This study provides a comparative assessment of how matrix type and expanded graphite (EG) addition influence the compression-after-impact (CAI) performance of carbon-fiber composites. EG was incorporated at 0.1 wt.% into thermoset epoxy and liquid thermoplastic Elium® matrices and evaluated under 30 and 100 J low-velocity impacts. The results indicate distinct impact-response characteristics at the two investigated impact energies. At 30 J, the CF/epoxy laminates exhibited slightly higher CAI strength retention than the corresponding CF/Elium® laminates, consistent with their higher mean stiffness and lower absorbed energy, while EG addition produced mean increases in ultimate CAI load of 5.6% for CF/epoxy and 7.2% for CF/Elium®. At 100 J, complete perforation occurred in all laminates; the neat CF/Elium® laminate retained 65.7% of its non-impacted CAI strength, compared with 60.7% for neat CF/epoxy, despite its lower initial stiffness. At 100 J, the addition of 0.1 wt.% EG produced mean increases in ultimate CAI load of approximately 9.6% for CF/epoxy and 8.9% for CF/Elium®. The two impact conditions produced distinct combinations of absorbed energy, damage extent, and residual CAI performance. Overall, post-impact performance varied with matrix type, EG addition, and impact condition. This work provides a quantitative assessment of how nanofiller reinforcement, matrix characteristics, and impact energy affect the impact and residual compressive responses of carbon-fiber composites.

Fiber-reinforced polymer composites (FRPCs) are highly efficient structural materials, yet they remain susceptible to subsurface damage caused by out-of-plane loading. Carbon-based nanomaterials have been extensively explored for intrinsic piezoresistive sensing under tensile and flexural loading, but their electromechanical response under indentation and impact remains less well established. In this study, the piezoresistive behavior of reduced graphene oxide (rGO)-coated glass-fiber laminates is examined through quasi-static indentation (QSI) and low-velocity impact (LVI) tests, complemented by X-ray Computed Tomography (XCT) to characterize the extent and morphology of internal damage. The QSI tests included loading to failure, sustained creep, and creep following cyclic conditioning, while the LVI tests were performed at multiple energy levels to induce different damage severities. Under monotonic QSI, sensors placed directly beneath the indenter recorded a sharp fractional change in resistance (FCR) drops up to –90%, compared to <12% in transverse sensors, with damage initiation being observed at a cross-head displacement ∼5.5–6 mm . Creep tests highlighted gauge length effects, and cyclic preconditioning reduced the peak FCR by more than 50% (from ∼13% to 6%) while stabilizing the piezoresistive response. During LVI, the peak forces reached ∼9.5 kN within ∼3 ms during a 100 J impact, and the sensors continued to capture delamination and fiber fracture even after perforation. XCT scans confirmed the progression of subsurface delamination and fiber breakage, providing a direct link between electromechanical signals and internal damage mechanisms. These findings demonstrate the potential of rGO-based sensing layers in detecting out-of-plane damage across diverse loading conditions and contribute to the development of multifunctional composites for structural health monitoring.

Graphene and related materials (GRMs) offer promising routes to enhance energy absorption in composites. This study investigates the morphological influence of two expanded graphite (EG) types i.e. nano-engineered wormlike EG (EG-W), and compact EG (EG-C) on microstructure, low-velocity impact, and thermomechanical performance used within recyclable liquid thermoplastic (Elium (R)) and carbon fiber composites (CF/Elium (R)). SEM, Raman, and XPS analyses reveal that EG-W's higher aspect ratio, interconnected morphology, and balanced surface chemistry provide superior dispersion and load-transfer capability compared to EG-C, despite the latter's higher oxygen functional content. Raman spectroscopy and 2D mapping further confirm notable differences in defect density, exfoliation, and spatial distribution across filler loadings (0-1.5 wt%). EG-W exhibits lower structural disorder (ID/IG = 0.06) and improved exfoliation (I2D/ID = 9.1), promoting uniform integration into the polymer matrix. Low-velocity impact tests (5-20 J) demonstrates that an optimal loading of 0.5 wt%, EG-W enhances peak force and energy absorption by 16.6 % and 18.9 %, respectively, compared to EG-C. At higher loadings (1-1.5 wt%), both systems exhibit reduced performance due to nanoparticle agglomeration. These findings highlight the critical role of EG morphology and concentration in tailoring impact resistance, enabling design of advanced recyclable composites for high-performance structural applications.

