PSI - Issue 71

Karthik K R. et al. / Procedia Structural Integrity 71 (2025) 210–217

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Surface cracks in steel pipes subjected to fatigue bending were investigated Zongchen et al. (2020). They have proposed an analytical approach to predict SIF with the help of ANSYS FE solution and subsequently predicted fatigue life. Gustav et al. (2023) studied the behaviour of surface cracks in steel plates under bending. Representative fracture toughness values have been derived from FE analysis and compared with experimental results of SEN(B) specimens. The structural behaviour of a cylindrical shell including longitudinal and circumferential external defects subjected to internal pressure was investigated by Moustabchir et al. (2018). The experimental setup employed strain gauges to monitor the evolution of the strain field near the defects. A detailed parametric finite element model simulated the experiments with a good match. Shlyannikov et al. (2016) generated experimental data for aluminium alloy D16 and low carbon steel on surface crack growth. Study clearly illustrates the effect of loading conditions as well as material properties on the crack growth in the samples with the same geometries Current study focuses on surface crack on AA2219 material. Fracture assessment is carried out for the biggest surface crack that can be missed during inspection at the critical weld location of the tank. Critical weld location is the location that is under maximum principal stress. 2. Background Fracture assessment can be made using either LEFM (Linear elastic fracture mechanics) or EPFM (Elastic plastic fracture mechanics) methods. LEFM is limited to structures that are made of materials behave linearly under loading and failure happens by limited or small amounts of plastic deformation (Ashok (2019)). Stress intensity factor (SIF) is a fracture parameter derived from the LEFM approach. It represents the amplitude of the crack tip stress singularity and is dependent on the geometry, crack size, load level, and loading configuration. Applicability of SIF parameter is limited to a small amount of plasticity near the crack tip. Viswanath et al. (2019) carried out a structural integrity evaluation of a cryogenic propellant tank using failure assessment diagram approach. This study has been carried out using stress intensity factor as fracture parameter. But metallic materials exhibit gross plasticity before failure, thus limiting the use of SIF to small group of materials. This limitation of LEFM has led to the development of EPFM. Christopher et al. (2022) explored both LEFM and EPFM methodologies and suggested criteria for applying EPFM to damage tolerance assessment. Based on their proposed methodology, propellant tank has to be assessed using EPFM approach.

Figure 3: Representation of a 2D crack tip (Ashok (2019)).

J-integral (referred to as J) is an EPFM parameter which can uniquely characterize the crack tip stress and strain field in a ductile material (Anderson (2017)). The definition of J is formulated for nonlinear elastic materials that are like metals deforming plastically when the loading is only in one direction (Ashok (2019). But metals follow a straight line while unloading happens. As long as the loading remains monotonic non-linear elastic assumption is valid for metallic materials. Representation of a 2D crack tip is shown in Figure 3. J is evaluated as = ∮ − Where, W = Elastic strain energy density T = Traction vector defined according to the outward normal n to the contour