PSI - Issue 21

Hande Yavuz / Procedia Structural Integrity 21 (2019) 112–119 H. Yavuz/Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Design criteria of aircrafts are still evolving due to their complexity and variety of challenges associated with safety issues (Bristow and Irving 2007). Complexity arises together with safety concerns in the form of several structural criteria: (1) external loads in terms of flight and ground loads have to be sufficiently defined, (2) internal loads should be balanced for each components on the account of good structural arrangement, (3) weight efficiency of structures should be controlled via properly located structural members, (4) material allowable should meet the design constraints and specifications, and (5) other safety issues would be considered for the protection against lightning strike, bird strike, and fatigue failure, etc. (Schijve 1994, Wanhill 2018, Degenhardt 2014, de Florio 2016). According to the published data concerning large transportation aircrafts, mass reduction of 1 kg achieved leads to decrease in fuel consumption around 120 L/year (Gay 2015). Arguing this fact would come along with the positive outcome of the use of polymer composites by means of contributing to payload gain and aircraft performance accordingly (Hinrichsen and Bautista 2001). However, with the increasing use of polymer composites in aircrafts, employing comprehensive testing issues for the determination of mechanical behavior at coupon-, element-, and component-level and performing extensive research to analyze their failure and damage characteristics possess the utmost importance especially prior to full-scale structural testing. Towards the acceptance issues of final composite product, coupon-level testing covers static strength, fatigue, damage sensitivity, moisture, and temperature effects. Joints and shear webs are usually grouped under the element-level testing items. Stiffened panels, major joints and full-scale sections such as nose-radomes are tested at the component-level (Johnson, Thomson, David, Joosten 2015). For the computational analysis of aircraft sections, idealization approach would also be referred in order to analyze the failure behavior of the structural components (Kaplan 2017, Dababneh and Kayran 2014). For the construction of aircraft components, materials selection plays a crucial role for the determination of candidate materials along with their material properties in specific structural applications. It is usually performed by considering proper objectives, constraints, and free variables with respect to functions of the components of a system at the preliminary design stage prior to computational analysis (Ashby 2011) . Basically, in Ashby’s methodology, the components are mainly considered in the form of beam, plate or column, etc. and they are often investigated under limited load case scenarios. Since widely recognized materials databases (e.g., CMH-17, MMPDS, PMP-HDBK) are available in materials selection software, best candidate materials would be easily identified using coupling constant(s) or exchange constant(s) depending on the way constraints and objectives organized (Ashby 2011, CES 2018). Combining strength and stiffness constraints within a single objective in order to minimize the mass of a component is regarded as multiple constraints design. Incorporating mass and cost objectives into single solution by defining a locally linear utility function is identified as conflicting objectives design. Nevertheless, it would worth mentioning flutter prevention in aircrafts while maximizing stiffness and strength properties of aircraft structures via Ashby’s methodology. Stiffness increase may not always be beneficial for the aircraft flutter, increase in bending mode frequency may positively affect bending strength; however, it would cause earlier bending-torsion coupling that may lead lower flutter speed (Zhu and Qui 1991). In lieu of introducing coupled set of complex design parameters to avoid panel flutter (Jorgensen 1991, Gou 2007) , materials selection using Ashby’s methodology was applied only on a uniform panel without extending the study towards the analysis of bending-torsion coupling. Besides, the design methodology may also be reconsidered using polymer composites in the form of dog-bone specimens (Monroy Aceves 2008). In this study, Ashby’s methodology was applied to determine best candidate materials for constructing stiff, strong, and light skin panels in an aircraft. According to materials selection approach, continuous fiber reinforced epoxy composites was found as one of the best candidate materials. Determination of proper mechanical properties of a material is essential when performing structural analysis on load-bearing components. Thus, elastic and strength properties of unidirectional continuous fiber reinforced epoxy composites were gathered from CES Edupack Material Universe Database (CES 2018). Various strength based failure criteria as regards classical lamination theory was also implemented in MATLAB in order to compare the effect of stacking sequence on the failure behavior of the laminated composite structures. Among those failure criteria, Hashin’s model was both adopted in MATLAB (Matlab 2015) and