In recent decades, the application of composite materials in aerostructures has significantly increased, with modern commercial aircraft progressively replacing aluminum alloys with composite components. This shift is exemplified by comparing the material compositions of the Boeing 777 and the Boeing 787 (Dreamliner). The Boeing 777 incorporates approximately 50% aluminum alloy and 12% composite materials, whereas the Dreamliner reverses this ratio, utilizing around 50% composites and 12% aluminum alloy. While metals remain advantageous due to their availability and ease of machining, composites offer greater potential for property tailoring to meet specific performance requirements. They also provide superior strength-to-weight ratios and enhanced resistance to corrosion and fatigue. To ensure the reliability of composites in aerospace applications, comprehensive testing under various loading conditions, particularly impact, is essential. Impacts were performed on quasi-isotropic (QIT) carbon-fiber reinforced epoxy panels with stainless steel, round-nosed and flat-ended impactors with rubber discs of 1-, 1.5- and 2 mm thickness, adhered to the flat-ended impactor to simulate the transition between hard and soft impact loading conditions. QIT composite panels were tested in this research employing similar lay-ups often being implemented in aircraft wings and other structures. The rubber discs were applied in the flat-ended impactor case but not for the round-nosed impactor due to the limited adhesion between the rubber and the rounded stainless-steel surface. Impact energies of 7.5, 15 and 30 J were investigated, and the performance of the panels was evaluated using force-time and force-displacement data alongside post-impact ultrasonic C-scan imaging to assess the damaged area. Damage was observed at all three energy values for the round-nosed impacts but only at the highest impact energy when using the flat-ended impactor, leading to the hardness study with adhered rubber discs being performed at 30 J. The most noticeable difference with the addition of rubber discs was the reduction in the damage in the plies nearest the top (impacted) surface. This suggests that the rubber reduces the severity of the impact, but increasing the thickness of the rubber from 1 to 2 mm does not notably increase this effect. Indentation clearly plays a significant role in promoting delamination at low-impact energies for the round-nosed impactors.

The present study investigates the effect of the impactor geometry on the impact performance, at relatively high impact energies, of carbon fibre-reinforced polymer (CFRP) laminates with a cross-ply configuration of [02/902], which were manufactured using unidirectional (UD) carbon-fibre epoxy-matrix plies. Drop-weight impact tests were performed using both round-nosed and flat-ended rigid impactors. White light interferometry (WLI), ultrasonic C-scan and scanning electron microscopy (SEM) were employed to assess the relationship between the indentation profile, the delamination footprint and the fracture morphology along the fracture plane. This study focusses on the coupling between the extent of indentation, the loading responses and the associated damage caused in the CFRP. This damage involved both intralaminar damage, including matrix cracking and fibre-kinking and fracture, and interlaminar, i.e. delamination, damage. This paper studies in more detail the finding that the flat-ended impactor only caused significant damage to the CFRP panel when an impact energy of 25 J and above was attained, which was previously observed by Y. Ding. J. Liu, Z.E.C. Hall. R.A. Brooks, H. Liu, A.J. Kinloch and J.P. Dear, “Damage and energy absorption behaviour of composite laminates under impact loading using different impactor geometries”, Composite Structures 321 (2023) 117259. At an impact energy of 25 J, a compressive kink-band fracture plane occurred around the periphery of the flat-ended impactor near the front surface of the laminate. A previously published numerical model has been extended to account for these experimental observations. The modelling accurately predicts the damage features observed for the two types of impactor geometry with indentation playing a significant role on the threshold for damage and the distribution of damage.

The present paper investigates the low-velocity impact behaviour and the compression-after-impact (CAI) behaviour of carbon-fibre reinforced-plastic (CFRP) composite panels after two impacts in different locations on a test coupon. The effect of changing the separation distance between the two impacts, as well as changing the impact energy, is examined by a comparison of C-scan damage maps and the CAI load versus deflection behaviour. It is shown that impacts that are close to one another lead to a linking of the delamination footprints at each impact site. Further, that increasing the impact separation distance, for the same impact energy, leads to an increase in the area of the initial delamination damage footprint, until a separation distance is reached when two completely separate damage zones are formed. As to be expected, the delamination damage area increases, and the residual compressive strength after impact (termed the 'CAI strength') decreases with an increasing impact energy. An interesting observation is that the residual compressive strength generally increases with increasing impact separation distance.

