Increasing lifetime by means of mechanical surface layer strengthening

Mechanical Surface Layer Strengthening

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Figure 1: Schematic representation of mechanical surface treatment methods
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Generation of strengthening and compressive residual stress by shot-peening
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The surface layer of metallic components and structures is of decisive importance for their lifetime. In all industrial sectors, where components are exposed to high stress such as automotive, vehicle and railway construction, aerospace, steel and bridge construction, drive technology and gear manufacturing, combustion engines, steam and gas turbines, compressor and pump manufacturing, tool and mold making as well as medical engineering, the material condition of the surface layer plays an elementary role. The reason for this is that damage processes such as fatigue and corrosion often initiate at the surface. Hence, they can be retarded or accelerated depending on the stress state of the surface layer, which has a significant impact on the component lifetime.

A lot of mechanical surface treatment methods (Figure 1) have been developed during the recent decades. These methods use the effect of the elastic-plastic deformation of the surface layers of metallic components by use of suitable impact tools such as balls, hammers, rollers, waterjet and laser. This deformation results in strengthening and compressive residual stress in a thin surface layer. Such beneficial layers may serve the purpose of significantly increasing the fatigue resistance of the entire component.

Therefore, we develop material models and simulation tools which help to optimize the material behavior and the component lifetime by use of a customized surface treatment. We derive correlations between material behavior, component geometry and process-related influencing factors of mechanical surface treatment methods on the residual stress field and the surface layer strengthening by combining experimental results with material and process simulation. Finally, we determine the increase of lifetime of cyclic loaded components as a function of the process parameters.

Increasing the Lifetime of Welded Joints by High Frequency Mechanical Impact

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Plastic deformation of the weld seam by means of High Frequency Mechanical Impact

In general, welds have significantly lower fatigue strength in comparison to the adjacent base material. This is mainly caused by the notch effect due to the weld seam geometry. Furthermore, changing material and microstructure conditions in the weld zone (metallurgical notch effect) and tensile residual stress in regions which are susceptible to cracking, reduce the fatigue strength of welded joints.

In recent years, High Frequency Mechanical Impact or the HFMI method has achieved great significance in practice. In this method, a hardened cylindrical pin with a round tip strikes on a component with high speed, respectively frequency >90 Hz, whereby the seam notch geometrically will be flattened, the surface will be strengthened and high compressive residual stresses will be induced. The effectiveness of the HFMI method to increase the service lifetime respectively the fatigue strength of welded joints as well as their applicability has been confirmed by numerous studies. By an effective post-treatment of welds not only constructive problems of new designs can be solved but also the use of high-strength steels will be facilitated.

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FEM simulation of the generation of compressive residual stress in a weld toe by use of high frequency mechanical impact method
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Numerical lifetime estimation of weld joints which were treated with high frequency mechanical impact method (red line) and their experimental verification (blue)



  • Buck, M.; Straub, T.; Eberl, C., Experimental investigation of damage detection and crack initiation up to the very high cycle fatigue regime, Fatigue of materials at very high numbers of loading cycles; Christ, H.-J. (Ed.); Springer Spektrum, Wiesbaden (2018) 365-393 Link
  • Schubnell, J.; Farajian, M.; Däuwel, T.; Shin, Y., Numerical fatigue life analysis of a high frequency mechanical impact treated industrial component based on damage mechanics models, Materialwissenschaft und Werkstofftechnik 49/1 (2018) 113-127 Link

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