Design, Analysis and Testing of Composite T-joints

Abstract

Thin shell aerostructures are prone to buckling and have low out-of-plane strength, which requires the use of stiffeners with various cross-sections attached to the skin. When manufactured from composite materials, these out-of-plane joints introduce a deltoid shaped void in the structure, often filled with a unidirectional fibre bundle or “noodle”. The production of this filler is labour-intensive, and the final product suffers from a knockdown in strength due to the low transverse tensile strength of the material and fabrication-induced defects.
This thesis introduces an experimentally validated numerical framework capable of accurately capturing the damage initiation of T-joins subjected to tensile (pull-off) loading. A new phenomenon called “filler in-situ strength” is proposed and demonstrated to be critical for defining whether the first event of damage occurs in the filler or in the laminate of a T-joint. Then, the model is used to tailor the failure behaviour of T-joints, and to propose a configuration for experimental testing of the mechanical behaviour of fillers.
Finally, eight alternative filler concepts are introduced and experimentally validated. Polyamide non-woven interleaved joints increase the damage tolerance of the structure at the cost of 25% damage initiation load reduction, vertical nanotubes yield no difference, and non-woven nanofibres match the strength within 6%. 3D printed fillers have 43% lower strength but demonstrate the possibility of thermoplastic-thermoset hybrid structures. Fillers made of chopped unidirectional and woven prepreg match the strength of the baseline noodle within -6% and 5%, respectively. A resin infused braided concept has 22% lower strength, but its counterpart made of multiple braids has the same strength (2% difference) as the rolled noodle.
The proposed solutions can serve as a basis to modify the strength, cost and manufacturing time of composite out-of-plane joints. In addition, the concept of hybridising thermoplastic and thermoset resins, and different type of thermoset resins within the same structure and in the same cure cycle opens up a wide range of new applications involving the combination of thermoplastic, infused braided, 3D woven and thermoset prepreg materials.

Date of Award	4 Sept 2019
Original language	English
Awarding Institution	University of Bath
Sponsors	GKN Aerospace
Supervisor	Chris Bowen (Supervisor), Richard Butler (Supervisor) & Andrew Rhead (Supervisor)