JEC COMPOSITES MAGAZINE - Issue #122 - July 2018 - 43

An aeronautics and space-oriented research group

Analysing the links between manufacturing/machining parameters and mechanical strength
When machining composite materials, the
main problem is the choice of criteria for the
qualification of machined surfaces. The current industrial criteria (for example, the average roughness Ra, Rp, etc.) do not appear
to be relevant for composites. In this context,
it is interesting to study the influence of the
microstructure (such as the presence of thermoplastic nodules between the plies) and the
conditions in which the composite is manufactured and cured on machining quality.
In particular, the surface defects and the extent of delamination during machining (drilling and milling) were analysed. In addition,
the influence of processes (drilling or trimming) on the defects and the mechanical resistance under static or fatigue conditions is
studied. New machining quality indicators,
such as the bearing surface, were proposed
and their links with the mechanical resistance were established.

Fig. 4: Example of ply thickness spatial evolution

Fig. 6: Hole roughness analysis to define drilling quality indicators

of a laminated plate's failure zones under
complex stresses (technological evaluators).
Analysis and modelling of shape defects
generated during the manufacture of
composite structures
During the manufacture of laminated composite structures, many physicochemical and
thermomechanical phenomena are at work
and can produce unwanted deformations of
the resulting parts.

A thermochemical modelling of the polymerization process was conducted to predict,
among other things, overshoots during the
polymerization of a thermoset resin for a
thick composite.
On another level, a semi-empirical thermomechanical spring-back model was developed and recently applied to a large part:
the A350 rib in the framework of CORAC
- these two studies are used to predict the
spring-back of bonded stiffeners.

Feature Aerospace

Fig. 5: A350 rib spring-back computation

Composites machining
The anisotropic and highly abrasive nature
of reinforcements, together with the heterogeneity of composite structures, makes
machining delicate. Moreover, for economic
reasons, one-shot drilling of hybrid materials (composite/aluminium, composite/
titanium, etc.) is a very complex machining
operation that damages the materials. As a
result, the ICA studies on the machining of
composite materials and multi-materials [2]
mainly focus on:
- the understanding of surface creation
mechanisms in relation to material removal processes (orthogonal cutting, drill,
bit and orbital drilling, and high-pressure
abrasive water jet cutting);
- in situ measurement of the drilling temperature by instrumentation at the core of
cutting tools using Bragg grating optical
fibres;
- the analysis of machined surfaces to improve the life of bolted structures;
- analytical and numerical modelling of the
conditions in which delamination appears
at the exit of the hole (drill and bit), taking
thermomechanical phenomena into
account;
- multiscale modelling of material removal
mechanisms and induced defects.

/ jec composites magazine

N°122 July 2018

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #122 - July 2018