Public Defense of Doctoral Thesis at the ETSi

Date: Wednesday, July 5, 2023.

Time: 5:00 p.m.

Place: Professor Juan Larrañeta Room, Higher Technical School of Engineering of the University of Seville.

The doctoral student Serafín Sánchez Carmona will publicly defend his doctoral thesis entitled "Damage and failure mechanisms under fatigue in long fiber composites with ultra-thin plies", which has been directed by professors Elena Correa Montoto and Alberto Barroso Caro , and supervised by Federico París Carballo , from the Department of Continuous Media Mechanics and Theory of Structures of the Higher Technical School of Engineering of the University of Seville

The study of the fatigue behavior of composite materials is of great interest in the scientific field due to the non-static nature of the loads suffered by a structure in service. In recent years, the appearance of ultrathin sheets, up to 15-20 µm thick, has taken on special relevance after finding a physically based explanation for the existence of the “scale effect” in composite materials.

This work presents a detailed study, both experimental and numerical, of the appearance of damage in cross-ply laminates of long carbon fiber and epoxy resin under cyclic traction-traction loading, varying the thickness of the layer of 90° (from 30 to 150 g/m2) in order to analyze whether there is a fatigue scale effect.

The extensive experimental work has consisted, first of all, in the characterization of the mechanical and thermal properties of the different prepregs used in addition to the analysis of the effect of different manufacturing and testing conditions on the fatigue life of 90° unidirectional laminates manufactured with conventional and ultra-thin thickness material; and, secondly, in an exhaustive microscopic observation of the appearance of damage in the different fatigue cross-ply laminates, varying the thickness of the 90° layer from ultra-thin thickness (30 g/m2) to conventional thickness (greater than 120 g/m2).

The appearance of an unconventional failure mechanism, specifically, longitudinal debonding at the applied load, has led to a detailed numerical analysis of the well-known “edge effect” when dealing with ultra-thin thicknesses, highlighting the appearance of a biaxial loading state at the edge. of the specimens even when only residual curing stresses exist. The results of this numerical study have been corroborated with 3D computed tomography.

All of this has led to corroboration of the existence of a “scale effect” in fatigue after the observation of a delay in the appearance of the different failure mechanisms when ultra-thin sheets are used with respect to those produced in composite material laminates with sheets of conventional thickness.