Unitized Textile Composite Stiffened Panels for Space Structures; Manufacturing, Characterization, Modeling, and Analysis

The primary goal to be attained with this research investigation is to demonstrate and gain an understanding of the mechanics of post-buckling behavior of unitized textile composite stiffened panels manufactured to remove stiffener separation as a primary failure mode. One significant failure mode of stiffened composite structures, particularly in compression, is stiffener separation leading to a catastrophic loss of load carrying capability. Much research has been done on the manufacturing and modeling sides of bolted composites, co-curing techniques, Z-pinned structures, and even completely stitching the stiffener preform to the shell. This fellowship work looks to remove the added machining required and corresponding modeling complexity by simplifying the structure via a unitization of the stiffener and shell. Each stiffener is formed from the same textile layer as that used to create the skin, thus removing possible areas for debonding. Tri-axially braided composite (TBC) textiles were used due to ease of handling the preform material in forming the stiffeners, as well as good suitability for the Vacuum-Assisted Resin Transfer Molding (VARTM) manufacturing process used to infuse matrix to form a fiber reinforced polymer composite. Textiles also offer potential for increased performance by taking advantage of manufacturing modifications such as variable angle tow braiding. To achieve this goal, four areas of research are conducted. Each area focuses on one aspect of the unitized textile stiffener concept and provides information, verification, or other data to quantify and qualify the success of the unitized concept. The four areas of research are manufacturing unitized stiffened textile panels, characterizing the manufacturing process and as manufactured panels, experimentally demonstrating the ability of the unitized concept to achieve stiffener separation removal, and computational verifying the ability to be able to model such a unitized structure. Manufacturing unitized stiffened panels was successful and currently represents a new development in active research. Stiffeners were formed from the same textile layers used to create the shell skin, thereby removing the need for co-cured, adhesively bonded, or other attachments in creating a stiffened shell structure. Other works have been labeled as a unitized structure, but this is the first demonstration where the preform stiffened structure cannot be separated from the underlying skin. Successful manufacturing also demonstrates that current manufacturing capabilities can be used, with no or slight modification, to implement the unitized Characterization of the unitized panels included geometric laser scanning using a Coordinate Measurement Machine (CMM) at NASA Langley to determine the effects of the unitized concept and VARTM process on the as-manufactured specimens, acid digestion to determine global fiber- and void- volume fractions and panel quality, and optical imaging to determine local tow-level fiber volume fraction. The information gathered from the characterization processes was used in the modeling of unitized structures. The result of the geometric scanning showed that there were systematic geometric differences between the nominal model and the as manufactured geometries. The stiffener spacing was the largest deviation from the nominal model, as the stiffeners were spaced farther apart than designed. The acid digestion and optical imaging tests confirmed that the panels were as high quality as currently possible with less than 1% void volume measured and unchanged fiber volume fractions as compared to previously studied manufacturing methods. Experimental compression tests were conducted on the manufactured unitized stiffened panels. Tests were conducted to capture buckling and post-buckling response, failure load, and failure mode data. Detailed analysis of the panels post-testing showed that the unitized stiffener concept succeeded shifting the primary failure mode from stiffener separation to a material failure mode. The primary failure mode was fiber rupture in compression and local tow buckling due to non-linear matrix (i.e. damage) behavior prior to ultimate failure. Computational efforts to capture the experimentally observed compression response are in good agreement, though the final level of modeling remains a work in progress. A finite element model developed using Abaqus shows a structural level model is able to capture up to 85% of the complete non-linear compression response of the stiffened panels without the need for special modeling detail of the stiffeners. To capture the final failure mode and loads accurately, a multiscale framework has been developed to include details of the TBC architecture. This framework aims to bring the last 15% of the compression response in agreement by capturing local stress concentrations and non-linear material behavior, both of which were excluded from the structural model. This effort demonstrates that a simple structural shell model can accurately model the response of unitized stiffened panels, while final panel failure aims to be captured using material failure properties rather than structural modeling methods. While the final computation modeling effort is still under development, the results from the manufacturing, characterization, experimental testing, and primary modeling shows strong potential for the unitized structure concept. All eight panels were successful in removing stiffener separation as a primary failure mode when loaded in uniaxial compression. An idea to simplify the manufacturing of composite structures shows that stiffness and strength are not the only parameters that can or should be used in design. Manufacturability, post-manufacturing preparation, overall failure mode, and other details can all be included in creating a structure to meet performance requirements. Designing a structure to remove a failure mode affects modeling efforts, and complex structural models are not necessarily needed to accurately capture the response when observed failure results from material behavior instead of structural phenomenon.