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Constitutive models dealing with the thermal and visco-plasticity of metals have seen wide applications in the automotive industry. A basic plasticity and fracture characterization of a 1.5 mm thick DP780 dual phase steel sheet based on uniaxial tensile (UT) experiments with seven distinct material orientations is complemented by low (∼0.001/s), intermediate (∼1/s) and high (∼150/s) strain rate experiments on notched tensile (NT) and shear (SH) specimens at temperatures ranging from 20 °C to 500 °C. At low strain rates, we observe a non-monotonic effect of the temperature on the force-displacement curves, with the highest curve obtained at 300 °C. Contrasting low speed tests, a monotonic effect of temperature is observed for intermediate and high strain rate experiments, with the highest curves obtained at 20 °C for both cases. Strain rate jump tests are performed proving the positive strain rate sensitivity of the steel. A machine-learning based plasticity model is developed to capture the observed complex strain rate- and temperature effect. The material is modeled as elasto-plastic, with a Hill'48 yield surface and a non-associated flow rule. The flow resistance is decoupled into a reference strain hardening term and a neural network term, which is a function of the plastic strain, strain rate, temperature, and an additional dynamic strain aging variable. The plasticity model is implemented into a material user subroutine and identified using a counterexample-guided hybrid experimental-numerical approach. The extracted loading paths reveal a complex rate and temperature effect on the ductility of DP780. A neural network based fracture initiation model is therefore adopted to describe the fracture onset across various stress states, strain rates and temperatures considered., International Journal of Plasticity, 151, ISSN:0749-6419, ISSN:1879-2154

The triply periodic minimal surface design (TPMS, Schwarz Primitive) is utilized to numerically investigate lattices with sandwich cell walls of constant macroscopic density. The sandwich construction of the cell walls involves a polymer (core material) and a ceramic material (skin). The ceramic (alumina) weight fraction in the sandwich construction is varied such that for each density a range of weight fractions from 0%wt (monolithic polymer) to 100%wt (monolithic alumina lattice) is under investigation in terms of the Young's modulus, peak stress and specific energy absorption. It is found that the specific energy absorption of sandwich TPMS lattices is higher compared to monolithic polymer or alumina TPMS lattices of equal density (42 kg m−3) with the 40%wt alumina lattice providing the highest specific energy absorption. At higher densities, the optimal energy absorption is obtained for 10% and 80%wt alumina lattices. Selected lattices of 10%, 30% and 40%wt alumina are fabricated via two-photon lithography and atomic layer deposition and are tested in situ under uniaxial compression. Utilizing the results of this study, design maps are constructed to provide guidelines fulfilling application-related requirements., Extreme Mechanics Letters, 53, ISSN:2352-4316

The dynamic perforation of 3 mm thick target plates extracted from ultra-high strength steel (Mars® 300) is investigated experimentally and computationally. Projectiles of 8 mm diameter are also extracted from Mars® 300 plates featuring blunt, conical and hemispherical tips. Using a single stage gas gun, impact experiments are performed with projectile velocities of up to 380 m/s. A minimum impact velocity of about 240 m/s (ballistic limit) was required to perforate the armor steel targets with conical and hemispherical projectiles of a mass of about 14 g. In all successful perforation experiments, the plate targets failed through plugging. At the same time, the conical and blunt projectiles are heavily deformed in a way that their final tip shape resembles that of the hemispherical projectiles. The latter turned out to be slightly more efficient than conical ones in the sense that their exit velocities are approximately 10% higher. Numerical simulations are performed of all impact experiments using the finite element software LS-DYNA. The plasticity model made use of a quadratic yield function with non-associated flow rule, a Swift-Voce strain hardening law and Johnson–Cook type of multipliers accounting for the effects of strain rate and temperature. The stress-triaxiality, Lode angle parameter and strain-rate dependent Hosford-Coulomb fracture initiation model is employed to predict the ductile failure of the Mars 300 steel. Overall, the simulation results are in good agreement with the experimental observations, including the mode of perforation (shear plugging) and the projectile exit velocities. The numerical simulations also show that a ring-like band of localized plastic deformation forms inside the targets with temperatures of up to 700 °C. Fracture initiates from the back face of the plate under biaxial tension, while the cracks propagate through the band of localized plastic deformation towards the impacted front face of the plate, thereby forming a plug as the projectile passes through the target.

Plate-lattices are a new emerging class of isotropic cellular solids that attain the theoretical limits for the stiffness of porous materials. For the same mass, they are significantly stiffer than random foams or optimal truss-lattice structures. Plate-lattice structures of cubic symmetry are fabricated from stainless steel 316L through selective laser melting. A special direct impact Hopkinson bar system is employed to perform dynamic compression experiments at strain rates of about 500/s. In addition, tensile specimens are manufactured for characterizing the stress–strain response of the additively-manufactured cell wall material for strain rates ranging from 10−3 to 103/s. The results show that plate-lattices of a relative density of 23% crush progressively when subject to large strain compression. Their specific energy absorption increases by about 8% when increasing the applied strain rate from 0.001 to 500/s, which is primarily attributed to the strain rate sensitivity of the base material. Good quantitative and qualitative agreement between the experiments and the simulations is observed when using a detailed finite element model of the plate structures in conjunction with a modified Johnson–Cook model. The comparison of the simulation results for plate- and truss-lattices of the equal-density reveal a 45% increase in specific energy absorption. Compression experiments on Ti–6Al–4V lattices revealed a low energy absorption due to the early fracture of the additively-manufactured cell wall material.

