Fetr6 is an underground mine using the stope-and-pillar mining method. As there was some evidence regarding pillar failure in this mine, improving works such as roof support and replacing existing pillars with concret...Fetr6 is an underground mine using the stope-and-pillar mining method. As there was some evidence regarding pillar failure in this mine, improving works such as roof support and replacing existing pillars with concrete pillars (CP) were carried out. During the construction of the second CP, in the space between the remaining pillars, one of the pillars failed leading to the progressive failure of other pillars until 4 000 m 2 of mine had collapsed within a few minutes. In this work, this phenomenon is described by applying both numerical and empirical methods and the respective results are compared. The results of numerical modelling are found to be closer to the actual condition than those of the empirical method. Also, a width-to-height (W/H) ratio less than 1, an inadequate support system and the absence of a detailed program for pillar recovery are shown to be the most important causes of the Domino failure in this mine.展开更多
In order to study the durability of sprayed concrete (shotcrete), effects of different hydration aging and water-binder ratio (w/b) on the microstructure of cement paste and basic mechanical properties of test spe...In order to study the durability of sprayed concrete (shotcrete), effects of different hydration aging and water-binder ratio (w/b) on the microstructure of cement paste and basic mechanical properties of test specimens were investigated. The phase composition, mass percentage of ettringite and portland in hydration production and microstructure were characterized by X-ray diffraction (XRD), thermo gravimetry-differential scanning calorimetry (TG-DSC) and scanning electron microscopy (SEM), respectively. The experimental results showed that changes in phase composition was more significant than those of water-binder ratio. With hydration aging and water-binder ratio increased, the mass percentage of ettringite and portland was decreased from 4.42%, 1.49% to 3.31%, 1.35%, respectively and the microstructure of paste was significantly compacted. Likewise, the mechanical properties including cubic compressive strength and splitting tensile strength were rised obviously.展开更多
The hydration process,hydration product and hydration heat of blended cement paste mixed with mineral admixture and expansive agent at low W/B ratio are studied by XRD,thermo analysis,and calorimetry instrument,and th...The hydration process,hydration product and hydration heat of blended cement paste mixed with mineral admixture and expansive agent at low W/B ratio are studied by XRD,thermo analysis,and calorimetry instrument,and they were compared with those of pure cement paste.The results show that pure cement and blended cement at low W/B ratio have the same types of hydration products,but their respective amounts of hydration products of various blended cements at same ages and the variation law of the amount of same hydration products with ages are different;The joint effect of tumefaction of gel-ettringite due to water absorption and the expansive pressure on the pore and rift caused by the crystalloid ettringite is the impetus of the volume expansion of cement paste,and the former effect is much greater than the latter one.展开更多
Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showe...Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showed that the violence of coal specimen failure depends on both the interface friction and width-to-height(W/H) ratio of coal specimen. The mode of failure for a uniaxially loaded coal specimen or a coal pillar is a combination of both shear failure along the interface and compressive failure in the coal. The shear failure along the interface triggered the compressive failure in coal. The compressive failure of a coal specimen or a coal pillar can be controlled by changing its W/H ratio. As the W/H ratio increases, the ultimate strength increases. Hence, with a proper combination of interface friction and the W/H ratio of pillar or coal specimen, the mode of failure will change from sudden violent failure which is brittle failure to non-violent failure which is ductile failure. The main objective of this paper is to determine at what W/H ratio and interface friction the mode of failure changes from violent to non-violent. In this research, coal specimens of W/H ratio ranging from 1 to 10 were uniaxially tested under two interface frictions of 0.1 and 0.25, and the results are presented and discussed.展开更多
Many experimental investigations have previously been performed and recentlydone on different shipbuilding structural steels where the specimens size and crack depth/specimenwidth (a/W) were varied. A series of intere...Many experimental investigations have previously been performed and recentlydone on different shipbuilding structural steels where the specimens size and crack depth/specimenwidth (a/W) were varied. A series of interesting results have been gained. It is worthwhile to havea review on the effect of a/W ratio on fracture toughness, and further theoretical analysis isnecessary. In this paper, experimental work in elastic-plastic fracture mechanics (EPFM) wasdiscussed. Tests had been carried out on 10 kinds of strength steels. Results showed that J_i andδ_i. values increased with decreasing a/W when a/W【0.3 for three-point bend specimens and thatshallow crack specimens which have less constrained flow field give markedly higher values oftoughness than deeply notched specimens. However, for a/W】0.3, the toughness was found to beindependent of a/W. Slip line field analysis shows that for shallow cracks, the hydrostatic stressis lower than that from standard deeply cracked bend specimen which develops a high level of cracktip constraint, provides a lower bound estimate of toughness, and will ensure an unduly conservativeapproach when applied to structure defects especially if initiation values of COD / J-integral areused.展开更多
