Not so fast: strike kinematics of the araneoid trap-jaw spider Pararchaea alba (Malkaridae: Pararchaeinae)

To capture prey otherwise unattainable by muscle function alone, some animal lineages have evolved movements that are driven by stored elastic energy, producing movements of remarkable speed and force. One such example that has evolved multiple times is a trap-jaw mechanism, in which the mouthparts of an animal are loaded with energy as they open to a wide gape and then, when triggered to close, produce a terrific force. Within the spiders (Araneae), this type of attack has thus far solely been documented in the palpimanoid family Mecysmaucheniidae but a similar morphology has also been observed in the distantly related araneoid subfamily Pararchaeinae, leading to speculation of a trap-jaw attack in that lineage as well. Here, using high-speed videography, we test whether cheliceral strike power output suggests elastic-driven movements in the pararchaeine Pararchaea alba. The strike speed attained place P. alba as a moderately fast strikers exceeding the slowest mecysmaucheniids, but failing to the reach the most extreme high-speed strikers that have elastic-driven mechanisms. Using micro-Computed-Tomography, we compare the morphology of P. alba chelicerae in the resting and open positions, and their related musculature, and based on results propose a mechanism for cheliceral strike function that includes a torque reversal latching mechanism. Similar to the distantly related trap-jaw mecysmaucheniid spiders, the unusual prosoma morphology in P. alba seemingly allows for highly maneuverable chelicerae with a much wider gape than typical spiders, suggestive that increasingly maneuverable joints coupled with a latching mechanism may serve as a precursor to elastic-driven movements.

Design of Stackable Origami Structures With Elastic Deployment Capabilities

This work presents a flexible type of origami structure that may be elastically deployed from a compact stacked form to a freeform target shape. The design process enables a target surface mesh to be converted into a compact stacked structure that may be deployed through the release of elastic energy stored in the folds. The process begins by finding a non-branching path passing once through each face in the target mesh. The edges of the target mesh not included in the path are cut and elastic smooth folds are introduced along those crossed by the path. The introduced smooth folds are folded in a sequence of ±180° along the path to create a stack. The structure transforms from the stacked form towards the target shape through the release of the stored elastic energy generated during stacking. The design framework considers the strain energy needed to sustain transformation and the required sizing of the smooth folds. The resemblance of the designed target shape with smooth folds compared to the target mesh is studied, and the significant volume saving when the structure is stowed in the stacked form is quantified. Examples showing the application of the design process to a diverse set of target meshes are provided. Proof-of-concept prototype fabrication using a 3D printer demonstrates the feasibility of the design approach. The results reflect the benefits of deployable stacked origami structures and show volumetric space savings from 50% to 90% while preserving around 80% of the target mesh area after the elastic smooth folds are introduced.

A physical model of mantis shrimp for exploring the dynamics of ultrafast systems

Efficient and effective generation of high-acceleration movement in biology requires a process to control energy flow and amplify mechanical power from power density–limited muscle. Until recently, this ability was exclusive to ultrafast, small organisms, and this process was largely ascribed to the high mechanical power density of small elastic recoil mechanisms. In several ultrafast organisms, linkages suddenly initiate rotation when they overcenter and reverse torque; this process mediates the release of stored elastic energy and enhances the mechanical power output of extremely fast, spring-actuated systems. Here we report the discovery of linkage dynamics and geometric latching that reveals how organisms and synthetic systems generate extremely high-acceleration, short-duration movements. Through synergistic analyses of mantis shrimp strikes, a synthetic mantis shrimp robot, and a dynamic mathematical model, we discover that linkages can exhibit distinct dynamic phases that control energy transfer from stored elastic energy to ultrafast movement. These design principles are embodied in a 1.5-g mantis shrimp scale mechanism capable of striking velocities over 26 m s−1 in air and 5 m s−1 in water. The physical, mathematical, and biological datasets establish latching mechanics with four temporal phases and identify a nondimensional performance metric to analyze potential energy transfer. These temporal phases enable control of an extreme cascade of mechanical power amplification. Linkage dynamics and temporal phase characteristics are easily adjusted through linkage design in robotic and mathematical systems and provide a framework to understand the function of linkages and latches in biological systems.

Energetic criterion for adhesion in viscoelastic contacts with non-entropic surface interaction

We suggest a detachment criterion for a viscoelastic elastomer contact based on Griffith's idea about the energy balance at an infinitesimal advancement of the boundary of an adhesive crack. At the moment of detachment of a surface element at the boundary of an adhesive contact, there is some quick (instant) relaxation of stored elastic energy which can be expressed in terms of the creep function of the material. We argue that it is only this "instant part" of stored energy which is available for doing work of adhesion and thus it is only this part of energy relaxation that must be used in Griffith's energy balance. The described idea has several restrictions. Firstly, in this pure form, it is only valid for adhesive forces having an infinitely small range of action (which we call the JKR-limit). Secondly, it is only applicable to non-entropic (energetic) interfaces, which detach "at once" and do not possess their own kinetics of detachment.