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.

Slow, intermediate and high strain rate experiments are carried out on flat smooth and notched tensile specimens extracted from dual phase steel sheets. A split Hopkinson pressure bar testing system with load inversion device is utilized to achieve high strain rates. Temperature dependent experiments ranging from 20 °C to 300 °C are performed at quasi-static strain rates. The experimental results reveal a non-monotonic effect of the temperature on the stress-strain curve for DP800 steel. For the temperatures considered, the lowest and highest curves are obtained for 120 °C and 300 °C, respectively. A modified Johnson-Cook plasticity model is developed to capture the observed unconventional effect of the strain rate- and temperature on the hardening response. It includes a von Mises yield surface, a non-associated Hill’48 flow rule, a Swift-Voce reference stress-strain curve describing the rate-independent behavior at room temperature, and a neural network function depending on the plastic strain, strain rate and temperature. This model is implemented into a material user subroutine and identified using a combination of analytical formulas along with a resilient back propagation algorithm. It is found that the obtained machine-learning based Johnson-Cook plasticity model can describe all experimental data with high accuracy, including both force-displacement and local surface strain measurements. Using a hybrid experimental-numerical approach, the loading paths to fracture are determined. It is found that the temperature not only affects the plasticity of the DP800 steel in a non-monotonic manner, but also its strain to fracture, with ductility loss of up to 30% when increasing the temperature from 20 to 120 °C, followed by a gain in ductility when increasing the temperature further to 300 °C.

Within the scope of the third Sandia Fracture Challenge the plasticity and ductile fracture behavior of an additively manufactured 316L stainless steel tensile specimen containing through holes and internal cavities is predicted in a blind round robin format. Only a limited number of experimental results, including flat dogbone-shaped and double-notch tension specimens, as well as EBSD maps of the Challenge geometry are provided by Sandia National Laboratory. A non-associated Hill’48 plasticity model with Swift-Voce strain hardening and Johnson–Cook strain rate hardening is used to accurately describe the large deformation response of the material. A special case of the recently developed Hosford–Coulomb model is used to predict fracture initiation and propagation by crack re-initiation. Very good qualitative and quantitative agreement of the blind prediction with the experimental results is obtained for both global force-displacement responses as well as the local surface strain evolution throughout the test. In a post challenge follow-up study, the role of the plasticity model is evaluated, focusing on the effect of the anisotropy and the strain-rate on the material response. Aside from considering the deterministic model, the statistical material properties of the additively manufactured structure are analyzed by defining a heterogeneous random media model. Probabilistic material properties for both plasticity and fracture are assigned to each element of the Challenge specimen. As an alternative, the role of intrinsic porosities is analyzed by randomly deleting 1% of the pristine geometry. The results of both approaches show that the presence of homogeneities follows a more realistic description of the material behavior, especially in the crack propagation regime post maximum force and when looking at local strains.

A new experimental technique is proposed for measuring the strain to fracture for sheet metal after proportional loading under plane strain conditions. The proposed technique makes use of a mini-Nakazima specimen that is clamped onto a 30 mm diameter die and subjected to out-of-plane loading through a dihedral punch. While other techniques for determining the strain to fracture for plane strain tension loading (e.g. notched tension or V-bending) suffer from limitations with regards to the thickness and ductility of the material to be characterized, the mini-Nakazima experiments are more robust and universally applicable. Experiments are performed on mini-Nakazima, notched tension and V-bending specimens extracted from 1.2 mm thick aluminum 2024-T351, 0.8 mm thick DP450 and 1.6 mm thick DP980 steel. In addition, detailed numerical simulations are performed for each experiment. The hybrid experimental-numerical results show the limitations of existing experimental techniques and demonstrate the reliability of the proposed stretch-bending technique. As a by-product, it is shown that the Yld2000-3D yield function with associated flow rule provides a reasonably accurate description of the large deformation response of aluminum 2024-T351. Equally good predictions are obtained for the DP450 and DP980 steels when using a von Mises yield function in conjunction with a non-associated Hill’48 flow rule. Furthermore, to characterize the effects of the stress triaxiality and the Lode angle parameter on the fracture response of the above materials, the strains to fracture are determined for simple shear and equi-biaxial tension.

