Refinements of empirical models to forecast the shear strength of persistent weak snow layers PART A: Layers of faceted crystals

This paper presents an adjustment for the normal load for persistent weak layers in the Columbia Mountains of Canada and an improved empirical model to forecast the shear strength of layers of faceted crystals for indexing the stability of the snowpack. This model is based on manual snowprofile observations, as was a previous model; however, the measured shear strength is adjusted for the normal load by a constant factor of ϕ = 0.21. The study includes the analysis of more field data for the rate of change of shear strength, which is assessed for daily and average loading rates. The Forecasting Model predicts the shear strength of layers of faceted crystals with an accuracy of 77–79% depending on whether the shear strength change between snowprofile observations is based on daily or average loading rates. A companion paper, Part B, develops a forecasting model for layers of buried surface hoar.     

An adjustment for kinetic growth metamorphism to improve shear strength parameterization in the SNOWPACK model

Shear strength is an important parameter for interpreting the stability of simulated snow covers. In the SNOWPACK model, snow shear strength is estimated as a function of snow density using expressions for different grain types. In the model, shear strength changes discontinuously as grain type changes from rounded to faceted grains (and vice versa), but in nature, shear strength changes take place more gradually. An experiment on the growth of depth hoar allows a new parameterization of continuous changes in shear strength. A parameter is induced in the expression for shear strength as a function of water vapor transport. It ranges from 0 (rounded grains) to 1 (depth hoar) depending on the metamorphic stage of the snow. The parameterization is incorporated into the SNOWPACK model to calculate the progressive change in shear strength during snow metamorphism. The calculated shear strength using the improved SNOWPACK model agreed well with that measured in cold-room experiments using artificial snow. This model, which can calculate gradually changing shear strength, is expected to improve the accuracy of avalanche forecasting.     

An adjustment for kinetic growth metamorphism to improve shear strength parameterization in the SNOWPACK model

Shear strength is an important parameter for interpreting the stability of simulated snow covers. In the SNOWPACK model, snow shear strength is estimated as a function of snow density using expressions for different grain types. In the model, shear strength changes discontinuously as grain type changes from rounded to faceted grains (and vice versa), but in nature, shear strength changes take place more gradually. An experiment on the growth of depth hoar allows a new parameterization of continuous changes in shear strength. A parameter is induced in the expression for shear strength as a function of water vapor transport. It ranges from 0 (rounded grains) to 1 (depth hoar) depending on the metamorphic stage of the snow. The parameterization is incorporated into the SNOWPACK model to calculate the progressive change in shear strength during snow metamorphism. The calculated shear strength using the improved SNOWPACK model agreed well with that measured in cold-room experiments using artificial snow. This model, which can calculate gradually changing shear strength, is expected to improve the accuracy of avalanche forecasting.     

基于黏塑性理论的碾压混凝土动态剪切本构模型

由于碾压混凝土大坝是逐层碾压而成,坝体层面处的静动抗剪强度均低于其本体强度,在地震、振动或冲击作用下,坝体层面(包括坝基界面)有可能发生沿层面的动态滑移失稳破坏。基于Perzyna黏塑性连续理论,提出了一个用于描述碾压混凝土层面动态剪切断裂行为的本构模型。该模型的特点有：混凝土材料的软化塑性和扩容特性直接与界面处断裂失效过程相联系;使用Carol率相关界面方程作为屈服判据来描述碾压混凝土材料的率相关性;使用经典塑性断裂理论来描述剪切面上的断裂失效和摩擦滑动过程,并且只需要较少的模型参数。利用该模型与含层面碾压混凝土的动态强度试验结果进行了对比分析,包括在不同的应力路径如单轴拉、压和压剪状态下和不同加载速率下的试验结果。结果表明模型与试验得出的结论吻合较好,这种弹-黏塑性动态剪切本构模型对预测包含薄弱层面的碾压混凝土动力破坏性能是有效的,这为相关工程问题的研究提供有益的思路和有效的工具。     

Loading rate effect as a function of the span-to-depth ratio in three-point bend testing of unidirectional pultruded composites

