| 1. |
- Hagman, Anton, 1984-
(författare)
-
Influence of inhomogeneities on the tensile and compressive mechanical properties of paperboard
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The in-plane properties of paperboard have always been of interest to paper scientists. Tensile properties are crucial when the board is fed through converting machines at high speeds. Compressive properties are essential in the later use. Inhomogeneities affect both the compressive and tensile properties. For the tensile properties, it is the inherent heterogeneity of the paperboard that might cause problems for the board-maker. Varying material properties, through the thickness of the paperboard, are on the other hand used to achieve high bending stiffness with low fiber usage. It is of interest to know how this practice affects the local compressive properties. Papers A and B aims to address this, while C, D and E focus on in-plane heterogeneities. Paper A investigates the mechanism that causes failure in the short span compression test (SCT). It was concluded that the main mechanism for failure in SCT is delamination due to shear damage. In paper B the effect of the through-thickness profiles on the local compression strength was examined. It was concluded that the local compression is governed by in-plane stiffness and through thickness delamination. The latter was in turn dependent on the local shear strength and in-plane stiffness gradients. In paper C the tensile test is investigated with focus on sample size and strain distributions. The strain behavior was dependent on the length to width ratio of the sample and was caused by activation of local zones with high strainability. Paper D focuses on the strain zones seen in C. The thermal response in paper was studied. It was observed that an inhomogeneous deformation pattern arose in the paper samples during tensile testing. It was concluded that the heat patterns observed coincided with the deformation patterns. It could be shown that the formation was the cause of the inhomogeneous deformation. In final paper, E, the virtual field method was applied on data from C.
|
|
| 2. |
- Marin, Gustav
(författare)
-
On the relation between paperboard properties and packaging performance
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper-based materials, such as paperboard, are commonly used as packaging materials. Inaddition to the advantage that wood as a raw material is renewable, there are also many otherbenefits of paperboard. From a mechanical point of view, paperboard has a high bendingstiffness compared to its relatively low weight and has a high foldability, which both areproperties of significance in the design of packages. However, a distinct drawback withpaperboard is its significant sensitivity to moisture. The moisture reduces the mechanicalproperties of the paperboard and consequently reduces the performance of the package. Thisthesis is starting with an investigation of the relation between moisture and differentmechanical properties on a continuum material level, and then these relations are applied onthe packaging design level through experimental testing and simulations.In Paper A, a material characterization was performed on a series of five paperboards withdifferent grammages from the same producer. Five types of mechanical tests to characterizethe paperboards’ material properties were performed:• In-plane tensile test,• Out-of-plane tensile test,• Short-span Compression Test (SCT),• Bending stiffness test,• Double-notch shear test.All tests were performed at several levels of relative humidity (RH). Linear relations betweenthe mechanical properties normalized with their respective value at 50 % RH and moistureratio were found.Paper B examined whether the linear relationships discovered in Paper A are true also forother paperboard series as well. Therefore, 15 paperboards from four producers wereinvestigated in this study, at the same levels of RH as before. Here, the in-plane stiffnessesand strengths and SCT-values were evaluated as a function of moisture. When also themoisture ratios in the investigated paperboards were normalized, it turned out that allpaperboards followed the same linear relationship between normalized mechanical propertyand normalized moisture ratio. Additionally, a bilinear elastic-plastic in-plane model wasdeveloped, that can predict the stress-strain relation of an arbitrary paperboard at an arbitrarymoisture level, and without requiring any mechanical testing except at standard condition(50% RH, 23 °C).In Paper C, this relation was used to estimate input material parameters for simulating a BoxCompression Test (BCT) at different moisture levels. The result showed that it was possibleto accurately predict the load-compression curve of a BCT when moisture was accountedfor.
