10 Juillet – Thesis defense - Torquato Garulli

10 h30 Full videoconferencing

Design and validation of fully-uncoupled multi-directional lay-ups to evaluate interlaminar fracture toughness.

The object of this study is the development of a novel class of stacking sequences
for the design of multidirectional polymer matrix laminated composite specimens for interlaminar fracture toughness (or delamination) tests. These sequences allow to obtain multidirectional specimens that, in the framework of Classic Laminated Plate Theory,  have a thermo-elastic behaviour that closely matches that of unidirectional specimens: they are completely free from elastic couplings and they do not develop laminate-level thermally-induced deformations due to the curing process. Furthermore, they allow to test delamination interfaces between plies of any desired orientation. Because of their properties, they were labelled Fully-Uncoupled Multi-Directional (FUMD).
In order to design these layups, Quasi-Trivial (QT) solutions were exploited.
Firstly, an algorithm for the creation of a database of such solutions was conceived and implemented.
Thanks to it, more and longer QT solutions were found than in previous studies.
Then, analytical rules were established allowing to obtain new QT solutions from the superposition of known ones.
These criteria allow to obtain QT sequences of any desired length, thus overcoming existing computational limitations arising when searching for QT solutions using the algorithm.
Combining QT solutions with a few basic laminate design principles and the superposition criteria, FUMD stacking sequences are designed.
In order to assess the properties of delamination specimens obtained with FUMD layups, a Finite Element model of a Double Cantilever Beam specimen was developed and used to compare the behaviour of a FUMD layup with that of other sequences proposed in relevant literature on the topic of delamination in multidirectional laminates.
By means of the standard and of a revised Virtual Crack Closure Technique formulations, Energy Release Rate distributions and modal partitions of the specimens were evaluated.
It emerged that the FUMD layup resulted in an optimal behaviour of the specimen.
Eventually, a mode I interlaminar fracture toughness experimental campaign was performed.
FUMD Double Cantilever Beam specimens were fabricated, along with with unidirectional ones.
A UD-fabric material was used to reduce the likelihood of delamination migration.
Rotations of the specimens arms and the shape of the delamination fronts were studied in order to assess the capability of the specimens to yield the correct mechanical behaviour for mode I delamination testing.
For both aspects, FUMD specimens yielded results similar to those obtained with unidirectional specimens.
With respect to interlaminar fracture toughness, specimens with identical delamination interface yielded similar values, even if their global stiffness was different.
On the other hand, different interfaces led to different interlaminar fracture toughness, related to different fracture behaviours.
While this work represents a preliminary study and further research is clearly required, FUMD delamination specimens have shown a good potential, and they may stand out as a viable solution for interlaminar fracture toughness tests.
Possibly, they could be considered for an extension of the scopes of existing standard test methods to multidirectional laminates and interfaces.

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