In order to evaluate contraction stress, which generates during photopolymerization of resin composites, transparent and photosensitive plates made of epoxy resin (Epidian 53, Organika-Sarzyna SA, Nowa Sarzyna, Poland) were used. The calibrated orifices (3 mm in diameter and 4 mm in thickness) in resin plates were prepared. The circular shape and size of orifices was meant to mimic an average tooth cavity. To obtain higher micromechanical retention, surface of the plates was sandblasted with a 50-μm grain corundum Cobra (Renfert, Hilzingen, Germany). Thus, prepared plates were immersed in distilled water for 3 months to eliminate errors associated with water sorption of resin. Next, dedicated bonding system was applied and cured with Elipar S10 lamp (3M ESPE, Landsberg am Lech, Germany) (Table 3 ). The orifices were filled with tested material in one layer. Three samples were prepared for each material. The polymerization was performed according to the manufacturer’s instructions (Table 2 and Table 3 ). Both light curing units (Mini L.E.D and Elipar S10) had an output irradiance of 1250 mW/cm2 and 1450 mW/cm2, respectively, as stated by the manufacturer. To ensure consistent irradiance values, the light curing units were calibrated with radiometer system (Digital Light Meter 200 from Rolence Enterprice Inc., Taoyuan, Taiwan).
Next, samples were stored in distilled water at room temperature. After selected period of time (30 min, 24 h, 72 h, 120 h, 168 h, 240 h, 336 h, 504 h, 672 h, and 1344 h), the generated strains in the plates were visualized in circular transmission polariscope FL200 (Gunt, Hamburg, Germany). Photoelastic images were registered by digital camera (Canon EOS 5D Mark II/Canon Inc., Tokyo, Japan), both in parallel and perpendicular orientation of filter polarization planes. Met-Ilo computer program (J. Szala, 2012, Poland) was applied to determine the arrangement and the dimension of interference fringes. The analysis of stress and strain was carried out in a two-dimensional state of the stress and three-dimensional state of deformations. The analysis of stress and strain was carried out in a two-dimensional state of the stresses and three-dimensional state of deformations. Additionally, the calculation was conducted following this assumption: the relative change in composite volume caused the extension of composite and the extension of base material being “tooth model” (epoxy resin plate). Accordingly, it was possible to determine the radial and circumferential stresses based on the Equations (4) and (5) given by Timoshenko [43 ]:
where σr—the radial stress, σθ—the circumferential stress, ps—the shrinkage stress around composite filling, a—the radius of the internal orifices in the plate, b—the radius of the largest of isochromatic fringe, and r—the radius contained in the region from a to b.
Upon calculating the shrinkage stress on the circumference of the orifices, the radial and circumferential stresses were determined on the basis of Equations (2) and (3).
Next, samples were stored in distilled water at room temperature. After selected period of time (30 min, 24 h, 72 h, 120 h, 168 h, 240 h, 336 h, 504 h, 672 h, and 1344 h), the generated strains in the plates were visualized in circular transmission polariscope FL200 (Gunt, Hamburg, Germany). Photoelastic images were registered by digital camera (Canon EOS 5D Mark II/Canon Inc., Tokyo, Japan), both in parallel and perpendicular orientation of filter polarization planes. Met-Ilo computer program (J. Szala, 2012, Poland) was applied to determine the arrangement and the dimension of interference fringes. The analysis of stress and strain was carried out in a two-dimensional state of the stress and three-dimensional state of deformations. The analysis of stress and strain was carried out in a two-dimensional state of the stresses and three-dimensional state of deformations. Additionally, the calculation was conducted following this assumption: the relative change in composite volume caused the extension of composite and the extension of base material being “tooth model” (epoxy resin plate). Accordingly, it was possible to determine the radial and circumferential stresses based on the Equations (4) and (5) given by Timoshenko [43 ]:
where σr—the radial stress, σθ—the circumferential stress, ps—the shrinkage stress around composite filling, a—the radius of the internal orifices in the plate, b—the radius of the largest of isochromatic fringe, and r—the radius contained in the region from a to b.
Upon calculating the shrinkage stress on the circumference of the orifices, the radial and circumferential stresses were determined on the basis of Equations (2) and (3).
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