scholarly journals Estimating the effective elastic modulus and specific fracture energy of snowpack layers from field experiments

2016 ◽  
Vol 62 (236) ◽  
pp. 997-1007 ◽  
Author(s):  
ALEC VAN HERWIJNEN ◽  
JOHAN GAUME ◽  
EDWARD H. BAIR ◽  
BENJAMIN REUTER ◽  
KARL W. BIRKELAND ◽  
...  
Keyword(s):  
Elastic Modulus ◽  
Fracture Energy ◽  
Field Experiments ◽  
Mechanical Energy ◽  
Low Frequency ◽  
Effective Elastic Modulus ◽  
Snow Layer ◽  
Specific Fracture Energy ◽  
Weak Layer ◽  
Specific Fracture

ABSTRACTMeasurements of the mechanical properties of snow are essential for improving our understanding and the prediction of snow failure and hence avalanche release. We performed fracture mechanical experiments in which a crack was initiated by a saw in a weak snow layer underlying cohesive snow slab layers. Using particle tracking velocimetry (PTV), the displacement field of the slab was determined and used to derive the mechanical energy of the system as a function of crack length. By fitting the estimates of mechanical energy to an analytical expression, we determined the slab effective elastic modulus and weak layer specific fracture energy for 80 different snowpack combinations, including persistent and nonpersistent weak snow layers. The effective elastic modulus of the slab ranged from 0.08 to 34 MPa and increased with mean slab density following a power-law relationship. The weak layer specific fracture energy ranged from 0.08 to 2.7 J m−2 and increased with overburden. While the values obtained for the effective elastic modulus of the slab agree with previously published low-frequency laboratory measurements over the entire density range, the values of the weak layer specific fracture energy are in some cases unrealistically high as they exceeded those of ice. We attribute this discrepancy to the fact that our linear elastic approach does not account for energy dissipation due to non-linear parts of the deformation in the slab and/or weak layer, which would undoubtedly decrease the amount of strain energy available for crack propagation.

scholarly journals Dynamic crack propagation in weak snowpack layers: Insights from high-resolution, high-speed photography

2021 ◽  
Author(s):  
Bastian Bergfeld ◽  
Alec van Herwijnen ◽  
Benjamin Reuter ◽  
Grégoire Bobillier ◽  
Jürg Dual ◽  
...  
Keyword(s):  
High Resolution ◽  
Crack Propagation ◽  
Fracture Energy ◽  
High Speed ◽  
Image Correlation ◽  
Crack Speed ◽  
Specific Fracture Energy ◽  
Weak Layer ◽  
Specific Fracture ◽  
Insight Into

Abstract. To assess snow avalanche release probability, information on failure initiation and crack propagation in weak snowpack layers underlying cohesive slab layers are required. With the introduction of the Propagation Saw Test (PST) in the mid-2000s, various studies used particle tracking analysis of high-speed video recordings of PST experiments to gain insight into crack propagation processes, including slab bending, weak layer collapse, crack propagation speed and the frictional behavior after weak layer fracture. However, the resolution of the videos and the methodology used did not allow insight into dynamic processes such as the evolution of crack speed within a PST or the touchdown distance, which is the length from the crack tip to the trailing point where the slab sits on the crushed weak layer at rest again. Therefore, to study the dynamics of crack propagation we recorded PST experiments using a powerful portable high-speed camera with a horizontal resolution of 1280 pixels at rates up to 20,000 frames per second. By applying a high-density speckling pattern on the entire PST column, we then used digital image correlation (DIC) to derive high-resolution displacement and strain fields in the slab, weak layer, and substrate. The high frame rates allowed time derivatives to obtain velocity and acceleration fields. On the one hand, we demonstrate the versatile capabilities and accuracy of the DIC method by showing three PST experiments resulting in slab fracture, crack arrest and full propagation. On the other hand, we present a methodology to determine relevant characteristics of crack propagation: the crack speed and touchdown distance within a PST, and the specific fracture energy of the weak layer. To estimate the effective elastic modulus of the slab and weak layer as well as the weak layer specific fracture energy we used a recently proposed mechanical model. A comparison to already established methods showed good agreement. Furthermore, our methodology also provides insight into the three different propagation results found with the PST and reveals intricate dynamics that are otherwise not accessible.

