Titanium micro-pins have been reported to enhance the modes I and II interlaminar fracture toughness of titanium-to-composite joints. In this study, we examined the enhancement of modes I and II interlaminar fatigue crack growth resistance of titanium-to-composite joints using 3D printed titanium micro-pins created using selective laser melting (SLM). The joints were formed by printing an orthogonal array of thin (1.0 mm diameter) titanium micro-pins over the titanium substrate using SLM, which were then embedded into a carbon-epoxy composite substrate to create a micro-pin reinforced bondline interface. Interlaminar fatigue experiments were conducted under cyclic displacement control conditions using the Double Cantilever Beam (DCB) and End Notch Flexure (ENF) test methods. Under modes I and II cyclic loading, pinned joints exhibited significantly higher strain energy release rates for equivalent crack growth rates due to the micro-pins forming a crack bridging zone behind the crack tip, enhancing fatigue crack growth resistance compared to unpinned joints. The micro-pins increased the modes I and II cyclic critical strain energy release rate value by 18-fold and 4-fold, respectively. A single micro-pin cyclic testing was also performed to investigate the efficacy of the SLM Ti-pins for generating bridging traction loads under fatigue loading. The fatigue test results are presented together with fractographic evidence of the fatigue strengthening and toughening mechanisms.

Until relatively recently most mechanical metamaterial classes being studied have been composed of a single solid constituent phase and design has focused almost exclusively on structural geometry. Additional design dimensions can be introduced by accepting heterogeneity and varying materiality, i.e., allowing mechanical properties to vary across the metamaterial's unit cells or even from cell to cell in the metamaterial domain, creating composite metamaterials. This higher dimensionality significantly expands the effective property envelope, but the additional complexity also presents a significant hurdle. To overcome the design challenge, an automated design framework is proposed that leverages modern evolutionary computation techniques, combined with finite element analysis for fitness evaluation, to a discretized or voxelated design domain. However, this approach introduces stochastic and statistical aspects to the design process, which requires additional processing to successfully extract useful solutions. A case study is presented in which the proposed automated design framework is used to generate 2D structures that exhibit pentamode-like behavior. Pentamode metamaterials, which are best known for extreme bulk-to-shear modulus ratios (B/G), offer unique control over effective elastic properties and make for a particularly interesting test case. The evolutionary objective was defined as maximizing B/G over a voxelated square 2D domain. It was found that the evolutionary process converges to a solution relatively rapidly, generally in less that a hundred generations. B/G ratio values of 10,000 and more were obtained, largely exceeding those commonly found in the literature for experimental pentamode metamaterials. These generated designs feature reduced stress concentrations due to the elimination of point-like connections between lattice struts, which addresses a key practical limitation of diamond lattice pentamodes. It was observed that whatever the initial variety of elastic moduli values across the voxels of the design domain, as evolution progressed this variety collapsed to a much smaller number, most often a binary composite of very stiff voxels with a limited number of much softer voxels at key locations that acted as hinges.

Metal-composite hybrid joints with through-thickness pins are high-performing, but a study is needed on manufacturing resin-infused composites with large pin angles. Single-pin metal-composite joints were investigated with pin angles of 0°, 20°, 45° and 80°. Microstructural characterisation revealed the pin insertion zone varied significantly with the pin angle and along the pin length. With increasing pin angle, the diamond-like resin-rich zone became triangular, asymmetric, and significantly reduced in size to effectively zero. Other asymmetric, complex and novel features were observed as well. Despite these features, the joints had suitably high quality and demonstrated the potential of resin-infused composites for high pin-angle joints.

Research in mechanical metamaterials has achieved extreme, unprecedented properties by exploiting inhomogeneity in the form of periodic structures, enabled by advances in digital computation and manufacturing. The result has been a proliferation of metamaterials and relatively recently, the emergence of non-periodic heterogeneous metamaterial systems, which we term metastructures. This represents the beginnings of the evolution of metamaterials from just materials with unique properties towards systems capable of more complex, machine-like functions. To better understand the essential features and relationships among this wide and growing variety of species within the classes of mechanical metamaterials and metastructures, a classification scheme that focuses on functionality is needed. Both periodic and non-periodic metamaterials systems can be abstracted as mechanisms that produce desired outputs by performing specified transformations on given mechanical inputs. Here we propose an approach to functional classification and comparison based on the deformation and force transformations that these metamaterials and metastructures can provide. Transformation- based approaches are prevalent in geometry processing, computer graphics and soft robotics. This review examines existing periodic and non-periodic metamaterials from this perspective, classifying them according to the quasi-static deformation output they can achieve, and identifying gaps, challenges and promising future directions. Two classes of transformations are defined, i.e., uniformly distributed (homogeneous) or spatially dependent (non-homogeneous), and their building blocks and mechanisms are discussed. This stance empowers a more efficient approach to inverse design of metamaterial systems, in which desired operations are realized through transformations produced by the combination of diverse building-block architectures.

