Presentazioni e Articoli Tecnici

Accurate determination of critical material properties is crucial in the nuclear industry. Ring tensile tests (RTT) have been extensively employed by Studsvik over the years to evaluate the mechanical properties of fuel cladding materials, with data analysis traditionally performed using Excel. However, the integration of COMSOL Multiphysics® now allows for the import of measured data into a simulation app, enabling 3D finite element modeling with plasticity and optimization routines to extract vital material information.

In Studsvik’s implementation of this test method, the specimen is fabricated by cutting a ring specimen from a cladding tube and machining two parallel lengths for a gauge section symmetrically on each side of the ring. This gauge section is similar to the parallel length in a standard tensile specimen, allowing the results to be analyzed in the same way.

The specimen is subjected to an increasing applied load, and force and displacement are recorded. Unlike an axial tensile test, the data cannot be directly translated into engineering stress and strain curves. A transformation algorithm is required that considers the specific geometry of the ring, loading supports, and a supporting dog-bone component. After a correct transformation, key material properties such as yield strength, ultimate strength, and elongation can be obtained. The former transformation algorithm relied on Excel® using parameters determined more than 20 years ago for specific ring dimensions. This limitation required updating and extending the transformation method for testing new geometries. COMSOL Multiphysics®, in conjunction with the Nonlinear Structural Materials Module, offers a convenient solution through a dedicated COMSOL® app created with the Application Builder.

The COMSOL® app enables effortless construction of the geometry. The mesh is automatically generated, allowing users to choose from predefined mesh sizes and element discretization options. Material parameters, including the friction coefficient between the specimen and moving parts, can be entered. Due to high test temperatures, accurate input of temperature and thermal expansion coefficients is crucial. Users can solve the problem by employing linear elasticity to quickly obtain Young's Modulus. Moreover, the app offers several isotropic hardening models for selection, as plasticity plays a pivotal role. Measurements can be directly imported, and compensation for rig deformations can be applied to the data. The app provides optimization routines utilizing the Optimization Module with the BOBYQA method, combined with an auxiliary sweep of the test structure's displacements, to automatically fit the simulated hardening parameters to the measured data.

To enhance accessibility, the application is transformed into a standalone compiled app using COMSOL Compiler™. By utilizing Java® programming code, many of the model setup tasks are simplified, making this an exemplary demonstration of how a challenging simulation can be made available to a broader audience.

The simulated results should be compared with the measured force and displacement diagram, and when a satisfactory agreement is reached, the stress versus strain curve is obtained from the simulation. The transformation is thus adapted to every new RTT measurement, lifting the current limitations and allowing testing of new dimensions, promising reduced uncertainties in the results.