There is a particularly high degree of experimental uncertainty associated with the determination of the strain to fracture for simple shear. In most shear specimens, different stress states coexist and it is usually impossible to prove experimentally that fracture indeed initiated in the sheared zones. Peirs’ simple shear specimen is analyzed in detail for nine generic materials covering different combinations from low to high strain hardening and from low ductility to high ductility. It is found that the validity of the shear fracture strain measurement depends on the strain hardening capability and the ductility of the material being tested, with a higher likelihood of validity for low hardening and low ductility materials. An extensive parametric study evaluating more than 600 distinct specimen geometries is performed in an attempt to identify a universal specimen geometry. It turns out that no single geometry exists which provides reliable measurements for all types of materials. While a single geometry can be found for different hardening behaviors, it seems to be necessary to use several distinct geometries to cover all levels of ductility. Since the ductility is usually not known prior to testing, we recommend testing three different types of specimen per material, reporting the highest measured strain among all specimens as strain to fracture for simple shear. Experiments are performed on specimens extracted from aluminum AA2024-T351, as well as DP800 and DP980 dual phase steels to demonstrate the validity of the proposed experimental methodology for identifying the strain to fracture for simple shear.

Slow, intermediate and high strain rate experiments with UT geometries are performed on aluminum AA7075-T6 sheet metal at various temperatures. The comprehensive experimental program characterizes the plasticity response at temperatures ranging from 20°C to 360°C and at strain rates ranging from 0.001/s to 150/s. The elevated temperature - elevated strain rate experiments are performed on a hydraulic tensile testing machine and a Split Hopkinson Pressure Bar system with a Load Inversion Device along with a custom-made induction heating system. A machine learning based modified Johnson-Cook plasticity model is calibrated to capture the complex strain rate and temperature effect of the observed hardening response., EPJ Web of Conferences, 250, ISSN:2100-014X, ISSN:2101-6275, 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 (

Both additively-manufactured and cast metals are known to exhibit stochastic mechanical properties at the macroscopic level. Using a robot-assisted mechanical testing system, more than 360 experiments are performed on flat specimens extracted from AlSi10Mg components that are either cast or made through laser powder bed fusion (LPBF), commonly known as selective laser melting (SLM). Aside from basic EBSD analysis of the respective microstructures, micro-computed tomography is performed revealing a significantly higher porosity and pore size for the cast material. The results from uniaxial tension experiments reveal a 10% higher yield strength (on average) and an about 20% higher ultimate tensile strength for the SLM made AlSi10Mg. The tracking of the specimen origin within the SLM component shows a clear location dependence of the observed hardening response on the build height. The shear and tension fracture experiments revealed a strong stress-state dependence and significantly higher fracture strains for the cast material as compared to its SLM-made counterpart. To facilitate the computer aided engineering of structures with additively-manufactured AlSiMg alloys, a build height dependent hardening model is proposed along with a probabilistic plasticity and fracture modeling framework., Additive Manufacturing, 43, ISSN:2214-8604

A novel experimental set-up is proposed to perform in-plane torsion experiments on grooved disk specimens. As compared to previously proposed systems, an enhanced clamping system is introduced to enable the torsion testing of high strength materials such as dual phase steels. The clamping is achieved without applying the clamping pressure through an axial actuator. Instead a custom-made washer-nut system is used which allows for the monitoring of the entire specimen front surface with cameras. Experiments are performed on three different steels (DC01, DP980, AISI301) to demonstrate the validity of the technique for measuring the stress-strain curve for strains of up to 2, for strain rate jump testing, for cyclic loading and for fracture testing. In particular, the demonstration experiments elucidate the importance of full field strain measurements during in-plane torsion. The strain fields vary along the gage section circumference in a periodic manner that is related to the anisotropy of the tested material. This important observation is confirmed though a finite element study covering a wide range of anisotropic Hill’48 materials with non-associated plastic flow.

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., Mechanics of Materials, 149, ISSN:0167-6636, ISSN:1872-7743, 149

Mars® 300 is an ultrahigh hard armor (UHHA) martensitic steel designed for ballistic protection. It is available in sheets of different thicknesses and also as perforated plates with a periodic pattern of cylindrical holes. Installed as an add-on layer in front of a main armor, its aim is to cause deflection or fragmentation of small-caliber projectiles. In the present study, the impact process is investigated with a hybrid experimental-numerical approach. Ductile fracture experiments are carried out at different stress states, strain rates and temperatures on a range of flat Mars® 300 steel specimens to identify the plasticity and fracture response. The plasticity model is composed of von Mises yield surface, a non-associated anisotropic flow rule, a combined Swift–Voce strain hardening law and a Johnson–Cook type of rate and temperature-dependency. To predict the onset of fracture, a stress state and strain rate-dependent Hosford–Coulomb fracture initiation model is used. Impact experiments are performed on targets of homogenous and perforated Mars® 300 plates by accelerating cylindrical Mars® 300 projectiles in a single-stage gas gun. Depending on the impact location, three different failure mechanisms are identified for the perforated plate. Subsequently, finite element simulations using the calibrated material model are carried out to thoroughly analyze the impact experiments. A very good prediction of the different impact cases and their fracture patterns is obtained, validating the applicability of the plasticity and the fracture model for impact loadings.

