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1. In-Situ Investigation of Resin Shrinkage in the Composite Manufacturing Environment

   https://doi.org/10.1007/s10443-021-09887-x  

   Motagi, Samarth ; Namilae, Sirish ( January 2021 , Applied Composite Materials)

   Cure shrinkage of the polymer matrix during the composite manufacturing process leads to residual stresses, which can adversely affect the structural integrity and dimensional stability of composite structures. In this paper, a novel approach is developed for measuring the resin shrinkage and strain evolution of an epoxy resin (EPON-862) in the composite manufacturing environment. The resin is cured in a custom-designed autoclave with borosilicate viewports, while digital image correlation (DIC) is used to analyze the strain evolution throughout the cure cycle. These processing induced strains are correlated to the cure-state using differential scanning calorimetery (DSC). The different mechanisms involved in the polymer strain evolution during composite processing are discussed.
2. Residual stress reduction during composite manufacturing through cure modification: In situ analysis

   https://doi.org/10.1177/00219983211066545  

   Chava, Sandeep ; Namilae, Sirish ; Al-Haik, Marwan ( March 2022 , Journal of Composite Materials)

   Residual stresses are detrimental to composite structures as they induce processing defects like debonding, delamination, and matrix cracking which significantly decrease their load-bearing capability. In this research, a new in-situ approach using digital image correlation is utilized to analyze the effect of the cure cycle modification on residual stress evolution during processing. It was found that the modified cure cycle comprising abrupt cooling after gelation reduces the residual stresses. Five different layup configurations are investigated to examine the effect of fiber direction. A maximum average residual stress reduction of 31.8% is observed for the balanced unsymmetric [30/-30/60/-60] laminate. The residual stress reduction results in an increase in failure strength between 4 and 12% in the different layups and can lead up to a 22% increase in first-ply failure strength.
3. Design of Four-Patch Multi-Stable Composite Laminates for Shape Morphing Applications

   https://doi.org/10.1115/DETC2021-67884  

   Biju, Jebin ; Fadel, Georges ; Li, Suyi ; Myers, Oliver ( August 2021 , Proceedings of the ASME 2021 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference)

   Thin bistable composite laminates can be used for shape morphing applications by virtue of their material properties and asymmetric ply layup. These laminates are called bistable because they can be snapped into two or more stable shapes. A single bistable patch can result in simple cylindrical shapes and when multiple such patches are assembled into a single multi-patch laminate they result in more complex shapes and multiple stable shapes that can find wide practical use in shape morphing applications. Analytical models exist that can approximate the stable shapes of the laminates from the input of material properties and laminate geometry. And these models correlate with FEA and experiment to a satisfactory degree and could be used for the design of multi patch laminates. In this research, we make use of these analytical models that solve for a four-patch grid laminate and create a design method based on optimization to solve the reverse problem to arrive at the laminate parameters given the target shape(s). Two approaches are presented wherein one targets a single stable shape and the other targets two stable shapes which are the shapes before and after snap through. This work would be useful to understand how multi-patch laminatesmore »could be designed using optimization.« less
4. DESIGN TOOL FOR DISCONTINUOUS RESIN PATTERNS IN VACUUM BAG-ONLY PREPREGS

   Schechter, Sarah G. ; Centea, T. ; Nutt, S. ( September 2019 , Proceedings of the Composites and Advanced Materials Expo (CAMX))

   Discontinuous resin distributions facilitate transverse air removal in vacuum bag-only prepregs during out-of-autoclave processing, and enable robust manufacturing. Methods to create discontinuous resin distributions with various pattern types and feature sizes have been demonstrated in recent reports. However, this new capability has expanded the design space for prepreg manufacturing, and optimum pattern characteristics have not been identified. In this work, a geometric model was developed to simulate prepregs and laminates with discontinuous resin distributions of various pattern type, feature size, stacking orientation, and ply count. Key metrics were employed to explore the capacity for air evacuation at room temperature. In particular, the projected surface area exposed was calculated to examine the fraction of uninhibited transverse air evacuation pathways. Secondly, sealed interfaces corresponding to the percentage of closed interlaminar regions within laminates were estimated. Finally, the tortuosity (the ratio of actual average gas transport path to straight-line path) of the dry pore network was calculated. A full factorial design, analyzed by n-way ANOVA and multi-comparison tests, was conducted to reveal the aspects of prepreg designs having the greatest influence on these metrics. Finally, these insights were used to fabricate prototype prepregs and experimentally measure their transverse permeability. Results revealed that amore »large number of sealed interfaces and high tortuosity were associated with lower permeability, indicating that these metrics can be used to screen resin patterns using the developed model. Broadly, the results validated a methodology to differentiate between discontinuous resin patterns with regards to air evacuation of the prepreg at room temperature, and therefore reduce the design space. Ultimately, this work can be used to guide prepreg design and to support manufacturing of high-quality composites by out-of- autoclave methods.« less
5. Defining relationships between geometry and behavior of bistable composite laminates

   https://doi.org/10.1177/00219983211005824  

   Phatak, Salil ; Myers, Oliver J ; Li, Suyi ; Fadel, George ( April 2021 , Journal of Composite Materials)

   Bistability is exhibited by an object when it can be resting in two stable equilibrium states. Certain composite laminates exhibit bistability by having two stable curvatures of opposite sign with the two axes of curvature perpendicular to each other. These laminates can be actuated from one state to the other. The actuation from the original post-cure shape to the second shape is called as ‘snap-through’ and the reverse actuation is called as ‘snap-back’. This phenomenon can be used in applications for morphing structures, energy harvesting, and other applications where there is a conflicting requirement of a structure that is load-carrying, light, and shape-adaptable. MW Hyer first reported this phenomenon in his paper in 1981. He found that thin unsymmetric laminates do not behave according to the predictions of the Classical Lamination Theory (CLT). The CLT is a linear theory and predicts the post-cure shape of thin unsymmetric laminates to be a saddle. MW Hyer developed a non-linear method called the “Extended Classical Lamination Theory” which accurately predicted the laminate to have two cylindrical shapes. Since then, a number of researchers have tried to identify the key parameters affecting the behavior of such laminates. Geometric parameters such as stacking sequence, fibremore »orientation, cure cycle, boundary conditions, and force of actuation, have all been studied. The objective of this research is to define a relation between the length, width and thickness of square and rectangular laminates required to achieve bistability. Using these relations, a 36 in × 36 in bistable laminate is fabricated with a thickness of 30 CFRP layers. Also, it is proved that a laminate does not lose bistability with an increase in aspect ratio, as long as both sides of the rectangular laminate are above a certain ‘critical length’. A bistable laminate with dimensions of 2 in × 50 in is fabricated. Further, for laminates that are bistable, it is necessary to be able to predict the curvature and force required for actuation. Therefore, a method is developed which allows us to predict the curvature of both stable shapes, as well as the force of actuation of laminates for which the thickness and dimensions are known. Finite Element Analysis is used to carry out the numerical calculations, which are validated by fabricating laminates. The curvature of these laminates is measured using a profilometer and the force of actuation is recorded using a universal test set-up.« less