TY - JOUR A1 - Kramer, Sharlotte L. B. A1 - Jones, Amanda A1 - Mostafa, Ahmed A1 - Ravaji, Babak A1 - Tancogne-Dejean, Thomas A1 - Roth, Christian C. A1 - Nonn, Aida T1 - The third Sandia fracture challenge: predictions of ductile fracture in additively manufactured metal JF - International Journal of Fracture N2 - 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. KW - Fracture KW - Rupture KW - Tearing KW - Deformation KW - Plasticity KW - Metal KW - Alloy KW - Additive manufacturing KW - Simulation KW - Prediction KW - Modeling Y1 - 2019 U6 - https://doi.org/10.1007/s10704-019-00361-1 VL - 218 SP - 5 EP - 61 PB - Springer Nature ER - TY - CHAP A1 - Schneider, Ingo A1 - Nonn, Aida A1 - Völling, Alexander A1 - Mondry, A. A1 - Kalwa, Christoph T1 - A damage mechanics based evaluation of dynamic fracture resistance in gas pipelines T2 - Procedia Materials Science, Volume 3 N2 - Investigation of running ductile fracture in gas transmission pipelines and the derivation of reliable crack arrest prediction methods belong to major topics in pipeline research. The yet available crack arrest criterion, known as the Battelle Two-Curve Method (BTCM), leads to reliable predictions up to grade X70 line pipe steels for which it has been validated. This includes specific limits in terms of mechanical properties, pressure and geometry. The application of this criterion to modern pipeline steels, i.e. especially grades X80 and beyond in combination with larger diameters and high pressure, has led to mispredictions of the BTCM. Hence, in order to ensure safe design of pipelines, new methods are required based on in depth knowledge and appropriate characterization of material resistance. This paper presents a procedure for the assessment of dynamic ductile fracture resistance based on combined experimental and numerical investigations. The procedure involves quasi-static and dynamic drop- weight tear testing (DWTT) on modified specimens with pre-fatigued crack for grades X65, X80 and X100 materials, and the application of cohesive zone (CZ) and Gurson-Tveergard-Needleman (GTN) models to describe ductile material damage. The damage model parameters are calibrated on basis of DWTT results and subsequently used to simulate dynamic crack propagation in a pipeline. The influence of material properties (strain hardening, toughness), pipe geometry, usage factor and decompression behaviour on ductile fracture propagation behaviour is studied and evaluated. The results will contribute to an enhanced understanding of major parameters controlling ductile fracture propagation and will help to establish a reliable procedure for safe design of new high-capacity pipelines with regard to crack arrest. KW - Pipeline KW - Dynamic ductile fracture KW - Crack arrest KW - Cohesive zone model KW - DWT testing Y1 - 2014 U6 - https://doi.org/10.1016/j.mspro.2014.06.315 VL - 3 SP - 1956 EP - 1964 ER - TY - JOUR A1 - Xue, Lufeng A1 - Keim, Vincent A1 - Paredes, M. A1 - Nonn, Aida A1 - Wierzbicki, Tomasz T1 - Anisotropic effects on crack propagation in pressurized line pipes under running ductile fracture scenarios JF - Engineering fracture mechanics N2 - 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. KW - BEHAVIOR KW - Fracture anisotropy KW - INITIATION KW - MODEL KW - Non-associated flow rule KW - PREDICTION KW - Rate dependent MMC Model KW - Running ductile fracture KW - STATE KW - STEEL KW - STRAIN-RATE KW - TOUGHNESS Y1 - 2021 U6 - https://doi.org/10.1016/j.engfracmech.2021.107748 VL - 249 PB - Elsevier ER - TY - JOUR A1 - Sadeghpour, Ebrahim A1 - Nonn, Aida T1 - Data-driven models for structure-property prediction in additively manufactured steels JF - Computational Materials Science N2 - 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. KW - Data-driven model KW - Neural networks KW - Polycrystalline materials KW - Structure-property relationships Y1 - 2022 U6 - https://doi.org/10.1016/j.commatsci.2022.111782 VL - 215 