The present paper investigates the low-velocity impact behaviour of carbon-fibre reinforced-plastic (CFRP) composite panels and the damage incurred when they are subjected to a single impact. The relationship between the depth of permanent surface indentation that results and the associated area of interlaminar delamination damage is investigated for two different thicknesses of composite panels. In particular, the delamination damage area increases with impact energy for both thicknesses of composite panel that were studied. Likewise, the indentation depth also increases with increasing impact energy, again for both thicknesses of CFRP panels. It is shown that the indentation depth, at the centre of the indentation, may be used to provide an indication of the extent of delamination damage within the CFRP panel after impact. Indeed, from plotting the indentation depth versus the interlaminar delamination normalised by the thickness of the panel area there is shown to be a unique ‘master’ relationship, with a positive intercept indicating that the indentation damage seems to result before delamination damage initiates. Thus, for both thicknesses of CFRP panels, it is suggested that the indentation process is a precursor to interlaminar delamination damage.

The current research uses the Digital Image Correlation (DIC) approach to study the fracture behaviour of Reaction-Bonded Silicon Carbide-Boron Carbide (SiC-B4C) ceramic (RBSBC). A Photron camera with 137,500 fps was used to capture the crack propagation at an interframe time frame of 2.6 ms before final fracture. The purpose was to capture the rapid brittle behaviour of the ceramic during the onset of crack propagation. The RBSBC failed in a transgranular manner like most monolithic SiC-based materials as Hexoloy-SA. The presence of randomly dispersed coarse B4C grains helps in deflecting the crack path which increases the overall crack length. The material's resistance to crack propagation (R-curve) was measured using the crack tip opening displacement, the stress intensity at the crack tip KI, and the J-integral against crack extension. Two methods were used to determine the KI: experimentally based on the displacement values at the crack tip as obtained from the DIC images, and quasistatic based on the fracture load and the geometrical dimension of the crack. The experimentally-determined KI and J resulted in higher values compared to the quasistatic ones. However, they reflect the actual conditions at the crack tip, while the quasistatic conditions underestimate the resistance-plot. The RBSBC has higher Rcurve compared to commercially available SiC-only materials as Hexoloy-SA and ABC-SiC. To improve the crack-resistance of RBSBC, the Si amount should decrease to a nm level, while the percentage of B4C grains should increase and be evenly dispersed across the material.

The focus of this study is on the nonlinear mechanical properties of epoxy and epoxy-based nanocomposites, exploring frequency and strain amplitude dependency. Nanocomposite samples of epoxy are reinforced with fumed silica (FS), halloysite nanotubes (HNT) and Albipox 1000 rubber (Evonik) nanoparticles. Considering these particles have different geometries and stiffnesses, they are expected to have significantly different influences on the mechanics of the resulting composite. To enhance the reliability of the results and to reveal the impact of nanofillers on the mechanics of the material more distinctly, the manufacturing process is designed to be the same for all the specimens within the same material groups to eliminate the effects of the manufacturing process. The comprehensive characterization process consists of Fourier-Transform InfraRed Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Dynamic Mechanical Analysis (DMA). The DMA tests are designed so that the material properties are measured depending on the vibration frequency and strain amplitude. Finally, the characterized nonlinear dynamic properties of these nanocomposites are used as the input material properties into a numerical model. In this simulation, a cantilever beam with representative nonlinear material properties, for these nanocomposites, is created, as example and its forced response is plotted under the same levels of excitation in the frequency domain. Key effects of the different nanofillers are identified using the resonance behavior, primarily focusing on the stiffness and damping of the epoxy-based nanocomposites. These experimental and numerical procedures followed show the significant impact of the nanoparticle reinforcements on the nonlinear nature of these epoxy-based composites.

This review paper comprehensively examines the dynamic landscape of 3D printing and Machine Learning utilizing biodegradable polymers and their composites, presenting a panoramic synthesis of research developments, technological achievements, and emerging applications. By investigating a multitude of biodegradable polymer types, the review paper delineates their suitability and compatibility with diverse 3D printing methodologies and demonstrates the merit of machine learning techniques, in future manufacturing processes. Moreover, this review paper focuses on the intricacies of material preparation, design adaptation as well as post-processing techniques tailored for biodegradable polymers, elucidating their pivotal role in achieving structural integrity and functional excellence. From biomedical implants and sustainable packaging solutions to artistic creations, the paper unveils the expansive spectrum of practical implementations, thus portraying the multifaceted impact of this technology. Whilst outlining prevalent challenges such as mechanical properties and recycling, this review paper concurrently surveys ongoing research endeavors aimed at addressing these limitations. In essence, this review encapsulates the transformative potential of 3D printing and Machine Learning with biodegradable polymers, providing a roadmap for future advancements and underscoring its pivotal role in fostering sustainable manufacturing/consumption for the future.

