Aida Nonn Stellv. Wissenschaftliche Leitung TCND und Leitung Werkstoffsimulation

@article{Nonn2023,

title = {Inferring mechanical properties of the SARS-CoV-2 virus particle with nano-indentation tests and numerical simulations},

author = {A. Nonn and B. Kiss and W. Pezeshkian and T. Tancogne-Dejean and A. Cerrone and M. Kellermayer and Y. Bai and W. Li and T. Wierzbicki},

doi = {10.1016/j.jmbbm.2023.106153},

issn = {1751-6161},

year  = {2023},

date = {2023-12-00},

urldate = {2023-12-00},

journal = {Journal of the Mechanical Behavior of Biomedical Materials},

volume = {148},

publisher = {Elsevier BV},

abstract = {The pandemic caused by the SARS-CoV-2 virus has claimed more than 6.5 million lives worldwide. This global challenge has led to accelerated development of highly effective vaccines tied to their ability to elicit a sustained immune response. While numerous studies have focused primarily on the spike (S) protein, less is known about the interior of the virus. Here we propose a methodology that combines several experimental and simulation techniques to elucidate the internal structure and mechanical properties of the SARS-CoV-2 virus. The mechanical response of the virus was analyzed by nanoindentation tests using a novel flat indenter and evaluated in comparison to a conventional sharp tip indentation. The elastic properties of the viral membrane were estimated by analytical solutions, molecular dynamics (MD) simulations on a membrane patch and by a 3D Finite Element (FE)-beam model of the virion's spike protein and membrane molecular structure. The FE-based inverse engineering approach provided a reasonable reproduction of the mechanical response of the virus from the sharp tip indentation and was successfully verified against the flat tip indentation results. The elastic modulus of the viral membrane was estimated in the range of 7–20 MPa. MD simulations showed that the presence of proteins significantly reduces the fracture strength of the membrane patch. However, FE simulations revealed an overall high fracture strength of the virus, with a mechanical behavior similar to the highly ductile behavior of engineering metallic materials. The failure mechanics of the membrane during sharp tip indentation includes progressive damage combined with localized collapse of the membrane due to severe bending. Furthermore, the results support the hypothesis of a close association of the long membrane proteins (M) with membrane-bound hexagonally packed ribonucleoproteins (RNPs). Beyond improved understanding of coronavirus structure, the present findings offer a knowledge base for the development of novel prevention and treatment methods that are independent of the immune system.},

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tppubtype = {article}

}

@article{WIESENT2023101774,

title = {Investigating the influence of geometric parameters on the deformation of laser powder bed fused stents using low-fidelity thermo-mechanical analysis},

author = {L. Wiesent and F. Stocker and A. Nonn},

url = {https://www.sciencedirect.com/science/article/pii/S2589152923001011},

doi = {https://doi.org/10.1016/j.mtla.2023.101774},

issn = {2589-1529},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {Materialia},

volume = {28},

pages = {101774},

abstract = {Maintaining dimensional accuracy is a major challenge of laser powder bed fusion (L-PBF) preventing its application for more complex and filigree L-PBF structures in industrial practice. Previous studies have shown that residual stresses and distortion of benchmark L-PBF components may be predicted by sequential thermo-mechanical analyses. However, the reliability of these analyses for more complex structures must be critically questioned, as comprehensive validation and sensitivity analyses are scarce. In this paper, we present a calibrated and validated low-fidelity sequential thermo-mechanical finite element analysis (FEA) of a tubular L-PBF lattice structure, i.e., an aortic stent, where pronounced local deformation is expected. As a first step, the finite element model was extensively calibrated using experimental data to ensure reproducibility of the simulation results. Thereupon, geometric features critical to the distortion of L-PBF lattice structures and measures to compensate for the distortion, such as inversion of the distorted L-PBF structure, were investigated. It was found that the distortion of the L-PBF lattice structures can be reduced, but not completely prevented, by increasing the strut angles, increasing the strut thickness, and decreasing the transition radius in the area of merging struts. FEA-based inversion of the numerically predicted deformed structure minimized distortion, resulting in the L-PBF aortic stent approximating the intended CAD geometry even with a small strut thickness. This work shows that low-fidelity sequential thermo-mechanical FEA can be used not only for the analysis and deformation compensation of reference structures, but also for the analysis of more complex filigree structures with pronounced local deformation.},

