Analysis of the Components of a Sandwich Composite Panel by DMA - METTLER TOLEDO

Analysis of the Components of a Sandwich Composite Panel by DMA

The mechanical properties of the individual components of a sandwich composite panel were investigated by DMA. The temperature-dependent shear moduli of the polyurethane foam core and the bending modulus of the carbon fiber epoxy composite skin sheets are important quantities that determine the mechanical behavior of the sandwich construction.

 A sandwich structure with bending stress applied at right angles to the panel..

 

Introduction

A sandwich composite panel consists of a lightweight core sandwiched between two thin, stiff skin sheets. Sandwich structures of this type are often employed when products have to exhibit high bending strength and stiffness but at the same time need to be light in weight. Typical examples of successful industrial applications are the wing flaps of aircraft, wind turbine blades, surfboards, and boat superstructures. 

Figure 1 shows a typical sandwich composite panel. When a bending load is applied, the skin sheets experience compressive or tensional stresses. They absorb the major part of the forces within the sandwich construction and are responsible for its high bending strength. The core must withstand shear stresses and compression. It supports and stabilizes the skin sheets so that they stay fixed in place relative to one another and do not buckle (deform). The material properties and geometries of the core and skin sheets are the most important factors that determine the strength and stiffness of such sandwich composite panels.

Information about the shear modulus of a potential core material in the temperature range in which the product is expected to be used is necessary in order to choose a suitable material and assess the mechanical properties of the sandwich structure. A soft core made of a material with a low shear modulus is easily deformed and the skin sheets would only be capable of absorbing a small amount of stress. A sandwich structure like this would be weak, too flexible, and easily deformed. A core material that cannot withstand a certain amount of shear stress is therefore unsuitable. 

When a core material with a relatively high shear modulus is used, the skin material takes up the bending stresses in such a way that the forces act as tensional or compressive stresses. Materials commonly used for cores are rigid foams (polyurethane, PVC, polystyrene), wood (balsa and cedar) or honeycomb structures (aluminum, PE, PP).

Similarly, knowledge of the Youngs modulus and temperature behavior of the skin sheets is important. A strong skin sheet with high Youngs modulus can absorb far more tensional and compressive stress than the core and makes the sandwich construction much stiffer. Glass or carbon fiber reinforced composites are often used as skin sheets. 

The skin and core materials should be balanced with regard to their mechanical properties – the material stress should be shared so that neither part prematurely fails. This applies not only at room temperature but also over the entire temperature range of the application. Softening of a material or major transitions should not occur in the application range. 

This article describes how the individual components of the sandwich construction (skin sheets and core) were measured by dynamic mechanical analysis, DMA. 

 

Experimental Details

The sandwich composite panel consisted of two 0.64-mm thick skin sheets and a 6-mm core made of polyurethane rigid foam. The skin sheets were made of a carbon fiber reinforced epoxy composite. Shear, compression and bending measurements were performed using a METTLER TOLEDO DMA/SDTA861e dynamic mechanical analyzer. 

Shear Measurement

The sample specimens were prepared as shown schematically in Figure 2 and loaded in the shear sample holder. 

Sample A: Two rectangular blocks, 6.3 mm long, 5.1 mm wide and 3.4 mm thick, were prepared by removing one of the skins. The skin thickness was 0.64 mm and the foam thickness 2.8 mm. The measurement conditions were: heating rate 2 K/min, maximum force amplitude 8 N, maximum displacement amplitude 5 µm. The experiments were performed at 1, 10 and 100 Hz.

Sample B:

Two rectangular blocks, 7.3 mm long, 5.4 mm wide and 4.3 mm thick, were prepared from the core by removing the skin sheets. The measurement conditions were: heating rate 2 K/min, maximum force amplitude 8 N, maximum displacement amplitude 5 µm. The experiments were performed at 1, 10 and 100 Hz. 

Sample C:

Two pieces of the skin sheet, 3.5 mm long, 2.0 mm wide and 0.60 mm thick, were cut out and the surfaces polished flat. The measurement conditions were: heating rate 2 K/min, maximum force amplitude 8 N, maximum displacement amplitude 5 µm. The experiments were performed at 10 Hz and 100 Hz.

Determination of Young's Modulus

A specimen of the skin sheet (the carbon fiber reinforced epoxy composite) was prepared without the rigid foam core for a 3-point bending measurement. The active length was 35.0 mm, the width 5.9 mm, and the thickness 0.60 mm. The specimen was heated at 2 K/min. A constant predeformation force of 0.2 N applied; the maximum force amplitude was 0.15 N, and the maximum displacement amplitude 20 µm. The measurements were performed at 1 Hz.

Conclusions

The mechanical properties and the performance of sandwich constructions are mainly influenced by the properties of the core and the skin sheets. The shear and Youngs moduli are easy to measure by DMA over a large temperature range.

Rigid polyurethane foam is used as core material and is capable of withstanding the shear stresses in the sandwich construction. Nevertheless, the shear storage modulus, G' , (14.9 MPa) is relatively small. In contrast, the bending modulus, E", of the skin sheet is large (18.1 GPa). This allows the desired stiffness of the sandwich panel to be achieved and at the same time with a lightweight structure. The glass transitions of the two components are close to one another, between 80 °C and 100 °C. The sandwich panel can therefore be used up to about 60 °C without loss of strength because the moduli are more or less constant up to this temperature. 

Analysis of the Components of a Sandwich Composite Panel by DMA | Thermal Analysis Application No. UC 314 | Application published in METTLER TOLEDO Thermal Analysis UserCom 31