PSI - Issue 42

Fatih Kocatürk et al. / Procedia Structural Integrity 42 (2022) 1206–1214 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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bolts that the fracture in the region of the thread was expected. The structural integrity of bolts was affected mostly by the internal socket depth of bolt head, as the fracture region of bolts can move from threaded region to under-head region depending on the depth of socket. However, some specially designed bolts are required to fracture in the under-head region, and these types of bolts must meet the minimum UTS under tension given in the ISO 898-1 (2004) standard. The Fracture Cone (FC) formed in the head region was investigated in Thomala and Kloos (2007) in terms of the failure mechanisms of bolts having greater SHaft Diameter (SHD) than the SOcket Diameter (SOD). To estimate the residual floor thickness, , representing the height between the end of the socket and under-head, an analytical formulation was introduced for the bolts with greater SHD than the SOD. The equation given in Eq. (1) was derived by Thomala and Kloos (2007) to estimate the MSD for bolts that have a SHD greater than the SOD, and is referenced in VDI 2230 standard: = √ 16 2 − 2 ( 2 ℎ − 2 ) 2 2. ∗ . .( ℎ + ) (1)

Fig. 1. Cross-section view of the bolt with a SHD larger than the SOD (Kocatürk et al., 2020). where is the nominal cross-section area of the shaft, ℎ is the SHD, is the average SOD, ∗ = / is the strength ratio where is the torsional strength and is the tensile strength (see Fig. 1). Bolts with a SHD less than the SOD are also widely used in industry over the type of bolt investigated in Eq. (1). In the authors' previous study (Kocatürk et al., 2020), an analytical formulation was introduced to estimate the MSD for bolts with a SHD less than the SOD. The developed analytical formulation was used to calculate the MSD for a representative bolt that satisfies the minimum UTS, the analytical results were validated with numerical simulations. To carry out the numerical simulations, Simufact.forming finite element analysis software was used. The estimated maximum depth of the socket with the developed analytical formulation was compatible with the numerical results. Within the scope of this study, a bolt with internal socket form was chosen and specimens with various values of socket depth were produced by the cold forging method. Then, the analytical model developed in Kocatürk et al. (2020) was validated by tensile tests performed using the produced bolt specimens. 2. Sample specification In this study, a representative sample of M8 bolts with a SHD less than the SOD was chosen. The selected bolts were produced by cold forging with various socket depth values. After forming, heat-treatment was applied to the bolts to achieve the mechanical properties for 8.8 grade defined in ISO 898-1 (2004). The specimens were produced by using 23MnB4 material which is widely used low alloyed steel in cold forging. One of the bolts and its cross sectional view was given in Fig. 2.