This study investigates one-part alkali-activated materials incorporating lead slag (LS) as fine aggregate, steel fibres (SF), and electric arc furnace (EAF) slag as partial replacement of natural coarse aggregate. An integrated experimental, statistical, analytical, and microstructural approach was adopted, with optimised mortars scaled to concrete and subjected to 10-time cyclic heating up to 400 degrees C. At the mortar scale, SF was identified as the dominant factor governing strength and thermal conductivity, enhancing crack resistance and forming effective heat-transfer pathways. Increasing the LS fine aggregate content improved thermal conductivity and specific heat capacity with a slight reduction in strength at higher levels. Analytical modelling showed that the parallel model well predicted thermal conductivity under ambient conditions, whereas none of the conventional models adequately captured post-heating behaviour. At the concrete scale, partial replacement of natural aggregate with EAF slag improved thermo-mechanical performance, with 10% replacement providing the most balanced enhancement in strength, thermal insulation, and heat storage capacity. These findings demonstrate the potential of waste-derived alkali-activated composites for sustainable concrete-based thermal energy storage applications.

Fracture mechanics, as an essential field of solid mechanics, has played a significant role in characterising and understanding cracking and failure processes of natural and engineered materials. With the emergence of the 4th paradigm: data-driven/physics-informed methods, machine learning (ML) offers transformative opportunities for this field. With the exponential growth of research leveraging ML in the fracture mechanics community, the need to ‘learn’ complex physics or to map from datasets becomes clearer. To this end, this paper presents a screenshot investigation at the intersection between ML and fracture mechanics. Firstly, the key ML algorithms are scrutinised, followed by the commonly used computational methods. Furthermore, we provide an in-depth review of ML applications across six significant aspects of fracture mechanics. Beyond these core findings, this work identifies underexplored research directions and offers actionable recommendations. This timely review highlights the potential of ML methods to advance fracture mechanics. It provides a practical guide for researchers to incorporate ML techniques into their studies, paving the way for the 4th paradigm in the fracture mechanics community.

The damage and failure of concrete under impact loads pose serious risks to the resilience and longevity of critical infrastructure. Classical continuum mechanics often falls short of accurately capturing the complex, discontinuous behaviour of concrete under such dynamic conditions. In contrast, the peridynamics theory, as a non-local approach, reformulates the equilibrium equations of continuum mechanics into an integral form, providing a unique alternative to address these existing issues. To this end, this paper provides a review of the latest advancements in this field, including theoretical developments, modelling techniques, and engineering applications. For the first time, a comparative study is conducted, including a vertical comparison between different PD methods, and a horizontal comparison between PD methods and other numerical methods frequently used in impact modelling. Lastly, we identified four critical areas that require further investigation: theoretical universality, algorithm development, computational efficiency, and practical validation, followed by a bottom-up, actionable roadmap. The insights offered aim to not only advance the scientific understanding of impact dynamics in concrete but also to guide engineers in applying peridynamics-based models in real-world applications.

Impact-contact events can significantly degrade structural integrity, making it important to evaluate how protective structures respond under such conditions. This study illustrates a novel boundary element method (BEM) for analysing the elastodynamic behaviour of cellular structures subjected to low-velocity impact. The proposed method builds on the conventional BEM for solving the transient analysis of anisotropic elastic materials in two dimensions and the contact constraint relations of elastodynamic impact-contact problems. By leveraging these relations, the impact-contact BEM can accommodate various impact-contact situations, e.g. when the indentation depth is prescribed and the initial velocity is given. Previous studies demonstrated that bone-inspired cellular structures (BCS) have superior mechanical performances and higher impact resistance than conventional honeycomb and auxetic structures. This paper further investigates the impact-contact behaviour of BCS materials under impact-contact conditions. The reliability and applicability of the proposed method are validated through comparisons with results obtained using the finite element method (FEM) in a commercial software. With the provided numerical results, the parametric studies of BCS under impact loads are further studied and discussed. These provide valuable insights and help deepen our understanding of the elastodynamic impact-contact responses of BCS under impact-contact situations.