Three-point bend tests were performed on pultruded half-round cross-section rods of glass fibre-reinforced polyester and glass fibre-reinforced epoxy, the tests being arranged so that the specimen was unloaded as soon as the deflection corresponding to the maximum load was reached. This testing method enabled the fracture mechanisms at maximum loads to be mapped as a function of the loading rate and the span-to-depth ratios. An effect highlighted by this map is that a specimen tested at intermediate span-to-depth ratios may result in a purely tensile fracture at low loading rates and a purely shear fracture at high loading rates. This behaviour is interpreted as being due to the fact that the increase in loading rate increases the brittleness of the material, subsequently increasing the defect sensitivity and leading to a shear fracture. This explanation is supported by fracture surface observations. In addition, it was found that the shear strength increases significantly with increasing loading rate up to 5000 mm min⁻¹ and then starts to decrease slightly. The decrease in shear strength at very high loading rates was found to correspond to multiple cracking behaviour rather than to a longitudinal fracture plane as for intermediate loading rates.     

Modeling the spatial distribution of surface hoar in complex topography

Buried surface hoar is a well-known weak snowpack layer, often associated with snow avalanches. Knowledge about the spatial distribution of surface hoar is therefore of great importance for avalanche forecasting. We investigate if spatial variations of surface hoar in mountainous terrain can be modeled based on terrain characteristics. Using a detailed radiation balance model, distributed radiation over an ensemble of 1800 simulated topographies, covering a wide range of terrain characteristics, was computed. Light winds and increased relative humidity were assumed to be favorable for surface hoar formation. To describe surface hoar formation, we derived a sky view factor threshold associated with the minimum snow surface cooling necessary for surface hoar formation based on laboratory measurements. To describe surface hoar destruction, as a first approach, we assumed that surface hoar only survives on shaded slopes. Applying two simple thresholds to our spatial radiation modelings, our results show that the spatial distribution of surface hoar is greatly affected by large-scale terrain roughness and sun elevation angle. Spatial correlation ranges for surface hoar, on the order of several hundred meters, were closely related to the typical spacing between mountains. Furthermore, correlation ranges of surface hoar decreased with increasing sun elevation angle. Overall, the modeled spatial patterns of surface hoar were in line with previously published spatial field observations, suggesting that simple terrain parameters can very well be used to describe the predominant surface hoar layer patterns in complex topography.     

Atomistic modeling of Mg/Nb interfaces: shear strength and interaction with lattice glide dislocations

Using a newly developed embedded-atom-method potential for Mg–Nb, the semi-coherent Mg/Nb interface with the Kurdjumov–Sachs orientation relationship is studied. Atomistic simulations have been carried out to understand the shear strength of the interface, as well as the interaction between lattice glide dislocations and the interface. The interface shear mechanisms are dependent on the shear loading directions, through either interface sliding between Mg and Nb atomic layers or nucleation and gliding of Shockley partial dislocations in between the first two atomic planes in Mg at the interface. The shear strength for the Mg/Nb interface is found to be generally high, in the range of 0.9–1.3 GPa depending on the shear direction. As a consequence, the extents of dislocation core spread into the interface are considerably small, especially when compared to the case of other “weak” interfaces such as the Cu/Nb interface.     

Fiber/matrix interphase characterization using the dynamic interphase loading apparatus

The dynamic interphase loading apparatus (DILA) developed to perform microdebonding push-out tests directly characterizes the fiber/matrix interphase properties of composites as a function of loading rate. The use of a piezoelectric transducer allows one to input a variety of displacements and loading rates (quasi-static to 50 mm/s) to the indenter. Transient force and displacement values, recorded during the test, are then used to determine the average shear strength and energy absorbed during debonding and frictional sliding during the microdebonding process. An E-glass/vinyl ester composite was tested under single microdebonding as well as fatigue loading. Test results showed that the strength and energy-absorbing capability of the interphase was sensitive to loading rate.     

A new technique to characterize the fiber/matrix interphase properties under high strain rates

Dynamic interphase-loading apparatus (DILA) has been developed to directly characterize the fiber/matrix interphase properties of composites under high loading rates. This apparatus uses a micro-mechanical method (micro-indentation) that is based on the debonding of a fiber from the matrix at the interphase region. Displacement rates up to 3000 μm/s that cause deformation of the interphase under high shear strain rates were obtained using the fast expansion capability of piezoelectric actuators (PZT). Transient force and fiber displacement for a specific displacement rate is measured during the test. The data are reduced to apparent average interphase shear strength and energy absorbed during debonding and frictional sliding during the micro-debonding process. An E-glass-fiber/epoxy-amine interphase was tested under various loading rates to demonstrate the capability of the test apparatus. Test results showed that the strength and energy-absorbing capability of the E-glass/epoxy-amine interphase are sensitive to loading rate.     

