Differences In Failure Behavior Research Articles

Failure Behavior and Damage Characteristics of Coal at Different Depths under Triaxial Unloading Based on Acoustic Emission

The depth effect of coal mechanical behavior seriously affects the safety and efficiency of deep coal mining. To explore the differences in failure behavior and damage characteristics of coal masses at different depths during the coal mining process, based on the consideration of in situ stress environment, physical properties, and mining disturbance of coal seams, triaxial unloading experiments with acoustic emission (AE) monitoring were conducted on coal samples at four different depths taken from the Pingdingshan coal mine area. The results showed that the AE activity of deep coal was more concentrated, and the cumulative AE energy of coal increased with increasing depth. The cumulative AE energy of the 1050-m coal sample was 69 times that of the 300-m coal sample. The b value representing the microcrack scale decreased with increasing depth, and the rupture degree of deep coal increased. The cracking mode of coal was classified and the failure behavior was analyzed. The cumulative tensile crack percentage of coal increased with increasing depth, and the tensile–shear composite failure occurred in the 300-m coal sample, whereas significant tensile failure occurred in the 1050-m coal sample. In addition, the damage evolution process of coal was divided into three stages, and the characteristic stress of coal was obtained. The ratio of crack initiation stress (σci) to peak stress (σc) increased with increasing depth, and the damage evolution process of deep coal was more rapid. The research results can provide useful guidance for disaster prevention and evaluation of surrounding rock stability during deep coal resource mining in the Pingdingshan coal mine area.

Influence of Fibre Inter-ply Orientation on the Mechanical and Free Vibration Properties of Banana Fibre Reinforced Polyester Composite Laminates

The mechanical properties and vibration damping characteristics of cross-ply laminates ([90°/0°/90°] and [0°/90°/0°]), [0°/45°/0°], four-layered angle ply laminate ([90°/0°]s) and Quasi-isotropic laminates ([0°/90°/45°]s, [0°/45°/90°]s,) containing 50 ± 2 wt% of banana fibres in polyester matrix were investigated. The experimental results indicate that the tensile strength, Young’s modulus, impact strength, and the natural frequency were superior for [0°/90°/0°] laminates. In the case of flexural properties, the Quasi-isotropic laminates displayed better performance than the other configurations. Tensile fractured specimens were examined under a scanning electron microscope (SEM) to understand the difference in failure behavior due to the inter-ply orientation in the laminate.

An investigation on static failure behaviour of CFRP quasi isotropic laminates under in-plane and out-of-plane loads

A study has been conducted to understand the differences in failure behaviour of a carbon fiber reinforced plastic (CFRP) material fabricated in two different laminate sequences. The material chosen was AS4 with the laminate systems as (0/45/−45/90) and (0/90/45/−45), both of quasi-isotropic (QI) nature. Carefully fabricated test coupons were tested under tensile, compression and flexure load conditions. The tensile and compression test results did not show any influence of the laminate system. However, the flexure loads show a consistent difference in failure behaviour of these two systems, owing to the role of out-of-plane-loading in QI ply. Classical lamination theory (CLT) and the numerical methods were employed to conduct stress analysis. Experimental results are discussed in comparison with the finite element method (FEM) and the analytical results.

Mechanics of a two-fiber model with one nested fiber network, as applied to the collagen-fibrin system

The mechanical behavior of collagen-fibrin (col-fib) co-gels is both scientifically interesting and clinically relevant. Collagen-fibrin networks are a staple of tissue engineering research, but the mechanical consequences of changes in co-gel composition have remained difficult to predict or even explain. We previously observed fundamental differences in failure behavior between collagen-rich and fibrin-rich co-gels, suggesting an essential change in how the two components interact as the co-gel’s composition changes.In this work, we explored the hypothesis that the co-gel behavior is due to a lack of percolation by the dilute component. We generated a series of computational models based on interpenetrating fiber networks. In these models, the major network component percolated the model space but the minor component did not, instead occupying a small island embedded within the larger network. Each component was assigned properties based on a fit of single-component gel data. Island size was varied to match the relative concentrations of the two components. The model predicted that networks rich in collagen, the stiffer component, would roughly match pure-collagen gel behavior with little additional stress due to the fibrin, as seen experimentally. For fibrin-rich gels, however, the model predicted a smooth increase in the overall network strength with added collagen, as seen experimentally but not consistent with an additive parallel model. We thus conclude that incomplete percolation by the low-concentration component of a co-gel is a major determinant of its macroscopic properties, especially if the low-concentration component is the stiffer component. Statement of significanceModels for the behavior of fibrous networks have useful applications in many different fields, including polymer science, textiles, and tissue engineering. In addition to being important structural components in soft tissues and blood clots, these protein networks can serve as scaffolds for bioartificial tissues. Thus, their mechanical behavior, especially in co-gels, is both interesting from a materials science standpoint and significant with regard to tissue engineering.

