Theses and Dissertations at Montana State University (MSU)
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Item Forming properties of stretch broken carbon fiber for aircraft structures(Montana State University - Bozeman, College of Engineering, 2023) Nold, Dalton Bradley; Chairperson, Graduate Committee: Dilpreet S. Bajwa; Douglas S. Cairns (co-chair); This is a manuscript style paper that includes co-authored chapters.Continuous carbon fiber is known to be a superior material for its strength, stiffness, and high strength-to-weight ratio and is often incorporated in aerospace composites. A challenge, however, is that it's not versatile in forming deep drawn geometries, which require convoluted manufacturing techniques resulting in expensive components. To overcome this, a type of carbon fiber with a random discontinuous fiber alignment called stretch broken carbon fiber (SBCF) is proposed. SBCF has potential to form parts with complex geometries with comparable or better mechanical properties to that of continuous carbon fiber. Montana State University (MSU) developed its own version of SBCF manufacturing processes, and research is being conducted to understand how SBCF prepreg tows react to stretch drawing at elevated temperatures using aerospace-grade epoxy resin systems. Currently, several new methods have been proposed to rapidly test these materials. This research revealed that SBCF forms with greater ease than continuous carbon fiber and is expected to substantially reduce manufacturing times for aircraft structures. To comprehend the material's behavior, simple tensile tests were coursed to understand how gauge length and temperature affected the peak loads when compared to continuous carbon fiber. It was discovered that on average, SBCF experienced stresses that were ten times less than continuous fibers. Additional tensile tests were conducted at elevated temperature to determine the true stress versus true strain. These tests are particularly important because they represent the material's most accurate mechanical properties. The results led to the discovery that SBCF experienced strain softening behavior. Furthermore, a series of forming tests using a novel "forming fixture" revealed that increasing the gap lowered the peak forming loads while the plunger geometry had little to no effect on peak forces at both room and elevated temperatures.Item Preliminary mechanical testing of continuous stretch broken carbon fiber cured laminates(Montana State University - Bozeman, College of Engineering, 2022) Loomis, Noah Michael; Chairperson, Graduate Committee: Dilpreet S. BajwaApplication of carbon fiber prepregs in cost-sensitive, high-volume structural applications are limited due to the difficulty to form deep drawn parts. Stretch broken carbon fiber (SBCF), is an aligned discontinuous form of carbon fiber that is under development at Montana State University (MSU). The improved SBCF has the potential to increase the formability of these carbon fiber prepregs. However, any formability benefits of SBCF would be limited if the laminates have reduced mechanical properties when compared to conventional continuous carbon fiber composites. Two studies were performed to evaluate the mechanical potential of MSU SBCF. The first study compared the 0° unidirectional tensile mechanical properties of continuous carbon fiber and SBCF laminates both manufactured by MSU at ambient room temperature. Materials included Hexcel IM7-G continuous carbon fibers and SBCF using a Solvay Cytec 977-3 resin as the matrix. The results of the study show that the unidirectional tensile mechanical properties of stretch broken carbon fiber laminates did not significantly differ from the continuous laminates. Normalized for a fiber volume of 60%, the MSU continuous and stretch broken materials had nearly equivalent tensile properties, and their properties were within 15% of stated values for commercial material. Failure modes and strain to failure were nearly identical between the two types of laminates. The second study compared tensile, compression and shear mechanical properties of commercial continuous carbon fiber and MSU SBCF laminates at ambient room temperature. Materials included Hexcel IM7-G carbon fiber with Hexcel 8552 resin as the matrix. The preliminary results of the study showed that the fiber dominated mechanical properties of stretch broken carbon fiber laminates were slightly lower when compared to the commercial continuous laminates. Fiber dominated mechanical properties had at least a 17% loss of strength while matrix dominant mechanical properties were found to be equivalent and unchanged.Item Forming evaluation metrics and methods for complex stretch broken and continuous carbon fiber parts(Montana State University - Bozeman, College of Engineering, 2023) Dube, Madison Eve; Chairperson, Graduate Committee: Cecily RyanCarbon fiber composites are an ideal material for aircraft structures due to their light weight and high strength and stiffness. Traditional continuous carbon fiber composites are limited by their ability to form into complex geometric features as they have minimal abilities to stretch into a mold. Stretch broken carbon fiber is discontinuous which allows for the different tows of carbon fiber to slip relative to each other allowing for laminate displacements unattainable to continuous carbon fiber. In order to develop and improve stretch broken carbon fiber manufacturing processes, a method is developed herein to analyze the forming quality of a complex carbon fiber part. This method quantifies common qualitative forming metrics by measuring, evaluating, and scoring carbon fiber defects including: bridging, wrinkling, variation in thickness, forming depth, corner thinning and thickening, resin pooling, and additional texture defects. The scores produced from evaluating the various forming metrics on formed carbon fiber parts are assembled in a weighted matrix to produce an overall forming score. This forming evaluation method is demonstrated on three different geometric mold designs to assess that it is quantitative, repeatable, accounts for many carbon fiber defects, and accounts for discrepancies between the formed part and the designed part. This method was also tested on different carbon fiber prepreg materials: MSU SBCF/Cycom 977-3 resin from Solvay, MSU SBCF/Hexcel 8552, and Hexcel IM7/8552. Through testing, the method is found to have achieved the method design goals as well as ascertaining areas of improvement for MSU SBCF prepreg which is still under development. Results show that stretch broken carbon fiber and traditional continuous carbon fiber achieve similar overarching forming scores, but outperform one another in different category metrics. MSU SBCF has higher scores in corner thickening and resin pooling; continuous carbon fiber has higher scores in wrinkling and bridging. The method forms the foundation to evaluate layup practices and autoclave cure cycles when systematically applied in a process development setting. This method is also extendable to dissimilar geometric part types with modifications in suitable metrics.Item Effects of manufacturing defects on the strength of toughened carbon/epoxy prepreg composites(Montana State University - Bozeman, College of Engineering, 2000) Turoski, Luke EverettItem Modeling of in-plane and interlaminar fatigue behavior of glass and carbon fiber composite materials(Montana State University - Bozeman, College of Engineering, 2007) Wilson, Timothy James; Chairperson, Graduate Committee: Douglas S. CairnsThis thesis presents the results of a modeling study of the fatigue behavior of fiberglass and carbon fiber reinforced epoxy composite materials intended primarily for wind turbine blades. The modeling effort is based on recent experimental results for infused glass fiber laminates typical of current blades, and hybrid carbon prepreg laminates of potential interest for future blades. There are two focus areas: in-plane performance represented by stress-life (S-N) curves, and out-of-plane ply delamination at details including ply drops and joints, based on fracture mechanics. In-plane fatigue models for both the mean performance and a statistically fit model with a 95/95 confidence limit were developed for three laminates, each representative of lower cost materials with applications in the wind turbine industry. These include polyester and epoxy resin infused glass fabrics and a hybrid carbon prepreg; two of the materials were tested in the axial and transverse directions. Models were adapted for the S-N results at several uniaxial loading conditions, including special treatment of the time dependence at high loads.