PSI - Issue 18

V. Dattoma et al. / Procedia Structural Integrity 18 (2019) 719–730 Author name / Structural Integrity Procedia 00 (2019) 000–000

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sufficient to reproduce experimental tests (Tong, 2000). Failure in fibre-reinforced composite structures containing stress concentration zones, such as bolt loaded holes under severe contact conditions, has been one of the main issues studied by researches during recent decades (Camano et al., 1997, Dattoma et al., 2013); in particular hole clearance and bolt geometrical characteristics have been enhanced to elevate the certified bearing load with stable and firm joint behavior before failure. Research efforts have been concentrated on the development of progressive failure analyses tools, which give the ability to model damage generation and propagation before failure, to better estimate the residual joint load (G. Scarselli et al., 2015). Chang and Chang (1987) developed a two-dimensional progressive damage model for notched laminate plates subjected to tensile loading, which was latter modified to predict net-tension and shear-out failure of loaded holes; both geometric and material non-linearity was considered. Failure analysis was performed using the Yamada-Sun (Yamada et al., 1987) and the Hashin criteria (Hashin, 1980). In a work of Shokrieh (1996) advancement on the development of a 3D fatigue progressive damage model has been successfully reported and the basis was somewhat set by giving a description of the model. In this work, simplified and advanced numerical models have been considered to describe the experimental mechanical behavior of a single shear bearing test, tested in accordance with ASTM Standard 5961-B. A simplified model has been carried out using homogeneous constitutive properties to determine the global behavior of the joint. Therefore, the orthotropic properties were assigned to each lamina and the resulting constitutive properties were calculated according to the classical laminate theory. The second model is built assembling the stacked laminae of orthotropic material, in a more realistic volumetric model of whole laminate, capable to simulate delaminations; the interface between laminae is modelled using a Cohesive Zone Model implementing an exponential law for interface delamination. The problem complexity arises with the significance of the friction and pre-tension load of the fastening system, affected by simultaneous contact non-linearity in extremely localized regions around the hole, which affects critical stresses in different way and according to single lamina orientation. The stiffness of the joint under load was evaluated and the analysis performed show both the coefficient of friction and preload induce small alteration of the composite joint behaviour. First investigations revealed the refined model results with higher rigidity than simplified model, whilst experimental data of tensile stress/strain curve are placed in between. Various failure modes were analysed from the results; it is observed the beginning of the macroscopic damage to occur already at small load levels, with a percentage error up to 20%, with respect to the experimental data, but the damage zone doesn’t affect the successive loading curve and doesn’t propagate fast. In order to get more reliable evaluations of significant progressive damage conditions, delaminations were finally introduced.

2. Materials and experimental methods 2.1. Specimen geometry and materials

The geometry of the bolted joint, considered in this work is shown in Fig. 1. The riveted joint in CFRP material consist in a single-lap configuration, with an upper and lower plates made of unidirectional CFRP composite laminate (16 layers) and an aeronautical bolt lock with protruding head.

Schematic views

Specimen Dimension [mm]

L

191 38.3 6.41

W

d

t

7

D H

10.8

3.3

Fig. 1. Geometry and dimensions of the bolted joint.