Long Wavelength Measurements of Surface Cracks in Silicon Nitride

1982 ◽  
pp. 569-571 ◽  
Author(s):  
J. J. Tien ◽  
B. T. Khuri-Yakub ◽  
G. S. Kino ◽  
A. G. Evans ◽  
D. Marshall
Keyword(s):  
Silicon Nitride ◽  
Surface Cracks ◽  
Long Wavelength

Long Wavelength Measurement of Surface Cracks in Silicon Nitride

1981 ◽  
Author(s):  
J.J. Tien ◽  
K. Liang ◽  
B.T. Khuri-Yakub ◽  
G.S. Kino ◽  
D. Marshall ◽  
...  
Keyword(s):  
Silicon Nitride ◽  
Surface Cracks ◽  
Long Wavelength ◽  
Wavelength Measurement

Detection of contact damage in ceramics by an ultrasonic method

1998 ◽  
Vol 13 (7) ◽  
pp. 1899-1904 ◽  
Author(s):  
Hyo Sok Ahn ◽  
Said Jahanmir ◽  
John A. Slotwinski ◽  
Gerald V. Blessing
Keyword(s):  
Silicon Nitride ◽  
Glass Ceramic ◽  
Ultrasonic Method ◽  
Circular Ring ◽  
Ultrasonic Technique ◽  
Surface Cracks ◽  
Compressional Waves ◽  
Different Depths ◽  
Pulse Echo ◽  
Hertzian Cone

A pulse-echo ultrasonic technique consisting of focused normal-incident compressional waves was used for the detection and evaluation of surface and subsurface damage in micaceous glass-ceramic and silicon nitride samples. The damage was produced by indentation with a tungsten carbide ball. The nature of the damage was found to be material-dependent and was classified into two types: Hertzian cone cracks in the silicon nitride, and distributed subsurface microcracks in the glass-ceramic. While the cone cracks were visible on the surface as circular ring cracks, the distributed subsurface microcracks were not associated with any visible surface cracks. Both the cone cracks and the distributed subsurface microcracks were easily detected by the ultrasonic technique. In addition, the ultrasonic beam was focused to different depths below the surface of the glass-ceramic sample to probe the subsurface region containing the microfracture damage.

Surface Cracks: Size and Failure Prediction Using Long Wavelength Measurements

1983 ◽  
pp. 175-192
Author(s):  
J. J. Tien ◽  
K. Liang ◽  
B. T. Khuri-Yakub ◽  
G. S. Kino ◽  
D. B. Marshall ◽  
...  
Keyword(s):  
Failure Prediction ◽  
Surface Cracks ◽  
Long Wavelength

Distinct Element Simulation of Laser Assisted Machining of Silicon Nitride Ceramics: Surface/Subsurface Cracks and Damage

2005 ◽  
Author(s):  
X. Shen ◽  
S. Lei
Keyword(s):  
Silicon Nitride ◽  
Distinct Element ◽  
Rake Angle ◽  
Effect Of Temperature ◽  
Depth Of Cut ◽  
Surface Cracks ◽  
Laser Assisted Machining ◽  
Subsurface Cracks ◽  
Element Simulation ◽  
Nitride Ceramics

This paper applies distinct element method (DEM) to simulate the material removal process of laser assisted machining of silicon nitride ceramics and show the formation and propagation of surface/sub-surface cracks and damage. A synthetic specimen is created using particle clusters to approximate the granular microstructure of β-type silicon nitride ceramics. The effect of temperature on machining is considered by its influence on the material properties. In addition, some other parameters such as rake angle, depth of cut, local damping coefficient and cluster size are also considered in a parametric study. It shows that all these parameters influence surface/sub-surface cracks and chip formation of silicon nitride ceramics in laser assisted machining.

Surface Crack Growth from Small Indentations in a Silicon Nitride Square Bar under Cyclic Reversed Torsion

2012 ◽  
Vol 566 ◽  
pp. 65-69
Author(s):  
Katsuyuki Kida ◽  
Takashi Honda ◽  
Edson Costa Santos
Keyword(s):  
Silicon Nitride ◽  
Crack Growth ◽  
Stress Ratio ◽  
Fatigue Testing ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Mode Ii ◽  
Surface Cracks ◽  
Silicon Nitride Ceramic ◽  
Surface Crack Growth

In order to investigate the mechanism of surface cracks in silicon nitride ceramic (HIP-Si3N4) from the viewpoint of shear stress, the authors focused on torsion fatigue testing and observed the crack growth behavior under conditions where the stress ratio was R = - 1. Furthermore the residual stresses around the cracks were measured. Based on these results, mode II growth of surface cracks is discussed and it was concluded that under stress ratio R = -1, surface cracks grow slowly in mode II, for ΔKⅡ less than 3.6MPam1/2.

Activated Prominences; Mechanisms for Activation and Eruption

1979 ◽  
Vol 44 ◽  
pp. 307-313
Author(s):  
D.S. Spicer
Keyword(s):  
Pressure Gradient ◽  
Density Gradient ◽  
Current Density ◽  
Convective Instability ◽  
Tearing Mode ◽  
Magnetic Shear ◽  
Long Wavelength ◽  
Ideal Mhd ◽  
Gradient Change ◽  
Accelerated Particles

A possible relationship between the hot prominence transition sheath, increased internal turbulent and/or helical motion prior to prominence eruption and the prominence eruption (“disparition brusque”) is discussed. The associated darkening of the filament or brightening of the prominence is interpreted as a change in the prominence’s internal pressure gradient which, if of the correct sign, can lead to short wavelength turbulent convection within the prominence. Associated with such a pressure gradient change may be the alteration of the current density gradient within the prominence. Such a change in the current density gradient may also be due to the relative motion of the neighbouring plages thereby increasing the magnetic shear within the prominence, i.e., steepening the current density gradient. Depending on the magnitude of the current density gradient, i.e., magnetic shear, disruption of the prominence can occur by either a long wavelength ideal MHD helical (“kink”) convective instability and/or a long wavelength resistive helical (“kink”) convective instability (tearing mode). The long wavelength ideal MHD helical instability will lead to helical rotation and thus unwinding due to diamagnetic effects and plasma ejections due to convection. The long wavelength resistive helical instability will lead to both unwinding and plasma ejections, but also to accelerated plasma flow, long wavelength magnetic field filamentation, accelerated particles and long wavelength heating internal to the prominence.

Lattice Imaging of Grain Boundaries in Ceramics

1977 ◽  
Vol 35 ◽  
pp. 114-115
Author(s):  
D. R. Clarke ◽  
G. Thomas
Keyword(s):  
Mechanical Properties ◽  
High Temperature ◽  
Silicon Nitride ◽  
Grain Boundaries ◽  
High Temperature Strength ◽  
Current Interest ◽  
Lattice Imaging ◽  
Special Significance ◽  
Structural Applications ◽  
Refractory Ceramics

Grain boundaries have long held a special significance to ceramicists. In part, this has been because it has been impossible until now to actually observe the boundaries themselves. Just as important, however, is the fact that the grain boundaries and their environs have a determing influence on both the mechanisms by which powder compaction occurs during fabrication, and on the overall mechanical properties of the material. One area where the grain boundary plays a particularly important role is in the high temperature strength of hot-pressed ceramics. This is a subject of current interest as extensive efforts are being made to develop ceramics, such as silicon nitride alloys, for high temperature structural applications. In this presentation we describe how the techniques of lattice fringe imaging have made it possible to study the grain boundaries in a number of refractory ceramics, and illustrate some of the findings.

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