|
|
| 3. |
- Borodulina, Svetlana
(författare)
-
Micromechanical Behavior of Fiber Networks
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper is used in a wide range of applications, each of which has specific requirements on mechanical and surface properties. The role of paper strength on paper performance is still not well understood. This work addresses the mechanical properties of paper by utilizing fiber network simulation and consists of two parts.In the first part, we use a three-dimensional model of a network of fibers to describe the fracture process of paper accounting for nonlinearities at the fiber level (material model and geometry) and bond failures. A stress-strain curve of paper in tensile loading is described with the help of the network of dry fibers; the parameters that dominate the shape of this curve are discussed. The evolution of network damage is simulated, the results of which are compared with digital speckle photography experiments on laboratory sheets. It is concluded that the original strain inhomogeneities due to the structure are transferred to the local bond failure dynamics. The effects of different conventional and unconventional bond parameters are analyzed. It has been shown that the number of bonds in paper is important and that the changes in bond strength influence paper mechanical properties significantly.In the second part, we proposed a constitutive model for a fiber suitable for cyclic loading applications. We based the development of the available literature data and on the detailed finite-element model of pulp fibers. The model provided insights into the effects of various parameters on the mechanical response of the pulp fibers. The study showed that the change in the microfibril orientation upon axial straining is mainly a geometrical effect and is independent of material properties of the fiber as long as the deformations are elastic. Plastic strains accelerate the change in microfibril orientation. The results also showed that the elastic modulus of the fiber has a non-linear dependency on a microfibril angle,with elastic modulus being more sensitive to the change of microfibril angle around small initial values of microfibril angles. These effects were incorporated into a non-linear isotropic hardening plasticity model for beams and tested in a fiber network in cycling loading application model, using the model we estimated the level of strains that fiber segments accumulate at the failure point in a fiber network.The main goal of this work is to create a tool that would act as a bridge between microscopic characterization of fiber and fiber bonds and the mechanical properties that are important in the papermaking industry. The results of this work provide a fundamental insight on mechanics of paper constituents in tensile as well as cyclic loading. This would eventually lead to a rational choice of raw materials in paper manufacturing and thus utilizing the environment in a balanced way.
|
|
| 4. |
- Huang, Hui
(författare)
-
Numerical and experimental investigation of paperboard creasing and folding
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis aims to increase the understanding of deformation and damage mechanisms of paperboard during converting, especially creasing and folding will be analyzed. A simple two dimensional creasing simulation was performed. In this model, paperboard was modeled as a combination of an anisotropic elastic-plastic continuum model with isotropic hardening and a softening cohesive interface model. The paperboard was composed of four plies with uniform material parameters. Creasing simulations were done on both machine direction (MD) and cross machine direction (CD) samples to two crease depths 0.0 mm and 0.2 mm, respectively. The simulation results showed good agreement with experimental results. The out-of-plane shear properties are dominating factors for creasing and folding. Therefore, a test method to determine shear properties was proposed. This part of the work is based on the most recently proposed test method, the laminated double notch shear test. To improve the technique, double notches with declined slopes, called tilted double notch shear test, were used instead of uniform depth double notches. The influence of shear zone length was also investigated. The results reveal the short shear zone lengths gave higher shear strength and more pronounced shear strength profile. The results from the rst two analyses were utilized to study folding of paperboard. The simulation model was the same as in the creasing simulations. However, to improve the model and better account the actual micro structure of paperboard a new material mapping method was proposed. The continuum properties of the plies were assumed to vary in the thickness direction. The shear strengths of the interfaces were determined by using the tilted double notch shear test using a short shear zone length, L= 5 mm. The agreement between simulation results and experiment results was good, and most of the folding properties were captured.
