Fluid Ingression Damage Mechanisms in Composite Sandwich Structures
Dr. John Tomblin, Executive Director, National Institute for Aviation Research, Wichita State University.
Allison Crockett, Research Engineer, Composites Laboratory, National Institute for Aviation Research, Wichita State University.
Honeycomb sandwich construction techniques have been and are currently being used widely throughout the aerospace industry. These types of sandwich constructions have been shown to be ideally suited for fuselage structures as well as control surfaces throughout the industry.
A critical issue for the sandwich construction concept is the environmental durability. Skin panels may be exposed to water and other fluids both interior and exterior of the aircraft in service. Exposure to these fluids in combination with thermal and mechanical loads may result in degradation of these structures over time.
Sandwich structures have been observed to absorb moisture, which results in increased weight, degradation of the core and facesheet materials, and degradation of the core-to-facesheet interface bond. In sandwich construction, the facesheets take the in-plane loads (tension/ compression) and the core takes the shear loads. It is important to understand the fluid ingression paths and damage mechanisms that result in decreased structural performance as related to continued airworthiness of the aircraft.
Historically, fluid ingression has been shown to enter the structure through several channels. Free-water or fluid can ingress easily through damaged facesheets/skins, near edges or penetrations through panels where seals are degraded or damaged. Wicking may occur through rivet holes or impact damage and by diffusion through the epoxy matrix on the composite skin, particularly in the case of thin facesheets, as shown in the image below. A number of reports have documented service problems due to water ingression and have described ways to avoid these types of problem in future designs.
Potential fluid ingression dut to thin facesheets.
The objective of the research program is to characterize the fluid ingression phenomenon in composite sandwich structures, as well as study the rate of ingression and document the damage mechanisms that allow the fluid ingression to propagate and potentially degrade the structural performance. There are various damage mechanisms that contribute to the moisture ingression problem, and different damage mechanisms will contribute differently to the rate of moisture ingression. The research will attempt to isolate each mechanism independently to study the effects on the rate of moisture ingression in typical honeycomb composite sandwich constructions. This act will provide key guidelines for where additional inspection intervals may need to be implemented in the fleet.
Panels from an Adam Aircraft fuselage and a Beechcraft Starship fuselage, tail and nose sections have been secured for the study. The Adam Aircraft panels not only are representative of a sandwich panel with honeycomb core, but more specifically they represent a sandwich panel with NOMEX core manufactured to higher quality control standards than in years past. The larger fuselage panels have been machined down to nominal 14” x 14” panels that are to be used for the fluid ingression study. These panels will be C-scanned and then damaged.
Various ways of impacting the panels were reviewed; the favored method is to use the INSTRON Impact Machine at NIAR to impact the panels with various energy levels. Using a 3-inch impactor, lower level of energy levels will be impacted first, increasing the energy until reaching a damage level that visibly damages the top facesheet so much that the bottom facesheet will be visible. Once the damage has occurred, the panels will be exposed to an aggressive and extreme ground-air-ground (GAG) cycle. This GAG cycle will not only saturate the damaged panels at a hot, wet condition but then also expose the damaged panels to a dry and brittle cold.
Once the fluid ingression rates can be isolated and quantified, there will be several other tasks performed to identify and explore using the new data, as well as known fluid ingression/migration mechanisms. Study of the GAG cycling of the disbonded sandwich constructions will allow standard test methods to be established. With standards in place, mechanics of disbond growth in sandwich panels can be understood. Intercellular diffusion and the permeability of the cell wall will be studied, as well as the filleting quality during the freeze thaw cycle. Identifying the effects of thermal exposure on the permeability and filleting quality of the sandwich panel will be key to understanding and aiding in the characterization of the fluid ingression phenomenon.
Characterization of fluid ingression phenomenon in composite sandwich structures Isolation of damage mechanisms and their effects of rate of moisture ingression Development of guidelines for implementing additional inspection intervals Standardization of test methods for GAG disbond sandwich panels Comprehension of mechanics of disbond growth in sandwich panels Identification of potential problems with the following related to fluid ingression: intercellular diffusion, cell-wall permeability, quality of machined honeycomb core, filleting quality and adhesive strength
The FAA's Center of Excellence for Composites and Advanced Materials (CECAM) provides the nation with a center for the validatino and quality assurance of composites and advanced materials to be applied in the construction of aircraft through research, testing, certfication and technology transfer; coordination and cooperation with the FAA, aircraft manufacturers, materials suppliers and airline companies; and education of the aircraft manufacturing and maintenance workforces. CECAM is part of the Joint Advanced Materials and Structures Center of Excellence (JAMS). CECAM is led by Wichita State University, with core members from Northwestern University, Purdue University, Tuskegee University, the University of Delaware and the University of California at Los Angeles.