Failure behavior of quasi-isotropic carbon fiber-reinforced polyamide composites under tension

In this paper, the microscopic failure behavior of quasi-isotropic carbon fiber-reinforced polyamide-6 (CF/PA6) laminates under tension was investigated experimentally. Laminates of two layups, namely [45°/0°/−45°/ 90°]s and [45°/0°/−45°/ 90°]2s, were made from CF/PA6 tapes of two different manufacturers and then subjected to tensile testing. Crack initiation and progression on the polished free edge of specimens were examined using optical microscopy, under several load levels. Crack growth behavior through the specimen width was also traced by observing the crack configurations in different sections in the specimen width direction. The effects of the spatial distribution of fiber on the microscopic damage events were elucidated. The difference in failure behavior between the present CF/PA6 laminates and conventional thermosetting CF/Epoxy laminate is discussed.

Influence of nano-silica on the failure mechanism of concrete specimens

Failure of basic structures material is usually accompanied by expansion of interior cracks due to stress concentration at the cracks tip. This phenomenon shows the importance of examination of the failure behavior of concrete structures. To this end, 4 types of mortar samples with different amounts of nano-silica (0%, 0.5%, 1%, and 1.5%) were made to prepare twelve 50x50x50 mm cubic samples. The goal of this study was to describe the failure and micro-crack growth behavior of the cement mortars in presence of nano-silica particles and control mortars during different curing days. Failure of mortar samples under compressive strength were sensed with acoustic emission technique (AET) at different curing days. It was concluded that the addition of nano-silica particles could modify failure and micro-crack growth behavior of mortar samples. Also, monitoring of acoustic emission parameters exposed differences in failure behavior due to the addition of the nanoparticles. Mortar samples of nano-silica particles revealed stronger shear mode characteristics than those without nanoparticles, which revealed high acoustic activity due to heterogeneous matrix. It is worth mentioning that the highest compressive strength for 3 and 7 test ages obtained from samples with the addition of 1.5% nano-silica particles. On the other hand maximum compressive strength of 28 curing days obtained from samples with 1% combination of nano-silica particles.

Analysis of failure of flexible geocell-reinforced retaining walls in the centrifuge

ABSTRACT: Since the 1980s, flexible geocell-reinforced retaining walls have been used more frequently for slope protection as an alternative to conventional retaining structures. The benefits of these geocell structures include light weight, rapid construction, cost-effectiveness, ecological protection and landscape enhancement. However, the deformation mechanisms and failure modes of geocell-reinforced retaining walls are often complicated, thereby restricting their use in engineering practice. The differences in failure behaviour between geocell-reinforced and rigid-facing retaining walls need to be identified. In the present study, a series of centrifuge model tests on retaining walls reinforced by geocells were performed. The tests varied with respect to aspect ratio (the reinforcement length to the height of the wall) and the slope inclination (horizontal to vertical). The experimental results indicate that the critical failure surface is a non-linear shape that passes through the toe of the wall and intersects with the back of the wall. As the aspect ratio decreased, the horizontal distance between the top of the critical failure surface and the wall increased, whereas the location of the critical failure surface intersecting with the wall moved downward. In the case of a smaller inclination angle, the critical failure plane had a compound shape that included part of the backfill material near the top portion of the wall. In the case of a larger inclination angle, the size of the sliding zone was small and the failure was mainly due to sliding of the wall along the slope. The internal displacement distributions in the sliding zone were measured using digital image analysis and indicated that the locations of the maximum lateral face displacement in geocell retaining walls differs from those in conventional rigid retaining walls.

Spacecraft attitude control subsystem: Reliability, multi-state analyses, and comparative failure behavior in LEO and GEO

The attitude control subsystem (ACS) on board a 3-axis stabilized spacecraft is critical for on-orbit operations; its degradation can significantly disrupt mission performance, and its failure is fatal to the spacecraft. An ACS that fails early on-orbit, for example in the first couple of years on a spacecraft with a 15-year design lifetime, results in significant losses to all stakeholders and constitutes a major financial blow to the satellite operator or insurer—losses can amount to a few hundred mission dollars. Understanding and preventing/mitigating ACS failure is an engineering and financial imperative.In this work, we first provide a comprehensive statistical analysis of ACS failure behavior on orbit. The results indicate that the ACS is a major driver of spacecraft unreliability and contributes up to 20% of all spacecraft failures. Nonparametric and parametric results, including mixture Weibull distributions, are provided for ACS reliability, and they are complemented with the subsystem’s multi-state failure analysis. It is shown for example that the ACS is 89% likely to be fully operational after 15 years on-orbit. Results also demonstrate that ACS suffers from both infant mortality and wear-out failures. The findings are important for satellite manufacturers and equipment providers as they indicate opportunities for improved ground testing and burn-in procedures for the ACS, and they highlight the need to consider additional redundancy, even in stand-by form, especially if increased spacecraft design lifetime is sought.Second, we conduct a comparative analysis of failure behavior of the ACS in Low Earth Orbit (LEO) and Geosynchronous orbit (GEO). The results demonstrate a marked difference in failure behavior in LEO and GEO: the ACS degrades and fails more frequently, harder (more severe anomalies), and earlier (more infant anomalies/failures) in LEO than in GEO. We expand on these results and conclude with hypotheses for causal factors of this difference in failure behavior.