In part I of this series, experimental investigation in EPFM (elastic-plastic fracture mechanics) had been discussed. In this paper, experimental investigation in LEFM ( linear elastic fracture mechanics) is given...In part I of this series, experimental investigation in EPFM (elastic-plastic fracture mechanics) had been discussed. In this paper, experimental investigation in LEFM ( linear elastic fracture mechanics) is given. Fracture toughness tests had been carried out on three different strength steels, using both through-cracked specimens with different α/W ratio and semi-elliptical cracked specimens with variable crack size and shape. Results show that the fracture toughness KIC increases with decreasing α/W when α/W 〈 0.3 for three-point-bend specimens, and that for α/W 〉 0.3, it is independent of α/W. Shallow crack specimens, both through-cracked and surface-cracked, gave markedly higher values than deeply notched specimens. However, the effect of crack shape on fracture toughness is negligible. Results also show that the LEFM approach to fracture is not tenable for design stresses where αc is often very small, far less than 2.5 ( KIC/σy)^2.展开更多
In part I and II of this series, experimental investigation in both EPFM and LEFM had been discussed. In this part, further theoretical analysis is given. The theoretical development of Two Parameter Fracture Mechanic...In part I and II of this series, experimental investigation in both EPFM and LEFM had been discussed. In this part, further theoretical analysis is given. The theoretical development of Two Parameter Fracture Mechanics by Hancock etc, has rationalized our experimental results. This method can be applied to engineering practice, and will allow the advantage of enhanced toughness for specimens with low levels of constraint to be taken into account for defect assessment.展开更多
The hardness variation of two kinds of alloys with 36 wt pct W content and 7/3, 9/1 Ni-to-Fe ratios during strain aging at 800℃ was studied. The microstructures of the aged alloys were analyzed by X-ray diffraction a...The hardness variation of two kinds of alloys with 36 wt pct W content and 7/3, 9/1 Ni-to-Fe ratios during strain aging at 800℃ was studied. The microstructures of the aged alloys were analyzed by X-ray diffraction and TEM. The results show that the strain aging hardness of W-Ni-Fe ternary alloy with 7/3 Ni-to-Fe ratio decreases monotonically with the increase of aging time. Under the same conditions, the hardness of 9/1 Ni-to-Fe ratio alloy decreases in the initial aging stage, but then increases as aging process goes on. X ray diffraction and TEM analysis show that there is not any precipitation depositing from the alloy with 7/3 Ni-to-Fe ratio during aging. The monotonic decrease in hardness of this alloy during aging process results from the recovery, recrystallization and solid solubility declining. In the alloy of 9/1 Ni-to-Fe ratio, the fine β phase precipitates dispersively during aging which hardens the alloy. The two different kinds of mechanisms (the softening one and the hardening one) decide the hardness variation of the alloy with 9/1 Ni-to-Fe ratio mentioned above.展开更多
文摘Fetr6 is an underground mine using the stope-and-pillar mining method. As there was some evidence regarding pillar failure in this mine, improving works such as roof support and replacing existing pillars with concrete pillars (CP) were carried out. During the construction of the second CP, in the space between the remaining pillars, one of the pillars failed leading to the progressive failure of other pillars until 4 000 m 2 of mine had collapsed within a few minutes. In this work, this phenomenon is described by applying both numerical and empirical methods and the respective results are compared. The results of numerical modelling are found to be closer to the actual condition than those of the empirical method. Also, a width-to-height (W/H) ratio less than 1, an inadequate support system and the absence of a detailed program for pillar recovery are shown to be the most important causes of the Domino failure in this mine.
基金Funded by the National Natural Science Foundation of China(Nos.51278403 and 51308445)the Program for Innovative Research Team in University(IRT 13089)
文摘In order to study the durability of sprayed concrete (shotcrete), effects of different hydration aging and water-binder ratio (w/b) on the microstructure of cement paste and basic mechanical properties of test specimens were investigated. The phase composition, mass percentage of ettringite and portland in hydration production and microstructure were characterized by X-ray diffraction (XRD), thermo gravimetry-differential scanning calorimetry (TG-DSC) and scanning electron microscopy (SEM), respectively. The experimental results showed that changes in phase composition was more significant than those of water-binder ratio. With hydration aging and water-binder ratio increased, the mass percentage of ettringite and portland was decreased from 4.42%, 1.49% to 3.31%, 1.35%, respectively and the microstructure of paste was significantly compacted. Likewise, the mechanical properties including cubic compressive strength and splitting tensile strength were rised obviously.
