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  • 1.
    Srinivasa, Prashanth
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Mechanics of Nanocellulose Foams: Experimental and Numerical Studies2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Nanofibrillar cellulose (NFC) foams are an interesting class of cellular materials that are being explored for a variety of applications, ranging from the automotive to the biomedical industries. The cellulose nanofibrils itself has unique and desirable mechanical properties. With recent advances in the preparation of these foams, it is anticipated that these foams will find applications in diverse areas, including those where the mechanical response is important. This macroscopic response is inextricably linked to the microstructure of the material. Thus, it is imperative to have numerical models that can not only predict the macroscopic response but can also provide insights towards tailoring the microstructure such that improved macroscopic properties can be sought. Towards this end, we study 2- and 3-D random cellular models along with characterising through experiments/simulations the macroscopic and cell wall material properties. 

    In Paper A, we explore the suitability of two-dimensional random structures in representing the macroscopic compressive response of foams. Though the two-dimensional model fails to capture the exact response, only an order of magnitude agreement is found, we map the effect of internal contact on the macroscopic response and study the effect of linear size, wall thickness and non-straightness of the cell walls. It is concluded that 2-D models are inadequate and that the out of plane connectivity is non-trivial. 

    In Paper B, NFC foams prepared from freeze-drying are experimentally characterised under uniaxial and biaxial loading conditions, with a view towards testing for structural anisotropy. It is found that the prepared foam is isotropic in the plane. The experiments also reveal that there are large irreversible deformations, when unloaded. A continuum hyperelastic model is fitted to the experimental data. 

    In Paper C, tomography based scans of the NFC foams are used to arrive at the material properties of the cell walls. We reconstruct the three-dimensional structure from the tomography scans and use it in finite element simulations to determine the elastic modulus and yield strength of the cell wall material. It is seen that the estimated elastic modulus is comparable to the upper limit for NFC paper, while the yield strength is comparable to estimates from indirect methods. The simulations also corroborate the damage mechanism, i.e. by plastic hinge formations followed by the collapse of the inner structure, as observed by experimental studies. 

    In Paper D, we utilise the material properties derived from the tomography-based work in simulating three-dimensional random structures. We validate the three-dimensional reconstruction method against the foam structures derived in microtomography. We then study the applicability of these random structures in representing the macroscopic response, together with studies on linear size and effect of partially open/closed cells. We also estimate the influence of cell face curvature on the elastic modulus and plateaus stress. It is concluded that 3-D models provide a reasonable representation of the response up to intermediate strain levels, but the densification regime is not captured by the considered representative size.

  • 2.
    Srinivasa, Prashanth
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Non-linear mechanics of nanocellulose foams2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    There has been a growing interest in nano-fibrillar cellulose (NFC), which has been fuelled not merely by the advantages it presents in terms of strength to weight ratio and biodegradability, but also owing to the recent advances in production techniques. NFC foam is essentially a hierarchical structure, wherein nanofibrils account for the smallest scale, with the pores/cell walls forming the meso scale. A complete scanning of the mechanical property space would require understanding of the contribution of each of these scales in these foams. We aim to understand these scale relationships, eventually allowing for the possibility of tailoring material properties at scales of interest.

    In paper A, we look at the applicability of two-dimensional random Voronoi structures in capturing the large-strain compressive response of these foams. We introduce internal contact, into the interiors of the cell walls, with the aim of capturing the densification regime. We then study the scaling effects associated with such a model, and, subsequently single out the contribution of internal contact on the overall compressive response. While it is seen that internal contact in random structures allow for capturing the densification regime, the model only provides an order of magnitude agreement with experimental data.

    In paper B, we characterize the NFC foam based on both uni-axial and bi-axial experiments. One of the aims is to ascertain if there are effects of directionality to the stress-strain response. For the two considered porosities, we do not find any evidence for directionality in the response. We then proceed to make the assumption of isotropy, and adopt the well-known Ogden-Roxburgh “pseudo-elastic” model - originally proposed for incompressible rubber like materials - for the particular case of highly compressible foams. The model allows to capture the damage observed in unloading and also the significant residual strains.

  • 3.
    Srinivasa, Prashanth
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A three-dimensional numerical model for large strain compression of nanofibrillar cellulose foams2018In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 33, no 2, p. 256-270Article in journal (Refereed)
    Abstract [en]

    We investigate the suitability of three-dimensional Voronoi structures in representing a large strain macroscopic compressive response of nanofibrillar cellulose foams and understanding the connection between the features of the response and details of the microstructure. We utilise Lloyd's algorithm to generate centroidal tessellations to relax the Voronoi structures and have reduced polydispersity. We begin by validating these structures against simulations of structures recreated from microtomography scans. We show that by controlling the cell face curvature, it is possible to match the compressive response for a 96.02% porous structure. For the structures of higher porosity (98.41%), the compressive response can only be matched up to strain levels of 0.4 with the densification stresses being overestimated. We then ascertain the representative volume element (RVE) size based on the measures of relative elastic modulus and relative yield strength. The effects of cell face curvature and partially closed cells on the elastic modulus and plateau stress is then estimated. Finally, the large strain response is compared against the two-dimensional Voronoi model and available experimental data for NFC foams. The results show that compared to the two-dimensional model, the three-dimensional analysis provides a stiffer response at a given porosity due to earlier self-contact.

