Effectiveness of the Process

 

Mode of operation of the high-frequency peening process

The effectiveness of the peening results from the combination of the geo­metric (inside-rounding), and the mechanical (work hardening), and the strain (residual compressive stress) aspects.
The HiFIT pin is ball-shaped with a defined radius. The defined impact leads to a reshaping and to an inside-rounding of the weld toe. The plastic reshaping of the surface causes a residual compressive stress in the border layer. This stress can be proved up to a depth of 2 mm. Superimposition of the residual compressive stress on the acting loads displaces the later into less critical areas. It prevents the formation or extension of micro cracks. As a result a uniform and continuous peening trace becomes visible on the weld toe.

 

Verification procedure

At present three different verification procedures are available:

  • nominal stress concept
  • structural stress concept
  • notch stress concept

Detailed descriptions in:

  • FKM guideline (German Forschungskuratorium Maschinenbau (FKM) e.V.
  • IIW recommendations (International Institute of Welding)
  • Eurocode

 

The Research Project REFRESH

The research project REFRESH was started on 01/01/2006 as a joint ­venture project among several well-known partners of the Industry and Scientific Institutes. The target of the project was the development of a holistic process for the extension of the service life of new and existing welded steel constructions. In doing so different aspects have been considered like

  • optimization of the process
  • quality assurance
  • further development of the methods of calculation

 

Experimental results about the fatigue strength

More than 1000 analyses made on butt seams (Fig. 1), on transverse ­stiffeners, and on longitudinal stiffeners (Fig. 2) impressively prove the effectiveness of the high-frequency peening processes.
The Wöhler lines of Fig. 3 present the results of dynamic analyses made on butt seam S690Q and show that the fatigue stability has almost been doubled. The notch class for this case of application gave 111 for the as-welded specimens and 201 for the treated specimens.
Also the results obtained on longitudinal stiffeners S690Q were impressive (Fig. 4).
Compared to the as-welded joints the course of the Wöhler lines could be improved from slope M = 3 to slope M = 4; we consequently obtained a manifold increase in the service life.