Non-linear deformation in fibre-reinforced epoxies: micromechanical characterisation and modelling by Nathan KLAVZER

Fibre-reinforced polymer (FRP) composites are omnipresent in structural applications requiring high stiffness or strength and low weight.  The matrix, typically made from highly cross-linked epoxies, often plays a crucial role in the damage and failure mechanisms dictating the overall strength of the composite. Hence, over the past two decades, bottom-up multiscale approaches have been at the heart of the research for composite materials. Understanding the mechanisms defining the deformation and failure of the constituents at the microscale is of key interest. This thesis provides experimental characterisation and modelling of an epoxy matrix at this scale. First, the macroscale properties of several highly cross-linked epoxies are compared and rationalised via the glass transition temperature. The aim is to determine if master trends exist in epoxies. (Non)-linear relationships between several mechanical properties, such as the yield stress, and the glass transition temperature are found, valid at different temperatures and strain rates. Second, the microscale mechanical response of the matrix in confined volumes between fibres is studied. A novel nano digital image correlation (DIC) method was developed. It relies on the creation of dense and homogeneous speckle pattern with nanoscale particles. Nano DIC allows quantifying strain fields at the submicron level and identifying regions of strain concentrations. Using the nano DIC results, a strain gradient plasticity model has been added to the constitutive model for the matrix material. This model resulted in accurate predictions of the matrix strains at the microscale and of the macroscale stress-strain response of the composite. Finally, nanoindentation was used to assess size effects within epoxies, namely the indentation size effect and the increase in the hardness of the matrix when in-between fibres of a composite. The influence of physical ageing and of the low strain rate response were carefully studied.

Jury members :

• Prof. Thomas Pardoen (UCLouvain, Belgium), supervisor
• Prof. Eric Deleersnijder (UCLouvain, Belgium), chairperson
• Prof. Hadrien Rattez (UCLouvain, Belgium)
• Prof. Pedro Camanho (University of Porto, Portugal)
• Dr. Jérémy Chevalier (Solvay, Belgium)
• Prof. Eric van der Giessen (University of Groningen, The Netherlands)
• Prof. Carlos Gonzàlez (IMDEA Materials Institute Madrid, Spain)

Visio conference link:  https://teams.microsoft.com/l/meetup-join/19%3ameeting_YjNlYzA4NjItMjEyYy00NDZjLTlkOGMtYjQzMzQ5NjYzNzc0%40thread.v2/0?context=%7b%22Tid%22%3a%227ab090d4-fa2e-4ecf-bc7c-4127b4d582ec%22%2c%22Oid%22%3a%22ece5769e-ff2b-448e-b50d-dbc1f9272aa5%22%7d