Stress-transfer mechanisms in wood-fibre composites

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STRESS-TRANSFER MECHANISMS

IN WOOD-FIBRE COMPOSITES

Karin Almgren

KTH Solid Mechanics Royal Institute of Technology

SE-100 44 Stockhol Sweden

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TRITA, HLF-0433 ISSN 1654-1472

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PREFACE

This project has been carried out within the VINNOVA project "Wet processing of Biocomposites for 3D shell applications" (P25749-1) at STFI-Packforsk AB as a co-operation between STFI-Packforsk AB and the Department for Solid Mechanics at KTH. Alfalaval, Stora Enso, Södra Cell, Billerud, Korsnäs-Frövi, Mondi Packaging, Br. Hartmann and M-real are gratefully acknowledged for co-financing the project.

I would also like to express my gratitude to Mikael Lindström at STFI-Packforsk AB for giving me the opportunity to work with this project in a most stimulating environment.

Assistant Professor Kristofer Gamstedt has been my supervisor, offering support and encouragement. Thank you for being such a patient teacher. You are truly a source of inspiration!

Mr Sune Karlsson and Mrs Anne-Mari Olsson are acknowledged for their help with the experimental part of the work. Thank you for sharing your knowledge, time and good mood with me. Thanks to colleagues and co-workers at STFI-Packforsk AB and at the Department of Solid Mechanics for your help and encouragement and for being such good friends.

Thank you Martin, my wonderful husband, for always being there for me, for your faith in me and for bringing so much love and joy to my life.

Karin Almgren March 2007, Stockholm

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LIST OF APPENDED PAPERS

Paper A. Dynamic-mechanical properties of wood-fibre reinforced polylactide: Experimental characterization and micro-mechanical modelling

Bogren, K.M., Gamstedt, E.K., Neagu, R.C., Åkerholm, M., and Lindström, M., (2006), Journal of Thermoplastic Composite Materials 19(6): 613-638

Paper B. Effects of moisture on dynamic mechanical properties of wood-fibre reinforced polylactide studied by dynamic FT-IR spectroscopy

Almgren, K.M., Åkerholm, M., Gamstedt, E.K., Salmén, L. and Lindström, M., (2007), Manuscript

Paper C. Comparison of stress-transfer mechanisms in Paper sheets and composites made from resin-impregnated sheets

Almgren, K.M., Gamstedt,E. K., Nygård, P., Malmberg, F., Lindblad, J. and Lindström, M., (2007), Submitted for publication

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In addition to this thesis, the work has resulted in the following publications

Dynamic-mechanical properties of wood fibre/polylactide

Bogren, K., Gamstedt, K., Neagu, C., Åkerholm, M. and Lindström, M., Proceedings of EcoComp 3rd_{International Conference on Eco-Composites,}

Stockholm, June 2005, poster

Dynamic-mechanical properties of wood-fibre reinforced polylactide: Experimental characterization and micromechanical modelling

Bogren, K.M., Gamstedt, E.K., Neagu, R.C., Åkerholm, M. and Lindström, M., Proceedings of Progress in Wood and Bio Fibre Plastic Composites 2006, Toronto, May 2006, 10 p

Micromechanical approaches to development of improved wood-fibre biocomposites

Gamstedt, E.K., Neagu, R.C., Bogren, K. and Lindström, M.,

Proceedings of the International Conference on Progress in Wood and Bio-Fibre Plastic Composites 2006, Toronto, 2006, 10 p

Effects of relative humidity on load redistribution in cyclic loading of wood-fibre composites analysed by dynamic Fourier transform infrared spectroscopy

Bogren, K.M., Gamstedt, E.K., Åkerholm, M., Salmén, L. and Lindström, M., Proceedings of Fifth Plant Biomechanics Conference, Stockholm, August 2006, 6 p

Stress transfer and failure in pulp-fiber reinforced composites: Effects of microstructure characterized by X-ray microtomography

