3D printing of patient-specific implants for osteochondral defects: workflow for an MRI-guided zonal design

Magnetic resonance imaging (MRI) is a common clinical practice to visualize defects and to distinguish different tissue types and pathologies in the human body. So far, MRI data have not been used to model and generate a patient-specific design of multilayered tissue substitutes in the case of interfacial defects. For orthopedic cases that require highly individual surgical treatment, implant fabrication by additive manufacturing holds great potential. Extrusion-based techniques like 3D plotting allow the spatially defined application of several materials, as well as implementation of bioprinting strategies. With the example of a typical multi-zonal osteochondral defect in an osteochondritis dissecans (OCD) patient, this study aimed to close the technological gap between MRI analysis and the additive manufacturing process of an implant based on different biomaterial inks. A workflow was developed which covers the processing steps of MRI-based defect identification, segmentation, modeling, implant design adjustment, and implant generation. A model implant was fabricated based on two biomaterial inks with clinically relevant properties that would allow for bioprinting, the direct embedding of a patient’s own cells in the printing process. As demonstrated by the geometric compatibility of the designed and fabricated model implant in a stereolithography (SLA) model of lesioned femoral condyles, a novel versatile CAD/CAM workflow was successfully established that opens up new perspectives for the treatment of multi-zonal (osteochondral) defects. Graphic abstract

SELECTION OF LOCATORS IN AUTOMATED DESIGN OF FIXTURES FOR LOCATING WORKPIECES DURING MACHINING

Using the systematic approach, the possible basing schemes of the workpieces in the machining attachments have been discovered and systematized with a view to their use in automated design. The analysis shows that the use of the proposed systematization in computer-aided design is significantly more rational, since the structure reflects the sequence of choice of the optimal basing scheme - analysis of the theoretical basing scheme, analysis of the geometry of the workpiece, formation of the possible list of the basing schemes. An analysis was performed with the help of which the criteria for geometric compatibility were revealed, allowing to make a choice of a basing scheme, satisfying the geometric shape of the workpiece. An algorithm for selection of locators has been developed. The development is part of a system for automated design of fixtures for locating of workpieces during machining.

Study Recognizes and Assists in Mitigation of Bottomhole Assembly Lateral Vibration Chatter

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 197231, “Recognition and Mitigation of the Bottomhole Assembly Lateral Vibration Chatter Mode,” by Jeffrey R. Bailey, SPE, and Harshit Lathi, ExxonMobil, and Matthew T. Prim, SPE, ADNOC, et al., prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. Lateral vibration modeling of certain bottomhole assembly (BHA) designs has shown great sensitivity to the proximity of stabilizer blades. This paper explores the nature of the vibrational dysfunction known as BHA chatter. A field-proven frequency-domain model illustrates the cause of the dysfunction, its rotary-speed dependence, and mitigation methods and results. The complete paper provides three case studies exploring this phenomenon, one of which is included in this synopsis. Introduction The authors first describe the similarities and differences of a BHA and a stringed instrument. The string of a violin, for example, typically has two fixed nodal points: the first at the bridge, which does not change, and the second at the position of the musician’s finger, which is moved along the fingerboard in order to play notes of different frequencies. The finger pressing on the string causes it to have zero displacement at that location, which defines a nodal point. Additional nodal points may occur in the motion of the string as harmonics of the fundamental mode, but these are not considered to be fixed nodes because the amplitudes of the harmonics vary. The string is relatively flexible, so it can be described adequately with a second-order differential equation. Significantly, a BHA typically has more than two nodes. A lateral wave propagating along the BHA must satisfy the nodal point constraint of zero lateral deflection at all these locations. These nodes typically are placed without regard to the frequency of the wave traveling along the string, which is governed by the rotary speed and the type of lateral excitation. The geometric compatibility requirement that the pipe has zero displacement at the fixed nodes has ramifications. The nodal point constraints force the pipe to adapt to the locations of these nodes through contact forces that literally push the pipe back into position to honor the constraints. In some scenarios, this process requires large forces. One consequence of large forces pushing the pipe to maintain geometric compatibility is that these forces are applied to the outer diameter of a body that is rotating, so this response may also generate torque and associated wear of the contacting surfaces. This observation applies to both static and dynamic forces but most commonly is recognized in the static domain. It is not typically recognized in dynamics as applied to BHA design.

