Norman A Fleck | University of Cambridge (original) (raw)

Papers by Norman A Fleck

Polymer, 2021

The mechanical properties of an ultrathin film made from a thermoplastic differ from the bulk due... more The mechanical properties of an ultrathin film made from a thermoplastic differ from the bulk due to the presence of the free surface. Here, molecular dynamics simulations are used to explore the thickness dependence of uniaxial and equi-biaxial tensile responses of polymethyl methacrylate (PMMA) films. The sensitivity of deformation response to temperature, molecular weight and the degree of side-branching is determined. We find that the tensile failure strain decreases with decreasing film thickness, temperature, and with decreasing molecular weight. The degree of side-branching plays a secondary role in dictating the tensile response. Failure is by the initiation of voids at the free surface, followed by the expansion of the voids in the thickness direction. Recent solid−state nanofoaming experiments and models suggest that the attainable porosity of nanofoams is less than that of macro−scale foams due to the reduced ductility of the cell walls of the nanofoam. Our results provide a physical explanation for this observation.

Journal of Physics: Energy, 2020

Li-ion batteries have revolutionized the portable electronics industry and empowered the electric... more Li-ion batteries have revolutionized the portable electronics industry and empowered the electric vehicle (EV) revolution. Unfortunately, traditional Li-ion chemistry is approaching its physicochemical limit. The demand for higher density (longer range), high power (fast charging), and safer EVs has recently created a resurgence of interest in solid state batteries (SSB). Historically, research has focused on improving the ionic conductivity of solid electrolytes, yet ceramic solids now deliver sufficient ionic conductivity. The barriers lie within the interfaces between the electrolyte and the two electrodes, in the mechanical properties throughout the device, and in processing scalability. In 2017 the Faraday Institution, the UK’s independent institute for electrochemical energy storage research, launched the SOLBAT (solid-state lithium metal anode battery) project, aimed at understanding the fundamental science underpinning the problems of SSBs, and recognising that the paucity o...

International Journal of Solids and Structures, 2019

The mechanism of load transfer within the bolted joint of a laminate sheet made from ultra-high m... more The mechanism of load transfer within the bolted joint of a laminate sheet made from ultra-high molecular weight polyethylene (UHMWPE) plies is investigated both experimentally and by an analytical model. The nature of load transfer and the active failure mechanisms are obtained as a function of joint geometry and of the lateral clamping force on the faces of the laminate (by pre-tensioning of the bolt). A combination of X-ray tomography and optical microscopy reveal that the dominant failure mechanism in the clamped joint is shear failure involving splits of the 0 o plies and sliding at the interface between the 0° and 90° plies. A simple analytical model is developed for this shear failure mechanism and, upon noting the competing failure mechanisms of bearing failure, bolt shear and of tensile failure of the 0° plies, a failure mechanism map is constructed in terms of the geometry of the bolted joint, for the case of no pre-tension of the bolt. The analytical model for shear failure suggests that the enhancement in joint strength with increased pre-tensioning of bolt is due to the fact that the shear strength of the UHMWPE increases with increasing hydrostatic pressure.

Journal of the Mechanics and Physics of Solids, 2018

The break-up of a nanowire with an organic ligand shell into discrete droplets is analysed in ter... more The break-up of a nanowire with an organic ligand shell into discrete droplets is analysed in terms of the Rayleigh-Plateau instability. Explicit account is taken of the effect of the organic ligand shell upon the energetics and kinetics of surface diffusion in the wire. Both an initial perturbation analysis and a full numerical analysis of the evolution in wire morphology are conducted, and the governing non-dimensional groups are identified. The perturbation analysis is remarkably accurate in obtaining the main features of the instability, including the pinch-off time and the resulting diameter of the droplets. It is conjectured that the surface energy of the wire and surrounding organic shell depends upon both the mean and deviatoric invariants of the curvature tensor. Such a behaviour allows for the possibility of a stable nanowire such that the Rayleigh-Plateau instability is not energetically favourable. A stability map illustrates this. Maps are also constructed for the final droplet size and pinch-off time as a function of two non-dimensional groups that characterise the energetics and kinetics of diffusion in the presence of the organic shell. These maps can guide future experimental activity on the stabilisation of nanowires by organic ligand shells.

