Hydroelastic response of concrete shells during impact on calm water

Many ocean structures located offshore are supported by large vertical concrete columns. High and steep storm waves – in the process of breaking – may induce large local slamming loads on these columns. The present work is related to the fundamental physics of the local hydroelastic shell response due to slamming. The concrete columns supporting typical offshore structures are large. The size means that full scale tests of a segment of the column is impractical and expensive. Model-scale testing in a wave tank is also challenging. Firstly, the scaling of structural properties need to adhere to the scaling laws of hydrodynamics. Secondly, the manufacturing of realistic Froude scaled elastic shell models is hard since curved shells carries loads by a combination of bending and membrane action. The challenge is to scale both the bending and membrane action properly. One part of this study shows how realistic Froude scaled elastic shells representing concrete shells can be designed. The second part of this study presents results from experimental and numerical analysis of drop tests. Numerical hydroelastic analyses of both the elastic model shells and the real concrete shells are presented. The results show that even large and thick concrete shells experience significant hydroelastic effects during slamming. The hydroelastic response of the concrete shells is dominated by only a few structural eigenmodes. This means that the calculated dynamic amplification factors, DAF, resemble those of one-degree-of-freedom mass–spring systems exposed to loads of finite duration. The structural responses are seen to significantly modify the hydrodynamic loads. This hydrodynamic load modification consists of the well-known added mass term but also a time dependent slam damping term which reduce the structural response when properly accounted for. Both terms are necessary to calculate the concrete shell response accurately.Hydroelastic response of concrete shells during impact on calm waterpublishedVersio

Samhandlingsreformen og bruk av digitale verktøy på Fosen. En innledende kartlegging

-Informasjons- og kommunikasjonsteknologi (IKT), som høykvalitets videokonferanser, sensorteknologi etc., kan være viktige hjelpemidler i gjennomføringen av Samhandlingsreformen (SHR). Gjennom kvalitative intervjuer av 53 personer ble det undersøkt hvilke strukturelle og kulturelle forhold som det er viktig å ta hensyn til i Fosenkommunene i gjennomføringen av SHR ved hjelp av digitale verktøy. Det eksisterende kommunesamarbeidet på Fosen er et meget godt utgangspunkt for realisering av SHR. Det gis også i dag et relativt avansert helsetilbud som inkluderer bruk av IKT, særlig gjennom Fosen Distriktsmedisinske Senter, men også ved sykehjemmene. Ansatte i helsetjenestene er generelt positive til utviklingsprosjekter og til å ta i bruk ny IKT. Det er imidlertid noe skepsis knyttet til bruk av IKT mellom behandler og pasient, og også generelt til hvorvidt brukerstøtten vil være tilstrekkelig. Det anbefales at innføring av nye IKT-løsninger tas i avgrensede steg. Ved hvert steg bør det sikres tilstrekkelig opplæring, mestring og brukerstøtte for å ivareta tilliten til løsningene. Sykehjemmene kan være ”noder” i utvidelsen av samarbeidet mellom Fosenkommunene ved hjelp av IKT. Det beskrives noen utfordringer som bør søkes løst, blant annet knyttet til opplæring av mange deltidsansatte, tilgjengelighet til videokonferanseutstyr, brukerstøtte og organisatoriske forhold i spesialisthelsetjenesten

Fuzzy variable linear programming with fuzzy technical coefficients

Fuzzy linear programming is an application of fuzzy set theory in linear decision making problems and most of these problems are related to linear programming with fuzzy variables. In this paper an approximate but convenient method for solving these problems with fuzzy non-negative technical coefficient and without using the ranking functions, is proposed. With the help of numerical examples, the method is illustrated

Low-Velocity Penetration of Aluminium Plates

The present thesis describes research on quasi-static and low velocity perforation of rolled aluminium plates, where the main objective has been to gain a better knowledge of the physical processes taking place during this type of structural problem. The objective has been met by a combination of laboratory tests, material modelling and non-linear finite element simulations. The thesis is organized in a synopsis, giving a brief introduction to the problem and summarising the main findings and conclusions, in addition to four independent papers. Paper I presents an experimental technique for measuring the deformations the plate undergoes during impact and perforation. This information can be used to validate numerical models and to increase the understanding of how energy is absorbed by the plate. Paper II presents an experimental and numerical investigation on the quasi-static perforation of AA5083-H116 aluminium plates. In the tests, square plates were mounted in a circular frame and penetrated by a cylindrical punch. A full factorial design was used to investigate the effects of varying plate thickness, boundary conditions, punch diameter and nose shape. Based on the obtained results, both the main and interaction effects on the maximum force, displacement at fracture and energy absorption until perforation were determined. The perforation process was then computer analysed using the nonlinear finite element code LS-DYNA. Simulations with axisymmetric elements, brick elements and shell elements were conducted. Slightly modified versions of the Johnson-Cook constitutive relation and fracture criterion were used to model the material behaviour. It was shown that the FEM models were able to predict the trends observed in experiments. Paper III evaluates methods for determination of the anisotropic properties of polycrystalline metallic materials. Four calibration methods were evaluated for the linear transformation-based anisotropic yield function YLD2004-18p (Barlat et al., 2005) and the aluminium alloy AA5083-H116. The different parameter identifications are based on least squares fits to combinations of uniaxial tensile tests in seven directions with respect to the rolling direction, compression (upsetting) tests in the normal direction and stress states found using the full-constraint (FC) Taylor model for 690 evenly distributed strain paths. An elastic-plastic constitutive model based on YLD2004-18p has been implemented in a non-linear finite element code and used in finite element simulations of plane-strain tension tests, shear tests and upsetting tests. The experimental results as well as the Taylor model predictions can be satisfactorily reproduced by the considered yield function. However, the lacking ability of the Taylor model to quantitatively reproduce the experiments calls for more advanced texture models. Paper IV presents an experimental and numerical investigation on low velocity perforation of AA5083-H116 aluminium plates. In the tests, square plates were mounted in a circular frame and penetrated by a cylindrical blunt-nosed projectile. The perforation process was then computer analysed using the nonlinear finite element code LS-DYNA, in order to investigate the effects of anisotropy, dynamic strain aging and thermal softening in low velocity impacts on the present aluminium alloy. Dynamic strain aging has been shown to influence both the predicted force level and fracture, while thermal softening only influences the fracture prediction. No effect of plastic anisotropy was observed

