Investigation of delamination crack under vibration fatigue exploiting simulation aided dynamic testing in digital environment
Frank Oosterveld (July 2021)
SUMMARY
This thesis work examines the damage propagation in composite laminates under vibration fatigue. Throughout the chapters, an overview of fatigue behaviour of composites will give an insight into the challenges of testing composites and a complete theoretical background will allow the reader to understand the method development. Nowadays, composite materials are used in all structural components of automotive, aerospace and, soon, space industry. However, fatigue is still an open challenge for structural components because of its underlying complex physics. One of the major challenges of testing in composites is to overcome the extremely time consuming testing procedure. Standard fracture mechanic tests under ASTM standards use a superposition of mode-I and mode-II to take into account the mode mixity ratio in real conditions. However, aerospace loading environments are dynamic and not quasi-static as in the testing practices. As an alternative, vibration testing is proven to be effective in monitoring the onset of delamination and its propagation. Experimental measurements underscore that the delamination growth can be described by the linear relationship that the vibration response phase has with the number of excitation cycles. Yet, these measurements are time consuming, as CT scans and microscopes are being used to identify the crack length. This leads to the exploitation of numerical models about the observed experimental behaviour.
Previous numerical simulations did not take into account the friction forces developed at the interfaces of the delamination. One of the main objectives is to construct a remastered finite element model including contact elements to simulate the friction effects. Consequently, a framework is developed to investigate the forces at the crack tip acting at two ends of the delamination, based on steady-state dynamics instead of transient analysis, allowing a rapid simulation process. With use of this novel numerical framework, the forces determining the failure mode-I and mode-II are investigated.
Due to the inclusion of the contact area in the FE model, a different numerical mode-mixity is present for the ‘crack-opening’ and ‘crack-closing’ deflection of the fully reverse cycle. For the crack-opening deflection, the GI (the mode I bending contribution to crack opening) has the most dominant influence in the total SERR, GT. However a combination of both GI and GII (the mode II shear contribution to crack opening) results in the crack growth. For the crackclosing deflection, it can be concluded that the GII does not have any contribution to the crack length. This means that, for opening the crack, all the strain energy comes from the in-plane shear sliding mode.
Finally, the numerical analysis constructs a relationship between the SERR and the vibration response phase over the total delamination length. A novel calibration factor/function is introduced to convert the measured response phase into the equivalent Strain Energy Release Rate. This results a broader understanding in the physical phenomena underneath the delamination crack under vibration fatigue will be gained. By obtaining a more profound understanding in the physics, the results of the experiments can be analyzed in a better way. This thesis represents a solid starting point to elaborate on fracture mechanics investigation, especially for aerospace applications where time saving, efficient and innovative techniques are required.