FCTM Esope

S2.11 - A damage mechanical approach to avoid cleavage fracture in bainitic HSLA pressure vessel steels

7 oct. 2021 | 12:00 - 12:30

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Description

Perhibitting cleavage fracture is a crucial safety factor in the design of pressure vessels. Damage mechanical simulations are utilized for the design of those components yielding to a better understanding of the mechanical properties. In the European standard DIN-EN 13445 the avoidance of cleavage fracture is utilized trough two different methodologies. For steels with a yield strength below 550MPa Charpy impact tests are sufficient to proof the toughness of the material. Above that, a costly fracture mechanical analysis using the Failure Assessment Diagram (FAD) has to be performed. To prevent those costs, a damage mechanical methodology shall be developed in this research. Cleavage fracture in damage mechanical models is yet mostly simulated by a combination with a Beremin model as a post-processor. Unfortunately, the Beremin model is not able to take stress triaxialities and the Lode angle into account, two parameters which are critical to adequately depict cleavage fracture. Thus, the generalized Orowan cleavage fracture criterion is used as it´s defines fracture in dependence of critical stress- and strain states for every element of the FEA calculation. The generalized Orowan criterion uses a combined damage criterion, taking into account both the technical cleavage fracture stress as well as the critical equivalent plastic strain. The equivalent plastic strain, at which crack initiation occurs in the material, will be evaluated and shall be coupled to the numerical model. This enhances the predictability of cleavage fracture. The Charpy impact test is much cheaper, compared to a full fracture mechanical analysis. Hence, a method using damage mechanical simulations and Charpy impact tests shall be established for steels with a strength greater than 550MPa. We assume that those steels offer way higher ductile resources and a higher toughness than expected, even at very low temperatures. Thus, a damage mechanical method, supporting the Charpy impact test for that material shall be established. High strain rates as well as the high triaxialities, yield to a both effective and conservative approach in Charpy impact tests. However, it also makes it very difficult to simulate. Challenges in the simulation of Charpy impact tests are the following: First of all, the materials yielding is changing a lot with temperature and strain rate, two parameters which need to be in cooperated in the model. Second, triaxiality and Lode angle need to be considered on both, ductile and brittle failure. This leads to the problem of transition zone behavior modeling, as the model needs to be able to distinguish between both failure mechanisms itself. The planed procedure however, only considers temperatures within the possible operating temperatures of the pressure vessel. All smaller temperatures in the transition curve are neglected. Consequently, the total amount of tests to calibrate the damage mechanical model is reduced to a minimum, saving both money and time. The evaluated simulations will be compared with results of Charpy impact test and shall be brought to nominal material parameters, so that they can be inserted into the according norms and standards. Furthermore, critical strains in dependency of triaxiality and Lode angle are determined at which cleavage fracture can be securely avoided. If that method proofs satisfactory toughness, results can be transferred to simulations of a full pressure vessel, to enhance its acceptance.

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