Ar:N2_2 - a non-universal glass

The bias energies of various two-level systems (TLSs) and their strengths of interactions with the strain are calculated for Ar:N$_2$ glass. Unlike the case in KBr:CN, a distinct class of TLSs having weak interaction with the strain and untypically small bias energies is not found. The addition of CO molecules introduces CO flips which form such a class of weakly interacting TLSs, albeit at much lower coupling than are typically observed in solids. We conclude that because of the absence of a distinct class of weakly interacting TLSs, Ar:N$_2$ is a non-universal glass, the first such system in three dimensions and in ambient pressure. Our results further suggest that Ar:N$_2$:CO may show universal properties, but at temperatures lower than $\approx 0.1$ K, much smaller than typical temperature $\approx 3$ K associated with universality, because of the untypical softness of this system. Our results thus shed light on two long standing questions regarding low temperature properties of glasses: the necessary and sufficient conditions for quantitative universality of phonon attenuation, and what dictates the energy scale of $\approx 3$ K below which universality it typically observed.Comment: 6 pages, 2 figures, results for excitations densities of states are added, presentation improve

Effects of embedded cracks and residual stresses on the integrity of a reactor pressure vessel

[EN] One potential challenge to the integrity of the reactor pressure vessel (RPV) of a pressurized water reactor is posed by a pressurized thermal shock (PTS), which is associated with rapid cooling of sections of the hot and still pressurized RPV by injection of relatively cold emergency coolant. PTS transients lead to high tensile circumferential and axial stresses in the RPV wall. If the stress intensity factor (SIF) is large enough, a critical crack may grow. Thus, the RPV has to be assessed against cleavage fracture. In this paper, two kinds of embedded cracks, i.e. semi-elliptical and elliptical crack with depth of 17 mm and length of 102 mm are considered. The extended finite element method (XFEM) is used to model such postulated cracks. The embedded crack with tip in the cladding/base interface causes a high K-I. This is due to the stress discontinuities at the interface between the materials. In the FAVOR (probabilistic fracture mechanics code) calculation, for such cracks the point closest to the inner surface is considered in order to be conservative. However, due to the highly ductile cladding material, it is unlikely that the embedded crack will propagate through the cladding. Thus, it is more appropriate to consider the outer surface point of the crack front. The effect of welding residual stress and cladding/base interface residual stress on the crack driving force is studied. Surface cracks are assumed in the study of residual stresses. Results show that considering realistic welding residual stresses may increase K-I by about 5 MPa.m(0.5), while the cladding/base interface residual stress has a negligible effect on K-I. The reason is that the cladding residual stress is only localized to the interface and it decreases significantly through the vessel wall. Considering welding residual stress increases the Weibull stress and fracture probability of the RPV.The authors are grateful for the financial support of the PROBAB Project provided by the Swiss Federal Nuclear Safety Inspectorate (ENSI) (DIS-Vertrag Nr. H-100668). V.F. Gonzalez-Albuixech is thankful for the research program Juan de la Cierva Incorporacion 2015, IJCI-2015-23245, financed by the Spanish Ministerio de Economia, Industria y Competitividad.Qian, G.; González Albuixech, VF.; Niffenegger, M. (2018). Effects of embedded cracks and residual stresses on the integrity of a reactor pressure vessel. Engineering Failure Analysis. 90:451-462. https://doi.org/10.1016/j.engfailanal.2018.04.009S4514629

Computing stress intensity factors for curvilinear cracks

The use of the interaction integral to compute stress intensity factors around a crack tip requires selecting an auxiliary field and a material variation field. We formulate a family of these fields accounting for the curvilinear nature of cracks that, in conjunction with a discrete formulation of the interaction integral, yield optimally convergent stress intensity factors. We formulate three pairs of auxiliary and material variation fields chosen to yield a simple expression of the interaction integral for different classes of problems. The formulation accounts for crack face tractions and body forces. Distinct features of the fields are their ease of construction and implementation. The resulting stress intensity factors are observed converging at a rate that doubles the one of the stress field. We provide a sketch of the theoretical justification for the observed convergence rates, and discuss issues such as quadratures and domain approximations needed to attain such convergent behavior. Through two representative examples, a circular arc crack and a loaded power function crack, we illustrate the convergence rates of the computed stress intensity factors. The numerical results also show the independence of the method on the size of the domain of integration

Estudio de las singularidades de frente de grieta y de esquina en grietas tridimensionales mediante el método de los elementos finitos extendido

