Toughening elastomers via microstructured thermoplastic fibers with sacrificial bonds and hidden lengths

Soft materials capable of large inelastic deformation play an essential role in high-performance nacre-inspired architectured materials with a combination of stiffness, strength and toughness. The rigid "building blocks" made from glass or ceramic in these architectured materials lack inelastic deformation capabilities and thus rely on the soft interface material that bonds together these building blocks to achieve large deformation and high toughness. Here, we demonstrate the concept of achieving large inelastic deformation and high energy dissipation in soft materials by embedding microstructured thermoplastic fibers with sacrificial bonds and hidden lengths in a widely used elastomer. The microstructured fibers are fabricated by harnessing the fluid-mechanical instability of a molten polycarbonate (PC) thread on a commercial 3D printer. Polydimethylsiloxane (PDMS) resin is infiltrated around the fibers, creating a soft composite after curing. The failure mechanism and damage tolerance of the composite are analyzed through fracture tests. The high energy dissipation is found to be related to the multiple fracture events of both the sacrificial bonds and elastomer matrix. Combining the microstructured fibers and straight fibers in the elastomer composite results in a ~ 17 times increase in stiffness and a ~ 7 times increase in total energy to failure compared to the neat elastomer. Our findings in applying the sacrificial bonds and hidden lengths toughening mechanism in soft materials at the microscopic scale will facilitate the development of novel bioinspired laminated composite materials with high mechanical performance

Efficient planning of peen-forming patterns via artificial neural networks

Robust automation of the shot peen forming process demands a closed-loop feedback in which a suitable treatment pattern needs to be found in real-time for each treatment iteration. In this work, we present a method for finding the peen-forming patterns, based on a neural network (NN), which learns the nonlinear function that relates a given target shape (input) to its optimal peening pattern (output), from data generated by finite element simulations. The trained NN yields patterns with an average binary accuracy of 98.8\% with respect to the ground truth in microseconds

Vortex-induced vibrations: a soft coral feeding strategy?

Soft corals, such as the bipinnate sea plume Antillogorgia bipinnata, are colony building animals that feed by catching food particles brought by currents. Because of their flexible skeleton, they bend and sway back and forth with the wave swell. In addition to this low-frequency sway of the whole colony, branches of A. bipinnata vibrate at high frequency with small amplitude and transverse to the flow as the wave flow speed peaks. In this paper, we investigate the origin of these yet unexplained vibrations and consider their effect on soft corals. Estimation of dynamical variables along with finite element implementation of the wake-oscillator model favour vortex-induced vibrations (VIVs) as the most probable origin of the observed rapid dynamics. To assess the impact of the dynamics on filter feeding, we simulated particles advected by the flow around a circular cylinder and calculated the capture rate with an in-house monolithic fluid-structure interaction (FSI) finite element solver and Python code. We observe that vibrating cylinders can capture up to 40% more particles than fixed ones at frequency lock-in. Therefore, VIVs plausibly offer soft corals a better food capture.Comment: 20 page

Simulating shot peen forming with eigenstrains

Shot peen forming is a cold work process used to shape thin metallic components by bombarding them with small shots at high velocities. Several simulation procedures have been reported in the literature for this process, but their predictive capabilities remain limited as they systematically require some form of calibration or empirical adjustments. We intend to show how procedures based on the concept of eigenstrains, which were initially developed for applications in other fields of residual stress engineering, can be adapted to peen forming and stress-peen forming. These tools prove to be able to reproduce experimental results when the plastic strain field that develop inside a part is known with sufficient accuracy. They are, however, not mature enough to address the forming of panels that are free to deform during peening. For validation purposes, we peen formed several 1 by 1 m 2024-T3 aluminum alloy panels. These experiments revealed a transition from spherical to cylindrical shapes as the panel thickness is decreased for a given treatment, that we show results from an elastic instability

Stability of a rotating cylindrical shell containing axial viscous flow

Un modèle d'écoulement visqueux a été développé pour étudier la stabilité d'une coque cylindrique contenant un écoulement axial parce que le modèle de fluide parfait a été démontré comme étant inadéquat. Il a été montré que la stabilité du système est très sensible à la modélisation de l'interface coque-fluide et qu'un faible taux de rotation tend à stabiliser le système

Flutter Limitation of Drag Reduction by Elastic Reconfiguration

Through experiments, we idealise a plant leaf as a flexible, thin, rectangular plate clamped at the midpoint and positioned perpendicular to an airflow. Flexibility of the structure is considered as an advantage at moderate flow speed because it allows drag reduction by elastic reconfiguration, but it can also be at the origin of several flow-induced vibration phenomena at higher flow speeds. A wind tunnel campaign is conducted to identify the limitation to elastic reconfiguration that dynamic instability imposes. Here we show by increasing the flow speed that the flexibility permits a considerable drag reduction by reconfiguration, compared to the rigid case. However, beyond the stability limit, vibrations occur and limit the reconfiguration. This limit is represented by two dimensionless numbers: the mass number, and the Cauchy number. Our results reveal the existence of a critical Cauchy number below which static reconfiguration with drag reduction is possible and above which a dynamic instability with important fluctuating loads is present. The critical dimensionless velocity is dependant on the mass number. Flexibility is related to the critical reduced velocity, and allows defining an optimal flexibility for the structure that leads to a drag reduction by reconfiguration while avoiding dynamic instability. Furthermore, experiments show that our flexible structure can exhibit two vibration modes: symmetric and anti-symmetric, depending on its mass number. Because the system we consider is bluff yet aligned with the flow, it is unclear whether the vibrations are due to a flutter instability or vortex-induced vibration or a combination of both phenomena.Comment: 10 pages, 11 figures, Published in Physics of Fluid

