Lars Pilgaard Mikkelsen

Elastocaloric effect of Ni-Ti wire for application in a cooling device

We report on the elastocaloric effect of a superelastic Ni-Ti wire to be used in a cooling device. Initially, each evaluated wire was subjected to 400 loading/unloading training cycles in order to stabilize its superelastic behavior. The wires were trained at different temperatures, which lead to different stabilized superelastic behaviors. The stabilized (trained) wires were further tested isothermally (at low strain-rate) and adiabatically (at high strain-rate) at different temperatures (from 312 K to 342 K). We studied the impact of the training temperature and resulting superelastic behavior on the adiabatic temperature changes. The largest measured adiabatic temperature change during loading was 25 K with a corresponding 21 K change during unloading (at 322 K). A special focus was put on the irreversibilities in the adiabatic temperature changes between loading and unloading. It was shown that there are two sources of the temperature irreversibilities: the hysteresis (and related entropy generation) ...

Post-necking behaviour modelled by a gradient dependent plasticity theory

Abstract The delay of the onset of localization and the post-necking behaviour for stretched thin sheets are determined by three-dimensional effects. Thus, a 2-D finite element analysis based on a local plasticity theory will give a physically unrealistic mesh dependent solution. This, in spite of the fact that the stress state, is essentially two-dimensional. By incorporating a length scale with relation to the thickness of the sheet, it is demonstrated how a 2-D finite element analysis based on a gradient dependent plasticity theory can give a good approximation of the post-necking behaviour. This is illustrated by numerical comparison of results from a full 3-D finite element analysis, with results from a 2-D finite element model based on a finite strain version of a gradient dependent J 2 -flow theory. Some numerical problems in the modeling will be discussed briefly.

The Effect of Silicon Addition on the High Temperature Oxidation of a Fe-Cr Alloy

The effect on the corrosion resistance of adding 0.15 wt.% silicon to a Fe-Cr alloy was examined at 1173 K in an H2/Ar atmosphere containing 0.7% water vapour for 24 h. The oxide scales formed on the Fe-Cr alloys with and without silicon addition consisted of chromia. Furthermore, the addition of silicon resulted in the formation of small distinct silica particles near the alloy/chromia interface. The amount of silicon added to the iron-chromium alloy appeared to be insufficient to form a continuous silica layer beneath the chromia scale under the present experimental conditions. Although the silica particles did not form a continuous silica layer beneath the chromia scale, the growth rate of the chromia scale decreased due to addition of silicon. Different oxidation mechanisms are proposed and discussed in order to explain the reduced growth rate of the chromia scale after the addition of silicon to the alloy.

Necking in rectangular tensile bars approximated by a 2-D gradient dependent plasticity model

Abstract Necking and post-necking of a uniaxially stretched tensile test specimen with a rectangular cross-section is investigated. The analysis is based on an enhanced 2-D plane stress finite element model, where the third dimension (the thickness effect) is taken into account by an incorporated length scale. The length scale, fixed with respect to the current deformed state, is incorporated in the continuum description by a finite strain version of a gradient dependent J 2 -flow theory. A convincing agreement with the true 3-D post-necking behaviour is obtained by this approximative 2-D plane stress model. The model gives a realistic prediction of the development of the post-necking zone in the tensile bar. Furthermore, the model is found to capture the transition from a dominating diffuse necking mode for a tensile bar with a square cross section to a dominating oblique localised necking mode for a wide strip; effects which can not be captured by an analysis based on a conventional (local) plane stress model.

A nonlocal two-dimensional analysis of instabilities in tubes under internal pressure

Abstract Studies of the post-bifurcation behaviour and imperfection-sensitivity of elastic–plastic tubes under internal pressure are carried out. When increasing enclosed volume is prescribed, the first critical bifurcation mode for cylindrical tubes is an axisymmetric non-cylindrical mode occurring shortly after the maximum pressure point. Thereafter, bifurcation into a non-axisymmetric mode occurs, leading to localized necking of the tube wall in the hoop direction. A 2-D membrane analysis based on a conventional (local) plasticity theory shows the bifurcations into the axisymmetric and non-axisymmetric modes. However, a full 3-D analysis is required to give an accurate description of the neck development on one side of the tube wall. If the axial variation in the deformation pattern is neglected, the analysis of neck development is reduced to a 2-D plane strain problem. In the present work, the 3-D post-bifurcation behaviour is approximated by a 2-D nonlocal membrane finite element analysis. The third dimension (the thickness direction) is taken into account in the continuum description by an incorporated length scale based on the current thickness of the tube wall. In the case of post-necking behaviour of stretched thin sheets, this has been found to give a good approximation of the real 3-D post-necking behaviour. Results for the post-bifurcation behaviour of the tube under internal pressure are presented.

