Public Defense of Doctoral Thesis at the ETSi

Date: Wednesday, July 5, 2023.

Time: 5:00 PM

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

Doctoral candidate 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 Mechanics of Continuous Media and Theory of Structures of the Higher Technical School of Engineering of the University of Seville

The study of fatigue behavior in composite materials is of great interest in the scientific field due to the non-static nature of the loads experienced by a structure in service. In recent years, the development of ultrathin sheets, as thin as 15-20 µm, has become particularly relevant after a physically based explanation was found 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 tensile-tensile loading, varying the thickness of the 90° layer (from 30 to 150 g/m2) in order to analyze whether there is a scale effect to fatigue.

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

The emergence of an unconventional failure mechanism, specifically longitudinal delamination under applied load, has led to a detailed numerical analysis of the well-known "edge effect" when dealing with ultrathin thicknesses. This analysis highlights the appearance of a biaxial loading state at the edge of the specimens even when only residual curing stresses are present. The results of this numerical study have been corroborated with 3D computed tomography.

All of this has led to corroborating the existence of a "scale effect" to fatigue after observing a delay in the appearance of the different failure mechanisms when ultrathin sheets are used compared to those that occur in composite laminates with sheets of conventional thickness.