Toughening mechanisms in bioinspired multilayered materials

Outstanding mechanical properties of biological multilayered materials are strongly influenced by nanoscale features in their structure. In this study, mechanical behavior and toughening mechanisms of abalone nacre-inspired multilayered materials are explored. In nacre’s structure, the organic matrix, pillars and the roughness of the -aragonite platelets play important roles in its overall mechanical performance. A micromechanics model for multilayered biological materials is proposed to simulate their mechanical deformation and toughening mechanisms. The fundamental hypothesis of the model is that nanoscale pillars and asperities have near theoretical strength. It has been shown earlier that organic matrix behaves stiffer in proximity of mineral platelets. However, the suggested values for stiffness and strength of organic matrix appear to be high. The proposed model assumes that pillars and the asperities confine the organic matrix to the proximity of the platelets and hence increase their stiffness and strength. The modeling results are in excellent agreement with the experimental results for abalone nacre. The results show that the stiffness of the organic matrix affects the stiffness of material, whereas platelets’ aspect ratio determines the ultimate strength. The pillars and the asperities are responsible for the ductility of multilayered material. In the proposed model, although all the components of the bioinspired structure have brittle behavior, the overall structure has a ductile response. The highly nonlinear behavior of the suggested multilayered material is a result of a distributed deformation in the nacre-like structure due to existence of nanoasperities and nanopillars with near theoretical strength

Toughening mechanisms in bioinspired multilayered materials

Outstanding mechanical properties of biological multilayered materials are strongly influenced by nanoscale features in their structure. In this study, mechanical behaviour and toughening mechanisms of abalone nacre-inspired multilayered materials are explored. In nacre's structure, the organic matrix, pillars and the roughness of the aragonite platelets play important roles in its overall mechanical performance. A micromechanical model for multilayered biological materials is proposed to simulate their mechanical deformation and toughening mechanisms. The fundamental hypothesis of the model is the inclusion of nanoscale pillars with near theoretical strength (σth ~ E/30). It is also assumed that pillars and asperities confine the organic matrix to the proximity of the platelets, and, hence, increase their stiffness, since it has been previously shown that the organic matrix behaves more stiffly in the proximity of mineral platelets. The modelling results are in excellent agreement with the available experimental data for abalone nacre. The results demonstrate that the aragonite platelets, pillars and organic matrix synergistically affect the stiffness of nacre, and the pillars significantly contribute to the mechanical performance of nacre. It is also shown that the roughness induced interactions between the organic matrix and aragonite platelet, represented in the model by asperity elements, play a key role in strength and toughness of abalone nacre. The highly nonlinear behaviour of the proposed multilayered material is the result of distributed deformation in the nacre-like structure due to the existence of nano-asperities and nanopillars with near theoretical strength. Finally, tensile toughness is studied as a function of the components in the microstructure of nacre

Comparison between the in situ and laboratory water retention curves for a silty sand

After an extreme rainfall event in May 2002 a series of landslides occurred in Ruedlingen in Canton Schaffhausen, North Switzerland. A 38° steep slope has been chosen in this area beside the river Rhine to carry out an artificial rainfall experiment to investigate the dependence between rainfall, suction, saturation and shear resistance. Two sprinkling experiments were conducted to represent an extreme rainfall event, the second of which resulted in failure of 130 m 3 of the slope. Several cycles of wetting and drying were applied to the soil and suction and volumetric water content were measured at different depths in three locations of the slope, by which in-situ Water Retention Curves (WRC) can be derived. The WRC of an undisturbed sample was also determined from laboratory test. The in situ and laboratory WRCs are compared in this paper and the differences will be discussed

The effects of hydraulic properties of bedrock on the stability of slopes

The transient process of rain infiltration in the soil and the effect of geometry and drainage properties of the bedrock on the pore pressure distribution and the stability of a slope are investigated. The simulated slope is a test field in northern Switzerland, where landslide triggering experiments were carried out. From geological point of view, the experimental site is located in the Swiss Molasse basin. The lithological units in the area are composed of horizontally layered and fractured sandstones intersected by marlstone. The stability of the slope is monitored at different stages of the infiltration using the limit equilibrium method of slices. Several cases were compared to study the effect of the fissures in the shallow bedrock on the stability of the slope. The approximate location and size of the fissures in the bedrock were determined by monitoring of spatial and temporal changes of electrical resistivity during rainfall and also geological investigations of the bedrock before and after the failure

Hydro-mechanical analysis of a surficial landslide triggered by artificial rainfall: the Ruedlingen field experiment

This paper interprets the hydromechanical behaviour of a steep, forested, instrumented slope during an artificial rainfall event, which triggered a shallow slope failure 15 h after rainfall initiation. The soil's mechanical response has been simulated by coupled hydro-mechanical finite-element analyses, using a critical state constitutive model that has been extended to unsaturated conditions. Failure occurs within a colluvium shallow soil cover, characterised as a silty sand of low plasticity. The hydraulic and mechanical parameters are calibrated, based on an extended set of experimental results, ranging from water retention curve measurements to triaxial stress path tests under both saturated and unsaturated conditions. Rainfall is simulated as a water flux at the soil surface and suitable boundary conditions account for the hydromechanical interaction between the soil cover and the underlying bedrock. The results are compared with field data of the mechanistic and the hydraulic responses up to failure and are found to provide a very satisfactory prediction. The study identifies water exfiltration from bedrock fissures as the main triggering agent, resulting in increased pore pressures along the soil-bedrock interface, reduced available shear strength and cause extensive plastic straining, leading to the formation and propagation of a failure surface.Accepted Author ManuscriptGeo-engineerin

