62
4 Mechanical characterization on constituent level
4.1.5 Conclusion
Meso-scale models have proven to be well suited to represent the honeycomb behavior
including realistic folding mechanisms. It is advisable to implement hexagons with
straight cell walls, while accounting for possible stretching of the entire hexagon grid.
This geometry along with a single layer orthotropic elasto-plastic material model leads
to a good approximation of experimental results at comparably low modelling and computational effort. The ML approaches have proven to require considerably more effort
in terms of modelling, calibration and computation, while providing little advantage in
model accuracy. Therefore, the SL approaches are recommended when implementing
meso scale models for sandwich panel joints. The macroscopic experimental results can
also be approximated with homogenized 3D-continuum elements in combination with
an orthotropic elasto-plastic model.
4.2 Face sheets
On constituent level, the face sheets can be treated as a regular composite material.
Therefore, standard test methods for determining the tensile properties of composite
materials, such as ASTM D 3039 [AST00] or EN ISO 527-5 [ISO527] were applied. Determining the compressive properties is more challenging, particularly for thin layups such
as sandwich face sheets commonly used in aircraft interior. Therefore, the four point
bending test on bonded sandwich panels has been established as common method for
determining the compressive properties of sandwich face sheets [Zen97]. In addition,
tests on the bonded panel have the advantage that the telegraphing effect is included in
the determined material properties. The same applies for determining the shear properties of the face sheets. Therefore, it is common practice in the sandwich construction
industry to determine the face sheet properties directly from structural tests on the
bonded sandwich panel. This is described in chapter 5. Despite this, the face sheets were
additionally investigated on constituent level in the framework of the present thesis. This
was done to validate existing material data and to generate a reference for the subsequent bending tests. In addition, the objective was to quantify the effect of face sheet
telegraphing on the mechanical properties. In the following, the performed experiments
and numerical studies are presented.
Materials
In the present work a total of three different E-glass prepreg fabric materials are investigated. All of which are typical in aircraft interior applications and comply with Airbus
material performance specifications, while there are different manufacturers that supply
prepregs according to these specifications. Table 13 summarizes the investigated materials. In the framework of the present thesis, the face sheets are generally referred to
4 Mechanical characterization on constituent level
4.1.5 Conclusion
Meso-scale models have proven to be well suited to represent the honeycomb behavior
including realistic folding mechanisms. It is advisable to implement hexagons with
straight cell walls, while accounting for possible stretching of the entire hexagon grid.
This geometry along with a single layer orthotropic elasto-plastic material model leads
to a good approximation of experimental results at comparably low modelling and computational effort. The ML approaches have proven to require considerably more effort
in terms of modelling, calibration and computation, while providing little advantage in
model accuracy. Therefore, the SL approaches are recommended when implementing
meso scale models for sandwich panel joints. The macroscopic experimental results can
also be approximated with homogenized 3D-continuum elements in combination with
an orthotropic elasto-plastic model.
4.2 Face sheets
On constituent level, the face sheets can be treated as a regular composite material.
Therefore, standard test methods for determining the tensile properties of composite
materials, such as ASTM D 3039 [AST00] or EN ISO 527-5 [ISO527] were applied. Determining the compressive properties is more challenging, particularly for thin layups such
as sandwich face sheets commonly used in aircraft interior. Therefore, the four point
bending test on bonded sandwich panels has been established as common method for
determining the compressive properties of sandwich face sheets [Zen97]. In addition,
tests on the bonded panel have the advantage that the telegraphing effect is included in
the determined material properties. The same applies for determining the shear properties of the face sheets. Therefore, it is common practice in the sandwich construction
industry to determine the face sheet properties directly from structural tests on the
bonded sandwich panel. This is described in chapter 5. Despite this, the face sheets were
additionally investigated on constituent level in the framework of the present thesis. This
was done to validate existing material data and to generate a reference for the subsequent bending tests. In addition, the objective was to quantify the effect of face sheet
telegraphing on the mechanical properties. In the following, the performed experiments
and numerical studies are presented.
Materials
In the present work a total of three different E-glass prepreg fabric materials are investigated. All of which are typical in aircraft interior applications and comply with Airbus
material performance specifications, while there are different manufacturers that supply
prepregs according to these specifications. Table 13 summarizes the investigated materials. In the framework of the present thesis, the face sheets are generally referred to