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Influence of molecular weight on strain-gradient yielding in polystyrene
KTH, Superseded Departments, Solid Mechanics.
KTH, Superseded Departments, Solid Mechanics.
KTH, Superseded Departments, Materials Science and Engineering.
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.

Place, publisher, year, edition, pages
2004. Vol. 44, no 10, 1987-1997 p.
Keyword [en]
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: urn:nbn:se:kth:diva-7943DOI: 10.1002/pen.20202ISI: 000224842300020Scopus ID: 2-s2.0-9144252706OAI: oai:DiVA.org:kth-7943DiVA: diva2:13129
Note
QC 20100910 QC 20110922Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2011-09-22Bibliographically approved
In thesis
1. Micro-mechanical mechanisms for deformation in polymer-material structures
Open this publication in new window or tab >>Micro-mechanical mechanisms for deformation in polymer-material structures
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
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:nbn:se:kth:diva-4626 (URN)
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
2. Length-scale effects in yielding and damage development in polymer materials
Open this publication in new window or tab >>Length-scale effects in yielding and damage development in polymer materials
2005 (English)Licentiate thesis, comprehensive summary (Other scientific)
Place, publisher, year, edition, pages
Stockholm: KTH, 2005. 13 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0394
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-485 (URN)
Presentation
2005-11-16, B3, KTH, Brinellvägen 23, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20101123

Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2013-01-15Bibliographically approved

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