En esta tesis se aborda primeramente el estudio de las grietas tridimensionales partiendo de las premisas de la MFEL y considerando la importancia de los términos de segundo orden del desarrollo de Williams para la correcta descripción del campo de tensiones en problemas tridimensionales. El estudio de las grietas tridimensionales se realiza mediante una extensión de los resultados bidimensionales en los que se suponen conceptos que en un estado tridimensional no pueden ser admitidos directamente. En la tesis se hace un análisis del efecto de la triaxialidad sobre estos resultados, poniéndose de manifiesto que no basta con aceptar los términos singulares, sino que al menos se deben incluir los términos constantes del desarrollo de Williams. El segundo punto lo constituye una introducción al XFEM para grietas tridimensionales, incluyendo algunos de los avances desarrollados en los últimos años. A continuación se aborda el cálculo de los FIT en grietas genéricas tridimensionales con curvatura. Para ello se proponen mejoras en la formulación de las integrales de dominio y se realizan verificaciones numéricas y estudios de convergencia. El cálculo de los FIT en grietas que presentan curvatura es el objetivo principal de la tesis. La formulación de la integral de interacción ha sido modificada para mejorar su eficacia. Además se ha introducido el efecto de la curvatura en los gradientes, para lo cual se han usado conceptos de geometría diferencial aprovechando la formulación mediante la LS. Esta propuesta mejora apreciablemente los resultados obtenidos mediante las integrales de dominio. El último punto es el estudio y propuesta de un enriquecimiento para mejorar la descripción del estado existente en las cercanías de la singularidad de esquina ---o de borde libre---. Además de una revisión bibliográfica del problema de esta singularidad, se ha introducido parte del efecto local de esta singularidad en la formulación de XFEM mediante una propuesta de enriqGonzález Albuixech, VF. (2012). Estudio de las singularidades de frente de grieta y de esquina en grietas tridimensionales mediante el método de los elementos finitos extendido [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/19116Palanci

Comparison of KI calculation methods

This paper compares different numerical methods for mode I stress intensity factor (SIF) calculations. Both 2D and 3D models are used to calculate KI for the compact tension specimens. J-integral and interaction integral provide relatively accurate results. The analysis of a reactor pressure vessel subjected to pressurized thermal shock is performed using the finite element method (FEM) and extended finite element method (XFEM). XFEM method shows advantages in modeling cracks but oscillations in 3D problems due to extraction domains for J and interaction integrals. The best results are obtained with domain integrals using a FEM with a refined mesh.The authors are grateful for the financial support of the PISA Project provided by the Swiss Federal Nuclear Safety Inspectorate (ENSI) (DIS-Vertrag Nr. H-100668). Guian Qian is also grateful for the visiting invitation provided by Key Laboratory of Pressurized Systems and Safety (East China University of Science and Technology), Ministry of Education, China.Qian, G.; González Albuixech, VF.; Niffenegger, M.; Giner Maravilla, E. (2016). Comparison of KI calculation methods. Engineering Fracture Mechanics. 156:52-67. https://doi.org/10.1016/j.engfracmech.2016.02.014S526715

Numerical analysis for design of bioinspired ceramic modular armors for ballistic protections

