Fast Fourier transform-based modelling for the determination of micromechanical fields in polycrystals

International audienceEmerging characterization methods in Experimental Mechanics pose a challenge to modelers to devise efficient formulations that permit interpretation and exploitation of the massive amount of data generated by these novel methods. In this overview we report on a numerical formulation based on Fast Fourier Transforms, developed over the last 15 years, which can use the voxelized microstructural images of heterogeneous materials as input to predict their micromechanical and effective response. The focus of this presentation is on applications of the method to plastically-deforming polycrystalline materials

Development of Recrystallization Texture and Microstructure in Cold Rolled Copper

Oxygen free electronic copper, 99.995% purity, of two initial grain sizes, 50 {mu}m and 100 {mu}m, has been cold rolled to six strains of 1.0, 1.5, 2.0, 2.65, 3.5 and 4.5 (von Mises equivalents). The rolled materials were partially and fully recrystallized to study the development of recrystallization textures as a function of grain size, strain and fraction recrystallized. The initial textures were relatively random and the deformation textures show the classic {beta} fiber development. As strain is increased both materials produce increasingly intense cube recrystallization textures, (100), as measured both by x-ray diffraction and the electron backscatter pattern (EBSP) techniques. The strong cube recrystallization textures are a product of a higher than random frequency of cube nucleation sites. An additional factor is that cube regions grow larger than non-cube regions. The explanation of the cube frequency advantage is based on the development of large stored energy differences between cube orientations and neighboring orientations due to recovery of cube sites. Of several possible explanations of the cube orientation size advantage, the most plausible one is solute entrapment. At the higher strains the boundaries of cube grains encounter the deformation texture S components, (123), changing the boundary character to one of 40{degrees}. These boundaries are more resistant to solute accumulation than random high angle boundaries, allowing the boundaries to migrate with less of a solute drag effect than a random high angle boundary

Formation of annealing twins during recrystallization and grain growth in 304L austenitic stainless steel

International audienceUnderstanding of the mechanisms of annealing twin formation is fundamental for grain boundary engineering. In this work, the formation of annealing twins in a 304L austenitic stainless steel is examined in relation to the thermo-mechanical history. The behavior of annealing twins of various morphologies is analyzed using an in-situ annealing device and EBSD. The results confirm that there is a synergistic effect of prior strain level on annealing twin density generated during recrystallization. The higher the prior strain level, the higher the velocity of grain boundary migration and the higher the annealing twin density in the recrystallized grains. This effect decreases as the recrystallization fraction increases. The existing mathematical models (Pande's model and Gleiter's model), which were established to predict annealing twin density in the grain growth regime, can not predict this phenomeno

Ultrafast X-Ray Imaging of Laser-Metal Additive Manufacturing Processes

The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large-scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts

Mortality and pulmonary complications in patients undergoing surgery with perioperative SARS-CoV-2 infection: an international cohort study

