摘要:

In‐service fracture of helicopter windshields was studied. Simulated catastrophic fracture tests were conducted by firing alumina and steel spheres onto stationary tempered and as‐received glass panels. The results were studied by Hertzian analysis and modified Auerbach's relations. Thermally tempered glass shows much higher impact resistance than that estimated from superposition of residual stresses. Subcritical impact sites exhibit slow crack growth in tempered plates, eventually leading to fracture of the entire pl

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15258.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Reactions and reaction mechanisms occurring at sessile drop interfacial regions of sodium disilicate glass on chromium were identified at 1000°C and an ambient atmosphere of 2.7 × 10−4Pa with a p(O2) of 1 × 10−15and 1 × 10−5Pa. Depending on the experimental conditions, the principal reactions were redox reactions of Cr0with Na+to form Cr2+ and Na0, Cr2+ with Na+to form Cr3+ and Na+, Cr0with Si4+ to form CrSix, alloy dendrites and Cr2+, and Cr2+with Si4+to form Cr3+and SiO(gas). Adherence was developed when the interfacial region was saturated with Cr3+, i.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15245.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

The kinetics of grain growth in fully dense Al2O3with and without MgO solute additions were measured for high‐purity samples containing no liquid phases. The MgO was found to suppress the grain‐boundary migration rate by a factor of 50. Compensating lattice defects are suggested to play a role in grain‐growth inhibition. Implications of these results to the sintering of Al2O3are disc

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15259.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Spin‐lattice relaxation times for7Li were measured as a function of temperature in two mixed‐isotope (7Li,6Li) triborate glasses and one mixed‐alkali (Li, Na) triborate glass. The rapid increase in relaxation rate above 400 K is believed to result from localized alkali motion. A mixed‐alkali effect was observed for such a motion, but it is shown not to be a mass

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15260.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

The fundamental question as to whether the tip structure of brittle cracks is atomically sharp or has a rounded contour is examined in relation to current descriptions of strength‐controlling flaws. The distinction between the two opposing viewpoints lies in the controlling flaw dimensions in the strength formulation; crack length in the first and tip radius in the second. Definitive evidence on the issue is obtained from aging tests on soda‐lime glass, using indentations as controlled flaws. An increase in the inert strength is observed with increased exposure of the newly created flaws to moist environments prior to stressing to failure. This strength increase saturates after approximately 1 day, depending on the environmental species. The trend mirrors that reported by Mould in an earlier aging study on abrasion flaws. However, whereas Mould concluded that the strengthening must be due to tip rounding, the present tests reveal that the indentation‐induced cracks actually extend during the aging period. A fracture mechanics analysis shows that such extension relaxes residual crack‐opening stresses associated with the central contact deformation zone. It is accordingly concluded that the cracks remain sharp throughout their postindentation evolution; the influence of extraneous conditions on the strength is manifested only through the driving forces on these cracks. Flaws which have been annealed (i.e., which have had their residual driving forces removed) show no such aging effects. The fracture mechanics analysis also shows that in the region of saturated aging, where the indentation cracks appear to stop growing, the fracture driving force is in the region of the zero‐velocity threshold described in the macroscopic crack growth studies by Michalske. Contrary to previous interpretation, the indication is that the cracks do not blunt out in this region, for otherwise the strengthening would steepen rather than level out. Implications of this result concerning the inviolate nature of basic crack growth laws are

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15246.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Ceramic‐matrix composites reinforced with SiC fibers were tested at room temperature both in, flexural and tensile configurations. The stress‐strain behavior for composites tested in tension was correlated with progressive microcracking and failure processes. Significant differences between failure modes in tension and flexure were obser

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15261.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

An Mn‐Fe‐rich slag forms near the tips of Si3N4‐based indexible cutting tools during the machining of cast iron. Manganese and iron were found to react chemically with Si3N4to form a liquid phase in the presence of air, but not in the absence of air, at temperatures comparable to those reached during machining operations. Thus, chemical reactions between Fe, Mn, Si3N4, and O2may be a factor in the wear of Si3N4tools against cast

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15262.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15263.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

The high‐temperature structure of zirconia was studied by powder neutron diffraction up to 2400°C. The boundaries of the domain of the nonstoichiometric tetragonal form are defined. They are consistent with a tetragonal‐cubic transition at 2350°C for stoichiometric zirconia. The changes in the structural parameters of the tetragonal form (unit‐cell, positional parameters and thermal B factors) with temperature give evidence of medium zirconium and oxygen mobilities. The oxygen ions are, however, always more mobile than the zirconium ions. An enhancement with temperature of the structural an‐isotropy tends to weaken the weaker of the two distinct Zr‐0 bonds of the tetragonal zirconia. It results in the transformation into the cubic form which is accompanied by a change in unit‐cell volume; this change becomes sharper as the composition tends toward stoichiometry. This transition is probably followed by an increase of the i

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15247.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Changes in resistivity and chemical changes in reducing gases were measured for porous zinc oxide ceramics with and without a platinum catalyst at 300° and 400°C to examine the gas sensing mechanism and the effect of platinum additions. Reducing gases were oxidized to CO2and Hz2on the sensor surfaces. Platinum addition promoted the oxidation of reducing gases but did not lead to an increase in the resistivity change at 400°C. The reaction sequences for the gas sensing process are proposed, taking into account partially oxidized intermediates of hydrocarbons and oxidation on platinum without an electron transfer proce

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1985.tb15248.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY