre metal laminates are an innovative and rather new material, the possible (local) effects of embedded heating on the stress–strain state have not yet been investigated. This research couples experimental characterisation of heated GLARE surface behaviour and numerical modelling analysis to investigate the surface and the through-the-thickness strain-stress state and temperature distributions due to the embedded heating. For the experimental part, the surface strains and the temperatures of a developed specimen were measured in a slow heating regime (temperature increase from 22.7 to 39.4 °C within 120 s) using, respectively, a developed shearography instrument and thermocouples with an infrared camera. Then a numerical model of heated GLARE was developed and verified with experimental results. Further, the numerical model was used to predict strains, stresses, and temperatures during a temperature increase similar to that used for de-icing in a real operation (temperature increase from −25 to 86.7 °C within 4.8 s).

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By integrating heater elements in Fiber Metal Laminates, such as GLARE, the material has a promising potential as de- or anti-icing system in aircraft structures. Application of this so called "heated GLARE" material will however result in increase and (up to ten fold) more frequent temperature loading compares to the regular flight conditions. To investigate the long-term effects thermal cycling tests are performed up to 36000 cycles in three different temperature ranges. Both heated GLARE and FM906 glass-fiber epoxy samples have been thermal cycled using an in-house developed thermal cycling setup. Heating is performed in two different ways, from the outside using Peltier elements and from the inside using integrated heater elements. In addition, continuous heating tests are performed to further examine the effect of aging and internal stress relaxation. FM906 glass-fiber epoxy samples cycled by external heating show an increase in interlaminar shear strength (ILSS) as a result of physical aging, whereas internal heating shows a decrease. Most GLARE samples cycled by internal heating shows an increase in ILSS. The continuous heating tests on heated GLARE confirm that aging and internal stress relief have counteracting effect on the ILSS. The temperature level determines, which effect is most dominant.

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Autoclave manufacturing of fibre metal laminates, such as GLARE, is an expensive process.Therefore, there is an increasing interest to find cost effective out-of-autoclave manufacturing processes without diminishing the laminate quality. The aim of this study is to evaluate the quality of fibre metal laminate panels adhesively bonded and cured using resistance heating. Three manufacturing processes are compared for different layups with an embedded steel mesh at the mid-plane:autoclave curing, resistance bonding of two (autoclave-cured) panels, and complete out-of-autoclave resistance curing of panels. Interlaminar shear strength tests and optical microscopy analysis showed that resistance bonding is a promising technique, leading to results comparable to autoclave curing.Resistance curing led to an interlaminar shear strength decrease of 30-60%. A study of the correlation between degree of cure and distance from the mesh revealed the potential of resistance bonding to be used for flexible embedded mesh geometries and on-site repairs.

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Fibre metal laminates with integrated heater elements have a promising potential as de- or anti-icing systems in aircraft structures. The alternating metal and composite lay-up in fibre metal laminates seems ideal for the development of a multifunctional skin with embedded heater elements. However, the long term durability needs to be carefully examined.

A unique thermal cycling setup has been designed and built to investigate the effects of thermal cycling on the material properties of GLARE (glass fibre reinforced aluminium). Peltier elements were used to provide external heating and external cooling by inverting the direction of the electrical current. With the same setup, heated GLARE samples can be internally heated using the integrated heater elements and externally cooled using the Peltier elements.

Glass-fibre epoxy composite, GLARE, and heated GLARE samples have been thermal cycled for 4000, 8000 and 12,000 cycles with temperature differences of 120 °C. The interlaminar shear strength (ILSS) increased by 6.9% after 8000 cycles for the glass-fibre epoxy composite material compared to the non-cycled samples. The GLARE samples showed a maximum ILSS increase of 4.2% after 12,000 cycles. However, the heated GLARE samples showed a continuous decrease of the ILSS with a maximum decrease of 7.8% after 12,000 cycles.@en

Nowadays, the application of glass-fibre composites in light-weight structures is growing. Although mechanical characterizations of those structures are commonly performed in testing, chemical changes of materials under stresses have not yet been well documented. In the present work coupon tests and Hyperspectral Imaging (HSI) have been used to categorise possible chemical changes of glass-fibre reinforced polymers (GFRP) which are currently used in the aircraft industry. HSI is a hybrid technique that combines spectroscopy with imaging. It is able to detect chemical degradation of surfaces and has already been successfully applied in a wide range of fields including astronomy, remote sensing, cultural heritage and medical sciences. GFRP specimens were exposed to two different thermal loading conditions. One thermal loading condition was a continuous thermal exposure at 120°C for 24h, 48 h and 96h, i.e. ageing at a constant temperature. The other thermal loading condition was thermal cycling with three different numbers of cycles (4000, 8000, 12000) and two temperature ranges (0°C to 120°C and -25°C to 95°C). The effects of both conditions were measured using both HSI and interlaminar shear (ILSS) tests. No significant changes of the physical properties of the thermally cycled GFRP specimens were detected using interlaminar shear strength tests and optical microscopy. However, when using HIS, differences of the surface conditions were detected. The results showed that the different thermal loading conditions could be successfully clustered in different colours, using the HSI linear unmixing technique. Each different thermal loading condition showed a different chemical degradation level on its surface which was indicated using different colours. @en

Current trends in aircraft design go towards smart materials and structures including the use of multi-purpose materials. Fiber Metal Laminates (FML) with embedded electrical heater elements in leading edges of aircraft used for anti- or de-icing follow those trends. The laminated structure of FMLs with layers of different materials leads to anisotropic material characteristics. The FML used in this study is GLARE (Glass Laminate Aluminum Reinforced Epoxy). The anisotropic structure raises questions concerning possible effects on the material characteristics when frequently heated by embedded heater elements and cooled by flight conditions. In order to investigate those possible effects on FMLs, knowledge about the thermal strains and stresses is important. Furthermore, non-destructive techniques are likely to be a future requirement to detect defective heater elements and delaminations at heated leading edges. Thus, this research uses a shearography (speckle pattern shearing interferometry) instrument in order to investigate the surface strain components of FMLs during thermal loading with the embedded heater elements. Parallel to the experiments, numerical analyses were conducted in order to investigate the strain-stress state due to thermal loading with embedded heater elements. The results of both, the strain measurement with the shearography instrument and the numerical analyses were analyzed and compared. The numerical results show how the embedded heater element affects the residual stress-strain state and the stresses due to thermal loading.

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