Correctly determining the onset of fracture is crucial when performing mechanical experiments. Commonly carried out by visual inspection, here an image-based machine learning approach is proposed to classify cracked and un-cracked specimens. It yields the potential to objectify and automate crack detection, thereby removing sources of uncertainty and error from the post-processing of experiments. More than 30’000 speckle-pattern images obtained from 77 experiments on three specimen geometries are evaluated. They comprise uniaxial tension, notched tension as well as axisymmetric V-bending experiments. Statistical texture features are extracted from all images. They include both first-order (variance, skewness, kurtosis) and higher-order statistical texture features, i.e. Haralick features. The discriminatory power of the texture information is evaluated based on the Fisher's Discriminant Ratio and feature correlations are identified and quantified. Image texture feature subsets of high discriminatory power are used to parse neural network architectures of different complexities from simple perceptron to feed-forward and cascade neural networks. It is found that a small subset of the investigated texture features is highly significant for all experiments. Using this feature subset in conjunction with multi-layer, non-linear and low complexity feed-forward network architectures classification accuracies in the order of 99% are obtained. At the same time, it is shown that linear classifiers are not sufficient to robustly distinguish the state of the specimens, even when high discriminatory power features are used.Graphical Abstract: [Formula presented], International Journal of Mechanical Sciences, 209, ISSN:0020-7403

Binder-jetting is an additive manufacturing process for metals which involves the rapid making of a binder bonded metal powder part followed by the removal of the binder and creation of a fully dense part in a subsequent sintering step. This emerging process has the potential of high cost efficiency as compared to selective laser melting (SLM) or shaped metal deposition. Here, we determine the rate-dependent plasticity and fracture properties of binder-jetted stainless steel 316L. Electron Back-Scattered Diffraction (EBSD) analysis and tomography revealed an equi-axed grain structure with an initial porosity of 3%. The latter is due to pores with an average size of 20μm that are clustered in layers perpendicular to the build direction. The observed stress-strain curve of the binder-jetted material always lies below that of wrought stainless steel 316L with a 50% lower initial yield stress and a 30% lower ultimate strength. Both the material anisotropy and strain rate sensitivity may be considered as second order effects in that comparison. The equivalent plastic strain obtained from fracture experiments for different stress states is also always lower for the binder-jetted material. The observations for the binder-jetted material therefore stand in stark contrast to those for SLM-made stainless steel which can provide an even higher yield strength than the wrought material. From a mechanism point of view, the low mechanical properties of the binder-jetted material may be explained by the high initial porosity which is reminiscent of cast metals., International Journal of Mechanical Sciences, 207, ISSN:0020-7403

A plane-strain-tension stress state leads to a minimum in ductility in most micro-mechanical and phenomenological fracture models as well as Forming Limit Curves. Hence, this stress state plays a crucial role in many applications and a reliable measurement of the strain to fracture under plane strain tension is of particular importance 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 novel setup includes a dihedral punch applying out-of-plane loading onto a Nakazima-type disc-shaped specimen with two symmetric circular cut-outs. 3D Digital Image Correlation (DIC) is used to measure the surface strains of the specimen up to fracture. In contrast to the widely used V-Bending test, the maximum obtainable strain is not limited when using this set up, and fracture will always initiate after proportional loading in a plane strain tension stress state. In the present study, a major simplification of the testing methodology is proposed, reducing from 3D DIC to a simple 2D DIC method for measuring the fracture strain. Comparisons are presented on three metals, a 1.5 mm thick DP600 steel, a 0.8 mm thick DP450 steel and a 1.2 mm thick AA2024 aluminum alloy., IOP Conference Series: Materials Science and Engineering, 1157, ISSN:1757-8981, ISSN:1757-899X

For calibrating sheet metal hardening and anisotropic constitutive models, the bulge test has proven to be an invaluable tool. In recent studies, the analysis of the test has been considerably improved by using Digital Image Correlation. Despite the progress achieved in the analysis of the measured data, based on membrane hypotheses it is still necessary to use large bulge diameter to sheet thickness ratios of more than 100 in order to ensure the validity of the standard equations. In order to overcome this constraint, a numerical study of a wide range of specimen material behaviors and various bulge geometries is performed. The numerical results obtained are used to draw up new estimates for the bending strain, the radii of curvature and the average bulge stress. The numerical and experimental data show that the inner and the outer radii of curvature do not share the same center. This means that the difference between outer and inner radii of curvature is not equal to the thickness of the apex. The accuracy of the approach presented here is assessed on the basis of numerical simulations and experiments. Numerical simulations involve six different types of Swift and Voce behavior. Experimental validation is performed from original bending and curvature measurements on aluminum and steel bulges. Lastly, a hardening law identification algorithm is presented and compared with experimental results obtained for bulge diameters as small as 50 mm and bulge size ratios ranging from 42 to 150.