  • 4.
    Srinivasa, Prashanth
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Analysis of the compressive response of Nano Fibrillar Cellulose foams2015In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 80, no Part A, p. 13-26Article in journal (Refereed)
    Abstract [en]

    Nano Fibrillar Cellulose (NFC) is fast emerging as a biomaterial with promising applications, one of which is cellular foam. The inner structure of the foam can take various shapes and hierarchical micro-structures depending on the manufacturing parameters. The compressive response of foams developed from these materials is currently a primary criterion for the material development. In this work, we focus on the connection between the non-linear part of the response and the inner structure of the material. We study the effect of internal contact and its contribution to gradual stiffening in the energy absorbing region and accelerated stiffening in the densification region of the large strain compressive response. We use the finite element method in this study and discuss the applicability and efficiency of different modelling techniques by considering well defined geometries and available experimental data. The relative contribution of internal contact is singled out and mapped onto the overall compressive response of the material. The effect of initial non-straightness of the cell walls is studied through superposing differing percentages of the buckling modes on the initial geometry. The initial non-straightness is seen to have a significant effect for only strains up to 1%. The secant modulus measured at slightly higher strains of 4%, demonstrates lesser effect from the non-straightness of cell walls. The simulations capture the compressive response well into the densification regime and there is an order of magnitude agreement in between simulations and experiments. We observed that internal contact is crucial for capturing the trend of compressive response.

  • 5.
    Srinivasa, Prashanth
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Three-dimensional random structure representation for nanofibrillar cellulose foams: Validation and representative volume simulations2017Report (Other academic)
  • 6.
    Srinivasa, Prashanth
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Aulin, C.
    Experimental characterisation of nanofibrillated cellulose foams2015In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882XArticle in journal (Refereed)
    Abstract [en]

    There is a growing interest in applications for nanofibrillated cellulose based materials owing to their exceptional mechanical properties. Nanofibrillated cellulose (NFC) foam is one such derivative which has potential applications in a wide array of fields. Here, we characterise the mechanical properties of two particular high porosity NFC foams (98.13 and 98.96 %) prepared by a freeze drying process. We evaluate their behaviour in uni-axial and bi-axial compression with cyclic loading. The secondary loading cycles reveal complete irreversible damage of the microstructure, with the secondary loading path being characterised by near zero plateau stress. In force controlled tests, negligible hysteresis corroborates the idea that there is no energy dissipation owing to near complete microstructural damage. Furthermore, we observe no indications of preferential orientation of the microstructure in these tests. The stress responses in mutually perpendicular directions are seen to be identical, within statistical considerations. We then utilise the “pseudo-elastic” model developed and adopt it to the case of highly compressible Ogden strain energy formulation with a modified neo-Hookean for the unloading, with a view of fitting a continuum hyperelastic model to the experimental data. The material parameters obtained from uni-axial data are seen to be insufficient to describe the more general bi-axial deformation. The parameters obtained from the bi-axial test describe uni-axial deformation up to stretches of 0.5 but overestimate the stress levels beyond that point.

  • 7.
    Srinivasa, Prashanth
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Aulin, Christian
    Innventia AB.
    Experimental characterisation of Nano Fibrillar Cellulose foamsManuscript (preprint) (Other academic)
  • 8.
    Srinivasa, Prashanth
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Karlberg, Filip
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Material properties of the cell walls in nanofibrillar cellulose foams from finite element modelling of tomography scans2017In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, no 24, p. 519-533Article in journal (Refereed)
    Abstract [en]

    The mechanical properties of the nanofibrillar cellulose foam depend on the microstructure of the foam and on the constituent solid properties. The latter are hard to extract experimentally due to difficulties in performing the experiments on the micro-scale. The aim of this work is to provide methodology for doing it indirectly using extracted geometry of the microstructure. X-ray computed tomography scans are used to reconstruct the microstructure of a nanofibrillar cellulose foam sample. By varying the levels of thresholding, structure of differing porosities of the same foam structure are obtained and their macroscopic properties of the uni-axial compression are computed by finite element simulations. A power law relation, equivalent to classical foam scaling laws, are fit to the data obtained from simulation at different relative densities for the same structure. The relation thus obtained, is used to determine the cell wall material properties, viz. elastic modulus and yield strength, by extrapolating it to the experimental porosity and using the measured response at this porosity. The simulations also provide qualitative insights into the nature of irreversible deformations, not only corroborating the experimental results, but also providing possible explanation to the mechanisms responsible for crushable behaviour of the nanofibrillar cellulose foams in compression.

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