Issue 49

L. Suchý et alii, Frattura ed Integrità Strutturale, 49 (2019) 429-434; DOI: 10.3221/IGF-ESIS.49.41 430 through the axial joining of a harder knurled component into a softer undersized component. While joining, a counter profile is incised by performing a plastic deformation on the surface of the softer component. For decades, the principle of self-forming connections has already been used for joining parts in fine mechanics [1]. Thomas [2] investigated a similar component connection back in the late 60s. He estimated the fundamental joining of torque characteristics of the so-called “press fit with discontinuous groove” [2]. Not until 8 years ago, various approaches have been proposed to describe the static and cyclic torque and bending load capacity of the KIF. Bader reported the minimal hardness ratio of 1.8 [3] and developed empirical calculations for several material combinations. Further calculations of the static load capacity in studies reviewed by the authors are predicated on the shear failure or equivalent stress of the counter profile [3–6]. Some of these studies regard the strain hardening in relation to the shaft chamfer angle φ , which divides the joining process into cutting and forming joining [7]. Similar to chip removing processes, a sharp edge ( φ = 90°, Figure 1b) of the harder knurled machine part cuts a groove into the softer part, forming a chip. In contrast, a chamfered edge ( φ < 60°) generates a smooth radial forming process, which leads to a massive local plastic strain and an additional elastic radial pressure due to the Young’s-modulus-related spring-back effect. Depending on the contact pressure, axial loads can also be transmitted due to friction between the components. Concerning the high-cycle fatigue, the calculation approach in [5] is based on the local stress gradient according to the guideline Analytical Strength Assessment (FKM-Guideline) [8] in combination with the criterion of maximum shear strain. In [9], the authors of the industrial application of KIF in differential gear emphasize the high power density of the connection with simultaneous cost efficiency. In contrast to the introductory literature, the tool part in the present study is a harder hub with an inner-knurled bore hole (Fig. 1). The counter-profile formation is carried out on the shaft surface. Subsequently, the softer shaft component of the inversed knurled interference fit (I-KIF) is subjected to a more advantageous pressure stress state in contrast to the tensile stress portion of the softer component of the KIF (radial hub expansion). Only the authors in [10] investigated the load capacity and assembling characteristics of I-KIF. Knurled interference fit with ductile hub Knurled interference fit with hardened hub Figure 1 : Geometry and material reversion of knurled interference fit Nevertheless, determining the fatigue strength of the KIF is challenging and has not yet been fully explored. In the present study, the high-cycle fatigue life of an inversed knurled interference fit (I-KIF) is investigated by means of experimental torque fatigue tests. Regarding the forming and cutting joining process, existing studies on fatigue strength of pre-strained specimen [11, 12] suggest a higher fatigue strength of the investigated materials. E XPERIMENTAL S ETUP he aim of the experimental investigations is to estimate the fatigue limit of the I-KIF with steel-steel pairing, distinguishing between the two different joining processes. Specimen specification and testing procedure Figure 1a-d shows the geometry of the investigated hub specimen made of 16MnCr5 case-hardened steel. Due to the pulsing axial forming procedure, the knurling is created prior to the heat treatment as described in [6]. The hardness of the shaft specimen made of C45 untreated steel was 233 HV. The connection interference is adjusted by the shaft diameter D oS . The slightly oversized shaft shoulder (Figure 1c) defines the supporting length of the connection by the parameter L j . Table 1 T