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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Each series of specimens are carried out following the indication of ASTM Standard 5961-B - Standard Test Method for Bearing Response of Polymer Matrix Composite Laminate with an extensometer of gage length of 50 mm installed in correspondence of the middle section (as in Fig. 2a). In particular, a test fixture reported in the standard has been used to avoid the specimen instability and ensure alignment (as shown in Fig. 2b). 3. Numerical models Two FEM model was realized with numerical software in order to predict the progressive damage of the riveted joint. The symmetrical "cross-ply" laminates have a deformation behaviour similar to that of a body made of homogeneous material orthotropic; for all others are present additional terms, which give for example couplings between bending stresses and deformations membranal. However, it is interesting to observe that the importance of these additional terms is quickly decreasing as the number of laminae, therefore a laminate with many plies essentially acts as if it were made of a homogeneous material orthotropic. These considerations justified the possibility to use homogeneous constitutive properties to build a simplified model describing the global behavior of the joint. Nevertheless, the classical theory of lamination is not able to evaluate the stress components along the thickness of the laminate; however, in certain situations, where the rivet will be in contact with the inner region of the hole’s laminate, such as in this work, these components can overtake high values and to result very important for the strength of the parts. The second model was therefore based on solid element in order to consider these aspects. 3.1. Model A The first model (denoted model A) was generated starting from the constitutive properties of the laminate calculated with Composite Design Simulation software, consisting of homogeneous material along all the thickness and orthotropic properties, it is generated in order to evaluate the general deformation behaviour and therefore the stiffness of the bolted joint, including contact features. The geometry of the joint is in accord with ASTM Standard D5961-B and is generated using Design Modeler software. The mesh of the two laminates is divided into two main regions: a square one corresponding to the overlap area with more detailed mesh around the bolt and a region with a coarser mesh away from the bolt. In the square area, the arcs that represent the edges of the hole was divided into 32 elements, while in the radial direction is adopted a division into 19 elements. The laminates are discretized with hexahedral elements while the rivet collar is discretized with tetrahedral elements. as in Figure 3a-b.

(a) (b) Fig. 3. (a) Mesh of the laminate and near the hole; (b) Mesh of the fastening system.

The contacts “contact” and “target” surfaces are modelled Between laminates and in the coupling “laminate-rivet” zone, where rivet has a greater stiffness, therefore the laminate is detected as “contact” and the rivet is detected as “target”. The influence of contact between the surfaces characterizes strongly the relative motion of two bodies under tension; in the specimen, the respective doublers are built as “bonded”, they are not allowed to slip or separate. The “laminate-laminate” and “laminate-rivet” contacts occur with friction, with different friction coefficient to be