J-Integral analysis of the mixed-mode fracture behaviour of composite bonded joints

• Autor(a) principal: Loureiro, Fernando José Carmona Freire de Bastos
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.22/15561
• topic: Adhesively-bonded joints; Single-Leg Bending; J-integral method; Fracture mechanics; Fracture toughness; Fracture envelope; Finite element method; Cohesive zone models; Cohesive laws; Juntas adesivas; Single-Leg Bending; Método do integral J; Mecânica da fratura; Tenacidade à fratura; Envelope de fratura; Método de Elementos Finitos; Modelo de dano coesivo; Leis coesivas

The adhesive technology has been constantly growing and expanding into industrial environments, not only for traditional applications but also for high-end applications, where it has been competing fairly with the conventional connection technologies, such as welding, brazing, bolting and riveting. Its unique key features allow it to raise the type of technology to unreachable levels, for certain applications, by its competitors. Some of the advantages are the lightness of the adhesively-bonded joints, good behaviour under cycling and fatigue loading conditions, flexibility in bonding several types of materials and low stress concentrations. However, in order to design and develop efficient adhesively-bonded joints, the strength prediction must be accurate for the assessment of the fracture properties, mainly the critical energy release rate for tensile (JIC) and shear (JIIC), associated to the mode I and II, respectively. For most of the adhesively-bonded joints applications, the loading conditions under operational service feature a combination of different stresses, for instance tensile and shear stresses, from which the concept of mixed-mode came to exist. For this reason, the assessment of fracture properties under those conditions is essential, especially the energy release rates related to different mode-mixities. The fracture properties are related to Fracture Mechanics and are obtained through energetic analyses, from which three methods are often used: models based on the measurement of the crack length during the damage propagation, models based on an equivalent crack length and methods based on the Jintegral formulation. In the specific case of the J-integral it is furthermore possible to obtain the cohesive laws of the adhesive, which can be later used in the design of adhesively-bonded joints. This current work presents an experimental and numerical analysis of a Single-Leg Bending (SLB) adhesively-bonded joint where the specimens were bonded with three distinct adhesives, in order to assess and compare their behaviour under mixed-mode load conditions, fracture properties and cohesive laws. For that purpose, the J-integral formulation of Ji et al. [1] was considered to obtain the energy release rate for mode I and II, tensile (JI) and shear (JII), respectively, whereas the cohesive laws are attained through direct differential operation of the JI-w0 and JII-δ0 curves, where w0 and δ0 are the local normal separation and local tangential slip between the two adherends at the cross-section of the crack tip, respectively. Afterwards, the fracture analysis was performed, where the experimental results were compared through load-displacement (P-δ) curves. The JI and JII values, obtained through correlation between experimental and numerical results incorporated into the J-integral formulation, were addressed by R curves and fracture envelopes. These latter were used to establish which criterion was more suitable for each adhesive type. For last, the tensile and shear stresses were determined through the cohesive laws, attained by the direct method. Overall, a good agreement on the fracture properties was obtained between the specimens of the same adhesive. Moreover, the cohesive laws also presented a good correspondence between specimens, and further enabled the design of adhesively-bonded joints with arbitrary geometry.