No abstract

The formulation of low order finite elements in the finite deformation range that exhibit improved properties over the basic Galerkin approach is of maximum interest. For instance, the locking-free response in the incompressible limit and in bending dominated problems, together with a good accuracy for coarse meshes, are some of the properties required for typical practical applications. A good response in localization problems is also a desired feature in problems involving large inelastic strains. These properties have to be combined with a simple numerical implementation, even in the general context of elastoplastic and fully coupled materials models, thus leading almost inevitably to formulations based on a strain driven structure. The effectiveness observed by low order elements in this general class of problems, especially in problems involving contact, has led to intensive research in this area.The formulation of bilinear quadrilaterals and trilinear bricks in the finite deformation range that exhibit these properties has been presented in Simo and Armero (1992) following the so-called enhanced strain formulation, originally proposed in Simo and Rifai (1990) for infinitesimal problems. To a large extent, this formulation is motivated by the excellent performance of the now classical QM6 element presented in Taylor et al. (1976). The finite deformation formulation consisted of a local additive enhancement of the deformation gradient, leading to a very simple and efficient numerical implementation. The elements developed in this framework were denoted by Q1/E4, Q1/E5, and Q1/E9 for plane strain, axisymmetric, and three dimensional problems

No abstract

The OpenWSN project is an open-source implementation of a fully standards-based protocol stack for capillary networks, rooted in the new IEEE802.15.4e Time Synchronized Channel Hopping standard. IEEE802.15.4e, coupled with Internet of Things standards, such as 6LoWPAN, RPL and CoAP, enables ultra-low-power and highly reliable mesh networks, which are fully integrated into the Internet. The resulting protocol stack will be cornerstone to the upcoming machine-to-machine revolution.This article gives an overview of the protocol stack, as well as key integration details and the platforms and tools developed around it. The pure-C OpenWSN stack was ported to four off-the-shelf platforms representative of hardware currently used, from older 16-bit microcontroller to state-of-the-art 32-bit Cortex-M architectures. The tools developed around the low-power mesh networks include visualisation and debugging software, a simulator to mimic OpenWSN networks on a PC, and the environment needed to connect those networks to the Internet.Experimental results presented in this article include a network where motes operate at an average radio duty cycle well below 0.1% and an average current draw of 68 A on off-the-shelf hardware. These ultra-low-power requirements enable a range of applications, with motes perpetually powered by micro-scavenging devices. OpenWSN is, to the best of our knowledge, the first open-source implementation of the IEEE802.15.4e standard.

Collapse of cliff faces by rockfall is a primary mode of bedrock erosion in alpine environments and exerts a first-order control on the morphologic development of these landscapes. In this work we investigate the influence of rock mass strength on the retreat rate of alpine cliffs. To quantify rockwall competence we employed the Slope Mass Rating (SMR) geomechanical strength index, a metric that combines numerous factors contributing to the strength of a rock mass. The magnitude of cliff retreat was calculated by estimating the volume of talus at the toe of each rockwall and projecting that material back on to the cliff face, while accounting for the loss of production area as talus buries the base of the wall. Selecting sites within basins swept clean by advancing Last Glacial Maximum (LGM) glaciers allowed us to estimate the time period over which talus accumulation occurred (i.e. the production time). Dividing the magnitude of normal cliff retreat by the production time, we calculated recession rates for each site. Our study area included a portion of the Sierra Nevada between Yosemite National Park and Lake Tahoe. Rockwall recession rates determined for 40 alpine cliffs in this region range from 0·02 to 1·22 mm/year, with an average value of 0·28 mm/ year. We found good correlation between rockwall recession rate and SMR which is best characterized by an exponential decrease in erosion rate with increasing rock mass strength. Analysis of the individual components of the SMR reveals that joint orientation (with respect to the cliff face) is the most important parameter affecting the rockwall erosion rate. The complete SMR score, however, best synthesizes the lithologic variables that contribute to the strength and erodibility of these rock slopes. Our data reveal no strong independent correlations between rockwall retreat rate and topographic attributes such as elevation, aspect, or slope angle.Intact rock strength, the presence and geometry of discontinuities (frequency, orientation, persistence, aperture, roughness, infilling), and the rockwall environment (degree of weathering, amount of water or vegetation) combine to control the mechanical behavior of a rock slope

