Rene Herrmann Compounding and Composites

Sandwich laminate analysis (1) The material properties of both core and skin varies. In a factory this material properties must be verified in a maaner as similar as possible to the actual sandwich structure. Having verified material parameters allows to model meaningfully with finite element method other products. Even if material porperties are acepted as reported in data sheets the real characteristics will differ. Core compressional strength may be overloaded for short sandwich laminates. However cores that have channels or perforations holes interconnecting the two skins will IMCREASE unexpectetly the comproessional strength and the specimen breaks instead on an UNEXPECTED place.

Sandwich laminate analysis (2) The shear strength of the core is especially in short specimen overloaded. However cleaness in the lamination may render the interface between the skin and the core so week that instead of shear overload the skin delaminates from the core. The skin on the bottom surface of the sandwich in often of higher quality than the top. Due to this difference the skins may not necessarily be regarded as equal or same.

Sandwich laminate analysis procedure (1) The studied model is 3 point bending with central load. The model is assumed known. For a composite sandwich the model has at least one limitation, the Youngs modulus of the skin is assumed uniform which it by definition can allmost never be. For the model to be applicable the user must insure a HOMOGENOUS loading of all fibers, eg. In the center between - 45/45 or in one direction of 0/90 degree. You need to break a beam in 3 point bending with little or no side effects in the bending. This mean the beam must be high as compared to wide. If this is not possible, width should not be more than TWICE the hight. Length should be at least 10 times width or hight, whichever is larger.

Sandwich laminate analysis procedure (1) The test must also reveal the comproessional properties of the core, a SEPARATE compression and SHEAR test on small specimen must be preformed, stress is the ration of force over cross section. The cross section is the loaded surface. The sandwich theory does not distinguish between compression and tensile loading in the skin surface. Most composites are weaker in compression than in tension. You must use the measured core properties and analyze the data of the 3 point bending for Youngs modulus of the skins. You will find the Youngs modulus and limiting stress and strain, probably for the compressed skin rather then the tensioned skin.

Sandwich laminate analysis procedure (1) The equations for comproession loading are: The equation for shear loading are: The measured data is force versus displacement, the equation is:

Data analysis Use the dimensions of the tested specimen and the test data from compression, shear and 3 point bending to extract all material properties. Measure also core density and find the datasheet for the CORECELL product with that density. Compare you measured values to the data sheet values. Use the excel based model implementation and find the Youngs modulus of the skin using 3 point bending data. Compare the experimental 3 point bending data to the sandwich model and define safe operation areas/parameters.