The brittle intermetallic Nb 3 Sn superconductor is currently being used to develop high field magnets in the framework of the Hi-Luminosity upgrade of the Large Hadron Collider at CERN. Despite its ...excellent superconductive properties,Nb 3 Sn wires suffer from significant critical current I c reduction due to the transverse load applied during the magnets' assembly and energization. In high critical current density ( J c >1200 A/mm 2 at 15 T, 4.22 K) RRP and PIT Rutherford cables when applying a transverse load of 150 MPa, the I c (12 T, 4.22 K) is reduced respectively by about 10-15% and 20%. At this level of transverse load, the I c reduction is practically reversible, and it is due to the strain induced in the superconductor, which reduces its upper critical field B c2 . Because of the B c2 reduction, at 19 T and 4.22 K, with a load of 150 MPa, the same cables are expected to experience an I c reduction up to 40%. Further increasing the transverse loads, cracks in the superconductor also start to reduce (irreversibly) the I c . A dedicated FEM 3D numerical model coupled with a J c scaling law has been developed to predict the electro-mechanical behaviour of RRP and PIT wires under transverse loads in the reversible regime. By using this model, the effects of different geometrical factors have been studied to identify the key parameters that allow limiting the effect of transverse loads on the I c reduction under transverse load. In particular, this paper deals with the role of the: production technologies diameter, sub-elements layout, heat treatment and precompression.
Superconducting magnets experience significant thermo-mechanical loads throughout their life cycle. These are introduced by the electro-magnetic forces during powering, but also by the prestress ...applied in many magnet designs. Further to this, the large thermal excursion that components of different materials experience can generate significant internal forces. The loads are also experienced by the superconducting coils, whose critical current can decrease as a consequence of the applied strain. It is then crucial to predict the overall mechanical behavior and conservatively design a magnet, avoiding failure of the mechanical components and of the superconducting coils. Finite Element Analysis (FEA) is generally used to perform these tasks, but its results rely heavily on the material properties and models used. This is in particular true for the coil composite, which is simplified to allow reasonable model sizes in full magnet models. In this paper, we present the state-of-art knowledge of the mechanical properties of the materials mostly used in superconducting magnet construction. We review elastic and plastic properties at room and cryogenic temperature, thermal contraction, and summarize the state-of-art failure criteria for these materials. Finally, the paper summarizes the present understanding of the mechanical behavior and limits of Nb 3 Sn coils. For the first time, an orthotropic failure criteria is proposed.
Purpose
The purpose of this study is to analyse long-term unicompartmental knee arthroplasty (UKA) focusing on survivorship, causes of failure and revision strategy.
Methods
This study is a ...retrospective analysis of data from a regional arthroplasty registry for cases performed between 2000 and 2017. A total of 6453 UKAs were identified and the following information was analysed: demographic data, diagnosis leading to primary implant, survivorship, complication rate, causes of failure, revision strategies. UKA registry data were compared with total knee arthroplasty (TKA) registry data of 54,012 prostheses, which were implanted in the same time period.
Results
6453 UKAs were included in the study: the vast majority of them (84.4%) were implanted due to primary osteoarthritis followed by deformity (7.1%) and necrosis of the condyle (5.1%). When compared to TKA, UKA showed lower perioperative complication rate (0.3% compared to 0.6%) but higher revision rate (18.2% at 15 years, compared to 6.2% for TKA). No correlation was found between diagnosis leading to primary implant and prosthesis survival. The most frequent cause of failure was total aseptic loosening (37.4%), followed by pain without loosening (19.8%). Of the 620 UKAs requiring revision, 485 were revised with a TKA and 61 of them required a re-revision; on the other hand, of the 35 cases where another UKA was implanted, 16 required a re-revision.
Conclusion
UKA is associated with fewer perioperative complications but higher revision rates when compared to TKA. Its survivorship is not affected by the diagnosis leading to primary implant. Revision surgery of a failed UKA should be performed implanting a TKA, which is associated with a lower re-revision rate when compared to another UKA.
Level of evidence
Level 3, therapeutic study.
