Now showing 1 - 10 of 43
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Dynamic crack bifurcation in PMMA

2006-11, Murphy, Neal, Ali, M., Ivankovic, Alojz

An investigation of the branching characteristics of small PMMA single edge notched tensile (SENT) specimens is presented. The influence of notch depth and specimen thickness was examined and it was found that branching only occurred for thicker specimens and very short notch depths. The location at which successful branching occurred was very consistent for a given notch depth. Subsequently, however, a statistical variation of branching patterns was observed. A series of simulations was then performed to provide further insight into these tests and in particular to examine the evolution of the fracture process region ahead of the running crack. A finite volume/cohesive zone formulation was used to model micro-crack nucleation and dynamic interaction in the process zone. The cohesive strength and fracture resistance were estimated from unnotched tensile tests and the application of LEFM to the notch test data. Even though a very simple criterion was used to govern the insertion and subsequent behaviour of the cohesive surfaces in the model, many of the experimental observations were reproduced, including high frequency oscillations in crack velocity, the substantial increase in the fracture surface area due to the formation of subsurface micro-cracks, and the location at which successful branching took place.

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Finite-volume stress analysis in multi-material linear elastic body

2012-07-05, Tuković, Željko, Ivankovic, Alojz, Karac, Aleksandar

Correct calculation of stresses at the interface of bonded or otherwise joined materials plays a significant role in many applications. It is therefore important that traction at the material interface is calculated as accurately as possible. This paper describes procedures that can be employed to achieve this goal by using centre-based finite-volume method. Total traction at the interface is calculated by decomposing it into normal and tangential components, both being calculated at each side of the interface, and applying the continuity assumption. The way in which the traction approximation is achieved depends on calculation of tangential gradient of displacement at the interface. To this end, three different methods are proposed and validated against problems with known solutions. It was shown that all methods can be successfully used to simulate problems with multi-material domains, with the procedure based on finite area method being most accurate.

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The prediction of dynamic fracture evolution in PMMA using a cohesive zone model

2005-04, Murphy, Neal, Ivankovic, Alojz

A cohesive zone model was used in conjunction with the finite volume method to model the dynamic fracture of single edge notched tensile specimens of PMMA under essentially static loading conditions. In this study, the influence of the shape of the cohesive law was investigated, whilst keeping the cohesive strength and separation energy constant. Cohesive cells were adaptively inserted between adjacent continuum cells when the normal traction across that face exceeded the cohesive strength of the material. The cohesive constitutive law was therefore initially rigid, and the effective elasticity of the material was unaltered prior to insertion of the cohesive cells. Notch depths ranging from 2.0 to 0.1 mm were considered. The numerical predictions were compared with experimental observations for each notch depth and excellent qualitative and quantitative agreement was achieved in most cases. Following an initial period of rapid crack tip acceleration up to terminal velocities well below the Rayleigh wave speed, subsequent propagation took place at a constant rate under conditions of increasing energy flux to an expanding process region. In addition, attempted and successful branching was predicted for the shorter notches. It was found that the shape of the cohesive law had a significant influence on the dynamic fracture behaviour. In particular, the value of the initial slope of the softening function was found to be an important parameter. As the slope became steeper, the predicted terminal crack speed increased and the extent of the damage decreased.

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Mechanical characterisation of polyurethane elastomer for biomedical applications

2010-01, Kanyanta, Valentine, Ivankovic, Alojz

Mechanical testing and modelling of a material for biomedical applications have to be based on conditions representative of the application of interest. In this work, an ether-based polyurethane elastomer is used to build mock arteries. The aim is to study the behaviour of arteries under pulsatile loading conditions and how that behaviour changes with the development and progression of atherosclerosis. Polyurethane elastomers are widely used as biomaterials, e.g. in tube form for bypasses and catheters. However, their mechanical behaviour has not been extensively characterised. This work establishes the variations in the behaviour of polyurethane elastomer with temperature, humidity and strain rate and also reports planar and equibiaxial tension, relaxation, creep and cyclic test results, providing a comprehensive characterisation of the material. Test results are used to determine the properties of the polyurethane elastomer and in the selection of a representative material model for future simulations of arterial behaviour and the development of atherosclerosis. The results show that the behaviour of the elastomer is significantly dependent on both humidity and temperature, with Young’s modulus of 7.4 MPa, 5.3 MPa and 4.7 MPa under dry-room temperature, wet-room temperature and wet at 37 ∘C conditions, respectively. The elastomer also exhibits rate-dependent viscoelastic behaviour. Yeoh’s hyperelastic material model provided the best fit to the entire range of experimental data. The Neo-Hookean model provides a good fit at small strain but significantly diverges at large strains. Nevertheless, in applications where deformations are relatively small, i.e. below 15%, the Neo-Hookean model can be used.

