Rainer Groh

Adaptive compliant structures for flow regulation

This paper introduces conceptual design principles for a novel class of adaptive structures that provide both flow regulation and control. While of general applicability, these design principles, which revolve around the idea of using the instabilities and elastically nonlinear behaviour of post-buckled panels, are exemplified through a case study: the design of a shape-adaptive air inlet. The inlet comprises a deformable post-buckled member that changes shape depending on the pressure field applied by the surrounding fluid, thereby regulating the inlet aperture. By tailoring the stress field in the post-buckled state and the geometry of the initial, stress-free configuration, the deformable section can snap through to close or open the inlet completely. Owing to its inherent ability to change shape in response to external stimuli—i.e. the aerodynamic loads imposed by different operating conditions—the inlet does not have to rely on linkages and mechanisms for actuation, unlike conventional flow-controlling devices.

Mass Optimisation of Variable Angle Tow, Variable Thickness Panels with Static Failure and Buckling Constraints

By taking advantage of curved fiber paths, Variable Angle Tow (VAT) laminates increase the design space for tailoring the structural behavior of thin-walled aerospace structures. In recent years, advancements in Automated Fiber Placement (AFP) and Continuous Tow Shearing (CTS) have facilitated the manufacture of these laminates. The CTS technique holds the advantage of reducing many of the manufacturing defects characteristic of the AFP process such as fiber wrinkling, tow gaps and tow overlaps, while also allowing for tighter steering radii. On the other hand, the CTS process features added complexity due to the coupling of fiber steering angle with tow thickness. In this study, a minimummass design of a typical aircraft wing panel under end-compression subject to pre-defined manufacturing, static failure and buckling load constraints is sought. The geometric effects of the asymmetric thickness distribution of the CTS panel on the critical buckling loads, postbuckling paths and static failure behavior are captured for the first time. A hybrid optimization scheme that couples a genetic algorithm with a pattern-search algorithm is used to define a VAT laminate that reduces the mass of both square and rectangular aircraft panels by 31% compared to a baseline straight fiber design. The optimization of the fiber paths is driven by two distinct requirements, namely local and global stiffness tailoring that influence the buckling performance and static strength, respectively. Finally, the initial postbuckling behavior of the optimized designs is investigated using Koiter’s perturbation approach, which reveals that postbuckling stability should be considered when optimising VAT panels manufactured by the CTS technique.

Deleterious localized stress fields: the effects of boundaries and stiffness tailoring in anisotropic laminated plates

The safe design of primary load-bearing structures requires accurate prediction of stresses, especially in the vicinity of geometric discontinuities where deleterious three-dimensional stress fields can be induced. Even for thin-walled structures significant through-thickness stresses arise at edges and boundaries, and this is especially precarious for laminates of advanced fibre-reinforced composites because through-thickness stresses are the predominant drivers in delamination failure. Here, we use a higher-order equivalent single-layer model derived from the Hellinger–Reissner mixed variational principle to examine boundary layer effects in laminated plates comprising constant-stiffness and variable-stiffness laminae and deforming statically in cylindrical bending. The results show that zigzag deformations, which arise due to layerwise differences in the transverse shear moduli, drive boundary layers towards clamped edges and are therefore critically important in quantifying localized stress gradients. The relative significance of the boundary layer scales with the degree of layerwise anisotropy and the thickness to characteristic length ratio. Finally, we demonstrate that the phenomenon of alternating positive and negative transverse shearing deformation through the thickness of composite laminates, previously only observed at clamped boundaries, can also occur at other locations as a result of smoothly varying the material properties over the in-plane dimensions of the laminate.