High strength 7xxx series aluminum alloys are being increasingly considered for automotive applications due to their superior specific strength compared to 6xxx series alloys. Complex structural components made from 7xxx series alloys need to be manufactured through warm forming or hot stamping due to their low ductility at room temperature. For hot stamping, the AA7075 blanks are heat treated to above 480°C, yielding a supersaturated solid solution, corresponding to a W temper state. The preheated blanks are then simultaneously formed and rapidly quenched using cooled dies. The entire forming process takes place in the W temper condition. The stamped parts are further subject to an artificial aging heat treatment to achieve the desired temper through precipitation hardening. Therefore, understanding the strain rate and temperature dependent plasticity behavior of AA7075-W is critical to establish optimum design parameters for the hot stamping process. The present work is concerned with the strain rate and temperature dependent plastic response of AA7075 in the W temper state. In particular, the results from a comprehensive experimental program on uniaxial tension specimens are presented. High temperature experiments are carried out at strain rates ranging from 0.001-2/s in a universal testing machine. An induction heating system with infrared high speed imaging is used to span the entire temperature range from 180°C to 480°C, over the full range of strain rates considered. Based on the experimental results, a candidate plasticity model with an empirical mixed Swift-Voce hardening law and a Johnson-Cook type strain rate and temperature dependence is calibrated., IOP Conference Series: Materials Science and Engineering, 651 (1), ISSN:1757-8981, ISSN:1757-899X

The stress state of plane strain tension plays a crucial role in many forming and crash applications. Under plane stress conditions (in thickness direction), most micromechanical and phenomenological models predict a minimum in ductility for a plane strain tension loading (in the plane of the sheet). When looking at Forming Limit Diagrams (FLD) or at modern stress state dependent fracture initiation models, a "plane strain ductility valley" exists between uniaxial and equi-biaxial tension. Given that the ductility reaches a minimum for plane strain tension, the reliable measurement of the strain to fracture for plane strain tension is particularly crucial when calibrating modern fracture initiation models. Many experimental techniques have been proposed in the past, but a standardized universal experimental technique for plane strain tension testing is still missing to date. It is the goal of the present work to develop a robust experimental technique for determining the strain to fracture for plane strain tension. Emphasis is placed on finding a technique that is universally applicable i.e. that provides reliable results irrespective of the material thickness or ductility. In addition, it should be easily automatized in order to be included in up-to-date test protocol dedicated to the prediction of the behavior from large database analysis. The experiments are designed such that the material is subject to proportional loading, i.e. the stress state remains constant throughout the entire loading history until fracture initiates. After identifying a suitable specimen geometry through finite element simulations, experiments are performed on specimens extracted from aluminum alloys and steels sheets. The experimental campaign includes three different types of plane strain tension experiments (flat notched tension, V-bending and the newly-proposed stretch-bending of mini-Nakazima specimens) to elucidate their differences and limitations, and to demonstrate that the newly-proposed technique is the only one that yields meaningful results for all three materials., IOP Conference Series: Materials Science and Engineering, 651 (1), ISSN:1757-8981, ISSN:1757-899X

The mechanisms leading to fracture of aluminum alloy AA2024-T3 under shear loading are investigated via X-ray synchrotron laminography. A 1mm-thick flat double-gage section shear specimen is loaded inside a synchrotron X-ray beamline. The microscale defects population is reconstructed on six loading steps as well as on the broken specimen. The material exhibits an initial void volume fraction of 0.7% as well as a high concentration of large Cu-rich intermetallic particles. Using 2D Digital Image Correlation of projected volume data, based on the void contrast, it is possible to track the evolution of both types of mesoscale defects throughout the loading and to correlate the deformation mechanisms with the local strains. Upon mechanical loading, pre-existing voids rotate and elongate following the deformation of the aluminum matrix. The intermetallic particles fail at early loading stages in a brittle manner, leading to the nucleation of voids normal to the maximum principal stress direction. The newly-created voids continue to grow during the subsequent loading steps impeded by the fragmented particles, eventually forming micro-cracks. A fracture mechanism is proposed based on these observations and assessed with a representative volume element simulation, pointing towards the formation of a shear localization band during the final loading step., Acta Materialia, 205, ISSN:1359-6454

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.

Predicting the failure of spot welds remains one of the most important modeling challenges in automotive engineering. In essence, the structure surrounding a spot weld corresponds to a graded material with severe variations in the plasticity and fracture properties as a function of the distance to the center of the weld nugget. While it is common practice to identify hardness variations around a spot weld, a newly developed micro-tensile testing technique is used to characterize spatial variations in both strain hardening and fracture strains. Furthermore, a new experimental device is presented to subject spot welds to combined tension, shear and bending loads. Following a careful experimental characterization of resistance spot welds connecting two 1.5 mm thick DP600 sheets, a detailed finite element model is built which accounts for the identified property gradients. In addition, a simplified surrogate model for use in conjunction with coarse shell element meshes is also calibrated. Simulations are performed of all structural experiments to assess the range of validity of the detailed and simplified FE models. It is found that the detailed model captures the experimentally observed failure modes including changes in the location of fracture initiation as a function of the loading, while the simplified model is able to predict the spot weld strength with reasonable accuracy.

Complex structural components made from 7xxx series alloys are usually manufactured through hot stamping due to their low ductility at room temperature. With the help of a custom-made induction heating system, experiments are performed on aluminum alloy 7075 in its W-temper. The comprehensive experimental program is executed comprising more than 100 specimens to characterize the plasticity and fracture response at temperatures ranging from 180 to 480 °C and strain rates ranging from 0.001 to 2/s. Stress states ranging from simple shear to biaxial tension are obtained through the use of planar shear specimens and tensile specimens with central holes and various notches. Based on the experimental results, an attempt is made to calibrate an enhanced Johnson-Cook type of constitutive model before employing an isotropic machine learning based constitutive model for hybrid experimental-numerical analysis of the fracture experiments. The extracted loading paths to fracture reveal a negative strain rate effect and a non-monotonic effect of temperature on the ductility of AA7075-W. An isotropic neural network based fracture initiation model is proposed, trained and validated to describe the onset of fracture across the range of stress states, strain rates and temperatures considered.

