The Anomalies of Polymers at Nanoscale as a Tool to Enhance the Fatigue Life of Structural Composites

Abstract

The “in service “properties' of high performance composites degrade due to the development of different damage modes occurring hierarchically at different length and time (cycles) scales, from diffuse crackand crazing within the polymer matrix, to fiber-matrix interface failure, to ply rupture and delamination where catastrophic failure occurs. The density of cracks within the matrix saturates at very early stage of the service life of the composite component and this “Critical Damage State” (CDS) triggers the other damage modes. Thus, shifting the CDS toward higher number of cycles allows enhancing the overall fatigue performances of composites, that is the main objective of our proposal. The matrix weakness is due to the residual stresses (of order of magnitude of the matrix strength) that accumulate as consequence of the manufacturing process. In order to relax the residual stresses we propose to take advantage of the anomalies of nanoconfined thermoplastic polymers that show a negative coefficient ofthermal expansion at temperatures below Tg. Silica microspheres can be covered by polymer shells of 10-40 nanometers and added to the reactive resin in order to counter balance the resin contraction through the expansion of the nanoconfined polymer, during cooling.