|
|
| 5. |
- Marin, Gustav
(författare)
-
Impact of paperboard deformation and damage mechanisms on packaging performance
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper-based materials, such as paperboard, are commonly used as packaging materials. In addition to the fact that paper is renewable, there are also many other benefits of paperboard. From a mechanical point of view, paperboard has a high bending stiffness compared to its relatively low weight and high foldability, which are properties of significance in the design of packages. However, a distinct drawback of paperboard is its significant sensitivity to moisture. The moisture reduces the mechanical properties of the paperboard and consequently reduces the performance of the package. This thesis addresses the impact of paperboard deformation and damage mechanisms on packaging performance, with the characterization of the material properties as a starting point. Initially, the relations between moisture and different mechanical properties on a continuum material level were investigated. Then, experimental testing and finite element (FE) simulations were applied to evaluate these relations at the packaging design level.In Paper A, a material characterization was performed on five commercial paperboards with different basis weights, from the same producer. Five types of mechanical tests to characterize the paperboards material properties were performed:In-plane tensile test,Out-of-plane tensile test,Short-span Compression Test (SCT),Two-point folding,Double-notch shear test.All tests were performed at several levels of relative humidity (RH). Linear relations between the mechanical properties normalized with their respective value at 50 % RH and moisture ratio were found. Paper B examined whether the linear relationships discovered in Paper A are also valid for other paperboard series. Therefore, this study investigated 15 paperboards from four producers at the same RH levels as in Paper A. The paperboards were chosen to be different in furnish and construction, where four recycled boards were included. Here, the in-plane stiffnesses, strengths and SCT values were evaluated as a function of moisture. When the investigated paperboards’ moisture ratios were also normalized, all paperboards followed a linear master curve between normalized mechanical property and normalized moisture ratio. Additionally, a bilinear elastic-plastic in‑plane model was developed to predict the stress-strain relation of an arbitrary paperboard at an arbitrary moisture level without requiring mechanical testing except at standard conditions (50% RH, 23 °C).In Paper C, the master curve developed in Paper B was used to estimate material input parameters for simulating a Box Compression Test (BCT) at different moisture levels by using an orthotropic material model with a stress-based failure criterion, i.e., a relatively simple material model with few input parameters. The result showed that it was possible to accurately predict the load-compression curve of a BCT when accounting for moisture. Furthermore, it was concluded that modeling the creases’ mechanical properties is vital for capturing the stiffness response of the package. Here, a measurable approach for reducing the creases’ mechanical properties was suggested, based on a folding test to obtain the relative creasing strength (RCS) and a short-span tensile test to obtain the relative tensile strength (RTS). It should be emphasized that the model does not include any fitting parameters. All input data is based on measured values. Due to the importance of creases, the RCS and RTS ratios were investigated further in Paper D. When evaluated against normative shear strength during creasing, the RCS and RTS values together formed a creasing window, where the RTS values corresponded to in-plane cracks (upper limit) and the RCS values corresponded to delamination damage (lower limit). It was observed that both the lower and upper limits exhibit linear relations as functions of shear strain. Since creases have an evident effect on the packaging performance from a stacking point of view, it was interesting to investigate a load case exposing the package to shear. Therefore, an additional load case was investigated in Paper E: torsion of paperboard packages, where the experimental data was accurately predicted. Additionally, the effect of bending stiffness was investigated by developing two FE models. Model 1 (used in Paper C) treated the paperboard as a homogeneous material, and Model 2 considered the paperboard a three-ply laminate structure. No significant effect was noted, and it was concluded that the strength has a more significant effect on the BCT than the bending stiffness. It should also be mentioned that there were no problems with cracks when the paperboards were creased and mounted to packages used in Papers C and E. This correlates to the creasing window developed in Paper D since the creasing depth used for the packages is located within the creasing window.To conclude, the primary procedure in this thesis is developing an easy-to-use model with few material parameters that demonstrably can predict the load-deformation curves for two different load cases. The purpose of the model is not to be used for the precise prediction of failure loads but to gain knowledge about damage mechanisms during the testing procedures. A clear advantage of this approach is that the model can be used to either change the package’s geometry or perform a parametric study on the ingoing material parameters. This can also be varied for each ply separately, which helps converters and paperboard producers. It has also been shown that the model can account for different moisture levels if the master curve developed within this thesis is applied. Finally, it should be emphasized again that the model does not include any fitting parameters. All input data is based on measured values.
|
|