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. ISSN:0734-743x ISSN:1879-3509

Basic ductile fracture experiments for sheet metal (or flat coupons extracted from bulk material) are presented to characterize the onset of fracture at different stress states. Special emphasis is placed on designing the experiments such that the stress triaxiality and the Lode angle parameter remain constant while the specimen is loaded all the way to fracture. A new in-plane specimen with two parallel gage sections is proposed to determine the strain to fracture for approximately zero stress triaxiality. A FEA based methodology is shown to identify the optimal specimen geometry as a function of the material's ductility and strain hardening. A tension specimen with a central hole is investigated in detail with regard to determining the strain to fracture for uniaxial tension. It is found that the required hole-to-ligament width ratio decreases as a function of the material ductility and increases as a function of the strain hardening exponent. The bending of a wide strip is pursued to prevent the necking prior to fracture under plane strain tension conditions, while an Erichsen-type of punch test is used to characterize the material response for equi-biaxial tension. It is worth noting that the strain to fracture can be directly determined from surface strain measurements in the cases of shear, plane strain tension and equi-biaxial tension loading, thereby removing the need to perform finite element simulations for extracting the loading path to fracture.

The large deformation response of Ti–6Al–4V parts made through additive layer manufacturing is investigated. A wire-feed process is chosen instead of a powder process in an attempt to reduce the oxide contaminations of the final part. The experimental program includes uniaxial tension experiments along different part directions and fracture experiments on flat specimens with cut-outs covering stress states ranging from pure shear to equi-biaxial tension. More than 100 experiments are performed in total to characterize the randomness in the material's fracture response. It is found that the stress-strain response of the ALM material is comparable to that of Ti–6Al–4V sheet stock, while its average ductility is substantially lower. For example, for pure shear loading, the average strain to fracture for the ALM material is 0.47, while the mill product of the same alloy failed at a strain of 0.65. A probabilistic extension of the stress state dependent Hosford–Coulomb fracture initiation model is proposed to account for the significant standard deviation in the identified strains to fracture. Microscopic and surface strain field analysis demonstrate that the initiation and propagation of ductile fracture in the ALM material is strongly affected by the presence of prior-beta grains.

In the context of the second Sandia Fracture Challenge, dynamic tensile experiments performed on a Ti–6Al–4V alloy with a complex fracture specimen geometry are modeled numerically. Sandia National Laboratories provided the participants with limited experimental data, comprising of uniaxial tensile test and V-notched rail shear test results. To model the material behavior up to large plastic strains, the flow stress is described with a linear combination of Swift and Voce strain hardening laws in conjunction with the inverse method. The effect of the strain rate and temperature is incorporated through the Johnson–Cook strain rate hardening and temperature softening functions. A strain rate dependent weighting function is used to compute the fraction of incremental plastic work converted to heat. The Hill’48 anisotropic yield function is adopted to capture weak deformation resistance under in-plane pure shear stress. Fracture initiation is predicted by the recently developed strain rate dependent Hosford–Coulomb fracture criterion. The calibration procedure is described in detail, and a good agreement between the blind prediction and the experiments at two different speeds is obtained for both the crack path and the force–crack opening displacement (COD) curve. A comprehensive experimental and numerical follow-up study on leftover material is conducted, and plasticity and fracture parameters are carefully re-calibrated. A more elaborate modeling approach using a non-associated flow rule is pursued, and the fracture locus of the Ti–6Al–4V is clearly identified by means of four different fracture specimens covering a wide range of stress states and strain rates. With the full characterization, a noticeable improvement in the force–COD curve is obtained. In addition, the effect of friction is studied numerically.

When a long slender structure such as a wire, strut or rope is loaded under tension all the way to fracture, upon failure an elastic unloading wave propagates down the wire and reflects as a compressive wave from the distant clamp end. The compressive effect of dynamic pulse buckling, amplitude and wave length depend on material properties and geometry features of the problem. Small scale experiments were performed on thin stainless steel wires of 0.127 mm diameter and varying lengths. The theory of dynamic pulse buckling, developed in the 1980s at the Stanford Research Institute, is used to study the initial phase of the deformation of the wire. In order to obtain a deeper insight in the mechanism of structural collapse, numerical simulations and high speed photography on small scale tests were employed. Both techniques revealed propagation of the bending disturbance (with increasing amplitude) and formation of the periodic helical shape wave with varying amplitude in space. A considerable axial momentum brings the wire into a complex process of elastic–plastic deformation with strain reversal. The numerical simulation was compared to high speed snapshots, showing good agreement. The implication of the presented results to structural integrity of deep water installations is also discussed.

