Christian Roth

Last Name

Roth

First Name

Christian

ORCID

0000-0001-7052-887X

Organisational unit

09473 - Mohr, Dirk / Mohr, Dirk

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Publications 1 - 10 of 29

Post-Necking full-field FEMU identification of anisotropic plasticity from flat notched tension experiments
Sakaridis, Emmanouil; Roth, Christian; Jordan, Benoit; et al. (2024)
International Journal of Solids and Structures
This work introduces a finite element model updating (FEMU) identification scheme to determine the material parameters of an anisotropic metal plasticity model. Surround digital image correlation (DIC) data is collected from tensile tests on mildly notched flat specimens and it is used to minimize specimen alignment errors when comparing simulations and experiments. The front surface displacement fields and resultant force history are leveraged to calibrate a Whip-Bezier based material model in a computationally-efficient procedure, which treats the pre- and post-necking regimes separately. Experimental data from specimens with a larger notch radius (NT20) serve as the training set, while data from specimens with a smaller notch radius (NT6) are used for validation. Analysis of identification methods using datasets from virtual experiments highlights the improved generalization ability of the full-field approach compared to solely using force–displacement curves. However, this work also demonstrates that through-thickness necking in real notched tensile experiments is asymmetric. This can hinder the identification of the large strain segment of hardening laws, especially when a FEMU approach incorporates full-field information from one specimen surface only. Consequently, it is recommended to use advanced finite element models that capture asymmetric localized strain fields or to base the identification of large strain hardening responses on experiments that achieve large strains without asymmetric through-thickness strain localization, such as in-plane torsion tests.
Neural network based rate- and temperature-dependent Hosford-Coulomb fracture initiation model
Li, Xueyang; Roth, Christian; Mohr, Dirk (2023)
International Journal of Mechanical Sciences
The accurate description of the strain rate and temperature dependent response of metals is a perpetual quest in crashworthiness and forming applications. In the present study, experiments are carried out to probe the onset of ductile fracture for an aluminum alloy AA7075-T6 for 136 combinations of stress state, strain rate and tem-perature. The experimental campaign covers strain rates ranging from 0.001/s to 100/s, and temperatures ranging from 20 degrees C to 360 degrees C. We combine a YLD2000 yield surface with a neural network based hardening law to describe the large deformation plasticity response of the material. The NN-based hardening law is trained on experimental data, achieving 3.9% accuracy on force predictions including the post-necking regime. The loading paths to fracture are extracted for each simulation, showcasing non-proportionally evolving stress triaxiality, Lode angle parameter, strain rate and temperature. A neural network parameterized Hosford-Coulomb fracture locus is proposed, which is trainable using these evolving loading histories. The accuracy of the proposed fracture model is validated against the experimental onset of fracture, predicting the fracture onset at an error of 8%.
Rate-and temperature-dependent plasticity of additivly manufacutred stainless steel 316L: Characterization, modeling and application to crushing of shell-lattices
Li, Xueyang; Roth, Christian; Tancongne-Dejean, Thomas; et al. (2020)
International Journal of Impact Engineering
A combined numerical and experimental investigation is carried out on the quasi-static and high strain rate response of additively manufactured stainless steel 316L obtained through selective laser melting. The experimental program comprises experiments on uniaxial tension, shear, notched tension and mini-Nakazima specimens, covering a wide range of stress states and strain rates (from 10−3 to 103/s). An anisotropic quadratic plasticity model with Swift-Voce hardening and Johnson-Cook rate- and temperature-dependence is identified to describe the behavior of the constituent base material under different stress-states and strain rates. Compression experiments at low and high loading speeds are conducted on elastically-isotropic shell-lattice structures to further validate the identified plasticity model in a structural application. It is found that the chosen plasticity model can describe the reaction force and deformation patterns of the smooth shell lattice loaded at different speeds and orientations with good accuracy. The experiments reveal that the additively-manufactured shell-lattices are capable of sustaining macroscopic compressive strains of more than 60% without visible fracture of the cell walls regardless of the loading speed. The comparison with the results for plate-lattice structures of the same mass elucidate the great energy absorption potential of shell-lattices.
