PSI - Issue 18

Mehdi Mokhtarishirazabad et al. / Procedia Structural Integrity 18 (2019) 457–471 M. Mokhtarishirazabad / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Fracture behaviour of stainless steel in plane strain condition is well established. This includes substantial studies on the effect of in-plane constraint (Picker 1983; Mills 1997; Ruggieri 2017; Hioe et al. 2017). Conversely, there only have been few studies of the fracture behaviour of stainless steel alloys with low out-of-plane constraint. This is because the majority of safety critical components have thick sections. However, there are components, such as thin tubes in heat exchangers, which may be subject to both a loss of in-plane and out-of-plane constraint. The thin components, which are believed to be more susceptible to plastic collapse when as-manufactured, can become prone to fracture after thermal ageing. Therefore, alternative approaches to assessing the integrity of thin components are required. The standard fracture toughness values according to standard methods such as ASTM E1820 are measured with the condition of high in-plane and out-of-plane constraints. However, this condition does not always apply to service components. There are studies which used the standard methods but did not follow the prescription on minimum crack length, for example, to evaluate the effects of loss of in- and out-of-plane constraint (Zhu 2016). We adopted the same approach to study the effects of loss of out-plane constraint. 2. Methodology The tensile properties of the material were evaluated by performing tensile tests on three flat dog-bone samples extracted from the middle of the plate with thickness of 5 mm, following the ASTM standard (ASTM-E8/E8M 2016). Fig. 1 shows the geometry of the tensile test sample. The loading for tensile tests was applied by an Instron 5969 Universal testing machine with a load cell capacity of 50 kN, under displacement control with a loading rate of 0.5 mm/min. All tensile experiments have been conducted with the same equipment and testing parameters in a single day.

Fig. 1. Geometry and dimensions of the tensile test sample.

Fracture experiments were carried out on plane sided SENB samples made of austenitic stainless steel 316L. The samples were machined in a way that the direction normal to the crack plane was parallel to the rolling direction while crack propagation direction was perpendicular to the rolling direction (LT direction). Side grooving was avoided to be able to have a full field displacement measurement near the crack tip area by employing 3D digital image correlation (DIC) method. Full-field measurements by DIC are under study and will be published separately. In this paper only some examples of the images taken from the crack region during loading are presented. The width of these samples was 50 mm. To study the effect of in plane and out of plane constraint, samples with four different initial crack lengths of a/W = 0.2, 0.3, 0.4 and 0.5 and three different thickness of 5, 10, 20 mm were machined, respectively. In addition, one CT samples were machined with thickness of 20 mm and a/W = 0.5. Therefore, 14 SENB samples and 1 CT samples were made in total (Table 1). Fig. 1 shows the geometry and dimensions of the specimens. Wire EDM was used to introduce initial cracks as previous research showed that considerable crack blunting prior to growth eliminates the effect of fatigue pre-crack (Mostafavi, Smith, and Pavier 2010). The dimensions of the specimens are provided in Table 1. A 3-point bending stage with rolling supports was used to carry out SENB tests. For the thinnest samples (thickness of 5 mm), anti-buckling plates were used (see Fig. 3). A sample naming convention SENB_BX_Y was adopted in which X is the sample thickness in mm and Y is a/W .