Natural fiber reinforced composites (NFRCs) suffer from water absorption and low temperature stability resulting in fiber degradation and subsequent material failure. Built—in piezoresistive sensors are investigated to monitor deformation/strain of the component. As a low-cost material from renewable resources biochar particles derived from olive stones were applied on flax plies and yarn bundles that served as model systems. Carbon black samples as prominent petrochemical variants were used as a reference. Biochar and carbon black covered fiber systems were laminated in epoxy resin followed by tensile tests. The electrical resistance was recorded simultaneously during testing. Biochar with a broad size distribution from nano to high micrometer range (D < 200 µm) was superior in sensor performance compared to carbon black and biochar with a smaller particle size range D < 20 µm. Gauge factors (GF) of NFRC samples with integrated biochar particles reached 30-80 while carbon black could not exceed a GF of 8. Sustainable biochar derived from olive stones exhibits therefore great potential as an alternative in carbon particle piezoresistive sensors to connect the mechanical state of a structural component with its electrical resistance.

The exploration of natural adhesives for wood-based products offers a sustainable solution to the environmental and health issues posed by synthetic adhesives. Among natural substances, lignin, a primary element of plant cell walls, stands out for its adhesive capabilities. We investigate the adhesive qualities of lignins sourced from a variety of plants and manufacturing methods, testing them in multiple forms, such as powder, liquid supernatant, and as a solvent for impregnating delignified wood veneers. The performance of lignin against urea-formaldehyde adhesive, with a particular focus on how pressing temperature and time influenced their adhesive strengths. Initial tests were carried out using a system inspired by the Automated Bonding Evaluation System (ABES) to measure the bond strength of urea-formaldehyde under various conditions, establishing a baseline for assessing lignin adhesives.

We evaluated lignin's adhesive potential by examining the effects of pressing duration, wood veneer treatment, and solvent choice on the adhesive properties of lignin. Lignins derived from Kraft and soda pulping processes showed encouraging adhesive characteristics under certain conditions. We highlight the significant impact of the lignin's botanical source, processing method, and pressing parameters on its effectiveness as an adhesive. We identified key challenges such as humidity control and the fine-tuning of pressing conditions that influence adhesive performance. Although lignin-based adhesives have not achieved the efficacy of synthetic alternatives, our findings emphasize lignin's potential as a sustainable adhesive, noting that specific pressing conditions and wood veneer treatments can enhance its adhesive quality.

Lignin stands as a pivotal constituent of wood, ranking as the second most prevalent organic material across the globe. The escalating demand for sustainable and renewable resources propels the exploration of innovative applications for technical lignins, such as their integration as a matrix in bio-composites. Nevertheless, the pursuit of modeling these bio-composites hinges on the precise identification of lignin's mechanical properties, a facet that remains relatively elusive at present. Complicating matters further, technical lignins sourced from lignocellulosic materials exhibit notable disparities in their chemical composition, size, cross-linking, and functional groups. These variations arise from discrepancies in the raw materials used and the isolation methods employed, including the pulping process and subsequent isolation and purification techniques. Hence, it becomes imperative to address these disparities when evaluating and understanding the mechanical characteristics of lignin. To tackle this challenge, our study delves into the examination of five distinct hot-pressed lignins, each derived through diverse extraction processes from varying feedstocks. The assessment employs microscopy-aided grid nanoindentation, aiming to unravel the nuanced mechanical properties of these lignins. Through such meticulous investigation, we endeavor to contribute valuable insights that will aid in comprehending the intricate interplay between lignin's structural variations and its mechanical behavior. The derived mechanical properties exhibit a robust correlation with the porosity observed in the lignin specimens. This correlation finds an apt description through the Mori-Tanaka homogenization scheme within the framework of continuum micromechanics. Employing this micromechanics fit, we conducted a reverse calculation to determine the stiffness of "solid" lignin devoid of any pore influence, revealing Young's modulus of 7.12 GPa. The noteworthy alignment between the micromechanics model and the experimentally measured indentation modulus validates that the indentation modulus of solid lignin remains consistent across all five variants. Remarkably, this consistency holds true irrespective of the extraction process or the specific feedstock employed.

The livMatS fiber Pavillon is an example of a bioinspired sustainable construction. It originates from the successful collaboration of an interdisciplinary team of architects and engineers from the Cluster of Excellence IntCDC, University of Stuttgart and biologists/biomimeticists and material scientists from Cluster of Excellence livMatS, University of Freiburg.