This study proposes a numerical method for calculating the stress fields in nano-scale multi-phase/composite materials, where the classical continuum theory is inadequate due to the small-scale effects, including intermolecular spaces. The method focuses on weakly nonlocal and inhomogeneous materials and involves post-processing the local stresses obtained using a conventional finite element approach, applying the classical continuum theory to calculate the nonlocal stresses. The capabilities of this method are demonstrated through some numerical examples, namely, a two-dimensional case with a circular inclusion and some commonly used scenarios to model nanocomposites. Representative volume elements of various nanocomposites, including epoxy-based materials reinforced with fumed silica, silica (Nanopox F700), and rubber (Albipox 1000) are subjected to uniaxial tensile deformation combined with periodic boundary conditions. The local and nonlocal stress fields are computed through numerical simulations and after post-processing are compared with each other. The results acquired through the nonlocal theory exhibit a softening effect, resulting in reduced stress concentration and less of a discontinuous behaviour. This research contributes to the literature by proposing an efficient and standardised numerical method for analysing the small-scale stress distribution in small-scale multi-phase materials, providing a method for more accurate design in the nano-scale regime. This proposed method is also easy to implement in standard finite element software that employs classical continuum theory.

This study investigates the vibration characteristics of rectangular cross-sectioned and straight beams with imperfect supports, focusing on the role of dry friction at the contact interfaces. The contact interactions are reduced to resultant point loads, and the friction at the contact interfaces is modelled using the Jenkins friction model introducing nonlinearity into the system. These nonlinear terms are included in solution-dependent boundary conditions for the governing differential equation of beam vibration. Two cases are considered in detail and solved: one where the beam is tightened between rigid clamps at both ends and excited from the middle with a harmonic displacement function, and another where only one end is clamped with the other end free but excited with an imposed harmonic displacement. The governing differential equation is solved analytically, separating the motion into two distinct regimes - full-stick and full-slip, using the Galerkin method. The results acquired from this analytical model are then compared to those from a numerical model, which is built and solved using the finite element method combined with a frequency sweep and time-marching.

The present study introduces a novel analytical solution to predict the nonlinear dynamic behaviour of bars under frictional clamping in axial and torsional motions. It investigates the vibration characteristics of straight bars with imperfect supports, which introduce dry friction at their contact interfaces. The bars are tightly clamped between rigid fixtures, with the tightening load acting as a normal load that induces friction, thus adding nonlinearity to the system. The model simplifies contact forces to point loads and utilises both the Jenkins and velocity-dependent friction models for simulating contact friction. These frictional forces are represented as solution-dependent external forces in the governing differential equation for bar vibration, which also includes appropriate boundary conditions. The equation is solved both analytically and through the numerical method of alternating frequency-time harmonic balance, to explore the influence of contact parameters on the bar's support system behaviour. Comparisons between the numerical and analytical results demonstrate strong agreement, confirming the model's accuracy and validity.

The present study investigates the effect of the impactor geometry on the impact performance, at relatively high impact energies, of carbon fibre-reinforced polymer (CFRP) laminates with a cross-ply configuration of [02/902], which were manufactured using unidirectional (UD) carbon-fibre epoxy-matrix plies. Drop-weight impact tests were performed using both round-nosed and flat-ended rigid impactors. White light interferometry (WLI), ultrasonic C-scan and scanning electron microscopy (SEM) were employed to assess the relationship between the indentation profile, the delamination footprint and the fracture morphology along the fracture plane. This study focusses on the coupling between the extent of indentation, the loading responses and the associated damage caused in the CFRP. This damage involved both intralaminar damage, including matrix cracking and fibre-kinking and fracture, and interlaminar, i.e. delamination, damage. A major finding was that the flat-ended impactor only caused significant damage to the CFRP panel at the relatively high impact energy of 25 J. At these high impact energies, a compressive kink-band fracture plane occurred around the periphery of the flat-ended impactor near the front surface of the laminate. A previously published numerical model has been extended to account for these experimental observations.

