Development of a through-process simulation workflow for spiral pipe forming including evolution of texture and dislocation substructure

Spiral forming is a widely used industrial method for the manufacture of large diameter welded pipes from levelled steel strip. However, the multi-step helical forming process and post-treatment of the pipe influence the material behavior and alter the final mechanical anisotropy of the product. In addition, the complex microstructures of modern pipeline steels contain various sources of anisotropy including residual stresses, crystallographic and morphologic textures, and directional dislocation substructures. They do not only affect the local and global pipe strength, ductility and toughness during monotonic loading but also cause strong strain path change effects, e.g. a pronounced Bauschinger effect. Anisotropy thus poses a true challenge to pipeline designers making it difficult to accurately predict the final pipe behavior from the known properties of the hot-rolled high-strength coil, the starting point of the forming process. Still, such predictive power is vital to guarantee the structural integrity of pipelines without failure in case of in-service loads beyond the elastic range. The pipe manufacturing process can be simplified into two steps: decoiling (incl. levelling) and spiral forming. For subsequent quality control, samples are extracted from the pipe, per standard flattened and tensile tested in hoop direction. With regard to developing a computational twin of the complete manufacturing and testing process, we represent each step by a separate finite element (FE) model. The full through-process simulation workflow thus necessitates tools to transfer the material state between individual FE models. Here, we present such a workflow for the dislocation substructural hardening model by [Peeters et al., Acta Mater. (2001) 49:1607- 1619]. For each material point, macroscopic and microscopic state variables, including residual stress, crystallographic texture and dislocation densities, are first interpolated and then transferred to the next model. In this way, the evolution of different anisotropy sources can be studied starting from coil via pipe and ending with tensile testin

Sports-based intervention and the problem of youth offending: a diverse enough tool for a diverse society?

This paper discusses sports-based interventions (SBIs) and the problem of youth crime. It notes the positive role sport can play in changing to better the lives of young people. However, there is a lack of robust evidence to support the argument that participation in sporting activity can lead to a reduction in anti-social and offending behaviour. The paper discusses how through focusing on ‘individual needs’ and ‘pathways to work’, SBIs can become overly reductionist and mask broader structural class-, gender- and race-based inequalities that permeate through neoliberal nation-states and western criminal justice systems. It concludes that SBI advocates must seek to promote a less homogeneous idea of what an SBI is, as well as be more sensitive to the diverse needs of young people, particularly if they are to tackle the underlying structural inequalities that arguably create the social problem, that is youth crime in the first place

Disclinations in large-strain plastic deformation and work-hardening

Large strain plastic deformation of f.c.c. metals at low homologous temperatures results in the subdivision of monocrystals or polycrystal grains into mesoscopic fragments and deformation bands. Stage IV of single crystal work-hardening and the substructural contribution to the mechanical anisotropy emerge at about the same equivalent strain at which the fragment structure becomes the dominant substructural feature. Therefore, the latter is likely to be the reason for the new features in the macroscopic mechanical response. The present paper reviews some experimental and theoretical work on deformation banding and fragmentation as well as a recent model which tackles the fragment structure development as well as its impact on the macroscopic mechanical response with the help of disclinations. Incidental or stress-induced formation of disclination dipoles and non-conservative propagation of disclinations are considered as “nucleation and growth” mechanisms for fragment boundaries. Propagating disclinations get immobilized in fragment boundaries to form new triple junctions with orientation mismatches and thus immobile disclinations with long-range stress fields. The substructure development is described in terms of dislocation and disclination density evolution equations; the immobile defect densities are coupled to flow or critical resolved shear stress contributions.status: publishe

A disclination-based approach for mesoscopic statistical modeling of grain subdivision in niobium

A mesoscopic statistical model for orientation fragmentation is presented and applied to a niobium polycrystal under uniaxial compression at room temperature. It is proposed that re-orientation bands nucleate at grain boundaries as a consequence of mismatch stresses and strains resulting from prior intersections of slip bands with these boundaries. In this view, the elementary process responsible for slip patterning also governs re-orientation banding. The model assumes that this elementary process be superjog formation in the bulk with subsequent flip-over of edge dislocations. Balance equations are formulated for the evolution of the densities of forest edge and screw dislocations, slip band edge and screw dislocations, and propagating and immobilized partial disclination dipoles. Microstructural parameters as well as work-hardening contributions can be predicted in reasonable agreement with experiment. The mismatch stress contribution to work-hardening is predicted to saturate, while a barrier contribution related to the newly formed re-orientation band boundaries may become decisive at large strains.status: publishe

Modeling grain fragmentation and deformation textures for titanium using a combined approach of the viscoplastic self-consistent model and a shear fluctuation model

