Issue 47

F. Moroni et alii, Frattura ed Integrità Strutturale, 47 (2019) 294-302; DOI: 10.3221/IGF-ESIS.47.22 298 The diagram of Fig. 5 shows the results of the 5 repetitions done with TRAZ-CI test specimen. The coefficient of variation (i.e. ratio of std. deviation to average) is 5.6%. Also the deformations at failure are pretty close to each other, that is a sign of a good reproducibility of the tests. The rupture surface is shown in Fig. 6, where it is evident the interlaminar failure mode that occurs between the two plies closest to the bonded interface with the aluminum attachments. This type of failure was detected in all the repetitions of the test. Figure 5 : Results of TRAZ-CI tests. Figure 6 : Example of rupture surface of TRAZ-CI tests. The diagram of Fig. 7 shows the results of the 5 repetitions done with TRAZ-BJ test specimen. The average value of strength is just the 68.5% of the average of TRAZ-CI tests. The coefficient of variation is 6.8% in this case. Also the deformation at failure are pretty close to each other, that is a sign of a good reproducibility of the tests. The rupture was always interlaminar in the composite and it occurred close to the root of the fillet between the reduced gage section and the specimen ends as shown in Fig. 8. The location of the failure and the lower value with respect to the TRAZ-CI tests can be related to both a slight stress concentration at the fillet root and to a possible damage caused by the tooling of the specimen done to reduce the gage section diameter. Unfortunately, this kind of failure does not allow to assess the strength of the adhesive in co-bonded joints, therefore a further TRAZ-BJ test has been designed were the threaded aluminum ends are bonded each other with the adhesive film (Fig. 9). The S max,norm is in this case 1.21 times the TRAZ-CI tests with a coefficient of variation of 3.9%. Failure was always cohesive in the adhesive layer. From the TRAZ-BJ (adhesive only) test it can be concluded that the tensile strength of the adhesive used here is higher than the ILTS of the composite. On the other hand, attention has to be paid in case of notches obtained by tooling operations on composite, since the comparison of TRAZ-CI and TRAZ-BJ tests it turns out a reduction of about 1/3 of the ILTS. Fracture toughness For the sake of confidentiality, results are scaled with respect to the mode I or mode II fracture toughness of the co-cured CFRP, respectively. Therefore, normalized values denoted as G I_norm and G II_norm are shown in diagrams. The value of force in force-opening diagrams is normalized, too, with respect to the maximum force of the same test, F max .