PSI - Issue 31

M. Dundović et al. / Procedia Structural Integrity 31 (2021) 111 – 115 M. Dundovi ć et al. / Structural Integrity Procedia 00 (2019) 000–000

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placed in different orientations to explore the influence of the layer printing direction in regard to the longitudinal axis of the specimen while preserving the equal building time. After printing the parts were removed from the build surface, cleaned from support and any uncured resin residue and left to dry for 72 hours at room temperature. Due to process parameters, the chosen specimens were completely cured and no post-curing was needed. 2.2. Experimental setup The photoelastic observations were performed using a Tiedemann circular polariscope. When the polarized light enters the stressed, transparent model the ray of light is double refracted, and these two rays continue to vibrate in mutually perpendicular planes with the same direction but different velocities. Upon emergence from the model the rays will have the relative retardation of δ depended on the difference between the two indices of refraction, i.e. principal strains, the wavelength of light and the thickness of model. The two rays emerging from the model are received by a second polarizer, termed the analyzer, which only transmits components of the two rays in its plane of polarization. The two emerging components can be added together, and cause extinction with monochromatic light if one component is half a wavelength behind the other, and maximum light intensity if one component is an integral number of wavelengths behind the other. The fringes in the interferogram produced are known as isochromatics. The interferogram is not generated at points where axes of the light ellipse coincide with the optical axes of the polarizer which allows the directions of the principal strains to be identified. The locus of these points is known as an isoclinic. In circular polariscope the isoclinic are removed using the quarter wave plates. The relation between the principal stress difference σ 1 - σ 2 , the thickness of model b , and the stress-optical sensitivity of the material is given by the stress-optic law: � − � = � �� �1� where f σ is the photoelastic constant and N is is the isochrome order. The photoelastic constant depends on material photoelastic properties as well as wavelength of light used and setup error. Therefore, before any measurements the calibration of whole setup and the material is needed, which is often done by the pure bending test. The pure bending test was performed by means of a custom-built special loading device consisting of two levers supported on one end and loaded with weights on the other. The support point of one lever must be fixed, while the other support must allow for horizontal movement in order to avoid unwanted compressional loads. For precision, the test samples were clamped using screws, small metal plates and 3D printed receptacles. High precision ball bearings were used at the support points to minimize the influence of friction on measurement results. The test samples were loaded incrementally until failure, the initial bending moment was M 0 = 970 Nmm due to the weight of the levers, while the moment is increased for 342 Nmm in each subsequent measurement step. 3. Results All test samples were examined under a monochromatic light source and the isochromes, which are directly representative of the strain level of the test specimen, have been observed. The observation was performed using both light and dark field measurement in order to verify the accuracy. In order to determine the photoelastic constant, the Young’s modulus must be calculated first. The strain measurements have been performed on photographs taken during tests, the stroke was measured on edges as well as on every isochrome and the average value was taken for modulus calculation. Additionally, the lengths of all isocromes were measured and compared with the length of the zero isochrome, obtained from dark field measurements, for strain determination, Fig. 2. The Young’s modulus obtained is E = 1214 N/mm 2 , what is an expected value for acrylic polymers. The photoelastic constant was determined on a vertically printed specimen loaded with a pure bending moment M 3 = 1639 Nmm by means of the stress-optic law, using the knowledge that one of the principal stresses must be equal to zero on all free outer boundaries of an object under load: � = � ���� �2