This paper describes sensor calibration and signal analysis techniques applicable to the method of acoustic emission (AE) and ultrasonic testing. They are particularly useful for obtaining absolute measurements of AE wave amplitude and shape, which can be used to constrain the physics and mechanics of the AE source. We illustrate how to perform calibration tests on a thick plate and how to implement two different mechanical calibration sources: ball impact and glass capillary fracture. In this way, the instrument response function can be estimated from theory, without the need for a reference transducer. We demonstrate the methodology by comparing calibration results for four different piezoelectric acoustic emission sensors: Physical Acoustics (PAC) PAC R15, PAC NANO30, DigitalWave B1025, and the Glaser-type conical sensor. From the results of these tests, sensor aperture effects are quantified and the accuracy of calibration source models is verified. Finally, this paper describes how the effects of the sensor can be modeled using an autoregressive-moving average (ARMA) model, and how this technique can be used to effectively remove sensor-induced distortion so that a displacement time history can be retrieved from recorded signals.

Abstract. Multi-year droughts in Mediterranean climates may shift the water balance, that is, the partitioning rule of precipitation across runoff, evapotranspiration, and sub-surface storage. Mechanisms causing these shifts remain largely unknown and are not well represented in hydrologic models. Focusing on measurements from the headwaters of California's Feather River, we found that also in these mixed rain–snow Mediterranean basins a lower fraction of precipitation was partitioned to runoff during multi-year droughts compared to non-drought years. This shift in the precipitation–runoff relationship was larger in the surface-runoff-dominated than subsurface-flow-dominated headwaters (−39 % vs. −18 % decline of runoff, respectively, for a representative precipitation amount). The predictive skill of the Precipitation Runoff Modeling System (PRMS) hydrologic model in these basins decreased during droughts, with evapotranspiration (ET) being the only water-balance component besides runoff for which the drop in predictive skill during drought vs. non-drought years was statistically significant. In particular, the model underestimated the response time required by ET to adjust to interannual climate variability, which we define as climate elasticity of ET. Differences between simulated and data-driven estimates of ET were well correlated with accompanying data-driven estimates of changes in sub-surface storage (ΔS, r=0.78). This correlation points to shifts in precipitation–runoff relationships being evidence of a hysteretic response of the water budget to climate elasticity of ET during and after multi-year droughts. This hysteresis is caused by carryover storage offsetting precipitation deficit during the initial drought period, followed by vegetation mortality when storage is depleted and subsequent post-drought vegetation expansion. Our results point to a general improvement in hydrologic predictions across drought and recovery cycles by including the climate elasticity of ET and better accounting for actual subsurface water storage in not only soil, but also deeper regolith that stores water accessible to roots. This can be done by explicitly parametrizing carryover storage and feedback mechanisms capturing vegetation response to atmospheric demand for moisture.

Faults strengthen or heal with time in stationary contact, and this healing may be an essential ingredient for the generation of earthquakes. In the laboratory, healing is thought to be the result of thermally activated mechanisms that weld together micrometre-sized asperity contacts on the fault surface, but the relationship between laboratory measures of fault healing and the seismically observable properties of earthquakes is at present not well defined. Here we report on laboratory experiments and seismological observations that show how the spectral properties of earthquakes vary as a function of fault healing time. In the laboratory, we find that increased healing causes a disproportionately large amount of high-frequency seismic radiation to be produced during fault rupture. We observe a similar connection between earthquake spectra and recurrence time for repeating earthquake sequences on natural faults. Healing rates depend on pressure, temperature and mineralogy, so the connection between seismicity and healing may help to explain recent observations of large megathrust earthquakes which indicate that energetic, high-frequency seismic radiation originates from locations that are distinct from the geodetically inferred locations of large-amplitude fault slip.