Journal of the Mechanics and Physics of Solids, 2017

The adhesion of micron-scale surfaces due to intermolecular interactions is a subject of intense ... more The adhesion of micron-scale surfaces due to intermolecular interactions is a subject of intense interest spanning electronics, biomechanics and the application of soft materials to engineering devices. The degree of adhesion is sensitive to the diameter of micro-pillars in addition to the degree of elastic mismatch between pillar and substrate. Adhesionstrength-controlled detachment of an elastic circular cylinder from a dissimilar substrate is predicted using a Dugdale-type of analysis, with a cohesive zone of uniform tensile strength emanating from the interface corner. Detachment initiates when the opening of the cohesive zone attains a critical value, giving way to crack formation. When the cohesive zone size at crack initiation is small compared to the pillar diameter, the initiation of detachment can be expressed in terms of a critical value H c of the corner stress intensity. The estimated pull-off force is somewhat sensitive to the choice of stick/slip boundary condition used on the cohesive zone, especially when the substrate material is much stiffer than the pillar material. The analysis can be used to predict the sensitivity of detachment force to the size of pillar and to the degree of elastic mismatch between pillar and substrate.

International Journal of Mechanical Sciences, 2019

There is a practical need to elevate both the indentation strength and level of energy absorption... more There is a practical need to elevate both the indentation strength and level of energy absorption of engineering foams by the addition of a stiff and strong face sheet for applications such as packaging and crash mitigation. In this study, the enhancement in plane strain indentation resistance of a polyvinyl chloride (PVC) foam by the presence of a polycarbonate (PC) face sheet is determined by experiment, finite element analysis and by an analytical model. Plane strain indentation is by a flat-bottom punch or by a cylindrical roller, and the strain distribution within the PC face sheet and in the foam substrate are measured by digital image correlation. With increasing indent depth, the face sheet bends and stretches elastically and then plastically until face sheet or substrate fail. The generation of membrane tension in the face sheet plays a major role in supporting the indentation load when the indent depth exceeds the thickness of the face sheet, and leads to a strong hardening behaviour beyond the initial collapse load for indentation. Finite element predictions of the full indentation response are based upon the measured tensile and compressive responses of the PVC foam and PC layer. An analytical model is developed by matching the stretching response of the PC face sheet to the indentation response of the underlying foam, with due consideration for load diffusion from membrane tension of the PC face sheet into the underlying foam substrate. The indentation model is calibrated by ancillary finite element simulations of the load diffusion problem, and they emphasise the role of a shear lag zone in dictating the large indentation resistance. The indentation response of the bi-layer is also compared with that of a sandwich beam in 3-point bending. Experiments, finite element simulations and an additional analytical model for indentation of the sandwich beam in 3-point bending reveal that strong hardening of the post-yield load versus displacement response is now absent, in contrast to that of the bi-layer. The lack of hardening in 3-point bending is traced to the relatively low value of plastic bending moment of the beam section.

SIGLEAvailable from British Library Document Supply Centre-DSC:9106.170(CUED/C-MICROMECH/TR35) / ... more SIGLEAvailable from British Library Document Supply Centre-DSC:9106.170(CUED/C-MICROMECH/TR35) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

International Journal of Solids and Structures, 2017

Butt joints have been manufactured from aluminium alloy substrates of interlocking square-wave ar... more Butt joints have been manufactured from aluminium alloy substrates of interlocking square-wave architecture and silyl-modified polymer (SMP) adhesive. The macroscopic traction versus opening separation responses of the butt joints have been measured for selected values of square-wave amplitude and thickness of adhesive layer. Insight into the failure mechanisms of the square-wave joint is obtained by performing tensile tests on planar butt joints and by performing double-lap shear tests on planar joints. The mechanisms of adhesive failure in a planar butt joint are determined by micro X-ray computed tomography. It is determined by optical fractography that the failure mechanisms in the square-wave joint are a combination of those observed in the butt and double-lap shear joints, and consequently a simple model for the strength and toughness of the square-wave joint is developed. A design map is constructed for the strength, toughness and damage-tolerance of the square-wave micro-architecture in order to generate the optimal topology for a given application.

Journal of the Mechanics and Physics of Solids, 2004

Predictions are made for the size effect on strength of a random, isotropic two phase composite. ... more Predictions are made for the size effect on strength of a random, isotropic two phase composite. Each phase is treated as an isotropic, elastic-plastic solid, with a response described by a modified deformation theory version of the Fleck-Hutchinson strain gradient plasticity formulation (Fleck N A and Hutchinson J W, 2001, J. Mech. Phys. Solids, 49, 2245-2272.) The essential feature of the new theory is that the plastic strain tensor is treated as a primary unknown on the same footing as the displacement. Minimum principles for the energy and for the complementary energy are stated for a composite, and these lead directly to elementary bounds analogous to those of Reuss and Voigt. For the case of a linear hardening solid, Hashin-Shtrikman bounds and self-consistent estimates are derived. A nonlinear variational principle is constructed by generalizing that of Ponte Castañeda (1992, J. Mech. Phys. Solids, 40, 1757-1788). The minimum principle is used to derive an upper bound, a lower estimate and a self-consistent estimate for the overall plastic response of a statistically homogeneous and isotropic strain gradient composite. Sample numerical calculations are performed to explore the dependence of the macroscopic uniaxial response upon the size scale of the microstructure, and upon the relative volume fraction of the two phases.

