br Thermal tempering But practically

Thermal tempering But practically, at any time the glass is in contact with the rollers somewhere at the surface. Furthermore the cooling is influenced by the flow of the air. Monnoyer and Lochegnies (2008) simulated the flow field of the quenching process. They showed that the air from the nozzles at the middle of the pane crosses the jets at the border area. Thus, the heat transfer is disturbed at the contact points as well as through the air flow and this process cannot lead to an absolutely homogeneous cool-down and thus anisotropy-free glass. With this knowledge, an attentive observer can determine on the building, the direction of oscillation, the reversal point, either the fabric is spiral or circular wrapped around the rollers or the distance between the fabric stripes of the rollers. One can also see sporadic problems due to blocked nozzles or recurring patterns through a poor parameterized and therefore non-optimized process. Commonly, hot bent and tempered glasses are more susceptible to anisotropies than flat glass, because the homogeneous cooling is more complicated (Bucak et al., 2009; Schuler et al., 2012). Newly, a generation of quenches is launched at the market which operates with an air cushion (LiSEC Group, 2011) instead of the rollers or the nozzles are controlled in dependence of the measured anisotropies (Arntzen et al., 2010). The last named system measures the anisotropies with a laser beam at discrete points of the glass pane. They are promising technologies regarding the prevention of anisotropies, but they have to prove the practical applicability concerning common dimensions, quantity and also prices for architectural purposes.
Photoelasticity The main effect for the formation of the colorful anisotropy pattern can be explained with the pka inhibitor of photoelasticity. Commonly, photoelasticity is used to determine the stress field in mechanical models with an experimental method. In Kuske and Robertson (1974), Wolf (1976), Föppl and Mönch (1972), Frocht (1948), and Dally and Riley (1978) the fundamentals and the application of this technique are explained. For the understanding of this effect, we shall follow a ray of light from the source to the observers eye. The ray of light prescribes the direction of propagation of the electromagnetic wave of the light. In Figure 3 a standard setting, named plane polariscope, is shown. The light emerges from a source with randomly oriented emitters and therefore the waves will have no preferred orientation of vibration. At the first polarizer only light that vibrates parallel to the axis of the polarizer (here parallel to the black bars; blue arrow) is transmitted. This linear polarized beam splits into the two principle axes (σ1 and σ2; red and green arrows respectively) of the glass specimen, because abnormal hemoglobin is a birefringent material when stresses are applied on it. During the transmission of these two rays a phase shift s can occur if the stresses in the two principle directions are not equal. When these two rays emerge the glass they will reach the analyzer. It is the same optical instrument as the polarizer but it is rotated 90°. Here only the components parallel to the analyzer axis of these two rays are transmitted. If there is no phase shift, the components eliminate each other and the observer will notice a black screen. On the opposite, if the phase shift is exactly a half wavelength the light intensity maximizes. Due to the fact that each wavelength of the visible spectrum experiences different retardation for the same stress state one can see colorful fringes by using white polarized light for this experiment. With the stress-optical-law the relative phase shift δ can be determined for the two-dimensional problem:But this is only valid, if the stress is constant through the thickness. For tempered glass this is not applicable. Here Eq. (1) must be transferred to the integral form for thin slices through the thickness:Due to the fact that the functions and are not exactly known, it is impossible to calculate stresses only from one fringe plot, which was taken in a transmission setup.