文摘The hydration process,hydration product and hydration heat of blended cement paste mixed with mineral admixture and expansive agent at low W/B ratio are studied by XRD,thermo analysis,and calorimetry instrument,and they were compared with those of pure cement paste.The results show that pure cement and blended cement at low W/B ratio have the same types of hydration products,but their respective amounts of hydration products of various blended cements at same ages and the variation law of the amount of same hydration products with ages are different;The joint effect of tumefaction of gel-ettringite due to water absorption and the expansive pressure on the pore and rift caused by the crystalloid ettringite is the impetus of the volume expansion of cement paste,and the former effect is much greater than the latter one.
基金sponsored by Coal and Energy Research Bureau and CDC-NIOSH under Grant No.R01OH009532
文摘Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showed that the violence of coal specimen failure depends on both the interface friction and width-to-height(W/H) ratio of coal specimen. The mode of failure for a uniaxially loaded coal specimen or a coal pillar is a combination of both shear failure along the interface and compressive failure in the coal. The shear failure along the interface triggered the compressive failure in coal. The compressive failure of a coal specimen or a coal pillar can be controlled by changing its W/H ratio. As the W/H ratio increases, the ultimate strength increases. Hence, with a proper combination of interface friction and the W/H ratio of pillar or coal specimen, the mode of failure will change from sudden violent failure which is brittle failure to non-violent failure which is ductile failure. The main objective of this paper is to determine at what W/H ratio and interface friction the mode of failure changes from violent to non-violent. In this research, coal specimens of W/H ratio ranging from 1 to 10 were uniaxially tested under two interface frictions of 0.1 and 0.25, and the results are presented and discussed.
文摘Many experimental investigations have previously been performed and recentlydone on different shipbuilding structural steels where the specimens size and crack depth/specimenwidth (a/W) were varied. A series of interesting results have been gained. It is worthwhile to havea review on the effect of a/W ratio on fracture toughness, and further theoretical analysis isnecessary. In this paper, experimental work in elastic-plastic fracture mechanics (EPFM) wasdiscussed. Tests had been carried out on 10 kinds of strength steels. Results showed that J_i andδ_i. values increased with decreasing a/W when a/W【0.3 for three-point bend specimens and thatshallow crack specimens which have less constrained flow field give markedly higher values oftoughness than deeply notched specimens. However, for a/W】0.3, the toughness was found to beindependent of a/W. Slip line field analysis shows that for shallow cracks, the hydrostatic stressis lower than that from standard deeply cracked bend specimen which develops a high level of cracktip constraint, provides a lower bound estimate of toughness, and will ensure an unduly conservativeapproach when applied to structure defects especially if initiation values of COD / J-integral areused.
文摘In part I of this series, experimental investigation in EPFM (elastic-plastic fracture mechanics) had been discussed. In this paper, experimental investigation in LEFM ( linear elastic fracture mechanics) is given. Fracture toughness tests had been carried out on three different strength steels, using both through-cracked specimens with different α/W ratio and semi-elliptical cracked specimens with variable crack size and shape. Results show that the fracture toughness KIC increases with decreasing α/W when α/W 〈 0.3 for three-point-bend specimens, and that for α/W 〉 0.3, it is independent of α/W. Shallow crack specimens, both through-cracked and surface-cracked, gave markedly higher values than deeply notched specimens. However, the effect of crack shape on fracture toughness is negligible. Results also show that the LEFM approach to fracture is not tenable for design stresses where αc is often very small, far less than 2.5 ( KIC/σy)^2.
文摘In part I and II of this series, experimental investigation in both EPFM and LEFM had been discussed. In this part, further theoretical analysis is given. The theoretical development of Two Parameter Fracture Mechanics by Hancock etc, has rationalized our experimental results. This method can be applied to engineering practice, and will allow the advantage of enhanced toughness for specimens with low levels of constraint to be taken into account for defect assessment.
基金This work was supported by the National Natural Science Foundation of China under grant No.59971007.
文摘The hardness variation of two kinds of alloys with 36 wt pct W content and 7/3, 9/1 Ni-to-Fe ratios during strain aging at 800℃ was studied. The microstructures of the aged alloys were analyzed by X-ray diffraction and TEM. The results show that the strain aging hardness of W-Ni-Fe ternary alloy with 7/3 Ni-to-Fe ratio decreases monotonically with the increase of aging time. Under the same conditions, the hardness of 9/1 Ni-to-Fe ratio alloy decreases in the initial aging stage, but then increases as aging process goes on. X ray diffraction and TEM analysis show that there is not any precipitation depositing from the alloy with 7/3 Ni-to-Fe ratio during aging. The monotonic decrease in hardness of this alloy during aging process results from the recovery, recrystallization and solid solubility declining. In the alloy of 9/1 Ni-to-Fe ratio, the fine β phase precipitates dispersively during aging which hardens the alloy. The two different kinds of mechanisms (the softening one and the hardening one) decide the hardness variation of the alloy with 9/1 Ni-to-Fe ratio mentioned above.