Decrease In Cohesive Strength Research Articles

Numerical simulation analysis of the influence of karst cave orientation and surrounding rock properties on tunnel excavation

Abstract The underground distribution of complex karst geology in the Zhangjiajie region of China poses a significant threat to the construction safety of tunnels, not only increasing project investments but also severely impacting the stable operation of engineering construction. This paper, based on ABAQUS numerical simulation, explores the influence of karst caves around tunnels in different orientations of surrounding rock environments through the rock strength, stress field, and dimension of surrounding rock mass. The results are as follows: (1) With the decrease in cohesive strength of the cave surrounding rock, the stability of the tunnel is enhanced. (2) The maximum displacement of the lower cave is 0.02011 m, greater than that of the side cave, which is 0.01965 m; the maximum principal stress of the lower cave is 7.457 MPa, greater than that of the side cave, which is 7.423 MPa. The lower cave has a greater impact on tunnel construction and can further reduce the stability of the karst tunnel surrounding rock. This paper has researched the influence of caves on tunneling through surrounding rock and investigated the effects of cave orientation, cohesive strength, and other parameters on tunnel construction. It provides theoretical guidance for tunnel alignment and pre-processing of concealed caves.

Distribution and characteristics of landslide in Loess Plateau: A case study in Shaanxi province

Every year about one third of the geohazards in China occur in the Loess Plateau causing human loss, damaging gas and oil pipelines, destroying highways, railways and degrading farmland. Field investigation and monitoring, in-situ tests and laboratory experiments were performed to improve our understanding of the factors effecting the distribution, characteristics and causes of loess landslides. First, we find that 79% of the landslides are shallower than10m, 85% have a volume of less than 100,000m3. Second, landslides on the Loess Plateau occur primarily on concave slope profiles that have slope angles of 20–35° and that face south-east. Third, the equivalent coefficient of friction of loess landslides is very low resulting in long run-out with a low angle sliding surface. Loess landslides generally transform into mud-flows resulting in an increase in volume in transit and forming a geohazard chain. Antecedent rainfall plays an important role in triggering loess landslides. Finally, clusters of landslides in the Loess Plateau occur because the loess easily disintegrates under high pressure due to its loose and highly porous structure. There is a sharp decrease in cohesive strength with increase in deformation and water content and thus landslides tend to undergo static liquefaction during sliding.

First principles calculations of hydrogen-induced decrease in the cohesive strength of α-Al 2O 3 single crystals

First principles calculations were conducted to investigate the effects of hydrogen on the cohesive strength of α-Al 2O 3 single crystals. The results show that the cohesive strength of α-Al 2O 3 single crystals decreases with increasing hydrogen concentration, C H. For example, the cohesive strength along the [0 0 1] direction, σ th(0 0 1), decreases from 48.9 GPa ( C H = 0) to 46.2 GPa ( C H = 1630 wppm) and 39.6 GPa ( C H = 4900 wppm). The lattice expansion and the changes in the local electronic structure caused by the hydrogen atoms are the two main reasons for the decrease in the cohesive strength. The results provide a computational evidence for the hydrogen-induced decrease in cohesive strength, which can explain the experimental phenomenon of hydrogen-assisted delayed fracture in α-Al 2O 3 ceramics during charging of hydrogen under a sustained load.

Tack and Shear Strength of Adhesives Prepared from Styrene-Butadiene Rubber (SBR) Using Gum Rosin and Petro Resin as Tackifiers

Tack and shear strength of styrene-butadiene rubber (SBR)-based pressure-sensitive adhesive were studied using gum rosin and petro resin as the tackifiers. The concentration of the tackifying resin was varied from 0 to 100 parts per hundred parts of rubber (phr). Toluene was used as the solvent throughout the experiment. The rolling ball technique was used to measure the tack of the adhesive, whereas, shear strength was determined by a TA-HDi Texture Analyser. Results show that the tack of the adhesive increases with increasing tackifier loadings for both tackifier systems. However, shear strength indicates the reverse behavior with increasing resin content, an observation which is attributed to the decrease in cohesive strength as the tackifier concentration is increased. Both tack and shear strength of the adhesives increases with molecular weight of SBR. Adhesive containing petro resin consistently exhibits higher values than the gum rosin system due to better wettability and compatibility in the former system.

Peel and shear strength of pressure‐sensitive adhesives prepared from epoxidized natural rubber

AbstractPeel and shear strength of two grades of epoxidized natural rubber (ENR 25 and ENR 50)‐based pressure‐sensitive adhesive was studied. Coumarone‐indene resin was used as the tackifier, whereas toluene was chosen as the solvent throughout the experiment. The tackifier loading was varied from 0 to 80 parts per hundred parts of rubber (phr). A SHEEN hand coater was used to coat the adhesive on substrate to give a coating thickness of 30, 60, 90, and 120 μm. Peel strength and shear strength of the adhesive were determined by using a Lloyd adhesion tester and Texture analyzer, respectively. Results show that maximum peel strength occurs at 40 phr of coumarone‐indene resin for both ENRs studied an observation, which is attributed to the maximum wettability of the substrate. However, the shear strength shows a gradual decrease with increasing tackifier loading because of the decrease in cohesive strength of adhesive. ENR 25 consistently indicates higher peel strength and shear strength than ENR 50. Generally, peel and shear strength increases with coating thickness. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007.

