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1. Experimental Analysis of Shell-like Deployable Anchors for Increased Tensile Resistance

   https://doi.org/10.20898/j.iass.2024.011  

   Tucker, Kaylee A; Sychterz, Ann C (December 2024, Journal of the International Association for Shell and Spatial Structures)

   Structures with deployable and compliant mechanisms are new to the domain of underground geotechnical systems. An anchor with rotationally deploying compliant thin-wall elements has been developed. This paper presents variations of this anchor that are targeted to increase the surface area associated with the anchor. This increased surface area correlates to higher skin friction to better resist tensile forces. The number and sizing of the deployable components, called awns, are investigated. The work presented here includes methods to change the deployment behavior of the awns by changing the shape of the awns and by using functionally graded materials for increased resistance when the anchor is subjected to uplift forces. Test members were fabricated from a combination of flexible and rigid polymers via additive manufacturing. Experimental testing included anchor deployment tests and awn tension tests. For deployment tests, torque was applied to an anchor placed in clear sand. Awn tension tests provided additional information about the deformation of functionally graded awns through isolated testing of the awns. The presented design and experimental methodologies give insights into the behavior of small-scale deployable anchors. 

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2. Lateral response of post-tensioned pendulum shear walls

   https://doi.org/10.1016/j.istruc.2024.106987  

   Silva, PF; Lagler, C; Burgueño, R (July 2024, Structures)

   This paper presents an experimental evaluation of a new interface configuration for unbonded post-tensioned shear (UPTS) walls. In the proposed configuration, the wall-base interface is shaped in a circular profile. This circular profile presents a major difference from the currently used flat profile at the wall-base interface of rocking UPTS walls. During lateral response, this circular profile induces the wall to predominantly rotate as a rigid body about a fixed point without uplift, and the system dissipates energy through the contact friction that develops at the wall-base interface. This rigid body motion resembles that of a pendulum, thereby designating this system as a pendulum UPTS wall. At this stage, research has demonstrated the many advantages of this system by proof-of-concept testing of a pendulum light-frame wood (LiF) UPTS wall specimen under increasing levels of post-tensioning force. Compared with rocking UPTS walls, experimental results demonstrate that besides performing damage-free when subjected to high drift levels, the proof-of-concept pendulum LiF-UPTS wall offers the following promising outcomes: (1) insignificant to no wall uplift, (2) insignificant to no wall base shear sliding, (3) reduced stress concentrations at the wall toes because contact stresses are distributed along the wall-base interface over a larger region, (4) nearly constant post-tensioning forces under high drift levels, which limits post-tensioning losses due to yielding of prestressing bars or tendons, (5) increase in energy dissipation capacity of the system through friction, and (6) decrease in the damping reduction factor and thus, reduction in the lateral displacement and force demands in pendulum LiF-UPTS walls. These research outcomes are likely to translate positively to other shear wall types, namely reinforced concrete (RC) precast pendulum UPTS walls. 

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3. Lateral response of post-tensioned pendulum shear walls

   Silva, Pedro; Lagler, Carmen; Burgueño, Rigoberto (September 2024, Structures)

   Leroy, Gardner (Ed.)

   This paper presents an experimental evaluation of a new interface configuration for unbonded post-tensioned shear (UPTS) walls. In the proposed configuration, the wall-base interface is shaped in a circular profile. This circular profile presents a major difference from the currently used flat profile at the wall-base interface of rocking UPTS walls. During lateral response, this circular profile induces the wall to predominantly rotate as a rigid body about a fixed point without uplift, and the system dissipates energy through the contact friction that develops at the wall-base interface. This rigid body motion resembles that of a pendulum, thereby designating this system as a pendulum UPTS wall. At this stage, research has demonstrated the many advantages of this system by proof-of-concept testing of a pendulum light-frame wood (LiF) UPTS wall specimen under increasing levels of post-tensioning force. Compared with rocking UPTS walls, experimental results demonstrate that besides performing dam-age-free when subjected to high drift levels, the proof-of-concept pendulum LiF-UPTS wall offers the following promising outcomes: (1) insignificant to no wall uplift, (2) insignificant to no wall base shear sliding, (3) reduced stress concentrations at the wall toes because contact stresses are distributed along the wall-base interface over a larger region, (4) nearly constant post-tensioning forces under high drift levels, which limits post-tensioning losses due to yielding of prestressing bars or tendons, (5) increase in energy dissipation capacity of the system through friction, and (6) decrease in the damping reduction factor and thus, reduction in the lateral displacement and force demands in pendulum LiF-UPTS walls. These research outcomes are likely to translate positively to other shear wall types, namely reinforced concrete (RC) precast pendulum UPTS walls. 

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4. Curvilinear Kirigami Skins Let Soft Bending Actuators Slither Faster

   https://doi.org/10.3389/frobt.2022.872007  

   Branyan, Callie; Rafsanjani, Ahmad; Bertoldi, Katia; Hatton, Ross L.; Mengüç, Yiğit (May 2022, Frontiers in Robotics and AI)

   The locomotion of soft snake robots is dependent on frictional interactions with the environment. Frictional anisotropy is a morphological characteristic of snakeskin that allows snakes to engage selectively with surfaces and generate propulsive forces. The prototypical slithering gait of most snakes is lateral undulation, which requires a significant lateral resistance that is lacking in artificial skins of existing soft snake robots. We designed a set of kirigami lattices with curvilinearly-arranged cuts to take advantage of in-plane rotations of the 3D structures when wrapped around a soft bending actuator. By changing the initial orientation of the scales, the kirigami skin produces high lateral friction upon engagement with surface asperities, with lateral to cranial anisotropic friction ratios above 4. The proposed design increased the overall velocity of the soft snake robot more than fivefold compared to robots without skin. 

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5. The evolution of rock friction is more sensitive to slip than elapsed time, even at near-zero slip rates

   https://doi.org/10.1073/pnas.2119462119  

   Bhattacharya, Pathikrit; Rubin, Allan M.; Tullis, Terry E.; Beeler, Nicholas M.; Okazaki, Keishi (July 2022, Proceedings of the National Academy of Sciences)

   Nearly all frictional interfaces strengthen as the logarithm of time when sliding at ultra-low speeds. Observations of also logarithmic-in-time growth of interfacial contact area under such conditions have led to constitutive models that assume that this frictional strengthening results from purely time-dependent, and slip-insensitive, contact-area growth. The main laboratory support for such strengthening has traditionally been derived from increases in friction during “load-point hold” experiments, wherein a sliding interface is allowed to gradually self-relax down to subnanometric slip rates. In contrast, following step decreases in the shear loading rate, friction is widely reported to increase over a characteristic slip scale, independent of the magnitude of the slip-rate decrease—a signature of slip-dependent strengthening. To investigate this apparent contradiction, we subjected granite samples to a series of step decreases in shear rate of up to 3.5 orders of magnitude and load-point holds of up to 10,000 s, such that both protocols accessed the phenomenological regime traditionally inferred to demonstrate time-dependent frictional strengthening. When modeling the resultant data, which probe interfacial slip rates ranging from 3 . μ m · s − 1 . to less than 10 − 5 μ m · s − 1 , we found that constitutive models where low slip-rate friction evolution mimics log-time contact-area growth require parameters that differ by orders of magnitude across the different experiments. In contrast, an alternative constitutive model, in which friction evolves only with interfacial slip, fits most of the data well with nearly identical parameters. This leads to the surprising conclusion that frictional strengthening is dominantly slip-dependent, even at subnanometric slip rates. 

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