Comparison of a Material Point Method and a Galerkin meshfree method for the simulation of cohesive-frictional materials

The simulation of large deformation problems, involving complex history-dependent constitutive laws, is of paramount importance in several engineering fields. Particular attention has to be paid to the choice of a suitable numerical technique such that reliable results can be obtained. In this paper, a Material Point Method (MPM) and a Galerkin Meshfree Method (GMM) are presented and verified against classical benchmarks in solid mechanics. The aim is to demonstrate the good behavior of the methods in the simulation of cohesive-frictional materials, both in static and dynamic regimes and in problems dealing with large deformations. The vast majority of MPM techniques in the literature are based on some sort of explicit time integration. The techniques proposed in the current work, on the contrary, are based on implicit approaches, which can also be easily adapted to the simulation of static cases. The two methods are presented so as to highlight the similarities to rather than the differences fromPeer ReviewedPostprint (published version

IDENTIFYING TARGET PROPERTIES FOR THE DESIGN OF META-MATERIAL TANK TRACK PADS

On track vehicle systems, track pads are designed to provide traction and support the weight of the vehicle, they have limited service life due to common failure by blowout. According to the literature, blowout is a failure mode caused by overheating due to hysteresis in elastomeric materials during high speed operations. Elastomers are used primarily for their high compliance, which is essential to protect the suspension components and maintain structural integrity of the track pad. The objective of the work is to explore the use of linear elastic meta-materials with optimized topology to replace elastomers and reduce or eliminate the effect of hysteretic loss. This work presents a methodology to design an alternate meta-material that can provide some of the desired elastic properties of the track pads. To determine the requirements for linear elastic meta-materials, dynamic analyses of a rollover event were conducted. From these analyses the complex dependence of the strain history on different strain components is understood. Due to the non-linearity of elastomers, tangent stiffness matrices are required to update the stress states at different strain increments. The elasticity tensors (tangent operators) determined at a set of strain levels, are used as prescribed constitutive parameters to tailor the meta-material unit-cell topology. The optimal material properties according to which the elastomeric track pad is designed with linear elastic material are identified in this work

Active elastohydrodynamics of vesicles in narrow, blind constrictions

Fluid-resistance limited transport of vesicles through narrow constrictions is a recurring theme in many biological and engineering applications. Inspired by the motor-driven movement of soft membrane-bound vesicles into closed neuronal dendritic spines, here we study this problem using a combination of passive three-dimensional simulations and a simplified semi-analytical theory for active transport of vesicles that are forced through such constrictions by molecular motors. We show that the motion of these objects is characterized by two dimensionless quantities related to the geometry and the strength of forcing relative to the vesicle elasticity. We use numerical simulations to characterize the transit time for a vesicle forced by fluid pressure through a constriction in a channel, and find that relative to an open channel, transport into a blind end leads to the formation of an effective lubrication layer that strongly impedes motion. When the fluid pressure forcing is complemented by forces due to molecular motors that are responsible for vesicle trafficking into dendritic spines, we find that the competition between motor forcing and fluid drag results in multistable dynamics reminiscent of the real system. Our study highlights the role of non-local hydrodynamic effects in determining the kinetics of vesicular transport in constricted geometries

A hybrid patient-specific biomechanical model based image registration method for the motion estimation of lungs

This paper presents a new hybrid biomechanical model-based non-rigid image registration method for lung motion estimation. In the proposed method, a patient-specific biomechanical modelling process captures major physically realistic deformations with explicit physical modelling of sliding motion, whilst a subsequent non-rigid image registration process compensates for small residuals. The proposed algorithm was evaluated with 10 4D CT datasets of lung cancer patients. The target registration error (TRE), defined as the Euclidean distance of landmark pairs, was significantly lower with the proposed method (TRE = 1.37 mm) than with biomechanical modelling (TRE = 3.81 mm) and intensity-based image registration without specific considerations for sliding motion (TRE = 4.57 mm). The proposed method achieved a comparable accuracy as several recently developed intensity-based registration algorithms with sliding handling on the same datasets. A detailed comparison on the distributions of TREs with three non-rigid intensity-based algorithms showed that the proposed method performed especially well on estimating the displacement field of lung surface regions (mean TRE = 1.33 mm, maximum TRE = 5.3 mm). The effects of biomechanical model parameters (such as Poisson’s ratio, friction and tissue heterogeneity) on displacement estimation were investigated. The potential of the algorithm in optimising biomechanical models of lungs through analysing the pattern of displacement compensation from the image registration process has also been demonstrated

Experimental and numerical investigation of soft impact loading on aircraft materials

Bird strike poses a great hazard to aircraft components, such as the engine and windshield, during flight. Thus, it is critical to understand the impact resistance of key aircraft components under bird strike. During this PhD study, Aluminium Alloy 2024-T3 and laminated glass interlayered with thermoplastic polyurethane (TPU), which are commonly used as the materials of the aircraft fuselage and windshield respectively, are selected as impact target materials. Both laboratory-based experiments and finite element simulations are performed using a light gas gun and ABAQUS/EXPLICIT respectively. A good agreement is achieved between the experimental work and numerical predictions for the impact response. Two bird substitute materials were selected: RTV rubber and ballistic gelatine, whose dynamic pressure profiles are similar to that of a real bird during a high speed impact. Mechanical properties of both materials were investigated by conducting compression tests at quasi-static (0.25, 2.5 and 25 min-1) and intermediate rate (2760 min-1 to 22500 min-1). As a result, the relationship between true stress and strain is obtained and constitutive equations are established using the hyperelastic models, i.e. the Ogden, Mooney-Rivlin and Neo-Hookean. The 3D Digital Image Correlation technique was employed in the gas gun test of the AA 2024-T3 and TPU interlayered laminated glass. Thus, the impact compliance of both targets from rubber and gelatine impacts are attained and found to be roughly equal to the same initial projectile momentum. An FE simulation was used to model the experimental process and was validated against the DIC results. Moreover, the Hugoniot pressure plays a predominant role in laminated glass fracture during the impact, with the rubber projectile leading to a larger damage than the gelatine projectile given the same momentum. This is because the shock wave speed in the rubber is larger than that in the gelatine projectile. A validated FE simulation is therefore presented, which can be used to simulate real bird impact on real aircraft structures in the future. Thus, this PhD work can be potentially implemented into industrial research programmes to aid design and optimisation.Open Acces