| 1. |
- Grassl, Peter, 1974, et al.
(författare)
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A damage-plasticity model for the dynamic failure of concrete
- 2011
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Ingår i: 8th International Conference on Structural Dynamics, Leuven, Belgium.
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Konferensbidrag (refereegranskat)abstract
- A constitutive model based on the combination of damage mechanics and plasticity is developed to analyse concretestructures subjected to dynamic loading. The aim is to obtain a model, which requires input parameters with clear physicalmeanings. The model should describe the important characteristics of concrete subjected to multiaxial and rate-depending loading.This is achieved by combining an effective stress based plasticity model with an isotropic damage model based on plastic andelastic strain measures. The model response in tension, uni-, bi- and tri-axial compression is compared to experimental results inthe literature.
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| 2. |
- Grassl, Peter, 1974, et al.
(författare)
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CDPM2: A damage-plasticity approach to modelling the failure of concrete
- 2013
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683. ; 50:24, s. 3805-3816
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Tidskriftsartikel (refereegranskat)abstract
- A constitutive model based on the combination of damage mechanics and plasticity is developed to analyse the failure of concrete structures. The aim is to obtain a model, which describes the important characteristics of the failure process of concrete subjected to multiaxial loading. This is achieved by combining an effective stress based plasticity model with a damage model based on plastic and elastic strain measures. The model response in tension, uni-, bi- and triaxial compression is compared to experimental results. The model describes well the increase in strength and displacement capacity for increasing confinement levels. Furthermore, the model is applied to the structural analyses of tensile and compressive failure.
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| 3. |
- Nemeth, Ulrika, 1968-
(författare)
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Det kritiska uppdraget : Diskurser och praktiker i gymnasieskolans svenskundervisning
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of the study is to investigate constructions and discourses of criticality, and how these are related to teachers’ conceived subject of Swedish concerning the development of students’ critical consciousness. The study is qualitative, and is based on three different materials: focus talks between teachers, international and national policy documents and frameworks, and classroom observations. The theoretical framework combines Bernstein’s code theory (classification framing, elaborated and restricted code, knowledge structures) and critical literacy as outlined by Freire, particularly the concept of critical transitivity. The analytical framework combines tools from systemic functional linguistics (transitivity, modality) and van Leeuwen’s discourse analysis (actors, activities and legitimations), where discourses are seen as recontextualizations of social practices. The research questions posed are:How are educational, critical practices constructed as discourses in international policy documents and frameworks and in the national curriculum for the subject of Swedish, and how do they relate to the critical task of the subject of Swedish?How do teachers of Swedish construct the critical task in relation to their conceptions of the nucleus and boundaries of the subject, and of its knowledge practices?Which critical textual practices are or could be shaped within the subject of Swedish?Analyses of policy documents show that high-status actors (OECD and EU) and the Swedish national curriculum constructions of the critical are legitimated by unquestionable reality claims, and are associated with clusters of discourses as aspects of competences and skills. The analysis of the focus talks and the classroom instruction shows that some of these discourses are included in conceptions and legitimations of the critical. Some of these are also included in teachers’ discourses of the critical tasks. These tasks are mostly seen as external and are mainly realized by unreflected doing, and are therefore not part of teachers’ intrinsic conceptions of what criticality in the subject of Swedish should embrace. Teachers’ intrinsic critical task is only enacted as an exception, turning the critical task into a wished one. The exceptions, however, seem to be realized in generally strongly framed practices, where students “see” things based on Swedish subject knowledge. The results indicate that the subject of Swedish might provide specific ways to move towards critical transitivity, and a fruitful encounter of such a transitivity and Bernstein’s vertical discourse.
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| 4. |
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| 5. |
- Nyström, Ulrika, 1981, et al.