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@article{SADEGHPOUR2022111782,

title = {Data-driven models for structure-property prediction in additively manufactured steels},

author = {E. Sadeghpour and A. Nonn},

url = {https://www.sciencedirect.com/science/article/pii/S0927025622004931},

doi = {https://doi.org/10.1016/j.commatsci.2022.111782},

issn = {0927-0256},

year  = {2022},

date = {2022-09-15},

urldate = {2022-01-01},

journal = {Computational Materials Science},

volume = {215},

pages = {111782},

abstract = {Data-driven models are developed to predict the mechanical properties of polycrystalline materials. The case study is the prediction of the yield strength of a 3D-printed 316L steel from morphological and crystallographic features. Three different artificial intelligence models including feed-forward (FNN), convolution (CNN), and graph (GNN) neural networks are employed to train the data-driven models and are compared in terms of performance and computational requirements. The dataset required for training is generated by performing crystal plasticity finite element simulations. The FNN model has the smallest input size and takes in some statistical parameters describing the material microstructure, but its accuracy is relatively low. The CNN approach inputs voxel-based realizations of the microstructure and is able to give accurate estimations; however, its training process is time-consuming and computationally expensive. In the GNN approach, the polycrystalline material is represented by a graph whose nodes and lines represent the grains and adjacency between grains. It is observed that GNN yields a better performance compared to the other two approaches and has the capability of handling complex tasks.},

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@article{Wiesent2022,

title = {Computational analysis of the effects of geometric irregularities on the interaction of an additively manufactured 316L stainless steel stent and a coronary artery},

author = {L. Wiesent and A. Spear and A. Nonn },

url = {https://www.sciencedirect.com/science/article/pii/S1751616121005117?dgcid=author

},

doi = {https://doi.org/10.1016/j.jmbbm.2021.104878},

year  = {2022},

date = {2022-01-01},

journal = {Journal of the Mechanical Behavior of Biomedical Materials},

volume = {Volume 125},

abstract = {Customized additively manufactured (laser powder bed fused (L-PBF)) stents could improve the treatment of complex lesions by enhancing stent-artery conformity. However, geometric irregularities inherent for L-PBF stents are expected to influence not only their mechanical behavior but also their interaction with the artery. In this study, the influence of geometrical irregularities on stent-artery interaction is evaluated within a numerical framework. Thus, computed arterial stresses induced by a reconstructed L-PBF stent model are compared to those induced by the intended stent model (also representing a stent geometry obtained from conventional manufacturing processes) and a modified CAD stent model that accounts for the increased strut thickness inherent for L-PBF stents. It was found that, similar to conventionally manufactured stents, arterial stresses are initially related to the basic stent design/topology, with the highest stresses occurring at the indentations of the stent struts. Compared to the stent CAD model, the L-PBF stent induces distinctly higher and more maximum volume stresses within the plaque and the arterial wall. In return, the modified CAD model overestimates the arterial stresses induced by the L-PBF stent due to its homogeneously increased strut thickness and thus its homogeneously increased geometric stiffness compared with the L-PBF stent. Therefore, the L-PBF-induced geometric irregularities must be explicitly considered when evaluating the L-PBF stent-induced stresses because the intended stent CAD model underestimates the arterial stresses, whereas the modified CAD model overestimates them. The arterial stresses induced by the L-PBF stent were still within the range of values reported for conventional stents in literature, suggesting that the use of L-PBF stents is conceivable in principle. However, because geometric irregularities, such as protruding features from the stent surface, could potentially damage the artery or lead to premature stent failure, further improvement of L-PBF stents is essential.},

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pubstate = {published},

tppubtype = {article}

}

@article{RAJARAMAN2021203712,

title = {Stress state characterization of ductile materials during scratch abrasion},

author = {D. Rajaraman and V. Keim and K. Pondicherry and A. Nonn and S. Hertelé and D. Fauconnier},

