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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)

   null (Ed.)

   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. 

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2. In-Situ Monitoring of the Manufacturing Process and Residual Stress Evolution in Thin-Ply Composites

   https://doi.org/10.2514/6.2021-1776  

   Chava, Sandeep; Namilae, Sirish (January 2021, AIAA Scitech 2021 Forum)

   null (Ed.)

   Thin-ply composite laminates are of interest for several applications in aerospace and other high-performance industries due to their ability to delay transverse microcracking and delamination in static, fatigue, and impact loadings. It is essential to understand the evolution of thermal residual stresses during cure to optimize the manufacturing process of thin-ply composites for deep-space applications. In this research, processing induced residual stresses in thin-ply laminates are evaluated by devising a novel in-situ experimental approach. Thin-ply prepreg laminates are cured in a specially designed autoclave with viewports with plies laid upon a flat tool and a curved tool. The curved tool configuration used in this research is designed to simulate cryogenic fuel tank surfaces. The evolution of residual stresses in terms of out-of-plane displacement is characterized using Digital Image Correlation (DIC) during the autoclave cure cycle. 

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3. A Review of Laser Peening Methods for Single Crystal Ni-Based Superalloys

   https://doi.org/10.3390/met12091414  

   Holtham, Noah; Davami, Keivan (September 2022, Metals)

   Single crystal Ni-based superalloys are often used to create gas turbine engine blades for their high strength under intense thermo-mechanical loading. Though they are remarkably capable under these conditions, a particular class of premature failure mechanisms known as surface-initiated damage mechanisms can lead to the early fracture of an otherwise healthy blade. This review paper discusses the current progress of post-processing techniques that can greatly mitigate the potency of surface-initiated damage mechanisms. In particular, laser peening (LP) is of significant interest due to the relatively low amount of cold work it induces, greater depth of compressive residual stresses than other cold working methods, ability to accommodate complex part geometries, and the minuscule effect it has on surface roughness. The residual stresses imparted by LP can greatly hinder crack growth and consequently allow for enhanced fatigue life. Given that turbine blades (constructed with single crystal Ni-based superalloys) are prone to fail by these mechanisms, LP could be a worthy choice for increasing their service lives. For this reason, initiative has been taken to better understand the mechanical and microstructural modifications imparted by LP on single crystal Ni-based superalloys and a summary of these investigations are presented in this review. Results from several works show that this class of alloy responds well to LP treatment with improvements such as ~30–50% increase in microhardness, 72% increase in low cycle fatigue life, and elevated resistance to hot corrosion. The primary objective of this review is to provide insight into current state-of-the-art LP techniques and summarize the findings of numerous works which have utilized LP for increasing the service lives of single crystal Ni-based superalloy components. 

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4. Chemical Shrinkage Characterization during Curing through Three-Dimensional Digital Image Correlation

   https://doi.org/10.26434/chemrxiv-2023-6xg1d  

   Bukenya, Kalima; Shah, Sagar; Sabato, Alessandro; Maiaru, Marianna (April 2023, ChemRxv)

   Chemical shrinkage in thermosetting polymers drives residual stress development and induces residual deformation in composite materials. Accurate characterization of chemical shrinkage during curing is therefore vital to minimize residual stresses through process modeling and optimize composite performance. This work introduces a novel methodology to measure the pre- and post-gelation chemical shrinkage of an epoxy resin using three-dimensional digital image correlation (3D-DIC). Differential scanning calorimetry (DSC) is employed to calculate reaction kinetics and correlate chemical shrinkage with the degree of cure. Rheology experiments are conducted to quantify gelation and validate post-gelation. 3D-DIC post-gelation results show excellent agreement with rheology. Pre-gelation results show the effect of the in-situ curing in the proximity of constraints on the global strain behavior. This work introduced an innovative approach to characterize the chemical shrinkage of thermosets during curing, which will enable accurate residual stress prediction for enhancing thermoset composite performance and provide insight into the in-situ polymer behavior during processing. 

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5. Effect of nanoscale interface modification on residual stress evolution during composite processing

   https://doi.org/10.1177/00219983231179522  

   Kumar, Deepak; Dusabimana, Marie Claire; Al-Haik, Marwan; Namilae, Sirish (June 2023, Journal of Composite Materials)

   The interface characteristics of the matrix and fibers significantly influence the evolution of residual stress in composite materials. In this study, we provide a methodology for reducing the residual stress in laminated composites by modifying the thermomechanical properties at the fiber–matrix interface. A hydrothermal chemical growth method was used to grow Zinc Oxide nanowires on the carbon fibers. We then utilized a novel digital image correlation approach to evaluate strains and residual stresses, in situ, throughout the autoclave curing of composites. We find that interface modification results in the reduction of residual stress and an increase in laminate strength and stiffness. Upon growing ZnO NWs on the carbon fibers, the maximum in situ in-plane strain components were reduced by approximately 55% and 31%, respectively, while the corresponding maximum residual stresses were decreased by 50.8% and 49.33% for the cross-play laminate [0°/90°] layup in the x and y directions, respectively. For the [45°/-45°] angle ply layup in the x-direction, the strain was decreased by 27.3%, and the maximum residual stress was reduced by 41.5%, whereas in the y-direction, the strain was decreased by 166.3%, and the maximum residual stress was reduced by 17.8%. Furthermore, mechanical testing revealed that the tensile strength for the [45°/-45°] and [0°/90°] laminates increased by 130% and 20%, respectively, with the interface modification. 

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