VTT Research System
Not a member yet
161399 research outputs found
Sort by
Property Variations of Binder-Free Lignin-Rich Fiber Networks Driven by Forming Processes and Hot Pressing
No full textSheets made from lignin-rich fiber raw materials can be bonded by hot pressing without external binders. This paper explores how air-laid, foam-laid, and water-laid web formation methods, initial sheet moisture content, as well as hot-pressing conditions (5 MPa, 100–260 °C, 1–60 s), impact the physical properties of board-like materials made of chemi-thermomechanical softwood fibers. In addition to the structural characterization of the hot-pressed materials by X-ray microtomography, air permeance, water contact angle, dry and wet tensile strength, and in-plane compression properties were measured. Despite the significant structural densification, characteristics of the forming method were retained after hot pressing in the final sheet properties. The compressed air-laid sheets had the highest air permeance and the smallest mean pore size, which could be beneficial for particle filtering. At moderate pressing temperatures and times, the significant proportion of large pores in the foam-laid sheets made them weaker than the corresponding water-laid sheets. However, under extreme pressing conditions, the foam- and water-laid sheets reached similar values of high tensile and in-plane compression strength. This suggests that polymer interdiffusion becomes the dominant factor for material strength under these conditions, superimposing the hydrogen bonding created during aqueous forming
A brittle failure evaluation of a semi-elliptical surface crack in a pipe mock-up using three variations of the advanced master curve assessment accounting for ductile crack growth, constraint loss and crack front length
No full textRecently, an experiment on a ferritic pipe mock-up, with a semi-elliptical surface crack, loaded in four-point bending, failed by brittle fracture following ductile crack growth. The experiment serves as a case-study to validate advanced methods for assessing brittle failure affected by constraint-loss, ductile crack growth and the crack front length. Previous work has focused on the determination of the component J-R curve. In this work, three variations of the advanced Master Curve assessment are conceptualized and applied to evaluate the probability of brittle failure of the pipe. The sensitivity of the brittle failure prediction to the three variations of the advanced method is investigated by gradually increasing the complexity of the assessment and by varying the input parameters related to constraint, ductility and the crack front length. The J-integral and constraint along the crack front are evaluated using finite element modelling. The results demonstrate the significance of the component J-R curve in the assessment and provide insight into how the input parameters and the assessment method affect the brittle failure prediction. The work contributes to the efficient operation of pressurized components and affects standards development
Solvent-Free Hydrogenation and Dehydrogenation of Quinoline and Quinaldine for the LOHC Concept
No full textAs presented herein, N-heterocyclic quinoline (Q) and quinaldine (MeQ) represent promising liquid organic hydrogen carriers (LOHCs). They can be quantitatively hydrogenated to tetrahydro (TH) forms under mild reaction conditions (100 °C and 10 bar of H2) using Pt/C and Pd/C catalysts. It is noteworthy that the hydrogenation occurs without a solvent, yielding hydrogen storage capacities of up to 2.9 wt·%. Further hydrogenation of Q and MeQ to their decahydro (DH) forms requires the presence of a solvent. In addition to the hydrogenation of Q and MeQ, we succeeded in the dehydrogenation of the TH forms under solvent-free reaction conditions. This suggests that a pressure-controlled system with a single catalyst for hydrogenation and dehydrogenation of the MeQ/Q system could enable hydrogen storage under solvent-free reaction conditions. We also examined various substituted pyridine structures to comprehend the role of protective alkyl groups in hydrogenation. However, solvent-free quinaldine hydrogenation reactions are sensitive to steric changes surrounding pyridine nitrogen. Among the structural variations studied, MeQ with one methyl group has proven to be the most favorable
Combining cellulose substrates and perovskites in sustainable solar cells is possible:a systematic literature review offering realistic solutions
No full textThe aim of this article is to provide direction for the advancement of cellulose films as sustainable substrates for perovskite solar cells (PSCs). Cellulose, the most abundant biopolymer on Earth, represents a viable, renewable alternative to glass and synthetic polymers when subjected to appropriate modifications. It can be customized via crosslinking, plasticization, and functionalization to increase flexibility and solvent resistance while decreasing gas permeation, surface roughness, and thermal expansion. The adoption of cellulose can drive transformative changes in PSC processing, facilitating the integration of sustainable electrode materials and greener alternatives to toxic solvents, as well as the replacement of high-temperature treatments. Although the literature contains numerous solutions to specific challenges, these findings are scattered across different fields and must be critically assessed for PSC suitability. In this article, we critically review alternative fabrication methods and form a step-by-step multidisciplinary strategy to alter both cellulose and PSC fabrication protocols for the development of sustainable next-generation solar cells.</p
Penalty force stabilization method for elasto-plastic correspondence models in peridynamics
No full textZero-energy modes are a challenge in non-ordinary state-based peridynamics. This work extends a penalty force approach to stabilize such models in both elastic and elasto-plastic simulations. The method penalizes nonuniform deformation directly through a tangent-modulus-based correction that adapts to the evolving material state. To evaluate its performance, we introduce two novel zero-energy mode measures — nodal and global nonuniform strain. We compare the method with first- and second-order bond-associated formulations in small and finite strain regimes and assess the influence of power-law, Gaussian, and uniform weight functions. In small strain tests, the penalty force method matches analytical solutions with displacement errors below 10 −9 with negligible zero-energy mode measures. In finite strain plasticity, the method converges reliably, reproduces finite element and experimental stress–strain responses, and maintains global displacement errors below 10 −4. It shows low sensitivity to the choice of a peridynamic weight function. The penalty force method requires only one deformation gradient evaluation per node, avoids tuning parameters, and suppresses zero-energy artifacts to negligible levels. The results show that it provides a stable and efficient alternative for correspondence-based peridynamic simulations across a wide range of deformation regimes.</p