Hidden symmetries generate rigid folding mechanisms in periodic origami

We consider the zero-energy deformations of periodic origami sheets with generic crease patterns. Using a mapping from the linear folding motions of such sheets to force-bearing modes in conjunction with the Maxwell–Calladine index theorem we derive a relation between the number of linear folding motions and the number of rigid body modes that depends only on the average coordination number of the origami’s vertices. This supports the recent result by Tachi [T. Tachi,Origami6, 97–108 (2015)] which shows periodic origami sheets with triangular faces exhibit two-dimensional spaces of rigidly foldable cylindrical configurations. We also find, through analytical calculation and numerical simulation, branching of this configuration space from the flat state due to geometric compatibility constraints that prohibit finite Gaussian curvature. The same counting argument leads to pairing of spatially varying modes at opposite wavenumber in triangulated origami, preventing topological polarization but permitting a family of zero-energy deformations in the bulk that may be used to reconfigure the origami sheet.

Solution to the Problem of a Mass Traveling on a Taut String via Integral Equation

The problem of a massive taut string, traveled by a heavy point mass, moving with an assigned law, is formulated in a linear context. Displacements are assumed to be transverse, and the dynamic tension is neglected. The equations governing the moving boundary problem are derived via a variational principle, in which the geometric compatibility between the point mass and the string is enforced via a Lagrange multiplier, having the meaning of transverse reactive force. The equations are rearranged in the form of a unique Volterra integral equation in the reactive force, which is solved numerically. A classical Galerkin solution is implemented for comparison. Numerical results throw light on the physics of the phenomenon and confirm the effectiveness of the algorithm.

Modeling Beam–Solid Finite Element Interfaces: A Stabilized Formulation for Contact and Coupled Systems

A number of engineering applications involve contact with bodies modeled using specialized theories of solid mechanics like beams or shells. While computational models for contact in 2D and 3D solid mechanics have been extensively developed in the literature, problems involving contact with beams or shells have received less attention. When modeling contact between a solid body represented with beam or shell theory and a domain discretized with solid finite elements, the contact model faces the typical challenges of enforcing geometric compatibility and the transfer of a complete pressure field along the contact interface, with the added complications stemming from the different underlying mathematical formulations and finite element discretizations in the connecting domains. Resultant-based beam and shell theories do not provide direct estimates of surface tractions, therefore rendering the issue of pressure transfer on beam–solid and shell–solid interfaces more problematic. In the absence of specialized contact formulations for solid–beam and solid–shell interfaces, contact models have relied almost exclusively on the Node-To-Surface (NTS) geometric compatibility approach. This formulation suffers from well-known drawbacks, including instability, surface locking and incomplete pressure fields on the interface. The NTS approach, however, remains the method most readily applicable to contact with beam or shell elements among the vast variety of available methods for computational contact modeling using finite elements. The goal of this paper is to bridge the gap in the literature on coupling domains with beam and solid finite element discretizations. We propose an interface formulation for beam–solid interfaces that ensures the transfer of a complete pressure field while enforcing geometric compatibility using standard NTS constraints. The formulation uses a stabilization approach, based on a special form of the Discontinuous Galerkin method, to enforce weak continuity between the stress fields on the solid side of the interface, and the moment and shear resultants in the contacting beam. We show that the proposed formulation is a robust approach for satisfying compatibility constraints while ensuring the transfer of a complete pressure field on beam–solid finite element interfaces that can be used with bilinear and quadratic interpolations in the solid, and Euler or Timoshenko formulations for the beam.

Effect of Geometric Error of Inner Raceway on Motion of Cylindrical Roller Bearings Under Radial Load

The motion error of bearing depends highly on the geometric profile of bearing components. Therefore, it is crucial to establish a correlation between the geometric error of bearing components and the motion error of an assembled bearing, which is required for designing and manufacturing bearings with high accuracy of motion. In this paper, authors derived a geometric compatibility equation for cylindrical roller bearing considering the geometric error of bearing inner raceway. Based on the load balance and the geometric compatibility derived, a mathematical model of motion accuracy is established, and the model is also validated. The effect of geometric error such as the amplitude of roundness error and dimension error of bearing inner raceway, and radial clearance on the bearing motion error is investigated. Results show that the motion error of the bearing increases with the amplitude of the roundness error of inner raceway, and reduces with the increase of radial load. The results indicated that the motion accuracy can be improved by controlling the distribution of machining tolerance of bearing components.