Mesoscale Simulation of Influence of Frost Damage on Mechanical Properties of Concrete

Mesoscale constitutive models of frost-damaged concrete are developed in this study through numerical simulation using a two-dimensional rigid body spring model (RBSM). The aim of the simulation is to predict the macrobehavior of frost-damaged concrete subjected to mechanical loading. The models also clarify the difference in failure behavior of concrete with and without frost damage. Zero strength elements and the concept of mesoscale plastic tensile strain are introduced into the normal RBSM springs to consider the experimentally observed cracking and plastic deformation caused by frost damage. The difference in the effect of frost damage on compression and tension behavior as found in the experiments is clearly predicted. Finally, analysis of a notched beam subjected to bending after different degrees of frost damage is carried out. The resulting load-deflection curves agree well with those obtained in the experiments. These good correlations confirm the applicability of the mesoscale model for predict...

Failure of Zircaloy cladding under transverse plane-strain deformation

Experiments have been performed to examine the ductility of Zircaloy 4 cladding tubes under conditions of near plane-strain deformation in the hoop direction (transverse to the tube axis) at temperatures of 25 and 300°C and at strain rates of 10 −3 and 10 2 s −1. To conduct these experiments, a specimen configuration was designed in which near plane-strain deformation is achieved, and a test methodology was established to determine two failure conditions: the limit strain at the onset of localized necking and the fracture strain. Experiments performed on cold-worked stress relieved material using the transverse plane-strain specimen geometry indicate major differences in failure behavior from that observed in uniaxial tension, although both test conditions result in failure by a localized necking process. The experimental results also indicate that while plane-strain fracture strains increase with temperature between 25 and 300°C, at a given temperature they are insensitive to strain rate. The limit strains at localized necking also increase with temperature but only at the high 10 2 s −1 strain rate. Finally, the failure data indicate a strong sensitivity to surface flaws, as predicted by localized necking theory.

Apparent Enhanced Fatigue Resistance under Cyclic Tensile Loading for a HIPed Silicon Nitride

Cyclic tensile loading tests of a commercial HIPed silicon nitride at elevated temperatures have indicated apparent “enhanced” fatigue resistance compared to static tensile loading tests under similar test conditions. At 1150°C, stress rupture results plotted as maximum stress versus time to failure did not show significant differences in failure behavior between static, dynamic, or cyclic loading conditions, with all failures originating from preexisting defects (slow crack growth failures). At 1260°C, the stress rupture results showed pronounced differences between static, dynamic, and cyclic loading conditions. Failures at low static stresses (<175 MPa) originated from environmentally assisted (oxidation) and generalized creep damage, while failures at similar times but much greater (up to 2 x) cyclic stresses originated from preexisting defects (slow crack growth failures). At 1370°C, stress rupture results did not show as pronounced differences between static, dynamic, and cyclic loading conditions, with most failures originating from environmentally assisted (oxidation) and generalized creep damage.

Network reliability with are failures and repairs : P. Simin Pulat. IEEE Trans. Reliab.R-37, 268 (1988)

The attitude control subsystem (ACS) on board a 3-axis stabilized spacecraft is critical for on-orbit operations; its degradation can significantly disrupt mission performance, and its failure is fatal to the spacecraft. An ACS that fails early on-orbit, for example in the first couple of years on a spacecraft with a 15-year design lifetime, results in significant losses to all stakeholders and constitutes a major financial blow to the satellite operator or insurer—losses can amount to a few hundred mission dollars. Understanding and preventing/mitigating ACS failure is an engineering and financial imperative.In this work, we first provide a comprehensive statistical analysis of ACS failure behavior on orbit. The results indicate that the ACS is a major driver of spacecraft unreliability and contributes up to 20% of all spacecraft failures. Nonparametric and parametric results, including mixture Weibull distributions, are provided for ACS reliability, and they are complemented with the subsystem’s multi-state failure analysis. It is shown for example that the ACS is 89% likely to be fully operational after 15 years on-orbit. Results also demonstrate that ACS suffers from both infant mortality and wear-out failures. The findings are important for satellite manufacturers and equipment providers as they indicate opportunities for improved ground testing and burn-in procedures for the ACS, and they highlight the need to consider additional redundancy, even in stand-by form, especially if increased spacecraft design lifetime is sought.Second, we conduct a comparative analysis of failure behavior of the ACS in Low Earth Orbit (LEO) and Geosynchronous orbit (GEO). The results demonstrate a marked difference in failure behavior in LEO and GEO: the ACS degrades and fails more frequently, harder (more severe anomalies), and earlier (more infant anomalies/failures) in LEO than in GEO. We expand on these results and conclude with hypotheses for causal factors of this difference in failure behavior.