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Karac, Aleksandar
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Karac, Aleksandar
Official Name
Karac, Aleksandar
Research Output
Now showing 1 - 10 of 24
- PublicationTransferability of Adhesive Fracture Toughness Measurements between Peel and TDCB Test Methods for a Nano-Toughened EpoxyOur previous work [1] on a nano rubber modified epoxy adhesive suggested that the observed bond thickness effect was due to the level of constraint (σhyd/σeq), a measure of the stress triaxiality, in the adhesive layer. In that study tapered double cantilever beam (TDCB) specimens were tested under quasi-static conditions for a range of bond gap thicknesses. The void diameters on the resulting fracture surfaces were measured from which the fracture strain was estimated in each case. The ratio of fracture strains corresponding to different constraint levels was found to agree with the predictions of the Rice and Tracey model. The current work attempts to further investigate the effects of constraint on adhesive joint fracture. Three experimental test methods are employed (i) the standardised LEFM tapered double cantilever beam (TDCB) test, in which the substrates experiences small elastic deformations, (ii) the fixed arm peel test where the substrate peel arm undergoes extensive plastic deformation and (iii) a recently developed circumferentially deep notched tensile (CDNT) test. Finite Volume simulations of the TDCB and CDNT tests were utilised to examine the role of constraint on the adhesive joint fracture.
371 - PublicationCharacterisation of a surrogate lung material made of polyurethane foam and fluid-filled gelatine microcapsules(Computational & Mathematical Biomedical Engineering (CMBE), 2009-07-01)
; ; In this study, a surrogate lung material, developed to mimic the lungs behaviour in low and high rate impact tests in order to better understand the damage mechanism in the lungs resulting from car crashes, collisions and explosion [1], is tested and characterised. This aims to eliminate the practice of live animal testing. The surrogate lung consists of polyurethane foam mixed with gelatine microcapsules filled with Barium Sulphate solution. Thus, both the foam and microcapsules must be individually characterised in addition to the surrogate lung itself when treated as a continuum material. For this, a number of compression tests were carried out on each material to ascertain their mechanical properties. On the other hand, the damage to the surrogate lung specimens as represented by burst microcapsules was analysed by carrying out CT scans before and after testing. The results show that the modulus of elasticity increases with the test speed. CT scan results clearly demonstrated the magnitude and distribution of damage within the specimen.199 - PublicationMicro-Mechanical Modelling of Void Growth, Damage and Fracture of Nano-Phase Structural Adhesives Using the Finite Volume Method(The European Structural Integrity Society Technical Committee, 2011)
; ; ; Significant toughening of structural adhesives is attainable with the addition of nano and/or micro particles1,2,3. A deep understanding of the effect of particle de-bonding and subsequent void growth to coalescence is key to evaluating the strengthening and failure mechanisms occurring in the damage and fracture of these adhesives. Tapered Double Cantilever Beam (TDCB) experiments, conducted at University College Dublin (UCD), have observed a significant dependence of the fracture toughness of these adhesives on bond gap thickness5. In conjunction with this change in fracture toughness, scanning electronmicroscopy (SEM) of the fracture surface has also revealed corresponding changes in void evolution as the bond gap is varied. Classical analysis suggests the change in toughness may be attributed to a physical constraint of the size to which the plastic zone around a crack tip may develop6. However, simulation of these TDCB tests using finite volume stress analysis has found that little plasticity develops in the bulk adhesive layer and is instead concentrated in the fracture process zone. The change in fracture toughness and void evolution present can be attributed to the change in triaxiality at different bond gap thicknesses and the results agree quite well with the void growth model of Rice & Tracey4. The variance of void growth with triaxiality is investigated here. The initial work considered here concerned 3D modelling of a void in an elastic perfectly plastic material with a view to verifying exponential dependence of void growth on the macroscopic stress triaxiality in the system in accordance with the Rice & Tracey model. The model examines void growth rate dependence on the stress triaxiality, for a given effective strain.421 - PublicationAn open-source finite volume toolbox for solid mechanics and fluid-solid interaction simulations(2018)
