PSI - Issue 17

Radek Doubrava et al. / Procedia Structural Integrity 17 (2019) 190–197 Radek Doubrava/ Structural Integrity Procedia 00 (2019) 000 – 000

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resistance to the impact of accidents, Cleary et al. (2005), Field et al. (2008). The risk of strange objects impact that can decrease local strength properties must be taken into account in the design of aircraft primary structures. Testing all impact scenarios on real structures is expensive and impractical. For elimination the damage effects of bird or hailstone strikes, two main approaches are typically implemented: 1. Bird or hailstone strike prevention strategies that attempt to reduce the probability of a bird or hailstone strike incident which includes flying trained falcons; training dogs; maintaining grass at certain heights; eliminating of food sources; using sonic cannons, lasers, recorded predator calls and other noise generators; as well as using radar equipment to detect and avoid storms. 2. Aircraft certification programs employ various measures to maintain the integrity of an aircraft against the high loads resulting from high-velocity impacts in accordance with international certification standards. The second approach is further developed in this study. International certification regulations require that all forward-facing aircraft components be proven capable of withstanding bird or hailstone strikes to a certain level before they can be used in an aircraft, per ASTM F330-10 (2010) and ASTM F320-05 (2005). A bird or hailstone impact test provides a direct method for determining the impact resistance; however, the design of aircraft structures typically involves much iteration, from design to manufacturing to testing and back, requiring that many bird strike tests be conducted. This is not only time-consuming but also costly. Furthermore, experimental data from these tests are often narrowly focused, constituting a barrier for their direct use in refining the structural design. Owing to these shortcomings, several numerical methods have been developed to simulate bird or hailstone strikes to reduce the number of intermediate tests required and subsequently shorten the duration of the component design phase as presented by Wilbeck et al. (1978) or Heimbs (2011). With regard to the high-speed effects over the course of the impact, it is a significant challenge to obtain data for the analysis of the mechanism of damage initialization and propagation, especially for composite materials ( Růžek (2014, 2017)).

Nomenclature E

Young’s modulus Shear modulus Finite element

G

FE

 Density Nu,  Poisson ’s ratio

BVID Barely Visible Impact Damage CFRP Carbon Fibre Reinforced Polymer NDT non-destructive technique  ut ultimate tension strength  u ultimate shear strength

2. Testing

Physical impact tests were performed at the Czech Aerospace Research Centre (VZLÚ) according to the ASTM F320-05 (2005) standard. The required impact velocity was achieved using a properly pressurized air gun-type pressure vessel (Fig. 1). The projectiles were accelerated by compressed air through the smooth borehole of a gun barrel to the required velocity according to the ASTM specifications. The tests were performed at perpendicular impact angles. The results from the perpendicular impact tests are more conservative than the real structural behaviour, which generally includes additional geometric stiffness, a shallower impact angle, or increased projectile fragmentation. The application of the highest impact energy allows a comparison of the material behaviour from the point of view of damage initiation and propagation.