Morphological and Mechanical Evaluation of Hybrid Organic−Inorganic Thermoset Copolymers of Dicyclopentadiene and Mono- or Tris(norbornenyl)-Substituted Polyhedral Oligomeric Silsesquioxanes

A new class of organic−inorganic hybrid thermoset copolymers has been prepared by ring-opening metathesis polymerization catalyzed with bis(tricyclohexylphosphine)benzylideneruthenium(II) dichloride (1). Dicyclopentadiene (DCPD) and norbornenylethyl polyhedral oligomeric silsesquioxane (1NB-POSS) and tris(norbornenylethyl)-POSS (3NB-POSS) with isobutyl pendent groups have been copolymerized at 60 °C over a range of POSS loadings. These copolymers contain small aggregates of 1NB-POSS, three to four molecules, at high loadings and uniform dispersions over all loadings of 3NB-POSS. The pendent group of 1NB-POSS decreases the cross-link density of the PDCPD matrix while 3NB-POSS increases the cross-link density. POSS incorporation has little effect on the glass transition temperature (Tg). Addition of 20 wt % 1NB-POSS decreases the Tg from 128 °C for PDCPD to 114 °C. Addition of 3NB-POSS has little effect on the Tg over the range of POSS loadings. The stiffness, in tension and compression, is observed to decrease with increasing POSS loading. Although the yield stress for both systems decreases, the toughness also decreases. The decrease in toughness of the 1NB-POSS copolymers is attributed to a loss of irreversible damage with increased POSS loading. Although a similar decrease in toughness is observed for the 3NB-POSS copolymers, the change in toughness is attributed to a decrease in cohesive strength with 3NB-POSS loading.

The effect of particle–matrix adhesion on the mechanical behavior of glass filled epoxies: Part 1. A study on yield behavior and cohesive strength

Basic properties of glass filled epoxy such as yield behavior and cohesive strength, which are important in understanding toughening mechanism, are studied using compression tests and double notch 4 point bending (DN-4PB) tests. Three different glass reinforcements, large glass spheres, small glass spheres, and glass fibers, were treated by different methods and used at different volume fractions ranging from 10 to 30 vol%. The effect of different surface treatments and moisture exposure on the yield behavior is studied. It was found that in all types of formulation the yield stress decreased after moisture exposure and that the yield stress was dependent on the surface treatment both before and after moisture exposure. No treatment and treatment of glass reinforcements with aminopropyltrimethoxysilane coupling agent resulted in relatively higher yield stress. Close observation of microstructure by transmission OM showed that the degree of debonding in the specimens for compression tests was quite dependent on the surface treatment after moisture exposure. The decrease in cohesive strength of the neat specimens were observed after moisture exposure.

Durability of resin bonds to a cobalt-chromium alloy

Common nickel-chromium-beryllium alloys used for resin-bonded fixed partial dentures have possible health hazards due to leaching of nickel and beryllium. For resin-bonded restorations corrosion resistant cobalt-chromium alloys (CoCr) are a suitable alternative material without sacrificing physical properties. This study evaluated the bond strength and bond durability of new adhesive systems to a CoCr alloy. Plexiglas (acrylic) tubes filled with composite were bonded to CoCr alloy discs. Groups of 24 samples were bonded using six different bonding systems. Subgroups of eight bonded samples were stored in an isotonic artificial saliva solution (37 °C) either for 1 day, 30 days or 150 days. In addition the 30- and 150-days samples were subjected to 7500 or 37 500 thermal cycles, respectively. The bond strength of a conventional BisGMA composite (Twinlook) to sandblasted CoCr was significant lower than when using chemomechanical bonding systems and decreased continuously during the storage time of 150 days. The additional use of silane on the sandblasted alloy resulted in an insignificant increase in bond strength. Statistically significant higher and more durable bonds to CoCr alloy were achieved either with the combination of silica coating and use of the conventional BisGMA composite or with the combination of sandblasting and the use of a composite modified with a phosphate monomer (Panavia EX). In the latter systems, the bond strengths were mainly limited by the cohesive strength of the resin composites; partial adhesive failures were only observed for a tribochemical silica coating system. A new composite also containing the active phosphate monomer (Panavia TPN-S) exhibited a significant decrease in cohesive strength over time.

Ｔｉ‐１３Ｖ‐１１Ｃｒ‐３Ａｌ合金の疲労き裂伝ぱ挙動に及ぼす水素チャージの影響

In order to investigate the influence of hydrogen on the fatigue crack propagation behavior of β-titanium alloy, two types of fatigue tests were carried out. One was the fatigue test in air on the hydrogen-precharged specimens, where the charging was carried out in H2SO4 aqueous solution electrolytically under the current density ic of 1000A/m2 for 260ks. Another was the fatigue test under electrolytic hydrogen-charging in H2SO4 aqueous solution at ic=1000 or 3000A/m2. Both test results were compared with the result obtained from the fatigue test of the non-charged specimen in air.The fatigue crack propagation rate da/dN of the precharged specimen was much larger than the da/dN of the non-charged specimen in all the range of stress intensity factor, ΔK. The crack propagation rate da/dN under hydrogen charging was larger than the da/dN of the non-charged specimen only when ΔK was smaller than 15MPa·m1/2, and the da/dN was much smaller than that of the precharged specimen. The fatigue fracture surface of the specimens which were hydrogen-charged by both methods revealed the cleavage fracture, which corresponded to the decrease in cohesive strength of the cleavage plane of β-phase by the dissolution of hydrogen atoms.