; ; ; ; Over the past 30 years, the cell-centred finite volume method has developed to become a viable alternative to the finite element method in the field of computational solid mechanics. The current article presents an open-source toolbox for solid mechanics and fluid-solid interaction simulations based on the finite volume library OpenFOAM. The object-oriented toolbox design is outlined, where emphasis has been given to code use, comprehension, maintenance and extension. The toolbox capabilities are demonstrated on a number of representative test problems, where comparisons are given with finite element solutions.1169 - Publication
304 - Publication
669 - PublicationModelling the Fracture Behaviour of Adhesively-Bonded Joints as a Function of Test Rate - A Rate Dependent CZM is Required to Predict the Full Range of BehaviourAdhesive bonding of lightweight, high-performance materials is regarded as a key enabling technology for the development of vehicles with increased crashworthiness, better fuel economy and reduced exhaust emissions. However, as automotive structures can be exposed to impact events during service, it is necessary to gain a sound understanding of the performance of adhesive joints under different rates of loading. Therefore, characterising the behaviour of adhesive joints as a function of loading rate is critical for assessing and predicting their performance and structural integrity over a wide range of conditions. The present work investigates the rate-dependent behaviour of adhesive joints under mode I loading conditions. A series of fracture tests were conducted using tapered double-cantilever beam (TDCB) specimens at various loading rates [1-2]. The experiments were analysed analytically and numerically. The full details of the analysis strategy employing analytical approaches for different types of fracture are presented in [1]. The numerical modelling of the TDCB experiments was performed using the finite-volume based package ‘OpenFOAM’ [3].
311 - PublicationAn Experimental and Numerical Investigation of the Mixed-mode Fracture Toughness and Lap Shear Strength of Aerospace Grade Composite JointsThe increasing use of composite materials in various industries, such as aerospace, automotive and renewable energy generation, has driven a need for a greater understanding of the fracture behaviour of bonded composite joints. An important prerequisite for the adhesive bonding of composites is the existence of a uniform surface free from contaminants and mould release agents. While there are several ways in which this may be achieved, the use of peel plies has emerged as the preferred choice for many industries due to the repeatable nature of the resulting surface, particularly in the highly regulated aerospace industry. However, the use of peel plies can present some problems. It is possible that contamination from the peel ply can be transferred to the composite substrate and adversely affects the adhesive joint [1]. Composite joints are typically evaluated using lap shear type tests. While these tests are relatively simple to perform and post-process compared to their fracture mechanics based counterparts, the results can often be misleading and are greatly dependent on the overlap length, the thickness of the substrate and the type of fillet employed [2, 3]. The aim of this work is to show that composite joint systems can be modelled using material properties determined from fracture mechanics based tests. The fracture parameters will be used to develop numerical models of the fracture tests that accurately predict the wide-area lapshear test.
661 - PublicationFinite-volume stress analysis in multi-material linear elastic body(Wiley Blackwell (John Wiley & Sons), 2012-07-05)
; ; 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.1231Scopus© Citations 46 - PublicationInvestigating the behaviour of fluid-filled polyethylene containers under base drop impact: A combined experimental/numerical approachIn this work, the behaviour of fluid-filled plastic containers under base drop impact is investigated using a combined experimental/numerical approach. In addition, theoretical predictions from two approaches, waterhammer theory and a mass-spring model, are also given. Experimental tests are conducted using a specially designed rig for testing plastic containers (bottles). Tested containers are fully instrumented with pressure transducers and strain gauges. The experiments are simulated using a two-system fluid-structure interaction procedure based on the Finite Volume Method. Good agreement is found between measured and predicted pressure and strain histories. Results obtained are in favour of waterhammer theory.
694Scopus© Citations 20
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