טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentFeldfogel Shai
SubjectDynamic Debonding in Plates Strengthened with Composite
Materials
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Oded Rabinovitch
Full Thesis textFull thesis text - English Version


Abstract

The need for strengthening of existing structures has been steadily growing in recent decades. Bonding of strips of composite FRP materials to deficient structural elements is often used to address this need. The main failure mechanism of the strengthened configuration is interfacial debonding. Experimental observations repeatedly point at the sudden, brittle, and unstable nature of these interfacial debonding mechanisms, which is by now well established in the context of on 1D beam members. In the much fewer studies on 2D strengthened plate members such as two-way slabs and shear walls, the sudden and brittle nature of the interfacial failure has been pointed at as an experimental observation, but it has not been explained, captured, or quantified analytically or numerically, constituting a gap of knowledge.

The objectives of this study are to qualitatively and quantitatively characterize the nature of interfacial debonding in 2D plate elements. This refers to a host of open questions regarding the inherently 2D and geometrically irregular evolution of plate-type debonding, the critical load levels that trigger it, the stability characterization, and, principally, the dynamic characterization of interfacial debonding in plate strengthened members. To achieve these objectives, a nonlinear and dynamic analytical-computational platform for the analysis of interfacial debonding in layered plates, comprising a multilayered plate model and a corresponding specially tailored in-house finite element formulation, is developed. This platform is based on the unique features of FRP strengthened plates which include a soft and vertically compressible adhesive layer sandwiched between the rigid substrate and FRP layers, a 2D debonding area and contour that constantly change their size, shape, location, morphology. highly localized interfacial tractions that govern the debonding evolution, irregular existing delaminations, geometrical nonlinearity, a large scatter of geometric, material, and stiffness scales between the layers, plate-type substrate cracking, non-monotonous equilibrium paths due to interfacial instabilities, and high-rate dynamic behavior complete the picture of physical phenomena. To account for these features, the platform integrates a variety of methodologies, including high-order adhesive modeling, nonlinear thermo-elasticity, 2D plate-type cohesive interfaces, bending-shear decomposition techniques, arc-length continuation methods, Newtonian dynamics, Newmark time-integration schemes, variational principles, and more.

This study investigates debonding phenomena in multiple structural. Numerous comparisons with experimental results and observations taken from the literature, closed-form solutions, and 2D and 3D FEA are scattered throughout the study and support the validity and veracity of the analytical-computational platform, while serving to shed light on and quantitatively explain many of the experimental observations. The analyses in this study show that the interfacial failure in FRP strengthened plates is unstable and dynamic by nature. They also serve to quantify, describe, and interpret the inextricable inter-relation between this unstable and dynamic nature and the inherently 2D and geometrically irregular evolution of the debonding area. The study raises and discusses the insufficiency of definitions and nomenclatures borrowed from the realm of 1D beam-type debonding to describe 2D plate-type debonding. This insufficiency raises the need for new terminology and definitions, which reflect renewed thinking and understanding of the 2D plate-type debonding phenomenon as such.