A Modeling Approach for the Fatigue Behavior of Laser Drilled Micro Perforated Structural Panels

Abstract

With hybrid laminar flow control the drag can be reduced for airfoils. This is done by boundary layer suction through millions of small holes in the skin panels. The goal of this paper is to assess the impact on fatigue properties and to describe a modeling approach to account for the laser drilled holes in the skin panels. Two fatigue test programs were performed to obtain inputs to model the fatigue behavior of micro perforated titanium panels. From the test data several SN curves were generated. The all the SN curves have a similar slope as the basic material. This allows for a factorization approach to model the fatigue behavior. Three factors are applied to the basic material properties. The first factor takes into account the hole geometry, which be captured analytically with a stress concentration factor. The pitch between the holes is large enough that there is no interference for fatigue initiation. Secondly multiple site damage was visually observed by multiple crack plateaus on the crack surface. The crack jumps between the rows of holes. This can be seen as shift in the SN curve which is in line with the knock down factor found by an analytical approach. And finally a technology factor to account for the manufacturing process was obtained. The manufacturing process for the coupons of the two test programs was different, resulting in a different technology factor. Therefore setting specific requirements for the manufacturing process can reduce the impact on the fatigue properties. By multiplying the basic material properties with these three factors according to Eq. (5) it is possible to do standard fatigue assessments in commonly used fatigue tools and with detailed spectra.