Slab Avalanche Modeling

The chapter starts with a review of a few unfounded arguments sometimes used to account for snow slab instability, and often resulting from the application of mechanical laws that are invalid in a granular, brittle, and healable material like snow. Statistical aspects are investigated using a two-threshold cellular automaton, one for basal instability, and the second one for crown crack opening. The results reproduce the power-law size distribution of starting zone sizes mentioned in chapter 4, and validate a “4-step” triggering scheme made of successive initiation and expansion events for both the basal crack and the crown crack. The possible sintering of collapsed weak layers is then analyzed. It is shown to flow as a slurry for shear strain rates less than a predetermined threshold, or to sinter in the opposite case, which provides a “joker” to any successful “4-step” scheme, turning an incipient avalanche into a simple “whumpf”.

Inflorescence and floral traits of the Colombian species of Tristerix (Loranthaceae) related to hummingbird pollination

Floral diversification in Loranthaceae reaches its highest peak in the Andes. The flowers of the exclusively Andean genus Tristerix exhibit tubular and vividly coloured flowers pollinated by hummingbirds. We studied inflorescence and flower morphoanatomy of the two Colombian species, T. longebracteatus and the highly endangered T. secundus. Both species have terminal racemes with up to 26 ebracteolate flowers, of which the proximal one opens and sets fruits first. The slightly irregular calyx initiation is followed by the simultaneous initiation of petals and the successive initiation of stamens. Anthesis is fenestrate, explosive, and triggered by the tension of the style against the abaxial petals, a mode so far not reported in Loranthaceae. Anthetic petals spread symmetrically in T. longebracteatus and asymmetrically in T. secundus. Nectar is produced by a supraovarial disk and by the petal mesophyll. Floral lifespan lasts up to 20 days. The hummingbirds Eriocnemis vestita and Pterophanes cyanopterus are the likely pollinators of T. secundus. Morphological traits are inconclusive to support one of the two competing sister group relationships that involve Tristerix, as the lack of cataphylls in renewal shoots links Ligaria and Tristerix, whereas the terminal inflorescences support its relationship with Desmaria and Tupeia.

Interaction of marine ice-sheet instabilities in two drainage basins: simple scaling of geometry and transition time

The initiation of a marine ice-sheet instability (MISI) is generally discussed from the ocean side of the ice sheet. It has been shown that the reduction in ice-shelf buttressing and softening of the coastal ice can destabilize a marine ice sheet if the bedrock is sloping upward towards the ocean. Using a conceptional flow-line geometry, we investigate the possibility of whether a MISI can be triggered from the direction of the ice divide as opposed to coastal forcing and explore the interaction between connected basins. We find that the initiation of a MISI in one basin can induce a destabilization in the other. The underlying mechanism of basin interaction is based on dynamic thinning and a consecutive motion of the ice divide which induces a thinning in the adjacent basin and a successive initiation of the instability. Our simplified and symmetric topographic setup allows scaling both the geometry and the transition time between both instabilities. We find that the ice profile follows a universal shape that is scaled with the horizontal extent of the ice sheet and that the same exponent of 1/2 applies for the scaling relation between central surface elevation and horizontal extent as in the pure shallow ice approximation (Vialov profile). Altering the central bed elevation, we find that the extent of grounding-line retreat in one basin determines the degree of interaction with the other. Different scenarios of basin interaction are discussed based on our modeling results as well as on a conceptual flux-balance analysis. We conclude that for the three-dimensional case, the possibility of drainage basin interaction on timescales on the order of 1 kyr or larger cannot be excluded and hence needs further investigation.

Dynamics of cavitation in a Douglas-fir tree-ring: transition-wood, the lord of the ring?

The objective of this work was to investigate the dynamics of embolism formation within a Douglas-fir tree-ring. Four resistant and four vulnerable 10-year-old trees were selected among 50 trees, based on their P50. Stem samples, taken next to those used to obtain the vulnerability to cavitation curves, were collected and submitted to increasing positive pressures, in order to simulate increasing tension caused by water stress in the xylem. Then the conductive surface of the samples was stained and scanned and the images were analyzed. X-ray microdensity profiles were obtained on the same samples. The microdensity profiles of the 2011 ring were analyzed in three parts, earlywood, transition-wood and latewood. The dynamics of embolism propagation was observed separately in these three parts. Our results showed that the initiation and the propagation of the cavitation follow a discrete trend, with at least two successive initiation events: first cavitation initiates and propagates rapidly in the latewood. Then, a second cavitation event begins and spreads in the earlywood and eventually propagates to the transition-wood, which remains the last conductive part in the ring before full embolism. We observed that resistant to cavitation trees showed lower transition-wood density than vulnerable to cavitation trees.

Successive Softening and Cyclic Damage in Viscoplastic Material

A successive initiation finite element modeling approach is presented in which an empirical continuum damage model, energy partitioning damage evolution model, developed by the author is used to update state of damage and constitutive properties of the material under thermomechanical cyclic loading and accumulate damage in the elements. Plastic and viscoplastic damages are evaluated based on the plastic and viscoplastic work densities obtained through finite element. Constitutive properties are updated elementwise at each step of the process based on the state of damage in each element. The elements that have reached the damage threshold are removed from the structure to initiate and propagate fatigue crack. This successive initiation approach is used to model crack initiation and propagation in Pb-free solder material under thermomechanical loading. A case study is presented, damage propagation path and pattern are compared with typical experimental results, and the accuracy of the model was verified.

A Safe Use of Explosives by Parting into a Small Amount of Powder

High explosives are useful material to generate great amount of energy in short time. Since controlling their releasing energy is hard, the use of explosive is limited to breaking and crushing the tough structures and processing of bulk materials. However, the reduction of mass of the explosive powders in a process and the arrangement of pellets with constant intervals provide us the safe handling, and then those lead the new utility of the explosion, while there are some difficulties encountered when a small amount of explosive powder is used, such as the initiation regularity of explosives and the protection of mechanical parts from impact damages. In this paper, the successive initiation of small explosive was tested by means of the wire explosion that is generated by the instant release of electric energy from high volt capacitors, and the successful results were obtained under the controlled condition. The damages of surrounding devices were avoided by using of the initiating head of the device that had small chamber isolated from the outer atmospheric environment.