Thermoplastic composites present considerable promise for the 3D printing of cryogenic fuel storage tanks , offering enhanced recycl ability and repairability compared to thermoset composites . However, a significant knowledge gap remains regarding their ability to withstand cryogenic environments without suffering ply cracking . This study investigates the microcracking behaviour of continuous carbon fibre reinforced thermoplastic (CFR TP) composites fabricated through extrusion -based 3D print ing. The e xperimental results reveal that CFRTP composites printed at room temperature exhibit a remarkable ability to withstand an applied strain of 0.60% without ply cracking at liquid nitrogen temperature . This performance surpasses that of conventional carbon fibre reinforced epoxy composites , which typically experience ply cracking even with any applied strain. Some of microcracks were traced back manufacturing defects . The defects were found to be fused by a post-heat treatment at 180℃ for 60 min. Unexpectedly, however, the treatment reduced the ply-cracking strain to 0.40% at the liquid nitrogen temperature . Computational micromechanical modelling reveal ed that this unexpected decline in ply -cracking resistance resulted from the increased thermal residual stresses induced by the heat treatment. The findings of this study suggest that 3D - printed thermoplastic composites exhibit robust resistance to microcracking at cryogenic temperatures, making them a promising solution in the quest for sustainable lightweight cryogenic fuel storage solutions . Keywords Three -Dimensional Printing; Carbon Fibre Reinforced Polymer Composite; Cryogenic Application ; Ply Cracking 1. Introduction Lightweight s torage of hydrogen fuel as a supercold cryogenic liquid is critical to the decarbonisation of the global aviation sector and other hard -to-decarbonised industries . To power future aircraft using hydrogen will require lightweight storage technologies to store liquid hydrogen at −252℃ [1, 2] . Fibre reinforced composites, particularly those made of carbon fibres, are a promis ing solution for this challenge , offering up to 40% weight reduction over metal lic tanks . However, existing aerospace -grade carbon fibre composites have been found to suffer matrix microcracking in the thickness direction of laminates, and these microcracks can link up to form interconnected pathways for hydrogen gas to escape [3-5]. To address this problem, nano -toughening techniques have b een found effective in improving the microcracking resistance of epoxy -based carbon fibre composites at cryogenic temperatures [6-9]. However, epoxy or thermoset polymer -based composites are more challenging to recycle or re -manufacture than thermoplastic composites [10-14]. In addition, thermoplastic composites have the added advantage of being made by direct three -dimensional (3D ) printing , offering a more rapid manufacturing process. Although thermoplastic composites have been successfully manufactured using 3D printing [15-19], there is a lack of understanding of their structural performance at cryogenic temperatures , especially their propensity to matrix cracking due to thermal residual stresses and mechanical loads [3]. 3D print ing of continuous carbon fibre reinforced thermoplastic composites , herein denoted as 3D -printed composites , offers a new route for manufacturing intricate design s and cylindrical vessels as shown in Fig. 1. However, a s-printed composites are known to contain high level of voids due to the entrapment of air between successive printed beads or filaments [16-21]. Furthermore, they suffer from high residual thermal stresses stem ming from the temperature discrepancies between the filaments extruded from the nozzle and the printed part [21]. Recent r esearch has shown that these issues can be addressed through heat treatments [22, 23] . However, it remains unknown how the voids affect the initiat ion and growth of microcracks and whether heat treatments can improve the composite ’s resistance to microcracking when subjected to the typical thermomechanical environment of liquid hydrogen vessels. To address these gaps, this study aims to elucidate the microcracking resistance of a 3D-printed thermoplastic composite with a particular focus on understanding the influences of voids and residual thermal stresses resulting from 3D printing on the microcracking behaviou r observed at cryogenic temperatures. 3D printed composites were produced using a continuous fibre 3D printer, Markforged® X7 from the USA, which employs the fused filament fabrication (FFF) process to extrude car bon fibre reinforced nylon filaments and deposit them on a bed at an elevated temperature. Optical microscop y was employed to study the voids in as -printed composites and after heat treatment. Three -point bending tests were conducted to assess the microcracking resistance of the as -printed and heat -treated composites at room and cryogenic temperatures. Fig. 1 3D printing of a thermoplastic composite vessel for storing cryogenic liquids. 2.Experimental 2.1. Manufacturing , heat treatment, and materials Two filament materials were employed , including a short -carbon -fibre (SCF) reinforced nylon 6 filament and a continuous -carbon -fibre (CCF) reinforced nylon filament from Markforged®, USA ; the product codes are SCF F-MF-0001 and F-MT-CFVX , respectively . The short -fibre filament was used to print the external surface layers while the interior of the composite was made using the continuous fibre filament. Printing of the two different filaments was carried out with different nozzle s, with t he nozzle temperatures being set at 270°C for SCF filament 252°C for the CCF filament , respectively . The chamber temperature of the print bed was maintained at room te mperature . During the extrusion process, the temperature of the extruded CCF filament decreased swiftly from its nozzle temperature to approximately 100°C just above the already printed section, as documented in previous literature [22]. As a result, the stress - free temperature of the melted filament directly above the printed portion was approximately 100°C. The h eat treatment of the printed CFRP composites was aimed at reduc ing porosity , employing a dual approach involving vacuum sealing and positive pressure within an autoclave. The composites were placed between two metal plates to ensure their flatness for subsequent mechanical test ing. Two PTFE films were inserted between the composite and the plates to aid in their retrieval post-treatment . The assembly was vacuum sealed within a vacuum bag and subsequently placed in an autoclave , where it underwent pressurization to two bar s using nitrogen gas. The heat treatment protocol involved a gradual heating rate of 10°C/min, starting from 25°C and reaching 180°C , and a hold time of 1 h at the peak temperature , followed by a controlled cooling at the same rate to 25°C. Since the peak temperature was above the glass transition temperature of the matrix, the heat treatment effectively reset the stress -free temperature to 180°C , substantially above that of the as -printed composites. 2.2 Characteri zation methods The microstructures of both the CCF and SCF filaments, as well as the resulting printed parts, were studied using an optical microscope (ZEISS Axio Zoom) at various magnifications (×10, ×50, ×100). Image analyses to evaluate porosity were carried out using ImageJ software. Pores in the images were distinctly color -coded. After the images were converted to binary format, the software calculated the areas represented by the black regions. For each analysis, ten images from each system were used. The Mettler -Toledo Thermal Analysis device (STARe System) was used for Dynamic Scanning Calorimetry (DSC) to identify the specific polyamide in the CCF and SCF filaments. An initial heating rate of 10 °C/min was used to heat the samples from 25°C to 300°C to eliminate any prior thermal history. The samples were then cooled back to 25°C at the same rate. Another identical temperature ramp was performed to evaluate key thermal properties of the polymer, such as the Glass transition temperature (T g), melting temperature (T m), crystallization temperature (T c), and enthalpy. Results f rom the thermograms suggest that the CCF filaments ’ matrix is polyamide 6 -I (PA6 -I), whereas the SCF filament s’ matrix is polyamide 6 (PA6). The ply cracking behavio ur of the CFRP composites was evaluated using a three -point bending test, conducted both at room temperature (RT) and cryogenic temperature (CT) , denoted as liquid nitrogen temperature (-196°C) . A minimum of three samples were tested for each condition . The 3D-printed CFRP composite sample s measured 153.6 mm in length, 13 mm in width, and 4 mm in thickness, with a nominal ply thickness of 0.125 mm. To ensure structural integrity during printing [24], four SCF layers were printed at both the top and bottom surfaces of the sample s, as well as around the perimeter of each printed CCF layer . These specific layers are termed "roof & floor layers" and "wall layers ," prevented part breakage and detachment during the printing process. Between the roof and floor layers , CCF layers were printed with a stacking sequence of [90 6/012/90 6], where the 0° fibre orientation aligned with sample’s length. During the three -point bending test s, the top surfaces of the sample were under compression while the bottom surface were under tension. The peak tensile strain in the CCF layer furthest away from the mid -plane of the composite can be calculated using the formula , 𝜀=6𝛿ℎ/𝐿2, where ε denotes the maximum strain in continuous fibre layer on the tension side, δ the mid- span deflection, L the span length , and h the thickness of beam , excluding the roof and floor layers. 3. Experimental r esults The r esults of the microstructural , thermal, and mechanical characteristics of the filaments and the 3D printed composites, before and after heat treatment , are presented and discussed below. 3.1 Microstructures and differential scanning calorimetry of the filaments The microstructures and thermal properties of both CCF and SCF filaments were analyzed to determine the distribution of carbon fibres , the shape and presence of voids, and the composition of the matrix. Fig.2 illustrates the microstructures of the (a -b) CCF and (c -d) SCF filaments, as well as the DSC results for both filaments during the (e) initial and (f) subsequent temperature cycles. This temperature cycle involved a ramp from 25°C to 300°C at a rate of 10°C/min, a 5 -minute hold, and then a cooling phase back to 25°C at the same rate. The CCF filaments consist of CCF embedded within a polyamide matrix, while SCF filaments have SCF within a polyamide 6 matrix. A more detailed discussion on the matrices of these two filaments will follow later. The diameters for the CCF and SCF filaments are approximately 380μm and 1720μm, respectively. The individual carbon fibre diameter in both filaments is around 7μm, within the normal range of 5-10μm. Observations from Fig.2a-b indicate that CCF distribution is not even , with areas being rich in either fib re or matrix. There are also visible voids in the filaments. The measured porosity and fibre volume fraction for the CCF filament were about 1.14% and 26.32%, respectively, using Image -J. This aligns closely with data from existing literature [26]. Conversely, the SCF filaments, as displayed in Fig.2c-d, exhibited an even distribution of SCF without any noticeable voids. The purpose of the DSC analysis for both filaments was to determine the characteristics of their matrix materials. The initial heating -cooling cycle aimed to eliminate the filaments' thermal history. As illustrated in Fig.2e, during the heating phase, the SCF filaments displayed a melting temperature (T m) of approximately 202℃ and a crystallization temperature (T c) of 161℃ in the co oling phase. This solidified the fact that the SCF filament's matrix is polyamide 6, which aligns well with findings from literature