Additive manufacturing (AM) enables the fabrication of complex, customized structures with efficient material use. In parallel, the global imperative for sustainable materials has intensified interest in bio-based composite materials derived from renewable biological sources and often reinforced with natural fibers or fillers. This review critically examines the convergence of AM and bio-based composites, offering a comprehensive analysis of current advancements, technical challenges, and future directions. It explores the suitability of various AM techniques, such as material extrusion (ME), vat photopolymerization, powder-based methods, etc., for processing bio-based polymers including cellulose, lignin, chitosan (CS), polylactic acid (PLA), etc. Key challenges such as poor thermal stability, low crosslinking density, and printability limitations are discussed alongside emerging strategies for overcoming them, including chemical modification, hybrid composite design, and parameter optimization. The review further delves into the evolution of 4D printing with bio-based stimuli-responsive materials, enabling applications in soft robotics, smart packaging, regenerative medicine, etc. By synthesizing interdisciplinary advances from materials science, mechanical engineering, and environmental design, this article highlights both the promise and complexity of integrating sustainability and functionality in AM. The insights provided serve as a roadmap for developing high-performance, eco-conscious systems that align with global sustainability goals.

Bamboo, a rapidly renewable lignocellulosic resource, is emerging as a sustainable alternative to conventional construction materials. Its high strength-to-weight ratio, short growth cycle, and carbon-sequestration capacity make it ideal for developing low-impact composite systems. This review presents a comprehensive overview of recent progress in bamboo fibre-reinforced composites (BFRCs), examining their structure, processing, properties, and potential in the building sector. The paper reviews the hierarchical and chemical characteristics of bamboo fibres that underpin their mechanical efficiency and discusses how treatments such as alkali, silane, and enzymatic modification improve fibre–matrix adhesion and composite performance. Advances in fabrication, including resin-transfer, compression moulding, and additive manufacturing, have expanded applications to structural, architectural, and prefabricated components. Key performance aspects, mechanical strength, thermal stability, environmental durability, and water resistance are analysed with emphasis on optimisation strategies through hybridisation, nanoparticle reinforcement, and surface functionalisation. Life-cycle studies indicate that bamboo composites exhibit significantly lower embodied energy and carbon emissions than glass-fibre system while maintaining comparable mechanical behaviour. Despite these advantages, challenges remain in achieving consistent fibre quality, long-term durability under humid and alkaline conditions, and standardisation of design and testing methods. Addressing these issues through improved processing control, modelling, and industrial collaboration will be vital for large-scale adoption. Overall, bamboo fibre composites represent a technically viable and environmentally responsible route toward circular, low-carbon construction materials capable of supporting next-generation sustainable infrastructure.

Low-velocity impact tests are carried out to explore the energy absorption characteristics of bio-inspired lattices, mimicking the architecture of the marine sponge organism Euplectella aspergillum. These sea sponge-inspired lattice structures feature a square-grid 2D lattice with double diagonal bracings and are additively manufactured via digital light processing (DLP). The collapse strength and energy absorption capacity of sea sponge lattice structures are evaluated under various impact conditions and are compared to those of their constituent square-grid and double diagonal lattices. This study demonstrates that sea sponge lattices can achieve an 11-fold increase in energy absorption compared to the square-grid lattice, due to the stabilizing effect of the double diagonal bracings prompting the structure to collapse layer-by-layer under impact. By adjusting the thickness ratio in the sea sponge lattice, up to 76.7% increment in energy absorption is attained. It is also shown that sea-sponge lattices outperform well-established energy-absorbing materials of equal weight, such as hexagonal honeycombs, confirming their significant potential for impact mitigation. Additionally, this research highlights the enhancements in energy absorption achieved by adding a small amount (0.015 phr) of Multi-Walled Carbon Nanotubes (MWCNTs) to the photocurable resin, thus unlocking new possibilities for the design of innovative lightweight structures with multifunctional attributes.

