Effects of pressure sensitivity on notch-tip fields in plastics.

The effects of pressure-sensitive yielding on the $\eta$ factor and the J integral estimation for compact tension specimens are investigated. The analytical expressions for $\eta$ and J for von Mises materials are generalized to pressure-sensitive materials under plane stress and plane strain conditions. Numerical results indicate that $\eta$ decreases as the pressure sensitivity increases. The effects are more pronounced under plane strain conditions than under plane stress conditions. For rigid perfectly-plastic materials, J for pressure-sensitive materials is reduced to a simple expression of the tensile yield stress times the crack tip opening displacement as for the von Mises materials. Also, plane-strain stress and slip-line fields near the sharp and round tips of wedge-shaped notches in perfectly plastic pressure-sensitive materials are investigated to illustrate the effects of pressure sensitivity and notch geometry on notch-tip fields. Results show that for sharp notches, as the pressure sensitivity or the wedge angle increases, the opening stress and hydrostatic tension ahead of the tip decrease. For round notches, as the pressure sensitivity increases, the extent of the exponential spiral zone ahead of the tip increases. As the wedge angle increases, the extent of the exponential spiral zone ahead of the tip decreases and the maximum opening stress and hydrostatic tension ahead of the tip decrease. The notch-tip fields in pressure-sensitive non-porous and porous solids are investigated by finite element analysis. The yielding behavior of porous solids which are used to represent rubber modified epoxies, is based on a generalized Gurson yield criterion that accounts for both the matrix pressure sensitivity and porosity effects. Due to the geometry and loading of the specimen modeled, some regions are under negative mean stresses, for which the yield criterion is modified. The computational results are compared with experimental cavitation and intense shear zones near the notch tip in specimens. The lowering of the mean stress ahead of the tip in epoxies with higher rubber contents changes the fracture mode from being controlled by mean stress at the elastic-plastic boundary to being controlled by plastic strain closer to the notch tip.Ph.D.Applied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130645/2/9811019.pd

An assessment of unstimulated salivary flow rate, IgA and clinical oral dryness among active and passive smokers

ObjectivesThe aim of this study was to assess the impact of smoking on the whole salivary flow rate (SFR), IgA levels and clinical oral dryness (COD) among active and passive smokers.Material and MethodsThe participants were categorized as active smokers (N = 54) or passive smokers (N = 163). Saliva was collected in tubes and placed in ice storage at –70°C. Salivary IgA levels were assessed in duplication using the enzyme linked immunosorbent assay (ELISA) method. Following the saliva sample collection, the subjects were assessed for COD using the COD score, SFR and caries. Chi-square test, the t-test and ANOVA were employed to compare the clinical impact of the smoking status associated with specific variables (smoking status, number of cigarettes, active caries, gender, age, COD score, IgA level and SFR). A p-value of 35 years age group (p < 0.05).ConclusionsThe study demonstrated significant differences in SFR, IgA and COD scores among the active and passive smokers. The number of cigarettes had a negative impact on saliva production, IgA levels, the oral health status, and the progression of caries with respect to age and gender. Smoking potentially leads to xerostomia associated with active caries