With the international effort to reach Net Zero by 2050, the aviation industry is in a race to adopt zero-carbon emission fuel sources within the coming decades. It is widely accepted that liquid hydrogen (LH2), a cryogenic liquid stored at 20 K (-253 degrees celsius), will serve as this fuel source for the majority of the aircraft market, as it offers a better payload and range to alternative solutions such as batteries or gaseous hydrogen. Nevertheless, there are many challenges associated with the transition to liquid hydrogen that the aviation industry will need to overcome. One significant challenge is the storage of LH2 aboard the aircraft. While metallic tanks are currently used in the space industry for single-use LH2 launch vehicles and are likely to be first to market in civil aviation; carbon fibre reinforced polymer (CFRP) tanks offer significant gravimetric efficiency benefits which, over the multi-cycle 20-25-year lifetime of a tank, translates to considerable cost savings. While CFRP tanks are considered promising long-term storage vessels, the susceptibility of the polymer matrix to microcracking at these extremely low temperatures is currently a primary barrier to adoption. There are a number of issues with microcracking in this application, not least the safety and thermal issues associated with increased hydrogen permeation, but also the integrity of the tank being compromised and the chance of liquid hydrogen boil off within crack networks causing delamination or tank rupture.

Matrix microcracking at cryogenic temperatures is understood to be caused by the build-up of thermally induced residual stresses through several possible mechanisms. On the microscopic level, the mismatch of co-efficient of thermal expansion (CTE) between the fibre and matrix leads to residual stresses in both constituents during thermal cycling. On the next level of structural hierarchy, the mismatch of effective CTE between adjacent plies with varying fibre orientation is a possible cause. In addition to this, when cooled down to cryogenic temperatures a material can experience thermal shock via inhomogeneous temperature distributions, where neighbouring domains encounter different temperatures, creating a steep temperature gradient across the material. This can also result in the development of transient thermally induced stresses and in turn cause microcracking. The overarching aim of this project is therefore to develop a polymer composite matrix which can withstand repeated exposure to a 20 K cryogenic environment without microcracking and be suitable for use in LH2 storage tanks. To address this, the key objectives of this project include:

- Determine, through a design of experiments testing approach, which polymeric molecular properties or toughening methods enable matrices with the desired thermomechanical and physical properties to supress microcracking

- Design and synthesise new matrices or adapt existing materials to incorporate these material properties or toughening methods

- Characterisation of these matrix materials to validate the design process and subsequent manufacture of composite panels using the best candidates

- Design and conduct a rigorous testing campaign to characterise the composite materials with respect to the key performance indicators for LH2 tanks, such as microcrack fracture toughness, hydrogen permeability and resistance to cracking under repeated cryogenic cycling