Issue 49

L. Suchý et alii, Frattura ed Integrità Strutturale, 49 (2019) 429-434; DOI: 10.3221/IGF-ESIS.49.41 433 At the end of the joining section at joining length L j the chip folds over (Figure 4) and remains a part of the shaft. The lower joining forces of cutting I-KIF indicate a lower local plastic strain at the groove root, which leads to lower strain hardening (Figure 3a). As reported by Lätzer [4], cutting joining of a KIF leads to a lower static torsional capacity. It is therefore assumed that both, the lower plastic strain and lower radial residual pressure, affect the cyclic properties of the cutting I- KIF. a) b) c) Figure 4: Detected fatigue cracks at the shaft notch, material C45, I geo = 0.4 mm, a) φ =90°, b) φ =15°, c) Global cracks. Figure 3b compares the results from fatigue testing, where the stress values are related to the smooth shaft surface of Ø45 mm. The arrow points represent a run-out specimen related to 10 7 cycles. Below this cycle limit, the depicted points correspond to a failure in the low-cycle fatigue region. The results of the magnetic crack detection show a typical crack propagation of 45° for the applied torsional load (Figure 4c). An approximately 14% higher fatigue limit of the strain-hardened test samples ( φ = 15°) is observed in comparison to the broaching type ( φ = 90°). This is similar to the static torque transmissibility of the I-KIF. Referring to the fatigue limit of the smooth shaft net area, the estimated high-cycle limits result in a fatigue notch factor of 1.7 for the cutting I-KIF, compared to 1.4 for a formed connection according to the German standard for load capacity calculation of shafts and axles DIN 743 [14]. Although a positive effect of plastic pre-strain on fatigue strength was observed in basic research [11], [12] the grain structure evaluation in Figure 5 shows similar grain refinement on the surface near the area of both, cutting and forming joining process. Therefore, the increase of fatigue limit rather has to be linked to the higher groove pressure of formed knurling. In [14], higher residual pressure stresses were estimated for the presented connection. Additionally in Figure 5b, the grain structure underneath the surface of the formed shaft knurling shows a stretched pattern, which can also be associated with higher radial pressure. a) b) c) Figure 5: Grain structure of a) the cut shaft knurling ( φ =90°), b) the formed shaft knurling ( φ =15°). c) Location of grain evaluation C ONCLUSION he present work has introduced the experimental investigations of the inversed knurled interference fit, which is a novel connection type for transmitting torque with high power density. By moving away from the pairing conditions of the well-known knurled interference fit (knurled and harder shaft) to the connection of the inner-knurled hardened hub with the softer smooth shaft, beneficial residual compression stresses are achieved in the shaft. The presented results also show the influence of the hub chamfer angle on the high-cycle fatigue of the connection. Concerning forming joining, a 14% higher endurance limit was achieved by using a hub chamfer angle of φ = 15° due to higher radial residual T