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Rock fatigue behaviour, including the fatigue threshold stress (FTS), post-peak instability and strength weakening/hardening during cyclic loading, is of paramount significance in terms of safety and stability assessment of underground openings. In this study, the evolution of the foregoing parameters for Gosford sandstone subjected to systematic cyclic loading, in the pre-peak and the post-peak regimes at different stress levels and under seven confinement levels ( σ 3 / UCS avg ) was evaluated comprehensively. The results showed that the FTS of rocks decreases exponentially from 97 to 80%, when σ 3 / UCS avg increases from 10 to 100%. The brittleness of rocks under monotonic and cyclic loading conditions increases with an increase in σ 3 / UCS avg when σ 3 / UCS avg ranging between 10 and 65% (known as the transition point). For higher confinements, however, the brittleness of rock transits from self-sustaining behaviour into ductile behaviour. The evolution of fatigue damage parameters for hardening tests showed that no critical damage happens within the specimens during cyclic loading; rather, they experience more compaction. This is while for weakening cyclic loading tests, continuous damage along with stiffness degradation was dominant. Furthermore, the variation of axial strain at failure point ( ε af ) shows that for lower confinement levels, the applied stress level does not affect the pre-peak irreversible deformation; its effect, however, becomes significant when confining pressure is high. For the specimens that did not fail in cycles, cyclic loading resulted in peak strength weakening or hardening depending on the applied stress level. The weakening effect was observed in higher confining pressures, which was mainly due to a higher amount of irreversible deformation accumulation in rocks in the pre-peak cyclic loading. An empirical model was proposed using classification and regression tree (CART) algorithm to estimate the peak strength variation of Gosford sandstone based on σ 3 / UCS avg and the applied stress level. Highlights: The effect of confining pressure and systematic cyclic loading history on the failure behaviour of rocks was investigated comprehensively. The lateral strain-controlled loading method and a modified triaxial testing procedure were used to measure rocks' failure behaviour. The evolution of fatigue threshold stress, rock brittleness, and peak strength with confining pressure and applied stress level was assessed. An empirical model was proposed to predict the peak strength variation using the classification and regression tree (CART) technique. [ABSTRACT FROM AUTHOR]

Abstract In recent years, the pre-notched or pre-cracked small punch test (P-SPT) has been successfully used to estimate the fracture properties of metallic materials for cases in which there is not sufficient material to identify these properties from standard tests, such as CT or SENB specimens. The P-SPT basically consists of deforming a pre-notched miniature specimen, whose edges are firmly gripped by a die, using a high strength punch. The novelty of this paper lies in the estimation of fracture properties using dog-bone-shaped specimens with different confinement levels. With these specimens, three confinement variations have been studied. The results obtained enable the establishment of a variation of fracture properties depending on the level of confinement of each miniature specimen and selection of the most appropriate confinement for this goal. Graphical abstract Unlabelled Image Highlights • Influence of confinement levels in the fracture properties has been studied. • New pre-notched dog-bone-shaped specimens has been used. • Different confinement levels tested on P-SPT specimens. • Relationship between fracture toughness and confinement levels has been analysed. [ABSTRACT FROM AUTHOR]

Nowadays, the small punch test is used successfully to characterize the mechanical behaviour of materials especially in those cases in which an insufficient amount of material is available to carry out standard tests. This test basically consists of deforming a miniature specimen whose edges are firmly gripped by a die. But, what happens if the level of confinement varies? This paper analyses the effect of varying the degree of confinement of an SPT specimen on its mechanical behaviour. This variation is achieved by modifying the geometry of the specimen from a conventional square shape to a dog bone shape, maintaining the same thickness as usual. With these specimens, confinement variations, ranging from the typical biaxial confinement (conventional SPT specimens) to an almost uniaxial confinement, as well as three intermediate values in between, have been achieved. The results obtained enable the establishment of a variation of mechanical properties depending on the level of confinement of each miniature specimen. [ABSTRACT FROM AUTHOR]

This experimental study highlights the confinement effect on a flame impinging a ceiling in confined or semi-confined environment. The enclosure used in this study represents a 1:10 scale model of a student compartment with two possible openings: a door and a window. It is developed based on the conservation of the Froude number in scaling law. The objective of this work is to provide explanations in terms of fire safety on thermal phenomena that can occur during fires in closed environments, for example in a room of a university residence, regarding eight heat rates release of fuel and five confinement levels. A flame oscillation modelling of propane-air flame is proposed and is verified by the experimental results. It is shown both by theoretical modeling and experimental results that the confinement of a compartment has an effect on period time of flame oscillation. Moreover, experimental results show that the confinement level of compartment is a key parameter to characterize vertical temperature evolution both in the center (impinging zone) and near the wall in the enclosure. The correlations for normalized gas temperature rise near the wall are proposed and these temperature evolutions are bounded by the ventilation conditions of the five configurations. Indeed, under condition of equivalence ratio greater than 1, the maximum gas temperature near the wall of a certain heat release rate decreases dramatically with the increasing of configurations' confinement levels, which is associated to the decrease in flame intensity by lack of oxygen with a great criterion of under ventilation. [ABSTRACT FROM AUTHOR]