High Frequency Impact Treatment

The English term High Frequency Impact Treatment, abbreviated as HiFIT procedures and called in the German High-frequency and high-frequency hammer peening process, refers to the treatment of welded steel structures at the weld toe to increase the fatigue strength.

Properties

The durability and service life of dynamically loaded, welded steel structures is determined in many cases by the welds, especially the weld transitions. Targeted treatment of transitions ( grinding, blasting, hammering, etc.) the life span can be increased by simple means in many constructions considerably. Peening methods have proved to be particularly efficient treatment methods and have been studied and developed in detail in the framework of the joint project REFRESH. The HiFIT process is one peening process that is universally applicable, requires little equipment expense and yet provides a high reproducibility and the possibility for quality control.

Operation

The HiFIT hammer works with a hardened pin with a rounded tip with a diameter of 3 mm. This pin is hammered with an adjustable intensity and a frequency of 150 .. 300 Hz to the transition area from the weld to the base material. It caused local transformations in the form of a post-treatment track.

In essence, the success of a HiFIT treatment is based on the following modes of action:

First reduce the notch effect by conversion or rounding of the suture transfer and removal of weld defects.

2 solidification of the outermost layer to increase the resistance to cracking and crack propagation.

3 introducing compressive residual stresses to increase the resistance to cracking and crack propagation through superposition of the notch stresses with compressive stresses.

Variants

In order to improve accessibility to the treatment site, and for rounding of component inside edges without welds, there are numerous variants of Spezialpins in various diameters and lengths. For the treatment of special materials Pin variants of " acceptable " materials are provided. Special versions for special applications are often deployed at the customer.

Evidence

In the research project "REFRESH - life extension of existing and new welded steel structures ( P702 ) from 2006 to 2009 could in numerous tests, a 80 - to 100 - percent increase in the weld fatigue strength and a 5 - be detected up to 15 - fold increase in the weld life. In this research project, the positive effect of the treatment could be detected on the service life of welded structures.

You could eat at the University of Duisburg are determined in experiments, a doubling of the fatigue strength and an increase in the lifetime by a factor of 10 compared to the untreated initial state.

At the Finnish Aalto University, a team led by Prof. Gary B. Marquis researched for years the benefits of the high frequency hammering. In the " International Journal of Fatigue ", he published a report with proposals for SN curves that are conservative with respect to the available results from 228 ​​trials recently.

Process steps

HiFIT the method can be applied both to existing and in new steel structures. However, the visibility and accessibility of the transition weld in the areas to be treated is required for a specific processing. At sample trays or in unstressed areas of the base material, the required shock intensity is determined by the trace depth. Existing structures must obtained in advance usually at the weld toe, a surface pretreatment. The parts must be free of loose rust and old paint. If necessary, previous sandblasting is required. For new constructions, no special preparation is required.

The HiFIT device is placed by hand on the treated weld toe and guided along during treatment at this. The device operates with a bar compressed air supply of 6-8. Through local transformations of the weld toe is plastically deformed ( rounded ) and solidified. The depth of the treatment track should be from 0.2 to 0.35 mm. The undercut at the weld toe is no longer visible after the treatment.

By visual inspection of the treated region can be examined. The treatment track is checked using a special template. One of the working pressure digital display allows the user to control the whole time.

Economic Importance

The economic potential of HiFIT treatments is discussed in detail in the REFRESH research report. The following are the main points are mentioned briefly.

Life Extension

For use on existing buildings, the service life can be extended significantly. Unless grossly visible cracks are present, HiFIT is a very suitable rehabilitation tool. With timely rehabilitation of existing designs is virtually no difference to the life of new treated welds. This results in the potential to use existing structures far beyond the previous lamp life. Is applied the HiFIT method very efficient eg highway bridges in steel hollow box profile design on the fly. The cost of the restoration is small compared to conventional methods. In the commercial vehicle industry and other industries highly stressed welds on existing and new structures can be successfully treated to increase durability.

Increase the transferable load levels

With new designs and some existing designs can be increased for the treated welds with the same life the load level. With the same life as the 1.6 times the expense of the weld are transferable. This has the very positive effect that larger lifting capacities can be moved eg in crane. The efficiency of the cranes increases with every stroke.

Lightweight

Taking into account the HiFIT process during development, the design, at the same load level and the same service life, to be selectively streamlined. Extensive experimental studies on structural details and FEM -based design method demonstrate the high efficiency of conventional S235, S355J2 and fine grain steels such as S460N, S690QL and even higher strength steels. The recoverable material savings make the application of HiFIT method in most applications already economically viable. Considering additionally the weight advantage achieved from the application, for example, the achievable payload increases in vehicle.