A hybrid rigid body rotation model for predicting a response of a temporary soil-filled wall subjected to blast loading

This study is aimed to provide an efficient analytical model to calculate a time history of response for a free-standing soil-filled HESCO Bastion concertainer^® wall subjected to air blast loading. The model is formulated based on the observations of the wall response to air blast loading in the experiments and on the deformation of a finite element model. This hybrid rigid body rotation model combines both a reverse Winkler foundation to model the distribution of pressure at the base of the wall and perfectly plastic shear resistance to model the shear deformation at the corner. The time histories of horizontal and vertical displacements calculated from the proposed analytical model are validated with displacements from both full-scale blast testing of free-standing simple straight walls and calculations using a finite element model.     

Torsional fatigue resistance of plasma sprayed HA coating on Ti–6Al–4V

The torsional strength of plasma sprayed hydroxyapatite (HA) coatings was studied under static and cyclic loading. The torsional shear tests were conducted in a frustum test device developed in this laboratory, which adapted to various coating thicknesses. The interfacial fatigue resistance was measured in terms of interfacial fatigue strength defined as the average maximum stress (_fmax). A staircase fatigue method was employed to determine the interfacial fatigue strength; this method resolved the uncertainty in detecting coating failure during torsion fatigue. The values for coating shear strength and shear fatigue strength obtained from the torsional tests did not differ from those obtained by previous tensional shear tests in this laboratory. The fatigue strength of one million cycles was about 35% lower than static shear strength. This finding might be used for estimating fatigue life span without cyclic loading tests.     

Influence of concrete matrix and type of fiber on the shear behavior of self-compacting fiber reinforced concrete beams

Steel fibers are known to improve shear behavior. The Design Codes (Eurocode 2 (EC2), Spanish EHE-08, Model Code 2010 and RILEM approach) have developed formulas to calculate the fiber contribution to shear, mainly focused on standard FRCs, i.e. medium strength concretes with a low content of normal strength steel fibers. However, in real applications other combinations are possible, such as high or medium strength concretes with high strength steel fibers of different lengths and geometry. An experimental program consisting of 12 self-compacting fiber reinforced concrete (SCFRC) I-type beams was carried out. All the beams had the same geometry and fiber content (50 kg/m³), and they were made with two different concrete compressive strength values and five different types of steel fibers and were tested for shear. The main conclusions reached were that the type of fiber substantially affects shear behavior, even when the Design Code formulas indicate similar contributions. The combination of high strength concrete matrixes with low strength fibers does not seem to be efficient. Also, the use of high residual flexural tensile strength values (e.g. f_R3 or f_R4) does not appear to be the most accurate reference value to calculate the beam shear strength in these cases. The present Design Codes consider standard FRCs, but their formulas should be revised for concretes with fibers of different strengths, slenderness and geometry, since these properties substantially affect shear behavior.     

纤维网格高延性混凝土加固RC柱抗剪性能试验研究

箍筋配置不当、剪跨比较小和轴压比较大的钢筋混凝土(RC)框架柱在地震作用下通常发生脆性剪切破坏。为提高框架柱的抗剪性能,提出采用碳纤维(CFRP)网格和高延性混凝土(HDC)复合加固RC柱。设计了6个RC柱试件,通过低周反复荷载试验,研究加固方式、纤维网格层数和轴压比对加固柱破坏形态、受剪承载力、延性及耗能能力的影响。结果表明：采用HDC和CFRP网格复合加固,可显著提高柱的抗剪承载力,明显改善其延性、变形性能和耗能能力;提高加固层的网格层数,对抗剪承载力影响较小,但加固柱的延性和变形能力得到较大改善;轴压比增大,复合柱的抗剪承载力稍有提高,但试件的延性、变形能力和耗能能力均降低;增加网格层数对高轴压比加固柱的增强效果和对低轴压比柱基本一致。最后基于桁架-拱模型,提出加固柱的抗剪承载力计算公式,计算结果与试验值吻合较好。     

