**5. Concluding Remarks**

The processes, effects, and applications of laser peening without coating (LPwC) were reviewed. A series of experimental studies clearly demonstrated that LPwC improves fatigue properties and reduces the susceptibility to stress corrosion cracking (SCC) through the impartment of compressive residual stresses (RSs) in the near surface layer of objects. LPwC has been applied to nuclear power reactors (NPRs) as a preventive maintenance against SCC of structural components since 1999 [1].

Low-energy LPwC was applied to welded joints of HT780 (780 MPa grade high-strength steel) structural steel with pulse energies down to 10 mJ. Fatigue testing revealed that the fatigue lives were sufficiently prolonged by LPwC even if 10 mJ pulse energy was used [16].

The Japanese governmen<sup>t</sup> launched a five-year national program, ImPACT in 2014 [17], which was designed to trigger off disruptive innovation for changes in society. In the program, compact high-power pulsed lasers including 20 mJ-class palmtop-sized handheld lasers has been developed. Due to the simplicity and robustness of handheld lasers, the application including LPwC necessarily expands in various fields, for example field maintenance of infrastructure such as bridges, windmills, power plants, etc.

**Funding:** This work was partially supported by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

**Conflicts of Interest:** The author declares no conflict of interest.