url = {https://www.sciencedirect.com/science/article/pii/S0043164821001010},

doi = {https://doi.org/10.1016/j.wear.2021.203712},

issn = {0043-1648},

year  = {2021},

date = {2021-01-01},

journal = {Wear},

pages = {203712},

abstract = {Abrasive wear limits the lifetime of many machine components. Most empirical models relate the abrasive wear resistance to material hardness. In reality, however, other material properties are also influencing as scratch abrasion damage follows from a highly complex stress trajectory upon scratching. Numerical (finite element) simulation of scratch abrasion requires the use of a material damage model, which translates this stress trajectory into material degradation and removal. Most damage models include the first two stress invariants. However, fully incorporating the complex stress trajectories that occur during scratch abrasion may require damage models with dependence of the third deviatoric parameter (Lode angle). This paper serves as an a-priori study to evaluate the stress states that may occur during scratch abrasion. Three mechanisms (ploughing, wedging, cutting) are considered. Hereto, the results of an extensive parametric study using elastic-plastic finite element simulations of a scratch indentation process are discussed. Complex, non-proportional variations in stress state values are observed to occur during scratch abrasion. Distinct stress state trajectories are identified for the three abovementioned mechanisms. These variations are critically discussed to motivate a selection of suitable damage models for rigorous finite element analysis of the wear processes associated with scratch abrasion.},

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pubstate = {published},

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@article{XUE2021107748,

title = {Anisotropic effects on crack propagation in pressurized line pipes under running ductile fracture scenarios},

author = {L. Xue and V. Keim and M. Paredes and A. Nonn and T. Wierzbicki},

url = {https://www.sciencedirect.com/science/article/pii/S001379442100196X},

doi = {https://doi.org/10.1016/j.engfracmech.2021.107748},

issn = {0013-7944},

year  = {2021},

date = {2021-01-01},

journal = {Engineering Fracture Mechanics},

volume = {249},

pages = {107748},

abstract = {The current analyses present results of running ductile fracture propagation in high strength X100 line pipe steels under the influence of anisotropy. Mechanical anisotropy is commonly available in pipe products as a result of the manufacturing process, especially, those subjected to hot/cold-worked deformation. The outcomes of the present analyses show that its effect on the behavior of running ductile fracture in cracked pipes undergoing depressurization is meaningful. For instance, the Crack-Tip Opening Angle (CTOA) not only exhibits a strong dependence to the pipe’s diameter size, but also to the material’s anisotropy nature when compared to a hypothetical isotropic material. Moreover, laboratory scale tests such as those performed on Battelle Drop Weight Tear (BDWT) samples provide useful information about initiation of ductile crack propagation when the anisotropy features are taken into account in the material description.},

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tppubtype = {article}

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@article{Wiesent2020a,

title = {Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents},

author = {L. Wiesent and U. Schultheiß and P. Lulla and U. Noster and T. Schratzenstaller and C. Schmid and A. Nonn and A. Spear},

editor = {A. Riveiro Rodr{'{i}}guez}},

url = {https://dx.plos.org/10.1371/journal.pone.0244463},

doi = {10.1371/journal.pone.0244463},

issn = {1932-6203},

year  = {2020},

date = {2020-12-29},

journal = {PLOS ONE},

volume = {15},

number = {12},

pages = {e0244463},

abstract = {Advances in additive manufacturing enable the production of tailored lattice structures and thus, in principle, coronary stents. This study investigates the effects of process-related irregularities, heat and surface treatment on the morphology, mechanical response, and expansion behavior of 316L stainless steel stents produced by laser powder bed fusion and provides a methodological approach for their numerical evaluation. A combined experimental and computational framework is used, based on both actual and computationally reconstructed laser powder bed fused stents. Process-related morphological deviations between the as-designed and actual laser powder bed fused stents were observed, resulting in a diameter increase by a factor of 2-2.6 for the stents without surface treatment and 1.3-2 for the electropolished stent compared to the as-designed stent. Thus, due to the increased geometrically induced stiffness, the laser powder bed fused stents in the as-built (7.11 ± 0.63 N) or the heat treated condition (5.87 ± 0.49 N) showed increased radial forces when compressed between two plates. After electropolishing, the heat treated stents exhibited radial forces (2.38 ± 0.23 N) comparable to conventional metallic stents. The laser powder bed fused stents were further affected by the size effect, resulting in a reduced yield strength by 41{%} in the as-built and by 59{%} in the heat treated condition compared to the bulk material obtained from tensile tests. The presented numerical approach was successful in predicting the macroscopic mechanical response of the stents under compression. During deformation, increased stiffness and local stress concentration were observed within the laser powder bed fused stents. Subsequent numerical expansion analysis of the derived stent models within a previously verified numerical model of stent expansion showed that electropolished and heat treated laser powder bed fused stents can exhibit comparable expansion behavior to conventional stents. The findings from this work motivate future experimental/numerical studies to quantify threshold values of critical geometric irregularities, which could be used to establish design guidelines for laser powder bed fused stents/lattice structures.},

