Rate Dependance of Forming UD Prepreg Laminates

The potential for reducing wrinkling during forming of a preconsolidated composite laminate by slowing the forming rate was investigated. A new manufacturing method was proposed for constructing a large aerospace component which upon initial trials suffered from severe wrinkling. This paper discusses the mechanisms behind wrinkle formation during the process, and cites the lack of interply slip as a key factor. A range of regulations and constrains meant that the only method by which the process could be modified was to dramatically reduce the forming rate, with the hope of utilizing the viscoelastic properties of epoxy resin and reduce the generated stresses in the resin to allow the prepreg to form wrinkle free. Bias extension and lap shear tests are used to study the materials behavior at a range of strain rates showing a significant reduction in the required forces. The effect of rate change was then investigated by forming circular laminates over a hemispherical shape. Cross-plied two ply blanks showed significant improvements in forming at slower forming rates, while full quasi-isotropic blanks continued to wrinkle even with vastly reduced forming rates.