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Micro-mechanical mechanisms for deformation in polymer-material structures
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In this thesis, the focus has been on micro-mechanical mechanisms in polymer-based materials and structures. The first part of the thesis treats length-scale effects on polymer materials. Experiments have showed that the smaller the specimen, the stronger is the material. The length-scale effect was examined experimentally in two different polymers materials, polystyrene and epoxy. First micro-indentations to various depths were made on polystyrene. The experiments showed that length-scale effects in inelastic deformations exist in polystyrene. It was also possible to show a connection between the experimental findings and the molecular length. The second experimental study was performed on glass-sphere filled epoxy, where the damage development for tensile loading was investigated. It could be showed that the debond stresses increased with decreasing sphere diameter. The debonding grew along the interface and eventually these cracks kinked out into the matrix. It was found that the length to diameter ratio of the matrix cracks increased with increasing diameter. The experimental findings may be explained by a length-scale effect in the yield process which depends on the strain gradients.

The second part of the thesis treats mechano-sorptive creep in paper, i.e. the acceleration of creep by moisture content changes. Paper can be seen as a polymer based composite that consists of a network of wood fibres, which in its turn are natural polymer composites. A simplified network model for mechano-sorptive creep has been developed. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre-fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys a constitutive law that is non-linear in stress. Fibre kinks are included in order to capture experimental observations of larger mechano-sorptive creep effects in compression than in tension. Furthermore, moisture dependent material parameters and anisotropy are taken into account. Theoretical predictions based on the developed model are compared to experimental results for anisotropic paper both under tensile and compressive loading at varying moisture content. The important features in the experiments are captured by the model. Different kinds of drying conditions have also been examined.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 40 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0442
Keyword [en]
Length-scale effects, Strain gradient, Moisture change, Humidity change, Network model, Fibre model, Mathematical model, Polymer, Micro-indentation, Particle composite, Interfacial debonding, Matrix cracking, Paper, Mechano-sorptive creep, Accelerated creep
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4626OAI: oai:DiVA.org:kth-4626DiVA: diva2:13133
Public defence
2008-02-22, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:15
Opponent
Supervisors
Note

QC 20100910

Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2013-01-14Bibliographically approved
List of papers
1. Influence of molecular weight on strain-gradient yielding in polystyrene
Open this publication in new window or tab >>Influence of molecular weight on strain-gradient yielding in polystyrene
2004 (English)In: Polymer Engineering and Science, ISSN 0032-3888, E-ISSN 1548-2634, Vol. 44, no 10, 1987-1997 p.Article in journal (Refereed) Published
Abstract [en]

Experimental observations have indicated that the presence of strain gradients has an influence on the inelastic behavior of polymers as well as in other materials such as ceramics and metals. The present study has experimentally quantified length-scale effects in inelastic deformations of the polymer material polystyrene (PS) with respect to the molecular length. The experimental technique that has been used is nano-indentation to various depths with a Berkovich indenter. The hardness has been calculated with the method by Oliver and Pharr, and also by direct measurements of the area from atomic force microscopy. The experiments showed that the length-scale effects in inelastic deformations exist in polystyrene at ambient conditions. The direct method gave a smaller hardness than the Oliver-Pharr method. It was also shown that the length-scale parameter according to Nix and Gao increases with increasing molecular weight. For high molecular weights above a critical value of entanglement, there was no pertinent increase in the length-scale parameter. The length-scale parameter for strain-gradient plasticity has a size of around 0.1 μm for polystyrene.

Keyword
Data reduction, Deformation, Hardness, Microelectromechanical devices, Molecular weight, Parameter estimation, Shear strength, Thin films, Molecular length, Strain-gradient yielding, Temperature fields
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7943 (URN)10.1002/pen.20202 (DOI)000224842300020 ()2-s2.0-9144252706 (Scopus ID)
Note
QC 20100910 QC 20110922Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2017-12-14Bibliographically approved
2. Length-scale effects on damage development in tensile loading of glass-sphere filled epoxy
Open this publication in new window or tab >>Length-scale effects on damage development in tensile loading of glass-sphere filled epoxy
2006 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 43, no 24, 7337-7357 p.Article in journal (Refereed) Published
Abstract [en]

