In this dissertation, theoretical, computational, and experimental methodologies are introduced to determine the rate-dependent material properties of the brain tissue. Experiments have shown that the brain tissue is significantly rate-dependent. To examine the range of strain rates at which trauma might happen, a validated finite element (FE) human head model was initially employed to examine the biomechanics and dynamic behavior of the head and brain under impact and blast loads. The strain rates to cause traumatic brain injury (TBI) were found to be in the range of 36 to 241 1/s, under these types of loadings. These findings provided a good estimation prior to exploring the required experiments for characterizing the brain tissue. The brain samples were tested by employing unconfined compression tests at three different deformation rates of 10 (n= 10 brain samples), 100 (n=8), and 1000 mm/sec (n=12). It was found that the tissue exhibited a significant rate-dependent behavior with various compression rates. Two different material characterization approaches were proposed to evaluate the rate-dependent mechanical responses of the brain. In the first approach, based on the parallel rheological framework, a single-phase viscoelastic model which captures the key aspects of the rate-dependency in large strain behavior was introduced. The extracted material parameters showed an excellent constitutive representation of tissue response in comparison with the experimental test results (R^2=0.999). The obtained material parameters were employed in the FE simulations of the brain tissue and successfully verified by the experimental results. In the second approach, the brain tissue is modeled as a biphasic continuum, consisting of a compressible solid matrix fully saturated with an incompressible interstitial fluid. The governing equations based on conservation of mass and momentum are used to describe the solid-fluid interactions. This viscoelastic biphasic model can effectively estimate the rate-dependent tissue deformations, the hydrostatic pressure as well as fluid diffusion through the tissue. Although both single-phasic, as well as bi-phasic models, can successfully capture the key aspects of the rate-dependency in large strain deformation, it was shown the biphasic model can demystify more phenomenological behavior of this tissue that could not be perceived with yet established, single-phasic approaches.

The project elaborated in frame of engineering studies branch M-STG describe characteristics of plasticity of non-anneal materials E 235, E 190 and E 220. Materials characteristic was receive by static and dynamical tests. Based on the literature study it was conducted survey of the current state of experimental techniques for high-speed deformation. The materials were designed by Holomon approximation of rigid-plastic material model without hardening. It was conducted force analyses for machine Unison MG 2790 for rewind bending and bend with compressive force considering the identified material model. It was describe the impact passive and active forces to move the neutral axis.

The Constant Rate of Strain (CRS) test is currently the most widely used method for determination of consolidation parameters in Sweden. These parameters are used to calculate the probable settlements and behavior of soils. With the Swedish standard strain rate, the duration of a single the test is about 24h, but a higher strain rate would allow for more tests to be performed in the same amount of time. For all clays in Sweden, the Swedish standard for the CRS test suggests a fixed rate of strain that is independent of soil properties, while the North American standard proposes a strain rate that generates a pore pressure ratio of 3-15%. Soil properties such as water content, liquid limit, sensitivity and shear strength vary greatly between different regions of Sweden. It would be beneficial if these properties could be used to find the ideal strain rate for the CRS test. Performing the tests at a higher strain rate, and thus performing more tests within a shorter amount of time, would save both time and money. In this report, 24 CRS tests are performed on three different clays with distinctive properties. Each clay is tested with three different strain rates: the Swedish standard rate of 0.680%/h, a higher rate of 3.00%/h and a lower rate of 0.154%/h. The results are evaluated according to both standards, and are compared and analyzed to determine whether there is any indication that the strain rate can be selected based on the soil properties. The results indicate that the selection of the strain rate is dependent on the soil properties. In addition, the majority of the tests can be conducted with higher strain rates than what is required by the Swedish standard and still manage to lie within the 3-15% limit of the pore pressure ratio, which is acceptable for the North American standard. However, the preconsolidation pressure does rise with increased strain rates, which can generate problems and erroneous results when calculating the settlements. A more extensive testing program with CRS tests and full-scale field tests must be carried out before any recommendation of a higher strain rate can be made.

