טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBen-Dror Efrat
SubjectDebonding Processes in Large Scale Structural Elements
Strengthened with Composite Materials - An XFEM
Approach
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Oded Rabinovitch
Full Thesis textFull thesis text - English Version


Abstract

The strengthening of beams using fiber reinforced polymer (FRP) composite materials has become a common method for structural upgrading. The strengthened beam consists of a composite FRP plate attached to the existing beam with a layer of adhesive. The debonding mechanism governed by the separation of the FRP plate from the original beam is probably one of the most critical failure modes of the strengthened beam.

The recognition of the debonding as a critical failure mechanism had led to a significant research effort. However, most of this effort has focused on small (lab-scale) beams. The few experimental studies on large beams indicate that such beams behave differently from small beam throughout the debonding process. However, analytical and numerical tools for the analysis of large beams are not found. Analytical solutions are based on exponential functions and applying them to large beams tends to fail, mainly due to the scatter of characteristic length scales. Numerical models and particularly Finite Element (FE) ones fail as well due to the same reason. On one hand, the mesh must agree with the characteristic length of the physical phenomena, which is few millimeters long. On the other hand, a realistic beam is few meters long and in order to keep the computational burden reasonable, the elements cannot be too small.

This study presents an analytical/numerical framework for the analysis of FRP strengthened beams. The framework combines the advantages of three methods: the high order model and its ability to reduce a 2D case into a 1D case and thus to reduce the number of degrees of freedom; the FE method; and the eXtended FE Method (XFEM), which takes into account the local phenomenon inside the element and spares the fine meshing. The three methods are used for the generation of a specially tailored high order 1D XFEM, which is capable for the analysis of large beams.

The study addresses the problem in two stages, the stress analysis stage and the debonding analysis stage. In the first stage, the peak stresses initiating the debonding process are quantified. The results reveal that up to a certain beam length the peak stresses change with the length of the beam. For longer beams the peak stresses keep constant regardless of the length. The second stage characterizes the debonding process through equilibrium path analysis. The results of the debonding process in large beam are in qualitative agreement with ones reported in literature. A scaled down model reveals that the behavior of a small beam throughout the debonding process may be fundamentally different from the one detected in the large beam. 

This research takes a step towards the understanding and quantifying the debonding process in large beams. In addition, it reveals some of the fundamental differences between the behavior of small beams and that of large ones. The implementation of the concept in a FE - XFEM platform enables a variety of future extensions such as non-linear, dynamic, and plate analyses and to the analysis of other layered structures.