PSI - Issue 13

Meike Funk et al. / Procedia Structural Integrity 13 (2018) 279–284 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

281

3

3. Results

3.1. Fractography

The investigations of the notch root and the fracture surfaces of broken specimens give information about the crack path and the coalescence of the initiated cracks. Obviously, the structure of the fracture surfaces has an influence on the cyclic lifetime. One characteristic feature are steps on the surface, as already reported by Bär and Wilhelm (2014). A detailed optical study was undertaken on the fracture surfaces recorded as a 3-dimensional false color picture (figure 1). The different heights are indicated by colors. Via contour lines parallel to the notch root the height of the steps was determined. The length of the step was measured perpendicular to the notch in the false color picture.

Specimen 3

-----------------

Specimen 2

Specimen 1

Specimen 3

notch ↓

Length of the step

Fig. 1. Notch root of specimens prepared with laser cuts fatigued with a maximum stress of 70 MPa. The 3-dimensional picture with false color shows the topology of the fracture surface.

The three specimens presented in Figure 1 show different types of crack paths in the notch root due to differences in the crack coalescence, influenced by the crack initiation sites. The crack initiation sites are forced by laser cuts at the edges of the root notch. For specimen 1, both cuts are directly in the root notch, whereas for specimen 2 and 3 the cuts are located in a certain (different) distance to the notch root (in the following, this will be denoted as “different levels”). Specimen 1 shows a straight crack path linking the laser cuts. The fracture surface of this specimen shows two small steps with a step length below 0.25 mm and a very small height of about 40 µm (table 1). Investigations with higher magnification show four crack initiation sites near the notch root as a reason for these steps. Specimen 2 shows two cracks starting from the laser cuts, which form one big step in the middle, as a reason of the coalescence. The height of the step was measured to 220 µm and the length was found to be 1.63 mm. The cracks in Specimen 3 are emanating from the laser cuts and are propagating independent from each other, until one of the cracks get dominant and a through the thickness crack is formed. The resulting step on the fracture surface has a length of 2.25 mm and a height of 300 µm.

Table 1. Different values of specimen 1 to 3 Lifetime

Length of step

Height of step

Point of coalescence

Specimen 1 Specimen 2 Specimen 3

90,000 cycles 107,000 cycles 127,000 cycles

0.25 mm 1.63 mm 2.25 mm

40 µm 220 µm 300 µm

52 % of N f 87 % of N f 91 % of N f

The lifetime was found to be rising with the step height as well as with the step length. Having the laser cuts directly in the notch root as in specimen 1, the cracks seem to exhibit a strong interaction, which accelerates the crack propagation. If the laser cuts are in different levels, the coalescence is hindered resulting in a retarded coagulation and therefore an increased lifetime. As shown in table 1 the cyclic lifetime is rising with the point of coalescence.