Fatigue behaviour of carbon fibre composites subjected to load reversals and variable mode ratios

Abstract

Abstract
The acceleration of development cycles in aerospace projects creates the need for a higher degree of ‘virtualisation’ in aero engine development, i.e. replacing physical testing by numerical modelling wherever it is deemed safe to do so. A major contributor to the development time in this industry is the testing of prototypes, especially for their long term properties and maintenance cycles. This project is aimed at providing a better understanding of fully-reversed composite fatigue to simplify and shorten the development process. A new method to allow testing of fully reversed fatigue in variable modes has been developed. Based on the standardised End-Loaded Split (ELS) test this method allows full reversal of through-thickness shear (mode-II) plus alternating crack opening (mode-I) and closure during the loading cycle. The new method has been subjected to extensive testing and verification to ensure the validity of the conducted tests. The specimen behaviour when subjected to cyclic loading has been evaluated using practical and numerical methods. The newly developed method was then used to investigate the fatigue behaviour of two different representative toughened composite materials, the Hexcel®IM7/8552 and IMA/M21 sys- tems. These have been tested in fully reversed conditions with the application of different combinations of mode-II and mixed-mode (I and II), at different ‘severities’ to achieve a comprehensive overview of the material degradation in these conditions. Great effort has been put into documenting the fatigue behaviour of these materials and comparing them against each other and against existing non-reversed fatigue data. In an effort to improve the understanding of the micro-mechanics of the material, the fracture surface and crack process zones have been investigated using a scanning electron microscope. The fracture surfaces have been analysed in terms of the mechanisms responsible for energy absorption during static and dynamic tests. The challenges encountered during development of test methods and an outlook of future work are also presented.

Date of Award	12 May 2020
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Luiz F Kawashita (Supervisor) & Stephen R Hallett (Supervisor)