International Journal of Latest Research in Engineering and Technology (IJLRET) ISSN: 2454-5031 www.ijlret.com || Volume 04 - Issue 08 || August 2018 || PP. 34-51 www.ijlret.com 34 | Page Geometry Effects on Failure Mechanisms of Hybrid Metal- Composite Bolted Joints Calin-Dumitru COMAN 1 1 Department of Structures and Materials, INCAS Bucharest, Romania Abstract: This paper presents the effects of geometry variation on the damage initiation and growth in the CFRP (Carbon Fiber Reinforced Polymer) composite laminated plate of the single-lap, single-bolt, hybrid metal-composite joints. A detailed 3D finite element model (FEM) incorporating geometric, material and friction-based contact full nonlinearities is developed to numerically investigate the geometry effects on the progressive damage analysis (PDA) of the orthotropic material. The PDA material model integrates the lamina nonlinear shear response, Hashin-tape failure criteria and strain-based continuum degradation rules, which were developed using the UMAT user subroutine in Nastran commercial software. In order to validate the geometry variation effects on the failure modes of the protruded head bolted joints, experiments were conducted using the SHM (Structural Health Monitoring) nondestructive testing technique. The results showed that the plate geometry is an important parameter in the design process of an adequate bolted joint and its effects on damage initiation and failure modes were quite accurately predicted by the PDA material model, which proved to be computationally efficient and can predict failure propagation and damage mechanisms in hybrid metal- composite bolted joints. Keywords: Progressive Failure, Nonlinear Shear Deformation, Hybrid Joints, SHM 1. INTRODUCTION The aerospace industry became the most common application field for fiber-reinforced polymer matrix composites (PMCs) due to their lightweight properties [1]. These structural components are often assembled in conjunction with metal parts using mechanically fastened joints resulting in hybrid metal-composite joints which generate some challenging issues for mechanical engineers. Poorly designed hybrid joints are not only a source of failure, but could lead to a reduction of the durability and reliability of the whole structure. Up to nowadays, the researchers studied the failure analysis of composite bolted joints using a method that combines continuum damage mechanics (CDM) [2] with finite element analysis (FEA). In the CDM models, the local damage onset appears at a low values of applied load and damage accumulation is developed with increasing load according to damage propagation laws, which makes the method accurate and able to predict various failure modes. The major disadvantage of the CDM models is the huge amount of test data required for model calibrations. The progressive damage analysis (PDA) of composite materials, which is based on the stress-strain failure criterion, showed that the material orthotropic properties reduction due to damage initiation is essential for the stress field analysis [3-7]. Many PDA models [8-11] incorporated lamina shear nonlinearity, Hashin-type failure criterion and constant elastic properties degradation laws for orthotropic materials, which make the method quite easy to implement and computationally efficient. Because these properties degradation models used constant factors for elastic properties reduction due to damage growth, the models weren’t be able to predict the bearing final failure. Models containing continuous degradation rules started to appear in the literature [12], [13] as to improve the numeric algorithm converge and to obtain a smoother load-displacement curve. One major lack of these models is that they focused only on few types of failure modes and were not considering the various joint failure modes. The composite progressive damage behavior is a complex nonlinear phenomena and in conjunction with geometric and contact nonlinearities can lead to loss of convergence in the finite element method analysis, mostly in implicit numerical algorithms which imply that many effortsare paid for obtaining a valid solution towards the ultimate global structure failure [14].The difficulties arising from composite materials usage on structural failure problems, are that these materials have anisotropic mechanical properties, brittle and low inter-laminar strength [15]. Awadhani et al. [16] presented a numerical and experimental study for the influence of a geometry parameter (edge distance to bolt diameter ratio, e/d) onto the stiffness, strength and failure modes of the hybrid metal-GFRP (Glass Fibers Reinforced Plastic) single-lap, single-bolt joints with steel bolts. The conclusion of the study was that the e/d ratio varied the failure modes from cleavage to bearing mode. It was also observed that the geometric parameter has increased the joint tensile strength; load at yield increased also. In this study it is presented and developed a progressive damage analysis using an adequate material