Full-scale experimental crash tests are important to determine the occupant risk factors and understand barrier system behaviour in vehicle-barrier impacts. Due to the expensive and time-consuming nature of such crash tests, finite element simulations of full-scale crash tests to analyse and design road safety structures have gained popularity as a more practical method of assessment. In this research, a simplified simulation technique was employed by dividing the barrier system into two sections called the Impact Zone and the Rigid Zone, which was adopted for concrete crash barriers for the first time. A previously performed experimental crash test was used to successfully validate the numerical model. The numerical crash model accurately predicted the critical parameters of the barrier performance. The numerical simulation was further used to extract otherwise difficult or impossible results from the experimental crash test, such as internal energies and exit angles. The importance of the use of optimised Karagozian and Case concrete material parameters was demonstrated through a parametric study carried out with autogenerated parameters. A parametric study performed with impact speed and angle proved that increased speed is more harmful for occupants than a more oblique impact angle. The potential benefits of replacing traditional concrete with improved energy absorbing concrete to reduce occupant risks were also investigated. The comparison of results between the numerical models and experimental crash test confirms the trustworthiness of the developed models to simulate the vehicle-barrier crashes for analysing existing barrier designs and further developing new barrier designs to increase road safety.

This paper aims to optimise a bulk scale design of a novel auxetic structure, the hourglass structure (HGS), through a multi-objective optimisation model to improve its protection performance. A 3D numerical model of the HGS under an in-plane quasi-static compression was developed and validated with experimental results. Based on the validated numerical model, a series of numerical analyses were conducted by automating the HGS design process with randomly generated design variables. The automated numerical analyses built a dataset of the selected protective performance indicators (namely, peak elastic stress, plateau stress, and energy absorption capacity). Then, the dataset was used to develop a high-accuracy surrogate model using a radial basis function (RBF) neural network. Afterward, the Pareto optimal solutions were searched with the non-dominated sorting genetic algorithm (NSGA-II). The best compromise design out of the Pareto optimal set was determined with the ideal point method. The performance of the optimum design was simulated under both quasi-static and blast loadings to comprehensively explore the protective performance. In addition, a correlation matrix was constructed to investigate the effects of each design parameter on the protective performance indicators quantitatively. The results showed that the obtained optimum design outperformed the baseline structure under both quasi-static and blast loadings. The optimum HGS design displayed a higher and more stable negative Poisson's ratio along with two deformation modes leading to two plateau stress levels. The optimised HGS design is applicable as the core of high-performance protective sandwich structures.

Hydrogen energy storage systems are expected to play a key role in supporting the net zero energy transition. Although the storage and utilization of hydrogen poses critical risks, current hydrogen energy storage system designs are primarily driven by cost considerations to achieve economic benefits without safety considerations. This paper aims to study the safety of hydrogen storage systems by conducting a quantitative risk assessment to investigate the effect of hydrogen storage systems design parameters such as storage size, mass flow rate, storage pressure and storage temperature. To this end, the quantitative risk assessment procedure, which includes data collection and hazard identification, frequency analysis, consequence analysis and risk analysis, was carried out for the hydrogen storage system presented in a previous study [1]. In the consequence analysis, the Millers model and TNO multi-energy were used to model the jet fire and explosion hazards, respectively. The results show that the storage capacity and pressure have the greatest influence on the hydrogen storage system risk assessment. More significantly, the design parameters may affect the acceptance criteria based on the gaseous hydrogen standard. In certain cases of large storage volume or high storage pressure, risk mitigation measures must be implemented since the risk of the hydrogen storage system is unacceptable in accordance with ISO 19880-1. The study highlights the significance of risk analysis conduction and the importance of considering costs associated with risk mitigation in the design of hydrogen storage system.

