PSI - Issue 59

Bernadett Spisák et al. / Procedia Structural Integrity 59 (2024) 3–10 B. Spisa ´ k et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2. The relevance of the hybrid specimen The options to carry out SCC measurements in Hungary are limited; one of the places it can be performed is in the autoclave equipment provided by VEIKI Energia+ Energetikai Fejleszt ő , Kivitelez ő Kft. For the determination of the stress intensity factor-crack propagation diagram it is advisable to use a pre-cracked specimen, but due to the equipment it was not feasible to measure on conventional compact tensile (CT) specimens, therefore the double cantilever bending (DCB) specimen was chosen as a first trial.

Fig. 2. Dimensions of the hybrid specimen.

Finite element simulations and analytical calculations can be used to determine the required pre-loading force for DCB specimens made of a given material. For the long (L=90 mm) DCB specimens, the magnitude of the force (14000 N) required to achieve the desired maximum stress intensity factor (K=35 MPa m 1/2 ) exceeded the maximum force that could be generated by the equipment (6000 N) which was used for the pre-loading. This required a reduction in specimen length, and the simulations were performed on DCB specimens of 80, 75, 70, 65 and 60 mm length. In the case of the 60 and 65 mm specimens, pre-loading not only produced ductile deformation in the vicinity of the crack tip, but also in the ligament part of the specimen, which is the distance between the crack tip and the back of the specimen. An additional problem was the placement of the bolts for the pre-loading. Since the internal diameter of the gauge was given, the way of pre-loading the specimen was modified. As a result, the modified dimensions illustrated in Figure 2 were determined, taking the dimensional limits imposed on the CT and DCB specimens into account in order to maximise the applicability of the subsequent specifications. The specimen contains a hole for the CT gripping pin to allow the pre-cracking to be achieved, and a hole has been provided for the preparation of the preload, which can be achieved with a countersink screw. The modified design was generated in sizes 1T, 0.5T, 0.25T and 0.16T, which are shown in Table 1. The magnitude of the preload can be given by the stress intensity factor. In the case of SCC testing, it is usually given between 15 and 40 MPa m 1/2 . There are two ways to determine the relationship between the magnitude of the load and the stress intensity factor. The first one is to make finite element simulations, which is a time consuming procedure as for every different pre-crack and geometry size the analysis has to be redone. The second one is the usage of an analytical formula where the stress intensity factor is given as the function of load and crack size. Several publications exist which summarise how the stress intensity factor can be determined based on the type of the specimen or the geometry, for example, the handbooks written by Murakami (1990) and Tada (2000) which include information regarding the calculation methods of SIF for different type of geometry. The BS 7910:2019 (2019) standard summarises the process of performing an assessment of an existing flaw and supporting the continued operation, repair, replacement decisions, and, or determining a limiting flaw size for a component subject to a given operating environment and to determine the time to reach the limiting state. However, none of the