Nonconventional tow-steered pressure vessels for hydrogen storage

Hydrogen has high gravimetric energy density with accompanying low carbon footprint with potential to replace fossil fuels. Hydrogen at ambient temperature is in the gaseous state and should be stored in pressure vessels that can withstand pressures as large as 70 MPa, making their design challenging. Developments in manufacturing techniques for composite structures enable varying the fibre tow trajectory throughout the structure to improve the structural performance. These structures are known as variable stiffness composite structures. This technique has recently been used to develop a design method for suppressing inefficient bending in non-spherical pressure vessels. This study employs the bend-free design method for gaseous hydrogen pressure vessels and investigates the potential advantages of this nonconventional design compared to conventional designs by assessing their Hydrogen Weight Efficiency (HWE). Results show that the HWE of the bend-free variable stiffness pressure vessel is 18.6% larger than the HWE of the best conventional design studied</p

Asymptotic homogenization for modeling of wingbox structures

The asymptotic homogenization technique has been used to analyze a wing box structure consisting of trapezoidally arranged reinforcements encased within thin rectangular plates. Ignoring stress concentration effects at the region of the overlap between the various components, the wingbox structure can be analyzed by handling each constituent independently from each other. To this end, a simpler structure was first considered which was made up of a base plate and a single stiffener web; the results were then extrapolated to those of the wingbox structure via superposition by adding in the contributions of each constituent of the overall unit cell. The work culminated in closed-form expressions for the effective in-plane elastic coefficients of the wingbox. This result demonstrates the attractiveness of the methodology in that it can be used in engineering analysis and design to customize the architecture of a thin-walled reinforced composite by changing some material or geometrical parameters of interest. Such parameters could be the material of the base plate, the spatial arrangement of the reinforcements, the relative sizes of the different constituents

Morphing composite cylindrical lattices with enhanced bending stiffness

A key aspect in the design of deployable space structures comprising slender elements such as booms is their deployed bending stiffness. In space, due to zero gravitational loading, a high level of bending stiff?ness is not required to support neighbouring structures, but instead is desirable to resist vibrations generated by the attitude control system. The morphing composite cylindrical lattice that is under development is a structure with significant potential for deployable applications in space, however, concepts developed so far may lack sufficient bending stiffness. Therefore, current work focuses on developing a method of increasing the lattice bending stiffness, while minimising any increase in both mass and stowed volume. These goals are achieved by using additional composite strips mounted adjacent and concentric to pre-existing strips. These strips are attached using pre-existing fasteners, thus, only increasing the weight of the structure by the mass of the composite strips. A finite element model of the new lattice configuration is developed and validated by comparison to experimental results. For this comparison, three different lattice configurations were manufactured, two lattices with a conventional strip configuration, an eight-strip lattice and a four-strip lattice, and a third using a new lattice configuration developed in this work. In comparison with the eight-strip lattice, the new lattice configuration is 32% less stiff, however it weights 33% less and stows to pproximately half the stowed height. Compared to the four-strip lattice, the new configuration weighs 75% more, but it is 281% stiffer while stowing to the same volume. By increasing the deployed bending stiffness, this work makes the morphing cylindrical lattice a more viable candidate for deployable space structures.</p

Dynamic analysis of prestressed variable stiffness composite shell structures

The design space for high-performance lightweight composite structures has grown considerably since the advent of the variable stiffness concept. In fact, variable stiffness composites have been found to improve buckling performance and dynamic stability, and to tune the structure’s dynamic response by tailoring structural stiffness. Thus, in order to exploit this wider design space, efficient linear analysis tools have an important role in preliminary design of variable stiffness structures, enabling designers to find more effective solutions when considering prestressed dynamically excited aerospace components. Considering this, a multi-domain Ritz method for eigenfrequency, transient and dynamic instability analysis of prestressed variable stiffness laminated doubly-curved shell structures is presented. Working within the first-order shear deformation theory, Sanders–Koiter shell kinematics allow general orthogonal surfaces to be modelled without further assumptions on the shallowness or on the thinness of the structure. The efficiency of the proposed Ritz method is granted by using Legendre orthogonal polynomials as displacement trial functions, while flexibility in modelling and design is given by penalty techniques that allow stiffened variable angle tow shell structures to be modelled as an assembly of shell-like domains. The proposed approach is verified by comparison with published benchmark results and finite element solutions. Original solutions are presented for a prestressed stiffened variable angle tow shell structure, which show great accuracy with an order of magnitude fewer variables when compared to standard finite element procedures, proving the reliability and efficiency of the present method in dealing with the dynamic analysis of multi-part aerospace structures.</p

