12
2 State of the art
Face wrinkling
Either due to Face debonding resulting from low adhesive strength between face and core
or due to core crushing resulting from
low compressive core strength
2.2 Sandwich structure joints
The design of sandwich panel joints is crucial in sandwich construction and a considerable share of the total development time in sandwich construction is typically attributed
to joint design. This is due to the sensitivity to localized loads, which is one of the major
short comings of sandwich structures. This is a result of the low-density core, which does
not provide enough stiffness to distribute loads adequately. High multiaxial stress concentrations in the vicinity of load introduction areas often lead to premature failure of
the core, which quickly propagates and eventually causes catastrophic failure of the entire panel [Zen97]. However, when it comes to joining sandwich panels, local external
load application is unavoidable. In order to prevent premature core failure, sandwich
panel joints are locally reinforced. Depending on the type of joint, an abundant variety
of joint designs can be found in the literature. In general sandwich panel joints can be
distinguished between inserts that enable fastening of components onto the panel and
joints along panel edges for assembling larger structures. A combination of both categories can also be found. In the following, the two categories are described in more detail.
2.2.1 Inserts
In Zenkert’s Handbook of Sandwich Construction [Zen97] inserts are defined as local
change in stiffness and strength of the sandwich panel, with the purpose of distributing
localized loads. There are three groups of such fastener elements [ESA11]. One of the
most common insert designs is illustrated in Figure 9. Here, a threaded circular insert is
bonded into a sandwich panel using adhesive. This process as well as the adhesive itself
are often referred to as “potting”. Since this is done post panel fabrication (post-fab),
this method requires to locally remove the face sheet and core, for instance by means
of milling, and to inject the adhesive into the cavity with the insert placed in its center.
This method enables a great variety in insert designs covering a wide range of requirements for the fastener element. There are different potting methods depending on the
2 State of the art
Face wrinkling
Either due to Face debonding resulting from low adhesive strength between face and core
or due to core crushing resulting from
low compressive core strength
2.2 Sandwich structure joints
The design of sandwich panel joints is crucial in sandwich construction and a considerable share of the total development time in sandwich construction is typically attributed
to joint design. This is due to the sensitivity to localized loads, which is one of the major
short comings of sandwich structures. This is a result of the low-density core, which does
not provide enough stiffness to distribute loads adequately. High multiaxial stress concentrations in the vicinity of load introduction areas often lead to premature failure of
the core, which quickly propagates and eventually causes catastrophic failure of the entire panel [Zen97]. However, when it comes to joining sandwich panels, local external
load application is unavoidable. In order to prevent premature core failure, sandwich
panel joints are locally reinforced. Depending on the type of joint, an abundant variety
of joint designs can be found in the literature. In general sandwich panel joints can be
distinguished between inserts that enable fastening of components onto the panel and
joints along panel edges for assembling larger structures. A combination of both categories can also be found. In the following, the two categories are described in more detail.
2.2.1 Inserts
In Zenkert’s Handbook of Sandwich Construction [Zen97] inserts are defined as local
change in stiffness and strength of the sandwich panel, with the purpose of distributing
localized loads. There are three groups of such fastener elements [ESA11]. One of the
most common insert designs is illustrated in Figure 9. Here, a threaded circular insert is
bonded into a sandwich panel using adhesive. This process as well as the adhesive itself
are often referred to as “potting”. Since this is done post panel fabrication (post-fab),
this method requires to locally remove the face sheet and core, for instance by means
of milling, and to inject the adhesive into the cavity with the insert placed in its center.
This method enables a great variety in insert designs covering a wide range of requirements for the fastener element. There are different potting methods depending on the