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The Effect of Prepeg Storage Humidity on Co-cured Composite Joints

2009, Mohan, Joseph, Murphy, Neal, Ivankovic, Alojz

The increasing use of composite materials in the aerospace industry has driven a need for a greater understanding of bonded composite joints. There are generally two types of composite joint used in the aerospace industry; secondary bonded joints and cocured joints. Secondary bonded joints are produced by bonding two cured composite laminates together with an adhesive. However, when composites and adhesives are used to manufacture large parts in the aerospace industry, it is often convenient to co-cure the two materials at the same time. This helps to reduce the high costs associated with autoclave curing and also to reduce processing time. However, despite the apparent advantages, co-curing is not without its drawbacks. Any moisture stored in the composite material prior to co-curing is released during the cure cycle and has a negative effect on the joint. This can also result in interfacial failure. A way around this problem is to either dry the composite material prior to curing or to engineer the composite surface using a variety of surface treatments to promote adhesion, such as an atmospheric pressure plasma treatment [1]. The former option will be investigated in this work. The effects of moisture on the fracture performance of secondary bonded composite joints is well publicised. Moisture can be introduced into the composite laminate prior to [2] or after [3] secondary bonding. The moisture can plasticize the adhesive and reduce the glass transition temperature of the adhesive [4]. However, compared to secondary bonded joints, relatively little work has been carried out on co-cured joints. In the present work, the effect of the level of moisture in the composite prepreg prior to co-curing will be examined.

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Fracture properties of PCBN as a function of loading rate and temperature

2010-11, Carolan, Declan, Petrović, Marin, Ivankovic, Alojz, et al.

Polycrystalline Cubic Boron Nitride (PCBN) is a superhard material which is used in machining of hardened steels and other abrasive and aerospace grade alloys. In these applications the tools are subjected to high operating temperatures, abrasive and impact loading. Impact loading can lead to the sudden fracture and hence failure of the tool. In this work the static and dynamic fracture toughness of PCBN is determined via a combined experimental-numerical approach. The results show that the fracture toughness of PCBN varies with loading rate

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A Block-Coupled Finite Volume Methodology for Linear Elasticity and Unstructured Meshes

2016-10-15, Cardiff, Philip, Tuković, Željko, Jasak, Hrvoje, Ivankovic, Alojz

The current article presents a new implicit cell-centred Finite Volume solution methodology for linear elasticity and unstructured meshes. Details are given of the implicit discretisation, including use of a Finite Area method for face tangential gradients and implicit non-orthogonal correction. A number of 2-D and 3-D linear-elastic benchmark test cases are examined using hexahedral, tetrahedral and general polyhedral meshes; solution accuracy and efficiency are compared with that of a segregated procedure and a commercial Finite Element software, where the new method is shown to be faster in all cases.

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Enhanced Carbon/Epoxy Composite Fracture Toughness Achieved Using Atmospheric Pressure Plasma Treatments

2010-02-22, Ramamoorthy, Amsarani, Mohan, Joseph, Ivankovic, Alojz, et al.

Composite materials are used in a wide range of industry sectors including automobiles, aeronautics and sports equipment. Two types of composite joints, co-cured and secondary bonded joints are used in the industries. Co-curing of composite joints is an efficient and cost-effective method of joining composites. The objective of this research is to enhance the bond strength between the composite material and adhesive, specifically in this study the bond between carbon-epoxy prepregs and an epoxy adhesive. The research investigated how the use of atmospheric plasma treatments of the uncured composite prepreg influenced the fracture toughness of the co-cured composite joints. The use of atmospheric pressure plasma treatment in the surface activation of prepregs was examined using contact angle measurements and X-ray photoelectron microscopy (XPS). Failure mechanism of the co-cured composite joints was studied by double cantilever beam (DCB) tests. Composite interface morphology was examined by scanning electron microscopy (SEM/FIB).

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Hierarchical RVE-based multiscale modelling of non-linear heterogeneous materials using the finite volume method

2022, Wu, Ke, Tuković, Željko, Cardiff, Philip, Ivankovic, Alojz

This paper describes the development of a hierarchical multiscale procedure within the finite volume OpenFOAM framework for modelling the mechanical response of non-linear heterogeneous solid materials. This is a first development of hierarchical multi-scale model for solid mechanics to use the finite volume discretisation method. In this computational procedure the information is passed between the macro and micro scales using representative volume elements (RVE), allowing for general, non-periodic microstructures to be considered. Each computational point at the macro scale is assigned an RVE with prescribed microstructural features. The overall macro response accounts for the microstructural effects through the coupling of macro and micro scales, i.e., the macro deformation gradient is passed to the RVE and in turn, the homogenised micro stress-strain response is passed back to the macro scale. The incremental total Lagrangian formulation is used to represent the equilibrium state of the solid domain at both scales and its integral equilibrium equation is discretised using the cell-centred finite volume (FV) method in OpenFOAM. The verification of the model is demonstrated using both 2D and 3D simulations of perforated elastic-plastic plates subjected to tensile loading.

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Thermal shock resistance of polycrystalline cubic boron nitride

2012-08, Carolan, Declan, Ivankovic, Alojz, Murphy, Neal

The effect of thermal shock on the exural strength has been investigated experimentally. It was found that the variation in exural strength with quench temperature was influenced by the CBN grain size. Polycrystalline material containing small CBN grains showed a discontinuous drop in measured exural strength above a material dependent critical quench temperature difference, delta Tc. The sharp decrease in measured strength is accompanied by unstable crack propagation. Material containing a significantly larger CBN grain size, exhibited a gradual decrease in strength above the critical quench conditions. The experimental observations agreed with an established theory developed for thermal shock of alumina. The theoretically calculated critical temperatures agree well with the observed experimental data for each material when a aw size equal to the CBN grain size is employed.