This study introduces a new class of additively manufactured origami-inspired hierarchical sandwich structures that exploit curved-crease geometry to achieve multiscale mechanical enhancement across diverse loading conditions. Three previously unreported core architectures, Hexavoid (HV), Rhomboid (RV), and Octavoid (OV), are developed and embedded into full sandwich panels, enabling a systematic investigation of how geometry-driven deformation governs stiffness, strength, and energy dissipation. Unlike conventional honeycomb cores that rely on localized wall buckling, the proposed architecture activates controlled in-plane contraction and distributed folding, producing stable load transfer between face sheets and delayed damage localization. Experiments reveal that the HV core delivers the most balanced performance, achieving a 40% increase in specific energy absorption and a 76% rise in elastic modulus under quasi-static compression relative to honeycomb benchmarks. Under flexural loading, the same architecture exhibits over threefold improvement in stiffness and fourfold enhancement in absorbed energy. Low-velocity impact and indentation tests further demonstrate that auxetic confinement and curved-crease deformation suppress premature failure, enabling up to 38% higher energy dissipation. Finite element analysis confirms that these gains arise from topology-induced stress redistribution and stretching-assisted deformation. The results identify curved-crease origami as a robust design strategy for lightweight sandwich structures combining impact resistance and energy absorption.

Recent research has focused on carbon fiber/epoxy laminate-based (CFRP) hydrogen storage tanks (HSTs), which offer superior structural strength, reduced weight, and enhanced safety. In this context, Graphene and Related Materials (GRMs) have emerged as promising reinforcements, enhancing the mechanical strength of the carbon fiber/epoxy laminates. However, a key challenge remains in scaling up graphene production for cost-effective, high-volume manufacturing. This study explores the potential of novel Microwave-assisted Graphene Intermediates (MGIs) as a sustainable alternative to commercial graphene materials for carbon fiber/epoxy composites. MGIs can drastically reduce the production time and cost of 2D materials. Notably, significant improvements in short beam shear strength (SBSS) were observed with MGI (similar to 68.9 +/- 0.5 MPa) compared to bottom-up graphene materials (BGM1-64.7 +/- 1.4 MPa), (BGM2-62.7 +/- 1.4 MPa), and top-down graphene material (TGM-62.3 +/- 1.0 MPa). Furthermore, low-velocity impact testing revealed that the MGI variant achieved superior performance with a peak force of 2.57 kN (similar to 25% enhancement) and absorbed energy of 13.3 J (similar to 14% improvement). Multi-criteria decision-making analysis further confirmed MGI as the top-ranked reinforcement among all tested GRMs, underscoring its superior mechanical performance and competitive market potential. This demonstrates MGI as a cost-effective and scalable alternative for the cost-effective and sustainable design of carbon fiber/epoxy laminate-based HSTs.

This study investigates the synergistic effects of expanded graphite (EG) reinforcement (0 - 1 wt%), structural geometry, and cryogenic conditioning on the low-velocity impact (LVI) performance of carbon fiber/epoxy (CF/ epoxy) composite laminates. Flat and curved CF/epoxy laminates were fabricated with varying EG loadings using in-house engineered worm-like EGs synthesized via microwave expansion of intercalated Ceylon vein graphite. Flat laminates served as baseline structures, while curved panels, manufactured using a custom mold, representing a composite pressure vessel segment, were cryogenically conditioned in liquid nitrogen at -196 degrees C and tested at 50 J impact energy using a customized curved fixture. Initial LVI tests to evaluate the optimal EG concentration revealed that 0.1 wt% EG loading exhibited the best performance, increasing peak contact force and energy absorption by 20.5% and 38%, respectively, relative to neat composites, due to enhanced particle dispersion and improved interfacial adhesion. This optimized system was then used to investigate the effects of structural geometry and cryogenic conditioning. While flat laminates showed a greater initial stiffness, the EGreinforced curved panels surpassed their neat flat equivalents, both in terms of the peak contact force and energy absorption, by over 11% in both cases, highlighting the toughening role of EGs. Cryogenic conditioning further improved performance, with the EG-modified curved laminates absorbing 26% more energy than their ambienttested counterparts. These results demonstrate the potential of EGs for enhancing the impact resistance of composite structures for use in extreme environments.

