Recently, microfluidic stretchable electronics has attracted great interest from academia since conductive liquids allow for larger cross-sections when stretched and hence low resistance at longer lengths. However, as a serial process it has suffered from low throughput, and a parallel processing technology is needed for more complex systems and production at low costs. In this work, we demonstrate such a technology to implement microfluidic electronics by stencil printing of a liquid alloy onto a semi-cured polydimethylsiloxane (PDMS) substrate, assembly of rigid active components, encapsulation by pouring uncured PDMS on-top and subsequent curing. The printing showed resolution of 200 μm and linear resistance increase of the liquid conductors when elongated up to 60%. No significant change of resistance was shown for a circuit with one LED after 1000 times of cycling between a 0% and an elongation of 60% every 2 s. A radio frequency identity (RFID) tag was demonstrated using the developed technology, showing that good performance could be maintained well into the radio frequency (RF) range.

The impact of sample size on in-plane strain behavior in paperboard was investigated, with the aim to explore the differences between local and global properties in paperboard, and try to pinpoint the mechanisms behind such differences. The local properties are of interest in converting as well as for future 3D forming of paperboard. It is important to identify differences in behavior between local and global properties since most paperboards are evaluated against the latter. The methods used for evaluation were tensile tests in controlled environment and speckle photography. The results show that there is a difference in strain behavior that is dependent of the length to width ratio of the sample, that this behavior cannot be predicted by standard tensile tests and that it depends on the board composition. The speckle analysis revealed that the behavior is a result of the activation of strain zones in the sample. These zones are relatively constant in size and therefore contribute differently to total strain in samples of different size.

The creasing and folding behavior of three paperboards have been studied both experimentally and numerically. Creasing and folding studies were performed on strips in both the machine direction and the cross machine direction. A finite element model that mimicked the experimental creasing and folding setup was developed, and the creasing and folding behavior could be well predicted for all three paperboards. An experimental characterization scheme consisting of three experiments was proposed, and was shown to be sufficient to predict the creasing and folding behavior. For the whole paperboard the shear strength profiles in the through thickness direction was determined with the notched shear test. Each ply was laid free by grinding, and density measurements and in-plane tension tests were performed on the bottom, middle and top plies of each paperboard. Instead of assuming uniform properties in each ply, the shear strength profiles were used to map the measured properties in the through thickness direction. Numerical simulations were performed when the ply and interface properties of the paperboards were altered to follow different shear strength profiles. This was done in order to mimic different production strategies. It was shown that the interface strengths mainly influenced the folding behavior. Whereas altered the ply properties affected the creasing force needed.

QC 20130815

The thermal response in paper has been studied by thermography. It was observed that an inhomogeneous deformation pattern arose in the paper samples during tensile testing. In the plastic regime a pattern of warmer streaks could be observed in the samples. On the same samples digital image correlation (DIC) was used to study local strain fields. It was concluded that the heat patterns observed by thermography coincided with the deformation patterns observed by DIC. Because of its fibrous network structure, paper has an inhomogeneous micro-structure, which is called formation. It could be shown that the formation was the cause of the inhomogeneous deformations in paper. Finite element simulations was used to show how papers with different degrees of heterogeneity would deform. Creped papers, where the strain at break has been increased, were analysed. For these paper it was seen that an overlaid compaction of the paper was created during the creping process. During tensile testing this was recovered as the paper network structure was strained.

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The present study investigates torsional and compressive loading of a paperboard package. Finite element (FE) analyses simulating the tests were performed to improve understanding of the stresses and deformations in the paperboard during loading. A simple experimental characterization of the necessary material properties could be performed to represent the multi‐ply paperboard as a single‐ply structure. The results from the single‐ply model were compared with a laminate model, and the differences between the models were small. Comparing experimental and FE simulations of box compression and torsion showed that the FE models could accurately predict the response curves. However, in the simulations, there was an overprediction of the maximum compressive force and maximum torque, which was expected since geometrical imperfections and the heterogeneous internal structure of the material were not accounted for in the material model or the FE model. Local yield lines formed at the onset of non‐linearities in the package load–displacement curves. Therefore, the strength of the paperboard affects the maximum compressive strength and maximum torque, and the bending stiffness of the paperboard only had a minor effect. When a first local maximum was reached, the number of FE that reached the failure stress increased exponentially. The simulations also showed that box compression was not an effect of package height, but higher packages had a lower maximum torque.

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