JEC COMPOSITES MAGAZINE - Issue #107 - August/September 2016 - 29

Multimaterial assembly - issues, problems and solutions

Tab. 2: SWOT matrix for mechanical assemblies

Fig. 2: Wedge test
for several surface
preparations
Tab. 1: SWOT matrix for bonded assemblies

Strengths
- Good fatigue resistance
- Flexible joint that can absorb
differential expansions
- Integrity of structures (no drilling,
no excessive temperature)
- Lighter structures than screwed or
riveted solutions
Weaknesses
- Lower mechanical resistance of the
adhesive than the parts to be bonded
- Strong influence of surface
preparation
- Limited temperature resistance
- Long curing time for some families
- Complex process
Opportunities
- Integration of added functions
(sealing, vibration absorption,
limiting galvanic corrosion)
- Hybrids can be produced:
bonding +...
Threats
- No NDT can ensure adhesion control
- Control of surface preparation
- Controlling and ensuring durability

Mechanical assembly techniques
such as screwing/riveting
Screwed and riveted assemblies are
widely used, for example when bonding cannot be used due to demountability criteria. These technologies are
well known for metal-to-metal assembly. In the case of an axial load, two
parameters need to be controlled to
ensure the durability of a screwed assembly:

- No detachment in the joint plan
between the assembled parts;
- No sliding between the assembled
parts.
To comply with these two parameters,
a preload is introduced in the bolt
when tightening, with an elastic deformation of the screw (a few dozen microns) so that it acts as a spring to filter
the external load. However, the use of
composites, especially thermoplastics,
involves several technical problems:
- The creeping of composite materials,
in particular thermoplastic matrices,
can cause spontaneous unscrewing;
- When subjected to moisture,
composites can swell;
- Composite materials are more
sensitive to the matting;
- The mechanical assembly is
weakened by the degradation caused
by drilling of the composite fibres.
As regards bolted assemblies with an
axial load, the following matrix was defined for multimaterial configurations
integrating composites (Table2).
The current trend is to tighten the assembly using a metal insert in the composite material to have a metal-to-metal situation and use the metal's rules.
Ecole Centrale de Nantes is currently
working to establish rules and a model
for thermoplastic composites without
insert screwing, which is the worst case
because it involves the highest creep
risk. In the current state, it appears
that the fibre content and orientation
affect the behaviour of the screwed assembly. New criteria with a new equa-

Strengths

Weaknesses

- Reproducibility
- Easy automation
- Well standardized
- Simple process
- Suitable for thermoset and thermoplastic
composites

- Fibres broken
by drilling
- Ageing of composites
(temperature, humidity)
- Different stiffness of
the materials to join
- Drilling cost

Opportunities

Threats

- Hybrid solution
(bonding-riveting or
bonding-bolting)

- The creeping of
thermoplastic matrices
can cause spontaneous unscrewing
- Need to control the
preload in the assembly
- Drilling defects

tion are being studied and will be proposed before the end of 2016.
CETIM has developed a new product
that can counter this phenomenon.
This way, the bolt keeps the preload
even at higher temperature.

Innovative multi-material
assemblies
Several technologies were specifically
developed for multi-material assembly,
such as BASF's collar joining, TWI's
Surfi-SculptTM, ComeldTM - the
friction spot welding method developed by HZG - GKSS's FricRiveting,
and nailing, clinching or hybrid solutions using overmoulding technology.
CETIM, who tested all these technologies to assess their potential, presents
three technologies developed through
partnerships with industry.

WARM-Bond hot bonding
assembly
An interesting property of thermoplastic composite materials is that the
thermoplastic matrix softens when
the temperature increases and freezes
when returning to room temperature,
this change being reversible. The thermoplastic composite matrix can be
used as a glue to adhere to the metal
(Figure 3).
The WARM-Bond process uses the
metal part to heat the thermoplastic
composite, the most effective solution

No107 August - September 2016

/ jec composites magazine

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Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #107 - August/September 2016
