An insert is a local change in stiffness and strength of the sandwich panel. The purpose of which is to distribute a localised load in an appropriate manner to the sandwich panel. Inserts in composite materials are spreadly use as they are necessary when perfoming a bolted joint.
Strength: the most important parameters related to strength are: Insert overall diameter di and Insert overall height h.
Materials: the standard commercially inserts are made from certain grades o metals like aluminum alloys, titanium alloys and steels.
Sandwich constructions consists of face sheets, core and adhesive film. They all contribute to the load capacity of an insert. In the following table is shown how contributes each part:
| LOAD TYPE | CORE | FACE SHEET | CORE/FACE BOND |
| Tension | High | Medium | Very low |
| Compression | High | Medium | Low |
| Shear | Low | High | Very low |
| Bending | High | Medium | Low |
| Torsion | High | Low | Low |
Types of inserts in composite materials
It exists 4 types of inserts depending on the loads applied to the sandwich structure:
- Self-tapping screws and rivets: they do not carry bending moments. Are used to attached light equipment to panels at points that are no subjected to any significant loads.
- Through the thickness inserts: transfer shear force to the core and bending moment and in -plane forces to the face sheets. They are used when high loads has to be applied to the sandwich panels.
- Partial insert: they are used when through the thickness inserts are undesirable. Partial inserts are attached to one face sheet and the core.
- Adhesive bonded cylinder: the insert is bonded in one face sheet and transfer loads of forces and bending moments. They are not as safer as partial insert when transferring in-plane forces. They are used when panels are subjected to high, uniform shear load as it does not disrupt the core.
Bonded inserts in composite materials
A) The adhesive usually is softer and weaker than both insert and face plate. The adhesive layer acts as a hinge between the face sheet and the insert in case of transverse load. The bending moment stresses may be break the adhesive causing a circular crack in the face sheet – insert interface.
B) The flared discs give a larger bonded surface and allows to transfer moment and significant transverse force by the face sheets. Although the face-sheet bending moment will certainly contribute to the stiffnesses, it may cause large bending stresses in the face sheets. The solution is to be increasing the thickness of the face sheets locally or reducing the stiffness of the adhesive layer.
C) Same as B)
D) This is used for cheap solutions. The radius of the insert is very small, a crew or rivet cannot transfer as much load as larger inserts. Only compressive loads can be considered, as neither the screws nor rivets are normally fixed by using adhesive. Bending moments should be avoided.
An insert is a disruption of the basic structure, and will cause stress concentration, even if no load is applied to the insert itself. This occurs because the core it the weaker part in the case of partial inserts in particular, stress concentrations in the core tens to cause cracks. Screws and rivets penetrating in the face sheet are a special case, since they are not normally connected to the core by adhesive, they may cause serious discontin uities in the core.
In shear, the face sheet of the sandwich panel will move in-plane relative to each other, causing large deformations compared to a thin steel plate. This can be improved depending on the type of insert used and the total shear stiffness of the panel will be increased.
Summary of inserts used in sandwich constructions
In this table, there is a summary about the types of inserts used in sandwich panels:
