Relation between interlaminar damage extension and fiber orientation with acoustic signal: quasi-static and cyclic loading

Author(s)

Roberto Ferreira Motta Júnior

• Document type: Master's Dissertation
• Press: Guaratinguetá. 2022-01-27.
• Institution: Universidade Estadual Paulista (Unesp). Faculdade de Engenharia. Guaratinguetá
• Defense date: 2021-11-25
• Advisor: Herman Jacobus Cornelis Voorwald; Marcos Yutaka Shiino

Abstract

Fiber-reinforced polymer (FRP) composites have a great potential to replace metals in applications that require lighter components and structures since these materials can reach high in-plane properties of specific strength and stiffness. The FRP composites frequently used in engineering structures are laminates comprised of continuous-fiber plies without reinforcement in the thickness direction, which reduces the material’s interlaminar strength leading to delamination susceptibility. Most of these structures operate under long-term cyclic loadings, resulting in a gradual delamination propagation. Hence, extensive research has been conducted to understand and predict fatigue delamination growth in FRPs over the past decades. However, most of the efforts were concentrated on the prediction itself rather than attributing physical explanations to the mechanisms associated with the propagation process. In order to contribute to this field, this research focuses on the assessment of delamination within a single loading cycle in FRP using double cantilever beam specimens with varying stress ratios (R). The acoustic emission (AE) technique was used to investigate damage propagation, and a new methodology was developed to quantify the strain energy release due to crack growth in fatigue. In addition, the development of fiber bridging in the crack propagation through different interface configurations was investigated with focus on the influence of bridging on the strain energy released. Results showed that under high R-ratios, the load cycle spends an increased time above the threshold energy (Uth: minimum amount of energy required to damage development) in terms of total strain energy, which affected the damage distribution within a single loading cycle. Besides that, the strain energy release behavior within the fatigue cycles indicated that different damage mechanisms are activated in different increments of the load cycle associated with different energy thresholds. The presence of multiple energy thresholds indicated that the application of different loading cycles results in distinct resistances to damage propagation (dU/dA) depending on which energy threshold is crossed. For example, the rupture of bridging fibers may impact dU/dA when the threshold energy to activate this damage mechanism is exceeded. It was observed that the angle of the fibers (α) in the interface where the crack propagates affected the stresses acting on the bridging fibers, leading to the rupture of more fibers when α was increased. In other words, α eases the activation of this specific damage mechanism. Hence, once fiber breakage releases strain energy, the material resistance to delamination growth is affected. (AU)

• FAPESP's process: 19/00846-3 - Relation between interlaminar damage extension and fibre orientation with acoustic signal: quasi-static and cyclic loading
• Grantee: Roberto Ferreira Motta Junior
• Support Opportunities: Scholarships in Brazil - Master