1. Development of Light Weight Composite Constraining Layers for Aircraft Skin Damping
RAM 7 Workshop
November 4 & 5, 2014
Nick Oosting
Roush Industries

2. Overview Background Introduction and Objectives
RAM 6 Workshop, October 2013
“Noise and Vibration Control with Constrained Layer Damping Systems”
Introduction and Objectives
Constrained Layer Damping Theory
Material Selection
Experimental FRF Testing
Conclusions and Next Steps 2

3. Introduction and Objectives
Constrained Layer Damping (CLD) treatments are a very efficient means of adding damping to aircraft skin panels.
Traditional CLD treatments use aluminum as the constraining layer material.
Previous work by Roush has shown that lightweight carbon fiber composite materials can be effectively used as constraining layers in CLD treatments while helping to reduce the overall weight of the treatment.
The objective of this work is to explore the use of high modulus carbon fiber materials to further reduce the weight while maintaining the damping performance of the CLD treatment.

4. Constrained-Layer Damping Theory
Energy dissipation using constrained-layer damping (CLD) is achieved by shearing a viscoelastic polymer between a base structure and a constraining layer as depicted below.
Viscoelastic Polymer
Constraining Layer
Base Structure
The energy dissipation created by a CLD is typically quantified in terms of loss factor (η), a dimensionless quantity that can be measured or predicted from the modal damping of a dynamic system.
Performance Variables:
Base Structure Dynamic Properties
Materials (modulus, damping and density)
Thicknesses
Coverage (location and coverage on base structure)
Temperature 4

5. CLD Advantages
Very high levels of damping compared to other damping methods
Can be very weight efficient
Many viscoelastic damping materials are available to choose from
Can be selectively applied to highly responsive areas
Does not require much packaging space
Easily applied to existing structures
Potential to increase impact dent resistance of aircraft skin panels 5

6. CLD Treatment Performance Assessment
RKU Modeling
Advantages:
Quick evaluation of many types of viscoelastic materials and their temperature effects
Quick evaluation of many types of constraining layers
Quick evaluation of viscoelastic material and constraining layer thickness effects
Limitations:
Complex shapes and boundary conditions can not be modeled
Not applicable for CLDs with less than 100% surface area coverage
FEA Modeling
Complex structural shapes and boundary conditions are easily modeled
CLD surface area coverage can be of any size
Computing resources and solve times are significantly greater
Modal loss factor is not a direct output of the model and needs to be computed using the half-power bandwidth method or the impulse response decay method. 6

7. CLD Treatment Performance Assessment
Experimental Testing
Advantages:
Complex structures and boundary conditions may be evaluated
CLD surface area coverage can be of any size
Accurate real world test results can be achieved if test is constructed properly
Limitations:
Slow/costly due to test fixture and sample construction time
Physical samples of constraining layer and damping material required
Difficult to evaluate temperature effects 7

8. Experimental Panel FRF Evaluations
Fixture was built for experimental FRF testing of sample plates of a typical aircraft skin panel size of 21” x 5.5”
Fixture provide clamped boundary conditions on all sides. Base plates were 2024-T6 aluminum, 0.025” thick
CLD constructed with Roush RA640 damping adhesive, 0.005” thick
FRF measurements were made at three locations on the panel
CLD treatments tested at various coverage levels (100%, 75%, 60%)
Driving
Point

9. Typical FRF Results
1,1
2,1
Typical Plate Bending Modes

10. Typical FRF Results

11. Typical FRF Results

12. Typical FRF Results

13. Effects of Constraining Layer Stiffness
Increasing the constraining layer thickness creates more damping and
increases the resonance frequencies(esp. at low temps), but will increase
the CLD weight and may be harder to adhere
Goal is to increase the stiffness of the constraining layer without increasing the mass, i.e. maximize the specific modulus (Young’s Modulus / Density) 13

14. Material Selection Material Selection Criteria: Current materials:
High specific tensile modulus (elastic modulus / density)
Lightweight / low density
Meets FAR requirements
Manufacturability/durability
Current materials:
Thickness (in)
Density (lb/in3)
Elastic Modulus (psi)
Specific Modulus (in x 10^8)
Areal Weight (lb/in^2)
Aluminum
0.0100
0.100
10.60E+06
1.06
10.00E-04
FMI Carbon Fiber
0.0110
0.043
5.30E+06
1.23
4.73E-04
Selected new CF materials:
Thickness (in)
Density (lb/in3)
Elastic Modulus (psi)
Specific Modulus (in x 10^8)
Areal Weight (lb/in^2)
FMI+Nano
0.0100
0.044
6.53E+06
1.48
4.40E-04
Granoc 1 Layer
0.0065
0.049
23.50E+06
4.80
3.18E-04
Granoc 2 Layer
0.0130
6.37E-04
Saati
0.0110
0.040
14.50E+06
3.63
Cytec 1 Layer
0.0055
0.043
7.98E+06
1.86
2.37E-04
Cytec 2 Layer
4.73E-04

15. Specific Modulus (in x 10^8)
Material Selection
Selected new CF materials:
Thickness (in)
Density (lb/in3)
Elastic Modulus (psi)
Specific Modulus (in x 10^8)
Areal Weight (lb/in^2)
FMI+Nano
0.0100
0.044
6.53E+06
1.48
4.40E-04
Granoc 1 Layer
0.0065
0.049
23.50E+06
4.80
3.18E-04
Granoc 2 Layer
0.0130
6.37E-04
Saati
0.0110
0.040
14.50E+06
3.63
Cytec 1 Layer
0.0055
0.043
7.98E+06
1.86
2.37E-04
Cytec 2 Layer
4.73E-04
Material Description:
FMI+Nano: standard modulus carbon fiber loaded with carbon nanotubes
Granoc: ultra high modulus carbon fiber (Pitch base fiber)
SAATI: very high modulus aerospace carbon fiber
Cytec: high modulus carbon fiber, spread weave

16. Damping for Full Octave Band Range
Averages of the loss factors were obtained for the modes within the 250, 500, 1000 and 2000 Hz octave bands
Experimental FRF data was processed through LMS Modal Parameter Estimation to obtain loss factor information

17. Performance Tradeoff
Higher
Performance
Higher
Performance

18. Optimized weight/performance FRF Results
Saati 75% coverage is approaching performance level of 75% aluminum coverage with ~28% weight reduction
Weight Savings*:
FMI CLD: 4.2%
Saati CLD: 28.3%
*vs 75% AL CLD

19. Conclusions
SAATI material shows improvement over the aluminum constraining layer system with weight reduction of approximately 28%
Granoc and Saati materials show significant improvement to damping performance
Very thin pitch based fiber systems, such as the Granoc material, may not be viable due to brittleness which can lead to cracking during manufacturing and installation
The dual layer Granoc system was stable but had a weight penalty when compared to the Saati material
Carbon nanotube loading of FMI material has negligible effect on damping performance
Cytec spread tow material shows no improvement

20. Next Steps Investigate Ultra High Modulus Saati materials
Investigate performance of graphene and bulk carbon nanotube sheet material in CLD applications
Continue work on impact dent resistance with carbon fiber CLD systems
Cost analysis

21. Questions?