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This paper presents an original procedure to determine the softening curve in concrete from a diametric Brazilian test and a three point bend test. An inverse procedure is proposed combining experimental results, numerical finite element computation and an iterative algorithm, called algorithm AMS-UPM, developed expressly for this research. The starting point of the algorithm is a bilinear softening curve, on which a successive transformations are applied decreasing the difference between the experimental and numerical results in each step. The procedure has been applied successfully to two conventional concretes. The final result is a softening curve that adjusts almost perfectly experimental data of the three point bending test. Resumen: En este trabajo se presenta un procedimiento original para obtener la curva de ablandamiento en hormigón a partir de un ensayo de compresión diametral y un ensayo de flexión en tres puntos. Se trata de un método inverso que combina resultados experimentales, cálculos numéricos por elementos finitos y un algoritmo iterativo desarrollado expresamente para la presente investigación. El punto de partida del algoritmo es la curva de ablandamiento bilineal, sobre la que se aplican una serie de transformaciones sucesivas reduciendo en cada paso la diferencia entre los resultados numéricos y experimentales. El procedimiento ha sido aplicado con éxito a dos hormigones convencionales, obteniendo en ambos casos una curva de ablandamiento que ajusta de forma prácticamente perfecta los registros experimentales del ensayo de flexión en tres puntos. Keywords: Concrete fracture, Cohesive model, Palabras clave: Fractura de hormigón, Modelo cohesivo

During some operations previous to the dry storage, which involve high temperature and hoop stress on the cladding, a part of the hydride platelets may precipitate along the radial direction. As a consequence, cladding embrittlement may occur. In this paper, the experimental techniques developed for the authors to produce hydride reorientation in unirradiated cladding are shown. Samples of fuel cladding with 150 and 500 ppm of hydrogen were prepared. Subsequently, these samples were subjected to reorientation treatments by applying high temperature (400 °C) and constant hoop stress (60, 90, 120 and 140 MPa). Finally, ring compression tests (RCT) were carried out to study the cladding embrittlement. Samples after the reorientation treatment (i.e. with radial hydrides) were subjected to RCT at 20, 135 and 300 °C and the results are discussed.

An original procedure to determine the softening curve in concrete has been proposed by the authors. This inverse method combines experimental results, finite element simulations and an iterative algorithm to adjust the experimental data. The end product of the process is a softening curve that allows us to very accurately reproduce the experimental curves. The proposed method calculates the fracture energy from the cohesive softening curve model, which in turn is iteratively determined by adjusting experimental load-displacement data of three-point bending tests. The procedure has been successfully applied to two conventional concretes.

In this work, a new methodology is devised to obtain the fracture properties of nuclear fuel cladding in the hoop direction. The proposed method combines ring compression tests and a finite element method that includes a damage model based on cohesive crack theory, applied to unirradiated hydrogen-charged ZIRLOTM nuclear fuel cladding. Samples with hydrogen concentrations from 0 to 2000 ppm were tested at 20 �C. Agreement between the finite element simulations and the experimental results is excellent in all cases. The parameters of the cohesive crack model are obtained from the simulations, with the fracture energy and fracture toughness being calculated in turn. The evolution of fracture toughness in the hoop direction with the hydrogen concentration (up to 2000 ppm) is reported for the first time for ZIRLOTM cladding. Additionally, the fracture micromechanisms are examined as a function of the hydrogen concentration. In the as-received samples, the micromechanism is the nucleation, growth and coalescence of voids, whereas in the samples with 2000 ppm, a combination of cuasicleavage and plastic deformation, along with secondary microcracking is observed.

The method reported in the literature to calculate the stress–strain curve of nuclear fuel cladding from ring tensile test is revisited in this paper and a new alternative is presented. In the former method, two universal curves are introduced under the assumption of small strain. In this paper it is shown that these curves are not universal, but material-dependent if geometric nonlinearity is taken into account. The new method is valid beyond small strains, takes geometric nonlinearity into consideration and does not need universal curves. The stress–strain curves in the hoop direction are determined by combining numerical calculations with experimental results in a convergent loop. To this end, ring tensile tests were performed in unirradiated hydrogen-charged samples. The agreement among the simulations and the experimental results is excellent for the range of concentrations tested (up to 2000 wppm hydrogen). The calculated stress–strain curves show that the mechanical properties do not depend strongly on the hydrogen concentration, and that no noticeable strain hardening occurs. However, ductility decreases with the hydrogen concentration, especially beyond 500 wppm hydrogen. The fractographic results indicate that as-received samples fail in a ductile fashion, whereas quasicleavage is observed in the hydrogen-charged samples.

The ring compression test applied to nuclear fuel cladding is relatively easy to perform but difficult to interpret. It can be representative of the loading state associated to a hypothetical spent fuel assembly drop accident. This is particularly important for spent fuel cladding subjected to drying operations previous to storage and transportation, because they may produce hydride reorientation along the radial direction of cladding. In this paper, experimental testing and numerical simulations are combined to obtain operative failure criteria from the results of the ring compression tests on unirradiated pre-hydrided samples with radial hydrides, simulating drying, storage and subsequent transport conditions.Copyright © 2015 by ASME

The structural integrity of nuclear fuel cladding is affected by the precipitation of hydrides during operation, which may embrittle the cladding. The aim of this work is to obtain the mechanical and fracture properties of the cladding as a function of the hydrogen content and testing temperature. To this end, the embrittlement caused by circumferential hydrides was simulated on unirradiated fuel cladding samples in the laboratory. The structural integrity of the cladding was assessed at different temperatures (20, 135 and 300ºC), by using the ring compression test. The mechanical properties and the fracture energy were calculated from the experimental load vs. displacement curves, by means of a finite element model which incorporates the cohesive crack model.