Christian Roth

Last Name

Roth

First Name

Christian

ORCID

0000-0001-7052-887X

Organisational unit

09473 - Mohr, Dirk / Mohr, Dirk

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Publications1 - 10 of 31

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.
Using miniature experiments to reveal strength gradients in battery casings
Tancogne-Dejean, Thomas; Roth, Christian; Grolleau, Vincent; et al. (2024)
International Journal of Mechanical Sciences
Complex metallic structures are often manufactured through multi-step forming or casting processes that induce severe mechanical property variations within the final part. Instead of using the virgin material properties to predict the final part performance, the local material properties after manufacturing need to be known. A novel comprehensive testing methodology is presented allowing the resolution of spatial gradients in the plasticity and fracture properties in metallic structures. It involves using a recently-developed miniature linear-motion system that probes the material within 2 mm wide gage sections (uniaxial, notched and central hole tension, in-plane shear). Additionally, a novel mini-bulge testing system is presented along with a mini-punch system to provide further insight into the local material response for biaxial loading conditions. A full testing campaign comprising 60 miniature samples is performed on all sides of a prismatic aluminum casing for Li-ion battery cells. The results demonstrate significant property variations with more than 30 % higher yield stresses for the side walls as compared to the bottom section. The miniature specimens allow identifying a location-dependent anisotropy, which would otherwise be missed by standard-sized samples. Furthermore, it is shown that the anisotropic Yld2000 plasticity model and the stress-state dependent Hosford-Coulomb fracture initiation model can be fully identified using results from experiments on miniature specimens only.
Mechanical properties and microstructure analysis of laser welded hybrid parts made of additively and conventionally manufactured 1.4313 soft martensitic steel
Dey, Indira; Floeder, Raphael; Kunze, Karsten; et al. (2025)
Materials & Design
This study investigates laser welding of additively (AM) and conventionally manufactured (CM) parts, aiming to enhance cost and energy efficiency for a diverse product range. In this context, hybrid specimens combining AM/CM subparts were produced, where AM subparts were created using DED, CM parts by hot forming, and the two were joined using laser welding. The material analysed is soft martensitic stainless steel. Mechanical characterisation was performed through tensile testing and hardness measurements and microstructure characterisation through EBSD, SEM, EDS, and light microscopy. The study reveals the presence of ultra-fine grains in the heat treated laser weld segments which suggests grain subdivision due to martensite deformation. As built hybrid specimens exhibited lower toughness due to the laser welds and lower strength due to the CM segments. The weakest point after the heat treatment was the HAZ of the CM segment. The best mechanical performance was observed in homogeneously heat-treated AM specimens. Moreover, the variability in grain size were examined but did not conform grain boundary strengthening, particularly after the heat treatment. This study highlights the critical influence of microstructural variations on the mechanical properties of hybrid welds, emphasizing the need for further investigation into strengthening mechanisms and individual heat treatments.
Robust characterization of anisotropic shear fracture strains with constant triaxiality using shape optimization of torsional twin bridge specimen
Kim, Yongnam; Zhang, Shunying; Grolleau, Vincent; et al. (2021)
CIRP Annals
In-plane torsion test has been drawing attention as an effective strategy to obtain good shear properties. Herein, a framework of shape optimization for a twin bridge specimen is presented to improve the performance while ensuring better machinability. The framework includes the consideration of material anisotropy and the determination of design variables. In this study, TRIP1180 sheet was employed as a target material. The optimized design was verified by an actual in-plane torsion test. The experimental results demonstrate that the proposed framework can optimize the shape of the twin bridge for maintaining zero triaxiality up to fracture occurrence.
Plasticity and fracture of AA7075 at elevated strain rates and temperatures
Li, Xueyang; Roth, Christian; Pandya, Kedar Sanjay; et al. (2022)
IOP Conference Series: Materials Science and Engineering
The accurate description of the strain rate and temperature dependent response of Aluminium alloys is a perpetual quest in the hot forming industry. In the present study, uniaxial tension, and notched tension experiments are conducted for an aluminium AA7075-T6 sheet metal at various temperatures and strain rates. The experimental campaign covers strain rates ranging from 0.001/s to 100/s, and temperatures ranging from 20°C to 360°C. We observe low strain rate sensitivity at room temperature, with an increase in strain rate sensitivity as temperature is increased up to 360°C. An YLD2000-3D model is employed to describe the anisotropy of the material. A machine learning based hardening model is employed to capture the complex strain rate and temperature effect on the observed hardening response. Counter-example regularization is utilized to guarantee a convergence in the numeric return-mapping algorithm. Comparing the experimental force-displacement curves with the numerical predictions, the neural network model accurately describes the large deformation response of the material in the post-necking range.
In-plane Torsion Tests, Toward Large Strains Under Monotonic and Cyclic Loading of Sheet Metals
Colon, Xavier; Grolleau, Vincent; Galpin, Bertrand; et al. (2024)
Lecture Notes in Mechanical Engineering ~ Proceedings of the 14th International Conference on the Technology of Plasticity - Current Trends in the Technology of Plasticity, ICTP 2023 - Volume 4
When compared to the simple shear test, the in-plane torsion test allows for large strains to be achieved without edge effects, specimen tearing from the jaws or buckling. The proposed device offers full optical access to the specimen and enables the use of 2D Digital Image Correlation. In this way, we can observe the effects of the material anisotropy on the strain all along the circumference of the specimen up to large strains. A series of radial grooves machined on the specimen’s inner clamped surface enables the transmission of large torques necessary for high-strength steels. The paper illustrates the potential of this test for the study of plasticity with a focus on anisotropy, large deformations, and cyclic testing. Two steel sheets are considered, a deep drawing low carbon steel DC01 and a stainless steel AISI304. The paper aims to establish a direct relation between Hills’ anisotropy model parameters identified from the standard uniaxial tests and the angular evolution of the effective strain along the shear gage section of the in-plane torsion specimen.
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.
Plane strain tension fracture a t high strain rate
Adlafi, Morwan; Galpin, Bertrand; Mahéo, Laurent; et al. (2021)
EPJ Web of Conferences ~ DYMAT 2021 - 13th International Conference on the Mechanical and Physical Behaviour of Materials under Dynamic Loading
Under plane stress conditions, most micromechanical and phenomenological models predict a minimum in ductility for plane strain tension stress state. Therefore, the stress state of plane strain tension plays a crucial role in many forming and crash applications and the reliable measurement of the strain to fracture for plane strain tension is particularly crucial when calibrating modern fracture initiation models. Recently, a new experimental technique has been proposed for measuring the strain to fracture for sheet metal after proportional loading under plane strain conditions. The basic configuration of the new setup includes a dihedral punch which applies out-of-plane loading onto a Nakazima-type of discshaped specimen with two symmetric holes and an outer diameter of 60 mm. In the present work, the applicability of the test is extended to high strain rates. High strain rates of about 100/s to 200/s are obtained using a drop weight tower device with an original sensor for load measurements. Quasi static tests are also performed for comparison, keeping the same specimen geometry, image recording parameters and set-up. The effective strains at fracture are compared from quasi-static to high strain rate loading for three different materials, i.e one aluminium alloy and two steels.
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.

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Christian Roth

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