Methodology for the experimental characterisation of mode i delamination under different loading rates
- Autores: Sergio Alonso Medina Escobar
- Directores de la Tesis: Jose Norberto Blanco Villaverde (dir. tes.), Emilio Vicente González Juan (codir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: Frédéric Laurin (presid.), Laura Carreras Blasco (secret.), Giuseppe Catalanotti (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
The rate-dependent behaviour of the interlaminar fracture toughness of fibre reinforced composites has been a matter of research during the last decades. However, the results obtained so far are not conclusive and further analysis is required. In parallel, the correct characterisation of these material properties should promote the development of reliable constitutive models for the simulation of dynamic events.
The research carried out in this thesis starts by trying to identify and understand the governing parameters involved in dynamic testing of mode I fracture toughness. With this in mind, a time-based threshold criterion is proposed to determine when dynamic effects might be neglected during the analysis of a high loading rate Double Cantilever Beam (DCB) test. The criterion compares the time after which inertia effects can be neglected in the specimen, known as transition time, versus the time for the initiation of fracture propagation. Three different methods are considered for determining the transition time: an analytical approach, a numerically-based method and a graphic method through FE simulations. Good agreement is found when comparing the derived expressions with the results of numerical simulations. It is also demonstrated that the transition time is affected by the velocity profile. The proposed criterion and approach to determine the transition time are useful tools to define when a quasi-static data reduction scheme can be used, providing an initial framework to mark off the analysis of high-rate tests.
An analysis of the specific literature regarding the test methods shows that there is no consensus on the appropriate methodology to characterise the interlaminar fracture toughness of composite materials under high-strain rates. In this thesis a new test method, the Guided Double Cantilever Beam (GDCB), is presented to measure the mode I fracture toughness in composites laminates and brittle adhesively bonded joints under intermediate/high loading rates. The proposed device guarantees a symmetric crack opening and thus pure mode I propagation during high loading rate testing. When used in a dynamic servo-hydraulic testing machine with controllable displacement rate, a constant opening velocity can be driven. The GDCB testing method is validated against a quasi-static DCB test, showing a good agreement between the results. In addition, the test method has been satisfactorily used under intermediate/high loading rates, showing the good performance of the device. The GDCB device has been patented with patent number WO/2022/003219.
The loading-rate mechanical response of the mode I delamination in composites and adhesively bonded joints has been also investigated. Using the GDCB test method, three different data reduction strategies have been assessed: a displacement-based formulation, a near-crack-tip displacement formulation, and a numerical assessment method based on the deformed shape of the arms of the specimen. The methods are developed to account for the dynamic effects which may be present during the test. Small differences between the three different methods can be noticed when comparing the results. Meanwhile, no clear rate-dependency of the fracture toughness has been evidenced for the composite laminate nor the adhesively bonded joint used.
Finally, a methodology for the dynamic mode I characterisation of composites and brittle adhesively bonded joints has been proposed. The methodology is established for a wide range of loading rates: from quasi-static to high loading rates (up to 30 m/s opening loading rates). The methodology covers the test methods, the data reduction methods, and the test set-up to obtain the parameters required in the data reduction.