Experimental investigation on shear fracture at high strain rates
Roth, Christian; Mohr, Dirk (2015)
EPJ Web of Conferences
Adiabatic shear banding is a well-understood failure mechanism of metals at high strain rates. In addition, recent research on the ductile fracture of metals has demonstrated that shear localization at the microscale is also an important precursor of fracture initiation at low strain rates. This talk presents a new shear fracture specimen which is used to conduct fracture experiments on advanced high strength steel sheets at strain rates of up to 1/s in a hydraulic testing machine and for strain rates of up to 2500/s in a Split Hopkinson Bar system. The experimental result for a 22 MnB5 steel show a significant increase in ductility as a function of strain rate. Results from scanning electron microscopy are also shown to provide insight into the effect of the strain rate on the shear localization at the microscale.
Site-specifically tailored microstructures with enhanced strength and hardening through laser powder bed fusion
Sofras, Christos; Čapek, Jan; Li, Xueyang; et al. (2024)
Materials & Design
Laser powder bed fusion (L-PBF) has emerged as an additive manufacturing technique that offers unprecedented design freedom. Besides being capable of producing complex and near net shape objects, L-PBF can impact tremendously the engineering materials community due to the possibility of locally manipulating metallic microstructures. Here we exploit the latter potentiality of L-PBF, to produce site-specifically tailored stainless steel components, in terms of their crystallographic texture. The tailored materials are tested and exhibit superior energy dissipation capabilities under bending deformation compared to uniformly textured materials. This is enabled by the strong dependence of the secondary hardening mechanisms, namely the deformation twinning and/or martensite formation, of these materials on the locally tuned microstructures. With the aid of finite element simulations, it is possible to identify the stress state and hence, the crystallographic orientations that facilitate twinning or martensite formation. Then, by engineering favorable crystallographic textures, matched to the complex stress state during bending, enhanced work hardening behavior is obtained. This site-specific microstructure design enabled by L-PBF provides a new pathway for the design of “smart” components that exhibit superior mechanical response under complex stress states.
Dynamic perforation of lightweight armor: Temperature-dependent plasticity and fracture of aluminum 7020-T6
Roth, Christian; Fras, Teresa; Mohr, Dirk (2020)
Mechanics of Materials ~ 149
The design of armored vehicles requires reliable constitutive models that are valid over a wide range of strain rates and temperatures. A comprehensive experimental program is executed to characterize the stress-strain response of high strength aluminum 7020-T6 at temperatures ranging from 20°C to 320°C. It includes tensile experiments on uniaxial, notched, central hole and shear specimens. Aside from low and intermediate strain rate experiments, high strain rate experiments are performed on a Split Hopkinson Pressure Bar (SHPB) system equipped with a load inversion device. Furthermore, hemispherical punch and V-bending experiments are performed to achieve equi-biaxial tension and transverse plane strain conditions. It is found that a Yld2000–3d plasticity model with isotropic strain hardening and thermal softening is suitable to describe the large deformation response, while a rate- and temperature-independent Hosford-Coulomb model is used to predict fracture. Impact experiments are performed on 4 mm thick targets with blunt, hemispherical and conical steel projectiles of 8 mm diameter and a mass of 13.8 g. The impact velocity is varied such that the full spectrum from the ballistic limit to complete penetration can be characterized. In addition, perpendicular and oblique configurations are considered. Numerical simulations are performed for all experiments confirming the validity of the identified constitutive model and providing unmatched insight into the dynamic penetration failure mechanism.
A strain-rate-dependent engineering approximation of the temperature rise in plastically-deforming DP steel
Li, Xueyang; Roth, Christian; Mohr, Dirk (2021)
EPJ Web of Conferences ~ DYMAT 2021 - 13th International Conference on the Mechanical and Physical Behaviour of Materials under Dynamic Loading
