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Topocal 2010 Crack
By 1988, cocaine was being widely available. In 12th grade students, the prevalence of use in 1988 was 30%. This prevalence rate continues to hover around 30% throughout the 1990s and well into the 2000s. Throughout this period, crack use has remained relatively stable, though its association with methamphetamine use has recently increased significantly. Among 12th grade students, in 1988, 15% reported ever having used methamphetamine, though its prevalence has increased to 42% in 2008, or about one-fifth of the student population (though again, a substantial fraction of this number report occasional use). Crack use in 1999 was 21% among 12th graders. From 1999 to 2008, it decreased to 17%. The 2000s has seen a steady increase of crack use, reaching 22% in 2008. However, it appears that this proportion of use is not as great among Whites as it is for Blacks (Crack Figure 2, CDC ).
As has been frequently noted, the white-to-black ratio of the percentage of students who use crack has increased from 4:1 in the early 1990s to 5:1 in the 2000s, with the increase among Blacks considerably greater (3:1) than that among Whites (2:1). There has also been an increase in Hispanic use, though the white-to-Hispanic ratio has not changed significantly. This pattern has not changed in the most recent years of this study. (Several explanations have been offered for this pattern of cocaine use.
In particular, there has been recent evidence that crack use is rising even among Whites, despite some widely held conceptions of crack users as predominantly Hispanic (see this report by the Race and Drugs website for a summary of evidence for this assertion).
The stress distributions generated by shear strain in a cubic lattice are shown in figure9. The stress in these regions decreases steadily with distance from the crack tip due to the action of the shear strains. The K-field lines intersect at the point of intersection of the diagonal bars. The distance from point of intersection to the crack tip is known as the spacing. The length of the bars is fixed, and the prismatic angle is given by the angle of intersection. These structures are constrained to maintain a fixed loading situation. Any change in loading conditions leads to a change in the local K-field. Bar rotation leads to a change in the K-field, since the K-field is continuous at the point of intersection of the bars, and the rotation of the bars modifies the K-field at this intersection. If an incipient crack is introduced at the location of the intersection, these structures tend to open, with stress concentration at the intersection point. If the intersection angle is greater than approximately 46, then the stress field at the intersection is produced by simple beam theory, and the stress at the intersection is K=Kt(BI)1/3 (equation 3). For a given value of Kt, the bar spacing must be chosen to satisfy equation 4: 1.8 If the value of Kt is chosen such that the stress at the intersection is less than f, then the lattice is not able to resist the applied stress. In the regime of f 5ec8ef588b