HCI meets Material Science: A Literature Review of Morphing Materials for the Design of Shape-Changing Interfaces

With the proliferation of flexible displays and the advances in smart materials, it is now possible to create interactive devices that are not only flexible but can reconfigure into any shape on demand. Several Human Computer Interaction (HCI) and robotics researchers have started designing, prototyping and evaluating shape-changing devices, realising, however, that this vision still requires many engineering challenges to be addressed. On the material science front, we need breakthroughs in stable and accessible materials to create novel, proof-of-concept devices. On the interactive devices side, we require a deeper appreciation for the material properties and an understanding of how exploiting material properties can provide affordances that unleash the human interactive potential. While these challenges are interesting for the respective research fields, we believe that the true power of shape-changing devices can be magnified by bringing together these communities. In this paper we therefore present a review of advances made in shape-changing materials and discuss their applications within an HCI context.

Morphing structures for flow regulation

This paper presents a progressive damage model of composite structures based on refined one-dimensional models. The damage model is implemented in conjunction with the Carrera Unified Formulation (CUF). In the CUF framework, the governing equations and finite element matrices are given via a few fundamental expressions, namely the fundamental nuclei, which are independent of the order of the structural model. The 1D models are built using expansions of the displacement field above the cross-section. Within the CUF framework, the Component-Wise refined beams are used to model every component of an engineering structure via Lagrange Expansion (LE) elements independently of their geometry, e.g. 2D transverse stiffeners and panels, and of their scale, e.g. fibre/matrix cells. The CUF allows the use of any order 1D structural models in a unified manner. A three-dimensional orthotropic elastic constitutive model with continuum damage based degradation is implemented. A stress-based failure envelope is predicted using Hashins criteria for uni-directional composites. The progression of damage is controlled by a linear damage evolution law and is based on fracture energy dissipation. A Newton-Raphson based iterative scheme is used to solve the non-linear problem. Damage propagation in laminated composite structures is obtained using highly accurate 3D displacement, strain, and stress fields given by 1D CUFGenerally, the development of aircraft structures consists of a conceptual, preliminary and detailed design phase. All of these require a multidisciplinary approach involving aerodynamics, structural and flight mechanics. One of the main challenges at the preliminary design stage is to tailor structural dimensions and material properties to minimise weight whilst simultaneously satisfying constraints such as allowable stresses, strains and buckling and meeting aerodynamic, manufacturing, certification and fuel requirements. The objective of this study is to develop a multilevel optimisation strategy for the aeroelastic tailoring of aircraft composite wings, including geometric and material uncertainties. In the proposed approach, the first level aims at minimising the weight of a baseline wing design for given static loads, subject to stress, buckling and aeroelastic (i.e. flutter and divergence) constraints. In second level, the design space is expanded to include parametric uncertainties and the structure is further optimised this time including gusts, flutter and divergence analyses. Polynomial Chaos Expansion is used here as the non-sampling-based method for uncertainty quantification and sensitivity analyses. A regional aircraft wing model is used as the baseline for comparison. The optimisation procedures are performed using MSc Nastran. The advantages of using a multilevel optimisation approach include: (i) the possibility to reduce the numbers of design variables at the various optimisation levels and (ii) the capability of performing optimisation on different discipline of studies simultaneously. Results show that the proposed method can provide an optimised wing structure whose robustness to uncertainties is both definite and measurable. ∗PhD Student (corresponding author). Email address: muhammad.othman@bristol.ac.uk (Muhammad F. Othman)Glass fibre reinforced polymer (GFRP) are being increasingly used in buildings construction as they offer several well-known advantages such as high strength to weight ratio, chemical resistance or ease to installation. However, their low stiffness and the induced instability phenomena limit their use. One of the solutions consists in associating GFRP pultruded beams with concrete slab in order to increase their stiffness and to decrease the risk of instability. Numerous studies showed the effectiveness of this innovative hybrid beams through bending tests under different loadings. In this study, we investigate the behavior of composite – concrete structures connected by means of studs or bonding. The material characteristics were first determined by means of tension or compression tests. The interface properties were also using push-out tests and provided for instance the average ultimate shear strength. Those results were used to design two composite-concrete