Sustainable polymer composite marine structures: Developments and challenges

Efficient recovery of carbon fibers from carbon fiber-reinforced polymers using direct discharge electrical pulses

Carbon fiber-reinforced polymers (CFRPs) are lightweight, high-strength composite materials that are widely used in various industries. However, recycling CFRPs remains a significant challenge because of the difficulty in separating carbon fibers (CFs) from the polymer matrix. This study compares two electrical pulse methods, namely direct discharge (DD) and electrohydraulic fragmentation (EHF), for the energy-efficient and precise recovery of CFs from CFRPs. The DD method involves the direct application of high-voltage pulses to the CFRPs, leveraging the Joule heat generation, thermal stress generation, and expansion force caused by plasma generation. In contrast, EHF is based on intensive shockwave impulses generated by high-voltage discharge plasmas along the interfaces of different materials. We examined the physical properties of the recovered CFs, namely their length, tensile strength, resin adhesion, and structural degradation, as well as the energy efficiency of the two methods in terms of CF separation. The results showed that DD is more effective for CF recovery, considering the preservation of long fibers with high strength and the separation of individual fibers without residual resin on the surface.

Development of Low-Pressure Die-Cast Al-Zn-Mg-Cu Alloy Propellers Part II: Simulations for Process Optimization

With the increasing demand for high-performance leisure boat propellers, this study explores the development of high-strength aluminum alloy propellers using the low-pressure die-casting (LPDC) process. In Part I of the study, we identified the optimal alloy compositions for Al-6Zn-2Mg-1.5Cu propellers and highlighted the challenges of hot tearing at the junction between the hub and blades. In this continuation, we developed a coupled thermal fluid stress analysis model using ProCAST software to optimize the LPDC process. By adjusting casting parameters such as the melt supply temperature, initial mold temperature, and curvature radius between the hub and blades, we minimized hot tearing and other casting defects. The results were validated through simulations and practical applications, showing significant improvements in the quality and structural integrity of the propellers. Non-destructive testing using X-ray CT confirmed the reduction in internal defects, demonstrating the effectiveness of the simulation-based approach for alloy design and process optimization.

Development of Low-Pressure Die-Cast Al-Zn-Mg-Cu Alloy Propellers-Part Ⅰ: Hot Tearing Simulations for Alloy Optimization

Recent advances in the leisure boat industry have spurred demand for improved materials for propeller manufacturing, particularly high-strength aluminum alloys. While traditional Al-Si alloys like A356 are commonly used due to their excellent castability, they have limited mechanical properties. In contrast, 7xxx series alloys (Al-Zn-Mg-Cu based) offer superior mechanical characteristics but present significant casting challenges, including hot-tearing susceptibility (HTS). This study investigates the optimization of 7xxx series aluminum alloys for low-pressure die-casting (LPDC) processes to enhance propeller performance and durability. Using a constrained rod-casting (CRC) method and finite element simulations, we evaluated the HTS of various alloy compositions. The results indicate that increasing Zn and Cu contents generally increase HTS, while a sufficient Mg content of 2 wt.% mitigates this effect. Two optimized quaternary Al-Zn-Mg-Cu alloys with relatively low HTS were selected for LPDC propeller production. Simulation and experimental results demonstrated the effectiveness of the proposed alloy compositions, highlighting the need for further process optimization to prevent hot tearing in high Mg and Cu content alloys.