Strain-rate-dependent mechanical properties and impact performance of manufactured epoxy-based nanocomposites are investigated. As reinforcements, fumed silica (FS) and halloysite nanotube (HNT) are used alongside Albipox 1000 and Nanopox F700. First, the internal structures of the composites are visualised using scanning electron microscopy (SEM). To identify the strain-rate-dependent mechanical properties, three-point bend tests are conducted at three different strain rate levels. For the impact resistance, Charpy impact tests are performed. For further investigations of the mechanical properties of the composites, mean-field homogenisation (MFH) and finite element (FE) analyses on the representative volume elements (RVE) are performed for each type of composite material. Overall, the modelling and experiments are in good agreement and account for the mechanical behaviour of these epoxy-based nanocomposites.

The patch-plug configuration has been widely used to repair composite structures and restore the structural integrity of damaged composites. In the present research, single-sided CFRP patch-repaired panels, with different patch-plug configurations, are prepared. This is where a circular-shaped damaged area has been removed and a CFRP patch has been adhesively-bonded onto the panel. In some cases, a CFRP plug is inserted into the hole, caused by removal of the damaged area, before the patch is applied. Such patch-repaired panels, and the pristine CFRP panel, are subjected to a low-velocity impact at an energy of 7.5 J. These impacted pristine and repaired panels are then examined using ultrasonic C-scan and optical microscopy to inspect the impact-associated permanent indentation, interlaminar and intralaminar damage. A finite element analysis (FEA) model, which significantly extends a previously validated elastic-plastic (E-P) numerical damage model, has been developed to predict the impact behaviour of the pristine CFRP panel and the various designs of patch-repaired CFRP panels. The comparison between the experimental and numerical results for all the studied cases shows the maximum deviations for the loading response and the damage area are 12% and 15%, respectively. The good agreement between the experimentally-measured impact properties and those predicted using the numerical model demonstrates that the model is a useful design tool.

Low density aramid and carbon fibre-reinforced epoxy foam has been synthesised with the aim of improving mechanical properties, principally fracture performance. The foam properties measured were fracture energy, compressive strength, and density. The influence of fibre type, loading, and length was investigated. In addition, composite face-sheet bond tests were performed to ascertain how effective toughness transferred from individual component to composite structure. In general, the addition of fibres improved the mechanical performance of reinforced samples compared to the control foam. Increases in compressive strength were moderate whilst fracture energy was increased by up to 107% from 124 J/m2 to 256 J/m2 by the addition of 0.75 mm aramid fibres. Increased fracture energy of the foam and the presence of fibres on the foam surface, caused an increase in face-sheet bond propagation fracture toughness of 50% from 277 J/m2 to 416 J/m2.

Auxetic cellular structures have the potential to revolutionise sandwich panel cores due to their potential superior energy absorption capability. Because of their negative Poisson’s ratio, auxetics behave counterintuitively and contract orthogonally under an applied compressive force, resulting in a densification of material in the vicinity of the applied load. This study investigates three cellular structures and compares their compressive energy absorbing characteristics under in-plane and axial loading conditions. Three unit cell topologies are considered; a conventional hexagonal, re-entrant and double arrowhead auxetic structures. The samples were additively manufactured using two different materials, a conventional Nylon and a carbon fibre reinforced composite alternative (Onyx). Finite element simulations are experimentally validated under out of and in-plane loading conditions and the double arrowhead (auxetic) structure is shown to exhibit comparatively superior energy absorption. For the carbon fibre reinforced material, Onyx, the specific energy absorbed by the double arrowhead geometry was 125% and 244% greater than the hexagonal (non-auxetic) and re-entrant (auxetic) structures respectively

The use of Digital Image Correlation (DIC) to reveal microstructural damage in cross-ply laminates was investigated. Matrix toughness plays a key role in governing microcracking at the tow level in near-surface plies. Experiments revealed that using a tough epoxy polymer as the matrix of the laminate resulted in increased laminate moduli in the principal directions. DIC provides insights into cross-ply laminate failure; the increase in modulus is attributed to microcrack formation in transverse plies. Early onset of matrix cracking around the tows is revealed by variations in the strain along the gauge length. The use of a tough epoxy polymer delays the load at which this cracking occurs. When an untoughened epoxy polymer is used as the matrix, microcracking can be observed at the beginning of the test, suggesting processing induced damage. The use of toughened polymers as the matrix of composite laminates is recommended to mitigate against this.

