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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Figure 12a-b shown the matrix tensile damage for different plies (-45, 0, 90). It can be observed that damaged begins around the hole in the contact zone with the bolt, and is evenly distributed in all the plies. However, it can observe how local damage occurs at a load equal to 5358N for which the first ply to failure is the ply 16. In Figure 13 fiber tensile damage for different plies for a level load equal to 8726 N is shown the. It can be observed that the plies more damaged are those arranged to 45, -45, 90, directions for which the fibers have a lower resistance than those oriented to zero coincident with the direction of load. However, it can be observed how local damage occurs at a load around to 6590N for which the first ply to failure is the ply16.

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Fig. 12. (a) Failure matrix and (b) damaged ply in tensile stress.

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Fig. 13. (a) Failure fibers and (b) damaged ply in tensile stress.

4.5. Model B – including Delamination onset In mechanically fastened joint in CFRP edge delamination is one of the first damage mechanism to be detected. Experimental results have shown that this delamination starting from hole boundary. Using these experimental results, it is not possible to obtain an accurate value for the delamination onset load. However, in detailed and refined FEM models it is possible to determine the presence of delamination at a given load level, as well as the interface and location where this form of damage occur. This in fact is the purpose of this delamination model. Figure 14a showed the results obtained modelling the delamination, compared to experimental data. From the results obtained it was observed as the delamination greatly reduces the stiffness of the model. This may be due to the fact that by introducing the cohesive zone between the layers of the laminate, with the relative characteristic coefficients, a further compliance is introduced into the model which allows a sliding movement of the laminae. From the results obtained, considering the characteristic lengths δ n and δ t equal respectively to 2,29×10 -6 m and 1,74×10 -6 m, it can be observed as in the model, a microscopic delamination begins around a load equal to 3457 N to the interface between the ply oriented to 45° and the ply oriented to 0° as shown in Figure 13b.