scholarly journals Temporal evolution of crack propagation propensity in snow in relation to slab and weak layer properties

2016 ◽  
Vol 10 (6) ◽  
pp. 2637-2653 ◽  
Author(s):  
Jürg Schweizer ◽  
Benjamin Reuter ◽  
Alec van Herwijnen ◽  
Bettina Richter ◽  
Johan Gaume
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Temporal Evolution ◽  
Effective Elastic Modulus ◽  
Swiss Alps ◽  
Snow Layer ◽  
Weak Layer ◽  
Failure Initiation ◽  
Specific Fracture ◽  
Measurement Period

Abstract. If a weak snow layer below a cohesive slab is present in the snow cover, unstable snow conditions can prevail for days or even weeks. We monitored the temporal evolution of a weak layer of faceted crystals as well as the overlaying slab layers at the location of an automatic weather station in the Steintälli field site above Davos (Eastern Swiss Alps). We focussed on the crack propagation propensity and performed propagation saw tests (PSTs) on 7 sampling days during a 2-month period from early January to early March 2015. Based on video images taken during the tests we determined the mechanical properties of the slab and the weak layer and compared them to the results derived from concurrently performed measurements of penetration resistance using the snow micro-penetrometer (SMP). The critical cut length, observed in PSTs, increased overall during the measurement period. The increase was not steady and the lowest values of critical cut length were observed around the middle of the measurement period. The relevant mechanical properties, the slab effective elastic modulus and the weak layer specific fracture, overall increased as well. However, the changes with time differed, suggesting that the critical cut length cannot be assessed by simply monitoring a single mechanical property such as slab load, slab modulus or weak layer specific fracture energy. Instead, crack propagation propensity is the result of a complex interplay between the mechanical properties of the slab and the weak layer. We then compared our field observations to newly developed metrics of snow instability related to either failure initiation or crack propagation propensity. The metrics were either derived from the SMP signal or calculated from simulated snow stratigraphy (SNOWPACK). They partially reproduced the observed temporal evolution of critical cut length and instability test scores. Whereas our unique dataset of quantitative measures of snow instability provides new insights into the complex slab-weak layer interaction, it also showed some deficiencies of the modelled metrics of instability – calling for an improved representation of the mechanical properties.

Fracture Behaviour of Modified Spruce Wood: A Study Using Linear and Non Linear Fracture Mechanics

Holzforschung ◽  
2002 ◽  
Vol 56 (2) ◽  
pp. 191-198 ◽  
Author(s):  
Alexander Reiterer ◽  
Gerhard Sinn
Keyword(s):  
Fracture Energy ◽  
Heat Treatments ◽  
Critical Stress Intensity Factor ◽  
Mode I ◽  
Mode Iii ◽  
Fracture Properties ◽  
Specific Fracture Energy ◽  
Wedge Splitting Test ◽  
Splitting Test ◽  
Specific Fracture

Summary The fracture properties of unmodified and modified (heat treatments under various conditions and acetylation) sprucewood are investigated using the wedge splitting test. Fracture parameters measured include critical stress intensity factor and specific fracture energy under Mode I loading and specific fracture energy under Mode III loading. The Mode I fracture properties are reduced by all kinds of modification. However, acetylation leads to a reduction of only 20%whereas heat treatments reduce the properties to a much greater extent, approximately 50%to 80%. The Mode III fracture properties are influenced less. SEM pictures of the fracture surfaces support the described findings.

scholarly journals Probabilistic Fracture Energy Assessment of Natural Fibre Reinforced Concrete by Two Parameter Weibull Distribution

2018 ◽  
Vol 7 (3.12) ◽  
pp. 407
Author(s):  
Neha P Asrani ◽  
Murali G ◽  
Arthika J ◽  
Karthikeyan. K ◽  
Haridharan. M.K
Keyword(s):  
Weibull Distribution ◽  
Fracture Energy ◽  
Plain Concrete ◽  
Natural Fibre ◽  
Test Results ◽  
Specific Fracture Energy ◽  
Energy Assessment ◽  
Experimental Process ◽  
Specific Fracture ◽  
Two Parameter