Bogren, K. Gamstedt, K., Berthold, F., Lindström, M., Nygård, P., Malmberg, F., Linblad, J., Axelsson, M., Svensson, S. and Borgefors, G.,

Proceedings of Progress in Paper Physics – A Seminar, Oxford, October, 2006, 4 p

Measuring fibre-fibre bonds in 3D images of fibrous materials

Malmberg, F., Lindblad, J., Östlund, C., Almgren, K.M. and Gamstedt, E.K., Proceedings of the 14th International Conference on Image Analysis and Processing, International Association on Pattern Recognition, Modena, 2007, (in press)

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INTRODUCTION

Fibre composite materials are popular for their high mechanical properties and low weight and are commonly used in the automotive and aircraft industry, in sport equipments, leisure boats etc. Due to the wide variety of matrix and reinforcement materials available, the design possibilities are unlimited. The matrix, often a polymeric material, surrounds the reinforcement and keeps it in place. The reinforcing fibres are chosen to improve the properties of the matrix, e.g. stiffness and strength. One or more material of each type is required to form a composite. The properties of the composite are dependent on the properties and fractions of the constituent materials, but also on geometry factors such as shape and orientation of the reinforcement

Short as well as continuous fibres are commonly used as reinforcement in composite materials. Continuous fibres provide high stiffness and strength. The good mechanical properties and the low density make them suitable for high performance applications. The mechanical properties of short fibre composites are not quite as good as those of composites reinforced with continuous fibres, but short fibres are still excellent reinforcing materials, widely used for structural components.

One of the most primitive composite materials is clay reinforced with straw, used by the early Egyptians to make building bricks. The development of synthetic, oil based materials gave the composite technology an upswing and today most fibre composites are produced from oil based materials, i.e. of synthetic fibres, such as carbon, glass and aramid fibres, and a polymeric matrix. Environmental issues and concerns about oil dependency are drawing the attention to alternative reinforcement materials and today natural fibres e.g. flax and hemp, are used again as reinforcement in composite materials, e.g. as panels in the automotive industry and in packaging materials. There are several advantages with natural fibres compared to the traditionally used synthetic fibres. Natural fibres are produced from renewable resources, are biodegradable, and their specific properties are excellent. The density and cost are low and the fibres are relatively non abrasive, which gives processing advantages [1,2].

Wood fibres are potential reinforcing fibres for short fibre composite applications. The variability of the mechanical properties is lower for wood fibres than for other types of natural fibres [3] and the raw material is already available in our forests today. The pulp industry provides an infrastructure to produce suitable fibres, not only for paper and board applications, but also for use as reinforcement in composite materials. The mechanical properties must however be improved if wood fibres are to reach their full potential as reinforcing material. Natural fibres tend to swell, deform and degenerate in contact with

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moisture. This phenomenon should also be suppressed to open up for applications in moist environments.

Stress transfer

Efficient stress transfer is important to the mechanical properties of composite materials. If the reinforcement is to be effective stress must transfer from the matrix to the reinforcement through a fibre-matrix interface. A strong interface will transfer stress from the matrix, generally rendering a stronger, stiffer and more brittle material. A weak interface on the other hand will not be able to transfer stress to the same extent, leading to a weaker and more ductile material. Controlling the interface is therefore an important task during material development.

Material development is an interdisciplinary research field that engages chemists, material scientists, physicists and processing engineers. Naturally, the different disciplines have different approaches to composite materials and molecular, microscopic and macroscopiclength scales are used. The fibre-matrix interface in wood-fibre composites can be improved on the molecular scale when chemical fibre modifications are made. The interface properties are also important parameters when describing the microstructure of the composite and when developing micromechanical models. Micromechanical models and laminate mechanics are commonly used to predict the mechanical properties of the composite material. These models are the link from microstructure to the macroscopic level, where mechanical properties such as Young’s modulus, shear modulus and damping properties are important engineering parameters.

