The Emergence of Sequential Buckling in Reconfigurable Hexagonal Networks Embedded into Soft Matrix

The extreme and unconventional properties of mechanical metamaterials originate in their sophisticated internal architectures. Traditionally, the architecture of mechanical metamaterials is decided on in the design stage and cannot be altered after fabrication. However, the phenomenon of elastic instability, usually accompanied by a reconfiguration in periodic lattices, can be harnessed to alter their mechanical properties. Here, we study the behavior of mechanical metamaterials consisting of hexagonal networks embedded into a soft matrix. Using finite element analysis, we reveal that under specific conditions, such metamaterials can undergo sequential buckling at two different strain levels. While the first reconfiguration keeps the periodicity of the metamaterial intact, the secondary buckling is accompanied by the change in the global periodicity and formation of a new periodic unit cell. We reveal that the critical strains for the first and the second buckling depend on the metamaterial geometry and the ratio between elastic moduli. Moreover, we demonstrate that the buckling behavior can be further controlled by the placement of the rigid circular inclusions in the rotation centers of order 6. The observed sequential buckling in bulk metamaterials can provide additional routes to program their mechanical behavior and control the propagation of elastic waves.

DEVELOPMENT OF THE COMBINED TECHNOLOGICAL PROCESS OF BLANK STACKS FLANGES FORMATION BY THE METHOD OF STAMPING BY ROLLING AND ROTARY DRAWING

The article presents the results of the development and research of the combined technological process of forming the outer and inner flanges of the lids of fractional and distillation columns on sheet blanks by the method of stamping by rolling and rotary drawing. For this purpose, equipment has been developed that allows to form both outer and inner flanges of the blank in one run of the conical roll. Studies have shown that technological capabilities of the process are limited by the risk of destruction of the top layers of the outer flange bending center and its corrugation, as well as by the neck formation or destruction of the peripheral areas of the inner flange. To assess the deformability of the outer flange, the stress-strain state of its bending center was investigated. According to the set stress values, the stress state of the material is determined, the maximum value of which on the surface of the bending zone is Formula for determining the minimum radius of the mandrel, which when using the values of the critical ductility of the material allows to prevent destruction. As well, an expression for determining the maximum width of the flange, provided that the destruction of peripheral areas is prevented, is obtained. As corrugations formation is the main danger in forming the external flanges by the stamping by rolling method (SR), the expression for determining the maximum width of the flange under the condition of a stable process is obtained. If it is necessary to get more developed flanges, it is proposed to provide thinning of their walls by rotary extraction at the second stage. When forming the inner flanges of the blank stacks radial compressive stresses and tangential tensile stresses in the material are brought about. The action of tangential stresses causes loss of stability of the flange by way of neck formation. The value of the critical strains increases with the approach to the state of plain-strain deformation. Therefore, it is recommended to develop process parameters based on construction of the critical strains diagrams.

Application of Two Conditions of Loss of Stability in Analysis of the Tube Bending Process

In this paper, the derivation of expressions for admissible values of strains and stresses for vertex points of layers subjected to tension during tube bending at bending machines is presented. The conditions of the dispersed and located loss of stability of the bent tube were assumed as criteria of instability. The original element of this paper is the extension of the criterion of strain location in a form of possible initiation of a neck or furrow (introduced by Marciniak for thin plates [1]) to bending thin- and thick-walled metal tubes at bending machines. The conditions of the dispersed and localized loss of stability together with formation of the plane state of deformation (PSD) in the plane stress state (PSS) were assumed as the criteria of instability. The calculation results were presented as graphs being useful nomograms. We present also simple examples of calculations of permissible and critical strains and values of bending angles including and not including displacement of the neutral axis y0, during cold bending metal thin-walled tubes at bending machines for bending angles <0o; 180o>.

Strain-Induced Ferrite Formation During Steckel Mill Simulations with Varying Roughing Pass Schedules

It has been previously demonstrated that austenite may undergo partial dynamic transformation (DT) during the plate rolling process. Austenite dynamically transforms into unstable ferrite during hot deformation even at very high temperatures. In this work, the plate rolling simulations, with emphasis on Steckel mill operations, through torsion testing under isothermal conditions were performed on an X70 steel. Four different roughing schedules were tested followed by five finishing passes with pass strains of 0.3 applied at 900 °C. The roughing schedules had zero, one, two and three roughing passes at a temperature of 1100 °C, strain of 0.4 and strain rate of 1 s−1. The stress–strain curves as well as the mean flow stress (MFS) behaviors indicated that both dynamic transformation (DT) and dynamic recrystallization (DRX) occurred during straining. The critical strains for the onset of DT and DRX were determined by means of the double differentiation method and the critical strain values decreased with the number of roughing and finishing strains from the first going to the last pass. It was observed that the volume fraction of the dynamically formed ferrite increased sharply during the finishing stage as the number of previous roughing passes increased, which can be attributed to higher strain accumulation. The results presented here indicate that improved models are needed to control the microstructure of the material during subsequent cooling.

Turning Maneuver Effect on Near-Surface Airfield Pavement Responses

Airport pavement structures experience heavy aircraft tire loading through a localized contact area. Distributed three-dimensionally and non-uniformly, tire-pavement contact stresses directly influence the near-surface behavior of flexible airfield pavements. The resulting high shear stress levels induced by aircraft tire loading may lead to instability through shoving or slippage cracking. As the tire turns during taxiing, the risk of near-surface damage is exacerbated. In this study, numerical modeling of an inverted pavement system and a conventional flexible pavement structure loaded with a single tire from the A-380 landing gear was developed. The analysis matrix included two tire-inflation pressures, two speeds, and rolling conditions that varied from free-rolling to two turning maneuvers. Two analysis approaches were performed: 1) use of traditional critical point strains, and 2) domain analysis, which characterizes bulk pavement behavior using multiaxial stresses and strains. The critical strains, which are used as inputs for airfield pavement design, changed negligibly under varying tire turning conditions despite the asymmetric contact stress distribution. On the other hand, domain analysis not only captured the asymmetric pavement behavior, but also identified that altering the tire movement from a free-rolling condition to turning could induce a significant increase in the potential damage.