Two different experimental techniques are employed to visualize the impact damage generated by a low-velocity impact on a carbon-fibre reinforced-polymer (CFRP) composite laminate. At the relatively low impact-velocity of 1.69 m.s −1 , and a corresponding impact energy of 7.5 J, used in the present work the damage induced in the CFRP panel is barely visible to the naked eye but the techniques of ultrasonic C-scan and X-ray computed tomography (CT) can detect the damage that has occurred. This damage is mostly interlaminar damage, i.e. delaminations, between the plies due to a change in modulus from one ply to the next in the laminate. This interlaminar damage is usually accompanied by intralaminar damage, e.g. matrix cracking, in the ply itself. The type and extent of damage detected from using these two techniques is discussed and the relative merits of these techniques are compared. In general, the CT gave the better resolved picture of damage but the lateral extent of the damage was underestimated relative to C-scan which was more sensitive to very fine delamination cracks. In addition, a numerical approach, based on a finite-element analysis model, is employed to predict the type, location and extent of damage generated by the impact event and the modelling predictions are compared to the experimental results.

Aircraft technologies and materials have been developing and improving drastically over the last hundred years. Over the last three decades, an interest in the use of composites for external structures has become prominent. For this to be possible, thorough research on the performance of composite materials, specifically the impact performance, have been carried out. For example, research of impact testing for pristine carbon-reinforced epoxy composites mentions matrix cracks, fibre fracture, and delamination as the failure modes that require monitoring. In addition, thorough testing has been carried out on composites repaired with an adhesive bond to observe the effects of conditioning on the adhesively bonded repair. The results suggest there are no major changes in the adhesive under the testing condition. By reviewing the impact testing on the pristine and repaired composite materials for aerostructures, this paper aims to illustrate the main findings and also explore the potential future work in this research scope.

This research investigates the performance of interlaminar hybrid composites as the skins of composite sandwich panels under blast loading with the aim of promoting delamination between dissimilar plies for energy absorption. The deformation of the composite panels was captured using high-speed digital image correlation (DIC). High-speed full-field DIC enables failure to be captured at the moment it occurs across the entire panel. X-ray micro-CT imaging was used to assess the post-blast damage sustained by particular areas of interest from each panel, which were selected based on DIC results. The combination of full-field DIC and detailed X-ray micro-CT scanning enabled a unique comparison of both the global and localised blast resilience of hybrid and conventional composite sandwich panels to be performed. Following a single blast load, the extent of damage to the Hybrid-3B skinned sandwich panel was found to lie between that of GFRP and CFRP skinned sandwich panels. X-ray micro-CT scanning of these panels reveals that there is no continuous damage path through the skin thickness of Hybrid-3B, whereas the GFRP and CFRP panels sustain damage in every ply. Following repeat blast loading, the Hybrid-4 skinned sandwich panel suffered from a front skin crack spanning the length of the panel. Post-blast compressive strength testing reveals that this skin crack and resulting core crack acted as a stress relief, limiting the damage sustained elsewhere in the panel. It was concluded that Hybrid-3B results in a good trade-off between strength and stiffness and is advantageous over conventional CFRP and GFRP panels under a single blast load. Under repeated loading Hybrid-4 offers advantages over Hybrid-3B. Finally, the design of the support structure can significantly aid in blast resilience, and, a holistic approach considering both panels and support should be taken when designing for blast resilience.

The present paper compares the damage and energy absorption behaviour of composites subjected to low-velocity impact using different frontal geometries for the impactor, with the composites possessing a layup of [02/902]2s. In this study, the rigid impactors with either round-nosed or flat-ended frontal geometry are employed to perform drop-weight tests at various impact energies ranging from 10 to 30 J. The measured loading response and energy absorption are analysed and compared. Additionally, the types and extent of impact-induced damage in the composite specimens are assessed via ultrasonic C-scan, optical microscopy (OM) and scanning electron microscopy (SEM) studies. It is shown that the impact energy threshold for damage initiation is greater than 20 J when using the flat-ended impactor but is less than 10 J when using the round-nosed impactor. In both cases, delamination initiates between the plies in the composite laminate. However, for the flat-ended impactor, the damage behaviour of the fibres exhibits kinking fracture, which differs from the pull-out fibre-fracture caused by the round-nosed impactor. These differences in behaviour are attributed to impactor/composite contact geometry effects which leads to different extents of indentation damage, which in turn directly affects the degree of delamination and fibre damage in the composite.