International audience; The nucleation, evolution and coalescence of voids is the well-established mechanism leading to ductile fracture under tension-dominated loading conditions. From a theoretical point of view, the same mechanism also applies to shear-dominated loading conditions. Here, an attempt is made to provide for the first time tomographic evidence of damage nucleation and evolution under shear-dominated loading in a modern engineering material. Monotonic experiments are performed on a flat double gage section smiley-shear specimen on the laminography stage of a synchrotron X-ray line. Based on fifteen scans of the entire gage section μm3, the mesostructural evolution inside a ferrite-bainite steel (FB600) is imaged in 3D up to the instant of specimen fracture. It is found that the as-received material includes a volume fraction of about 0.015% CaO particles. Upon mechanical loading at stress triaxialities evolving from 0 to 0.3, the ductile matrix detaches from these second phase particles, creating a prolate void space whose principal axis is aligned with the principal direction of the applied macroscopic field of deformation. The void space continues to grow while developing micro-crack like features. A large deformation analysis is performed on a representative volume element of the particle-matrix mesostructure replicating the experimental observations of void growth in an approximate manner. Furthermore, the simulation results suggest that a porosity as low as 0.05% is already sufficient to cause the ductile failure under shear-dominated loading through the formation of a band of localized plastic deformation at the mesoscale.

In-plane shear specimens typically feature free gage section boundaries along which the state of stress deviates from that of pure or simple shear. As a consequence, the geometry of in-plane shear specimens needs to be carefully chosen to avoid any early fracture initiation from the free boundaries, before the actual failure strain for shear is reached at the specimen center. From this perspective, disc specimens for in-plane torsion experiments offer a significant advantage: they do not feature any free boundaries. However, detailed analysis suggests that circular groove need to be introduced (local thickness reduction) to ensure a strain localization away from the clamped specimen shoulders. In most existing in-plane torsion tests, the specimen is clamped on the inner diameter by applying out-of-plane compression to avoid any slipping. In such configurations, it is impossible to monitor the entire sheared circumference with cameras for digital image correlation. It is the goal of the present work to develop in-plane torsion test using grooved specimens with full optical access to the specimen for 2D or 3D DIC measurements. Furthermore, the experimental set-up will be designed for plasticity and fracture characterization at strain rates of up a few 100/s. Its main feature is a new clamping technique. After identifying a suitable specimen geometry through finite element simulations, experiments are performed on specimens extracted from aluminum alloys and steels sheets. The experimental campaign includes proportional loading, reversed loading and strain rate jumps. The full optical access to the sheared gage section area also enables the discussion of the effects of plastic anisotropy on the strain fields in in-plane torsion experiments. The results from the in-plane torsion experiments are also compared with the fracture strain measurements from in-plane shear experiments performed in a conventional uniaxial loading frame., IOP Conference Series: Materials Science and Engineering, 651 (1), ISSN:1757-8981, ISSN:1757-899X

Ductile fracture experiments are carried out at different stress states, strain rates and temperatures on a range of flat Mars 300 steel specimens to calibrate both a plasticity and a fracture model. To predict the onset of fracture a stress state and strain rate-dependent Hosford–Coulomb fracture initiation model is used. Single material impact experiments are performed on targets of homogenous and perforated Mars 300 plates by accelerating cylindrical Mars 300 impactors in a single-stage gas gun. It is shown that the chosen modeling approach allows accurate modeling of the plastic response as well as the fracture patterns., EPJ Web of Conferences, 183, ISSN:2100-014X, ISSN:2101-6275, DYMAT 2018 - 12th International Conference on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, ISBN:978-2-7598-9053-8

Among all other stress states achievable under plane stress conditions, the lowest ductility is consistently observed for plane strain tension. For static loading conditions, V-bending of small sheet coupons is the most reliable way of characterising the strain to fracture for plane strain tension. Different from conventional notched tension specimens, necking is suppressed during V-bending which results in a remarkably constant stress state all the way until fracture initiation. The present DYMAT talk is concerned with the extension of the V-bending technique from low to high strain rate experiments. A new technique is designed with the help of finite element simulations. It makes use of modified Nakazima specimens that are subjected to V-bending. Irrespective of the loading velocity, plane strain tension conditions are maintained throughout the entire loading history up to fracture initiation. Experiments are performed on specimens extracted from aluminum 2024-T3 and dual phase DP450 steel sheets. The experimental program includes quasi static loading conditions which are achieved on a universal testing machine. In addition, high strain rate experiments are performed using a specially-designed drop tower system. In all experiments, images are acquired with two cameras to determine the surface strain history through stereo Digital Image Correlation (DIC). The experimental observations are discussed in detail and also compared with the numerical simulations to validate the proposed experimental technique, EPJ Web of Conferences, 183 (), ISSN:2100-014X, ISSN:2101-6275

