PSI - Issue 50

S.V. Panin et al. / Procedia Structural Integrity 50 (2023) 220–227

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S.V. Panin et al. / Structural Integrity Procedia 00 (2019) 000 – 000

this could contribute to an increase in the mechanical properties due to additional reinforcement with damaged fibers. However, such a mode of the welded joint formation is unacceptable for any practical applications.  Modes 1 – 3 (the USW duration of 800 ms) Mode 1. In this case, shear strength was only 68 MPa and elongation at break was 3.5% (Fig. 1, a; curve No. 50). This indicated brittle fracture at the low strength level. At the minimum values of both USW duration of 800 ms and the compression force of 2 atm, there was a decrease in the welded joint thickness by 400 µm (Fig. 1, b). It was found on the cross-section micrograph (Fig. 3, a) that the prepreg retained its integrity but both ED films were melted. Mode 2. An increase in P compress up to 3 atm at the same =800 ms was accompanied by a multiple increase in shear strength up to 94 MPa and elongation at break up to 8.5%. A decrease in the sample thickness at the welded joint zone was 350 µm (Fig. 1, b). The sample was fractured with the formation of a neck (Fig. 1, a; curve No. 51). The cross-section micrograph showed that the prepreg retained its integrity, and the ED films had a residual thickness of at least 50 µm (Fig. 3, b). Mode 3. A further increase in P compress up to 4 atm at =800 ms, on the contrary, caused a significant decrease in the mechanical properties of the welded joint (  shear =29 MPa and  =1.2%). A change in the sample thickness at the welded joint zone was 260 μm (Fig. 1, b). Apparently, the key root was a ‘ weak ’ melting of the ED films and, accordingly, a low interlayer adhesion. For the same reason, the prepreg layer was even and wide (Fig. 3, c). For all subsequent USW modes, it was found on the cross-section micrographs that the prepreg did lost its integrity to some extent (Fig. 3, d, e, f, g, h and i). This meant that the formation of the welded joints was accompanied by melting both ED films and (partially) the prepreg. Nevertheless, the results of the analysis of their structure and the mechanical properties are presented below.  Modes 4 – 6 (the USW duration of 1000 ms) Mode 4. At the low compression force of 2 atm, the high mechanical properties were registered (  shear =92 MPa and  =6.5%). The sample thickness at the welded joint zone reduced by 380 μm (Fig. 1, d), i.e. both ED films were melted. The sample fracture was not accompanied by the formation of a neck (Fig. 1, a; curve No. 54). In general, the prepreg retained its integrity. Mode 5. Increasing P compress up to 3 atm at the same USW duration of 1000 ms resulted in a noticeable decrease in the mechanical properties (  shear =55 MPa and  =2.7%), although the sample thickness at the welded joint zone of 370 µm almost did not change compared to Mode 4 (Fig. 1, b). The sample fractured in a brittle manner (Fig. 1, a; curve No. 55). It was found on the cross-section micrograph (Fig. 3, e) that the prepreg only partially retained its integrity, while the ED films did not melt completely. Mode 6. A further increase in P compress up to 4 atm at =1000 ms gave rise to a significant improvement of both shear strength and elongation at break (96 MPa and 10.1%, respectively). A change in the sample thickness at the welded joint zone was 540 μm (Fig. 1, b) that had to be accompanied by melting both ED films and the prepreg (Fig. 3, f). Thus, high mechanical properties during the shear tests of the lap welded joints were not a sign of maintaining the integrity of the interlayer structure at the interface.  Modes 7 – 9 (the USW duration of 1,200 ms). At theses modes, the shear strength values were high enough (above 90 MPa). Mode 7. At the low compression force of 2 atm, the sample possessed high both shear strength of 94 MPa and elongation at break of 12.5%. The sample thickness at the welded joint zone reduced by 500 μm (Fig. 1, d), i.e. both ED films and (partially) the prepreg were melted. The sample fracture occurred with the formation of a neck (Fig. 1, a; curve No. 57). The prepreg lost its integrity in some regions (Fig. 3, g). Mode 8. The increase in P compress up to 3 atm at the USW duration of 1200 ms led to the high shear strength level of 91 MPa, but the elongation at break value of 5.3% was rather low. The sample thickness at the welded joint zone reduced by the maximum degree of 870 µm, which meant melting not only of both ED films and the prepreg, but partially the welded plates (adherends, Fig. 1, b). The sample fractured brittlely (Fig. 1, a; curve No. 58). It was found on the cross-section micrograph (Fig. 3, h) that the prepreg mostly retained its integrity, but there were pores in the fusion zone. Mode 9. At the maximum USW duration of 1200 ms and the compression force of 4 atm, the greatest shear strength and elongation at break were observed (  shear =96 MPa and  =16.0%, respectively). A change in the sample thickness at the welded joint zone was 620 µm (Fig. 1, b), which had to be accompanied by melting of both ED