This paper aims to investigate the crushing behaviour and protective performance of an innovative auxetic structure, named the hourglass structure (HGS), under in-plane compressive loads. The HGS was fabricated with a 3D printing method through fused filament fabrication technique. The 3D printed HGS specimens were subjected to a displacement-controlled in-plane quasi-static compressive load. A 3D comprehensive numerical model was developed and calibrated with the experimental results. The calibrated numerical model was used for a parametric study to explore the effects of relative density, functional grading, and crushing velocity on the protective performance of the HGS. Furthermore, a theoretical model was proposed to correlate plateau stress of the HGS with geometric design parameters, effective Poisson's ratio, and matrix material properties. Verified with a series of numerical analyses of different HGS designs, the proposed theoretical model predicted plateau stress with less than 10% relative error. In addition, experimental, numerical, and theoretical analyses uncovered load-deformation relationship, Poisson's ratio, crushing mechanism, and energy absorption capacity of the novel auxetic. Overall, the HGS exhibited excellent features for protective engineering applications such as shock mitigation and blast energy absorption.

Hydrogen is one of the new energy solutions to powering several of the world’s needs in the pursuit of a net-zero carbon world. However, with the proliferation of hydrogen infrastructure (generation, transport, storage, etc), the risk of accidents, particularly in residential areas, due to hydrogen’s explosion characteristics cannot be ignored. A recent review of hydrogen incidents collected by H2 Tools shows that just over one fifth of incidents result in an explosion, and over a quarter in fire events. It is thus imperative that the risks associated with hydrogen explosions are better understood. Vapour cloud explosions (VCEs) are complex phenomena which depend on a wide variety of factors which are not present for other high-explosive explosions (e.g. TNT). Examples of this are seen in how current hydrogen explosion tests are conducted, including the preference for unobstructed vapour cloud tests, and a focus on overpressures with less consideration for impulse. Even among hydrogen risk analysis papers, there is a tendency to only report on the overpressures for the consequence. As VCEs can take place as deflagration or detonation events, the duration, or impulse, of the explosion must be considered to properly assess the consequence. To better understand the type of data available from recent hydrogen explosion tests, a comparison is made with commercial analysis software, DNV Phast, and the TNO Multi-energy (ME) and Baker-Strehlow-Tang (BST) blast models. The need to better understand impulse is highlighted here, with a clear discrepancy between the actual and predicted impulse for several tests. Finally, this review covers some of the design philosophies for blast protection from high-explosives and how they can be utilised in hydrogen VCEs.

Auxetics are a class of structural metamaterials with a negative Poisson’s ratio. In comparison to conventional structures, auxetic structures are proven to exhibit several superior properties including higher energy absorption, enhanced indentation resistance, and improved mechanical properties. As a result, auxetic structures are gaining popularity as lightweight, high-performance protective structures to resist blast and impact loads. Although several past reviews on auxetics partially covered different aspects of auxetics, such as classification of architectures, mechanisms, mechanical properties, energy absorption behaviours, and applications, this field still requires a comprehensive overview of the anti-blast and -impact performances of the different auxetic structures. Therefore, this paper aims to review recent advances in auxetic-based lightweight, high-energy absorbing protective structures. Different design factors affecting the performance of the protective structures are critically discussed. Moreover, a classification of modern auxetic topologies, the status of large-scale auxetic fabrication, and anti-ballistic performance evaluation methods are presented in this paper. Overall, auxetic core sandwich panels offer superior protective performance than equivalent conventional protective armours. However, several limitations and challenges exist with the design, fabrication, and implementation of auxetic-based protective structures.

Rubberised concrete has been suggested for structures subjected to dynamic loadings, such as impact and blast, however, the understanding of the dynamic behaviour with rubber replacement and the influence of the particle size is still limited. This paper investigates the impact behaviour of rubberised concrete using both experimental and numerical studies. Split Hopkinson Pressure Bar tests under higher strain rates up to 181 s-1 were used to analyse the experimental dynamic behaviour using concrete specimens with a low volume of coarse and fine aggregates replaced by tyre shreds (15 mm) and tyre crumbs (2-4 mm), respectively. Under the same impact, rubberised concrete specimens demonstrated ductile failures, high dynamic increase factors, and increased normalised toughness moduli. Furthermore, rubberised concrete with the larger tyre particles showed increased ductility and normalised toughness moduli than that with the smaller tyre particles. Conversely, the specimens with smaller tyre particles were revealed to have higher dynamic strengths and dynamic increase factors. The numerical model in LS-DYNA, using the Karagozian and Case material model, was calibrated to reflect the observed rubberised concrete experimental performance. The calibration technique was validated by accurately predicting the dynamic tyre particle size effect of rubberised concrete numerically. The results proved that lowlevel rubber replaced concrete can be potentially used in impact resistant structures as there is a good balance between quasi-static and increased dynamic impact resistance for more sustainable constructions.