composite beams, one connected by means of shear studs, the other one by epoxy bonding. Both those beams and the same GFRP profile without the slab were statically loaded up to failure. Their behaviors and measurements were analyzed and compared. They show the interest of the slab on the failure load and on the failure mechanism. Furthermore, bonding connection allowed a greater stiffness than shear studs connection and a stress distribution at the GFRP profile – concrete interface limiting the risk of cracking. Existing models from literature are finally used to simulate the behavior of the three beams. Numerical results are compared to measurements showing their enough accuracy to predict the displacements and the state of stress in that new type of hybrid beams.Reinforced structures are mandatory in the space structures on which the lightweight is the main project parameter. The coupling between simple thin-walled plate and different systems of ribs or beams along one or more directions make it possible to meet the requirements of lightness and strength. During the project phase a structure is usually analysed via Finite Element Method (FEM), where different approaches can be used but the pointed out one common essential characteristic, a mesh discretization of a continuous domain into a set of discrete subdomains, usually called elements. Three main finite elements (FEs) are widely used in the commercial code, but only the Solid (3D) FE represents more faithfully the behaviour of a real structure. The solid FE models require a large number of degrees of freedoms (DOFs) and therefore the analyses are computational expensive [1]. For these reason that usually the reduced models are used as substitute of solid models. The reduced models are made using shell (2D) and beam (1D) FEs, and they are suitable to build a reinforced structure, in fact the shell are used for the skin and the beam for the stringers. The present work uses a refined 1D model based on the Carrera Unified Formulation (CUF) [2] to analyse space structures made coupling skin and stringers. Thanks to its refined cinematic the present model can be used to represent both skin and stringers. The whole structure is obtained connecting simple one-dimensional structures using a new approach called Component-Wise (CW) [3]. This is possible because the unknowns are only displacements. Free-vibration analysis of isotropic and composite space structures with non-structural masses and loading factor are considered. A space vehicle is inspired to Arian 5 with a central body, on which the cryogenic fuel and the payload are accommodated, and two lateral boosters, on which solid fuel is stored. The results show the quasi-3D capabilities of the present 1D CUF model and the coupling with the CW approach provide accurate results nearest to solid FE results than the classical refined FEs models. In conclusion the present 1D refined model appears suitable for the analysis of reinforced thin-walled structures, it provides accurate results with the benefit to reduce the computational costs with respect to the classical refined FE approaches. References [1] E. Carrera, E. Zappino and T. Cavallo. Accurate free vibration analysis of launcher structures using refined 1D models. International Journal of Aeronautical and Space Sciences,vol. 16(2) 206-222, 2015. [2] E. Carrera, G. Giunta and M. Petrolo. Beam Structures: Classical and Advanced Theories. Jhon Wiley & Sons Ltd, 2011. [3] E. Carrera, A. Pagani and M. Petrolo. Component-wise Method Applied to Vibration of Wing Structures. J Appl Mech, vol. 80(4), 041012-1-041012-15, 2013The design of structures, made of composite materials, requires the use of advanced structural models such as 3D finite elements. This approach provides accurate results regarding displacements, strains and stresses but, on the other hand, requires a huge computational cost. The research of new structural models able to provide reliable analyses reducing the computational cost is an challenging task. The present work proposes the use of an advanced one-dimensional FE model based on the Carrera Unified Formulation (CUF) that allows increasing the accuracy of the results maintaining a lower number of DOFs. These beam models concern the use of several expansions to describe the displacement field over the cross-section. In this particular case, the Lagrange Expansions are used [1]. This feature allows to maintain the geometric characteristics of the studied structure and, therefore, provide 3D-like results. Through the formulation presented, different structures can be arbitrary oriented in a global reference system and joined via the assembly procedure. In this way, this approach can deal with the analysis of complex structures made of several components. This work focuses on the analyses of tapered beams with different cross-sections. Composite materials have been mostly used in this work but, during the assessment, isotropic metallic materials are also used. Both static and dynamic analyses have been performed to evaluate the modal frequencies and the stress fields under simple load cases. The results have been compared with solid FEM solutions from commercial FE codes. Where possible, the results have also been compared with literature results. The research demonstrates that the advanced FE model used can provide accurate analyses and 3-D-like results in spite of the use of 1-D finite elements. Tapered structures can be easily studied considering an high-fidelity geometrical description