Particle-reinforced polymers are widely used in load-carrying applications. The effect of particle size on damage development in the polymer is still relatively unexplored. In this study, the effect of glass-sphere size on the damage development in tensile loaded epoxy has been investigated. The diameter of the glass spheres ranged from approximately 0.5-50 mu m. The first type of damage observed was debonding at the sphere poles, which subsequently grew along the interface between the glass spheres and epoxy matrix. These cracks were observed to kink out into the matrix in the radial direction perpendicular to the applied load. The debonding stresses increased with decreasing sphere diameter, whereas the length to diameter ratio of the resulting matrix cracks increased with increasing sphere diameter. These effects could not be explained by elastic stress analysis and linear-elastic fracture mechanics. Possible explanations are that a thin interphase shell may form in the epoxy close to the glass spheres, and that there is a length-scale effect in the yield process which depends on the strain gradients. Cohesive fracture processes can contribute to the influence of sphere size on matrix-crack length. Better knowledge on these underlying size-dependent mechanisms that control damage development in polymers and polymer composites is useful in development of stronger materials. From a methodology point of view, the glass-sphere composite test can be used as an alternative technique (although still in a qualitative way) to hardness vs. indentation depth to quantify length-scale effects in inelastic deformation of polymers.

Keyword
length-scale effects, particle composite, polymer matrix, interfacial debonding, matrix cracking
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7944 (URN)10.1016/j.ijsolstr.2006.05.026 (DOI)000241539200008 ()2-s2.0-33749350399 (Scopus ID)
Note
QC 20100910. Conference: Colloquium on Size-Dependent Mechanics of Materials. Groningen, NETHERLANDS. JUN 13-15, 2005Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2017-12-14Bibliographically approved
3. Mechano-sorptive creep under compressive loading: a micromechanical model
Open this publication in new window or tab >>Mechano-sorptive creep under compressive loading: a micromechanical model
2008 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 45, no 9, 2420-2450 p.Article in journal (Refereed) Published
Abstract [en]

The creep of paper is accelerated by moisture cycling, an effect known as mechano-sorptive creep. It has also been observed that the mechano-sorptive effects are larger in compression than in tension. In this paper a simplified network model for mechano-sorptive creep is presented. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre-fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys constitutive laws that are non-linear in stress. Geometrical fibre effects are included in the model in order to capture experimental observations of the differences between paper loaded in tension and compression. Theoretical predictions based on the developed model are compared to experimental results for paper both under tensile and compressive loading at varying moisture content. The important features in the experiments are captured by the model, i.e. the creep is accelerated by the moisture cycling and the mechano-sorptive effects are larger in compression than in tension.

Keyword
mechano-sorptive creep, accelerated creep, paper, modelling, moisture changes, humidity change, sorption, fibres, creep, network model, fibre network, mathematical model, fibre bonding, tension, compression
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7945 (URN)10.1016/j.ijsolstr.2007.12.002 (DOI)000254982400003 ()2-s2.0-40649090096 (Scopus ID)
Note
QC 20100910. Uppdaterad från in press till published (20100910).Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2017-12-14Bibliographically approved
4. An anisotropic fibre-network model for mechano-sorptive creep in paper
Open this publication in new window or tab >>An anisotropic fibre-network model for mechano-sorptive creep in paper
2008 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 45, no 22-23, 5765-5787 p.Article in journal (Refereed) Published
Abstract [en]

In this paper a simplified network model for mechano-sorptive creep is presented, which is a further development of an earlier paper [Strombro, J., Gudmundson, P., 2008. Mechanosorptive creep under compressive loading - a micromechanical model. International journal of Solids and Structures 45 (9), 2420-2450.]. It is assumed that the anisotropic hygro-expansion of the fibres leads to large stresses at the fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys a constitutive law that is non-linear. Fibre kinks are included in order to capture experimental observations of larger mechano-sorptive effects in compression than in tension. Moisture dependent material parameters and anisotropy in the fibre distribution have been introduced. Theoretical predictions based on the model are compared to experimental results for an anisotropic paper both under tensile and compressive loading at varying moisture content and it is found that the important features in the experiments are captured by the model. Different kinds of drying conditions have also been examined.

Keyword
Mechano-sorptive creep, Accelerated creep, Paper, Modelling, Moisture changes, Humidity change, Sorption, Fibres, Creep, Network model, Fibre network, Mathematical model, Fibre bonding, Tension, Compression, Anisotropy
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7946 (URN)10.1016/j.ijsolstr.2008.06.010 (DOI)000260273300008 ()2-s2.0-51649092462 (Scopus ID)
Note
QC 20100910. Uppdaterad från submitted till published (20100910).Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2017-12-14Bibliographically approved

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