This research investigates a novel 3D-printed sandwich lattice sheet-embedded composite laminate designed for enhanced impact resistance. Additive manufacturing parameters have been established for two corrugated, nanoengineered sandwich lattice sheet geometries: triangular and curved topologies-in addition to reference bulk structures. Nanocomposite sandwich lattice sheets with varying Graphene Nano Platelet (GNP) concentrations (0-0.5 wt%) have been manufactured and integrated into glass fiber-reinforced laminates using co-infusion and co-curing techniques. This innovative approach enables seamless integration of 3D-printed, nanoengineered lattice sheets, preserving in-plane properties while localizing GNP reinforcement for enhanced energy absorption and offering a scalable, industrially compatible toughening strategy. The investigation involved analyzing the molecular composition, microstructure, and bulk properties of the constituent materials used in the lattice sheet fabrication, before subjecting the lattice sheet-integrated laminates to drop-weight impact loading. The laminates exhibited an excellent improvement in impact resistance, showing up to a similar to 170 % increase in initial collapse load compared to baseline samples. These sandwich lattice structures effectively reduced damage propagation and displayed superior energy absorbing characteristics, notably in the case of the triangular sandwich lattice-embedded laminates. The study highlights the potential of triangular sandwich lattice sheet-embedded laminates, specifically those with optimized GNP concentrations, for applications requiring an enhanced impact resistance.

Carbon fibre-reinforced polymer (CFRP) composites are widely used in hydrogen storage systems due to their high strength-to-weight ratio and durability. However, repeated cryogenic thermal cycling, as experienced in hydrogen fuel applications, can induce thermal stresses that lead to microcracking, matrix-fibre debonding, and reduced mechanical performance. This study investigates the influence of graphene nanoplatelets (GNPs) on enhancing the cryogenic durability of CFRPs. GNPs were incorporated into the epoxy matrix at varying concentrations (0-0.75 wt%), and composites were subjected to controlled thermal cycling between -196 degrees C and 60 degrees C. Short-beam strength testing assessed mechanical performance, while scanning electron microscopy (SEM) and Raman mapping examined microstructural damage and dispersion. Results showed that, compared to unmodified CFRP, the incorporation of 0.1 wt% and 0.25 wt% GNPs increased strength by approximately 9 % and 10 %, respectively, before cycling, and by up to 20 % after 25 cycles. Strength reductions due to cycling were limited to 4.9 % and 5.7 % for the GNP-modified composites, compared to a 13.3 % loss observed in the unmodified composites. In contrast, 0.75 wt% GNPs led to agglomeration, increased cracking, and lower performance. SEM confirmed fewer cracks and improved interfacial adhesion at optimal GNP concentrations. These findings demonstrate that well-dispersed GNPs significantly enhance the mechanical resilience of CFRPs under cryogenic cycling, making them promising for hydrogen storage applications.

Advanced composites are increasingly used in aircraft and automotive structures, highlighting the need to address delamination, a critical failure mode caused by interlaminar weakness. Also the end-of-life of composite material is challenging. The interlaminar fracture toughness, impact resistance and residual flexural performance of glass/epoxy composites with recycled milled carbon fibre (rmCF) and recycled milled Kevlar fibre (rmKF) fillers are examined in this study. This novel approach blends recycled fillers (rmBF) and tests their effects on interlaminar fracture toughness, crash performance, and residual strength using modes I, II, mixed mode I/II and drop weight impact tests. Filler-loaded samples show a remarkable increase in fracture toughness, with initiation improvements of 319, 31 and 200

This study presents a novel investigation into the impact energy absorption characteristics of graphene nanoplatelet (GNP)-enhanced carbon fiber-reinforced liquid thermoplastic composites under various cryo-thermal cycling conditions. For the first time, we evaluate the performance of carbon fiber reinforced composites composed of liquid thermoplastic as a matrix and graphene nano platelets (GNPs) as additives for hydrogen storage applications. Laminates with various GNP concentrations (0, 0.25, 0.5, 1, and 1.5 wt.%) in liquid thermoplastic resin (Elium) were prepared. The laminates were subjected to cryo-thermal cycling (0, 1, 10, and 25 cycles) before testing at low-velocity impact (5, 10 and 20 J). The results showed that GNP-reinforced composites exhibited a superior retention of impact resistance under cryo-thermal cycling, with the 0.5 wt.% GNP composite demonstrating the best overall impact performance. Specifically, this composite achieved an 8 % increase in peak contact force and a 10 % increase in absorbed energy over the neat composite, due to its ability to alleviate thermal stresses. However, increasing the GNP content beyond this threshold resulted in particle aggregation, which reduced the mechanical properties. After extended cryo-cycling, all composites exhibited a decline in performance, with the neat samples experiencing the greatest reductions: 18 % in peak contact force and 14 % in absorbed energy. In contrast, the 1.5 wt.% GNP samples displayed better resilience, with reductions of only 6.5 % in the peak contact force and 4 % in absorbed energy.