Influence of snowpack layering on human-triggered snow slab avalanche release

Dry-snow slab avalanches involve the release of a cohesive slab over an extended plane of weakness. In most fatal avalanches, the triggering of the initial failure occurred by localized rapid near-surface loading by people — followed by fracture propagation. Whereas a limit-equilibrium (LE) approach to snow slope failure only takes into account slab depth, slab density and weak layer strength, it omits properties such as the stiffness of adjacent layers and the fracture propagation process. Nevertheless, LE has been applied with some success to the frequency of skier triggering, suggesting that it is relevant to failure initiation. Since field studies have shown that, for a given slab thickness, stiffer slabs are less likely to be triggered, slab properties influence failure initiation, fracture propagation or both. A highly simplified finite element (FE) model of static skier loading was used to assess the effect of slab and substratum properties on skier-induced stresses in the weak layer. Compared to a uniform slab, the skier-induced stress at the depth of the weak layer varied by a factor of 2 due to layering. In particular, the simplified FE model suggests that while stiffer layers in the slab will reduce the skier-induced stress in the weak layer, stiff layers just below the weak layer can increase the shear stress. These results were incorporated into a modified stability index and compared to stability test results. However, by taking into account snowpack layering the correlation between the modified stability index and stability test results did not improve. While our simulations suggest that less stress penetrates through stiffer slabs and thus fracture initiation is less likely, other studies show that, once initiated, fractures under stiffer slabs have high propagation propensity.     

Mixed-mode fracture load prediction in lead-free solder joints

Double cantilever beam (DCB) fracture specimens were made by joining copper bars with both continuous and discrete SAC305 solder layers of different lengths under standard surface mount (SMT) processing conditions. The specimens were then fractured under mode-I and various mixed-mode loading conditions. The loads corresponding to crack initiation in the continuous joints were used to calculate the critical strain energy release rate, J_ci, at the various mode ratios using elastic–plastic finite element analysis (FEA). It was found that the J_ci from the continuous joint DCBs provided a lower bound strength prediction for discrete 2 mm and 5 mm long joints at the various mode ratios. Additionally, these J_ci values calculated from FEA using the measured fracture loads agreed reasonably with J_ci estimated from measured crack opening displacements at crack initiation in both the continuous and discrete joints. Therefore, the critical strain energy release rate as a function of the mode ratio of loading is a promising fracture criterion that can be used to predict the strength of solder joints of arbitrary geometry subject to combined tensile and shear loads.     

Dynamic enhancement of blast-resistant ultra high performance fibre-reinforced concrete under flexural and shear loading

Two independent projects are described in which drop-hammer techniques are used to investigate the dynamic increase factor (DIF) under both flexural and shear high-speed loading of a new ultra high performance fibre reinforced blast-resistant concrete. The results from both studies correlate well. The results show that a DIF of the flexural tensile strength rising from 1.0 at 1 s⁻¹ on a slope of 1/3 on a log (strain rate) versus log (DIF) plot can be used for design purposes. The results also show that no DIF should be used to increase the shear strength at high loading rates.     

Interpretation of the human skin biotribological behaviour after tape stripping

The present study deals with the modification of the human skin biotribological behaviour after tape stripping. The tape-stripping procedure consists in the sequential application and removal of adhesive tapes on the skin surface in order to remove stratum corneum (SC) layers, which electrically charges the skin surface. The skin electric charges generated by tape stripping highly change the skin friction behaviour by increasing the adhesion component of the skin friction coefficient. It has been proposed to rewrite the friction adhesion component as the sum of two terms: the first classical adhesion term depending on the intrinsic shear strength, τ₀, and the second term depending on the electric shear strength, τ_elec. The experimental results allowed to estimate a numerical value of the electric shear strength τ_elec. Moreover, a plan capacitor model with a dielectric material inside was used to modelize the experimental system. This physical model permitted to evaluate the friction electric force and the electric shear strength values to calculate the skin friction coefficient after the tape stripping. The comparison between the experimental and the theoretical value of the skin friction coefficient after the tape stripping has shown the importance of the electric charges on skin biotribological behaviour. The static electric charges produced by tape stripping on the skin surface are probably able to highly modify the interaction of formulations with the skin surface and their spreading properties. This phenomenon, generally overlooked, should be taken into consideration as it could be involved in alteration of drug absorption.     

