JEC COMPOSITES MAGAZINE - Issue #111 - March 2017 - Special JEC World - o - 111

A virtual test lab for structural composite materials

Fig. 2: Local-to-global multiscale simulation strategy to
conduct virtual mechanical tests of composite materials
that explores the hierarchical structure of composite
materials

Complex design and certification

Multiscale simulation

However, precisely because of the hierarchical structure of
FRP that reflects in an orthotropic behaviour regarding stiffness, strength and damage mechanisms, as well as because of
uncertainties introduced by their manufacturing processes,
the design and certification of composite structures are more
complex than equivalent approaches followed for metallic
structures. In the aerospace industry, for example, the certification of composite components is traditionally carried out
by means of experimental campaigns following a bottom-up
approach and advanced numerical simulations following a
top-down strategy. The experimental tests span from small
coupons to panels, and from subcomponents up to the final
global structure to obtain the mechanical properties at the
different length scales. In parallel, the design is carried out
using a top-down approach beginning by the analysis of the
whole structure [1]. The results of these analyses are used to
provide the boundary conditions for more refined analyses
at lower length scales, using the material properties obtained
from the corresponding mechanical tests. The modelling detail is increased as successive analyses "zoom in" on structural regions, identified as being potentially strength or toughness critical, e.g. likely to suffer drop-weight or bird impact.
While this approach has been successful from the design and
certification viewpoint, it has several serious drawbacks in
terms of cost and materials optimization. Firstly, the pyramid
of mechanical tests (from coupons to structures) is very expensive. Moreover, certification should be achieved for different operating environmental conditions and ageing stages. Due to the lengthy sample conditioning process, these
requirements can significantly extend the complete certification of a new material to several months or even years.
Secondly, any change in the properties of either matrix or
fibres requires repeating the entire certification campaign,
leading to a large inertia to introduce new materials. Finally,
FRP properties are used as input at every length scale, but
are not optimized for each situation, leading to sub-optimum
designs.

These limitations are being overcome by the novel bottom-up,
multiscale simulation strategies that follow the hierarchical
structure of FRP [2]. This new strategy comes about as a result
of recent advances in different areas, including micromechanical characterization of constituent properties, development
of accurate numerical modelling tools for composites at the
micro- and mesoscale, coherent strategies to pass information
between length scales, and the use of computer parallelization
techniques to further expand the increasing power of microprocessors (CPU) accelerated by graphics processing units
(GPU). In consequence, "virtual tests" are starting to be used
in engineering applications to limit the number of costly experimental tests used to certify the safety of composite structures, and to reduce development time.
The overall multiscale simulation scheme takes advantage
of the natural separation of length scales between composite entities: ply, laminate and structural elements (Figure 2).
This allows multiscale modelling to be carried out by computing the properties of one entity (e.g. individual plies)
at the relevant length scale, homogenizing the results into
a constitutive model, and passing this information to the
next length scale to determine the mechanical behaviour of
the larger entity (e.g. laminate). This procedure is repeated
between laminates and structural elements. At each length
scale, the main material properties (stiffness, strength and
toughness) are determined for different loading conditions
and passed as input for the simulations at the next length
scale. Thus, there are two main ingredients to this multiscale
modelling strategy. First, the determination of the FRP constituent properties (matrix, fibre and fibre/matrix interface)
by means of experimental micromechanical tests on small
composite material samples, which are rapidly conditioned
if required. Second, the simulation of the mechanical properties at the ply, laminate and structural component levels
using the FE method by means of computational micromechanics, mesomechanics and structural mechanics, respectively.
No111 March 2017

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Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #111 - March 2017 - Special JEC World - o