This document discusses research into the vibro-acoustic design of aluminum sandwich plates. A new concept called an ECOCELL core made of aluminum plates aims to simplify existing resin core sandwich plates. Various core configurations and face sheet thicknesses were modeled and tested numerically. Randomizing core stiffness properties and optimizing the ECOCELL design were found to improve acoustic performance. An initial prototype was manufactured with issues. Future work includes optimizing designs using genetic algorithms and experimental validation.

1. DEMEASS IV, March 26-30 2011 Urspelt (Luxemburg) Progresses on the vibro-acoustic design of a class of aluminium sandwich plates Vincenzo D’Alessandro, Francesco Franco, Sergio De Rosa, TizianoPolito ælab‐Vibrations and AcousticsLaboratory Departmentof AerospaceEngineering Università degli Studi di Napoli “Federico II” Via Claudio 21, 80125, Napoli, Italy www.dias.unina.it

3. References

4. DEMEASS III: A Review

5. ECOCELL Core

6. Investigated Configurations

7. Numerical Results

8. Experimental Tests

9. Conclusions and Future WorkProgresses on the vibro-acoustic design of a class of aluminium sandwich plates 2

11. A new concept of sandwich plate (all aluminum based) was tested in order to get the typical results available with more complicated configurations.

12. This work is the straight continuation of the work presented in the last DEMEASS.

13. Activities herein presented will be continued and extended under the project SUPERPANELS (www.superpanels.unina.it).Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 3

14. References Kurtze and Watters (1959) studied the application of sandwich panels to increase the sound insulation between adjoining spaces. They investigated the relation between bending and shear waves and TL characteristics. Lang and Dym(1975) optimized the design of a sandwich panel with the goal to exceed the TL values predicted by the mass law by at least 20 dB in a selected frequency range. Barton and Grosveld(1981) considered an aeronautical application of honeycomb panels to improve sidewall attenuation in a light twin-engine. Thamburaj and Sun (2002) demonstrated that an anisotropic core can lead to higher TL and that the proper design of face sheet thickness can further improve the performance. NASA tests (2002) about the performance of sandwich structures with a core made of a lattice of truss elements are available. Cunefare et al. (2003) gave additional indication that structural acoustic optimization has the potential to achieve significant gains to reduce interior noise levels in aerospace structures. Franco et al. (2007) analyzed the optimization of the structural-acoustic characteristics of various and innovative sandwich configurations. They considered different core configurations, and those having a truss geometry have been very promising configurations, since it is possible control the stiffness along the two direction in-plane of the panel. Franco, De Rosa and Polito(2010) demonstrated that the vibro-acoustic responses can improve if a random stiffness is imposed over an optimized configuration, highlighting that randomization represents a very cheap simulation step. Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 4

15. DEMEASS III: A Review Sandwich panel with truss-core 18128 and 23664 rodsalong the X and Z axes, respectively Effect of a randomization of the stiffness properties on the optimized configuration has been analyzed. Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 5

16. DEMEASS III – A Review @ 740 Hz Optimized resin Optimized+Randomized resin Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 6

17. ECOCELL Core: Concept Stereolithography is a costly and complicated procedure. Need for both simplified plates and related models able to take into account the peculiarities of the configurations and results of the sandwich plate with resin core. Thinking to the LEGO© elementary brick concept and classical carton filler for packaging . . . . Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 7

18. ECOCELL Core: Concept – Basic Unit Equivalent COreCELL: an aluminium (plate-like) basic element able to reproduce the complexity of the resin core cells. Core modular element (dimensions in mm) Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 8

19. ECOCELL Core: Global view Truss-like core 16 stiffeners along x-axis 11 stiffeners along z-axis Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 9

20. ECOCELL Core: Global view of the sandwich Basic Sandwich Panel Configuration Lx=0.640 m, Lz=0.420 m FE mesh: 10965 grid points 10752 four-point plate element. 2 m Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 10

21. Vibro-acoustic Indicators Continuos Discrete v(ω) is the velocity vector R(ω) is the radiation resistance matrix Ais the nodal equivalent areas matrix Thus, it is possible, on the base of the results achieved from a frequency response analysis of the finite element model of a generic plane structure, to calculate the radiated acoustic power. Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 11

