PSI - Issue 4

Stefan Kolitsch et al. / Procedia Structural Integrity 4 (2017) 95–105 Stefan Kolitsch/ Structural Integrity Procedia 00 (2017) 000 – 000

104 10

Under consideration of a semi-elliptical crack, the SIF range can be calculated depending on the applied stress range from the experiment, the crack length and the geometry factor from Newman and Raju (1977, 1979 and 1981):

W a

B a

     

c K Y a

a    . 

, , ,

    

(15)

Using Eq. 15 the experimental data can then be displayed in the Kitagawa-Takahashi diagram and compared for all materials. It is seen that the proposed approach gives a good, albeit somewhat conservative estimate of the safe region. Finally, instead of choosing the stress ratio R as a parameter for visualizing Eq. 14, one can also take the crack size. This leads to the crack size dependent Smith diagram displayed in Fig. 7, where the endurable stresses are plotted for crack lengths 0.1 mm ≤ a ≤ 0.5 mm.

Fig. 7. Endurable stress depending on the defect size from Eq. 16 plotted for the pearlitic material in the Smith diagram

Conclusion

In this publication, methods for the assessment of high strength materials in switch components have been presented. Regarding the manufacturing process, commonly used standards prescribe a minimum fracture strain in tensile experiments; however, in consideration of small defects the fracture strain depends on the fracture mechanism. Therefore a diagram of failure strain vs. defect size is presented, thereby facilitating the decision about a required thermo-mechanical bending process. Here the bainitic material has the highest and the ferrite-martensite the lowest fracture strain in consideration of very small defects. For increasing the fracture strain the ferrite martensite material has to be bent within a thermo-mechanical process.