2.2 Sandwich structure joints
13
insert type. Generally, it is distinguished between, partially and fully potted as well as
through-the-thickness bonded. In addition, there are further post-fab bonded insert designs such as so called “onserts” or flanged bushings.
Figure 9 Typical constituents of sandwich panel inserts [See14]
However, bonding is not essential and there are insert designs that do not require any
adhesive. This second group comprises mechanically fastened inserts, which generally
have a low structural performance, since they are lacking a planar load transmission into
the face sheet. As third group, there are insert designs that require reinforcement of the
panel during panel manufacturing (co-fab). This can be implemented using low density
core filling or by bonding a rigid section as local core replacement during panel manufacturing. This method has the advantage that large high strength fastener elements can
be implemented anywhere in the panel without damaging the face sheets. However, the
panel manufacturing process is considerably more complicated and the positioning of
the fastener elements tends to be less accurate. In addition, co-fab methods are generally less flexible in the overall sandwich structure production. Figure 10 illustrates the
three different fastener methods with multiple examples for each group. This summary
represents the view of the author and is derived from available literature, most notably
[Hei09], [Zen97], [ESA11] and [Bit97]. The literature also provides guidance on the relative load carrying capabilities of the different insert types and in case of the Insert Design
Handbook (IDH) [ESA11] comprehensive resources on design considerations for the selection of inserts are given. Regarding the loads, it can be distinguished between four
loading conditions for sandwich fasteners, namely tension/compression, shear, torsion
and bending (Figure 11). The latter two loading conditions are unfavorable for inserts
and they are therefore avoided. This is achieved by fastening components onto the panel
using multiple inserts. Such arrangements transform torsional loading of the component
into shear for the individual inserts while bending is transformed into tension/compression [ESA11]. Therefore, shear and tension/compression are of primary interest in the
design phase.
13
insert type. Generally, it is distinguished between, partially and fully potted as well as
through-the-thickness bonded. In addition, there are further post-fab bonded insert designs such as so called “onserts” or flanged bushings.
Figure 9 Typical constituents of sandwich panel inserts [See14]
However, bonding is not essential and there are insert designs that do not require any
adhesive. This second group comprises mechanically fastened inserts, which generally
have a low structural performance, since they are lacking a planar load transmission into
the face sheet. As third group, there are insert designs that require reinforcement of the
panel during panel manufacturing (co-fab). This can be implemented using low density
core filling or by bonding a rigid section as local core replacement during panel manufacturing. This method has the advantage that large high strength fastener elements can
be implemented anywhere in the panel without damaging the face sheets. However, the
panel manufacturing process is considerably more complicated and the positioning of
the fastener elements tends to be less accurate. In addition, co-fab methods are generally less flexible in the overall sandwich structure production. Figure 10 illustrates the
three different fastener methods with multiple examples for each group. This summary
represents the view of the author and is derived from available literature, most notably
[Hei09], [Zen97], [ESA11] and [Bit97]. The literature also provides guidance on the relative load carrying capabilities of the different insert types and in case of the Insert Design
Handbook (IDH) [ESA11] comprehensive resources on design considerations for the selection of inserts are given. Regarding the loads, it can be distinguished between four
loading conditions for sandwich fasteners, namely tension/compression, shear, torsion
and bending (Figure 11). The latter two loading conditions are unfavorable for inserts
and they are therefore avoided. This is achieved by fastening components onto the panel
using multiple inserts. Such arrangements transform torsional loading of the component
into shear for the individual inserts while bending is transformed into tension/compression [ESA11]. Therefore, shear and tension/compression are of primary interest in the
design phase.