22. Investigated Configurations Coding: CODE_THmm Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 12

23. Numerical Results (cont’d) Influence of CoreConfiguration 106 Base_1mm D_1mm E_1mm 105 C_1mm 104 V2MS [m2/s2] 103 102 101 01000 2000300040005000 Frequency [Hz] Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 13

24. Numerical Results (cont’d) Influence of CoreConfiguration 80 Base_1mm D_1mm E_1mm 70 C_1mm dB 60 50 40 C_1mm configuration 30 01000 2000300040005000 Frequency [Hz] Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 14

25. Numerical Results (cont’d) Influence of Face-Sheet Thickness 80 Base_1mm Base_2mm Base_3mm 70 dB 60 50 40 30 01000 2000300040005000 Frequency [Hz] Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 15

26. Numerical Results (cont’d) Randomness of the Core Stiffness (1/4) It was investigated the effect of the addition of a random distribution of the core stiffness. The random distributions were based on two (uncoupled) Gaussian functions. 106 Base_1mm sr = 40% sr = 60% 105 sr = 70% 104 V2MS [m2/s2] 103 102 sr = 70% 101 01000 2000300040005000 Frequency [Hz] Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 16

27. Numerical Results (cont’d) Randomness of the Core Stiffness (2/4) Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 17

28. Numerical Results (cont’d) Randomness of the Core Stiffness (3/4) 106 C_1mm C7_1mm C1_1mm C2_1mm 105 C3_1mm C4_1mm C5_1mm C6_1mm 104 C7_1mm V2MS [m2/s2] 103 102 101 01000 2000300040005000 Frequency [Hz] Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 18

29. Numerical Results (cont’d) Randomness of the Core Stiffness (4/4) 106 80 C_1mm C1_1mm C2_1mm 70 C3_1mm C4_1mm C5_1mm 60 C6_1mm dB C7_1mm 50 40 30 20 01000 2000300040005000 Frequency [Hz] Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 19

31. Twenty-seven design variables: thickness of the stiffeners

32. Constraint: weight of the core, i.e. the thickness of the stiffeners are explicitly linked because is imposed a constraint on their variations so as to keep the core weight less or equal to the value of the initial configuration

33. Objective Functions: the simplest objective function is the average of the square structural velocity on the radiating face sheet and over the chosen frequency range

34. NG = number of discrete sample points on the face of the panel

35. NF = number of frequency sample pointsProgresses on the vibro-acoustic design of a class of aluminium sandwich plates 20

36. Numerical Results (cont’d) Optimization of the ECOCELL Core (2/2) 104 C7_1mm Optimizerconfiguration 103 V2MS [m2/s2] 102 101 1800 200022002400 Frequency [Hz] Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 21

37. Experimental Tests – The 1st prototype Manufacturing Problems ECOCELL core in steel instead of aluminum alloy Face sheet 3 mm thick and grooved in correspondence of the stiffeners positions to facilitate their installation Pods filled with epoxy glue There is contact in the intersection of stiffeners! Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 22

38. Experimental Tests – The 1st prototype Modal analysis – Impact testing by using LMS TESTLab Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 23

40. Vibro-acoustic response of several articles with common structure and difference in skin and core have been analyzed in terms of mean square velocity and radiated acoustic power, evaluating the influence of core configurations.

41. Confirm that randomization approach allows an improvement of vibro-acoustic behavior.

43. Find the right technology to manufacture the sandwich panel, in order to compare numerical and experimental tests.Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 24

44. Thanks foryourattention Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 25

46. The imposed boundary condition has both the face-sheet simply supported on their four edges.

47. In all models the in-plane dimensions of the panel are kept constant.

48. Sandwich panels with the same total mass are compared since the mass variation is a critical design parameter due to the characteristics of panel TL vs. mass.

49. The dynamic load was a pressure distribution according to a monopole acoustic source located 2 m. from the plate.

50. The structural-acoustic response is characterized in terms of mean square velocity - averaged over the spatial domain – and radiated acoustic power, calculated by using a finite element approach.

51. Numerical Optimization: MSC/NASTRAN optimizer (SOL 200) - Modified Method of Feasible Directions.;

52. Randomization: effect of the addition of a random distribution of the core stiffness. The random distributions were based on two (uncoupled) Gaussian functions.Progresses on the vibro-acoustic design of a class of aluminium sandwich plates 26