Fracture analysis of carbon fiber/epoxy matrix interface through microbond and cruciform tests

Potukuchi, Sri Krishna Sasidhar

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https://hdl.handle.net/2142/90456 Copy

Description

Title

Fracture analysis of carbon fiber/epoxy matrix interface through microbond and cruciform tests

Author(s)

Potukuchi, Sri Krishna Sasidhar

Issue Date

2015-12-22

Director of Research (if dissertation) or Advisor (if thesis)

Geubelle, Philippe H.

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• carbon
• epoxy
• cohesive failure model
• microbond test
• cruciform test
• Abaqus CAE

Abstract

In fiber-reinforced polymeric-matrix composites, the fiber/matrix interface plays a key role in transferring loads from the fibers to the matrix through shear. In this project, we investigate numerically two tests, used to characterize the normal and shear interfacial failure of a carbon fiber/epoxy matrix system. The first part of this study is devoted to the simulation of the microbond test, in which, a drop of epoxy deposited on a carbon fiber is subjected to a longitudinal load, which eventually leads to the shear failure of the interface. An axisymmetric finite element analysis is carried out with ABAQUS [2] CAE to extract the parameters (failure strength, fracture toughness, friction coefficient and the final displacement to failure), that define the cohesive failure model used to simulate the initiation and propagation of the crack front along the interface. Emphasis is placed in this study on characterizing the interfacial failure properties of three composite systems, defined by the surface treatment of the carbon fiber. Special care is taken to capture accurately, the shape of the epoxy bead, and in particular, the meniscus created by surface tension effect during the deposition of the bead on the carbon fiber. The nonlinear finite element analysis also takes into consideration, the residual stresses present along the fiber/matrix interface due to the mismatch in the coefficient of thermal expansion between the fiber and the matrix. The parameters defining the bilinear cohesive failure law are extracted through a comparison between numerical predictions and experimental measurements of the axial force vs. displacement curve. The cohesive model is then validated by simulating the shear failure of other bead/ fiber systems with the same surface treatment. Results show a very strong dependence of the interfacial failure strength and fracture toughness on the surface treatment of the carbon fiber. The numerical analysis also investigates the sensitivity of the solution on the cohesive model parameters. The second part of this study involves a detailed 3D linear finite element analysis, again using ABAQUS [2] CAE, of the cruciform test, which aims at extracting the transverse (normal) failure property of the fiber/matrix interface. The focus of the work is placed on investigating the effect of key geometrical parameters, such as the thickness of the cruciform specimen and the gap between the fiber and the face-sheets, on the ratio between the maximum transverse traction acting on the fiber and the maximum principal stress present along the fillet of the cruciform. This ratio plays a critical role in determining the location of the failure process, and therefore, the success of the experiment. We show numerically that, while the thickness of the specimen does not seem to affect that ratio, decreasing the distance between the fiber and the face-sheets strongly favors a fiber/matrix interface failure.

Graduation Semester

2016-05

Type of Resource

text

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http://hdl.handle.net/2142/90456

Copyright and License Information

Copyright 2016 Sri Krishna Potukuchi

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