In the dry-forming process, paper pulp is formed without adding water, making it more resource-effective than traditional papermaking. It is a relatively new technology, patented only in recent years, and very few material investigations exist in the literature; hence, little is known of the constitutive behaviour. The stress state during forming is highly complex, including multiaxial loading, extreme densification, friction, large strains, and fibre-joint formation. This paper studies dry-formed materials at different compression levels, from the sparse mat to the highly densified network. Three primary loading modes are investigated: in-plane tension, out-of-plane shear and out-of-plane compression. The results indicate that the tensile modulus and strength scale quadratically and cubically to the density, respectively, while the shear properties start developing after the density passes a threshold value. The compressive properties proved difficult to quantify, mainly because of the discrepancy between the density before and after the compressive test. The dry-formed material was compared to wet-formed paper materials in the literature. This showed that the in-plane (tensile) properties and the out-of-plane shear strength are visibly lower while the shear stiffness is similar, compared to wet-formed materials. Nonetheless, the findings set a starting point for numerical simulations of the dry-forming process.

During the manufacturing of a power transformer, typically, a fixed clamping pressure is exerted on the winding structure to keep its mechanical integrity during short circuit faults. Various factors, e.g., breathing during load cycles, poor sealing, aging of cellulose materials, etc., introduce moisture in the transformer during its long service life. A typical power transformer operation has a continuous moisture exchange between the cellulose material and the oil. The pressboard samples and controllable heated brass plates are stacked in a test rig. This paper investigates the effect of moisture and load variation on the development of clamping pressure in the test rig. Pressboard samples with two different moisture contents are studied: dry (0.6%) and wet (4.7%). Two different heating cycles are investigated: a constant load and a frequent start-stop load. The temperature of the brass plate and oil, relative humidity of oil and pressboard, and clamping force are measured throughout the experiment. It has been observed that the clamping pressure follows the temperature profile of the brass plates. A significant loss of clamping pressure is observed after mounting the wet samples in the rig. The clamping pressure drops further after the first heating cycle as the rig temperature returns to the ambient. However, when the rig returns to room temperature, there is no noticeable decrease in clamping pressure, neither for the dry sample nor for the wet sample after its first heating cycle.

In this work we have investigated the effect of surface modification of fibres on the overall mechanical properties of high-density papers. Paper sheets were prepared by a combination of heat-pressing and polyelectrolyte Layer-by-Layer (LbL) modification of different softwood fibres. LbLs of Polyallylamine Hydrochloride (PAH) and Hyaluronic Acid (HA) were adsorbed onto unbleached kraft fibres and bleached Chemo-ThermoMechanical Pulp (CTMP) to improve the strength of the fibre–fibre joints in papers made from these fibres. Additionally, different sheet-making procedures were used to prepare a range of network densities with different degrees of fibre–fibre interaction in the system. The results demonstrate that interfacial adhesion within fibre–fibre joints plays a pivotal role in the network's performance, even at higher paper densities. Hygroexpansion measurements and fracture zone imaging with Scanning Electron Microscopy (SEM) further support the claim that stronger interactions between the fibres allow for a better utilisation of the inherent fibre properties. Surface treatments and network densification significantly improved the paper sheets' mechanical properties. Specifically, LbL-treatments alone increased specific stiffness up to 60% and specific strength by over 100%. This improvement is linked to the build-up of residual stresses during drying. Due to a high interaction between the fibres during water removal the fibres become constrained, leading to increased stretching of fibre segments. Strengthened fibre joints intensify this constraint, further increasing the stretch and, consequently, the paper's strength.

Cracks oriented in the toughest direction across the grain of wood (0°) tend to deflect at 90° to the precrack rather than extending in 0° direction. Fracture toughness data across the grain are therefore difficult to interpret. Crack growth mechanisms and effects from replacing wood pore space with a polymer are investigated. Crack growth is analyzed in four-point bending fracture mechanics specimens of birch and two different polymer-filled birch composites using strain-field measurements and finite element analysis (FEA). Calibrated cohesive zone models in both precrack and 90°-directions describe fracture process zone properties in orthotropic FEA-models. Conditions for 0° crack penetration versus 90° crack deflection are analyzed based on cohesive zone properties. Stable, subcritical crack deflection takes place at low load, reduces crack tip stress concentration, and contributes to high structural toughness, provided the 90° toughness is not too low. Polymer-filled neat birch composites have the best structural toughness properties in the present investigation, since 90° toughness is not compromised by any chemical treatment.

Adhesion is crucial for the development of mechanical properties in fibre-network materials, such as paper or other cellulose fibre biocomposites. The stress transfer within the network is possible through the fibre-fibre joints, which develop their strength during drying. Model surfaces are useful for studying the adhesive strength of joints by excluding other parameters influencing global performance, such as geometry, fibre fibrillation, or surface roughness. Here, a numerical model describes the development of adhesion between a cellulose bead and a rigid surface using an axisymmetric formulation, including moisture diffusion, hygroexpansion, and cohesive surfaces. It is useful for studying the development of stresses during drying. A calibration of model parameters against previously published contact and geometry measurements shows that the model can replicate the observed behaviour. A parameter study shows the influence of cohesive and material parameters on the contact area. The developed model opens possibilities for further studies on model surfaces, with quantification of the adhesion during pull-off measurements.