This paper introduces a protective sandwich structure with simultaneous auxetic compression and tension behaviour (termed as the dual-mechanism) in the core. To achieve the proposed mechanism, a novel architected cellular structure, named as the dual-mechanism auxetic (DMA), was developed. The protective performance of the DMA was examined by subjecting the DMA-core sandwich panel (DMASP) to blast loading through computational modelling. A series of dynamic numerical models were developed in LS-DYNA, and validated with the past experimental results. The protective performance of the DMASP was evaluated comprehensively based on energy absorption, stress mitigation, and deflection control. Moreover, the protective performance of the DMASP was compared with an equivalent areal density solid plate. In addition, parametric studies were conducted to investigate effects of various design parameters and explosive environments on the protective efficacy of the DMASP. The numerical simulations uncovered that, in addition to high energy absorption capacity of the DMA core, the dual-mechanism was beneficial to mitigate high stress transfer to the protected structure by redirecting stress in the lateral direction through auxetic tensile behaviour. Furthermore, the DMASP offered better protective performance than the equivalent solid structure with respect to all the evaluation criteria under different blast loading scenarios.

Re-entrant and honeycomb cellular structures have shown potential for mitigating the effects of extreme loadings such as those imposed by impacts and near-range air blast. However, these cellular geometries can buckle locally and collapse in the immediate vicinity of the loading, which can limit their effectiveness as a protective element. These deficiencies can be addressed by mimicking alternate, naturally occurring, cellular structures, including that of the porcupine quill, which is studied here. The quill possesses several distinct features that effectively counteract buckling and bending, and minimise weight. This study mimics several structural features of the quill to develop a novel cellular design for counteracting air blast loads such as those associated with detonations of high explosives. The performance of the bio-mimetic structure is benchmarked against traditional hexagonal and re-entrant designs, which have been documented in the archival literature. The quill-inspired structure offers more design freedom than the traditional cellular geometries. By iteratively mimicking several of the structural features of the porcupine quill, an optimal balance between local buckling and collapse can be realised, which minimises the reaction on the target below and maximises energy dissipation.

Auxetics are structures and materials with a negative Poisson's ratio, meaning they contract in the direction perpendicular to the applied force under a compressive load. This phenomenon can lead to increased energy absorption, among other favourable parameters to protect against impulsive loadings. In this research, a structure with alternating oval perforations was investigated which gives rise to an auxetic geometry, referred to as an 'auxetic oval' design within this paper. Quasi-static (strain rate = 0.001/s) and dynamic (strain rate = 100/s) experiments have been conducted on small-scale specimens, manufactured using subtractive fabrication (laser cutting). Different design criteria were examined to understand the mechanisms behind the energy absorption of the auxetic structures. A modified version of the energy efficiency method was developed to determine the densification strain of the auxetics. A shift from strain-softening to strain-hardening behaviour is seen as the number of cell layers increases, with the perfectly-plastic response ideal for energy absorbing structures. Inertia effects are present in the dynamic tests, showing an increase in plateau stress for several of the designs. The quasi-static and dynamic responses are fundamentally identical, meaning that the performance of the auxetics do not change under the range of strain rates examined. Digital image correlation has been successfully utilised in the analysis process. This includes confirmation of the mechanism giving rise to the auxetic behaviour (rigid rotating squares) and the calculation of the negative Poisson's ratio. Strain fields have been examined showing the regions of energy dissipation. Based on these results the auxetic oval design experiences bending dominated behaviour. Finally, a unique design has been developed, dubbed the 'Hybrid Auxetic Oval', which shows favourable behaviour compared to the equivalent base auxetic oval design by mitigating the effects of fracture.