An experimental investigation is presented into the enhancement of interfacial toughness properties of hybrid titanium-to-composite structural joints using 3D printed metal pins created using selective laser melting (SLM). The joints were formed by printing an orthogonal array of thin (1.0 mm) diameter titanium pins over the titanium substrate using SLM, which were then embedded into a carbon-epoxy composite substrate to create a high toughness interface. SLM Ti-pinned hybrid joints were evaluated for the first time with and without film adhesive, which is co-bonded between the titanium and composite substrates. The SLM Ti-pins increased the modes I and II interfacial toughness of the hybrid joints by up to about 19- and 11-fold, respectively. The study reveals that pins created using SLM are highly effective at the strengthening and toughening of hybrid metal-composite joints, with the need for adhesive bonding dependent on the loading condition.

High-fidelity simulation tools have significant potential to support composite aircraft sustainment, though further study is required on incorporating the complex impact damage field. In this paper, compressive residual strength assessment is investigated using the high-fidelity computational tool BSAM (TM). The experimental impact damage was mapped and modelled at a high-fidelity level, which included ply-by-ply definition of the geometry of the impact indentation, fibre fracture in the plies and delamination in ply interfaces. It was shown that applying a small lateral displacement or 'pseudo-impact' step was highly effective in generating matrix cracks in the impact region, which provided a suitably realistic representation of the interconnected damage map through-the-thickness. It was found that inclusion of all damage modes in the post-image damage map at a high-fidelity definition was essential due to the strong degree of interaction between damage modes. The results support improved sustainment of defence platforms, through enhanced predictive capability and understanding.

Metamaterials and architected materials derive their effective properties from their internal structure more than from their composition. While the variety of metamaterial structures is limitless in principle, in practice, metamaterials have tended to be based on periodic structures with a homogeneous topology, which limits the range of properties that can be achieved. The metamaterial design space can be expanded by accepting irregularity, a step towards discarding the bonds of periodicity and topological uniformity. Varying materiality, that is allowing the mechanical properties of the metamaterial’s constituent material to vary, introduces additional design dimensions. Here, an evolutionary framework is proposed to search for solutions in these expanded design spaces. It relies on a modern evolutionary computation technique combined with conventional deterministic modelling, in the form of finite element analysis, executed on a massively parallel computing system. This approach produces large amounts of data, introducing stochastic and statistical aspects to the design process, and the extraction of useful solutions is discussed. A case study is presented in which the bulk-to-shear modulus ratio (B/G) of a voxelised square 2D domain is maximised by our evolutionary framework. Extreme values the B/G ratio are the defining characteristic of pentamode metamaterials, which make for a particularly interesting test case because they offer a path towards unparalleled control over effective mechanical properties. B/G ratio values of 10,000 and more were obtained, largely exceeding those commonly found in the literature for pentamode metamaterials that have been tested. In addition, the designs generated by this evolutionary approach eliminate the need for point-like connections between lattice struts, which addresses a key practical limitation of existing pentamodes.

The effect of in-plane compression loading on the low energy impact response of fibre metal laminates (FML) is investigated. A GLARE (R)-like FML consisting of bonded layers of aluminium alloy and glass-epoxy composite was impacted at different incident energy levels (up to ~20 J) while under different axial compression strain levels (up to 8000 mu e). Experimental testing and finite element (FE) analysis reveal that compression preloading increases (up to 50%) the maximum impact force experienced by the FML due to shortening of the impact contact time. The out-of-plane deformation of the FML, the amount of plasticity damage to the aluminium layers, and the shape and size of delamination damage to the glass-epoxy layer are dependent on the compression strain level. The FE model predicted the impact response and impact damage to the FML. The FE model is a validated numerical tool to analyse the low energy impact response of FMLs used in compression load-bearing structures.