Effects of humidity on shear behavior of bamboo

Bamboo is a naturally occurring biological composite, however its microstructure and hence its properties are very complex compared to the manmade composites. Due to optimization, it can be assumed that the variation in properties along the thickness of the culm be a smooth transition for better bonding strength between layers and to prevent non uniformity in stress concentration. As a consequence, biological structures are complicated and functionally graded. Hence, a realistic model that can capture the mechanical performance of bamboo is valuable in future design of robust multifunctional composites. This paper presents the results of experimental and numerical studies on the torsional (shear) properties of bamboo. The hierarchical and multi-scale structure of bamboo and the distribution of micro-scale fibers are revealed via laser scanning and atomic force microscopy. This information was incorporated into a finite element model to analyze the mechanical behavior of bamboo under torsion and to estimate the shear modulus of bamboo along the fibers. Moreover, the effects of humidity and therefore water content on the mechanical properties of bamboo were evaluated by performing torsion tests on samples maintained in environments with different humidities. Increasing the humidity does not cause a drop in the shear modulus, however, a jump in the shear modulus did occur at around 60% humidity. Results of this study indicate that the highest strength values in samples occurred in environments with humidity levels between 60% and 80% and undergo a significant drop after that. In higher humidities, the samples behave more ductile

Interfacial delamination of a sandwich layer by aqueous corrosion

The mechanism of aqueous delamination of a methyl methacrylate -based adhesive layer, sandwiched between two steel plates, is investigated by a systematic series of critical tests. These tests include starving the specimen of oxygen, varying the aqueous environment from de-ionised water to water with a high concentration of salt, and varying the mechanical constraint imposed by the sandwich. Sandwich construction starves the delamination crack tip of oxygen, and delamination occurs by water attack of the interface. In contrast, an adhesive coating on a steel substrate undergoes cathodic delamination when oxygen is present at the crack tip and the delamination crack is filled with salt water.<br/

Mountain Risks: two case histories of landslides induced by artificial rainfall on steep slopes

Mountainous areas tend to be exposed to an enhanced risk of damage caused by natural hazards; most often exacerbated by the topography (leading to gravitational mass movements such as avalanches, landslides, mud and debris flows). This contribution compares landslides induced by artificial rainfall on two different areas located in Switzerland. One field test site was located on slopes above Saas Balen (Gruben glacier, Canton Wallis, Switzerland) and was instrumented. Artificial rainfall tests were carried out in the summers of 1999 and 2000 to investigate hydro-mechanical mechanisms of instability (Teysseire et al., 2000). Shallow failure occurred in the steeper instrumented slope in 2000. The second test field is located near Ruedlingen (Canton Schaffhausen, Switzerland). A landslide triggering experiment was carried out there in autumn 2008 and spring 2009 to replicate the effects of a heavy rainfall event of May 2002, in which 100 mm rain fell in 40 minutes, causing 42 superficial landslides. The slope was subjected to extreme rainfall by artificial means in October 2008 and in March 2009, triggering about 130 m3 of debris. Infiltration of rainfall has led to surface instability slopes in an alpine moraine (Gruben) and in silty sand (Ruedlingen). Both slopes were steeper than the internal angle of friction, having different initial degrees of saturation and suction. The hydromechanical behaviour of these two field full scale landslides will be compared, trying to expose a deeper understanding of the rainfall induced failure mechanisms

Unsaturated hydraulic conductivity of a silty sand with the instantaneous profile method

The unsaturated hydraulic conductivity of a silty sand at different initial void ratios is measured using the instantaneous profile method. The variation of the suction and volumetric water content is recorded during the infiltration process as a function of time. Accordingly, an infiltration column was developed with a height of 600 mm and an inner diameter of 170 mm. The suction and volumetric water content were measured simultaneously every 100 mm along the column by means of small tensiometers and TDRs, respectively. Hydraulic conductivity is calculated by dividing the water flow velocity by the hydraulic gradient. The soil is reconstituted from Ruedlingen (Canton Schaffhausen, Switzerland), where landslide triggering experiments were carried out in October 2008 and March 2009. The hydraulic conductivity functions are determined and the laboratory values are compared to the in-situ measurements of hydraulic conductivity carried out in the course of the landslide triggering experiments

Poly[(μ5-3,5-dinitro­benzoato)rubidium]

The asymmetric unit of the title compound, [Rb(C7H3N2O6)]n, comprises an Rb cation and a 3,5-dinitro­benzoate anion. The Rb cation is eight-coordinated by O atoms from five 3,5-dinitro­benzoate anions. On the other hand, each 3,5-dinitro­benzoate anion links five Rb cations with the carboxyl­ate groups as μ3-bridging. The metal atom is firstly linked by the carboxyl­ate groups into a chain along the c-axis direction, which is further linked by bonds between the Rb and nitro O atoms, giving a three-dimensional framework