(författare)
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Comparative numerical studies of projectile impacts on plain and steel-fibre reinforced concrete
- 2011
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X. ; 38:2-3, s. 95-105
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Tidskriftsartikel (refereegranskat)abstract
- The enhanced energy absorption characteristics of fibre-reinforced concrete, compared to plain concrete, has in experimentally studies been shown to improve the projectile resistance and motivate its increased usage in protective structures. However, the high cost of undertaking experiments and the high parameter variation and dependency of the experimental setups and results, respectively, make it difficult to draw generic conclusions of how the addition and increased amount of fibres affects the local damage caused by projectile impact, which motivates the use of numerical simulations to study this. The numerical hydrocode AUTODYN was used in a qualitative study of how the addition of different amounts of fibres, modelled as different post-crack relations, influence the depth of penetration and crater formation on the front and rear face of a concrete target. Fibres added to the concrete mix had a minor influence on the depth of penetration while the crater size on both front and rear faces of the target decreases. The crack propagation beyond the crater on the front face was also reduced when fibres were added to the concrete. An increased amount of fibres in the concrete showed no effect on the size of the front-face crater, but led to further decreased size of the crater on the rear face of the concrete cylinder. It is concluded that the scabbing crater can be reduced in size and prevented by usage of fibre-reinforced concrete even if the depth of penetration is only slightly less than to penetration depth in plain concrete.
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| 6. |
- Nyström, Ulrika, 1981
(författare)
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Concrete Structures Subjected to Blast and Fragment Impacts - Numerical Simulations of Reinforced and Fibre-reinforced Concrete
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Concrete is widely used in design of protective structures due to its good energy-absorbing characteristics under high pressures and, when properly reinforced, ductile behaviour. Nevertheless, the response of concrete structures subjected to severe dynamic loading differs from their static behaviour, on a structural level but also on a material level. The addition of steel fibres in the concrete may improve the energy-absorbing characteristics of plain concrete, which is especially true for the tensile behaviour. The fracture energy for steel-fibre reinforced concrete may be many times higher already for low dosages of fibres compared to plain concrete. In design of protective structures it is important to identify the possible threats and their risk of occurrence to be able to characterise the design loads. Often this involves the effects of cased charges, i.e. combined blast and fragment loading. While the structural behaviour for blast load and single fragment impacts is relatively well understood, the response under combined loading, including the blast and multiple impacts of fragments, is not yet clear.The theoretical bases for concrete material behaviour, weapon load characteristics, and their effect on the structural response are treated in this licentiate thesis. In addition, three numerical studies are presented, whose aim is to increase the understanding of impact and impulsive loading and the subsequent response of a concrete element. The first numerical study was a comparative investigation of the relative effect on the impact resistance when adding steel fibres to concrete. It was concluded that the depth of penetration of the striking projectile was only slightly influenced by the addition of fibres, while the sizes of the front- and rear-face craters were decreased. The second numerical study involved combined blast and fragment loading of a reinforced concrete wall strip, and it was seen that the total damage of the wall strip subjected to the combined loads was highly related to the damage caused by the fragment impact alone. Furthermore, the mid-point deflection in combined loading was larger than the sum of mid-point deflections in blast and fragment loading, indicating synergetic effects of the two loads. In the third numerical study the effect of reinforcement on the projectile impact resistance was studied. It was concluded that the presence of reinforcement may improve the impact resistance of the concrete if a suitable reinforcement detailing is used.
