ANALYSIS OF THE MICROBOND TEST USING
NONLINEAR FRACTURE MECHANICS
J. Novák1, C. P. Pearce1 and P. Grassl1
L. Yang2, J. L. Thomason2
1 University of Glasgow
Dept. of Civil Engineering, Glasgow G12 8LT
2 University of Strathclyde,
Dept. of Mechanical Engineering, Glasgow G1 1XQ
SUMMARY
Microbond tests composed of single fibre and matrix droplet are often used to determine
the properties of fibre reinforced composites. Interfacial shear strength is quantified
by the maximum pull-out force assuming a uniform stress distribution along the fibre.
Here, finite element analyses are performed to investigate the validity of this assumption.
Keywords: Microbond, pull-out, debonding, bond strength, FE simulation
INTRODUCTION
Microbond pull-out tests composed of a single fibre and a matrix droplet are often used to quantify the interfacial properties of fibre reinforced composites (Fig.1). The main parameter provided by the test is a value representing the bond strength between fibre and matrix, which is determined by the peak of the load-displacement curve assuming a uniform stress distribution along the fibre. However, local stress concentrations due to the geometry and loading setup of the test might play a significant role for the determination of the bond strength. The aim of the present work is to study these effects by detailed two-and-three-dimensional nonlinear finite element analyses. The importance of the effects is presented and possible improvements of the shear strength evaluation are discussed.
Figure 1: Microbond test sample and 2D FE discretisation.
NONLINEAR FINITE ELEMENT ANALYSIS
The microbond test is modelled by means of the nonlinear finite element method. Fibre and droplet are represented by continuum finite elements, and the interaction between the two phases is described by interface elements [1]. The influence of droplet diameter and length, fibre diameter and knife position on the value obtained for the average bond strength is investigated by means of a parameter study (Fig.1).
Special attention is paid to the stress distribution and debonding process and its influence on the load-displacement curve. Debonding at the matrix/fibre interface is described by a new scalar damage-plasticity constitutive model which is used in the interface elements. This constitutive model is capable of taking into account confinement effects caused by thermal expansion and contractions. Stiffness reduction is described by the damage part of the constitutive model.
Input parameters for the finite element analyses are determined by experiments. Furthermore, the results of the parameter study are compared to trends observed experimentally in [2].
ACKNOWLEDGEMENTS
Funding by the Glasgow Research Partnership in Engineering (GRPE) for the present research is gratefully acknowledged.
The finite element analyses are performed by means of the object oriented finite element program OOFEM [3], which was extended by the authors for the present study.
References
1. N. Nishikawa, T. Okabe, K. Hemmi, N. Takeda, 2008. Micromechanical modelling of the microbond test to quantify the interfacial properties of fibre-reinforced composites. International Journal of Solids and Structures 45, 4098-4113.
2. L. Yang, J. L. Thomason, 2008. Interface strength in glass fibre-polypropylene measured using the fibre pull-out and microdebond methods. ICCM17 Conference Proceedings.
3. B. Patzak, Z. Bittnar, 2001. Design of object oriented finite element code. Advances in Engineering Software 32(10-11), 759-767.