Recycling CF/PEEK offcut waste from laser assisted tape placement: Influence of overlaps and gaps

A recycling method for waste CF/PEEK prepreg tapes is proposed that uses offcuts from laser assisted tape placement (LATP) processing, without shredding, grinding or further cutting. Laminates (using the prepreg waste) representing different configurations were manufactured considering overlaps and gaps reflecting observed defects in LATP. Interlaminar shear strengths (ILSS) with 0◦ fibre (Parallel) and bending strength for 90◦ fibre (Normal) directions were measured. The parallel samples with 3 mm overlap had the highest ILSS (100.3 MPa) while the 2 mm gapped samples had the lowest ILSS (58.7 MPa). The Normal samples failed at the surface due to matrix-dominated failure providing bending strengths between 44.0 MPa and 82.1 MPa. Failure mechanisms were identified similar to that of non-recycled composites reported in the literature (50–120 MPa), indicating that the recycled prepreg tapes retained approximately 84 % of the ILSS of the highest reported values.</p

Size-dependent bending modulus of fibre composite laminates comprising unidirectional plies

Heterogeneous materials can show size-dependent behaviour in which the bending modulus depends on sample size. In fibre composite materials the interaction between fibre and matrix can lead to such a size effect. Then the effective modulus calculated by the rule of mixtures can either underestimate or overestimate the bending modulus of the laminate, depending on the fibre/matrix material mismatch, the microstructural morphology (fibre distribution) and the laminate thickness. In this work, the bending behaviour of a laminate comprising unidirectional fibre composite plies is considered using Euler Bernoulli beam theory and the influence of size on the bending modulus is investigated. The effective bending modulus of each ply is calculated and used to formulate the overall bending modulus of the laminate. The results show that the laminate bending modulus depends on the number of plies, the number of fibres through the thickness of each ply, the fibre spacing and radius, and the mismatch of fibre and matrix material properties in each ply. Our analysis shows that accounting for the ply microstructure (fibre spacing and radius and number of fibres per ply) can lead to a 10% difference in the predicted bending modulus in a three ply laminate, when there are less than four fibres through the thickness in each pl

The effect of laser assisted tape placement processing conditions on microstructural evolution, residual stress and interlaminar shear strength of carbon fibre/PEEK laminates

In the present study, both experiments and thermo-mechanical coupled simulations were conducted to characterise the diverse crystallisation behaviours and the processing parameter-microstructure-mechanical property relationships occurring in laser-assisted tape placement (LATP) manufacturing of carbon fibre (CF)/Polyetheretherketone (PEEK) laminates. Specifically, at various processing temperatures (350 ◦C or 400 o C), increasing the compaction pressure from 2 to 4 bar causes distinct defect distribution behaviours. However, variations in processing parameters show minimal effect on the morphology and size of crystallised spherulites, which were consistently around 2–3 μm in size, resulting in a final crystallinity of manufactured laminates within 30%–35%. It was found that the cold crystallisation processes occurring in PEEK during LATP play an important role in determining the final degree of crystallinity. Experimental measurements and simulations indicate that changes in processing parameters have a negligible effect on residual stress levels, especially regarding interlaminar residual stresses. A processing temperature of 400 ◦C was found to generate a diffuse, yet coherent, interphase spanning the fibre/matrix interface with a thickness approximately 70 nm. In contrast, at a processing temperature of 350 ◦C, a distinct, incoherent interface was confirmed between fibre and matrix. The formation of the interphase, coupled with fewer defects, leading to a relatively high interlaminar shear strength (78 MPa) of manufactured laminates under appropriate processing conditions. Therefore, it is suggested that regulating the degree of cold crystallisation in polymer matrices while ensuring a strong fibre/matrix interfacial bond by the optimisation of processing temperature, will enable the tailoring of microstructure and design of composites to meet specific strength property requirements.</p