This paper aims to develop lightweight high-performance auxetic structures for protective applications such as blast and impact energy absorption. In this study, novel auxetic topologies were computed with an objective of energy absorption maximisation through the topology optimisation method. Analytical design formulations with the consideration of fabrication practicality were derived for the novel auxetic structures; named as the hourglass structure (HGS), braced cross-petal structure (BCPS) and cross-petal structure (CPS). Based on the formulated design parameters, rectangular scaffolds of each structure were designed to investigate bulk mechanical properties and energy absorption under a compressive load. Validated nonlinear finite element (FE) models developed in LS-DYNA FE code were used to examine the mechanical performance of the novel structures. The FE simulations revealed the deformation mechanism, negative Poisson's ratio, stress-strain behaviour and energy absorption. The protective performances of the developed auxetic structure were evaluated with respect to the peak elastic stress, plateau stress, onset of the densification strain and energy absorption capacity. The HGS outperformed the other structures in term of the protective performance. In addition, comparison of specific energy absorption at the onset of densification showed that the HGS, BCPS and CPS possessed superior energy absorption capacities than the conventional auxetic structures.

This paper aims to scrutinise the effectiveness of a re-entrant auxetic honeycomb-core sandwich panel (AHSP) to protect reinforced concrete (RC) slab under close-in and far-field detonations of high explosive. A series of quarter symmetric 3D comprehensive numerical models were developed by using Arbitrary Lagrangian Eulerian (ALE) formulation in LS-DYNA hydrocode. Strain rate effects of the rate sensitive materials were employed in the model to consider dynamic behaviour of the materials during blast loading. The developed numerical model was validated with the documented experimental results. Deformation patterns, energy absorption, overpressure damping, blast pressure deflection, and stress-transmission to the protected structure were investigated with the computational models. Furthermore, the performance of AHSP sacrificial protective structure was compared with an equivalent areal density conventional honeycomb-core sandwich panel (CHSP) structure. The results showed that AHSP not only absorbed higher blast energy but also acted as a pressure deflector by forming a densified concave depression under close-in blast load. AHSP damped higher magnitude of blast overpressure in both close-in and far-field blasts. Moreover, a uniform redistribution of the stress on the protected structure was observed with AHSP protection. The overall response of AHSP was found to be better than CHSP to protect the RC slab.

Rubberised concrete (RC) is a green and environmentally friendly concrete type that can be used as a sustainable production material. However, investigations on the structural applications of RC are limited due to its increased cost and decrease in strength. In this study, economical mixture designs were formulated with low-level replacements of rubber aggregates from recycled end-of-life tyres, as a prospective approach. The size effect with coarse (15 mm tyre shreds) and fine (2-4 mm tyre crumbs) rubber aggregates on the fresh and mechanical properties of structural concrete, specifically its fracture properties, was investigated, incorporating preliminary testing and constitutive models. The damage evaluation and characteristics of Mode I fracture behaviour of RC were investigated using three-point bending tests on notched beams. Using digital image correlation which possibly can be used to replace the conventional tools, the crack patterns on the beams were detected and crack mouth opening displacements were measured. The rubber replacements led to crack bridging effects. Overall, the results indicate that addition of recycled tyre aggregates at low percentages results in improved ductility and higher energy absorption and thereby improving the toughness of concrete, with marginal impacts on its strength and other properties. Furthermore, the selected mathematical models for compression and fracture can be used to evaluate the performance of RC structural elements with different rubber replacement levels. The modified RC can be potentially applied to structural components for earthquake resistance, integral bridges, or impact resistant road barriers.

