Search Results (2)

Parachute suspension lines shed vortices during descent, and these vortices develop oscillating aerodynamic forces that can induce forced parasitic vibrations of the lines, which can have an adverse impact on the parachute system. Understanding the line’s mechanical behavior can assist in studying the vibrations experienced by the suspension lines. A well-calibrated structural model of the suspension line could be used to help to identify how the braid’s architecture contributes to its mechanical behavior and to explore if and how a suspension line can be designed to mitigate these parasitic vibrations. In the current study, a mesomechanical finite element model of a polyester braided parachute suspension line was constructed. The line geometry was built in the Virtual Textile Morphology Suite (VTMS), and a user material model (UMAT) was implemented in LS-DYNA^® release 14 to describe the material behavior of the individual tows. The material properties were initially calibrated using experimental tension tests on individual tows, which exhibited an initial modulus of ~4100 MPa before transitioning to ~3200 MPa at a stress of 30 MPa. When these properties were applied to the full braid model, slight adjustments were made to account for geometric complexities in the braid structure, improving the correlation between the model and experimental tensile tests. The final calibrated model captured the bilinear tensile behavior of the braid, with an initial modulus of 2219 MPa and a secondary modulus of 1350 MPa, compared to experimental values of 2253 MPa and 1420 MPa, respectively, showing 2% and 5% differences. The calibrated model of the braided cord was then subjected to torsion, and the results showed good agreement with dynamic and static experimental torsion tests, with a difference of 8–19% for dynamic tests and 13–27% for static tests when compared to experimental values. The availability of virtual models of suspension lines can ultimately assist in the design of suspension lines that mitigate flow-induced vibration. Full article

This paper delves into the impact of the damage accumulation effect, which leads to the degradation of material strength and stiffness, on the seismic resistance of transmission towers. Building upon the elastic–plastic finite element theory, a mixed hardening constitutive model is derived for circular steel tubes, standard elements in transmission towers, incorporating the damage accumulation effect. A user material subroutine, UMAT, is created within the LS–DYNA framework. The program’s validity and reliability are established through axial constant–amplitude loading tests on single steel tubes. The subroutine is employed to conduct the incremental dynamic analysis (IDA) of an individual transmission tower and to contrast it with the structure utilizing the Plastic Kinematic material model, assessing the discrepancies in tower top displacements and segment damage indices (SDIs) at both macroscopic and microscopic scales. The results shows that the Plastic Kinematic model inflates the seismic performance of the transmission tower. When considering the damage accumulation effect in structural failure, the damage index of the members increases, leading to a reduction in both the structural strength and stiffness. The dynamic response in the plastic phase becomes more pronounced, and the onset of structural failure is accelerated. Consequently, structural analysis under seismic conditions should account for the damage accumulation process. Through the delineation of member and segment damage, the extent of damage to transmission tower segments can be quantitatively assessed. Subsequently, the ultimate load–bearing capacity and the most vulnerable location of the transmission tower can be ascertained. Finally, this paper provides a detailed analysis of the transmission tower collapse process under seismic action and summarizes the mechanism of collapse for the structure. Full article