The Effect of Diaphragm Stiffness on the Quality of Diaphragm Formed Thermoplastic Composite Components

Abstract

An experimental autoclave with a forming rate control system and sophisticated data acquisition facilities has been employed for polymeric diaphragm form ing of carbon fibre reinforced polyetheretherketone (PEEK). Central displacement mea surements were recorded using a high temperature linear variable differential transformer (LVDT), while forming into a hemispherical mould. Using this facility the pressure versus displacement relationships were established for Upilex-R, Upilex-S, and Kapton-H and superplastic aluminium (Supral) diaphragms. Both Upilex-S and Kapton-H were found to have similar stiffness values, about eight times greater than Upilex-R, while the Supral is about sixteen times stiffer than Upilex-R. Pressure versus displacement characteristics are reported for (0°)8, (0°)16, (0°/90°) 2 s, (0°/90°)4 s and (0°/+45°/-45°/90°) s carbon fibre reinforced PEEK (APC-2) layups, which show that the laminate stiffness at forming tem perature is very low relative to the diaphragm stiffness of Upilex-S, Kapton or Supral. The effect of diaphragm stiffness on squeeze flow is clearly demonstrated as the stiffer di aphragms, Upilex-S and Supral, require higher pressures to complete the forming and consequently cause increased squeeze flow over that shown when using Upilex-R. How ever, it has also been demonstrated that the use of these stiffer diaphragms reduces the risk of laminate buckling. When forming with Upilex-R diaphragms, out of plane buckling was eliminated for a (0°/+45°/-45°/90°), APC-2 layup by reducing the forming rate to 6 mm/mm