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@article{Wiesent2020,

title = {Mechanical properties of small structures built by selective laser melting 316 L stainless steel – a phenomenological approach to improve component design},

author = {L. Wiesent and U. Schultheiß and P. Lulla and A. Nonn and U. Noster},

doi = {10.1002/mawe.202000038},

year  = {2020},

date = {2020-12-17},

journal = {Materialwissenschaft und Werkstofftechnik},

volume = {51},

number = {12},

pages = {1615--1629},

abstract = {Experimental investigations are conducted to quantify the influence of specimen thickness and orientation on the mechanical properties of selective laser melted stainless steel 316 L. The results indicate that the mechanical strength and ductility increase with increasing specimen thickness until a saturation value is reached from a specimen thickness of about 2 mm. Specimen orientation dependency is pronounced for thin specimens (< 1.5 mm), whereas only small deviations in strength are observed for thicker specimens with orientations of 30°, 45° and 90° to build direction. The mechanical properties of the specimen orientation of 0° to build direction shows great deviation to the other orientations and the smallest overall strength. A reliable design of selective laser melted components should account for specimen thickness and orientation, e. g. by a correction factor. Furthermore, it is recommended to avoid loads vertical (90°) and parallel (0°) to build direction to guarantee higher ductility and strength.},

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@article{Keim2020b,

title = {FSI-simulation of ductile fracture propagation and arrest in pipelines: Comparison with existing data of full-scale burst tests},

author = {V. Keim and M. Paredes and A. Nonn and S. Münstermann},

doi = {10.1016/j.ijpvp.2020.104067},

issn = {0308-0161},

year  = {2020},

date = {2020-01-01},

journal = {International Journal of Pressure Vessels and Piping},

volume = {182},

pages = {104067},

abstract = {The fracture propagation and arrest control for pipelines transporting rich natural gases and high vapor pressure liquids is based on the Battelle Two-Curve Model (BTCM). Distinct limitations of this model were demonstrated for past and modern steels and gas mixtures. These can be related to the insufficient description of individual physical processes and interactions between the pipe material and transported mixture during the running ductile fracture. In the past, fluid-structure interaction (FSI) models enabled a more sophisticated, coupled analysis of the failure scenario. To quantify their capability of describing the multi-physical processes, the FSI models need to be verified by experimental data from full-scale burst tests (FSBT). Therefore, this paper deals with the simulation of five FSBTs from the literature on API grade X65 pipes with different pipe geometries, mixtures and initial conditions. The FSI is modeled by the coupled Euler-Lagrange (CEL) method. The modified Mohr-Coulomb (MMC) model is implemented in the CEL framework to describe the deformation and ductile fracture in the X65/L450 pipes. 3D Euler equations are used to calculate the mixture decompression with the GERG-2008 equation of state defining the volumetric behavior of a CO2-rich mixture, CH4 and H2. The extended model considers the effect of soil backfill on the pipe deformation and inertia. The numerical predictions agree well with the experimental findings in terms of the crack propagation speed and arrest length underlining the capability of the developed numerical tool.},

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@article{Keim2019d,

title = {Application of the modified Bai-Wierzbicki model for the prediction of ductile fracture in pipelines},

