Interlaminar fracture toughness of layered materials under high-speed loading conditions
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Date
2024-07-24
Authors
Κοτσίνης, Γεώργιος
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Abstract
This thesis delves into the fracture toughness of composite laminates and adhesive joints under high-speed loading conditions, with a particular focus on the elastic vibrations of the specimen during testing. It develops analytical tools for calculating dynamic fracture toughness and for describing the vibrational response of beam-type delamination specimens. These tools are then validated using numerical methods, and the thesis proposes a new experimental setup for testing.
The current understanding of how fracture toughness is affected by loading rate is ambiguous. Various experimental setups, analytical models, and numerical tools have been employed to investigate this relationship. However, each method has its advantages and limitations, which may influence the observed dependency of fracture toughness on loading rate by introducing additional effects from the rapidly applied load, such as elastic vibrations of the specimen.
The first part of this work concentrates on creating analytical tools to calculate fracture toughness at the initiation, propagation, and arrest of a crack. The general scenario involves a beam-type delamination specimen subjected to an external, time-dependent input. This specimen can experience mode I, mode II, and mixed-mode loading conditions. It is assumed that the specimen is loaded using a wedge insertion method, where changing the wedge angle can produce the aforementioned loading types. The initial step is to analyse the specimen as a waveguide to identify the different types of waves that can propagate through it. Subsequently, by applying the appropriate boundary conditions, the vibrational response of the specimen is examined. The analytical model incorporates shear effects and crack tip root rotations, extending existing analytical models and resolving previous issues. At this stage, the frequency equation and the normal vibration modes are derived and categorized based on the excited fracture mode. Finally, dynamic fracture toughness at the initiation, propagation, and arrest of a crack is calculated using energy methods or contour integrals. The Virtual Crack Closure Technique (VCCT) applied to 2D numerical models is used to verify the analytical model, along with published numerical results based on the Interfacial Thick Level Set (ITLS) model.
The second part addresses scenarios where creating an analytical model is challenging, such as for beam-type delamination specimens with geometric asymmetries, different joint materials, or residual thermal stresses. A hybrid method is introduced, which integrates experimentally measured inputs obtained using the Digital Image Correlation (DIC) method into analytical models. The method is applied on a metal-composite bonded joint. Testing is conducted using a novel experimental setup that involves accelerating a wedge with a gas gun. Additionally, the aforementioned analytical model is applied to a composite laminate specimen and a composite adhesive joint to assess the impact of accounting for elastic vibrations on the values of mode I dynamic fracture toughness.
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Keywords
Dynamic fracture toughness, Elastic vibration, Beam waveguide, Wedge insert method, Rapid crack propagation, Digital image correlation, Dynamic testing, Metal-composite bonded joint