Effectiveness of the Process

Mode of operation of the high­frequency 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 ball­shaped with a defined radius. The defined impact leads to a reshaping and to an inside­rounding 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 them 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. Three different verification procedures are available at present:

  • 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

Discussions of the IIW

The International Institut of Welding IIW published two papers in 2013 concerning the quality assurance and fatigue assessment. The proposed procedures lead to even higher tolerable stresses as already given by the REFRESH project. 

 

Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed fatigue assessment guidelines Gary B. Marquis & Eeva Mikkola & Halid Can Yildirim & Zuheir Barsoum available under:

http://link.springer.com/article/10.1007/s40194-013-0075-x

Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed procedures and quality assurance guidelines Gary Marquis & Zuheir Barsoum

http://link.springer.com/article/10.1007/s40194-013-0077-8

Prof. Gary Marquis and colleagues (University Aalto, Helsinki, Finland) produced many publications in this concern where they always proved the reliability, effectivity and user friendliness of the post-weld-treatment by High Frequency Hammer Peening. They presented 46 documents to commission XIII of the IIW that verified the improvement of the fatigue strength of welded constructions with the help of the HFMI technology.

Design Examples

(According to IIW Recommendation HFMI

In the following examples were no thickness, size effect correction and no mean stress effects (e.g. R≤0.15) taken into consideration.

 

Example 1

The weld detail is categorised into FAT-Class 63 according to its characteristics. Stress range is 63 MPa @ 2 Million load changes (Fig. 8).

Through HiFIT Treatment the Class is improved by 4 steps (Fig. 7 blue arrow) to FAT 100. Stress range is 100 MPa @ 2 Million load changes.

The improvement is approx. 60%

At the same stress level (63 MPa) the fatigue life time improves from 2 Mio to 40 Mio load changes!

The Factor is 20! (Fig 8) 

Example 2

The same weld construction is now from a steel fy ≥ 950 MPa. Will this not be treated the as welded status will not be changed and stays with a FAT Class of 63 (Fig. 9). There is no improvement just by using high tensile steel. 

Through HiFIT Treatment the Class is now improved by 8 classes (Fig. 7 red arrow) to FAT 160. The stress range is now 160 MPa @ 2 Million load changes.

The improvement is approx. 150%  (Fig. 9)

At the original stress level (63 MPa) the fatigue life time improves from 2 Mio to more then 100 Million load changes! Probably to the status that it will never brake! (Fig. 9)

 

 

Fig. 7: Improvement of FAT Classes by HiFIT Treatment
Fig. 8: S/N Curve for fy < 355 MPa; R ≤ 0,15
Fig. 9: S/N Curve for fy ≥ 950 MPa; R ≤ 0,15