Plasticity and fracture experiments are carried out on flat smooth and notched tensile specimens extracted from DP800 steel sheets. A split Hopkinson pressure bar testing system equipped with a load inversion device is utilized to reach high strain rates. Temperature dependent experiments ranging from 20°C to 300°C are performed at quasi-static strain rates. The material exposes a monotonic strain hardening behaviour with a non-monotonic temperature dependency. The rate-independent material behaviour at room-temperature is described with a non-associated Hill’48 plasticity model and an Swift-Voce strain hardening. A machine learning based model is used multiplicatively to capture the rate and temperature responses. A good agreement between measured and simulated force-displacement curves as well as local surface is obtained. The loading paths to fracture are then extracted to facilitate further development of a temperature dependent fracture initiation model., EPJ Web of Conferences, 183, ISSN:2100-014X, ISSN:2101-6275, DYMAT 2018 - 12th International Conference on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, ISBN:978-2-7598-9053-8

A new set-up is proposed to perform high strain rate tension experiments on sheet metal using a compression Hopkinson bar system on the input side. With the help of a custom-made load inversion device, the compression loading pulse is converted into tensile loading of the specimen boundary. A tensile output bar is used to measure the tensile force acting on the specimen boundaries. A high speed camera system is employed to measure the displacement history at the specimen level through planar digital image correlation. The output bar is positioned on top of the input bar. As a result, the valid experiment duration of the proposed system is twice as long as that of conventional Kolsky systems. It therefore facilitates the execution of intermediate strain rate (~100/s) experiments without increasing total system length. Numerical simulations are carried out to assess the effect of spurious bending effects that are introduced through the eccentricity of the input and output bar axes. In addition, experiments are performed on straight and notched specimens to demonstrate the characterization of the rate dependent plasticity and fracture properties of a 1.06 mm thick DP780 steel.

Low, intermediate and high strain rate tensile experiments are carried out on flat smooth, notched and central-hole tensile specimens extracted from advanced high strength steel sheets. A split Hopkinson pressure bar testing system is used in conjunction with a load inversion device to perform the high strain rate tension experiments. Selected surface strains, as well as local displacements, are measured using high speed photography in conjunction with planar digital image correlation (video extensometer). Through thickness necking precedes fracture in all experiments. A hybrid experimental–numerical approach is therefore employed to determine the strain to fracture inside the neck. To obtain an accurate description of the local strain fields at very large deformations, a plasticity model with a Johnson–Cook type of rate and temperature-dependency and a combined Swift–Voce strain hardening law is used in conjunction with a non-associated anisotropic flow rule. The incremental change in temperature is computed using a strain rate dependent weighting function instead of solving the thermal field equations. The comparison of the computed and measured force–displacement curves and surface strain histories shows good agreement before and after the onset of necking. From each experiment, the loading path to fracture is determined describing the evolution of the equivalent plastic strain in terms of the stress triaxiality, Lode angle parameter, strain rate and temperature. An empirical extension of the stress-state dependent Hosford–Coulomb fracture initiation model is proposed to account for the effect of strain rate on the onset of ductile fracture. The model is subsequently calibrated and successfully validated using the results from fracture experiments on DP590 and TRIP780 steels.

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., EPJ Web of Conferences, 94, ISSN:2100-014X, ISSN:2101-6275, ISBN:978-2-7598-1817-4

The Hosford-Coulomb model incorporates the important effect of the Lode angle parameter in addition to the stress triaxiality to predict the initiation of ductile fracture. A strain-rate dependent extension of the Hosford-Coulomb model is presented to describe the results from low, intermediate and high strain rate fracture experiments on advanced high strength steels (DP590 and TRIP780). The model predictions agree well with the experimental observation of an increase in ductility as function of strain rate for stress states ranging from uniaxial to equi-biaxial tension., EPJ Web of Conferences, 94, ISSN:2100-014X, ISSN:2101-6275, ISBN:978-2-7598-1817-4

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., EPJ Web of Conferences, 94, ISSN:2100-014X, ISSN:2101-6275, ISBN:978-2-7598-1817-4

Rubredoxins (Rds) are essential electron transfer components of bacterial membrane-bound alkane hydroxylase systems. Several Rd genes associated with alkane hydroxylase or Rd reductase genes were cloned from gram-positive and gram-negative organisms able to grow on n- alkanes (Alk-Rds). Complementation tests in an Escherichia coli recombinant containing all Pseudomonas putida GPo1 genes necessary for growth on alkanes except Rd 2 (AlkG) and sequence comparisons showed that the Alk-Rds can be divided in AlkG1- and AlkG2-type Rds. All alkane-degrading strains contain AlkG2-type Rds, which are able to replace the GPo1 Rd 2 in n- octane hydroxylation. Most strains also contain AlkG1-type Rds, which do not complement the deletion mutant but are highly conserved among gram-positive and gram-negative bacteria. Common to most Rds are the two iron-binding CXXCG motifs. All Alk-Rds possess four negatively charged residues that are not conserved in other Rds. The AlkG1-type Rds can be distinguished from the AlkG2-type Rds by the insertion of an arginine downstream of the second CXXCG motif. In addition, the glycines in the two CXXCG motifs are usually replaced by other amino acids. Mutagenesis of residues conserved in either the AlkG1- or the AlkG2-type Rds, but not between both types, shows that AlkG1 is unable to transfer electrons to the alkane hydroxylase mainly due to the insertion of the arginine, whereas the exchange of the glycines in the two CXXCG motifs only has a limited effect.