TR73

  • Increase font size
  • Default font size
  • Decrease font size


C6 - Fatigue Behavior



Fatigue Behavior of Sheet-Bulk Metal Formed Parts

Project Status: Active

Last Update: 04.10.2018



Members


Components produced by sheet-bulk metal forming typically show locally inhomogeneous material properties. Due to different process routes and forming parameters, a locally varying degree of deformation and thus differently work hardened areas are generated. An additional consequence of cold forming is the formation of ductile damage in the form of voids in the microstructure. Investigations of the 2nd funding period on specimens taken from sheet-bulk metal formed components have shown that a higher work hardening has a positive influence on the fatigue life. On the other hand, an increase of voids due to cyclic loading and thus a negative influence due to increasing ductile damage could be shown. In addition, residual stresses remain in the workpieces as a result of the forming process. Tensile stresses favor the crack formation and crack propagation during service and have a negative effect on the components' fatigue life.

Fatigue tests:

The described material properties have to be taken into consideration for a component layout with respect to their application, for example, as transmission components. Under cyclic loads, these properties interact with one another and thus require a service life analysis under application conditions, since in uniaxial fatigue experiments these interactions can not be fully taken into account. For this purpose, in the third funding period, fatigue tests are provided in a gearing test bench, whereby gear pairings can be tested in use and limit loads can be determined as a function of the forming parameters during production.

Pruefstand en

Figure 1: Gearing test bench

Material characterization:

In addition to the fatigue testing, the components material state is analyzed with regard to hardness and residual stresses in order to determine input parameters for fatigue life modeling. Therefore, mechanical test methods such as micro hardness tests and tensile tests are used. In order to determine the residual stress state of the components, microscopic residual stress measurements are also provided by means of a microscopic hole drilling method using focused ion beam technique and digital image correlation (FIB-DIC).

fib3

Figure 2: DIC analysis of strain distribution around a microscopic slot induced by FIB in a surface, which is subjected to residual stresses

Surface treatment:

By an additional mechanical or thermal treatment of the components' surface, the potential for inducing compressive residual stresses and the adjustment of the microstructure is to be analyzed. In this way, crack initiation can be counteracted or a possible crack propagation should be inhibited in order to positively influence the components fatigue life.

The aim is to investigate the influence of the chosen process route or forming parameters and, if appropriate, subsequent treatment of the components to improve the fatigue life and to analyze the occurrence of various types of failure, such as pitting formation or tooth fracture. On this basis, existing fracture mechanic approaches for fatigue life modeling are to be extended by the consideration of the residual stress state in the surface layer.


Working Groups


Publications

    2018

    • Besserer, H.; Rodman, D.: Micro-Scale Residual Stress Measurement Using Focused Ion Beam Techniques and Digital Image Correlation. In: QDE2018, International Conference on Quenching and Distortion Engineering, Nagoya, Japan, 27.11.-29.11.18, (2018), pp. 32

    2017

    • Besserer, H.; Rodman, D.: Fatigue Behavior of Sheet-Bulk Metal Formed Components. In: MS&T17 - Materials Science and Technology 2017, Pittsburgh, 08.-12.10.2017, (2017), pp. 859–864