This study investigates the quasi-static and dynamic compression performance of a newly designed stacked pyramidal lattice (SPL) structure composed of struts that resemble I-beams. These novel lattice structures are 3D-printed considering three different stacking sequences, and their stiffness, strength, and energy absorption properties are experimentally assessed through low-velocity impact (1.54 m/s) and quasi-static compression tests. Additionally, dynamic finite element (FE) simulations are carried out to delve deeper into the collapse mechanisms and failure processes. The findings indicate that the SPLs with I-beam struts outperform conventional SPLs with square struts of same mass showcasing superior rigidity, durability, and energy absorption. Specifically, we report enhancements in strength and energy absorption of 26% and 109% under quasi-static compression and 34% and 74% under low-velocity impact, respectively. The latter enhancements are attributed to the improved transverse bending stiffness of the I-shaped cross-section, resulting in lateral (sideward) buckling of the lattice struts. Both experimental and numerical findings demonstrate that altering the stacking sequence of the SPL can lead to significant improvements in the dynamic compression performance, with enhancements of up to 84% in collapse strength.

This study employs a novel co-curing approach to fabricate 3D glass fiber reinforced polymer (GFRP) composites with enhanced damage tolerance by incorporating graphene nanoplatelet (GNP) surfacing films. Low-velocity impact (LVI) tests were conducted on four different samples: pristine GFRP and GFRP composites with 1 wt%, 2 wt%, and 5 wt% GNP surfacing films co-cured with the laminate during resin infusion. The impact tests were performed at energy levels of 20 J, 30 J, and 50 J using a 6.20 mm diameter impactor. Moreover, a flexure-afterimpact (FAI) test was also performed to assess the effects of impact-induced damage on the residual strength of LVI samples. The LVI and FAI results demonstrated that 2 wt % GNP-GFRP composites exhibited slightly better damage performance compared to the pristine GFRP composite. This improvement in LVI and post-FAI performance is attributed to the toughening effect of the 2 wt % GNP surfacing film and the enhanced interfacial adhesion between the co-cured GNP film and the 3D GFRP composite. Additionally, short beam shear testing was conducted to evaluate the interlaminar shear strength (ILSS) of the films with the composites. Notably, the ILSS of the 2 wt% GNP-GFRP composite improved by 10 % compared to the pristine GFRP composite. These findings suggest that incorporating GNP surfacing films is a promising strategy for damage-resistant structures in aerospace applications.

This study investigates the repair of damaged curved glass/epoxy composite laminates through various adhesive reinforcements and adhesively bonded external patches. The experimental procedures involve fabricating laminates, conducting four-point bending tests, quasi-static indentation tests, and optimizing damaged areas. Results demonstrate the influence of repairs on curved beam strength and interlaminar tensile strength. Adhesive repairs with chopped fiber reinforcement outperform plain epoxy and particle fiber-reinforced repairs, preserving 51.28% of curved beam strength. Adhesively bonded external patches, both single and double, with chopped fiber reinforcement exhibit significant strength recovery 72.08% and 80.78% compared to virgin samples. The study emphasizes the effectiveness of adhesively bonded external patches, particularly those with chopped glass fiber reinforcement, in enhancing curved beam strength and interlaminar tensile strength. This research provides valuable insights for repairing angled structures, such as those in airplane components, showcasing potential applications of these techniques for structural maintenance. Investigated adhesive reinforcements and bonded external patches for curved composites.Laminates fabricated, tested for bending, indentation, and optimized in damaged regionChopped fiber adhesive repairs preserve significant 51.28% strength.Adhesively bonded patches exhibit 72.08% and 80.78% strength recovery.