Characterisation of interface properties

Test methods used today to determine interface properties are typically single fibre tests [4]. They are originally developed for longer fibres, such as glass and carbon fibres and are rather cumbersome and time consuming when working with wood fibres with lengths of a few millimetres. Less time consuming and more straight-forward methods to study interface and stress transfer would simplify the material development of wood-fibre composites.

In this thesis efforts have been made to gain information about interface properties and stress transfer without single-fibre methods. Different approaches have been used to gain understanding of the stress transfer mechanisms at the different length scales mentioned above. Mechanical testing has been performed and evaluated through micromechanical modelling and image analysis. The dynamic Fourier Transform Infra Red (FT-IR) technique has been evaluated as a tool for evaluation of stress transfer at the molecular level.

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SUMMARY OF PAPERS

Paper A

Dynamic-mechanical properties of wood-fibre reinforced polylactide: Experimental characterization and micro-mechanical modelling

To avoid single fibre testing, new methods for evaluation of stress transfer and interface properties were investigated. Halpin-Tsai’s micromechanical models were used in the complex form together with laminate analogy to predict mechanical properties, i.e. Young’s modulus and damping, of a wood fibre composite material. The model assume perfect interface between fibres and matrix meaning that losses due to imperfections in the interface will be neglected. The predicted data was then compared to experimentally determined data. The predicted and experimentally determined data correlated well for the elastic property, Young’s modulus while the experimentally determined damping was significantly larger than the predicted damping, se Figure 1. The energy loss in the interface is an important factor to this mismatch which therefore can be used as a measure of interface properties. The effect of moist was also studied by performing the study at dry and humid conditions.

0 0.05 0.1 0.15 0.2 0.25 0.3 Experimental Predicted Lo s s f a ct or Dry Humid

Figure 1: Experimentally determined and predicted loss factors at dry and humid conditions

A minor investigation of the use of the dynamic FT-IR (Fourier Transform Infra Red) technique for stress transfer studies was done. The FT-IR results showed the same trends at the molecular level as the mechanical measurements at the macroscopic level. The results are promising but the technique is not yet fully developed for stress transfer evaluation of composite materials.

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Paper B

Effects of moisture on dynamic mechanical properties of wood-fibre reinforced polylactide studied by dynamic FT-IR spectroscopy

The dynamic FT-IR (Fourier Transform Infra Red) technique was further evaluated for stress transfer investigations in a wood fibre composite material. The FT-IR technique is commonly used to characterise polymer samples at the molecular level. The application to wood-fibre composites is, however, relatively new. With this technique specific molecular bonds are detected and studied as the sample is stretched. This gives the possibility to study which molecular bonds are affected when the sample is strained, which provide information of stress transfer at the molecular level. Mechanical properties, such as Young’s modulus and loss factor, were measured simultaneously at the macroscopic level to provide comparison

The effect of humidity on load redistribution between fibres and matrix in the composite material was studied on the molecular level.. Dynamic FTIR spectra for composite material are presented in Figure 2.

-1 0 1 2 3 4 5 1250 1300 1350 1400 1450 1500 Wavenumber (cm-1) Dynamic FTI R respons e 0 % RH 60 % RH 80 % RH 90 % RH

Figure 2: Dynamic FTIR spectra of composite material, in-phase response, 90° polarization

Another finding was that the matrix showed an increased elastic response with increased relative humidity, i.e. with increasing relative humidity, the dynamic loss increases and load is transferred from the fibres to the matrix. An explanation of the relative stress redistribution from the fibres to the matrix is that moisture makes the interfacial stress transfer less efficient.

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Paper C

Comparison of stress-transfer mechanisms in Paper sheets and composites made from resin-impregnated sheets

The stiffness and strength of paper is highly dependent on the degree of fibre-fibre bonding in the paper sheet. Fibre-fibre-fibre bonds are not present in synthetic composite materials, where the stress is transferred between fibres and matrix through a fibre-matrix interface. In wood-fibre composites it is not obvious which of these mechanisms that is dominating. To determine the relative importance of the two mechanisms the effect of fibre-fibre bonding to stiffness and strength of wood-fibre composites was studied.