Modelling and simulation of the human body during an impact situation such as a car accident, can lead to better designed safety features on vehicles. In order to achieve this, investigation into the material properties and the creation of a numerical model of cortical bone is needed. One approach to creating a material model of cortical bone suitable for these situations is to describe the material model as visco-elastic, as reported by Shim et al. [1], Bekker et al. [2] and Cloete et al. [3]. The work by Shim et al. and Bekker et al. developed three-dimensional models, but do not accurately capture the transition in behaviour in the intermediate strain rate region, while Cloete et al. developed a phenomenological model which captures the intermediate strain rate behaviour in one dimension. This work aims to verify and extend these models. The intermediate strain rate regime (1 s−1 to 100 s−1 ) is of particular interest because it is a key characteristic of the behaviour of cortical bone and several studies have been conducted to gather experimental data in this region [3, 4, 5, 6]. The behaviour can be captured using non-linear viscoelastic models. This dissertation focuses on the development and implementation of a material model of cortical bone based on non-linear visco-elastic models to capture the intermediate strain rate regime behaviour. The material model was developed using uni-axial test results from cortical bone. The model by Cloete et al. has been improved and extended, and issues of local and global strain rate with regards to the viscosity have been clarified. A hereditary integral approach was taken in the analysis and implementation of discrete models and was found to be consistent with mathematical models. The model developed was extended to three dimensions in a manner similar to that of Shim et al. and Bekker et al. for implementation in commercial finite element software (LS-Dyna and Abaqus).

Antarctic mass balance is mainly controlled by surface mass balance (SMB, i.e. the net effect of precipitations at the surface of the ice sheet) and ice discharge at its margins, mostly through ice shelves. These floating ice bodies made from ice flowing from the continent to the ocean are buttressed by ice rises (elevation of the sea floor on which ice shelf re-grounds) such as the Derwael Ice Rise (DIR) in Dronning Maud Land (DML). In addition to this role important to consider in the future contribution of Antarctica to sea level rise, ice rises are also “climate dipsticks” helping to reconstruct the climate of the past centuries to millennia at high resolution. Due to their coastal location, they witness the changes happening there more rapidly than inland. Furthermore, their internal stratigraphy forms arches that allow to assess their stability, to date their own formation and therefore, in some cases, to constrain the past extension of the ice sheet at the scale of several millennia. As part of the IceCon project :Constraining ice mass changes in Antarctica, this thesis aimed to drill a 120 m ice core (named IC12 for the IceCon project, 2012) at the divide of the DIR and perform physico-chemical analyses to study its density and its internal annual layering with the aim of reconstructing SMB of the last two centuries. We also recorded a virtual image of the borehole using an optical televiewer (OPTV) to assess the ability of this instrument to reconstruct a density profile and measure vertical strain rates when the logging is repeated in the same borehole after a sufficient period of time (here, 2 years).The results show a general increase in snow accumulation rates (SMB) of 30-40% during the 20th century, particularly marked during the last 20-50 years. SMB variability is governed to a large extent by atmospheric circulation and to a lesser extent by variations in sea ice cover. The vertical velocity profile measured from repeat borehole OPTV was applied to refine SMB correction and the results fall in the error range of the corrections made using a model previously developed to study the DIR’s stability. This thesis also contributed to characterizing the spatial variability of SMB across the DIR by dating internal reflection horizons (IRHs), former surfaces of the DIR buried under subsequent snow layers and detected using radio-echo-sounding, and by measuring the density profile of IC12. SMB is found to be 2.5 times higher on the upwind slope than on the downwind slope due to the orographic effect. This pattern is regularly observed on ice rises in DML and stresses the importance of adopting a sufficient spatial resolution (5 km) in climate models.Finally, the technical developments allowing to rapidly reconstruct a density profile from the OPTV image of a borehole contributed to improving our knowledge of two features of Antarctic ice shelves, namely melt ponds, influencing surface mass balance and subglacial channels, influencing basal mass balance. Specifically, the results show that density is 5 % higher in surface trenches associated with subglacial channels, and that ice below melt ponds can reach the density of bubble-free ice due to melting and refreezing processes, with implications on ice shelf viscosity. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The main intention of this thesis work was to investigate the effect of strain rate on continuum and pre-cracked polymer failure. Low-Density Polyethylene (LDPE) was chosen to study experimentally and numerically. In order to cover wide range of strain rates, four specific strain rates were selected for the uniaxial tensile tests. To perform the tests, cyclic loading and unloading with relaxation was utilized in the room temperature for continuum specimen and for pre-cracked specimen monotonic tensile test till failure was utilized. Through Digital Image Correlation (DIC) the local strain distribution was assessed through the specimen and the deformation was compared with simulation results. Based on the extensive literature review of material models from PolyUMod library among Viscoplastic models, the Three Network Viscoplastic (TNV) model was selected to proceed with the calibration. The motivation behind choosing TNV model is it's capability of capturing load-unload curves, different strain rates as well as non-linear responses. Furthermore, it was seen that among Viscoplastic models, TNV has the lowest average errors which plays a vital role in this case as the accuracy of FE simulation directly depends on the calibration results. From the experimental results it was safe to say that with increasing strain rates LDPE films tend to get stiffer and stronger both in continuum and pre-cracked. Through the calibration it was seen that the predicted curves were in reasonable agreement with experimental ones. Hence,the calibrated model was exported as python script into Abaqus CAE to perform the simulations. The comparison was done and discussed in details between the simulation and experimental data in three orientations; MD (Machine Direction), CD (Cross Direction) and 45 direction.