author = {V. Keim and A. Nonn and S. Münstermann},

doi = {10.1016/j.ijpvp.2019.02.010},

issn = {0308-0161},

year  = {2019},

date = {2019-01-01},

journal = {International Journal of Pressure Vessels and Piping},

volume = {171},

pages = {104 - 116},

abstract = {The complex mechanical and corrosive loads of modern pipeline systems transporting oil, natural gas and CO2 impose steadily increasing requirements on material properties. The majority of current design standards still limit the application of modern high toughness linepipe steels due to the simple specification of material requirements in terms of energy levels from Charpy impact or Battelle Drop-Weight-Tear (BDWT) tests. In consequence, research activities have been conducted recently aiming at developing modified or novel experimental methods for the characterization of the ductile fracture behavior. To quantify the effects of various parameters on fracture behavior and derive suitable correlations, it is necessary to accompany these activities by numerical simulations with appropriate ductile damage models. In this paper, the MBW model is applied to study the structural behavior of pipelines in ductile fracture regime. Due to its precise incorporation of the underlying load conditions, the damage model is successfully used to simulate the slant fracture behavior in Battelle Drop weight tear test specimens and pipe sections. In comparison to ductile damage models applied in former studies, namely the Gurson-Tvergaard-Needleman and Cohesive Zone model, the presented numerical methodology allows for a more detailed investigation of loading, material and geometry effects on fracture and crack arrest behavior of pipelines.},

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@article{Wiesent2019,

title = {Experimentally validated simulation of coronary stents considering different dogboning ratios and asymmetric stent positioning},

author = {L. Wiesent and U. Schultheiß and C. Schmid and T. Schratzenstaller and A. Nonn},

doi = {10.1371/journal.pone.0224026},

year  = {2019},

date = {2019-01-01},

journal = {PLOS ONE},

volume = {14},

number = {10},

pages = {1-25},

publisher = {Public Library of Science},

abstract = {In-stent restenosis remains a major problem of arteriosclerosis treatment by stenting. Expansion-optimized stents could reduce this problem. With numerical simulations, stent designs/ expansion behaviours can be effectively analyzed. For reasons of efficiency, simplified models of balloon-expandable stents are often used, but their accuracy must be challenged due to insufficient experimental validation. In this work, a realistic stent life-cycle simulation has been performed including balloon folding, stent crimping and free expansion of the balloon-stent-system. The successful simulation and validation of two stent designs with homogenous and heterogeneous stent stiffness and an asymmetrically positioned stent on the balloon catheter confirm the universal applicability of the simulation approach. Dogboning ratio, as well as the final dimensions of the folded balloon, the crimped and expanded stent, correspond well to the experimental dimensions with only slight deviations. In contrast to the detailed stent life-cycle simulation, a displacement-controlled simulation can not predict the transient stent expansion, but is suitable to reproduce the final expanded stent shape and the associated stress states. The detailed stent life-cycle simulation is thus essential for stent expansion analysis/optimization, whereas for reasons of computational efficiency, the displacement-controlled approach can be considered in the context of pure stress analysis.},

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@article{KEIM2019103934,

title = {Fluid-structure-interaction modeling of dynamic fracture propagation in pipelines transporting natural gases and CO2-mixtures},

author = {V. Keim and P. Marx and A. Nonn and S. Münstermann},

doi = {10.1016/j.ijpvp.2019.103934},

issn = {0308-0161},

year  = {2019},

date = {2019-01-01},

journal = {International Journal of Pressure Vessels and Piping},

volume = {175},

pages = {103934},

abstract = {As part of current design standards, the Battelle Two-Curve Model (BTCM) is still widely used to predict and secure ductile crack arrest in gas transmission pipelines. For modern linepipe steels and rich natural gases or CO2 mixtures, the BTCM might lead to incorrect predictions. On the one hand, it suffers from the insufficient description of the individual physical processes in the pipe material and fluid itself. Furthermore, the model does not account for fluid-structure-interaction (FSI) effects during simultaneous running-ductile fracture (RDF) and mixture decompression. Numerical FSI models allow for a more sophisticated, coupled analysis of the driving forces for the failure of pipelines. This paper deals with the development of an FSI model for the coupled prediction of 3D pressure profiles acting on the inner pipe wall during crack propagation. The coupled Euler-Lagrange (CEL) method is used to link the fluid and structure models. In a Lagrange formulation, the modified Bai-Wierzbicki (MBW) model describes the plastic deformation and ductile fracture as a function of the underlying stress/strain conditions. The fluid behavior is calculated in a 3D model space by Euler equations and the GERG-2008 reference equation of state (EOS). The coupled CEL model is used to predict the RDF in small-diameter pipe sections for different fluid mixtures. The calculated 3D pressure distributions ahead and behind the running crack tip (CT) significantly differ in axial and circumferential directions depending on the mixture composition. The predicted FSI between the pipe wall and fluid decompression in 3D CEL/FSI model provides reliable knowledge about the pressure loading of the pipeline during RDF.},

