6.2.2 Isochromatic Evaluation Using Three Fringe Photoelsaticity Three-fringe photoelasticity [10] uses a single isochromatic color image for finding the total fringe order at each point in the model domain. This is done by comparing RGB intensity values in the dark field isochromatic image of the application specimen with that of a calibration table. To address the issue of fringe order continuity in complex specimen shapes, Sohan and Ramesh [11] has proposed a new scanning method with mask based algorithm known as Advancing Front Scanning for TFP (AFSTFP). Modified window search method is used for refinement which is based on the fringe order increments and is given as ei ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffifffii R Ri ð Þ 2 þ G Gi ð Þ 2 þ B Bi ð Þ 2 q ; N2 Np ΔN, Np þΔN ð6:1Þ where R, G, Bcorresponds to the intensities of the pixel under consideration andRi, Gi, Bi is the corresponding intensities in the calibration table. The average value of the fringe orders of all the neighboring resolved pixels is used as Np in Eq. (6.1) and a value of 0.4 for ΔNis found to be suitable for the current study. The isochromatic fringe orders are evaluated using TFP for a quantitative appreciation of the isochromatic data. Maximum fringe order is observed in the coronal region of the model. Figure 6.2a shows the magnified experimental dark field isochromatic for the coronal region shown in Fig. 6.1b. It is interesting to note that the fringe features shown in Fig. 6.2a are similar to that of an isochromatics corresponding to a typical contact problems. This needs further investigation on coronal region to understand the mechanics involved better. The next section of this paper deals with the evaluation of the high stressed areas in the coronal region with a focus on analyzing the contact zone using photoelasticity. 6.3 Contact Zone Evaluation of the Coronal Region Structural integrity between the bone and the implants has great importance, as higher stresses around the implants can lead to bone resorption. It is also stated that the total area of contact between the implant and bone is significant in deciding the osseointegration strength of implant-bone interface [12]. The higher stresses produced in the coronal region is very crucial as it can lead to peri-implant bone lose. Generally, the stresses near the contact zones are characterized by the contact length and frictional coefficient. The potential of digital photoelasticity can be used to determine these contact parameters from the isochromatic data in the coronal area of the model. Fig. 6.2 (a) Magnified view of dark field isochromatic for the coronal region shown in 6.1 (b). (b) Schematic representation of two bodies in contact. (c) Reconstructed dark field isochromatics in monochrome from the results of least squares analysis with data points echoed back for the contact zone shown in 6.2 (a) 6 Contact Zone Evaluation of Dental Implants Using Digital Photoelasticity 41
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