Background: The impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on postoperative recovery needs to be understood to inform clinical decision making during and after the COVID-19 pandemic. This study reports 30-day mortality and pulmonary complication rates in patients with perioperative SARS-CoV-2 infection. Methods: This international, multicentre, cohort study at 235 hospitals in 24 countries included all patients undergoing surgery who had SARS-CoV-2 infection confirmed within 7 days before or 30 days after surgery. The primary outcome measure was 30-day postoperative mortality and was assessed in all enrolled patients. The main secondary outcome measure was pulmonary complications, defined as pneumonia, acute respiratory distress syndrome, or unexpected postoperative ventilation. Findings: This analysis includes 1128 patients who had surgery between Jan 1 and March 31, 2020, of whom 835 (74·0%) had emergency surgery and 280 (24·8%) had elective surgery. SARS-CoV-2 infection was confirmed preoperatively in 294 (26·1%) patients. 30-day mortality was 23·8% (268 of 1128). Pulmonary complications occurred in 577 (51·2%) of 1128 patients; 30-day mortality in these patients was 38·0% (219 of 577), accounting for 81·7% (219 of 268) of all deaths. In adjusted analyses, 30-day mortality was associated with male sex (odds ratio 1·75 [95% CI 1·28–2·40], p\textless0·0001), age 70 years or older versus younger than 70 years (2·30 [1·65–3·22], p\textless0·0001), American Society of Anesthesiologists grades 3–5 versus grades 1–2 (2·35 [1·57–3·53], p\textless0·0001), malignant versus benign or obstetric diagnosis (1·55 [1·01–2·39], p=0·046), emergency versus elective surgery (1·67 [1·06–2·63], p=0·026), and major versus minor surgery (1·52 [1·01–2·31], p=0·047). Interpretation: Postoperative pulmonary complications occur in half of patients with perioperative SARS-CoV-2 infection and are associated with high mortality. Thresholds for surgery during the COVID-19 pandemic should be higher than during normal practice, particularly in men aged 70 years and older. Consideration should be given for postponing non-urgent procedures and promoting non-operative treatment to delay or avoid the need for surgery. Funding: National Institute for Health Research (NIHR), Association of Coloproctology of Great Britain and Ireland, Bowel and Cancer Research, Bowel Disease Research Foundation, Association of Upper Gastrointestinal Surgeons, British Association of Surgical Oncology, British Gynaecological Cancer Society, European Society of Coloproctology, NIHR Academy, Sarcoma UK, Vascular Society for Great Britain and Ireland, and Yorkshire Cancer Research

(13) 3D Characterization and Modeling of Fatigue Cracks

Anthony D. Rollett - Professor of Material Science and Engineering, Carnegie Mellon UniversityEnormous strides have been made in quantifying the growth of fatigue cracks over the years and incorporating that understanding in predictions of component lifetime. Nevertheless, it is clear that the behavior of short cracks is less well quantified, where short is relative to the length scale(s) found in materials microstructure, e.g. grain size. Ultimately, materials science seeks to predict the location and growth of fatigue cracks in order to design materials microstructure to maximize fatigue lifetime. Towards that end, it is interesting to study the relationship between cracks and microstructure near the initiation point. Short fatigue cracks in nickel-based superalloys have been characterized using conventional SEM and orientation mapping. 3D characterization used High Energy Diffraction Microscopy (HEDM), and computed tomography (CT) to map out the crack positions within their embedding grain structure. The main finding is that cracks develop most readily along long twin boundaries with high resolved shear stress on the slip systems parallel to the twin plane. Also, both halves of a different superalloy, fully fractured sample have been fully characterized in 3D using the same tools. The HEDM and CT were performed with high energy x-rays on beamline 1ID at the Advanced Photon Source (APS). The 3D orientation maps are used as input to computations of the full field stress-strain response. The fracture surface is analyzed with respect to local orientation and inter- versus trans-granular character. The likely origins of fatigue crack initiation in these cases are discussed.1_pkz6t2h

(14) 3D Characterization and Modeling of Fatigue Cracks (slides)

Enormous strides have been made in quantifying the growth of fatigue cracks over the years and incorporating that understanding in predictions of component lifetime. Nevertheless, it is clear that the behavior of short cracks is less well quantified, where short is relative to the length scale(s) found in materials microstructure, e.g. grain size. Ultimately, materials science seeks to predict the location and growth of fatigue cracks in order to design materials microstructure to maximize fatigue lifetime. Towards that end, it is interesting to study the relationship between cracks and microstructure near the initiation point. Short fatigue cracks in nickel-based superalloys have been characterized using conventional SEM and orientation mapping. 3D characterization used High Energy Diffraction Microscopy (HEDM), and computed tomography (CT) to map out the crack positions within their embedding grain structure. The main finding is that cracks develop most readily along long twin boundaries with high resolved shear stress on the slip systems parallel to the twin plane. Also, both halves of a different superalloy, fully fractured sample have been fully characterized in 3D using the same tools. The HEDM and CT were performed with high energy x-rays on beamline 1ID at the Advanced Photon Source (APS). The 3D orientation maps are used as input to computations of the full field stress-strain response. The fracture surface is analyzed with respect to local orientation and inter- versus trans-granular character. The likely origins of fatigue crack initiation in these cases are discussed