JEC COMPOSITES MAGAZINE - Issue #93 - December 2014 - 54

multimaterial

Fig.4: Welding methods clissified

Chemical assemblies
Adhesive bonding can be used to join all types
of metal alloys (aluminium, titanium, stainless steels, etc.) to thermoset-matrix (mainly
epoxies) and thermoplastic-matrix (PEEK, etc.)
composite materials.
A major development goal in the optimization
of adhesive-bonded systems is to optimize the
adhesion of structural glues to metals and composites. Dry processes (laser and cold plasma)
help comply with new European health-safetyenvironment regulations (VOCs, RoHS and
REACH) and boost adherence to metals and
composites.
For composite materials, plasma processing (at
atmospheric pressure or under low pressure in Fig. 5: the latest Cetim study
a working gas or gas mixture) helps increase
the surface energy by modifying the chempressive stress to the bonding surface to increase the tensile shear
istry of the extreme surface of organic materials (in particular of
strength of bonded assemblies with single or double lap welds by as
the thermoplastic matrices of composite materials). Reproducible
much as 20 to 40%, depending on the case.
laser processes (Excimer or YAG) can be used effectively to clean
and/or abrade composite surfaces to eliminate the layers of mould
Mechanical assemblies
release agents without damaging the fibres.
Structural assemblies use two main technologies, as a function of
For metals, recent research has shown that YAG or Excimer laser
the mechanical stress tensor:
processes can clean the surfaces and modify the oxide film on alu- Riveting with pre-drilling for fuselage components, e.g. LGP®
minium and titanium alloys, enabling adhesive bonds with better
lockbolt fasteners (shear stress) or Hi-Lite® fasteners (both shear
performance in terms of mechanical strength and durability in hot,
and tensile stress);
humid environments - equivalent to the performance obtained
- Tension bolt: high tensile stress; mainly for mechanical compowith the best chemical conversion processes.
nents.
However, the latest Cetim study presented at the 2014 World
- The main challenges with riveted assemblies are to minimize the
Congress on Adhesion and Related Phenomena (WCARP) shows
assembly constraints (e.g. geometric accuracy of the bores), to
that phosphoric sulphuric acid (PSA) anodizing and SOCOGEL
reduce cycle times, and to robotize.
B0102® sol-gel solutions are still more effective for a 2024 T3
For screw assemblies with filtering prestress, potential developaluminium alloy than YAG and Excimer laser processes (with the
ment goals are :
tested parameters), which themselves are still not as effective as
sandblasting (fig 5).
- implementing pre-torque + angle tightening ranges (elastic or
elasto-plastic tightening)
Higher mechanical strength for bonded assemblies can be obtained - in situ tensile measurement (based, for example, on ultrasonic
by optimizing the geometry of the line of glue, or by applying comwaves) during tightening, then monitoring over time - notion of

54 jec composites magazine / No93 December 2014
JEC COMPOSITES MAGAZINE - Issue #93 - December 2014

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