Inverse differential quadrature solutions for free vibration of arbitrary shaped laminated plate structures

An essential aspect of design of laminated plate structures in many engineering applications is the analysis of free vibration behaviour in order to model the structure for random excitations. In this regard, numerical solutions to the systems of high-order partial differential equations governing free vibration response of the structure become important. Direct approximation of such high-order systems are prone to error arising from the sensitivity of high-order numerical differentiation to noise necessitating the demand for improved solution techniques. In this work, a novel generalised inverse differential quadrature method is developed to study the dynamic behaviour of first-order shear deformable arbitrary-shaped laminated plates. The ensuing underdetermined system is operated upon by Moore–Penrose pseudo-inverse preconditioning to form a squared eigenvalue system. Free vibration solutions of square, skew, circular, and annular sector plates for different boundary conditions are obtained and validated against exact and numerical solutions in the literature and ABAQUS. It is demonstrated with numerous examples that iDQM solutions are in excellent agreement with exact solutions for square plates and the results for arbitrary shaped plates are comparable with solutions in the literature while saving up to 96% degrees of freedom required for ABAQUS solution. Finally, refined parametric studies conducted reveal that, subject to varying geometric configurations, iDQM solutions are numerically stable and potentially converge faster than DQM </p

Efficient three-dimensional geometrically nonlinear analysis of variable stiffness composite beams using strong unified formulation

The use of composite laminates for advanced structural applications has increased recently, due in part to their ability for tailoring material properties to meet specific requirements. In this regard, variable stiffness (VS) designs have potential for improved performance over constant stiffness designs, made possible by fibre placement technologies which permit steering of the fibre path to achieve variable in-plane orientation. However, due to the expanded, large design space, computationally expensive routines are required to fully explore the potential of VS designs. This computational requirement is further complicated when VS composites are deployed for applications involving nonlinear large deflections which often necessitate complex 3D stress predictions to accurately account for localised stresses. In this work, we develop a geometrically nonlinear strong Unified Formulation (SUF) for the 3D stress analysis of VS composite structures undergoing large deflections. A single domain differential quadrature method-based 1D element coupled with a serendipity Lagrange-based 2D finite element are used to capture the kinematics of the 3D structure in the axial and cross sectional dimensions, respectively. Predictions from SUF compare favourably against those in the literature as well as with those from ABAQUS 3D finite element models, yet also show significant enhanced computational efficiency. Results from the nonlinear large deflection analysis demonstrate the potential of variable stiffness properties to achieve enhanced structural response of composite laminates due to the variation of coupling effects in different loading regimes

An adaptive metastructure concept using bistable composite laminates

Concepts involving adaptive and morphing structures offer us the possibility to realise shape transformation with each shape imparting an individual functionality. There is an increased demand in the design of structural components that are suitable for diverse operational conditions rather than limited to a unique one. However, most concepts of shape adaptive structures require a “holding force” or a continuous supply of energy to maintain a targeted 3D shape. As a result, structures that can lead to desired shape transformation with self-locking capabilities are more desirable. In this work, the concept of multistability is employed to demonstrate a novel class of metastructures that can tackle this challenge. The metastructure is constructed with a periodic arrangement of bistable unit cells made of highly anisotropic composite laminates. Each unit cell comprises multi-sectioned rectangular composite plates exhibiting bistable behaviour due to the thermal residual stresses engendered during the cool-down process from curing to room temperature. To analyse the proposed metastructure, a finite element model has been developed at three hierarchical levels: a plate level, a unit cell level, and a lattice level. At the plate level, a corresponding semi-analytical model using the Rayleigh-Ritz method has also been formulated to validate the finite element models. By carefully tuning the size and spacing of the unit cell, a desired response of the metastructure can be achieved. Additionally, by changing the ply layup and the fibre orientation of each layer of constituent composite plates, conflicting requirements, including load-carrying, shape-adaptive and lightweight, at the same time can be addressed simultaneously. From an extensive parametric study, few designs have been selected for its application in a load-carrying morphing structure.</p