Physics-Based Estimates of the Maximum Magnitude of Induced Earthquakes in the Groningen Gas Field

<p>The induced seismicity in the Groningen gas field, The Netherlands, has led to intense public concerns and comprehensive investigations. One of the main challenges for assessing future seismic hazard in the Groningen gas field is to estimate the maximum possible earthquake magnitude (Mmax) that could be induced by gas extraction. Previous methods are strongly rooted in empirical and statistical approaches that are inherently limited by the scarcity of data. Here, we combine a physics-based dynamic rupture model based on the 3D theory of fracture mechanics with field-based and lab-based constraints to estimate Mmax in the Groningen gas field. If earthquakes in the reservoir have a rupture depth extension constrained by the reservoir thickness, the largest earthquakes should develop a large aspect ratio (longer horizontally than vertically). The model is thus an extension of the 3D theoretical rupture model on long faults with uniform stress and strength developed by Weng & Ampuero (2019), in which we have incorporated spatial heterogeneities, such as along-strike variable fault width, depth-dependent initial stresses and friction properties. The essential parameters that control rupture propagation and earthquake magnitude are the stored elastic energy and the fracture energy. Our method requires estimates of the stored elastic energy on reservoir faults as a result of the stresses induced by differential reservoir compaction during depletion. The fracture energy is constrained by laboratory experiments and theoretical frictional models. Coupling physics-based rupture models with field and lab observations provides an estimate of Mmax in the Groningen gas field and serves as a practical step toward physics-based seismic hazard assessment for other gas fields in the world.</p><p> </p><p>Citation:</p><p>Weng, H. and J. P. Ampuero (2019). "The Dynamics of Elongated Earthquake Ruptures." Journal of Geophysical Research: Solid Earth.</p><p><br><br></p>

An Experimental Method for Estimating the Tearing Energy in Rubber-like Materials Using the True Stored Energy

A method for determining the critical tearing energy in rubber-like materials is proposed. In this method, the energy required for crack propagation in a rubber-like material is determined by the change of the recovered elastic energy. Hence, the dissipated energy due to different inelastic processes is deducted from the total strain energy applied to the system. Therefore, the classical method proposed by Rivlin and Thomas using the pure shear tear test is modified using the actual stored elastic energy. The elastically stored energy in a pure shear is determined experimentally using cyclic loading under quasi-static loading rate of 0.01 s-1 for different unloading rates, i.e. 0.01, 0.1 and 1.0 s-1. The experimental results show that the classical method overestimates the critical tearing energy by approximately 18% and the unloading rate is minimal which suggests that the dissipation depends only on the loading path.

Size Effect in Plastic Deformation and Failure of Metallic Glasses

Depending on the composition and structure of metallic glasses cells with the dimensions in the range from tenths nanometers to tenths micrometers were observed on the ductile fracture surface. The variation in dimple size was compared with the serrations presented on the loading curve at the nanoindentation of the metallic glasses with different compositions. Higher instantaneous deformation can be connected with simultaneous shearing at more suitable shear band configurations. The cell morphology with the various cell sizes is observed at the failure of the metallic glasses. At the failure of high strength metallic glasses, the cells are formed in short time due to the release of high amount of stored elastic energy. In this case the uniform cell morphology with the cell size of about 20 nm is observed.

The control of nocifensive movements in the caterpillar Manduca sexta

ABSTRACTIn response to a noxious stimulus on the abdomen, caterpillars lunge their head towards the site of stimulation. This nocifensive ‘strike’ behavior is fast (∼0.5 s duration), targeted and usually unilateral. It is not clear how the fast strike movement is generated and controlled, because caterpillar muscle develops peak force relatively slowly (∼1 s) and the baseline hemolymph pressure is low (<2 kPa). Here, we show that strike movements are largely driven by ipsilateral muscle activation that propagates from anterior to posterior segments. There is no sustained pre-strike muscle activation that would be expected for movements powered by the rapid release of stored elastic energy. Although muscle activation on the ipsilateral side is correlated with segment shortening, activity on the contralateral side consists of two phases of muscle stimulation and a marked decline between them. This decrease in motor activity precedes rapid expansion of the segment on the contralateral side, presumably allowing the body wall to stretch more easily. The subsequent increase in contralateral motor activation may slow or stabilize movements as the head reaches its target. Strike behavior is therefore a controlled fast movement involving the coordination of muscle activity on each side and along the length of the body.

Microtubule instability driven by longitudinal and lateral strain propagation

Tubulin dimers associate longitudinally and laterally to form metastable microtubules (MTs). MT disassembly is preceded by subtle structural changes in tubulin fueled by GTP hydrolysis. These changes render the MT lattice unstable, but it is unclear exactly how they affect lattice energetics and strain. We performed long-time atomistic simulations to interrogate the impacts of GTP hydrolysis on tubulin lattice conformation, lateral inter-dimer interactions, and (non-)local lateral coordination of dimer motions. The simulations suggest that most of the hydrolysis energy is stored in the lattice in the form of longitudinal strain. While not significantly affecting lateral bond stability, the stored elastic energy results in more strongly confined and correlated dynamics of GDP-tubulins, thereby entropically destabilizing the MT lattice.

The Effects of Augmented Eccentric Loading upon Kinematics and Muscle Activation in Bench Press Performance

The aim of this study was to investigate the effects of an augmented eccentric load upon the kinematics and muscle activation of bench press, and to investigate possible mechanisms behind augmented eccentric loading during the lift. Sixteen resistance-trained males (age 28.5 ± 7.7 years, height 1.78 ± 0.08 m, body mass 80.7 ± 14.3 kg) performed three repetitions at 95/85% of 1RM (augmented eccentric loading), and 85/85% of 1RM (control) in bench press, while barbell kinematics and muscle activation of eight muscles were measured. The main findings were that no kinematic differences between the augmented and control condition were found, only an effect of repetition. Furthermore, augmented loading caused a higher activation of the biceps brachii during the pre-sticking and sticking region, while a lower activation in the sternal part of pectoralis major during the eccentric phase was observed. Based on the present findings, it can be concluded that augmented eccentric loading with 95% of 1RM in bench press did not have any acute positive effect upon the concentric phase of the lift (85% of 1RM) and that the proposed underlying mechanisms like potentiation, increased neural stimulation and preload, and recovery of stored elastic energy does not seem to occur with these loads.