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| 7. |
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| 8. |
- Nyström, Ulrika, 1981
(författare)
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Modelling of Concrete Structures Subjected to Blast and Fragment Loading
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Betong är tack vare sin låga kostnad och sitt stora användningsområde ett av de mest använda byggmaterialen i modern tid. Dess stora energiabsorberande förmåga vid höga tryck, samt det sega beteende betongkonstruktioner har vid väl utformad armering, gör betongen också lämplig i skyddande konstruktioner. Responsen hos konstruktioner utsatta för extrem dynamisk belastning, så som stötvågs- och splitterbelastning, är dock förknippad med hög komplexitet och trots många år av forskning världen över är de fenomen som uppstår ännu inte helt klarlagda.Med målsättningen att öka kunskapen om betongkonstruktioners respons vid stötvågs- och splitterbelastning har litteraturstudier, teoretiska analyser och numeriska simuleringar använts på ett iterativt sätt. Den teoretiska grunden för betongmaterialets beteende, karaktärisering av stötvågs- och splitterlaster, lasternas verkan på betongkonstruktioner och dynamiska aspekter på vågpropagering behandlas i sammanläggningsdelen i denna avhandling. Därtill presenteras två numeriska studier. Den första numeriska studien var en jämförande studie av vilken relativ inverkan som tillsättning av stålfibrer i betong har på dess motståndsförmåga mot projektilinträngning. Det kunde konstateras att inträngningsdjupet var relativt opåverkat av fibertillsättningen medan kraterstorleken, på både fram och baksidan av den beskjutna kroppen, minskade. I den andra studien undersöktes responsen hos en armerad betongväggsstrimla vid kombinerad belastning av stötvåg och splitter. Det kunde konstateras att den totala skadan i väggelementet var starkt relaterat till skadan orsakad av enbart splitterbelastningen. Vidare framgick att väggstrimlans mittnedböjning var större för det kombinerade lastfallet än för den sammanlagda nedböjningen orsakad av stötvågs- respektive splitterbelastningarna separat. Detta tyder på en synergieffekt för de kombinerade lasterna. En tredje numerisk studie har utförts inom ramen för det presenterade arbetet. I denna studie har armeringens inverkan på betongens motståndsförmåga vid projektilbeskjutning undersökts. Slutsatsen drogs att armering kan ha viss betydelse för detta motstånd förutsatt att armeringen är lämpligt inlagd.I avhandlingen presenteras även en ny materialmodell (CDPM2) för dynamiskt belastad betong. Modellen har visat god förmåga att beskriva responsen vid ett stort antal lastfall, statiska så väl som dynamiska. I utvärdering av modellen har det visats att genom vissa modifieringar skulle modellens förmåga att beskriva responsen vid stora dynamiska laster kunna förbättras. Vissa riktlinjer för möjliga modifieringar har presenterats, men implementeringen och den vidare utvärderingen av dessa är inte en del av det presenterade arbetet.
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| 9. |
- Nyström, Ulrika, 1981, et al.
(författare)
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Numerical Studies of Projectile Impacts on Reinforced Concrete
- 2006
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Ingår i: Proceedings of the 2nd International Conference on Design and Analysis of Protective Structures 2006, 13-15 November 2006, Singapore / Fan S. C., Chua H. K.. - 9810570007 ; , s. 310-319
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Konferensbidrag (refereegranskat)
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| 10. |
- Nyström, Ulrika, 1981, et al.
(författare)
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Numerical studies of the combined effects of blast and fragment loading
- 2009
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X. ; 36:8, s. 995-1005
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Tidskriftsartikel (refereegranskat)abstract
- The well-known synergetic effect of blast and fragment loading, observed in numerous experiments, is often pointed out in design manuals for protective structures. However, since this synergetic effect is not well understood it is often not taken into account, or is treated in a very simplified manner in the design process itself. A numerical-simulation tool has been used to further study the combined blast and fragment loading effects on a reinforced concrete wall. Simulations of the response of a wall strip subjected to blast loading, fragment loading, and combined blast and fragment loading were conducted and the results were compared. Most damage caused by the impact of fragments occurred within the first 0.2 ms after fragments' arrival, and in the case of fragment loading (both alone and combined with blast) the number of flexural cracks formed was larger than in the case of blast loading alone. The overall damage of the wall strip subjected to combined loading was more severe than if adding the damages caused by blast and fragment loading treated separately, which also indicates the synergetic effect of the combined loading. (C) 2009 Elsevier Ltd. All rights reserved.
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