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tppubtype = {article}

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@article{Kramer2019,

title = {The third Sandia fracture challenge: predictions of ductile fracture in additively manufactured metal},

author = {S. L. B. Kramer and A. Jones and A. Mostafa and B. Ravaji and T. Tancogne-Dejean and C. C. Roth and M. Gorji and K. Pack and J. T. Foster and M. Behzadinasab and J. C. Sobotka and J. M. McFarland and J. Stein and A. D. Spear and P. Newell and M. W. Czabaj and B. Williams and H. Simha and M. Gesing and L. N. Gilkey and C. A. Jones and R. Dingreville and S. E. Sanborn and J. L. Bignell and A. R. Cerrone and V. Keim and A. Nonn and S. Cooreman and P. Thibaux and N. Ames and D. O. Connor and M. Parno and B. Davis and J. Tucker and B. Coudrillier and K. N. Karlson and J. T. Ostien and J. W. Foulk and C. I. Hammetter and S. Grange and J. M. Emery and J. A. Brown and J. E. Bishop and K. L. Johnson and K. R. Ford and S. Brinckmann and M. K. Neilsen and J. Jackiewicz and K. Ravi-Chandar and T. Ivanoff and B. C. Salzbrenner and B. L. Boyce},

doi = {10.1007/s10704-019-00361-1},

issn = {1573-2673},

year  = {2019},

date = {2019-01-01},

journal = {International Journal of Fracture},

volume = {218},

number = {1},

pages = {5-61},

abstract = {The Sandia Fracture Challenges provide a forum for the mechanics community to assess its ability to predict ductile fracture through a blind, round-robin format where mechanicians are challenged to predict the deformation and failure of an arbitrary geometry given experimental calibration data. The Third Challenge (SFC3) required participants to predict fracture in an additively manufactured (AM) 316L stainless steel bar containing through holes and internal cavities that could not have been conventionally machined. The volunteer participants were provided extensive data including tension and notched tensions tests of 316L specimens built on the same build-plate as the Challenge geometry, micro-CT scans of the Challenge specimens and geometric measurements of the feature based on the scans, electron backscatter diffraction (EBSD) information on grain texture, and post-test fractography of the calibration specimens. Surprisingly, the global behavior of the SFC3 geometry specimens had modest variability despite being made of AM metal, with all of the SFC3 geometry specimens failing under the same failure mode. This is attributed to the large stress concentrations from the holes overwhelming the stochastic local influence of the AM voids and surface roughness. The teams were asked to predict a number of quantities of interest in the response based on global and local measures that were compared to experimental data, based partly on Digital Image Correlation (DIC) measurements of surface displacements and strains, including predictions of variability in the resulting fracture response, as the basis for assessment of the predictive capabilities of the modeling and simulation strategies. Twenty-one teams submitted predictions obtained from a variety of methods: the finite element method (FEM) or the mesh-free, peridynamic method; solvers with explicit time integration, implicit time integration, or quasi-statics; fracture methods including element deletion, peridynamics with bond damage, XFEM, damage (stiffness degradation), and adaptive remeshing. These predictions utilized many different material models: plasticity models including J2 plasticity or Hill yield with isotropic hardening, mixed Swift-Voce hardening, kinematic hardening, or custom hardening curves; fracture criteria including GTN model, Hosford-Coulomb, triaxiality-dependent strain, critical fracture energy, damage-based model, critical void volume fraction, and Johnson-Cook model; and damage evolution models including damage accumulation and evolution, crack band model, fracture energy, displacement value threshold, incremental stress triaxiality, Cocks-Ashby void growth, and void nucleation, growth, and coalescence. Teams used various combinations of calibration data from tensile specimens, the notched tensile specimens, and literature data. A detailed comparison of results based of these different methods is presented in this paper to suggest a set of best practices for modeling ductile fracture in situations like the SFC3 AM-material problem. All blind predictions identified the nominal crack path and initiation location correctly. The SFC3 participants generally fared better in their global predictions of deformation and failure than the participants in the previous Challenges, suggesting the relative maturity of the models used and adoption of best practices from previous Challenges. This paper provides detailed analyses of the results, including discussion of the utility of the provided data, challenges of the experimental-numerical comparison, defects in the AM material, and human factors.},

