Compressive behavior of phenol-formaldehyde impregnated paper composites is studied in creep and strain recovery tests observing large nonlinear viscoelastic strains and irreversible strains, describing the latter as viscoplasticity. Stiffness reduction was not observed in experiments and therefore is not included in the material model. Schapery's nonlinear viscoelastic and nonlinear viscoplastic constitutive law is used as a material model and the stress dependent non-linearity functions are determined. First, the time and stress dependence of viscoplastic strains is described by Zapas et al. model and identified measuring the irreversible strains after creep tests of different length at the same stress and doing the same for creep tests of a fixed length but at different stress. Then, the determination of nonlinear viscoelastic stress dependent parameters is performed.

The present study describes the mechanical response of thermoset polymers under high compressive loads. A well-defined free radically cured vinyl ester resin has been used and studied in six different geometries in order to determine the dependence of apparent mechanical properties on the particular size and shape of a sample. The mechanical response in compression has also been compared to the response in tensile tests. Variation of the film thickness, boundary conditions and loading conditions reveal that there is a significant effect on the mechanical performance (apparent properties) of the polymer. When the thickness-to-width ratio of the sample is reduced in a compression test, the friction between the sample and the compression plates proves to be of great importance. The yield stress increases dramatically when the thickness of the sample is reduced, whereas it decreases when the friction between sample and the compression plate is reduced. The creep decreases when the thickness of the material is reduced and it decreases even more due to reaction of the material surrounding the compressed part of the sample. The described test conditions and observed phenomena will be subject to simulation in Part 2 of this study.

A nonlinear viscoelastic material model was used to describe the experimental behaviour of thin vinyl ester specimens subjected to compression in thickness direction. The stress-dependent material functions in the model were found in creep and strain recovery tests on thick cylindrical specimens. The elastic and creep response of thin thermoset polymer specimens subjected to compressive loads was simulated while varying the geometry of the test set samples. The calculated increase in the apparent elastic modulus and decrease of the creep-strain rate due to reduced thickness-to-width ratio is in a good qualitative correlation with experimental results for corresponding geometries. The constraint due to friction and interaction with the material outside the loaded surface area were identified as the cause for high apparent stiffness, which converges with decreasing thickness to an asymptotic value dependent on the modulus and Poisson's ratio of the material.

Two methodologies for identification of material functions in Schapery's nonlinear viscoelastic material model are compared in context to their ability to deal with deviations from Heaviside stepwise load application and unloading in real test conditions where the time intervals for load increase to plateau value and to unloading to zero are finite. In the first method the description of the whole loading, creep, unloading and recovery process is given by one-step load application and one-step unloading whereas in the second method the load increase and decrease intervals are approximated by two-step load application with 0.5 of the load applied in the increase region. Vinyl ester with known viscoelastic properties and incremental form of Schapery's constitutive equation is used to simulate "experimental data" for several length of load application and unloading. The two data reduction methodologies are applied to these "data" and the accuracy of identified material functions is compared with the true values (input data).

Anisotropic material model for composites made from phenol-formaldehyde impregnated paper is developed starting from Schapery's nonlinear viscoelastic and nonlinear viscoplastic constitutive law derived from thermodynamics. Methodology for determination of the included stress-dependent functions has been developed and used to determine properties of two composites in two directions related to material symmetry. Creep with following strain recovery tests have been performed at several levels of stress. Data reduction is based on two-step load application methodology to include effect of finite time of load increase to the plateau value and its removal. No stiffness reducing micro damage was detected and the plastic deformations were considered negligible or accounted for in a simplified manner. The material model was used to simulate a constant stress rate loading and unloading test. Model is validated comparing simulations with results from test.

