Development of a test device to determine the frictional behavior between honeycomb and prepreg layers under realistic manufacturing conditions

Abstract

During the autoclave manufacturing of sandwich components, several defects may arise. A typical defect in the use of honeycombs is core crush. The occurrence of this defect depends, among other things, on the angle of the honeycomb chamfer. To design this angle, the friction between honeycomb and prepreg must be known. Due to the low rigidity of Nomex® honeycomb in lateral direction, the determination of the frictional behavior between face sheets and honeycomb is a particular challenge. This paper shows the development of a test device for measuring the friction between honeycomb with film adhesive and carbon/epoxy prepreg for different temperatures and compressions corresponding to realistic manufacturing conditions. The device is also suitable for the evaluation of interlaminar friction between prepreg plies and tool-part-interaction. The requirements for the device are shown in detail. Advantages and disadvantages of various concepts for the individual modules are discussed in order to select the most promising design. As proof of concept, friction measurements between honeycomb and prepreg as well as prepreg to prepreg measurements are carried out. In the case of interlaminar measurements, the friction coefficient determined at 1 mm deformation deviates by 5.3% from already published values obtained with another device and is within the stated scatter range. The measurement results for honeycomb against prepreg are in direct correlation with manufacturing trials performed. Using the lowest coefficient of friction established, a critical angle of the honeycomb chamfer was determined. Subsequently, component samples were manufactured with an inclination angle of 7.5° above and 7.5° below the critical angle. The specimens with larger angle showed a strong core crush, whereas those with smaller angle exhibited no core crush. This demonstrates that with this device the honeycomb chamfer of sandwich components can be dimensioned based on the lowest friction coefficient determined between honeycomb and prepreg.