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Humidity induced creep and it’s relation to the dilatancy boundary
Published in N.D. Cristescu, H.R. Hardy, R.O. Simionescu, Basic and Applied Salt Mechanics, 2020
In accordance with the model of Pharr & Ashby (1983) and the general ideas of Horseman (1988) we explain our observations as follows: If rock salt is permeable, then water vapor can propagate through the network of open grain boundaries and microcracks. The water molecules in the humid air are preferably adsorbed at places of increased energy, i.e. at sharp convex surfaces and in strain hardened regions at contact areas in cracks and pores. At these sites the solubility is locally raised (and the critical relative humidity is decreased) and a thin brine film is formed. The stored mechanical energy, predominantly produced by strain hardening due to pile up dislocations will be dissipated by recovery, thus facilitating creep. This mechanism is strongly influenced by the local relative air humidity. Because moisture acts as a catalyst, only very small amounts are needed. Thus recovery of strain hardened material steadily takes place at varying places causing a great increase in creep rate. It results that a small range around open grain boundaries and cracks becomes ductile and cames the humidity induced creep. It is still unclear how far this effect extends into the grains. The marked transient creep phases after a humidity change show that moisture needs some time to come to a steady state.
Moisture behavior of slab-on-ground, structures
Published in J. Carmeliet, H. Hens, G. Vermeir, Research in Building Physics, 2020
Long-term phase design ensures that under thermal and moisture equilibrium conditions moisture cannot condense at any level inside the examined structure and that the critical relative humidity levels of the used materials are not exceeded (especially under the coating materials, Table 3). Figure 6 shows a calculation example of a typical slab-on-ground structure with varying insulation thickness and vapor resistance of the coating layer. Figure 6 indicates that doubling the insulation thickness decreases the relative humidity at the lower surface of the coating approximately by 5% units. The increased thickness of the insulation has also an indirect effect on the moisture balance of the structure by reducing the heat flow into the subsoil and lowering the temperature and partial pressure of the water vapor of the fill layers. The worst structural combination is the low thermal resistance of the insulation combined with highly vapor resistant coating material. If the critical humidity of the used coating material (for example plastic carpet, Table 3) is RH = 80%, the humidity at the lower surface of the coating exceeds this limit when the soil temperature reaches Ts = +18°C. According to the performed survey (Fig. 4) this kind of subgrade temperature is inevitable under the poorly insulated slab structure.
Fundamental Forces in Powder Flow
Published in Wolfgang Sigmund, Hassan El-Shall, Dinesh O. Shah, Brij M. Moudgil, Particulate Systems in Nano- and Biotechnologies, 2008
N. Stevens, S. Tedeschi, M. Djomlija, B. Moudgil
Rabinovich and coworkers have also investigated the effect of nanoscale surface roughness on the critical humidity required for the onset of capillary forces in many systems (38,41–44). The presence of surface roughness on a nanoscale has been shown to have a profound effect on the critical humidity required to induce capillary adhesion. This can be seen in Figure 8.15, a graph of the adhesion force measured by atomic force microscopy of silica surfaces of varying roughness over a range of relative humidity values. Increasing roughness increased the critical relative humidity required for the onset of capillary adhesion as well as decreased the overall adhesion force. Surface roughness will decrease the minimum amount of water needed to form liquid bridges at particle contact points since the capillary can form on the asperities as shown in Figure 8.16a. However, the volume of liquid present is small and the onset of capillary adhesion shifts to higher relative humidity levels such that the liquid bridges form between the two particle surfaces. This is shown in Figure 8.16b. The effect is seen regardless of substrate, as shown in Figure 8.17. Their study also validated the use of equation (8.16) to predict the force of adhesion as a function of particle size, humidity, and roughness for practical systems.
Alternative solutions for the physicochemical evaluation and improvement of the caking properties of calcium ammonium nitrate fertilizer as a quality problem under atmospheric conditions
Published in Journal of the Chinese Institute of Engineers, 2023
As the storage temperature increases, the critical relative humidity value (CRH) at which the fertilizer starts to retain moisture decreases, leading to increased, which increases moisture absorption and caking. Even fertilizers are stored at the appropriate temperature, they may be exposed to weather conditions at temperatures of 40–45°C during transportation. In this case, it is necessary to use additives or hydrophobic coatings that can inhibit the mechanisms that can be triggered in the fertilizer structure, particularly for sensitive fertilizers like AN. The another recommend could be climate-controlled transportation even though it has high costs. It is important to examine the impact force of the temperature on caking. Temperature, as a parameter that triggers moisture retention, is as important as relative humidity, depending on the fertilizer composition. At least the limit temperatures presented in this study can be used as a storage limit in the CAN production sector. Temperature is the most crucial ambient condition, especially for ammonium nitrate fertilizers, as it is a special compound with four crystalline phase transitions. Therefore, temperature is a critical parameter for ammonium nitrate, which can cause cracking in the granule structure under normal conditions, and it must be a parameter that must be strictly controlled. It has been observed that the tendency toward caking of CAN fertilizer supplemented with additives increases rapidly, especially after 40°C. Inappropriate stabilizer content or increased free moisture content may lower this temperature limit. In addition, it was observed that the fertilizer structure started to dissolve above 55°C. Relative humidity is an important parameter that comes after temperature. While the CRH value is around 55% specific to CAN, a fertilizer developed with additives becomes resistant to humidity up to 70%. However, it has been observed that the fertilizer retains less than 2% moisture during import transport, where conditions such as 80% humidity are quite possible, and this does not significantly increase caking. Even if the storage humidity and temperature are optimum, stacking above 0.28 kg/cm2 limit pressure can create deformations that may cause caking of the fertilizer. Finally, it has been observed that the mechanical properties of fertilizers exposed to critical temperature and humidity values, such as abrasion resistance and fracture strength, decrease due to weakening, breaking and degradation of the granule structure.