PSI - Issue 37

4

A. Prokhorov et al. / Procedia Structural Integrity 37 (2022) 540–546 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

543

a

b

Fig. 3. Residual stresses vs depth in Armco-iron (a), Ti-grade2 (b).

Maximum compressive residual stress for Armco-iron specimen is 250 MPa at the depth of 0.17 mm. The compressive residual stress is observed up to 1mm depth. Difference between x and y directions may be associated with direction of laser beam trajectory and crystalline orientation. The results obtained for Titanium Grade 2 is different. Maximum compressive stress is 780 MPa and has the depth of 0.12 – 0.27 mm. The diagram has a linear character of compressive stress distribution. Compressive residual stress after 0.9 mm after transforms into tensile. In contrast with Armco-iron residual stresses in x and y directions are the same up to the depth of 0.55 mm. 3. Results The results of fatigue tests are shown in table 3. All specimens treated by laser shock peening have low durability in comparison to samples in the initial state. In case of Armco-iron specimen the durability drops up to 10 times after the LSP treatment. This unexpected result can be explained by the idea that the geometry of Armco-iron specimens and configuration of LSP treatment area on titanium specimens have a bad design. Similar findings about the degradation of fatigue properties for corner cracks after the LSP treatment of fastener holes have been observed in [Ivetic G, Meneghin I, Troiani E, Molinari G et al.].

Table 3. Results of fatigue tests. Specimen

Armco-iron

Titanium Grade 2

LSP treatment

No

Yes

No

Yes

Number of cycles

7.50 × 10 5

9.12 × 10 4

5.86 × 10 4

7.80 × 10 4

Another interesting point of this research is the character of crack initiation in Armco-iron specimen after the LSP treatment. Figure 4(a) sows that fatigue crack initiation occurred in the flat area of the specimen close to the fillet radius. A single corner crack has been observed. On the contrary, Figure 4 (b) displays a number of fatigue cracks initiated in different areas and the main crack on the border of cubic zone. Authors suppose that this effect is attributed to possible tensile residual stresses at the specimen corners. These tensile residual stresses result in earlier crack initiation after the LSP treatment and multiple crack initiation.