This research developed the quasi-static and dynamic equipment and protocols for tests of both Very-High-Strength Concrete (VHSC) and High-Strength High-Ductility Concrete (HSHDC) to predict blast performance. VHSC was developed for high compressive strength (> 200 MPa). Using VHSC as the baseline material, HSHDC was developed and exhibits comparable compressive strength (> 150 MPa) and high tensile ductility (> 3% tensile strain). This research investigated quasi-static material properties including compression, tension, and flexure (third-point and pressure loadings). Additionally, dynamic blast load simulator (shock tube) tests were performed on simply-supported one-way panels in flexure. Subsequently, the material response in flexure was predicted using the Wall Analysis Code (WAC). Although VHSC has a higher peak flexural strength capacity, HSHDC exhibits higher ductility through multiple parallel micro-cracks transverse to loading. The equipment and test protocols proved to be successful in providing ways to test scaled concrete specimens quasi-statically and dynamically.

The present study develops a phenomenological adaptation to an internal state variable (ISV) model that incorporates the influence of dynamic recrystallization (DRX) in a material’s evolving microstructure and flow stress response. During metal forming and joining processes that promote internal heat distributions and large strains, microstructural processes often occur that result in a transformation of the evolving microstructure away from the base distribution. In an effort to lower the stored energy accumulated in the material’s lattice and grain structure, the deformed material may undergo a type of dynamic recovery process, such as DRX. In this study, the ISV model’s flow stress output is modified to include a phenomenological DRX softening and hardening term internal to the isotropic hardening rate ISV. The flow stress thus directly includes the influence of microstructure evolution. The evolving grain size is modeled such that an inverse relation exists between strain hardening and average grain size.

The thesis focus on simulating one-dimensional flame subjected to a microwave and nanosecond pulse. We modified open-source codes Cantera and Ember to perform one-dimensional flame simulations for steady and unsteady state, respectively. Our model is computationally efficient to perform simulations in a range of parameters such as electric field strength, flow strain rate, and pulse repetitive frequency. Our model for the one-dimensional flame subjected to a microwave predicted flame speed enhancement more accurately than the previous studies. <br>

No description available.