Tesi di laurea magistrale
Progressive Failure Analysis of Composite Countersunk Bolted Joints Tests using an Energy Based Continuum Damage Model
INGEGNERIA CIVILE E INDUSTRIALE
Corso di studi
relatore Prof.ssa Lazzeri, Roberta
- Continuum Damage Model
- Bolted Joints
- Progressive Damage
Data inizio appello
Data di rilascio
The thesis discusses the progressive failure analysis of composite bolted joint specimens with countersunk fasteners. Previous work on Bolted Joint stress analysis demonstrates that the out-of-plane stresses can be significant, for this reason a 3D framework analysis must be considered in studying this problem. The progressive failure analysis uses an energy-based continuum damage model for material stiffness degradation under three-dimensional states of stress. This model consists in a bilinear softening behavior of the elements, in particular, the first stage of the material behavior is considered linear until failure initiation occurs. After that the material follows a linear degradation until zero. The most important thing about this method is that a mesh independent model, or rather the mesh size effect on the results is alleviated using the CDM model. This is provided by the scaled fracture toughness over the element length. The material model is implemented, first as a user defined material UMAT and then secondly as VUMAT subroutine for use with the ABAQUS™ finite element software. The developed methodology was validated using open-hole tension and open hole compression tests results, then used to predict progressive failure of bolted joints. The test fixtures studied in this work are the procedures described in the ASTM D569 normative, in particular, the Double shear tensile loading configuration (Procedure A) and the Single shear tensile loading of one-piece specimen (Procedure C). Finally, a new test fixture procedure is presented in this work obtained by modifying the configuration of ASTM D5961 Procedure A, introducing a countersunk region in the specimen, obtaining the same configuration of Procedure C. The purpose of this modification is to allow the use of a bigger pin to avoid plasticity and consequently, to prevent failure of the pin before the specimen failure. This procedure will be named Procedure A modified and it will be performed to obtain the bearing strength of a composite laminate with a countersunk fastener under double-shear tensile load. The goal of the thesis is to validate the experimental results and to give support to the experimental evidence, about the behavior in the bearing region during the damage process.
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