Abstract Plekhov

The effect of laser induced residual stress on fatigue properties of titanium alloys

It is well known that the creation of compressive residual stresses in the surface layer of materials leads to a change in the nature of the process of propagation of fatigue cracks both at the stage of their initiation and propagation. At the same time, the magnitude and depth of the residual stress layer affects the sensitivity of the structural element to the magnitude of initial damage and can serve as an effective method of increasing the resource of structures under combined dynamic and cyclic loads. The laser shock peening (LSP) is one of the most effective technique for creating residual compressive stresses. In comparison of other conventional technique, compressive residual stress generated by LSP can reach high amplitude of about  ̴ 1 GPa and extend into material up to depth of several millimeters.

In the present work, LSP is investigated both theoretically and experimentally. The theoretical description of the effect of laser impact on the substance includes the solution of a complex of related physical and mechanical problems. Four key problems can be identified as follows. The first problem is material evaporation (a plasma torch formation). This torch generates a stress wave in the processed material with an amplitude significantly exceeding the Hugoniot elastic limit. The second problem is elastic-plastic wave propagation in the material with a characteristic duration of nanoseconds. This wave generates plastically deformed surface layer. The third problem is calculation of self-balanced field of residual stresses in the sample induced by the plastically deformed surface layer. The fourth problem is the effect of field residual stresses on processes of fatigue cracks initiation and propagation.

The experimental setup includes experimental facilities for verification of numerical model based on results of both real-time stress-time measurement obtained by photonic Doppler velocimeter (PDV) and residual stress-depth distribution measurement obtained by drilling technique. To show the effectiveness of the technique, the peculiarities of fatigue crack propagation in notched titanium plane samples were investigated. The possibility of increasing of the fatigue life has been experimentally shown. It particular, the optimal LSP regimes provides the increasing in the fatigue life by more than seven times increase compared to the base specimens without treatment.

Keywords: Laser shock peening; residual stress; fatigue crack; photonic Doppler velocimeter.