PB - Elsevier ER - TY - CHAP A1 - Xue, Lufeng A1 - Paredes, Marcelo A1 - Nonn, Aida A1 - Wierzbicki, Tomasz T1 - Modeling of Crack Propagation in Defective X100 Line Pipes T2 - ASME 2020 39th International Conference on Ocean, Offshore and Arctic Engineering: August 3-7, 2020, virtual, online N2 - A comprehensive experimental program is carried out to determine material parameters for fracture initiation and propagation in X100 pipeline steels. The quadratic Hill’48 yield function along with an isotropic hardening are used to describe plastic flow at large deformation and a phenomenological fracture criterion to predict fracture initiation. Fracture mechanics SENT specimens are used to calibrate post-initiation softening parameters necessary for ductile crack propagation in thick components. Once the material model parameters set is complete a final comparison is conducted with ring expansion test on same material. KW - Berg KW - Bruchmechanik KW - Pipeline KW - plastisches Fließen KW - Rissausbreitung KW - Versuchsprogramm KW - Werkstoffmodell KW - Werkstoffparameter Y1 - 2020 SN - 978-0-7918-8434-8 U6 - https://doi.org/10.1115/OMAE2020-18048 PB - American Society of Mechanical Engineers CY - New York ER - TY - JOUR A1 - Wiesent, Lisa A1 - Schultheiß, Ulrich A1 - Lulla, Philipp A1 - Noster, Ulf A1 - Schratzenstaller, Thomas A1 - Schmid, Christof A1 - Nonn, Aida A1 - Spear, Ashley T1 - Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents JF - PLoS ONE N2 - 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. KW - Heat treatment KW - Lasers KW - Surface treatments KW - Specimen preparation and treatment KW - Material properties KW - Stiffness KW - Deformation KW - Powders KW - Koronarendoprothese KW - Rapid prototyping KW - Numerische Methode Y1 - 2020 U6 - https://doi.org/10.1371/journal.pone.0244463 N1 - Corresponding author: Lisa Wiesent VL - 15 IS - 12 SP - 1 EP - 30 PB - PLOS ER - TY - JOUR A1 - Wiesent, Lisa A1 - Schultheiss, Ulrich A1 - Schmid, Christof A1 - Schratzenstaller, Thomas A1 - Nonn, Aida T1 - Experimentally validated simulation of coronary stents considering different dogboning ratios and asymmetric stent positioning JF - PlOS One N2 - 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. KW - Catheters KW - Coronary stenting KW - Deformation KW - Stent implantation KW - Stiffness KW - Stent KW - Ballondilatation KW - Numerische Strömungssimulation Y1 - 2019 U6 - https://doi.org/10.1371/journal.pone.0224026 VL - 14 IS - 10 SP - 1 EP - 25 PB - PLOS ER - TY - JOUR A1 - Kofiani, Kirki A1 - Nonn, Aida A1 - Wierzbicki, Tomasz T1 - New calibration method for high and low triaxiality and validation on SENT specimens of API X70 JF - International Journal of Pressure Vessels and Piping N2 - 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. KW - Ductile fracture KW - SENT tests KW - Offshore API grade KW - X70 KW - Modified Mohr – Coulomb model KW - Triaxiality and Lode angle dependence Y1 - 2013 U6 - https://doi.org/10.1016/j.ijpvp.2013.07.004 VL - 111-112 SP - 187 EP - 201 ER - TY - JOUR A1 - Bleck, Wolfgang Peter A1 - Dahl, Winfried A1 - Nonn, Aida A1 - Amlung, Lars A1 - Feldmann, Markus A1 - Schäfer, Dirk A1 - Eichler, Björn T1 - Numerical and experimental analyses of damage behaviour of steel moment connection JF - Engineering Fracture Mechanics N2 - 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. KW - Beam column connections KW - Seismic loading KW - LPD model Y1 - 2009 U6 - https://doi.org/10.1016/j.engfracmech.2009.03.004 VL - 76 IS - 10 SP - 1531 EP - 1547 ER - TY - JOUR A1 - Nonn, Aida A1 - Dahl, Winfried A1 - Bleck, Wolfgang Peter T1 - Numerical modelling of damage behaviour of laser-hybrid welds JF - Engineering Fracture Mechanics N2 - 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. KW - GTN model KW - Laser-hybrid welds KW - Damage KW - Crack path deviation Y1 - 2008 U6 - https://doi.org/10.1016/j.engfracmech.2007.10.015 VL - 75 SP - 3251 EP - 3263 ER -