Akademik Veri Yönetim Sistemi
12th EASN International Conference on "Innovation in Aviation & Space for opening New Horizons", Barcelona, İspanya, 18 - 21 Ekim 2022, ss.1
Carbon
fibre composite structures have been extensively applied in various fields including
aerospace, marine, and civil industries due to their advantages which include high
strength and stiffness to weight ratio. Often to meet specific needs, requires
an integrated approach to designing composite structures including material
selection, manufacturing method, and appropriate testing prior to final use. Regarding
testing, fracture toughness is an important property that dictates the service
life of composites. Composites do not exhibit fatigue failure like metals yet
they are more prone to failure by delamination. This can be due to several
factors including but not limited to manufacturing defects such as voids, machining-induced
stress-concentrated zones, insufficient wetting of fibres, or service
conditions such as humidity, moisture absorption, cyclic loadings, impact, wear
etc. In addition, interlaminar stresses are generated due to significant differences
in moduli values of resin and fibre which leads to excessive strain in the
resin. Delamination is possible in such cases where through-thickness
reinforcement is insufficient.
Fracture
is divided into three pure failure modes, opening (Mode I), sliding (Mode II),
and tearing (Mode III) with Mode I being the most commonly conducted fracture
toughness testing approach. The Double Cantilever Beam (DCB) test method was
standardized as ASTM D 5528 in 1994. Standards are also available for Mode-II
end notched flexure (ENF) and Mixed-Mode Bending (MMB) tests on unidirectional
(UD) laminates. However, most of the laminate designs of composite structures
involve multidirectional (MD) laminates, and delamination usually occurs at the
interfaces between differently oriented plies.
Service
conditions most of the time, do not involve a single mode of fracture but a
combination of those as previously mentioned. Therefore, MMB tests have been
increasingly implemented in fracture studies. Even though there is considerable
literature on the experimental aspects of MMB, numerical modeling studies are
quite limited. This study, therefore, first aims to experimentally investigate
the mixed-mode fracture behaviour of 20 plies of USN150B laminated composite
structure which was also used in a representative wing structure built in
Cardiff University under the context of a COST Action CA 18203 Optimising
Design for Inspection. In addition to several mechanical characterization tests
for representative wing structure components, experiments will be conducted by
means of an MMB fracture toughness test fixture (ASTM D 6671) for the unidirectional
composite sample with the conditions provided in Table 1. Then, the numerical
models will be established to simulate the experimental response of the
composite panel. To this goal, different techniques including virtual crack
closure technique (VCCT), cohesive zone modeling (CZM), and extended finite
element method (X-FEM) will be employed and their results will be compared.
Table 1. MMB testing specifications for 20-ply
composite sample
|
Sample Width, mm |
20 |
|
Total length, mm |
160 |
|
Thickness, mm |
3.8 (20 plies) |
|
Pre-cracked length, mm |
63 |
|
Solid length, mm |
117 |
|
Pre-crack insert thickness,
μm |
13 |