JEC COMPOSITES MAGAZINE - Issue #113 - June 2017 - 63

simulation TECHNOLOGY

X-ray computed tomography for assessment of
mechanical properties and monitoring damage

aBDErrahmanE ayaDi,
hEDi nouri, FréDéric rogEr,
IMT Lille Douai,
Institut Mines-Télécom
haBiBou maitournam,
ENSTA
SoFianE guESSaSma,
INRA Nantes

A coupled experimental and numerical approach was developed to overcome
the limits of integrative simulation methods in the case of short glass fibre-reinforced thermoplastics. On the one hand, laboratory X-ray computed tomography
and finite element modelling were combined to assess local mechanical properties in strongly anisotropic zones where weld lines potentially form. On the
other hand, an original experimental protocol was developed to monitor damage
evolution during continuous tensile loading. The experimental results open new
perspectives to formulate more physically based local damage criteria.

n order to meet the new over-demanding environmental standards,
car manufacturers have to rethink the design methods of their brandnew series to reduce both CO2 emissions and fossil fuel consumption [1, 2]. One of the adopted solutions is to lower overall weight by
reducing the use of metallic materials. In this context, short glass fibre-reinforced thermoplastics (SFRTs) have been increasingly used for
semi-structural applications due to their attractive mechanical properties and lower weight. Furthermore, SFRT components have the advantage of being easily shaped by injection moulding. However, when
filling moulds of irregular geometric shapes featuring ribs, stress concentration zones or abrupt thickness variations, microstructural heterogeneities can occur. These local heterogeneities commonly cause
sudden changes in glass fibre orientation and density distributions.
In addition, the presence of micrometric porosity is highly probable
when the mould filling flow is further disrupted by macroscopic vortices, as in the case of weld lines [3]. In the particular case of short glass
fibre-reinforced polyamide, the resulting components are mainly used
for under-the-hood applications near the engine such as different parts
of the air intake circuit and the fuel injection rail. In such environments,
these materials are continuously subjected to fluctuating environmental conditions and pressure resulting in prompt failure. The integrative
simulation approach was developed to take into account all the previous aspects as well as microstructural heterogeneities, especially local
fibre orientations. It integrates a simulation of the mould filling process
to compute average fibre orientations used to evaluate a specific anisotropic constitutive model for each integration point of the final finite

element model [4]. Despite the advances of computational frameworks, exceptions still exist. For instance, weld lines are an unresolved
engineering challenge because of their multi-scale extended degree of
anisotropy that cannot be accurately estimated by process simulation
packages [5, 6]. Moreover, in the context of SFRTs, local damage criteria do not integrate the effect of local fibre heterogeneities such as
the presence of out-of-plane oriented fibres or fibre clusters, which are
known to be the precursors of premature damage.
Recent advances in microstructure characterization techniques have
extended the level of precision of structural arrangement analyses
within SFRTs. The X-ray micro-tomography (µCT) technique proved
its ability to provide detailed microstructural data based on X-ray absorption differences between the material constituents. Furthermore,

Fig. 1: Schematic representation of the suggested approach based on experimental X-ray characterization of zones where injection moulding packages
may lack precision [3]

No113 June 2017

/ jec composites magazine

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #113 - June 2017