The work presented in this thesis has been focused on the mechanical properties of wood fiber composites. One goal has been to investigate the processing of wood polymer composite materials with good mechanical properties, and how to tailor these properties for different applications. Different kinds of composites were manufactured and characterized regarding microstructure and mechanical properties. In the first study, it was investigated how to utilize the high stiffness and strength of the wood cell wall material to get good mechanical properties of the composite material. Wood chips were cooked with a sulfite technique for different cooking times in an attempt to extract relatively undamaged fibers with high stiffness and strength and study the influence of different degree of fiber separation. Composites were made by impregnating oriented fiber mats with an epoxy resin and curing in a laboratory press. The best properties were achieved with completely separated fibers. The apparent Young’s modulus of the fibers was calculated by a micromechanical model to approximately the same level as is measured on single wood fibers, which indicates a good utilization of stiffness of the wood cell wall material. Recognizing that high stiffness and strength requires high volume fraction of fibers without getting porosity in the material, the manufacturing process was developed for high volume fraction of fibers. Commercial kraftliner and saturating papers were impregnated with low viscosity phenol- formaldehyde resin and laminates were made by pressing a stack of these papers in a hot press. Fiber volume fractions of about 70% were achieved, which is high compared to for example conventional glass fiber composites. It was also shown that high values of stiffness (Young’s modulus >19GPa) and strength (tensile strength >190MPa) are obtainable with this type of material. By further development of the paper, higher strength and stiffness could probably be achieved The general conclusions are that it is favorable to use carefully separated single wood fibers to make a wood composite material with good mechanical properties. To be able to tailor properties, fibers in the form of thin and relatively dense fiber mats or paper with high degree of orientation could be used. Composite materials could then be made from these mats by impregnation with a pre-polymer with low viscosity and good compatibility with wood to get a material free from porosity. Since the mechanical properties of both wood fibers and polymer matrix material are time dependent, wood fiber composites have complex time dependent stress-strain response. For wood fiber composites to be used in load bearing applications, their time dependent behavior has to be known and be able to predict. Starting from a general non-linear viscoelastic and non- linear viscoplastic constitutive law derived from thermodynamics, a methodology for determination of the included nonlinearity functions for a specific material has been developed. Creep - strain recovery tests have been performed and the developed methodology was used to determine a constitutive model. An incremental form of the model was also developed to simulate arbitrary loading conditions and to implement the model in finite element software. The material model was verified by simulating a linear load ramp and compared with results from a corresponding experiment. The material tested was characterized as a nonlinear viscoelastic viscoplastic material, with no stiffness reducing micro damage detected in the load range tested.

Wood-based paper fiber composites are of interest due to favorable mechanical properties of the fibers. Wood fibers were separated from sapwood of spruce by a sulfite cooking procedure with 2, 3, 4 and 5 hrs cooking times. Oriented fiber mats were manufactured from these fibers, which were mildly treated as compared with commercial paper fibers. Composites were produced with fiber volume fractions in the range 17.6-19.5%. Scanning electron microscopy, mechanical testing and micromechanics modeling of composites modulus were methods used. Mechanical property data from the literature were compiled and compared with the present results. Young's modulus and strength were 6.4-8.4 GPa and 59-115 MPa respectively. Differences were mainly attributed to differences in fiber orientation distribution, but advantages were noted with longer cooking time fibers separated into individual wood cells. If the low fiber content is taken into account, this material has high stiffness and strength compared to results found in the literature.

The goal of this work has been to investigate how to make wood polymer composite materials with good mechanical properties, which also could be tailored for different applications. The fundamental idea behind the work was that good mechanical properties of the composite material require that the stiffness and strength of the wood cell wall material are really utilized. Furthermore, the volume fraction of fibers has to be high and the fibers have to be oriented in the loading direction. Different kinds of experimental materials were manufactured and characterized. In the first paper, wood chips were cooked with a sulfite technique for different cooking times in an attempt to extract relatively undamaged fibers with high stiffness and strength and study the influence of different degree of fiber separation. Composites were made by impregnating oriented fiber mats with an epoxy resin and curing in a laboratory press. The best properties were achieved with completely separated fibers. The apparent Young's modulus of the fibers was calculated to a level which indicates a good utilization of stiffness of the wood cell wall material. In the second paper experiments were made in order to study the manufacturing process for wood fiber composites with high volume fraction of fibers. Commercial papers were impregnated with a phenol-formaldehyde resin and laminates were made by pressing a stack of these papers in a hot press. Fiber volume fractions of about 70% were achieved. It was also shown that values of stiffness (Young's modulus >19GPa) and strength (tensile strength >190MPa) are obtainable with this type of material. The general conclusions are that to make a wood composite material with good mechanical properties in combination with ability to tailor properties, is to use carefully separated single wood fiber in the form of dense fiber mats or paper with high degree of fiber orientation as reinforcement. Composite materials are then made by impregnation with a compatible low viscosity resin and curing in a hot press.