Piezoelectric actuators have been highly successful in a wide range of structural control applications. As such, there is an ongoing need for rapid and accurate structural analysis techniques, particularly for highly heterogeneous composite materials and accounting for the actuator as a patch. Here, a new model based on the Refined Zigzag Theory (RZT) formulation that includes geometric nonlinearities is proposed for buckling, postbuckling and nonlinear static response analyses of geometrically imperfect composite beams with piezoelectric actuators. Both the analytical and the finite element (FE) formulation are presented for symmetrically and non-symmetrically laminated beams. The FE approximation is further generalised to the case of beams with geometric discontinuities to model composite beams with piezoelectric actuator patches. The new RZT model is numerically verified through comparisons to Abaqus solutions for buckling and postbuckling analyses and for the geometrically nonlinear response to an applied voltage of geometrically imperfect composite beams with piezoelectric actuator patches. This work presents a new model for composite beams with piezoelectric actuators and confirms the remarkable advantages of RZT in terms of accuracy and computational efficiency also for challenging nonlinear analyses, where the RZT computational time is generally less than half the time required by the FE commercial code.

In this work, a coupled experimental and numerical investigation into compression after impact of composites at elevated temperature is presented. Quasi-isotropic composite specimens were subjected to low velocity impact (with approximately 8 J impact energy) to generate barely visible impact damage. Impact damage was characterised non-destructively using ultrasonic inspection and X-ray micro computed tomography to create a post-impact damage map. The impacted specimens were loaded in compression until failure, with specimens tested at room temperature and an elevated temperature of 70°C. The numerical analysis utilised BSAM™ for high fidelity computational analysis at ply level incorporating all critical damage and structural mechanisms. The post-impact damage map was incorporated into the model, which included capturing the matrix cracking, delamination, fibre fracture and indentation. The residual strength analysis captured progression and interaction of damage mechanisms, as well as sub-laminate buckling and contact across the delaminated interfaces. The effect of temperature on material properties was incorporated based on material data from standardised tests. The numerical results were correlated to the experimental results, which demonstrated the effectiveness and capabilities of the modelling approach, and also allowed for novel insights into the effect of elevated temperature on compression after impact strength. The outcomes of the work contribute towards improved sustainment of aircraft, by supporting a physics-based composite strength assessment, which increases reliability and accuracy in predictive tools. This leads towards less conservative assessments and less reliance on experimental testing, which in turn can translate to an increase in the service life for aircraft and reduction in the cost of operation.

The effect of in-plane loading on the low energy impact damage tolerance of a fibre metal laminate (FML) is experimentally investigated. The FML consisted of stacked layers of aluminium alloy (AA2024-T3) and S2-glass/ epoxy (GFRP) composite. The FML was impacted in the transverse direction at increasing energy levels (up to ~ 20 J) while preloaded in tension or compression at different levels of elastic strain (up to 8000 mu epsilon). The amount of impact-induced damage and the post-impact compressive strength of the FML were both dependent on the type and magnitude of the preload. The FML sustained plasticity damage and cracking to the aluminium layers; fibre damage, matrix cracking and delamination cracking within the GFRP layer; and debonding cracking between the metal and composite layers under impact loading. The amount of impact damage to the FML increased with the tension and compression strain levels, with more damage created under the tension preload condition. The post-impact compressive strength of the FML also decreased with increasing preload strain, with the reduction being greater for tension compared to compression. The research provides new insights into the impact damage tolerance of FMLs used as load-bearing structural materials.

A finite element (FE) model to accurately compute the low energy impact response and impact damage to fibre metal laminates (FMLs) under tension preloading is described. The FE model can predict the initiation and progression of impact damage that includes metal plastic deformation, delamination damage, and debonding along the metal-composite interfaces. The accuracy of the FE model is evaluated using experimental data for an aluminium/glass-epoxy FML. The FML was impacted at different energy levels while preloaded to different tension strain levels. The FE model accurately predicted the peak impact load, impact-time response, absorbed impact energy and bulk deformation of the FML for the different impact energy and preload conditions. The FE model also accurately calculated the amount of plastic deformation damage to the metal sheets and delamination damage to the composite sheet, which were dependent on the tension preload strain.

In this study, compression after impact fatigue test results were obtained experimentally for the composite material system. A novel approach was developed using a compact low-cost thermoelastic stress analysis system developed by the Australian Defence Science and Technology Group to quantify the development of damage and propose a new methodology for in-situ damage tracking in composite structures by measuring the change in damage area on the front impact surface. The post-impact compression fatigue damage morphology was characterised using XmCT. Damage captured by XmCT correlated well with the damage observed through TSA, which offers confidence in the use of TSA to monitor the progression of low velocity impact damage.