Quantifying changes in weak layer microstructure associated with artificial load changes

Researchers and practitioners have long utilized a variety of penetrometers to investigate the snowpack. Identifying definitive relationships between penetrometer-derived microstructural information and stability has been challenging. The purpose of this study is two-fold: 1. We propose a simple field test to establish relationships between load and penetrometer-derived microstructural estimates, 2. We utilize the SnowMicroPen (SMP) to quantify changes in weak layer residual strength and microstructural dimension associated with an artificial loading event. Our dataset is from Moonlight Basin, Montana and includes three modified loaded-column tests, each paired with 5 SMP profiles. Depth hoar comprised the targeted weak layer. Results indicate that loading caused the residual strength and rupture frequency to decrease significantly. Much like a compression test at a micro-scale, the force required for the SMP to rupture individual structures as well as the micro-scale strength decreased significantly when the slab stress was increased by artificially adding blocks of snow. A decrease in observed rupture frequency within the weak layer (or an increase in the distance between ruptured structures) also occurred after the loading event, probably because some structures within the weak layer had already failed or were so close to failing that the penetrometer could not detect their rupture. Due in part to the large difference in loads, microstructural differences between the natural and loaded columns were significant enough that only one profile would have been necessary to determine a significant difference in residual strength. Artificial removal of slab stress resulted in greater rupture forces and larger microstructures, likely due to elastic rebound.     

Quantifying changes in weak layer microstructure associated with artificial load changes

Researchers and practitioners have long utilized a variety of penetrometers to investigate the snowpack. Identifying definitive relationships between penetrometer-derived microstructural information and stability has been challenging. The purpose of this study is two-fold: 1. We propose a simple field test to establish relationships between load and penetrometer-derived microstructural estimates, 2. We utilize the SnowMicroPen (SMP) to quantify changes in weak layer residual strength and microstructural dimension associated with an artificial loading event. Our dataset is from Moonlight Basin, Montana and includes three modified loaded-column tests, each paired with 5 SMP profiles. Depth hoar comprised the targeted weak layer. Results indicate that loading caused the residual strength and rupture frequency to decrease significantly. Much like a compression test at a micro-scale, the force required for the SMP to rupture individual structures as well as the micro-scale strength decreased significantly when the slab stress was increased by artificially adding blocks of snow. A decrease in observed rupture frequency within the weak layer (or an increase in the distance between ruptured structures) also occurred after the loading event, probably because some structures within the weak layer had already failed or were so close to failing that the penetrometer could not detect their rupture. Due in part to the large difference in loads, microstructural differences between the natural and loaded columns were significant enough that only one profile would have been necessary to determine a significant difference in residual strength. Artificial removal of slab stress resulted in greater rupture forces and larger microstructures, likely due to elastic rebound.     

Simple-shear box experiments with floating ice rubble

A simple-shear box was used to study the shear strength characteristics of floating layers of vertically unconstrained ice rubble comprised of parallelpiped ice blocks. A comparative set of experiments was also performed using floating layers of parallelpiped plastic blocks in order to determine the origin of cohesion in ice rubble. Experiments were also performed using mushy ice. However, the shear-box proved not to be useful for determining the shear testing of mushy ice.The shear strength of a layer of ice rubble was found to depend on normal stress, which in turn was found to depend on rubble thickness, layer porosity, and shear rate. The dependence on shear rate of normal stress and, as a consequence, of shear strength of a layer of floating ice rubble is attributed to the development of freeze-bonds between the ice blocks comprising the rubble layer. It is argued that, at slower shear rates, more and stronger freeze-bonds develop than at higher shear rates, thus enabling the layer to withstand larger normal stresses and, consequently, shear strengths that increase with decreasing shear rates. If the influence of freeze-bonding on normal stress is taken into account, and if a Mohr-Coulomb failure criterion is used to characterize shear strength, it is found that a floating layer of ice rubble undergoing continuous-shear deforms as a cohesionless material; or at least as a material with unique cohesive properties.