Experiments at ten strain rates ranging from 0.001/s to 4/s are carried out on uniaxial tension specimens extracted from DP800 metal sheets. Digital Image Correlation (DIC) is used to obtain surface strain fields and a high speed infrared (IR) camera is employed to measure the corresponding temperature rise due to plastic dissipation. A temperature rise of 60K is witnessed for the highest loading speed whereas minimal temperature rise (<1K) is seen for the lowest loading speed. To minimize the computational cost by treating the temperature as an internal state variable, (effectively avoiding more complex coupled thermo-mechanical analyses), a logarithm based function is proposed that models the transition from isothermal to adiabatic conditions. The proposed function exhibits a higher accuracy compared to literature formulations.
Axisymmetric V-bending: A single experiment to determine the fracture strain and weakest sheet material direction for plane strain tension
Beerli, Thomas; Grolleau, Vincent; Mohr, Dirk; et al. (2022)
International Journal of Solids and Structures
Most metals exhibit a critical minimum in ductility for plane strain tension. The fracture strain for this particular state of loading is an important reference point for the calibration of ductile fracture models. Here, an axisymmetric V-bending technique is proposed to identify the strain to fracture for plane strain tension for the weakest in-plane direction of sheet metal. After clamping the center and outer boundary of a disc specimen onto an axisymmetric fixture, the specimen is drawn over a tubular knife until fracture initiates on the specimen surface. Throughout the experiment, the specimen surface is monitored with a digital camera for timely crack detection and strain measurement by planar Digital Image Correlation (DIC). It is shown through numerical simulations that the experimental setup is remarkably robust with the results exhibiting a low sensitivity to geometric imperfections. Experiments are performed on aluminum AA2024 and two dual phase steels (DP780 and DP 980) to demonstrate the validity of the proposed technique through comparison with conventional V-bending and dihedral mini-Nakazima experiments.
Dynamic ductile fracture
Mohr, Dirk; Roth, Christian (2023)
Dynamic Behavior of Materials
Ductile fracture is the main mechanism leading to the formation and propagation of cracks in metallic structures. It is a physical process that involves the large deformation and progressive damage of materials under extreme loading conditions. The prediction of ductile fracture by means of finite element computations requires plasticity models that predict the three-dimensional deformation response of solids. In the case of dynamic loading, the temperature- and rate-dependent mechanisms described in the previous chapter need to be captured through advanced plasticity models. Two distinct approaches to predict ductile fracture are covered here. The first consists of using porous plasticity models in conjunction with coalescence criteria. The second is based on nonporous plasticity models that can be used in conjunction with damage indicator models to predict the initiation of ductile fracture. With a view to analyzing crash and impact problems, the Johnson-Cook plasticity model and its fracture counterpart are discussed. Furthermore, the Lode angle and stress triaxiality sensitive Hosford-Coulomb fracture initiation model is introduced.
On loading velocity oscillations during dynamic tensile testing with flying wheel systems
Erice, Borja; Roth, Christian; Gary, Gerard; et al. (2015)
EPJ Web of Conferences
Flying Wheels (FW) provide a space-saving alternative to Split Hopkinson Bar (SHB) systems for generating the loading pulse for intermediate and high strain rate material testing. This is particularly attractive in view of performing ductile fracture experiments at intermediate strain rates that require a several milliseconds long loading pulse. More than 50 m long Hopkinson bars are required in that case, whereas the same kinetic energy (for a given loading velocity) can be stored in rather compact flying wheels (e.g. diameter of less than 1.5 m). To gain more insight into the loading capabilities of FW tensile testing systems, a simple analytical model is presented to analyze the loading history applied by a FW system. It is found that due to the presence of a puller bar that transmits the tensile load from the rotating wheel to the specimen, the loading velocity applied onto the specimen oscillates between about zero and twice the tangential loading speed applied by the FW. The theoretical and numerical evaluation for a specific 1.1 m diameter FW system revealed that these oscillations occur at a frequency in the kHz range, thereby questioning the approximate engineering assumption of a constant strain rate in FW tensile experiments at strain rates of the order of 100/s.

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