The low temperature crystallization of hydrogenated nitrile butadiene rubber (HNBR)

HNBR was analyzed following low temperature storage (between 0 °C and −50 °C) in order to measure the effects of cold crystallization during exposure close to, and below, the glass transition temperature (Tg). Differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and hardness testing were performed to measure the changes in melting enthalpy, shear stiffness and hardness as a result of low temperature exposure. An increase in crystallinity was measured even when the HNBR was held well below the Tg of the HNBR. Although the degree of crystallinity due to low temperature exposure is estimated to be quite small, a significant increase in hardness was seen after 24 h exposure. The changes in properties due to the presence of “microcrystalline” regions are especially relevant for permanently low temperature applications, since the material properties over longer timescales at low temperatures may deviate significantly from the material properties measured immediately after cooling.acceptedVersio

Numerical/experimental impact events on filament wound composite pressure vessel

Impacts on pressure vessels, produced by winding glass fibre with vinyl ester resin over a polyethylene liner, were numerically and experimentally investigated in the current work. Pressure vessels were experimentally tested under low velocity impact loads. Different locations and incident energies were tested in order to evaluate the induced damage and the capability of the developed numerical model. An advanced 3-D FE model was used for simulating the impact events. It is based on the combined use of interlaminar and intralaminar damage models. Puck and Hashin failure theories were used to evaluate the intralaminar damages (matrix cracking and fibre failure). Cohesive zone theory, by mean of cohesive elements, was used for modelling delamination onset and propagation. The experimental impact curves were accurately predicted by the numerical model for the different impact locations and energies. The overall damages, both intralaminar and interlaminar, were instead slightly over predicted for all the configurations. The model capabilities to simulate the low velocity impact events on the full scale composite structures were proved.acceptedVersio

Influence of strain rate and temperature on the mechanical behaviour of rubber-modified polypropylene and cross-linked polyethylene

acceptedVersio

Material characterisation and failure envelope evaluation of filament wound GFRP and CFRP composite tubes

The full procedure for material characterisation of filament wound composite pipe is reported. Two different typologies of composite were used in order to evaluate the performance of the developed test methodology. Test samples were produced with glass/vinylester and carbon/epoxy in tubular section by filament winding. Split disk and biaxial tests were used to evaluate the basic in plane material properties. A new design for the biaxial test was developed. The end tabs and fixture were made in order to reduce the stress concentration at the edges of the samples and to remove any possibility of sample misalignment. The influence of the sample length as well as the sample preparation was investigated and the best solution reported. Moreover, an innovative optical method was developed for the evaluation of the void content of the produced material. In addition to the basic strength data, the complete failure envelopes in the plane s2 t12 were also evaluated for both materials by the use of the biaxial test procedure here developed. The experimental failure envelopes were also compared with the prediction made with some of the most common failure theories currently available. The results clearly showed the ability of the Puck criterion to accurately predict the failure envelope (especially when torsion plus axial compressive loads were applied to the samples).acceptedVersio

A micromechanical model of fiber bridging including effects of large deflections of the bridging fibers

A micromechanical model of cross-over fiber bridging is developed for the prediction of macroscopic mixed-mode bridging laws (traction-separation laws). The model is based on non-linear beam theory and takes into account debonding between fiber and matrix as well as buckling of fibers in compression. Further, it is shown how failure of the bridging fibers can be taken into account through a Weibull distributed failure strain. Predictions made by the proposed model are compared with predictions made by detailed 3D finite element models, and a very good agreement was observed. It is shown that models based on linear beam theory are only valid for small transverse deflections of the bridging ligament and greatly underestimate the force transferred by ligaments subjected to moderately large deflections. The novel model, on the other hand, is applicable in the entire range where the bridging problem transitions from a beam bending problem to a bar-like problem. Finally, an example of how the proposed model can be used for parameter/sensitivity studies is given. A conclusion from this study is that reducing the fracture toughness, G_c , of the interface between fibers and matrix may lead to increased energy dissipation through cross-over fiber bridging as more fibres remain intact longer.publishedVersio