Composite Honeycomb Treatment via Non-Obstructive Particle Damping (NOPD)

Abstract

The objective of this work was to study the effectiveness of non -obstructive particle damping (NOPD) treatment in honeycomb (HC) structures. The NOPD approach entails development of a design and procedure for such a treatment, based on finite element modeling (FEM) analyses and tests. Laboratory tests were conducted to evaluate the performance of various particles u nder different vibration environments. Modeling and predictions were carried out simultaneously as tests were in progress and the model was correlated with the test data. This paper describes results of modal tests carried out at the Boeing RPP (Rocketdyne Propulsion a nd Power) Engineering Development Laboratory (EDL) on honeycomb panels and FEM analyses for the prediction of its modal characteristics. Introduction: Honeycomb structures are typically stiff. Therefore, flexural waves induced by loads, from such source s as turbulent boundary layers, have structural velocities that travel faster than the speed of sound in the surrounding air. The latter makes the honeycomb structures efficient radiators of noise at most frequencies, even down to the low end of the spect rum. Damping can help reduce the amplitudes of these waves at all frequencies. The higher the damping achieved, with a reasonably low weight penalty, the more beneficial it is in reducing the amplitude of vibration. It is relatively easy to absorb vibra tion energy in the high frequencies. Many damping treatment options exist, including viscoelastic materials. NOPD, on the other hand, could provide significant damping even at low frequencies, which could make its use very desirable 1,2,3 . To study the dam ping effectiveness of light particles placed inside the cells of stiff honeycomb structures a test and analysis program was carried out at RPP in the Fall of 2002: finite element analyses were carried out to predict the modal characteristics of HC panels, and to correlate those characteristics with laboratory modal test results. Tests were conducted with different particle materials filling the honeycomb cells. The panels were suspended with rubber bungee chords and structurally excited by shakers. The resp onse amplitudes and damping values, predicted using the test -data were compared under no particles with the ones with various particles filling the cells. Statistical Energy Analysis (SEA) was then carried out to predict the acoustic attenuation profile in the frequency range of interest. Analytical Design Developed and Correlated with Test Results: Square pieces of 2 ft. x 2 ft. x ½ in. HC were tested in the lab for modal characterization with different particle fillings. Several analyses were performed to estimate the passive damping material properties of the various NOPD materials based on test data obtained in earlier tests. These properties were then used to predict the damping of different panels with different particle treatments. The process requ ired several finite element models created using Abaqus (a commercially available FEM code with attractive non -linear analysis capabilities) composite laminate shells to represent the honeycomb structure with fiberglass face -sheets. The face -sheets were t reated as orthotropic since they were layered symmetrically across their thickness with respect to orientation angle of the fibers of individual layers. The honeycomb core itself was modeled as an orthotropic material with perturbations in density and dam ping to represent the assortment of NOPD materials tested. For correlation with tests and determination of particular modal damping, modal frequency dependent composite damping was used. Also, the quasi -analytical design sensitivity for frequency respons e in Abaqus was found to be useful during the correlation steps. The measured weight of the fiberglass panel was used in all of the analyses. Stiffness of the composite panel was correlated through the empty panel frequencies and mode shapes. The dampi ng values were then matched by test/analysis correlation. The values for the general modal characteristics of the various particles were then used in the acceleration response predictions, as is described below: The FEM analyses and test results indicated numerous modes starting at around 63 Hz. The FEM was used to predict the modal characteristics, with the cells modeled as individual solid elements, and with the face sheets modeled as separate solid elements. The FEM predictions showed a first bending mo de at 75 Hz with the uncorrelated model, and after correlation with test data at 63 Hz. The FEM was modified to reflect the mass and damping effects of each kind of particle on the honeycomb response and correlated with test data. Figure 1 shows the variou s FEM mode shapes of the baseline (empty) panel side -by -side with the measured mode shapes from the test results. The panels were analyzed and the results were correlated with the test data from previous tests as follows: