Failure mechanisms of corrugated sandwich panels under transverse three-point bending

The failure mechanism of corrugated sandwich panels under three-point bending is investigated experimentally and numerically using ABAQUS/Standard. Three dominant modes of failure, viz. face buckling, face yielding and core buckling, previously reported by others, were also identified here and they were found to be affected by the face-sheet thickness, core height and the corrugation angle of the panel. It will be shown that the deformation map, and regime boundaries, theoretically predicted previously by Valdevit et al. are valid only for restricted cases when the indenter and support pins are simultaneously located at a vertex; their prediction breaks down if this condition is not met and alternative failure maps will be developed here. The indenter nose shape (flat and cylindrical), size of the indenter and the location of indentation – these were not previously investigated by others – will be shown to play a significant role on the mode of failure that a panel subsequently develops

Large deformation, damage evolution and failure of ductile structures to pulse-pressure loading

In this paper, a model is developed for an elastic perfectly-plastic structural beam system subjected to general pulse-pressure loadings - this may be either impulsive or non-impulsive - which is capable of capturing large non-linear deformation, ductile damage evolution and its consequential failure. The proposed model is an extension of Schleyer and Hsu (2000) by incorporating interactions between bending, membrane stretch and transverse shear in the fully plastic stress state, and uses damage mechanics to capture the loss of integrity at the supports and the subsequent beam detachment. Predictions by the model were validated against existing experimental data from literature and to three-dimensional finite element models developed in this paper. Parametric studies were performed to elucidate the effects of loading duration on the mode of deformation by the beam and the critical conditions governing their transition. The efficacy of Youngdahl’s (1970; 1971) technique on desensitising pulse shape effects is also investigated using different pressure pulse profiles and it will be shown that the technique is successful only for monotonically decaying pulse-pressures

The influence of deformation limits on fluid-structure interactions in underwater blasts

This paper revisits a classical fluid-structure interaction (FSI) problem on the momentum and energy transfer to a structure from an underwater blast. Hitherto, the majority of analytical models assume a rigid (non-deformable) and free-standing (unsupported) structure where resistance to its translational motion - apart from that offered by its inertial mass - comes from ‘ad-hoc’ backing spring(s) introduced to simulate compression of the fluid medium and/or the resistance to transverse deformation encountered by a real structure. These limitations/assumptions are relaxed in this paper by adopting a physically realistic fully-clamped ductile beam system that takes into account large elasto-plastic deformation, limits to material deformation, boundary compliance and boundary failure; the analytical framework was developed previously by Yuan et al. [1]. By coupling the fluid (water) domain to the analytical model of the ductile beam system, the momentum and energy transferred by the blast wave are critically re-evaluated for non-impulsive loading régime; in particular, on how the beam’s deformation mode and boundary compliance affects fluid and structure interaction, up until the point of complete beam detachment from its supports. Detailed finite-element models were also developed to simulate the interactions between the fluid and structural beam where predictions were in good agreement with those by the analytical model. Sensitivity analyses were carried out that offer new insights on the influence of the beam’s aspect ratio and inertial mass

In-plane pipe whip: Post-failure dynamic response

Pressurized pipes are ubiquitous in chemical, nuclear and power plants. A sudden failure and release of high-speed fluid cause large inelastic displacements characterized by a whipping-type motion, which can ultimately hinder the surrounding structural and functional systems. In this paper, a beam user element has been developed, implemented and applied to analyze the in-plane flexural dynamic response of pipe whip. The two-dimensional Euler–Bernoulli beam element is based on the corotational kinematic formulation and elastoplastic constitutive models that include metal plasticity and moment–curvature relationships obtained from numerical bending tests, which highlighted the existence of two new dimensionless groups that govern the flexural response of slender pipes and enable the creation of moment–curvature master curves for thick and thin pipes. The corotational beam element formulation is compared against an analytical rigid-perfectly plastic model, numerical simulations employing shell elements and available experimental results, showing very good accuracy in the prediction of the inelastic deformation response of pipe whip and its hazardous area of influence. Furthermore, parametric studies are performed to investigate the effect of load intensity, cross-sectional geometry and concentrated tip mass on the post-failure deformation modes, plastic hinge formation and extension of the hazard zone. The presented results represent valid tools to assess the safety of industrial piping systems undergoing failure, and to optimally design pipe whip restraints