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@article{Keim2019b,

title = {Using local damage models to predict fracture in additively manufactured specimens},

author = {V. Keim and A. Cerrone and A. Nonn},

doi = {10.1007/s10704-019-00371-z},

issn = {1573-2673},

year  = {2019},

date = {2019-01-01},

journal = {International Journal of Fracture},

volume = {218},

number = {1},

pages = {135-147},

abstract = {This paper explores the efficacy of employing local damage models, normally applied to ductile material systems manufactured by subtractive techniques, to additively manufactured laboratory specimens. While these specimens were ductile and metallic, their additive character (i.e. porosity and surface roughness) could have had potential to activate multiple life-limiting failure paths, thus obfuscating failure prediction. Herein, two damage models are considered and compared: the micromechanical Gurson-Tvergaard-Needleman model and a Crack Band model of the strain-based, phenomenological genre. Simulations used to calibrate elastic and plastic material properties and predict damage in a novel, non-standard specimen were quasi-static, explicit. Both damage models proved capable in resolving the experimentally-observed failure path and associated loading conditions. The analyses described herein were made as part of the Third Sandia Fracture Challenge.},

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@article{Cerrone2016,

title = {Predicting failure of the Second Sandia Fracture Challenge geometry with a real-world, time constrained, over-the-counter methodology},

author = {A. R. Cerrone and A. Nonn and J. D. Hochhalter and G. F. Bomarito and J. E. Warner and B. J. Carter and D. H. Warner and A. R. Ingraffea},

url = {https://doi.org/10.1007/s10704-016-0086-x},

doi = {10.1007/s10704-016-0086-x},

issn = {1573-2673},

year  = {2016},

date = {2016-03-01},

journal = {International Journal of Fracture},

volume = {198},

number = {1},

pages = {117-126},

abstract = {An over-the-counter methodology to predict fracture initiation and propagation in the challenge specimen of the Second Sandia Fracture Challenge is detailed herein. This pragmatic approach mimics that of an engineer subjected to real-world time constraints and unquantified uncertainty. First, during the blind prediction phase of the challenge, flow and failure locus curves were calibrated for Ti--6Al--4V with provided tensile and shear test data for slow (0.0254 mm/s) and fast (25.4 mm/s) loading rates. Thereafter, these models were applied to a 3D finite-element mesh of the non-standardized challenge geometry with nominal dimensions to predict, among other items, crack path and specimen response. After the blind predictions were submitted to Sandia National Labs, they were improved upon by addressing anisotropic yielding, damage initiation under shear dominance, and boundary condition selection.},

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@article{Cerrone2014b,

title = {Implementation and verification of the Park–Paulino–Roesler cohesive zone model in 3D},

author = {A. Cerrone and P. Wawrzynek and A. Nonn and G. H. Paulino and A. Ingraffea},

url = {http://www.sciencedirect.com/science/article/pii/S0013794414000770},

doi = {https://doi.org/10.1016/j.engfracmech.2014.03.010},

issn = {0013-7944},

year  = {2014},

date = {2014-01-01},

journal = {Engineering Fracture Mechanics},

volume = {120},

pages = {26 - 42},

abstract = {The Park–Paulino–Roesler (PPR) potential-based model is a cohesive constitutive model formulated to be consistent under a high degree of mode-mixity. Herein, the PPR’s generalization to three-dimensions is detailed, its implementation in a finite element framework is discussed, and its use in single-core and high performance computing (HPC) applications is demonstrated. The PPR model is shown to be an effective constitutive model to account for crack nucleation and propagation in a variety of applications including adhesives, composites, linepipe steel, and microstructures.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kofiani2013b,

title = {New calibration method for high and low triaxiality and validation on SENT specimens of API X70},

