IMPROVING MODAL PARAMETER PREDICTIONS FOR JOINTED AIRFRAME PANELS - PART I - EXPERIMENTS

This paper describes a combined experimental and numerical investigati on aimed at identifying the physical mechanisms and modelling errors t hat are the culprits for the lack of correlation between experimental measurements and analytical predictions of the modal parameters of hel icopter airframes. The identification of the major contributors to thi s lack of correlation is an indispensable first step to actual modific ation and improvement of modelling processes and tools. The study focu ses on simple configurations: panels with lap and butt joints fastened with HI-LITE fasteners or rivets. Impact hammer tests were used to me asure their natural frequencies, mode shapes, and damping ratios. The results were compared with numerical predictions based on current indu stry modelling practices. Next, shaker tests were used to investigate the nonlinear behavior of the fastened connections. The most significa nt discrepancy between measurements and predictions were due to modell ing idealizations of the joints. Nonlinearities were also important, t hough to a lesser extent.