Failure and detachment path of impulsively loaded plates

The deformation and detachment path of simply-supported and fully clamped mild steel quadrangular plates subjected to impulsive blast loads are investigated numerically. A comprehensive failure model that incorporates two competing mechanisms of damage due to ductile and shear failure is employed to simulate the progression of tearing within the plate. The stress triaxiality dependent Modified Mohr-Coulomb (MMC) fracture criterion is implemented in a finite element model, and experimental tensile and shear tests were carried out to calibrate the material parameters. The direction of crack propagation along the clamped plate support and the residual length are predicted for a wide range of impulse intensity, reproducing the failure modes observed experimentally. Furthermore, the developed model highlights the different failure mechanisms that occur in simply-supported plates, similar to those of panels subjected to localised blast loadings. Parametric studies are performed to establish dimensionless failure maps and investigate the influence of plate topology and boundary conditions on the dynamic response, for both square and rectangular geometries, thus offering an effective support for the design of impulsively loaded plates for advanced engineering applications

Experimental investigation and modelling of T-stubs undergoing large displacements

This paper investigates the development of second (2nd) order effects, arising from geometric and material non-linearities of T-stubs bolted to a rigid support, through a combination of experimental, numerical and analytical approaches. Experimental data is presented for a broad range of T-stub geometries, designed to ensure that significant 2nd order effects always develop, that will complement the existing library of limited test results. Finite element models, incorporating combined tensile (ductile) and shear damage initiation, evolution and failure in both the flange and bolt, are also developed to elucidate how key geometric/material parameters influence the resistance and ductility of T-stubs undergoing large displacement. It will be shown that the restraining effect from the bolt is integral to the activation of catenary action in the flange and the development of a second hardening branch in the tensile response, leading to identification of two new modes of failure that are not currently considered in classical theory or by EC3 (Part 1.8). A mechanical model is formulated to identify the key geometric and material parameters controlling the initiation, and development, of the second hardening branch. Finally, a criterion is proposed to estimate the critical displacement from when 2nd order effects become active

Bayesian optimisation of hexagonal honeycomb metamaterial

Periodic mechanical metamaterials, such as hexagonal honeycombs, have traditionally been designed with uniform cell walls to simplify manufacturing and modelling. However, recent research has suggested that varying strut thickness within the lattice could improve its mechanical properties. To fully explore this design space, we developed a computational framework that leverages Bayesian optimisation to identify configurations with increased uniaxial effective elastic stiffness and plastic or buckling strength. The best topologies found, representative of relative densities with distinct failure modes, were additively manufactured and tested, resulting in a 54% increase in stiffness without compromising the buckling strength for slender architectures, and a 63% increase in elastic modulus and a 88% increase in plastic strength for higher volume fractions. Our results demonstrate the potential of Bayesian optimisation and solid material redistribution to enhance the performance of mechanical metamaterials

Nasal Lipopolysaccharide Challenge and Cytokine Measurement Reflects Innate Mucosal Immune Responsiveness

<div><p>Background</p><p><b>P</b>ractical methods of monitoring innate immune mucosal responsiveness are lacking. Lipopolysaccharide (LPS) is a component of the cell wall of Gram negative bacteria and a potent activator of Toll-like receptor (TLR)-4. To measure LPS responsiveness of the nasal mucosa, we administered LPS as a nasal spray and quantified chemokine and cytokine levels in mucosal lining fluid (MLF).</p><p>Methods</p><p>We performed a 5-way cross-over, single blind, placebo-controlled study in 15 healthy non-atopic subjects (n = 14 <i>per protocol</i>). Doses of ultrapure LPS (1, 10, 30 or 100μg/100μl) or placebo were administered by a single nasal spray to each nostril. Using the recently developed method of nasosorption with synthetic adsorptive matrices (SAM), a series of samples were taken. A panel of seven cytokines/chemokines were measured by multiplex immunoassay in MLF. mRNA for intercellular cell adhesion molecule-1 (ICAM-1) was quantified from nasal epithelial curettage samples taken before and after challenge.</p><p>Results</p><p>Topical nasal LPS was well tolerated, causing no symptoms and no visible changes to the nasal mucosa. LPS induced dose-related increases in MLF levels of IL-1β, IL-6, CXCL8 (IL-8) and CCL3 (MIP-1α) (AUC at 0.5 to 10h, compared to placebo, p<0.05 at 30 and 100μg LPS). At 100μg LPS, IL-10, IFN-α and TNF-α were also increased (p<0.05). Dose-related changes in mucosal ICAM-1 mRNA were also seen after challenge, and neutrophils appeared to peak in MLF at 8h. However, 2 subjects with high baseline cytokine levels showed prominent cytokine and chemokine responses to relatively low LPS doses (10μg and 30μg LPS).</p><p>Conclusions</p><p>Topical nasal LPS causes dose-dependent increases in cytokines, chemokines, mRNA and cells. However, responsiveness can show unpredictable variations, possibly because baseline innate tone is affected by environmental factors. We believe that this new technique will have wide application in the study of the innate immune responses of the respiratory mucosa.</p><p>Key Messages</p><p>Ultrapure LPS was used as innate immune stimulus in a human nasal challenge model, with serial sampling of nasal mucosal lining fluid (MLF) by nasosorption using a synthetic absorptive matrix (SAM), and nasal curettage of mucosal cells. A dose response could be demonstrated in terms of levels of IL-1β, IL-6, CXCL8 and CCL3 in MLF, as well as ICAM-1 mRNA in nasal curettage specimens, and levels of neutrophils in nasal lavage. Depending on higher baseline levels of inflammation, there were occasional magnified innate inflammatory responses to LPS.</p><p>Trial Registration</p><p>Clinical Trials.gov <a href="https://clinicaltrials.gov/ct2/show/NCT02284074?term=nasal+lipopolysaccharide&rank=1" target="_blank">NCT02284074</a></p></div

Crack initiation and fracture toughness of random Voronoi honeycombs

The competing effects of cell-regularity and relative density upon the toughness of Voronoi honeycombs are investigated for different loading modes using finite elements. Mode I toughness is shown to be the more sensitive to microstructural variations than mode II although both retain a strong quadratic dependence upon relative density. Crack initiation is shown to occur at up to six cells from the crack-tip in regions of high localised strain and/or high strain gradient. The inclusion of T-stress dramatically changes the location of ligament fracture and the normalised effective toughness of a lattice. Ligament fracture is predominantly due to bending

Deformation and failure of rectangular plates subjected to impulsive loadings

The deformation and failure of fully-clamped rectangular plates subjected to zero-period, uniform-momentum impulsive loads are studied. Analytical predictions are given for the critical velocities corresponding to the transition between deformation modes. Three-dimensional (3D) numerical analyses were performed using the non-linear finite element (FE) code ABAQUS/Explicit® to predict the maximum central deflection and deformation mode of rectangular plates for different combinations of aspect ratios and impulses. Two competing mechanisms of bulk material failure, viz. by the nucleation, coalescence and growth of voids and by shear band localisation, are implemented in the FE model to simulate tensile tearing, resulting in progressive ductile fracture, at the support. The numerical results are validated against experimental data for square mild-steel and aluminium plates where they are found to be in good agreement. Deformation maps delineating the different deformation régimes for different combinations of blast impulse and aspect ratio are constructed for plates of equal mass. The effects of imposing a finite-period, as opposed to a zero-period, impulsive load upon the deformation mode and maximum deflection are also discussed