University roles and responsibilities

  • Level 4 (Year 1) Tutor
  • FEPS Mathematics Education Working Group

    My qualifications

    PhD in Engineering
    University of Cambridge
    MASc in Civil Engineering (research based)
    University of Toronto
    BASc in Civil Engineering
    University of Waterloo

    Previous roles

    2018 - 2020
    Mary Ewart Junior Research Fellow
    Department of Engineering Science & Somerville College, University of Oxford


    Research interests


    Postgraduate research supervision



    Martin G Walker, Keith A Seffen (2017)Localization in Thin Metallic Annuli due to Diametrical Extension, In: AIAA journal55(11)pp. 3980-3989 American Institute of Aeronautics and Astronautics

    The art form of kirigami has recently attracted interest from engineers and scientists for generating complex three-dimensional structures from flat sheet-like materials. When thin metal sheets are used, the deformation can become plastic and localized, allowing for permanent intricate shapes to be formed. In this study, the illustrative case of an annular plate under diametral tension is considered, and it is shown that the deformed shape can be considered as a spatial mechanism of localized plastic yield lines connected to largely undeformed regions. This technique provides useful information for the design of novel permanently deployable structures.

    M G Walker (2020)Mechanics of generically creased disks, In: Physical review. E101(4-1)pp. 043001-043001

    Folded structures are often idealized as a series of rigid faces connected by creases acting as revolute hinges. However, real folded structures can deform between creases. An example of particular interest is a disk decorated by multiple radial creases. Such disks are bistable, snapping between a "natural" and "inverted" shape. We investigate the mechanical behavior of these creased disks and propose a new analytical approach to describe their mechanics. Detailed experiments are performed which show that, when indented at the center, a localized dimple forms, precluding the conical shape assumed in previous studies. As the indentation depth increases this dimple expands radially until reaching the disk edge when it snaps to the inverted shape, which has a conical form. We develop an analytical model which approximates each face as a series of rigid facets connected by hinges that can both rotate and stretch. Energy expressions are derived relating hinge rotation and stretching to compatible shell deformations of the facets and equilibrium enforced by minimizing the total strain energy. By increasing the number of facets, the mechanics of the continuum shell is approached asymptotically. The analysis shows that membrane stretching of the faces is required when a conical form of deformation is enforced. However, in the limit of zero thickness, the forming and propagation of a localized dimple is inextensional. This new approach relates the kinematic analysis of rigid origami to the mechanics of thin shells, offering an efficient method to predict the behavior of folded structures.

    Michael Seica, Jeffrey Packer, MARTIN G. WALKER, Matthew Gow (2022)Mitigation of Blast Effects through Novel Energy-Dissipating Connectors, In: International journal of protective structures. Sage

    Explosions generate overpressures that can cause irreparable damage to structures. For many buildings, especially critical infrastructure, continued operation after an explosive attack is essential. The use of energy-dissipating methods will enable the protection of a structure and occupants from a blast and permit the timely repair and re-occupation of the building after an event. The concept behind the system presented is the creation of panels that can be used as cladding for structures. The panels are connected to the main structure using energy-dissipating component assemblies around the panel edge. When subjected to a blast load the panels transfer the blast pressure through the assemblies, thereby reducing the forces transmitted to the underlying structure. After an event, the panels and energy-dissipating component assemblies can be replaced quickly and easily, allowing the building to be reoccupied in a short time after an attack. This study focuses on the characterization of energy-dissipating component assemblies using static and dynamic laboratory testing. A predictive theory, supported by a single degree of freedom model, is developed and a general evaluation method proposed. Further laboratory testing expands the characterization of behaviour of the assemblies through experiments, with a blast generator in tension tests and in simulated blast panel tests. The time histories developed from tension tests are then compared to examine the effect of loading rate. The investigations on blast panels also include a comparison with predictions to determine whether the latter can describe the global behaviour of the system. Lastly, the response of the energy-dissipating component assemblies is evaluated in full-scale field blast tests on cladding panels.

    M.G Walker, K.A Seffen (2019)The flexural mechanics of creased thin strips, In: International journal of solids and structures167pp. 192-201 Elsevier Ltd

    Many structures in Nature and Engineering are dominated by the influence of folds. A very narrow fold is a crease, which may be treated with infinitesimal width for a relatively simple geometry; commensurately, it operates as a singular hinge line with torsional elastic properties. However, real creases have a finite width and thus continuous structural properties. We therefore consider the influence of the crease geometry on the large-displacement flexural behaviour of a thin creased strip. First, we model the crease as a shallow cylindrical segment connected to initially flat side panels. We develop a theoretical model of their coupled flexural behaviour and, by adjusting the relative panel size, we capture responses from a nearly singular crease up to a full tape-spring. Precise experiments show good agreement compared to predictions.

