The Effect of Prepeg Storage Humidity on Co-cured Composite Joints

The increasing use of composite materials in the aerospace
industry has driven a need for a greater understanding
of bonded composite joints.
There are generally two types of composite joint used
in the aerospace industry; secondary bonded joints and cocured
joints. Secondary bonded joints are produced by
bonding two cured composite laminates together with an
adhesive. However, when composites and adhesives are
used to manufacture large parts in the aerospace industry,
it is often convenient to co-cure the two materials at the
same time. This helps to reduce the high costs associated
with autoclave curing and also to reduce processing time.
However, despite the apparent advantages, co-curing
is not without its drawbacks. Any moisture stored in the
composite material prior to co-curing is released during the
cure cycle and has a negative effect on the joint. This can
also result in interfacial failure. A way around this problem
is to either dry the composite material prior to curing or to
engineer the composite surface using a variety of surface
treatments to promote adhesion, such as an atmospheric
pressure plasma treatment [1]. The former option will be
investigated in this work.
The effects of moisture on the fracture performance of
secondary bonded composite joints is well publicised.
Moisture can be introduced into the composite laminate prior to [2] or after [3] secondary bonding. The moisture
can plasticize the adhesive and reduce the glass transition
temperature of the adhesive [4]. However, compared to
secondary bonded joints, relatively little work has been
carried out on co-cured joints.
In the present work, the effect of the level of moisture
in the composite prepreg prior to co-curing will be examined.