Composite Materials Research Progress

Multi-scale Analysis of Fiber-Reinforced Composite Parts… 43
failure envelope of the matrix entering in the composition of a composite structure, in the
cases that a pure mechanical load is applied. The identification method was built around an
inverse scale transition method which requires the knowledge of the macroscopic strengths,
and both the macroscopic and microscopic elastic stiffnesses. Besides, it was necessary to
consider some hypotheses in order to proceed to the identification of the coefficients of the
microscopic quadratic failure criteria. In the present work, it was assumed that the
macroscopic failure of a uni-directionally reinforced ply is dominated by the local failure of
the matrix when the external load is applied in planes perpendicular to the fiber axis.
Numerical applications of the proposed inverse method were made considering the cases
of two high-strength composites structures: AS4/3501-6 and T300/N5208. The determination
of the microscopic quadratic failure criterion of the pure epoxies (3501-6 and N5208,
respectively) was achieved. The obtained results are close together and present a good
agreement with ultimate strengths measured on reduced sized plain resins (available from
already published literature). This demonstrates the reliability of the present predictive
method for estimating the local failure behaviour of epoxies whose experimental failure
criterion has not yet been determined.
In further works, the proposed approach will be extended to the more general case of
hygro-thermo-mechanical loads. This will imply to take into account the stress free strains in
order to keep consistency between the failure envelopes expressed in stress and strain spaces.
Besides, the rigorous treatment of the hygro-thermo-mechanical load requires to consider the
dependence on the temperature and moisture content of a) the elastic stiffness, coefficients of
thermal expansion and coefficients of moisture expansion and b) the ultimate strength (and in
general, the coefficients of the considered failure criterion), at both macroscopic and
microscopic scales. Others perspectives of research are proposed in the following section
below.
7. Perspectives
Scale transition modelling based theoretical analysis of composite structures constitutes an
overexpanding field of research, due to multiple factors. Among them, the emergence of new
materials exhibiting a specific, more advanced microstructure, the ambition to account for
additional, sometimes only recently discovered, physical phenomena and the relentless
research for building faster, more convenient but still reliable models stand for the three
essential motivations for achieving further developments in the incoming years.
7.1. Emergence of New Materials
The present development stage of Eshelby’s single inclusion theory involved in the
mechanical modeling of composites is not intended for a rigorous treatment of the
morphology presented by the reinforcements used for manufacturing woven-composites. As a
consequence, answering to the question of a theoretical study, through scale transition
models, of mechanical parts made of such composites will require a specific and still missing
solution.