Median crack

The indentation fracture mechanics approach likens abrasive-workpiece interactions for grinding of ceramics to small-scale indentation events. The deformation and fracture patterns observed for normal contact with a Vickers pyramidal indentor under an applied load P are illustrated in Figure 3.1. A zone of plastic deformation is foimd directly imder the indentor. Two principal crack systems emanate from the plastic zone med-ian/radial and lateral cracks. Median/radial cracks are usually associated with strength degradation and lateral cracks with material removal. 

Samples of banded coals show in the vitrinite portion transversal bands of high relief (owing to polishing) with a fine median crack filled with uranium minerals (Figure 4a, b). 

Fig. 6.2.10. (a) A schematic diagram of the composite block illustrating the grain orientation and the indentations Q and R respectively, along and away from the bulk interface, a—a. (b) A schematic diagram of flow lines in the porous zone, pz, and radial, median and lateral cracks, rc, me and 1c respectively within the deformed zone. (After van der Zwaag et al., 1980)... 

Lawn B. R., Evans A. G., Marshall D. B., 1980, Elastic-plastic indentation damage in ceramics. The median/radial crack system, J. Am. Ceram. Soc., 63, (9-10), 574-581. 

Fig. 6.2.6. Diagrammatic representation of median, lateral and chevron cracks formed by scribing with a sharp indenter. (After Misra and Finnie, 1979)...

Fig. 6.2.8. Median crack parameters under sharp indenters, general model. (After Lawn et al., 1976)...

Fig. 6.2.9. Indentation /strength model system, (a) Vickers indenter, peak load P, generates median/radial crack, characteristic dimensions (value c0 at completion of contact cycle, with further post-indentation, supercritical extension to c0, if exposed to moisture), (b) Tensile field a in combination with residual ( ghost ) contact field, drives crack system to failure. (After Marshall and Lawn, 1980)...

The median is robust to extreme outliers The term robust is used to indicate that a statistic or a procedure will continue to give a reasonable outcome even if some of the data are aberrant. Think what would happen if the slow-coach who took 24.8 s to crack the child-proof container had instead taken a week to get it open. This character, who is currentiy ranked 15th, would still be ranked 15th and the median, which is established by the eighth ranker, would be quite unchanged at 4.1 s. In contrast, the mean would be hugely inflated if somebody took a week. 

For theoretical prediction of value x50, characterizing stability to cracking, two main assumptions will be made. Firstly, it is assumed, that the samples fracture occurs, when active environment in diffusion process reaches their median plate. This assumption is based on the... 

In collaboration with Magura et al., we examined the utility of hair analysis for determining the prevalence of cocaine use among criminally involved youth. Personal interviews and scalp hair specimens were obtained from 121 male youths (median age = 19) who had been in jail in New York City and were followed up in the community after their release. Of these hair specimens, 67% were positive for cocaine at a mean concentration of 68 ng/10 mg of hair. Only 23% of the youths reported any use of cocaine or crack during the previous 3 months, and 36% reported any lifetime use. Associations were found between cocaine in hair and several behavioral variables prior number of arrests, rearrest after release from jail, not continuing education, and no legal employment. 

When a crack propagates in polystyrene at low crack velocities, the craze ruptures close to its median plane by a mechanism having the approximate characteristics of viscous flow. Each fracture surface is then covered by a thin layer of craze. At higher crack velocities, however, failure occurs along the boundaries between the craze and the adjacent bulk polymer by practically brittle fracture (I). The change in fracture... 

A significant, independent association between ilhcit drug use and a mild decline in renal function was found in a study of six hundred and forty seven patients followed in the Hypertension Clinic of the VA Medical Center in New Orleans, Louisiana [5]. These patients used a number of illicit substances (heroin, cocaine or crack, psychedelics, amphetamines, marijuana) and were followed for a median of 7 years. They were all hypertensive and the data was adjusted for systohc blood pressure, use of antihypertensive medication. 

The three attrition mechanisms are in turn governed by different failure modes brittle, semibrittle, and ductile (Ghadiri, 1997). Brittle failure occurs when internal or surface cracks already exist and is dominant at low elastic deformation at the powder contact surface (Shipway and Hutchings, 1993). Semibrittle failure, at limited plastic deformation, is responsible for flaw initiation and occurs when the impact forces surpass the yield point. In fact, median and radial cracks cause particle fragmentation, and lateral cracks cause chipping. Soft materials are usually ductile and the mechanisms for particulate solids under ductile mode have not yet been elucidated (Ghadiri, 1997). 

Figure 11.7 Crack systems developed from the Vickers indents, a) Side and top views of a median crack, h) Top and side views of a Palmqvist crack.

Figure 7. Optical micrograph of a Vickers hardness impression and the radial/median cracks generated on the corners of impression on the polished surface of a dental porcelain (a), and schematic image showing the dimensions a and c used to calculate the hardness, H V, and fracture toughness, by indentation fracture (IF) method...

Figure 9. Schematic images of radial/median cracks generated at the corners of Vickers impression (a) before ion exchange, without residual stresses and (b) after ion exchange, indicating the shortening of the cracks due to the residual compressive stress fields (indicated by arrows)...

The erosion of brittle materials has been described in terms of interacting Hertzian crack systems, or by the formation of lateral and median vents for sharp indentors. This is indicated in Figure 9.4. It has also been observed that brittle materials are capable of undergoing plastic deformation particularly when under erosion by very small particles.This is important because, at high temperature, some oxides formed as scales may show a ductile reaction to erosive impact. 

---