[EN] The exigent requirements for personal protections in terms of energy absorption and ergonomics have led to increasing interest in bioinspired protections. This work focuses on the numerical analysis of ballistic behavior of different bioinspired geometries under impact loadings. Ceramic armors based on ganoid fish scales (the type exhibited by gars, bichirs and reedfishes), placoid fish scales (characterizing sharks and rays) and armadillo natural protection have been considered. Different impact conditions are studied, including perpendicular and oblique impacts to surface protection, different yaw angle, and multiple impacts. Main conclusion is related to the improved efficiency of modular armors against multiple shots exhibiting more localized damage and crack arrest properties. Moreover, its potential ergonomic is a promising characteristic justifying a deeper study.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work has been carried out within the framework of the research program Juan de la Cierva Incorporacion 2015, and research projects DPI2017-88166-R, and RTC-2015-3887-8 of FEDER program financed by the Ministerio de Economia, Industria y Competitividad of Spain. The support of the Generalitat Valenciana, Programme PROMETEO 2016/007 is also acknowledged.González Albuixech, VF.; Rodríguez-Millán, M.; Ito, T.; Loya, JA.; Miguélez, MH. (2019). Numerical analysis for design of bioinspired ceramic modular armors for ballistic protections. International Journal of Damage Mechanics. 28(6):815-837. https://doi.org/10.1177/1056789518795203S815837286Chen, I. H., Kiang, J. H., Correa, V., Lopez, M. I., Chen, P.-Y., McKittrick, J., & Meyers, M. A. (2011). Armadillo armor: Mechanical testing and micro-structural evaluation. Journal of the Mechanical Behavior of Biomedical Materials, 4(5), 713-722. doi:10.1016/j.jmbbm.2010.12.013Chintapalli, R. K., Mirkhalaf, M., Dastjerdi, A. K., & Barthelat, F. (2014). Fabrication, testing and modeling of a new flexible armor inspired from natural fish scales and osteoderms. Bioinspiration & Biomimetics, 9(3), 036005. doi:10.1088/1748-3182/9/3/036005Deka, L. J., Bartus, S. D., & Vaidya, U. K. (2009). Multi-site impact response of S2-glass/epoxy composite laminates. Composites Science and Technology, 69(6), 725-735. doi:10.1016/j.compscitech.2008.03.002Duro-Royo, J., Zolotovsky, K., Mogas-Soldevila, L., Varshney, S., Oxman, N., Boyce, M. C., & Ortiz, C. (2015). MetaMesh: A hierarchical computational model for design and fabrication of biomimetic armored surfaces. Computer-Aided Design, 60, 14-27. doi:10.1016/j.cad.2014.05.005Flores-Johnson, E. A., Shen, L., Guiamatsia, I., & Nguyen, G. D. (2014). Numerical investigation of the impact behaviour of bioinspired nacre-like aluminium composite plates. Composites Science and Technology, 96, 13-22. doi:10.1016/j.compscitech.2014.03.001Grujicic, M., Pandurangan, B., & Coutris, N. (2011). A Computational Investigation of the Multi-Hit Ballistic-Protection Performance of Laminated Transparent-armor Systems. Journal of Materials Engineering and Performance, 21(6), 837-848. doi:10.1007/s11665-011-0004-3Grunenfelder, L. K., Suksangpanya, N., Salinas, C., Milliron, G., Yaraghi, N., Herrera, S., … Kisailus, D. (2014). Bio-inspired impact-resistant composites. Acta Biomaterialia, 10(9), 3997-4008. doi:10.1016/j.actbio.2014.03.022Klasztorny, M., & Świerczewski, M. (2015). NUMERICAL MODELLING AND VALIDATION OF 12.7 MM FSP IMPACT INTO ALFC SHIELD – ARMOX 500T STEEL PLATE SYSTEM. Journal of KONES. Powertrain and Transport, 19(4), 291-299. doi:10.5604/12314005.1138463Liu, P., Zhu, D., Yao, Y., Wang, J., & Bui, T. Q. (2016). Numerical simulation of ballistic impact behavior of bio-inspired scale-like protection system. Materials & Design, 99, 201-210. doi:10.1016/j.matdes.2016.03.040Morka, A., & Nowak, J. (2015). NUMERICAL ANALYSES OF CERAMIC/METAL BALLISTIC PANELS SUBJECTED TO PROJECTILE IMPACT. Journal of KONES. Powertrain and Transport, 19(4), 465-472. doi:10.5604/12314005.1138618Pandya, K., Kumar, C. V. S., Nair, N., Patil, P., & Naik, N. (2014). Analytical and experimental studies on ballistic impact behavior of 2D woven fabric composites. International Journal of Damage Mechanics, 24(4), 471-511. doi:10.1177/1056789514531440Poniżnik, Z., Nowak, Z., & Basista, M. (2015). Numerical modeling of deformation and fracture of reinforcing fibers in ceramic–metal composites. International Journal of Damage Mechanics, 26(5), 711-734. doi:10.1177/1056789515611945Porter, M. M., Ravikumar, N., Barthelat, F., & Martini, R. (2017). 3D-printing and mechanics of bio-inspired articulated and multi-material structures. Journal of the Mechanical Behavior of Biomedical Materials, 73, 114-126. doi:10.1016/j.jmbbm.2016.12.016Reaugh, J. E., Holt, A. C., Welkins, M. L., Cunningham, B. J., Hord, B. L., & Kusubov, A. S. (1999). Impact studies of five ceramic materials and pyrex. International Journal of Impact Engineering, 23(1), 771-782. doi:10.1016/s0734-743x(99)00121-9Rostamiyan, Y., & Ferasat, A. (2016). High-speed impact and mechanical strength of ZrO2/polycarbonate nanocomposite. International Journal of Damage Mechanics, 26(7), 989-1002. doi:10.1177/1056789516644312Russell, B. P. (2014). Multi-hit ballistic damage characterisation of 304 stainless steel plates with finite elements. Materials & Design, 58, 252-264. doi:10.1016/j.matdes.2014.01.074Serjouei, A., Chi, R., Sridhar, I., & Tan, G. E. B. (2015). Empirical Ballistic Limit Velocity Model for Bi-Layer Ceramic–Metal Armor. International Journal of Protective Structures, 6(3), 509-527. doi:10.1260/2041-4196.6.3.509Shaktivesh, Nair, N., & Naik, N. (2014). Ballistic impact behavior of 2D plain weave fabric targets with multiple layers: Analytical formulation. International Journal of Damage Mechanics, 24(1), 116-150. doi:10.1177/1056789514524074Yang, W., Chen, I. H., Gludovatz, B., Zimmermann, E. A., Ritchie, R. O., & Meyers, M. A. (2012). Natural Flexible Dermal Armor. Advanced Materials, 25(1), 31-48. doi:10.1002/adma.20120271