This study investigates a novel self-sensing honeycomb composite structure composed of two distinct cellular layers with differing unit cell architectures, specifically hexagonal and re-entrant designs. Short carbon fiber (CF)/polyamide 12 (PA12) composite filaments with 0, 5 or 15 wt.% CF content were utilized to additively manufacture the honeycomb structures via Fused Filament Fabrication (FFF), and their mechanical and piezoresistive self-sensing characteristics were experimentally investigated under quasi-static in-plane and out-ofplane compression at both room temperature and elevated temperatures. The results reveal that the hybrid hexagonal/re-entrant (HR) honeycombs mechanically outperform their non-hybrid double-layer and single-layer counterparts under in-plane loading, reporting an increase in collapse strength and energy absorption by factors of 1.64 and 2.25, respectively. These improvements are attributed to the mechanical interactions occurring at the interface between the auxetic and non-auxetic layers within the hybrid structure, effectively enhancing its structural attributes. Furthermore, the double-layer honeycombs display excellent strain-sensing capabilities within the elastic regime, with gauge factors reaching values as high as 146. Mechanical tests conducted at elevated temperatures reveal that the CF/PA12 honeycombs retain a significant portion of their elastic modulus, strength and energy absorption even at 125 degrees C, while maintaining high gauge factors of up to 72.4. These honeycombs also exhibit pronounced thermoresistive behavior, evidenced by a decrease in electrical resistance of up to 41.3 % with increasing temperatures from 25 to 125 degrees C. Considering their exceptional combination of thermo-mechanical, thermoresistive and piezoresistive characteristics, these hybrid double-layered CF/PA12 honeycombs hold promise for potential applications in multifunctional lightweight structures, offering integrated temperature and strain-sensing capabilities.

This paper introduces a novel hybrid honeycomb (HC) design achieved by continuously blending non-auxetic hexagonal and auxetic re-entrant cell geometries along the out-of-plane direction. These novel hybrid HCs are additively manufactured via fused deposition modelling (FDM) using PA12 polymer reinforced with 15 wt.% of discontinuous carbon fibres. We study the mechanical and piezoresistive performance of hybrid HCs under quasi-static in-plane and out-of-plane loading performed at temperatures ranging between 25-125 degrees C. The results demonstrate significant in-plane compression performance enhancements in the hybrid configuration, achieving up to 43% increase in the collapse strength and 119% in absorbed energy. The incorporation of multiple hybrid layers in the honeycomb structure further enhanced the in-plane properties, ultimately achieving a 181% enhancement in energy absorption. The hybrid honeycombs also showed a pronounced piezoresistive response with gauge factors in the range of 18-37 within the elastic regime, making them suitable for a wide range of multifunctional applications.

There is a growing drive in replacing conventional non-renewable fibres such as glass and carbon reinforced composites with more sustainable and renewable reinforcements such as flax, hemp, jute in biobased composites in key industry sectors such as automotive, marine, building and construction motivated by lower carbon footprint and sustainability. Despite this drive, flammability characteristics of these sustainable biobased composites are not fully understood. Through an up-to-date review, this article meticulously discusses the theme of bio-based and eco-friendly flame retardants (FRs), delving into their intricate mechanisms, flammability testing methodologies, and emerging research trends. It underscores the pivotal necessity of developing tailor-made eco-friendly FRs customised for a diverse range of materials to bolster fire safety in sustainable material applications. Furthermore, it illuminates the limitations associated with prevalent methods for assessing flammability and advocates for advancements in dynamic testing and multi-scale analyses to more accurately simulate real-world fire scenarios. This review also highlights the key characteristics of natural plant fibres and their composites critical for the development of sustainable and fire safe materials for key applications areas. Additionally, it highlights the diverse spectrum of strategies employed in the realm of flame-retardant materials research, emphasizing a pronounced shift towards eco-friendly alternatives, innovative coatings, and the ongoing exploration of synthetic biopolymers, nanocomposites, and fibres in the pursuit of heightened fire safety. Amidst the inherent challenges, this comprehensive review unequivocally underscores the pivotal role of interdisciplinary research collaboration in driving forward fire safety within the domain of sustainable materials.