PSI - Issue 5

Lars Sieber et al. / Procedia Structural Integrity 5 (2017) 1019–1026 Sieber, Stroetmann / Structural Integrity Procedia 00 (2017) 000 – 000

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The vulnerability to brittle fracture increases enormously due to strain ageing in particular in the region of plastic deformation. Such strain hardening can appear as cold deformation e.g. in the peripheral areas of punched holes. At the beginning of the 20th century punched holes at cyclic stressed steel structures were only allowed in secondary structural components. In contrast, in the 19th century the rivet holes for joints in structural steelwork were almost exclusively punched. The punching of rivet holes was permitted e.g. for the construction of steel grid pylons with up to 8 mm material thickness. The cold deformations due to punching combined with steels that have a tendency to ageing lead to a material embrittlement at the edge of the holes. The notch effect of holes causes stress concentrations which have to be reduced by local plastification to obtain the calculative load carrying capacity of the structural components. If this local yielding is not possible adequately due to embrittlement of the steel tensile stress leads to a deformation-free failure in the connection (see e.g. Klinger et al. (2011)). To confirm and to examine the extent of this material influence extensive metallographic investigations were carried out Stroetmann et al. (2015) to analyze the structural differences between the deformed edges and the undeformed base material. Furthermore, strain hardening was analysed by hardness measurements and the width of influenced zones was determined. In the micro-sections (see Figure 5) the typical area of penetration and the plastic deformations in the direction of movement of the punching tool is visible. It is identifiable at the direction of the stretched ferrite grain. Similar textur conditions were documented at all punched edges. The micrographs illustrated that the visually deformed zones of the grain texture have a maximum witdh of 1.5 mm.

Fig. 5 . Micrographs at the punched edge of a hole after micro etching in alcoholic HNO 3 -solution

Already in the examinations by Huhn (2004) the strain hardening at the punched holes was quantified with the help of Vickers hardness measurement. However, the object was always current structural steels without any significant tendency of strain ageing. To examine the transferability of the results onto old steel, Vickers hardness measurements (HV0.2 according to EN ISO 6507) were carried out on components with punched holes made of old mild steel. In the hardened peripheral zones the hardness increases up to 2.5 times in relation to the base material. This hardness value of up to 350 HV0.2 indicates a initiating susceptibility to cold cracking. The influence of the punching cover an periphical zone of approximately 3 mm depth. Accordingly, the hardened edge zone is wider than the area in which a deformed grain texture can be visibly proven on a micro-section.