The inspiration for the pavilion came from columnar cacti such as the saguaro cactus (Carnegia gigantea), and the prickly pear cactus (Opuntia sp.), which are characterized by their special wood structure. The up to 20 m tall saguaro cactus has – like many other columnar cacti – a cylindrical wooden body that is hollow on the inside and thus particularly light. The individual wooden elements grow together to form a net-like structure, which gives the wooden body additional mechanical stability. By analyzing the (micro-)arrangement of the reticulated wood with µCT and MRI, and abstracting these net structures, the structural and mechanical properties of biological wood structures could be transferred to the pavilion's lightweight load-bearing elements. The supporting structure of the livMatS pavilion consists of 15 flax fiber elements designed by digital planning and prefabricated exclusively from continuous spun natural fibers in a robot-assisted coreless fiber winding process, replacing the intergrowth that causes the netlike structures in the biological models. With a total area of 46 m², the entire fiber structure weighs only about 1.5 t and is designed to withstand the full snow and wind loads of the applicable building codes.

The pavilion, made (nearly) entirely of recyclable materials, points the way to a new greener architecture of the 21st century. Contributing to the resource-efficient architectural approach are the load-bearing elements made of renewable raw materials (flax, sisal) and the transparent ceiling elements made of polycarbonate. The ceiling elements can be shredded and remelted at the end of their useful life.

Plant fiber-based biocomposites are a sustainable alternative to traditional materials in several industries, including the construction sector. The excellent mechanical performance of the cellulosic fibers results in a reinforced composite that can be used for structural elements, such as beams, plates, or shells. To ensure the serviceability of biocomposite structures, the characterization of the (time-dependent) viscoelastic material behavior is essential. In this contribution, we aim to tackle the characterization by multiscale micromechanics modeling.

Both the polymer and the plant fiber are viscoelastic materials. Plant fibers deform over time due to the viscous nature of lignin (and hemicellulose) as well as through sliding phenomena occurring in the interfaces between cellulosic fibrils. The microscopic mechanisms and the viscoelastic properties of lignin and of the polymer matrix are then incorporated into a multiscale model to upscale them to the composite scale. Thereby, the correspondence principle of viscoelasticity is exploited to perform the homogenization using classical continuum micromechanics homogenization theory, albeit in the Laplace Carson space.

Comparison of model results with creep test data from single-fiber creep tests and creep tests on wood allows for back-calculation of the missing interfacial viscoelastic parameters. Then, the model is used to predict the creep behavior of several different biocomposites, varying in terms of plant source, polymer material, production method, and fiber orientation distribution. Model predictions are successfully compared to results from experimental testing campaigns. Sensitivity studies demonstrate that typically, the polymer creep dominates the composite creep, particularly at high temperatures. At high moisture contents of the plant fibers, however, the amplified creep of the fibers leads to a pronounced increase in the composite creep.

This study presents an advanced numerical model for plant fiber-reinforced composites, addressing a significant gap in predictive modeling for these environmentally friendly materials. Our model describes the complex interactions between cellulosic fibers and matrix in biocomposites, accounting for all major failure mechanisms: matrix softening, fiber breakage, and fiber-matrix debonding.
Employing nonlinear plasticity, XFEM, and cohesive zone models, we simulate failure in a unit cell with two fibers and periodic boundary conditions [1]. This approach enables a precise prediction of nonlinear macroscopic behavior in biocomposites. Validated against experimental data, the model accurately predicts tensile and compressive properties of both short- and long-fiber composites.
The unit cell method also enables further sensitivity analyses, providing valuable insights into effects such as the softening related to decreased interfacial shear strength and the strengthening impact of longer fibers. This research paves the way for future studies on lignin-based biocomposites [2] and aims to establish a comprehensive link between analytical [3] and numerical modeling approaches for a robust mechanical prediction model for complex biocomposite materials.

[1] Senk, V.; Königsberger, M.; Pech, S.; Lukacevic, M; Schwaighofer, M.; Zelaya-Lainez, L; Füssl, J. Advanced numerical modeling of plant fiber-reinforced composites: Predicting macroscopic nonlinear behavior through fiber, matrix, and interface failure. Submitted to Elsevier, 2024.

[2] Schwaighofer, M.; Zelaya-Lainez, L; Königsberger, M.; Lukacevic, M.; Serna Loaiza, S.; Harasek, M.; Lahayne, O.; Senk, V.; Füssl, J. Characterization of mechanical properties of five hot-pressed lignins extracted from different feedstocks by microscopy-aided nanoindentation. In: Materials & Design, 227(5):111765, 2023.

[3] Königsberger, M.; Lukacevic, M.; Füssl, J. Multiscale micromechanics modeling of plant fibers: upscaling of stiffness and elastic limits from cellulose nanofibrils to technical fibers. In: Materials and Structures, 56(13), 2023.