Mechanical Property Characterization of Polymeric Composite Materials for Hydrogen Storage Tanks

Abstract

The surge in hydrogen tank popularity, driven by the push for carbon neutrality, has emphasized the necessity for precise determination of input material properties to design high-performance tanks. Unfortunately, conventional characterization methods often fall short in accurately estimating these material properties. The properties of carbon/resin in the Unidirectional (UD) directions are crucial design factors for ensuring the safety and cost-effectiveness of hydrogen tanks. Characterizing composite materials is a fundamental step in the research and development of hydrogen tanks, with the accuracy of material properties being essential for predicting tank behavior and burst pressure. However, conventional methods, such as the ASTM D2290 split disk method, face limitations, particularly in accurately measuring hoop tensile strength. The current ASTM testing method presents two significant issues. Firstly, the design of the specimen generates non-uniform stress distribution due to stress concentration. Secondly, bending occurs at the split area during testing. Challenges arise from the use of notched specimens, where the measured hoop tensile strength consistently falls below the actual strength due to non-uniform stress distribution and stress concentration. Researchers have observed premature splitting along the hoop direction, leading to lower tensile strength predictions, with the design of the ring tensile specimen, incorporating notches, contributing to these challenges. Another issue is the forced failure along the circumference of the ring, occurring parallel to the split line of the disk during the test. The stress condition at this location is not exclusively tensile under the specified loading conditions. Tensile testing induces bending stress, causing a non-uniform distribution of strain along the thickness direction of the specimen. This results in an apparent tensile strength rather than an accurate representation of true tensile strength. Hence, there is a pressing need for redesigning testing specimens and revising the testing methodology to address these issues. The study addresses these limitations by proposing updated designs for ring tensile specimens. These designs eliminate notches, incorporate tabs to maintain stress uniformity, and transform the curved gauge area into a flat one. Numerical simulations and experimental data demonstrate that these design modifications lead to more evenly distributed hoop stress and improved tensile strength. The novel ring tensile specimens are employed to characterize filament-wound composites derived from Elium® 591 resin, a thermoplastic resin with unique properties. The conventional ASTM D2290 split disk method is used for measuring composite material properties, but it presents challenges. The method forces failure along the ring's circumference parallel to the disk split line, generating bending stress during tensile testing and affecting strain distribution, resulting in a lower prediction of the hoop strength. To address these challenges, an alternative testing device named the Mini Burst chamber is introduced in this research. Unlike the split disk method, the Mini Burst chamber applies uniform internal radial pressure, ensuring an even distribution of hoop stresses. This multitasking tool predicts the mechanical strength and cyclic behavior of filament-wound specimens across a wide temperature range (+100°C to -60°C) with greater accuracy than the traditional split disk method. Recyclability is a critical concern in the hydrogen tank industry, as traditional tanks made with thermosetting resin are non-recyclable. Introducing thermoplastic resin can address this issue, but the material properties of thermoplastic-based composites are not as readily available as those of epoxy-based composites. The Mini Burst chamber is employed to predict the mechanical and cyclic characteristics of thermoplastic composites for hydrogen storage tanks, demonstrating the accurate determination of mechanical hoop strengths. This research contributes to advancing the understanding and design considerations in the development of hydrogen tanks. In conclusion, this study seeks to enhance hydrogen storage tank technology by addressing critical challenges related to design, safety, and recyclability. The suggested alterations to ring tensile specimen designs and the introduction of an alternative testing method (utilizing a mini burst chamber) present a promising approach for precise mechanical property determination. These innovations have the potential to transform the design and performance evaluation of hydrogen storage tanks.