Sulfur fatigue

Conventional cure systems use relatively high levels (2.5 + phr) of sulfur combkied with lower levels of accelerator(s). These typically provide high initial physical properties, tensile and tear strengths, and good initial fatigue life, but with a greater tendency to lose these properties after heat aging. 

To explain the mechanism by which the formation of Ni3S2 scale can result in the fatigue failure of the blade disc, the effect of sulfur on the mechanical properties of nickel base alloys must be understood. Since the fracture modes are totally intergranular with evidence of... 

Colorless gas with a strong odor of rotten eggs detectable at 0.005 ppm. However, it can cause olfactory fatigue and the sense of smell is not reliable. Used industrially to produce elemental sulfur, sulfuric acid, and heavy water for nuclear reactors. 

The addition of tellurium has a deoxidizing action and confers on lead useful work and precipitation-hardening properties. It also improves resistance to wear, vibration, and mechanical breakdown. These properties, along with improved corrosion resistance, are utilized in the sheathing of power communication and marine cables, in chemical equipment (especially those exposed to sulfuric acid), or where resistance to fatigue is important. Tellurium has successfully replaced tin (1—3% Sn) in the sheathing. Thinner sheaths can be extruded, and the extrusion pressure for 0.05% Te-beafing lead is about the same as for a 3% Sn alloy. 

SAS mixes are optimized using a slightly modified Marshall Method (ASTM Method D 1559) and represent a series of trade-offs between stability and flow, air voids content and permeability and fatigue life [15]. The major departure from the Marshall Method is that only 2 hammer blows rather than the conventional 75 blows per face are used. The light compaction is used only to expel entrapped air and to achieve more uniform sample configurations without disturbing sulfur soldification within the aggregate voids. 

The stability and permeability of a mix are not sufficient criteria to reflect how a material will perform under the repeated loads generated by traffic. Therefore, any mix design selection process should include an examination of the material s fatigue resistance. Figure 5 shows the relationship between sulfur content and fatigue life for some SAS mixtures at two strain levels [15]. These tests were run at constant stress in third point flexure. Both curves go through a maximum at a sulfur content of 14 percent. 

Figure 5. Fatigue life as a function of sulfur content for a sand-asphalt-sulfur mix. Test temperature 50°F (10°C). Test frequency 60 Hz. Materials medium-coarse sandt and 150/180 pen. asphalt. Asphalt content 6% wt (15),...

The engineering properties of aggregate mixtures employing elemental sulfur as a binder are not suitable for highway pavement applications such mixtures possess adequate compressive strength, but are extremely brittle and lack satisfactory fatigue behavior to resist repetitive traffic loading. 

The fibers of aromatic PODs are known to have a combination of good properties, such as strength and stiffness, fatigue resistance, and relatively low density, in the range of 1.2 to 1.4 g/cm3. PODs have been used to improve the heat resistance of many synthetic fibers. This is usually done by dissolving the POD in sulfuric acid and then treating the fibers with this solution. Poly( >-phenylene-l,3,4-oxadiazole) is the most commonly used commercial polyoxadiazole, and the fiber spun from this polymer is called Oksalon. 

Sometimes the opposite is also the case, that is, the boiler itself restricts the type of flue gas treatment system that can be installed. Most electric power generating facilities have coal specifications required by the boiler design and the manufacturer s warranty. These sulfur and ash specifications are designed to optimize boiler performance and avoid fatigue of boiler materials. Boiler design and coal specifications can limit the type of system that can be applied. The construction date of a boiler determines the level of pollutant control required, and it also goes far toward determining the type of flue gas treatment system that can achieve this level of emissions control. 

Direct Substitution Method. Sulfur and asphalt were poured together at 300°F and were added to the heated aggregate at 300°-320°F. The three materials were mixed in the Hobart mixer for 2 min. Standard Marshall test samples for Marshall and dynamic stiffness tests were prepared as in the emulsification method. Bar samples for fatigue testing were prepared by kneading the mix into 3 X 3 X 12-in. bars to obtain compaction equal to the Marshall compacts. 

It is therefore recommended that the sulfur concentration in the binder be maintained at a low level, sufficiently high to meet mix stability requirements but without lowering the standard of mix flexibility i.e., 20-30 wt %. Flexibility should not be confused with fatigue resistance, which... 

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