Analysis of failure mechanisms associated with the unfolding failure in CFRP Profiles

• Autores: Sindhu Bushpalli Shiva Reddy
• Directores de la Tesis: Enrique Graciani Díaz (dir. tes.)
• Lectura: En la Universidad de Sevilla ( España ) en 2024
• Idioma: inglés
• Número de páginas: 230
• Enlaces
  • Tesis en acceso abierto en: Idus
• Resumen

  • The rapid increase in the usage of composite materials in the aerospace sector demands their introduction into primary and secondary structures involving complex geometries. Considering one such application of highly curved composite laminates as stiffening agents (spars/rib configurations in airplanes), these laminates are subjected to delaminations under opening bending loads due to their relatively weak out of plane properties. This type of failure is termed unfolding failure, occurring when the laminate's loading attempts to open the curvature. This failure is typically associated with the interlaminar normal stress (INS) characterized by the interlaminar tensile strength (ILTS), generally obtained by a fourpoint bending test.

    The four-point bending test, originally designed to obtain ILTS in unidirectional curved composite laminates, provides an apparent strength when applied to non-UD curved laminates, exhibiting a thickness-dependance of the ILTS with thickness of the specimens. Several authors have associated this dependency with manufacturing defects or porosity, but results are not conclusive. Therefore, the aim of this project is to analyze both numerically and experimentally the failure mechanisms involved in unfolding failure in order to demonstrate a novel idea that the onset of unfolding failure is associated with intralaminar stresses instead of the interlaminar stresses.

    Preliminary analyses of existing experimental results have shown a good agreement with this new hypothesis that, in most cases, unfolding failure starts with an intralaminar failure which, under the presence of sufficiently high intralaminar stresses, propagates as a delamination. This failure mechanism is called induced unfolding. The current study is based on a set of new experimental results specifically oriented to observe the effect of intralaminar failures and establish a stress criterion for predicting the induced unfolding failure loads in curved laminates.

    Since the failure is catastrophic and difficult to observe precisely, the experimental results are complemented by numerical simulations using FEM models, in which crack onset and its subsequent propagation (including possible migration from layers to interfaces and vice-versa) are simulated using Phase Field and Cohesive Zone Modeling approaches. Correlation between experimental and numerical results provide important proof of existence of this novel failure mechanism. Furthermore, this study presents analytical methodologies within the framework of Finite Fracture Mechanics to determine the critical loads responsible for the onset of first transverse cracks in highly curved composite laminates considering different hypotheses. Both Phase Field and Finite Fracture mechanics combine energy and strength criteria in the analysis of unfolding failure.