Fracture energy is the post-crack energy absorption ability of the material that represents the energy absorbed by the structure at the time of failure. Its analysis has gained importance and hence requires a powerfulmethod for its development. A two parameter Weibull distribution proves to be an efficient tool in analysing the scattered experimental test results. In this paper, the specific fracture energy of plain concrete and concrete reinforced with natural fibres of hemp, wheat straw and elephant grass are statistically analysed by two parameter Weibull distribution by using graphical method. For determining Weibull parameters, 21 equations have been used and the best equation is taken for the reliability analysis. A Weibull reliability curve is plotted, which shows the specific fracture energy at each reliability level. This curve enables an engineer to choose the fracture energy of a particular mix based on its reliability requirement and safety limit. Therefore, reliability curves are a pioneer in statistical analysis as they eliminate the time-consuming and costly experimental process. This method can be applied in areas with similar uncertainties.  

Damage and Fracture Mechanisms during Mode I and III Loading of Wood

Holzforschung ◽  
2001 ◽  
Vol 55 (5) ◽  
pp. 525-533 ◽  
Author(s):  
E.K. Tschegg ◽  
K. Frühmann ◽  
S.E. Stanzl-Tschegg
Keyword(s):  
Crack Initiation ◽  
Crack Propagation ◽  
Fracture Energy ◽  
Fracture Mechanisms ◽  
Mode I ◽  
Mode Iii ◽  
Initiation Energy ◽  
Specific Fracture Energy ◽  
Damage And Fracture ◽  
Specific Fracture

Summary Tests under mode I and mode III loading were performed on side grooved Compact-Tension specimens of larch and beech under steady state crack propagation to study the damage and fracture behaviour and the influence of two fibre orientations. From the complete load-displacement diagram, all important damage and fracture mechanical values (stiffness/compliance, microstructural damage, crack initiation energy, specific fracture energy, etc.) have been determined. Crack initiation energy and specific fracture energy are approximately ten times higher for mode III loading than for mode I loading in both wood species. Crack initiation occurs in mode III under external mode III loading, crack propagation, however, takes place under mode I, owing to crack surface interference. The influence of fibre orientation on the (fracture) mechanical properties of beech and larch is different. This difference may be explained mainly by the high number of rays in beech.

Identification of AAAS Composites Mechanical Fracture Parameters

2021 ◽  
Vol 322 ◽  
pp. 66-71
Author(s):  
Martin Lipowczan ◽  
David Lehký ◽  
Iva Rozsypalová ◽  
Petr Daněk ◽  
Pavla Rovnaníková ◽  
...  
Keyword(s):  
Fracture Energy ◽  
Modulus Of Elasticity ◽  
Crack Mouth Opening Displacement ◽  
Crack Model ◽  
Mechanical Fracture ◽  
Specific Fracture Energy ◽  
Static Modulus ◽  
Fracture Parameters ◽  
Static Modulus Of Elasticity ◽  
Specific Fracture

The paper deals with selected alkali-activated aluminosilicate (AAAS) composites based on ceramic precursors in terms of characterization by mechanical fracture parameters. Two composites made of brick dust as a precursor and an alkaline activator with a silicate modulus Ms = 1.0 were investigated. The composites differed in the fineness of grinding of the precursor – in the first set it was 0 to 1 mm, in the second set 0 to 0.3 mm. The filler was crushed brick. The test specimens had nominal dimensions of 40 × 40 × 160 mm and were provided with notches in the middle of the span up to 1/3 of the depth of the specimens after 28 days of hardening. Five to six specimens from each composite set were tested. The specimens were subjected to three-point bending tests, in which force vs. displacement (deflection in the middle of the span) diagrams (F–d diagrams) and force vs. crack mouth opening displacement (F–CMOD) diagrams were recorded. After correction of these diagrams, the values of static modulus of elasticity, effective fracture toughness, effective toughness and specific fracture energy were determined using the Effective Crack Model and the Work-of-Fracture method. After the fracture experiments, the values of informative compressive strength were determined on one of the fractured parts. At the same time, the values of static modulus of elasticity, tensile strength and specific fracture energy were identified using artificial neural networks and F–d diagrams measured and simulated in the ATENA FEM software. All evaluations included the determination of basic statistics of parameters.

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