Geometric description scheme for crack propagation by a geometric description based on Level Sets and Fast Marching Method

The Level Set Method is used to describe mathematically the propagation of surfaces, by the resolution and approximation of a set of differential equations. This technique has been widely used on the XFEM framework, specially for crack propagation. However, there are other techniques based on geometrical considerations, as the Fast Marching Method. In this study, it is proposed a geometrical related technique based on the Fast Marching Method and Level Set Method for crack growth description in XFEM. Therefore, the results could be used directly on fracture mechanics analysis using XFEM.El level set method es una técnica que se utiliza habitualmente para describir matemáticamente la propagación de superficies mediante la aproximación y resolución de ecuaciones diferenciales. Esta técnica ha sido ampliamente utilizada en combinación con el XFEM, especialmente en el análisis del crecimento de grietas. Sin embargo, existen otras opciones que se basan en consideraciones geometricas como el fast marching method. En este trabajo se implementa una técnica geométrica basada en el fast marching method y level sets para la descripción de propagación de grietas en XFEM y, por tanto, compatible para su uso directo con esta técnica en cálculos de mecánica de la fractura

Geometric description scheme for crack propagation by a geometric description based on Level Sets and Fast Marching Method

The Level Set Method is used to describe mathematically the propagation of surfaces, by the resolution and approximation of a set of differential equations. This technique has been widely used on the XFEM framework, specially for crack propagation. However, there are other techniques based on geometrical considerations, as the Fast Marching Method. In this study, it is proposed a geometrical related technique based on the Fast Marching Method and Level Set Method for crack growth description in XFEM. Therefore, the results could be used directly on fracture mechanics analysis using XFEM.El level set method es una técnica que se utiliza habitualmente para describir matemáticamente la propagación de superficies mediante la aproximación y resolución de ecuaciones diferenciales. Esta técnica ha sido ampliamente utilizada en combinación con el XFEM, especialmente en el análisis del crecimento de grietas. Sin embargo, existen otras opciones que se basan en consideraciones geometricas como el fast marching method. En este trabajo se implementa una técnica geométrica basada en el fast marching method y level sets para la descripción de propagación de grietas en XFEM y, por tanto, compatible para su uso directo con esta técnica en cálculos de mecánica de la fractura

Domain integral formulation for 3-D curved and non-planar cracks with the extended finite element method

The computation of stress intensity factors (SIFS) in curved and non-planar cracks using domain integrals introduces some difficulties related to the use of curvilinear gradients. Several approaches exist in the literature that consider curvilinear corrections within a finite element framework, but these depend on each particular crack configuration and they are not general. In this work, we introduce the curvilinear gradient correction within the extended finite element method framework (XFEM), based only on the level set information used for the crack description and the local coordinate system definition. Our formulation depends only on the level set coordinates and, therefore, an explicit analytical description of the crack is not needed. It is shown that this curvilinear correction improves the results and enables the study of generic cracks. In addition, we have introduced a simple error indicator for improving the SIF computed via the interaction integral, thanks to the better behavior of the J-integral as it does not need auxiliary extraction fields.This work has been carried out within the framework of the research projects DPI2007-66995-C03-02 and DPI2010-20990 financed by the Ministerio de Economia y Competitividad. The support of the Generalitat Valenciana, Programme PROMETEO 2012/023 is also acknowledged.González Albuixech, VF.; Giner Maravilla, E.; Tarancón Caro, JE.; Fuenmayor Fernández, FJ.; Gravouil, A. (2013). Domain integral formulation for 3-D curved and non-planar cracks with the extended finite element method. Computer Methods in Applied Mechanics and Engineering. 264:129-144. https://doi.org/10.1016/j.cma.2013.05.016S12914426