A systematic investigation for mode-I fracture properties of stitched composites

Epoxy resin offers promising properties in composites, while its cross-linked structure prevents the disposal of waste composites. Pyrolysis is an essential technique for the breakdown of epoxy resin polymers, but the fundamental reaction mechanisms need to be clarified. In this study, the combination of experimental studies and the reactive force field molecular dynamic (ReaxFF-MD) simulation were performed to determine the formation mechanisms of pyrolysis products considering the secondary reactions of intermediates. The pyrolysis process was divided into the cleavage and polycondensation stages based on the carbon atom numbers of the largest cluster. The cleavage stage began with the removal of side-chain substituents, followed by the breakdown of the primary epoxy resin chain, resulting in the generation of pyrolysis gas products, of which the OH, C-C-O, and CH3 groups were identified as the critical intermediates in the creation of H2O, CO, and CH4, respectively. Meanwhile, the simultaneous evolution of liquid pyrolysis products (bisphenol A, phenol, and benzene) was established. After the epoxy resin chains were completely cracked, the unsaturated hydrocarbon intermediates underwent the dehydrogenation and polymerization reactions, causing the formation of pyrolysis char and the continuous generation of H2, H2O, and CO. The findings could provide some guidance to selectively regulate the pyrolysis of epoxy resin matrix composites. 

Insights into the pyrolysis mechanisms of epoxy resin polymers based on the combination of experiments and ReaxFF-MD simulation

Terahertz (THz) technology has firmly established itself as an effective sensing and nondestructive testing (NDT) technique for the detection of substances and physicochemical evaluation of materials and structural systems since its first emergence almost three decades ago. To date, both the effectiveness and accuracy of this technology have been extensively demonstrated in a myriad of applications across the spectrum of research and development all the way to process analytical technology (PAT), quality control, NDT, and structural health monitoring. These applications are generally enabled by the production and availability of advanced, versatile, robust, highly accurate, and industrially rugged THz spectroscopy and imaging (THz-SI) systems, the unique properties of THz waves compared with other electromagnetic waves, as well as the advancements in electronics, photonics, and THz metamaterial systems development. This article presents a comprehensive state-of-the-art and state-of-the-practice review of sensing and NDT applications of THz technology and analyzes the role of THz metamaterials in enhancing the resolution and sensitivity of THz systems. The study also provides a general overview of the fundamentals of THz-SI systems and discusses the suitability of THz sensing and NDT in a variety of real-world application scenarios (e.g., composites’ defect detection and evaluation, paints and coatings thickness measurement and characterization, and biomolecule detection). Aspects, such as the noise caused by the presence of barriers, challenges with experimental implementations and operability of THz systems, long times required to acquire THz images, as well as limited customizability and portability of currently available THz systems, are also discussed.

Sensing and Nondestructive Testing Applications of Terahertz Spectroscopy and Imaging Systems: State-of-the-Art and State-of-the-Practice

Tensile behavior and failure mechanism of 3D woven fabric reinforced aluminum composites

Abstract The use of terahertz time-domain spectroscopy (THz-TDS) for the nondestructive testing and evaluation (NDT&amp;E) of materials and structural systems has attracted significant attention over the past two decades due to its superior spatial resolution and capabilities of detecting and characterizing defects and structural damage in non-conducting materials. In this study, the THz-TDS system is used to detect, localize and evaluate hidden multi-delamination defects (i.e., a three-level multi-delamination system) in multilayered GFRP composite laminates. To obtain accurate results, a wavelet shrinkage de-noising algorithm is used to remove the noise from the measured time-of-flight (TOF) signals. The thickness and location of each delamination defect in the z -direction (i.e., through-the-thickness direction) are calculated from the de-noised TOF signals considering the interaction between the pulsed THz waves and the different interfaces in the GFRP composite laminates. A comparison between the actual and the measured thickness values of the delamination defects before and after the wavelet shrinkage denoising process indicates that the latter provides better results with less than 3.712% relative error, while the relative error of the non-de-noised signals reaches 16.388%. Also, the power and absorbance levels of the THz waves at every interface with different refractive indices in the GFRP composite laminates are evaluated based on analytical and experimental approaches. The present study provides an adequate theoretical analysis that could help NDT&amp;E specialists to estimate the maximum thickness of GFRP composite materials and/or structures with different interfaces that can be evaluated by the THz-TDS. Also, the accuracy of the obtained results highlights the capabilities of the THz-TDS for the NDT&amp;E of multilayered GFRP composite laminates. 

/pdf/terahertz-spectroscopic-characterization-and-thickness-3vo80i9l.pdf

Terahertz Spectroscopic Characterization and Thickness Evaluation of Internal Delamination Defects in GFRP Composites

Insight into Pyrolysis Behavior of Silicone-Phenolic Hybrid Aerogel Through Thermal Kinetic Analysis and Reaxff Md Simulations

The pyrolysis characteristics of the polymeric nanocomposites are dominantly regulated by the pyrolyzable components to accommodate a specific thermodynamic environment. The pyrolysis mechanisms of the pyrolyzable system including silicone and phenolic aerogels (SiOC/PR) of silicone-modified phenolic aerogel matrix nanocomposites are carried out by kinetic analysis, ReaxFF molecular dynamics (MD) simulations, and artificial neural network (ANN). The experimental results show that the SiOC/PR systems exhibit better thermal stability when the mass ratio of SiOC to PR is 1:3. A distributed activation energy model with five pseudo-components is employed to calculate the kinetic parameters of each decomposition stage of SiOC/PR systems for describing the decomposition characteristics. The significant pyrolysis products detected by TG-FTIR are examined by ReaxFF MD simulations. MD results reveal that the methanol evolution of SiOC/PR systems is distinct in a high-temperature oxygen-free environment, where a mass ratio 1:3 of SiOC to PR is more likely to release methanol and forms a silicon oxide-rich phase with high bond energy. Two reaction mechanisms for methanol identified by the MD trajectory are the extraction reactions of methyl intermediates as well as hydroxymethyl intermediates. The mixture ratio of the pyrolyzable system of the SiOC/PR nanocomposites with optimal thermal resistance equaling 0.22 is predicted by ANN. This work can help understand the system’s chemical features and guide the pyrolyzable system design of the SiOC/PR nanocomposites. 

A comprehensive study of pyrolysis characteristics of silicone-modified phenolic aerogel matrix Nanocomposites: Kinetic Analysis, ReaxFF MD Simulations, and ANN prediction

Non-orthogonal FE model with stress smoothing for progressive damage analysis of shear pre-deformed 3D angle-interlock woven composites under compression

Kinematic modeling with Micro-CT slice auxiliary calibration for 2.5D woven SiO2f/SiO2 composites

The effect of fiber preform configuration on in-plane compressive behavior of high porosity needled carbon/carbon composites in elevated temperature environment

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