author = {K. Kofiani and A. Nonn and T. Wierzbicki},

url = {http://www.sciencedirect.com/science/article/pii/S0308016113001130},

doi = {https://doi.org/10.1016/j.ijpvp.2013.07.004},

issn = {0308-0161},

year  = {2013},

date = {2013-01-01},

journal = {International Journal of Pressure Vessels and Piping},

volume = {111-112},

pages = {187 - 201},

abstract = {The determination of the exact mechanical properties of material is essential for an optimal and safe design of linepipes. It is especially important for the prevention of over-engineering and the reliable assessment of complex accidental loading, such as extreme bending due to loss of buoyancy of support, or abrupt ground movement. Currently, the focus of research in offshore deepwater installations and linepipes is towards pre-cracked structures with high triaxiality stress states and complex loading histories. At the same time, low triaxiality stress states must be correctly studied in order to represent shear dominated failure in pipes. A comprehensive experimental and numerical program was undertaken to determine the mechanical properties of the traditional API X70 grade of steel. The material was characterized for anisotropic plasticity, fracture initiation and uncracked ductility for various states of stress. The same material was also used for pre-cracked fracture toughness assessment. The experimental program included flat and round specimens. The first type of tests on flat butterfly-shaped, central hole, notched and circular disk specimens; were selected to address the low stress triaxiality range. Tests on round notched bar specimens and SENT fracture mechanics tests extended the characterization and verification process to higher stress triaxiality values. This program covered a wide range of stress conditions and demonstrated their effect on the material resistance to crack extension. Each test conducted was numerically simulated using solid finite element models, matching the exact geometric and loading history features. The numerical simulation provided information on the local stress and strain fields around the location of the potential or existing cracks. Based on the above hybrid experimental/numerical technique tailored for pipe applications, the MMC fracture model was calibrated. The model relates the material ductility not only to stress triaxiality but also to the Lode parameter. The predictive capabilities of the MMC were then evaluated in the case of SENT testing, used extensively in the pipeline industry. It was shown that the present fracture model calibration can describe fracture behavior of SENT experiments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{BLECK20091531,

title = {Numerical and experimental analyses of damage behaviour of steel moment connection},

author = {W. Bleck and W. Dahl and A. Nonn and L. Amlung and M. Feldmann and D. Schäfer and B. Eichler},

url = {http://www.sciencedirect.com/science/article/pii/S001379440900085X},

doi = {https://doi.org/10.1016/j.engfracmech.2009.03.004},

issn = {0013-7944},

year  = {2009},

date = {2009-01-01},

journal = {Engineering Fracture Mechanics},

volume = {76},

number = {10},

pages = {1531 - 1547},

abstract = {Plastic design allows the exploitation of the full resistance of steel structures by taking advantage of stress–redistributions due to plastic strains exceeding the yield strain. Especially in seismic design the utilization of material reserves and the formation of plastic hinges play an important role. In devastating earthquakes in Northridge (USA) and Kobe (Japan) brittle fracture of welded connections in steel moment frames occurred prior to formation of plastic hinges and utilization of plastic material reserves. The subsequent research works resulted in improved design rules and recommendations for these kinds of failure. But to guarantee sufficient ductile performance of these connections also in the upper shelf region, plastic and earthquake resistant design rules should take into account degradation of strain capacity and toughness properties due to quasi static and especially seismic loading. In the scope of the current European project “Plastotough”, the main objective is to derive quantified toughness design rules in the upper shelf based on the strain requirements opposed to strain capacities. This paper gives an overview over the research work in performance and shows recent results from experimental and numerical analyses performed within this project for monotonic and cyclic loading.},

note = {MatModels 2007},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{NONN20083251,

title = {Numerical modelling of damage behaviour of laser-hybrid welds},

author = {A. Nonn and W. Dahl and W. Bleck},

url = {http://www.sciencedirect.com/science/article/pii/S0013794407003943},

doi = {https://doi.org/10.1016/j.engfracmech.2007.10.015},

issn = {0013-7944},

year  = {2008},

date = {2008-01-01},

journal = {Engineering Fracture Mechanics},

volume = {75},

number = {11},

pages = {3251 - 3263},

abstract = {The effect of laser-hybrid welds on deformation and failure behaviour of fracture mechanics specimens is investigated in order to provide quantitative prediction of damage tolerance and residual strength. The simulation of crack initiation and crack extension in hybrid welds is performed by applying GTN damage model. The identification of damage parameters requires combined numerical and experimental analyses. The tendency to crack path deviation during crack growth depends strongly on the constraint development at the interface between base and weld metal. In order to quantify the influence of local stress state on the crack path deviation, the initial crack location is varied. Finally, the results from fracture mechanics tests are compared to real component, beam-column-connection, with respect to fracture resistance.},

note = {Local Approach to Fracture (1986–2006): Selected papers from the 9th European Mechanics of Materials Conference},

keywords = {},

pubstate = {published},

tppubtype = {article}

}