We investigate the compressive failure mechanisms in flax fiber composites, a promising eco-friendly alternative to synthetic composite materials, both numerically and experimentally, and explain their low compressive-compared-to-tensile strength, the compressive-to-tensile strength ratio being 0.28 -0.6. We present a novel thermodynamically consistent continuum damage micromechanics model capturing events on the fiber-matrix scale. It describes the microstructure of a unidirectional composite and includes the instantaneous constitutive behavior of matrix and fibers. We show that flax fibers behave as elastic-plastic-damaged solids in compression. Furthermore, we show that fiber damage plays an utmost role in the compressive failure of flax fiber composites - it is a major determinant of the material's compressive stress-strain response. Using X-ray Computed Tomography (XCT) and Scanning Electron Microscopy (SEM), we identify the fiber damage as intra-technical fiber splitting and elementary fiber crushing. Due to micro -structural similarities among natural fibers, the same micro-mechanisms are likely to appear in other bio-based fibers and their composites.

Fracture toughness and mechanisms of crack growth are characterized for transparent wood polymer biocomposites and compared to native wood, with the crack normal to the fiber direction (LT fracture plane). Side-grooved specimen geometries generated pure mode I crack growth, whereas previous investigations commonly report 90° crack path deflection. Crack growth micromechanisms were analyzed by experimental fracture tests and in-situ microscopy observations. Large damage zones around the crack tip with fiber bundle bridging and pull-out were observed in the crack wake, justifying more advanced cohesive zone modeling suitable for composite materials design. The polymer matrix resulted in much higher fracture energy of the biocomposites compared to native wood due to increased local cohesive strength. This strength increased from the polymer contribution and more homogeneous stress distribution in the wood fibers.

We introduce a three-dimensional geometrically nonlinear Reissner beam theory with embedded strong discontinuities for the modeling of failure in structures and discuss its finite element implementation. Existing embedded beam theories are geometrically linear or two-dimensional, motivating the need for the present work. We propose a geometrically nonlinear beam theory that accounts for cracks through displacement discontinuities in 3D. To represent the three modes of fracture inside an element, we enrich each displacement field with an incompatible mode parameter in the form of a jump, an additional degree of freedom. We then eliminate these nodeless degrees of freedom through static condensation and evaluate them within the framework of inelasticity by utilizing an operator split solution procedure. Seeing that the coupled strain-softening problem is non-convex, we present an alternating minimization (staggered) algorithm, thus retaining a positive definite stiffness matrix. Finally, the four-parameter representation by quaternions describes a three-dimensional finite rotation. We demonstrate a very satisfying and robust performance of these new finite elements in several numerical examples, including the fracture of random lattice structures with application to fibrous materials. We show that accounting for geometrical nonlinearity in the beam formulation is necessary for direct numerical simulations of fiber networks regardless of the density.

With material properties as a starting point, this study focuses on analyzing the performance of a paperboard package. Torsional and compressive loading of a paperboard package have been investigated through physical experiments and finite element (FE) simulations, where an orthotropic linear elastic material model with a stress-based failure criterion was used. Comparing physical experiments and FE simulations of box compression and torsion showed that the finite element models could accurately predict the response curves. Additionally, the model was utilized to investigate which impact variations in moisture, bending stiffness, and crease quality had on packaging performance. The effect of moisture was examined through an established master curve, where the necessary mechanical properties could be expressed as linear functions of moisture ratio. The impact of creases was evaluated by varying previously established ratios (relative crease strength, RCS, tensile strength, RTS) for reducing the creases’ mechanical properties in the simulations. Furthermore, the results showed that the strength of the paperboard affects the maximum compressive strength and maximum torque. Still, the bending stiffness of the paperboard only had a minor effect on box compression strength. To conclude, the model accurately predicted how moisture, crease quality, and bending stiffness affected packaging performance.

Creasing is an essential process to convert paperboards into packages since it enables folding along well-defined lines. The creasing process relies on purpose-made damage that is initiated in the paperboard structure: delamination. However, creasing might also cause in-plane cracks, which must be avoided. In this laboratory study, three paperboards were creased at six different depths, respectively. Two mechanical tests were performed to characterize the creases at standard climate (23°C and 50% RH): 2-point folding, to examine the bending force and short-span in-plane tensile test to evaluate the strength. The results were normalized with the values for the uncreased boards, which gave the relative strength ratios: relative creasing strength (RCS) and relative tensile strength (RTS). When the relative strengths were evaluated against the normative shear strains, a creasing window was formed. This window has an upper limit given by the RTS values, corresponding to the in-plane cracks, and a lower limit given by the RCS values, corresponding to the delamination damage initiated in the paperboard during creasing. It was observed that both the RCS and RTS values exhibit a linear relation against normative shear strain. From this, it was concluded that performing tests at two creasing depths might be sufficient to estimate the lower, and upper, limits for the creasing window in future studies. Finally, the effect of moisture was investigated by creasing, folding and tensile testing at 23°C and 90% RH, which showed that moisture had no clear effect on the RCS or the RTS values.