| 1. |
|
|
| 2. |
- Fredriksson, Per, 1976-
(författare)
-
Modelling and simulation of plastic deformation on small scales : interface conditions and size effects of thin films
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Contrary to elastic deformation, plastic deformation of crystalline materials, such as metals, is size-dependent. Most commonly, this phenomenon is present but unnoticed, such as the effect of microstructural length scales. The grain size in metallic materials is a length scale that affects material parameters such as yield stress and hardening moduli. In addition, several experiments performed in recent years on specimens with geometrical dimensions on the micron scale have shown that these dimensions also influence the mechanical behaviour. The work presented in this thesis involves continuum modelling and simulation of size-dependent plastic deformation, with emphasis on thin films and the formulation of interface conditions.A recently published strain gradient plasticity framework for isotropic materials [Gudmundson, P., 2004. A unified treatment of strain gradient plasticity. Journal of the Mechanics and Physics of Solids 52, 1379-1406] is used as a basis for the work. The theory is higher-order in the sense that additional boundary conditions are required and, as a consequence, higher-order stresses appear in the theory. For dimensional consistency, length scale parameters enter the theory, which is not the case for conventional plasticity theory. In Paper A and B, interface conditions are formulated in terms of a surface energy. The surface energy is assumed to depend on the plastic strain state at the interface and different functional forms are investigated. Numerical results are generated with the finite element method and it is found that this type of interface condition can capture the boundary layers that develop at the substrate interface in thin films. Size-effects are captured in the hardening behaviour as well as the yield strength. In addition, it is shown that there is an equivalence between a surface energy varying linearly in plastic strain and a viscoplastic interface law for monotonous loading.In paper C, a framework of finite element equations is formulated, of which a plane strain version is implemented in a commercial finite element program. Results are presented for an idealized problem of a metal matrix composite and several element types are examined numerically. In paper D, the implementation is used in a numerical study of wedge indentation of a thin film on an elastic substrate. Several trends that have been observed experimentally are captured in the theoretical predictions. Increased hardness at shallow depths due to gradient effects as well as increased hardness at more significant depths due to the presence of the substrate are found. It is shown that the hardening behaviour of the film has a large impact on the substrate effect and that either pile-up or sink-in deformation modes may be obtained depending on the material length scale parameter. Finally, it is qualitatively demonstrated that the substrate compliance has a significant effect on the calculated hardness of the film.
|
|
| 3. |
|
|
| 4. |
- Johansson, Niclas, 1976-
(författare)
-
Neonatal Exposure to Highly Brominated Diphenyl Ethers and Perfluorinated Compounds : Developmental Dependent Toxicity and Interaction
- 2009
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigated the developmental neurotoxic effects of neonatal exposure to highly brominated diphenyl ethers (PBDEs) and perfluorinated compounds (PFCs), alone or in combinations, during a critical period of the brains’ rapid growth and development, in mice. The compounds investigated were the decaBDE (PBDE 209), nonaBDE (PBDE 206), octaBDE (PBDE 203), heptaBDE (PBDE 183), and three PFCs, PFOS, PFOA, and PFDA. PBDEs and PFCs have been identified as emerging classes of persistent environmental compounds, present in wildlife as well as humans, and present at higher levels in infants/children, compared to older persons. Individuals can be exposed to these compounds throughout her/his lifetime and newborn/children can be exposed to toxicants both via the mothers’ milk and directly via ingestion and inhalation. The brain growth spurt (BGS) is defined by rapid growth and developmental of the brain. For rodents (mice and rats), the BGS is postnatal spanning the first 3-4 weeks after birth. In humans this period begins during the third trimester of pregnancy and continues throughout the first two years of life. It has been shown that several environmental toxicants can induce permanent disorders in brain function when administered to the neonatal mouse, during the BGS. This thesis shows that highly brominated PBDEs, including PBDE 209, PBDE 206, and PBDE 203 can cause developmental neurotoxic effects, when given directly to the neonatal mouse. Of the investigated PFCs, PFOS and PFOA were shown to cause similar effects as the PBDEs. Furthermore, PBDE 209 and PFOA can at low doses interact and enhance the neurotoxic effects in mice. Effects in the adult animal included; deranged spontaneous behavior, reduced or lack of habituation, decreased learning and memory abilities, and increased susceptibility of the cholinergic system. Both classes of compounds were shown to affect proteins (CaMKII, GAP-43, synaptophysin, and tau) important for neuronal growth and synaptogenesis in the neonatal mouse brain.
|
|
| 5. |
- Johansson, Niclas, 1976-, et al.
(författare)
-
Neontatal exposure to deca-brominated diphenyl ether (PBDE 209) causes dose-response changes in spontaneous behaviour and cholinergic susceptibility in adult mice
- 2008
-
Ingår i: Neurotoxicology. - 0161-813X .- 1872-9711. ; 29:6, s. 911-919
-
Tidskriftsartikel (refereegranskat)abstract
- Polybrominated diphenyl ethers (PBDEs), used as additive flame-retardants, are increasing in the environment and are present in human mother's milk, newborns and toddlers. We reported earlier that several PBDEs, highly brominated PBDEs, caused developmental neurotoxic effects in mice, manifested as persistent aberrations in spontaneous behaviour, habituation capability, learning and memory, and changes in the cholinergic system.The present study was undertaken to explore the dose–response effects of PBDE 209 on spontaneous behaviour, habituation and its effects on the murine cholinergic system. Neonatal male NMRI mice were given 1.4, 2.3, 14 or 21 μmol PBDE 209/kg body weight, when 3 days old. The agent was administered as a single oral dose via a metal gastric tube. Spontaneous behaviour and response to the cholinergic agonist nicotine were observed in adult mice at 2 and 4 months of age. Mice were also observed for anxiety-like behaviour in an elevated plus-maze. Adult mice, 2 and 4 months old, showed a dose–response related change in spontaneous behaviour, viz. were hyperactive and showed reduced or lack of habituation, effects that worsen with age. At the adult age of 4 months the susceptibility of the cholinergic system was also affected in a dose–response related manner, viz. reduced and/or hypoactive response to nicotine. This shows that PBDE 209 can be as potent as the lower brominated PBDEs in causing developmental neurotoxic defects.
|
|
