PSI - Issue 5

Shayan Eslami et al. / Procedia Structural Integrity 5 (2017) 1433–1438 Shayan Eslami et al./ Structural Integrity Procedia 00 (2017) 000 – 000

1436

4

(a)

(b)

(c)

(d)

Figure 3- Prepared specimens ready for the fatigue test (a); Remote stress vs Strain for PP, PE and optimized weld (b) [5], Polypropyelene dogbone (c); welded specimen dimensions (d).

This article focuses on the fatigue evaluation of the optimized welded specimens and its comparison with the highest performing base material. The obtained results is explained in the following section of this manuscript.

3. Results and discussion

The SN fatigue curve for dissimilar polymer lap-joints was obtained. The manufactured specimens were tested under three different loading levels (R=0.1 ) with respect to the parent materials’ characteristics [5]. Figure 4 shows the S-N plots of the different tested specimens; Polypropylene dogbone and FSW specimens. The horizontal axis is the number of cycles and the vertical axis represents the maximum stress. It can be determined that the fatigue life of polymeric FSW is comparable with the base materials characteristic, but in a brittle behavior. The polypropylene specimens failed thermally instead of fatigue failure due to the high frequency of cyclic loading, while the FSW joints failed from retreating side of the weld in a brittle behavior, using the same testing frequency (25 Hz). The PP dogbone specimens failed due to the thermal softening, which led to excessive deformation and necking. Using high testing frequency, the generated viscous heat is greater than the heat removes from the specimen during the test, and leads temperature to rise until thermal failure occurs. The FSW specimens had competitive fatigue life in compare with the parent material, as well as good resistance to the thermal failure. The resistance of FSW joints to the thermal failure can be explained due to polymer degradation and mixing of two different polymers under high axial force and temperature.