Date of Award

2018

Document Type

Open Access Thesis

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Prasun Majumdar

Abstract

Interfaces are often the most critical part of the heterogeneous materials and their structures. Such interfaces appear a multiple length scales and significantly affect bulk properties in different material systems. Fiber reinforced composite materials are one such example of heterogeneous materials with interfaces at constituent scale (fiber-matrix interface and interphase) to laminate scale (secondary adhesive joints). Manufacturing induced defects and subsequent in-service damage at these interfaces can severely affect the durability of fiber reinforced laminated composite materials and their joints. In addition to lamination process, defects can also form during secondary joining process (e.g., adhesive bonding and repair) which can be very detrimental to the performance of the composite structure as it can become the “weak link”. Adhesive bonding has wide applications in different disciplines including bio-medical, energy, automotive, civil and aerospace structural composites. With growing popularity of adhesive bonding, manufacturing defects at the interfaces of joints have attracted increased attention of many researchers in the recent years. Although significant progress has been made in detecting specific types of defects such as voids and large debonding using different NDE methods, there is a lack of understanding of so called “zero volume” defects which forms a weak interface. The purpose of this study is to examine the role of defects at the adhesive-to-laminate interface on multi-physical properties, and specifically understand how a weak interface may affect mechanical durability.

In this study, a multi-faceted approach has been taken to understand fundamental scientific challenge of surface properties and its implications on bonded interface. Controlled experiments have been designed to create weak interface by different surface modification technique and validated with indirect measurement of work of adhesion. Using broadband dielectric spectroscopy (BbDS) technique, the effect of surface modification on dielectric properties are quantified. This modification is then translated in formation of a truly “weak interface” with “zero volume” unlike a traditional disbond or delamination type scenario. The laminated composite joint is then studied for both mechanical and dielectric property changes due to the formation of weak interface. Carbon fiber reinforced laminate and epoxy adhesive were taken as test bench material systems although the fundamental focus is not on the material itself but the interface

Interface strength of bonded joint was determined experimentally and quantitatively linked to dielectric properties. Several analytical models were developed to understand the effect of weak interface on dielectric properties and results demonstrate that the model captures observed experimental trend of property changes. Moreover, failure of bonded structure under tension loading was also predicted with analytical tools available in the literature and it was observed that only shear dominated brittle fracture represents the weak interface adequately. This is an interesting shift as a strong joint will show ductility in terms of both shear and peel stresses due to adherend bending effect. Details of experimental results and analyses of weak interface have been included in this thesis

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