Superconducting magnet coils are subject to large thermo-mechanical loads applied during magnet assembly, cooldown and operation. These loads can cause the reduction of their critical current due to ...mechanical strains or local filament failures. Measurements on longitudinally stretched strands and Rutherford cables under transverse pressure have allowed exploration of material limits in two directions. However, no systematic study of the effect of multi-axial loading conditions has been done. Finite Element (FE) models show that, indeed, the actual limits of the material are strongly dependent on the nature of the applied load and that the strength under multi-axial loading can be significantly higher with respect to uniaxial loading conditions. In this paper, we try, for the first time, to measure the effect of multi-direction loading conditions on Nb 3 Sn Rutherford cables. The experiments are performed on impregnated cable stacks under transverse, lateral, and longitudinal constraints. The integrity of the cables is verified by destructive metallography inspection, evaluating the damage as a function of the applied loading condition.
The international particle physics community considers a Muon Collider (MC) as a possible option for the successor of the Large Hadron Collider (LHC) at CERN. An international collaboration has ...recently been set up to produce a conceptual design study of a Muon Collider. One of the main challenges is the need for an ultra-high magnetic field solenoid for the final cooling of the muons. This magnet must have a bore aperture of about 5 cm and a 1% magnetic field homogeneity over 0.5 m of length. CERN is exploring the possibility of developing such a magnet by only using a stack of Rare-earth Barium Copper Oxide (ReBCO) tapes as a conductor. The study's main idea is to produce a modular compact magnet constituted by an assembly of identical pancakes electrically connected in series. Quench protection and stress management are the biggest design challenges. To cope with them, we are investigating the option of Non/Metal-Insulated (N/M-I) pancakes, each made of a single coil (i.e., discarding concentric, nested coils), inserted in a stiff outer ring that provides a sufficient precompression to the coil (∼200 MPa). To protect the magnet after a quench and to ramp up the field sufficiently fast, the N/M-I coil interlayer electrical resistance must be optimized and controlled. In this paper, we present a preliminary design of this concept. If successful, this type of design will contribute to developing high-field solenoids for particle accelerators and promote the use of ReBCO tapes in compact windings needed for different applications, such as electrical machines and fusion reactors based on magnetic confinement.
Abstract Introduction The survival of total knee arthroplasty (TKA) in patients with poliomyelitis remains a debated topic due to the high recurrence of postoperative genu recurvatum. This study aims ...to report the long-term survival of TKA in patients with poliomyelitis, using data from the Italian Register of Prosthetic Implantology. Materials and methods A registry-based population study was conducted, utilizing data from the Emilia Romagna orthopedic arthroplasty implants registry (RIPO - Registro Implantologia Protesica Ortopedica). The cohort consisted of 71 patients with poliomyelitis-related arthritis who underwent TKA. The study assessed and analyzed demographic data, implant type, fixation method, insert type, and level of constraint. Additionally, variations in preoperative and postoperative both clinical and functional Knee Society Scores (KSS) were collected. Results Eight implants required revision surgery (16%), and three patients died (6.1%), resulting in a 10-year survival rate of 86.6% and a 15-year survival rate of 53.9%. Aseptic loosening was the primary cause of revision, accounting for 37.5% of failures, followed by insert wear (25%). No statistically significant correlation was found between the level of constraint and implant survival (p=0.0887, log-rank). Both the clinical and functional KSS improved postoperatively. Conclusion TKA is a viable alternative to knee arthrodesis and, in properly selected patients, might represent the first-choice treatment for articular degeneration due to its high survivorship. Despite the complexity of these cases, TKA can effectively alleviate articular pain, instability, and angular deviation, thereby preserving knee functionality.
High-performance Nb3Sn superconducting wires have become one of the key technologies for the development of next generation accelerator magnets. While their large critical current densities enable ...the design of compact accelerator-quality magnets for their operation above 10 T, the noticeable reduction of the conductor performance due to mechanical strain appears as a new essential characteristic in magnet design. In this work, we extensively investigate the effect of transverse loads, up to 250 MPa, in state-of-the-art Nb3Sn Restacked-Rod-Process round superconducting wires. The tests are performed using a compressive Walters spring device, where the force is applied to the resin-impregnated wire, and the critical current is measured under magnetic fields ranging from 16 to 19 T. As a complement, critical current measurements under axial strain are also performed using a standard Walters spring. Interestingly, the study shows that the wire's electro-mechanical response under transverse stress depends on the initial axial strain condition. Nonetheless, when the main direction load becomes predominant, all tested wires converge to a common behavior. This observation allowed us to combine the results from critical current measurements under the loads exerted in both directions (axial and transverse), shedding some new light on the mechanisms behind critical current degradation.
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