International Journal of Fracture, 1993

The compressive failure of carbon fibre-epoxy laminates is investigated theoretically and experim... more The compressive failure of carbon fibre-epoxy laminates is investigated theoretically and experimentally. Panels with a single edge notch, a central notch or a central hole are considered. The failure mechanism is by microbuckling in the 0 ° plies and is accompanied by delamination and plastic deformation in the off-axis plies [1]. To predict the critical length of the microbuckle and the failure load, the microbuckle is modelled as a cohesive zone. The magnitude of the normal compressive traction across the microbuckle is assumed to decrease linearly with increasing overlap of material on either side of the microbuckle. The relative effect of the specimen size and a bridging length scale is investigated to illustrate the transition between small-scale and large-scale bridging. If the bridging length scale is small compared with the specimen dimensions, the specimen fails when the stress intensity at the notch tip equals a critical compressive stress intensity factor K~c. When the bridging length scale is not small compared with either the initial defect size or the unnotched ligament length then it is necessary to include the details of the traction across the mierobuckle to predict the failure load accurately.

Inelastic Deformation of Composite Materials

Polymer Science and Technology Series

Basic Questions in Fatigue: Volume I

Norman A. Fleck1 Influence of Stress State on Crack Growth Retardation REFERENCE: Fleck, NA,&quot... more Norman A. Fleck1 Influence of Stress State on Crack Growth Retardation REFERENCE: Fleck, NA," Influence of Stress State on Crack ... Blazewicz [35] has conducted a critical experiment where he made ball impressions on uncracked 2024-T3 aluminum alloy sheet specimens. ...

KSME Journal

The machining process observed during orthogonal cutting tests for sintered bronze of l 1%, 23% a... more The machining process observed during orthogonal cutting tests for sintered bronze of l 1%, 23% and 33% porosity is analyzed. Each step of the machining process, that is, compaction due to the indentation by a cutting tool, the shear local to the tip of the tool and the fracture near the chip root are simulated by a mechanical model based on classical theories of indentation, mackining and fracture. The presented models make it possible to predict the several behaviors of the machining, such as the types of chip formation, maximum cutting pressure and chip wave length. Good agreement between the experimental results and the predictions is found.

The Fifth International Conference on Advanced Semiconductor Devices and Microsystems, 2004. ASDAM 2004.

Anovel nanually closed microcage has been fabricated and cl1aracterized. This device was made fro... more Anovel nanually closed microcage has been fabricated and cl1aracterized. This device was made from a highly compressively stressed diamond like carbon (DLC) and electroplated Ni bimorph structure. The large stress io the DLC causes the bimorph layer to curve Once it is released from the substrate. The radius of curvature is in the range of 18 -50f.lm, and can be controlled by varying the DLC and the Ni thicknesses. The devices can be operated iu a pulsed mode current with low operation temperature, and can' be opened by -60flIU laterally with a power coosumption of only �16mW.

International Journal of Solids and Structures

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

Solid-state nanofoaming experiments are conducted on two polymethyl methacrylate (PMMA) grades of... more Solid-state nanofoaming experiments are conducted on two polymethyl methacrylate (PMMA) grades of markedly different molecular weight using CO 2 as the blowing agent. The sensitivity of porosity to foaming time and foaming temperature is measured. Also, the microstructure of the PMMA nanofoams is characterized in terms of cell size and cell nucleation density. A one-dimensional numerical model is developed to predict the growth of spherical, gas-filled voids during the solid-state foaming process. Diffusion of CO 2 within the PMMA matrix is sufficiently rapid for the concentration of CO 2 to remain almost uniform spatially. The foaming model makes use of experimentally calibrated constitutive laws for the uniaxial stress versus strain response of the PMMA grades as a function of strain rate and temperature, and the effect of dissolved CO 2 is accounted for by a shift in the glass transition temperature of the PMMA. The maximum achievable porosity is interpreted in terms of cell wall...