SBIR-STTR Award

Sandwich structural designs are the most weight efficient concepts for stability-critical components such as control surfaces. The objective of this proposal is to demonstrate the feasibility of producing honeycomb sandwich structure using a unique Double Walled Honeycomb design. This new ARDL honeycomb design is called Hexagonal Double walled structure or "HexD" This is depicted in Figure 1. HexD honeycomb core exhibits a unique cell structure. This design promises a higher stiffness to weight ratio, barrier resistance, and higher impact resistance. Higher stiffness has already been demonstrated using finite element analysis and prototype testing. Figure 2 illustrates the significance of the using HexD over a common regular honeycomb in almost all the aspects of its usage as a control surface. The primary function of the face sheets is to provide the required bending and in-plane shear stiffness, and to carry the axial, bending, and in-plane shear loading. The primary function of the core is to stabilize the facings and carry most of the shear loads through the thickness. In order to perform this function the core must be as rigid and as light as possible. HexD performs these functions in a rather unique manner by having 86% more bond lengths and twice the walls. The primary structural role of the adhesive, which is the face/core interface in sandwich construction, is to transfer shear stresses between faces and core. This condition stabilizes the faces against rupture or buckling away from the core. It also carries loads normally applied to the panel surface and prevents the moisture intrusion and subsequent degradation. Overall stiffness of the sandwich component is also a key consideration in design for general instability of elements in compression. Prior applications of sandwich structures on in-service vehicles have resulted in costly manufacturing and supportability problems. HexD components resist transverse shear and normal compressive and tensile stress resultants. Manufacturing problems have been caused by core materials that are difficult to machine and difficult to bond, difficulties in panel joining, and difficulties with the integration of hard points for concentrated load introductions. Supportability problems have resulted largely due to the integration of materials and concepts that are highly susceptible to impact damage and moisture intrusion that leads to corrosion, mechanical degradation of the sandwich core, and delamination in core-to-face sheet bonds. This proposal is for developing a new innovative sandwich structure solution that is producible, mitigate the historical shortfalls of sandwich, and provides high structural efficiency. This sandwich structure design will feature double layer core and skins, and damage and moisture resistant materials. HexD is further accentuated by using the revolutionary technology of nanocomposite adhesion. It is developed for the aerospace industry that relies heavily on the structural integrity and optimum strength to weight advantage of rigid honeycomb composite. Improved moisture resistance and stability are the unique twin characteristics of the proposed design, is developed based on the double honeycomb. HexD core cells have a unique 12 walled design compared to the 6 walled design common with the regular honeycomb. Shear strength is also improved due to the self-stiffening mechanics unique to a double walled structure with the edge of the inner honeycomb reinforcing the outer cell structure. Effectively each cell has a double wall layer with higher stability. Optimizing the sandwich structure and making an unusually strong structure will enhance safety and probably will be directed toward molding advanced composites made of polymeric materials such as nanoparticle filled adhesives. The new materials and special manufacturing methods adapted from other fields (racecars, aerospace, boat building, etc) to achieve high volume and low cost would completely change the way of making sandwich structures. The new methods could offer the manufacturer a much easier, in some cases, an order of magnitude lower product cycle time, capital investment, assembly effort, product reliability and product cost. The facility cost, risk and simplification of production would greatly improve under this concept. ARDL's commercialization strategy rests on the premise of such potentially decisive competitive advantages that would easily reward adopters and encourage rapid market entry. However, while this proposed design is intended for military aircrafts, the adoption of this design concept would require major technological changes in the automotive industry in regard to overall weight and impact resistance of the automobile.

Sandwich structural designs are the most weight efficient concepts for stability-critical components such as control surfaces. The objective of Phase I of this study was to demonstrate the feasibility of producing a honeycomb sandwich structure using a unique Double Wall Honeycomb design. This new ARDL honeycomb design is called Hexagonal Double wall structure or "HexD". This is depicted in Figure 1. The Phase II project will involve further development and demonstrate the producibility and robustness of the HexD Sandwich Structure concept on a generic component whose design is derived from a current or emerging aircraft. This effort will include the configuration, analysis, testing and fabrication of a subcomponent. This subcomponent should be subjected to environmental conditioning, simulated flight loads and impact. A repair concept will be developed and demonstrated in Phase II. HexD honeycomb core exhibits a unique cell structure. This structure promises a higher stiffness to weight ratio, improved barrier resistance, and higher impact resistance. Higher stiffness has been demonstrated using finite element analysis and prototype testing. Preliminary results from Phase I illustrate the benefit of using HexD over a common regular honeycomb in almost all the aspects of its usage as a control surface. The primary function of the face sheets is to provide the required bending and in-plane shear stiffness, and to carry the axial, bending, and in-plane shear loading. The primary function of the core is to stabilize the facings and carry most of the shear loads through the thickness. In order to perform this function the core must be as rigid and as light as possible. HexD performs these functions in a rather unique manner by having 86% more bond lengths and twice the number of walls of a conventional honeycomb structure. The primary structural role of the adhesive, which is the face/core interface in a sandwich construction, is to transfer shear stresses between faces and core. This condition stabilizes the faces against rupture or buckling away from the core. It also carries loads normally applied to the panel surface and prevents the moisture intrusion and subsequent degradation. Overall stiffness of the sandwich component is also a key consideration for stability of elements in compression. Prior applications of sandwich structures on in-service vehicles have resulted in costly manufacturing and supportability problems. HexD components resist transverse shear and normal compressive and tensile stress resultants. Manufacturing problems have been caused by core materials that are difficult to machine and difficult to bond, difficulties in panel joining, and difficulties with the integration of hard points for concentrated load introductions. Supportability problems have resulted largely due to the integration of materials and concepts that are highly susceptible to impact damage and moisture intrusion that leads to corrosion, mechanical degradation of the sandwich core, and delamination in core-to-face sheet bonds. This proposal is for developing a new innovative sandwich structure solution that is machineable, mitigates the historical shortfalls of conventional sandwich constructions, and provides high structural efficiency. This sandwich structure design will feature double layer core and skins, and utilizes damage and moisture resistant materials. HexD is further accentuated by using the revolutionary technology of nanocomposite adhesion. Nanocomposite adhesives were developed for the aerospace industry which relies heavily on the structural integrity and optimum strength to weight advantage of rigid honeycomb composites. Improved moisture resistance and stability are the unique twin characteristics of the proposed adhered structure based on the double honeycomb. HexD core cells have a unique 12 walled design compared to the 6 walled design common with the regular honeycomb. Shear strength is also improved due to the self-stiffening mechanics unique to a double walled structure with the edge of the inner honeycomb reinforcing the outer cell structure. Effectively each cell has a double wall layer with resultant higher stability.

Keywords:
sandwich structural, control surfaces, double wall structure, hexd, honeycomb, stiffness to weight ratio, barrier resistance, impact resistance