Pre-existing fractures

Clinical Effectiveness. Teriparatide works equally well in women and men with osteoporosis. Teriparatide has reduced the risk of new vertebral fractnres by 65% compared with placebo (actnal fracture incidence 5% vs. 14%, respectively) in postmenopausal women with osteoporosis and pre-existing fractures. New nonvertebral fracture risk was reduced by 53% (3% fractures with teriparatide vs. 6% fractures with placebo) with the 20-mcg/day dosage. Teriparatide can... 

As shown in Figure 7 (a), multiplet planes are almost vertical and their strikes are approximately NNW-SSE. Similar distribution is observed for the pre-existing fractures (Figure 7 (b)). The direction... 

The geological conditions, parameters and stress conditions have been presented elsewhere in this proceeding, see Andersson et. al (2003). Two kinds of models have been established to simulate the fracturing process (See Figure 4.). The Far-field model" simulates the effect of excavations in infinite medium without external boundaries. It is used to investigate parameter sensitivity and the effect of pre-existing fractures. The Pillar model is used to simulate real heating process and 3D excavation effects, with the aid of the stress reconstruction technique described above. 

The effect of pre-existing fractures was preliminary studied by using FRACOD models. Large fractures may reduce the stress concentration in the critical pillar region because they allow larger deformation compared to intact rock. Fracturing is reduced compared to the intact case due to lower stress level at the borehole wall. 

Figure 7. Stress distribution and fracturing of a model without pre-exiting fracture. Right, with an inclined pre-existing fracture.

However, instability may occur due to fracture initiation and propagation when pre-existing fractures are located inside the pillar in a way that they cause high stress concentration near the borehole wall (See Figure 7.). 

Size Distribution Relationships. Different models have been used to describe the size distribution of particles experiencing single and multiple fractures. A model based on fracture at the site of the weakest link and a distribution of weakest links in the system gave results that could be described as well by the Rosin-Rammler relation (56). The latter is based on the concept that fracture takes place at pre-existing flaws that are distributed randomly throughout the particle. 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

Intergranular fracture along pre-existing paths Transgranular fracture along straingenerated paths Mixed crack paths by adsorption, decohesion or fracture of brittle phase ... 

Cummings SR, Black DM, Thompson DE, Applegate WB, Barret-Connor E, Musliner TA, Palermo L (1998) Alendronate reduces the risk of vertebral fractures in women witout pre-existing vertebral fractures results of the fracture intervention Trial. J Am Med Assoc 280 2077-2078... 

Dislocations as introduced in Section 3.2 have been postulated in order to explain the low yield strength of a crystal (if we compare it to the theoretical shear strength). Likewise, cracks are postulated in order to explain fracturing of crystals well below the theoretical tensile strength of the atomic bonds. Pre-existing cracks can easily magnify low applied stresses at their tips to the maximal atomic bond strength. 

Schematic diagram illustrating brittle fracture. It is typical of the temperature range below 0.87g. A crack nucleates at a pre-existing flow or craze and propagates unstably. 

As pointed out above, boron fiber is also a composite fiber like any other CVD-type fiber. During fracture, cracks frequently originate at pre-existing defects located at the boron/core interface or at the surface. Commonly, a radial crack originates at the boron/core interface and leads to a brittle fracture. 

In a disordered solid, as modelled by the percolation model (discussed in Section 1.2) with intact bond concentration p on a lattice, there occur various vacancy clusters (voids), or pre-existing cracks, of different sizes and shapes. The typical crack size in such a percolating solid being of the size of the correlation length (see Section 1.2.1) the application of the Griffith fracture criterion gives (Chakrabarti 1988, Ray and Chakrabarti 1985a,b) from equation (3.4)... 

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