A new grain fragmentation model is proposed for the formation of grain substructures in polycrystalline materials during plastic deformation. Micro-shear bands and twins are assumed to originate from grain boundaries due to the local stress concentrations caused by the interaction of grain boundaries and slip bands. These stress concentrations will impose shear fluctuations on the neighboring grains, which can be relaxed by the formation of corresponding substructures, i.e., micro-shear bands and twins, in these neighboring grains. Texture simulations for cold rolling and equal channel angular pressing (ECAP) of commercially pure titanium are performed using a combined approach of the viscoplastic self-consistent (VPSC) model and the shear fluctuation model. The texture update scheme in the combined approach is presented. Experimental textures at different thickness reductions of cold rolling at room temperature (300 K) and after the first pass of ECAP at 473 K are measured and compared with simulated textures obtained by the standard VPSC approach and the combined approach. In comparison with the standard VPSC approach, the combined approach gives better texture predictions for the two cases: The missing components at low to intermediate rolling reductions in the standard VPSC approach are reproduced; at high rolling reductions and after ECAP, the dominating fibers are also essentially smoothened in the combined approach, in better agreements with the experimental textures.status: publishe

Modeling of distortions after carburization and quenching processes of large gears

A new finite element model is developed to predict the deformations, stresses, phase compositions and carbon concentration gradients that arise as a consequence of the physical processes involved in a carburization and quenching process of a large steel gear. Firstly, the diffusion of carbon at elevated temperatures in the austenitic range is studied in a diffusion model. Secondly, the calculated carbon concentration distribution is used as an input for a model that couples the thermal, metallographic and mechanical effects during the quenching process and calculates the evolution of the temperature, phase composition and deformation history at any point in the gear. Two effects typical for oil quenching of large gears are incorporated in the model. The first is the influence of the gear’s own weight while hanging on chains before, during and after entering the quench bath. The second is the three-dimensional effect that it takes time between the moment the gear enters the oil quenching bath and the moment when the gear is fully immersed. The non-uniform temperature distribution over the gear’s axis causes a non-homogeneous plastic deformation. A diffusion-thermo-metallo-mechanical model that takes these effects into account is compared with a model that does not. The results show that these effects should be incorporated.status: publishe

Online use of physically based plasticity models for steady state cold rolling processes

A procedure has been developed to incorporate computationally costly physically based crystal plasticity models to calculate texture and anisotropy for steady state forming processes online. When using these models, at every point in the deformed zone, an average and a nonlinear solution procedure for stresses and/or strains in all these grains is required. The online calculation cost is avoided by offline creating a database with texture and anisotropy data for all possible deformation modes of the process. The case studied is a cold rolling process, but can easily be extended to any type of forming process, when the deformation field is known in advance. Textures and anisotropy data are predicted using a viscoplastic selfconsistent model, but the method is suitable for any kind of crystal plasticity model. Single crystal plastic parameters, such as the critical resolved shear stress, the single crystal hardening parameters, and the strain-rate sensitivity, have been calibrated based on mechanical tests by means of a direct search simplex algorithm. The online calculated deformation history is compared to the histories stored in the database and the best match is selected. The deformation history is divided in two zones, the one before the neutral point where forward shearing occurs and the one after the neutral point where backward shearing occurs. One online deformation generation and selection procedure requires 0.005 s of CPU time for a database with a division in deformation gradients fine enough to accurately cover all deformations. The method allows calculating yield surfaces at any point in space based on microstructural effects modeled by crystal plasticity, without incremental material updating and necessity to define a kinematic and isotropic hardening, which makes the method suitable for fast models to calculate rolling forces and torques online.status: publishe

Simulation of the effects of interstitial content and temperature on texture and substructure evolution of commercially pure titanium during ECAP

© Published under licence by IOP Publishing Ltd. The effects of interstitial content and temperature on texture and substructure evolution of commercially pure titanium during the first pass of equal channel angular pressing were investigated. Different values of critical resolved shear stresses were proposed for different interstitial contents and processing temperatures. Simulation results show that texture was affected by both interstitial element and temperature. Also, in substructure simulation, the cell size was affected by interstitial content, while the fragment size was more dependent on temperature change.status: publishe

The sound of metal: Acoustic Emission during the deformation of commercially pure titanium

Acoustic emission (AE) was monitored in situ during in-plane tensile deformation of a commercially pure titanium plate. Different load orientations were considered: parallel to the rolling direction (RD), transversal direction (TD) and under an angle of 45 degrees (45D) with RD. The AE event rate, peak amplitude, signal duration, rise time and ring-down counts were monitored during deformation. Orientation dependent mechanical behavior, AE signal characteristics and microstructural evolution was observed and correlated to the difference in texture and twinning phenomena. The intensity and characteristics of AE signals in RD and 45D do not change considerably during deformation, which was attributed to the continuous nucleation and growth of compression twins. Anomalous AE behavior in TD was observed, showing two distinct peaks. All signal parameters show significantly decreased values during a first peak, which was correlated to massive nucleation of small tensile twins. These tensile twins disappear with increasing strain, indicating the end of the first peak. The second peak is comparable to the plateau behavior in RD and 45D and occurs during necking. Compression twinning is dominant, but limited to the necking zone. An unfavorable textural effect in TD has been identified, which hinders the formation of compression twins during the homogeneous deformation, as opposed to RD and 45D, resulting in anomalous AE behavior. The acoustic emission technique proves to be a complementary technique to electron diffraction as it allows monitoring and identifying the twinning modes of commercially pure titanium in situ.status: Published onlin