Stiffness and strength of paper sheets and wood fibre composites with varying degree of fibre-fibre bonding were experimentally determined. The results showed that in contrast to paper properties, composite properties are not dependent on degree of fibre-fibre bonding, Figure 3.

0 20 40 60 80 100 120 140

Degree of fibre-fibre bonding

S tre ng th (MP a ) 0 20 40 60 80 100 120 Spe cific strengt h (kN m/ kg ) ↓↓ ↓ ↑ ↑↑ Ø Ø Matrix material ↓↓ Lowest degree of bonding ↓ Low degree of bonding ↑ High degree of bonding ↑↑ Highest degree of bonding

Figure 3: Tensile strength and specific strength of composite materials with different degree of fibre-fibre bonding.

Image analysis studies show that the fibre network in the paper sheets is not destroyed during the manufacturing of the composites. The degree of fibre-fibre bonding is therefore believed to be the same in the composite and paper samples.

It is therefore suggested that the load in wood-fibre composites is mainly transferred from the matrix to the fibres and not from fibre to fibre, which means that the fibre-fibre bonds have little importance on the strength of this kind of composite. Focus should therefore be on fibre properties and fibre-matrix interfacial properties rather than fibre-fibre bonds when developing pulp-fibre composites with high polymer-fibre interaction.

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CONCLUDING REMARKS

The relationship between microstructure and mechanical properties is of interest in development of novel wood-fibre composite materials. In particular, the role of the interface on the stress-transfer ability between the load-carrying fibres under the influence of moisture is of importance. Since it is cumbersome to investigate interfacial behaviour on the local microscopic level due to unwieldy handling and large local material variability, a more direct macroscopic approach has been opted for. The combination of dynamic mechanical analysis, dynamic FT-IR spectroscopy as well as X-ray microtomography has been used to study the stress-transfer mechanisms. So far, these tools have only relatively sparsely been used to measure stress-transfer phenomena in cellulosic fibre composites. Using these experimental techniques, it has been shown that moisture affects the stress-transfer efficiency of the interface leading to a relative load redistribution from the fibres onto the matrix. In particular, the dynamic FT-IR investigations show that the stiffness is decreased and the loss factor increase as the moisture content of the material increases.

Furthermore, a relative measure of the degree of fibre-fibre bonding has been quantified by x-ray microtomography for differently consolidated paper webs used as precursors in manufacturing of composite materials. It has been shown that the degree of fibre-fibre bonding has negligible influence on the strength and stiffness of composite materials, in contrast to the unimpregnated paper sheets. The fibre-matrix interface is thus of more importance than the fibre-fibre bonds for composite strength and stiffness.

SUGGESTIONS FOR FUTURE WORK

The micromechanical models used today are often based on the assumption that the fibre-matrix interface, and thereby the stress transfer between fibres and matrix, is perfect [5-10]. This assumption is better for some materials than others, but is never a correct description of the actual conditions. In material development, when the properties of the interface can be regarded as design parameters, this assumption is however not adequate. Development and application of models that account for the imperfections in interface is therefore useful in evaluation of interface properties in the materials development.

Wood fibres are sensitive to moisture and water. The problems of swelling and deformation of wood-fibre composite materials will therefore also be addressed. If the problems of moisture were reduced, wood fibres would surely become more attractive as reinforcing material. This warrants development and use of

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mechanical methods to assess the relative influence of fibres, matrix and microstructure on the hygroexpansion and moisture uptake.

A close cooperation between chemists, making modifications of the material, and mechanical engineers, investigating where modifications should be made to improve mechanical properties of the material, could lead to efficient material development of wood fibre reinforced composite materials.

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Stress-transfer mechanisms in wood-fibre composites