Investigation of the fracture surface

Useful results were obtained in the analysis of fatigue striations. Figure 3 shows the surfaces of specimens tested at a maximum stress of 500 MPa. Both specimens show clearly visible striations. The distance between these striations is closely related to the rate of propagation of the fatigue crack - the smaller the distance, the lower the rate of crack growth. The treated specimen has a higher fatigue groove density than the base material, indicating slower fatigue crack growth. On average, the distance between grooves decreased by 0.39 μm.

Figure 3. Fatigue striations in specimens: a - BM, b - with LSP

Conclusions

Based on the results obtained, the following conclusions can be drawn:

• For the successful application of LSP, it is extremely important to select the appropriate parameters of the laser shock peening, otherwise the processing may lead to a deterioration in fatigue characteristics.

• Optimal parameters of LSP for titanium alloy OT4-0 are - laser energy 3 J, pulse duration 10 ns and single forging.

• Improvement of fatigue characteristics after LSP was recorded in different zones of the fatigue curve, which indicates the possibility of laser shock peening of parts operating under different operating conditions.

• LSP significantly reduces the growth rate of fatigue cracks.

Reference list

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2. Hertzberg, R. W. Deformation and fracture mechanics of engineering materials / R. W. Hertzberg. – Fourth Edition. – New York : John & Sons. 1996. – 786 р.

3. Ding, K. Laser Shock Peening: performance and Process Simulation / K. Ding, L. Ye. – Cambridge : Woodhead Publishing Limited, 2006. – 172 p. – ISBN 9781845691097.

4. Impact toughness and microstructural response of Ti-17 titanium alloy subjected to laser shock peening / S. Huang, Y. Zhu, W. Guo [et al.] // Surface and Coatings Technology. 2017. V. 327. P. 32–41.

5. Fatigue behavior of geometric features subjected to laser shock peening: Experiments and modeling / M Achintha, D. Nowell, D. Furfari [et al.] // International Journal of Fatigue. 2014. V. 62. P. 171–179.

6. Effect of laser shock processing on fatigue life of 2205 duplex stainless steel notched specimens / C. A. Vázquez Jiménez, Rosas G. Gómez, C. Rubio González [et al.] // Optics & Laser Technology. 2017. V. 97. P. 308–315.

7. Fomin, F. Surface modification methods for fatigue properties improvement of laser-beam-welded Ti-6Al-4V butt joints / F. Fomin, B. Klusemann, N. Kashaev // Procedia Structural Integrity. 2018. V. 13. P. 273–278.

8. Petan, L. Influence of laser shock peening pulse density and spot size on the surface integrity of X2NiCoMo18-9-5 maraging steel / L. Petan, J. L. Ocaña, J. Grum // Surface and Coatings Technology. 2016. V. 307. P. 262–270.

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Introduction

One of the approaches providing necessary strength characteristics of metal machine parts is surface hardening which leads to a nanostructured surface layer and improved component hardness. As a result of surface treatment, the wear resistance, the fatigue strength and the corrosion resistance are increased. The last two properties are highly important to prevent the stress corrosion cracking because it is frequently the failure reason of machine assemblies subjected to the high temperature effect. The stress corrosion cracking is observed when the fatigue stretching load interacts with the aggressive external environment. This kind of failure is dangerous due to its brittle character resulting in dramatic loss of the strength.

There are several treatment methods, which vary in physical principles of the specimen surface layer change. These include the thermal, thermochemical and mechanical treatment. Moreover, in each case engineers have offered several treatment ways (technical operations) that differently influence the surface morphology of diverse chemical composition materials. It requires extensive scientific investigations. In this field of knowledge, one of the advanced engineering ideas is to couple two technical operations in such a manner that the final material properties will turn out better than if each method were used separately.

In the current work, the main methods of surface hardness improvement, including combined ones, are considered. Results of strengthened surface layer depth and the extent of surface hardness augmenting after utilizing these methods are given.