Modal analysis of a spinning disk in a dense fluid as a model for high head hydraulic turbines

In high head Francis turbines and pump-turbines in particular, Rotor Stator Interaction (RSI) is an unavoidable source of excitation that needs to be predicted accurately. Precise knowledge of turbine dynamic characteristics, notably the variation of the rotor natural frequencies with rotation speed and added mass of the surrounding water, is essential to assess potential resonance and resulting amplification of vibrations. In these machines, the disk-like structures of the runner crown and band as well as the head cover and bottom ring give rise to the emergence of diametrical modes and a mode split phenomenon for which no efficient prediction method exists to date. Fully coupled Fluid-Structure Interaction (FSI) methods are too computationally expensive; hence, we seek a simplified modelling tool for the design and the expected-life prediction of these turbines. We present the development of both an analytical modal analysis based on the assumed mode approach and potential flow theory, and a modal force Computational Fluid Dynamics (CFD) approach for rotating disks in dense fluid. Both methods accurately predict the natural frequency split as well as the natural frequency drift within 7.9% of the values measured experimentally. The analytical model explains how mode split and drift are respectively caused by linear and quadratic dependence of the added mass with relative circumferential velocity between flexural waves and fluid rotation

Failure mechanisms of coiling fibers with sacrificial bonds made by instability-assisted fused deposition modeling

Instability-assisted 3D printing is a method for producing microstructured fibers with sacrificial bonds and hidden lengths which mimic nature’s toughening mechanisms found in spider silk. This hierarchical structure increases the effective toughness of poly(lactic acid) (PLA) fibers by 240% − 340% in some specimens. Nevertheless, many specimens show worse toughness as low as 25% of that of the benchmark straight fiber due to the incomplete release of hidden lengths caused by premature failures. Here, we report mechanical tests and simulations of microstructured fibers with coiling loops that identify the material plastic deformation as being crucial to fully release the hidden lengths. Without sufficient material yielding, high local tensile stress results from the bending-torsion-tension coupled deformation of the coiling loop and induces crack initiation at the fiber backbone during the loop unfolding process. On the other hand, the influence of bond-breaking defect is found to be negligible here. Moreover, for a number of broken bonds beyond a critical value, the accumulated elastic energy along the released loops induces a high strain rate (~ 1500 mm/mm/s) in quasi-static tensile test, which fractures the fiber backbone within 0.1 ms after the breaking of a new bond. We also show a size effect in fused deposition modeling (FDM) extruded PLA fibers, which results in higher effective toughness (~ 5 times the performance of the straight fiber benchmark) in small coiling fibers (dia. = 0.37 mm), due to the better ductility in bending and torsion than large fibers (dia. = 1.20 mm). The failure mechanisms of single microstructured fiber presented here lay the groundwork for further optimizations of fiber arrays in the next generation of high energy-absorption composites for impact protection and safety-critical applications

Spiderweb-inspired, transparent, impact-absorbing composite

Transparent materials with high impact absorption are required for many safety-critical engineering systems. Existing transparent tough composites have increased impact resistance but often fail catastrophically because of poor impact absorption. We propose a transparent impact-absorbing composite that reproduces the toughening mechanism involving sacrificial bonds and hidden lengths in spider silk. Our material consists of an elastomer matrix and an instability-assisted, 3D-printed, bidirectional fabric of microstructured fibers with sacrificial bonds and alternating loops. Under impact, the hidden loops unfold after bond breaking and matrix cracking, resisting impactor penetration with graceful failure. The large-scale plastic deformation of the unfolding loops significantly increases energy dissipation and leads to hysteresis of 95.6% (dissipated energy/total absorbed energy × 100%), minimizing the released elastic energy and reducing the rebounding damage. Our approach opens a new avenue for designing and manufacturing transparent high-energy-absorbing composites for impact protection applications

Mechanical stress initiates and sustains the morphogenesis of wavy leaf epidermal cells

Pavement cells form wavy interlocking patterns in the leaf epidermis of many plants. We use computational mechanics to simulate the morphogenetic process based on microtubule organization and cell wall chemistry. Based on the in silico simulations and experimental evidence, we suggest that a multistep process underlies the morphogenesis of pavement cells. The in silico model predicts alternatingly located, feedback-augmented mechanical heterogeneity of the periclinal and anticlinal walls. It suggests that the emergence of waves is created by a stiffening of the emerging indented sides, an effect that matches cellulose and de-esterified pectin patterns in the cell wall. Further, conceptual evidence for mechanical buckling of the cell walls is provided, a mechanism that has the potential to initiate wavy patterns de novo and may precede chemical and geometrical symmetry breaking