Analysis of forming loads for thermoplastic composite laminates

Abstract

Using Mindlin plate theory, a model for the forming of thermoplastic composite laminates is presented. The transverse strain in the plate theory is used to account for the slippage between adjacent plies. This interply slip is characterized by a rate dependent constitutive model that is adopted from an experimentally determined relation between the interply shear stress and interply slip velocity. A laminate under cylindrical bending assumptions is used to demonstrate the methodology and to examine the contributions of temperature and loading rate to the amount of compressive stress in the surface ply located on the inner radius of curvature. An experiment which allows measurement of the load histories required to form a thermoplastic composite laminate by the matched die molding process is described. Load histories for two laminates and two forming rates at a typical melt-phase forming temperature are presented along with the histories given by the forming model. Using a hydraulic hot press and two-stage matched die mold, the upper limit on forming rate at a specific forming temperature is demonstrated. Forming above this rate results in wrinkling of the plies on the inner radius of curvature.