    Martin G Walker, Keith A Seffen (2018)On the shape of bistable creased strips, In: Thin-walled structures124pp. 538-545 Elsevier Ltd

    We investigate the bistable behaviour of folded thin strips bent along their central crease. Making use of a simple Gauss mapping, we describe the kinematics of a hinge and facet model, which forms a discrete version of the bistable creased strip. The Gauss mapping technique is then generalised for an arbitrary number of hinge lines, which become the generators of a developable surface as the number becomes large. Predictions made for both the discrete model and the creased strip match experimental results well. This study will contribute to the understanding of shell damage mechanisms; bistable creased strips may also be used in novel multistable systems. •Gauss mapping approach is used to describe the kinematics of a hinge and facet model of a bistable creased strip.•The approach is generalised for an arbitrary number of hinge lines approaching the generators of a developable surface.•Bistability is maintained when a hole is introduced removing the stress singularity.•Predictions are compared to experiment showing good agreement.

    Michael V Seica, Michal Krynski, Martin Walker, Jeffrey A Packer (2011)Analysis of Dynamic Response of Architectural Glazing Subject to Blast Loading, In: Journal of architectural engineering17(2)pp. 59-74 American Society of Civil Engineers

    The effect of blast loading on civilian structures has received much attention over the past several years. The behavior of architectural glazing is of particular interest owing to the disproportionate amount of damage often associated with the failure of this component in a blast situation. This paper presents the development of a simple yet accurate finite element-based tool for the analysis of architectural glazing subjected to blast loading. This has been achieved through the creation of a user-friendly computer program employing the explicit finite-element method to solve for the displacements and stresses in a pane of glass. Both monolithic and laminated panes have been considered, in single and insulated unit configurations, and employing several types of glass. In all cases, the pane of glass has been modeled as a plate supported by an array of boundary conditions that include spring supports, and two failure criteria are employed. Furthermore, the program is designed to predict the hazard level, given a particular glazing configuration and blast load.

    Matteo Taffetani, M. G. Walker (2022)Axisymmetric ridges and circumferential buckling of indented shells of revolution, In: Physical Review E: Statistical, Nonlinear and Soft Matter Physics

    When poking a thin shell-like structure, like a plastic water bottle, experience shows that an initial axisymmetric dimple forms around the indentation point. The ridge of this dimple, with increasing indentation, eventually buckles into a polygonal shape. The polygon order generally continues to increase with further indentation. In the case of spherical shells, both the underlying axisymmetric deformation and the buckling evolution have been studied in detail. However, little is known about the behaviour of general geometries. In this work we describe the geometrical and mechanical features of the axisymmet-ric ridge that forms in indented general shells of revolution with non-negative Gaussian curvature and the conditions for circumferential buckling of this ridge. We show that, under the assumption of 'mirror buckling' a single unified description of this ridge can be written if the problem is non-dimensionalised using the local slope of the undeformed shell mid-profile at the ridge radial location. However, in dimensional form the ridge properties evolve in quite different ways for different mid-profiles. Focusing on the indentation of shallow shells of revolution with constant Gaussian curvature, we use our theoretical framework to study the properties of the ridge at the circumferential buckling threshold and evaluate the validity of the mirror buckling assumption against a linear stability analysis on the shallow shell equations, showing very good agreement. Our results highlight that circumferential buckling in indented thin shells is controlled by a complex interplay between the geometry and the stress state in the ridge. The results of our study will provide greater insight into the mechanics of thin shells. This could enable indentation to be used as a means to measure the mechanical properties of a wide range of shell geometries or used to design shells with specific mechanical behaviours.

    Souhayl Sadik, Martin G. Walker, Marcelo A. Dias (2022)On local kirigami mechanics II: Stretchable creased solutions, In: Journal of the Mechanics and Physics of Solids161104812 Elsevier

    Following on Part I of this work series on local kirigami mechanics, we present a study of a discretely creased mechanism as a model to investigate the mechanics of the basic geometric building block of kirigami - the e-cone. We consider an annular disk with a single radial slit discritised by a series of radial creases connecting kinematically flat rigid panels. The creases allow both relative rotation and separation between panels, capturing both bending and stretching deformations. Admissible equilibrium con gurations are obtained by penalising these deformations using elastic springs with stiffnesses derived from compatible continuum plate deformations. This provides a tool to study both inextensible and extensible e-cone con gurations due to opening of the slit and rotation of its lips. This creased model hence offers the possibility to study the e-cone away from its isometric limit, i.e., for plates with finite thickness, and explore the full range of post-buckling (far-from-threshold) behaviour as well as initial buckling (near-threshold) instability. Our local approach provides a fundamental understanding of kirigami phenomenology, underpinned by a proper theoretical approach to geometry and mechanics.