Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

Development of testing methods for measuring the effect of mode III loading at the crack tip on
fracture toughness and fatigue crack growth rates, is necessary for the development of accurate
damage assessment tools for analysis of the residual strength and lifetime of debonded sandwich
composites.
In the last chapters of the thesis, fatigue experiments were conducted on sandwich panels with a
circular debond to validate the developed 3D numerical scheme. However, to fully examine the
effectiveness of the developed numerical scheme and its limitations, more experiments on more
complex geometries like curved structures and loading conditions, such as e.g. lateral pressure
and in-plane compression, should be conducted. Additionally, since direct access to the crack in
debonded panels is not possible, methods of measurement such as FBG sensors or ultra-sonics
should be used instead of DIC technique to obtain better measurement of the debond growth.
Different finite element based models have been put forward in this thesis based on fracture
mechanics tools. However, all these models are limited to very simple, small geometries due to
the need for high-density mesh at the crack tip (front) and practical/geometrical limitations
regarding the generation of 3D finite element models in complex large geometries. In summary,
in the case of damage assessment of large structures the global response of the structure should
be accounted for in these models making the resulting finite element model extremely
computationally heavy. An important step in further development of the devised damage
assessment schemes is to develop new methodologies for the analysis of delamination/debonding
in composite structures, taking into account the global response of the structures. The
submodelling concept, as used in Chapters 2 and 3 of this thesis, can be used in a new way which
allows for coupling between the global response of the structure and the local effects of the
damage. As schematically described in Figure 6.1, an iterative procedure may be devised which
couples the global complete model of structures and a detailed model of the damaged region as
developed in this thesis. The global stiffness properties and behaviour of a structure, e.g. a wind
turbine blade, can be determined using finite element modelling of the blade by shell elements as
the first step. By reducing the stiffness of the elements in the damaged zone the effect of initial
damage on the global behaviour of the structure can be estimated. The displacement boundary
conditions of the 3D submodel, detailing the damaged zone of the structure, are subsequently
updated on the basis of the results from the global shell finite element model by shell to solid
submodelling technique, which is available in most of the commercial finite element software
like ANSYS. The detailed analysis of the damaged zone can be conducted by means of state-ofthe-art
fracture mechanics tools developed in this thesis. In the case of cyclic loading by
determining the debond growth at the end of each cycle (cycle jump), the geometry of the
debond can be updated in the global shell model accordingly. Finally, the global shell FE model
of the blade can be reconstructed once again and the procedure is repeated for the next iteration,
e.g. loading cycle. Thus, it is possible to overcome the scale limitations and couple the local
scale effects of the debond damage with the global scale response of the blade.
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