Preforming is a common process in the manufacture of net-shaped composites, and there is a critical need for technologies that improve manufacturing speed without adversely impacting the interlaminar fracture toughness. Furthermore, composite structures that are repairable in-situ have potential for significant cost and performance benefits. This paper presents a new multi-functional composite system that combines ultra-high interlaminar fracture toughness with exceptional repair efficiency and is seamlessly integrated into the braiding process for rapid preform manufacture. The co-braided repair agent, EMAA, is shown to improve the interlaminar fracture toughness by over threefold, while enabling full recovery of the fracture properties via a thermal repair process. The resistance to crack initiation and crack propagation were strongly influenced by the geometric shape and spatial distribution of the EMAA which, in turn, was dictated by the preforming time. A new strategy is developed to produce a 3D fused EMAA network that enables the effective delivery of the repair agent into the delamination cracks and a greater than 100% recovery to the steady-state fracture toughness following delamination repair. The EMAA composites developed in this study exhibited repeated (at least three cycles) in-situ delamination repair abilities.

A penetration model for composites reinforced with high tenacity fibres like ultra-high molecular weight polyethylene (UHMW-PE) is proposed based on the modified Bernoulli theory typically used for metallic targets. The model describes penetration by tensile failure of the composite material through compression loading in an infinitely thick target and includes consideration for projectile deformation. The predicted projectile velocity and axial interface loads shows good agreement with numerical simulations for a deforming and rigid projectile condition against two grades of UHMW-PE composite. The model can also be used to predict transient projectile velocity and interface loads during the first phase of penetration (which acts over most of the penetration event) for a target of finite thickness. It was shown that transition to bulging occurs upon the arrival of the rarefaction wave, generated from the back of an unsupported target, at the projectile-target interface. The stress relief can lead to a change in penetration mode when the magnitude of release is sufficient to reduce the load at the interface below that of the target strength. This change in mode occurs late in the penetration event, and the model developed in this work is demonstrated to be valid up to this point.

Environmental degradation of transparent thermoplastic polyurethane (TPU) is an issue of growing concern. Measured increased in the mechanical properties of nano-reinforced TPUs have been reported, however, limited research is available regarding their adhesive properties and resistance to environmental aging. By incorporating cellulose nanocrystals (CNCs) into a TPU via planetary ball milling, an increase to both the mechanical and adhesive properties was obtained. The TPU/CNC nanocomposite showed bond integrity and lap shear strengths were increased by 34% and 25% respectively relative to the unmodified nanocomposite, and further improvements in mechanical performance were seen. The TPU/CNC nanocomposite showed higher melting temperature and increased melting enthalpy indicating improved thermal aging resistance. The restriction of TPU segments and changes in chemical bond state and crystallinity were identified as key mechanisms driving performance improvements.

There is a lack of understanding on the damage tolerant design of bonded scarf joints, specifically when an existing bondline flaw affects the performance. This paper presents an improved progressive damage modelling methodology for assessing the damage tolerant performance of composite scarf joints containing artificial flaws in the bondline. The developed methodology is validated using an experimental study investigating the influences of design parameters on the strength of scarf joints under quasi-static tensile loading. The presented work gives excellent predictions of joint strength, and significant insight into the initiation, interaction and progression of damage. A new damage tolerant design approach is subsequently proposed for the application of bonded composite scarf joints, particularly in primary aerospace structures. The proposed design approach assesses the state of the stress at the failure-critical regions of adhesive and adherend to inform on damage-tolerant safe design loads, thus improving the future application of bonded composite scarf joints.

The Refined Zigzag Theory (RZT) is a structural theory developed for the analysis of composite multilayer and sandwich beams. However, the accuracy of RZT for buckling analysis of sandwich beams has not been experimentally investigated, and for RZT and Timoshenko Beam Theory (TBT) the effect of the degree of heterogeneity on their accuracy requires further study. The aim of this work was to validate the use of the RZT for predicting the critical buckling loads of sandwich beams, even with highly heterogeneous material properties, and to assess the use of the TBT for the same application. Buckling experiments were conducted on five foam-core sandwich beams, which varied in geometry and included highly heterogeneous configurations. For each beam, two finite element (FE) models were analyzed using RZT- and TBT-beam FEs. The comparison between the numerical and the experimental results highlighted a major capability of RZT to correctly predict the critical buckling load for all the beams considered. The dependence of the TBT results on the beam characteristics was further investigated through a parametric analysis, which showed the dominant effect to be a close to linear relationship between the TBT error and the beam face-to-core thickness ratio. The work demonstrated the outstanding accuracy of the RZT predictions, including the superior capabilities with respect to TBT, and has application for rapid and accurate analysis of industrial structures.