A Numerical Axisymmetric Collapse Analysis of Viscoplastic Cylindrical Shells under Axial Compression

Abstract Circularcylindrical shells are frequently used as structural components because of their high strength and their ability to absorb energy during complete structural collapse. Total collapse analyses have mainly been based on experimental work and approaches inspired by this. However, in the last few years, powerful numerical tools have been available and numerical collapse analyses have become more attractive. This paper presents results from an axisymmetric numerical collapse analysis. The analysis is based on a finite rotation shell theory accounting for contact between the shell walls. The strains are assumed to remain small and the shell material is described by an elastic–viscoplastic model. The sensitivity of the collapse behaviour is demonstrated with respect to parameters such as initial imperfections, thickness of the shell, material parameters and rate of deformation. Comparisons between the results numerically obtained and approaches found in the literature are presented. Good agreement was found for the folding length of the developed collapse pattern whereas small differences between the mean crushing loads was observed. Furthermore, it was noted that the developed collapse pattern was strongly dependent on the strain hardening of the material.

On the analysis of viscoplastic buckling

Abstract For elastic-viscoplastic structures the classical elastic-plastic bifurcation approach to inelastic buckling is not valid. Only an elastic bifurcation point exists in the elastic-viscoplastic case, and the inelastic buckling behaviour is controlled by a strong sensitivity to small imperfections. However, in the last few years some papers have been published on an approximation that leads to a so-called viscoplastic bifurcation point. Results of accurate numerical analyses for elasticviscoplastic columns are compared with predictions based on the approximate bifurcation approach. In some cases the bifurcation approach gives a poor approximation of the actual elastic-viscoplastic column behaviour, whereas in other cases the discrepancy is less pronounced. The simple column model gives a clear illustration of the effect, but similar results for plates are also mentioned briefly.

Initiation and Performance of a Coating for Countering Chromium Poisoning in a SOFC-stack

Minimising transport of chromium from the metallic interconnect (e.g. of Crofer 22APU) to the cathode in a planar solid oxide fuel cell is done by application of a coating between the two parts. The coating is applied by slurry coating, and taken through stack initialisation it transforms into a stable and densely grown barrier layer, which minimises both the evaporation of chromium from the interconnect surface and the electrical contact resistance between the interconnect and the cathode. Between comparable stack element tests with and without coatings at 750oC, the degradation rate in terms of power density decreased from 6.5% per 1000h to very low levels (<0.84% per1000 hour) during the first 1000 hours of stack element testing when using such type of coating, indicating that the cathode poisoning from chromium has reached levels interesting for commercial development.

A simplified model predicting the weight of the load carrying beam in a wind turbine blade

Based on a simplified beam model, the loads, stresses and deflections experienced by a wind turbine blade of a given length is estimated. Due to the simplicity of the model used, the model is well suited for work investigating scaling effects of wind turbine blades. Presently, the model is used to predict the weight of the load carrying beam when using glass fibre reinforced polymers, carbon fibre reinforced polymers or an aluminium alloy as the construction material. Thereby, it is found that the weight of a glass fibre wind turbine blade is increased from 0.5 to 33 tons when the blade length grows from 20 to 90 m. In addition, it can be seen that for a blade using glass fibre reinforced polymers, the design is controlled by the deflection and thereby the material stiffness in order to avoid the blade to hit the tower. On the other hand if using aluminium, the design will be controlled by the fatigue resistance in order to making the material survive the 100 to 500 million load cycles experience of the wind turbine blade throughout the lifetime. The aluminium blade is also found to be considerably heavier compared with the composite blades.

Fatigue damage observed non-destructively in fibre composite coupon test specimens by X-ray CT

This study presents a method for monitoring the 3D fatigue damage progression on a micro-structural level in a glass fibre/polymer coupon test specimen by means of laboratory X-ray Computed Tomography (CT). A modified mount and holder made for the standard test samples to fit into the X-ray CT scanner along with a tension clamp solution is presented. Initially, the same location of the test specimen is inspected by ex-situ X-ray CT during the fatigue loading history, which shows the damage progression on a micro-structural level. The openings of individual uni-directional (UD) fibre fractures are seen to generally increase with the number of cycles, and new regions of UD fibre fractures also appear. There are some UD fibre fractures that are difficult to detect since their opening is small. Therefore, the effect of tension on the crack visibility is examined afterwards using a tension clamp solution. With applied tension some additional cracks become visible and the openings of fibre fractures increases, which shows the importance of applied tension during the scan.