PSI - Issue 17

D.G. Papageorgiou et al. / Procedia Structural Integrity 17 (2019) 532–538 D.G. Papageorgiou, H. Bravos, C. Medrea/ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 4. (a) The second piece as received, (b,c) The cleaned fracture surface on the left side and on the right side.

The third part breaks to the semicircle area after limited plastic deformation (Fig. 5a). The fracture initiates to the circumference of the round cross section (point 1 on Fig. 5b,c), propagating rapidly to a relatively small detachment area. The fracture is characteristic of low cycle fatigue (point 2 on Fig. 5b). The relative positions of crack initiation and the detachment point are in acute angle, which is characteristic of the presence of torsional stresses (Fig 5b.) In the fracture initiating point, the material has a bluish color, characteristic of temperature increase (point 2 on Fig. 5c). It is probably due to excessive friction forces exerted to the fractured area while working, or heat that was dispersed during crack propagation (Halford and Gallagher 2000).

Fig. 5. (a) The third piece as received, (b) The cleaned fracture surface, (c) The fracture surface as received.

The material (42CrMo4) used for the manufacturing of the first two holders was engraved on their surface (Fig. 6a). As shown on figure 7b, the chemical analysis performed verified the mark. It is a Chromium-Molybdenum low alloy steel (4140 ASTM, DIN EN 42CrMo4). It is delivered in the hardening and tempering condition and is suitable for applications where high tensile strength as well high toughness is required, mainly in heavy cross sections (Wegst 2007). The mechanical properties referred above are mandatory for the application of U-bolt holders. According to the optical emission spectroscopy the third holder was manufactured from ASTM 304L (Fig. 6c). It is a low carbon, Cr/Ni (18/8) austenitic stainless steel. Main properties of the specific steel grade are the excellent welding ability and toughness as well as the high corrosion resistance (Wegst 2007). It is used mainly after cold forming, due to increased strength and hardness after cold working. Its fatigue properties are very limited (absence of endurance limit) so it is inappropriate for the particular application. Use of different materials for the manufacturing of the U-bolts holders indicates that either the technical specifications from designer for the material selection did not followed or there was lack of technical specifications. The hardness of all three holders was measured by the Vickers method. The hardness range of the first two holders was 300-320 HV30 (30-32HRC). The specific steel is usually delivered after hardening and tempering to 28-32HRC (Böhler V320, 2018). Practically, the hardness range of the holders is verified also. The third holder was 250HV30 ( ≈ 23HRC) on the surface and 227HV30 ( ≈ 20HRC) to the core. The acquired hardness is well above the