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

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

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.

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

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

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

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

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

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

Typical FRF Results

Typical FRF Results

Typical FRF Results

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

Material Selection Material Selection Criteria: Current materials: High specific tensile modulus (elastic modulus / density) Lightweight / low density Meets FAR 29.853 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

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

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

Performance Tradeoff Higher Performance Higher Performance

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

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

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

Questions?