Extreme loading threats arising from terrorism, war zones, natural disasters, and even accidents, require specialised protection in order to limit and prevent death and damage to infrastructure and vehicles. In particular, soil blast loading is of concern due to the increased loading imparted by the dual phase explosive/soil mix. Smart, more efficient structures are required to better protect against these kinds of threats. Auxetics, structures and materials with a negative Poisson’s ratio, are one such novel structure which have seen increasing research as protective structures. Their counterintuitive nature means they contract under a compressive load and vice versa. This phenomenon draws materials towards the point of impact and can lead to increased energy absorption, indentation resistance, and fracture toughness. This research investigates the auxetic oval structure for its protective capabilities with a focus on energy absorption. Quasi-static and dynamic testing was conducted on samples to quantify the response over strain rates ranging between 0.001-100 /s. It was found that the auxetic oval structure displayed the classic stress-strain response for energy absorption with a flat plateau stress region. However, fracture was present in the experiments and shown to negatively affect the energy absorption. A Hybrid design utilising another auxetic geometry, the re-entrant honeycomb, was developed to remedy this issue. Digital image correlation (DIC) was also used in the analysis of the experiments to confirm the presence of the rigid rotating square mechanism controlling the auxetic behaviour. Numerical models were then developed in finite element software LS-DYNA and validated against the stress-strain response and DIC analysis from the experiments. Further understanding of the energy absorption mechanisms were identified through the numerical models, including the locations of plastic energy dissipation. Parametric studies were then conducted from which different behaviour characteristics were tied to geometric features. Most results were intuitive, such as the plateau stress being tied to the thickness of the material between holes. However, additional insight was revealed about how the thickness negatively affected the Poisson’s ratio, i.e. thicker sections changed the load transfer response and thus negated the auxetic effect. Only one parameter, the oval ratio, was shown to directly affect the densification strain, which was due to the available space the holes could collapse in. Finally, load rate studies showed how the auxetic oval structure was characterised by uniform crushing for load rates up to 10 m/s. Above 200 m/s the auxetic displayed localised crush band formation. Soil blast loading was quantified through flying plate tests conducted with 0.5 kg TNT charges buried within an unsaturated soil. The impulse imparted to the plates can be used to determine the loading imparted to a protective structure. It was also used to validate numerical models of soil blast events in LS-DYNA using the Arbitrary Lagrangian-Eulerian (ALE) method. The soil model was populated with data from soil characterisation tests. Good agreement between the experiments and models was generally achieved. Further work on understanding the loading mechanism of soil blast was then conducted. It was determined that ~63% of the impulse was from the soil alone. Furthermore, the spatial and temporal loading was determined through additional numerical models, highlighting the localised nature of soil blast events. The final body of research investigated the auxetic oval geometry with soil blast loading considered. Simplified localised loading on long auxetic sections showed the benefit of a negative Poisson’s ratio over conventional positive Poisson’s ratio equivalent structures. This included an increase in relative energy absorption due to the ability to draw material towards the load point. Loading was then simulated recreating the failure scenarios of a vehicle subject to a soil blast event. This was conducted using simplified load models (ConWep) on the auxetic structure, and ALE loading on homogenised foam material models representing the auxetics. Both methods highlighted the benefits of the auxetic oval structure over conventional protective structures, including; reduced stress transmission, increased energy absorption, and decreased global body acceleration. In conclusion, the auxetic oval structure displays an ideal response for energy absorption, and the localised nature of soil blast is ideal for activating the main benefits of an auxetic structure though material densification and load redistribution.

The complex cellular structure of trabecular bone possesses lightweight and superior energy absorption capabilities. By mimicking this novel high-performance structure, engineered cellular structures can be advanced into a new generation of protective systems. The goal of this research is to develop an analytical framework for predicting the critical buckling load, Young’s modulus and energy absorption of a 3D printed bone-like cellular structure. This is achieved by conducting extensive analytical simulations of the bone-inspired unit cell in parallel to traverse every possible combination of its key design parameters. The analytical framework is validated using experimental data and used to evolve the most optimal cellular structure, with the maximum energy absorption as the key performance criterion. The design charts developed in this work can be used to guide the development of a futuristic engineered cellular structure with superior performance and protective capabilities against extreme loads.