PSI - Issue 5

N.A. Kosheleva et al. / Procedia Structural Integrity 5 (2017) 99–106 V.P. Matveenko et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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this task. Zhao et al. [12] proposed rounding of the internal right angle at the point C . The rounding radius in this work equals the plate thickness. Fig. 3 shows the stress intensity distribution σ i on the adhesion surface for the optimal variant of the spew geometry with g А = 0° and g В = 63° in the case of rounding at the point C and the distribution σ i for the variant of the external surface of the spew fillet with the coupling angles g А = g В = 9.3°, which, according to [11], provides the lowest stress concentration. In contrast to [11], this paper has an additional rounding of the right angle at the point C . The analysis of the results shows that the stresses in the vicinity of singular points for the obtained optimal geometry are much smaller than for the spew fillet geometries discussed in [6 – 9, 11, 12].

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4 12 x , mm Fig. 3. Stress intensity distribution σ i over the adhesive joint surface: curve 1 refers to the variant of the spew geometry [11] ( g А = g В = 9.3°) and curve 2 refers to the optimal variant of the spew geometry ( g А = 0°, g В = 63°). 3. Optimization of stress state in the output zone of a fiber optic sensor embedded into composite material One of the problems of using fiber optic sensors embedded into the polymer composite material is to ensure their connection to the measuring equipment. This goal can be achieved through physical contact or without it. In both cases, there is an input-output of optical fiber or dismountable elements on the surface of the composite material. Due to the properties heterogeneity of the optical fiber on the surface, special points are appeared in the schemes for stress state calculating, where singular solutions can exist in the framework of linear theory of elasticity, reflecting in the actual product the presence of pronounced stress concentration zones. According to this and developing the approaches outlined in the previous chapter, it is possible to propose options for optimizing the stress state in the fiber-optic input output zone of the composite material. For the numerical simulation of the stressed state, consider the subregion in the input-output zone with the following dimensions 2 l by d , where l = 1.5 mm, d = 3 mm (highlighted in color in Fig. 4a). Two variants of a fiber optic sensor in the form of a quartz fiber and a quartz fiber with a polymer coating are considered. Here, the quartz fiber has a diameter of 0.124 mm and the mechanical characteristics E q = 71.4 · 10 9 Pa,  q = 0.17, thickness of polymer coating 0.012 mm and the mechanical characteristics E pc = 5 · 10 9 Pa,  pc = 0.35. Mechanical characteristics of the material: E m = 26.3 · 10 9 Pa,  m = 0.18. Fig. 5 shows the stress intensity distribution over the surface of a quartz fiber without and with polymer coating under loading. The results demonstrate the presence of the high stress concentration zone in the vicinity of a singular point on the surface where various materials (composite material – quartz, composite material – polymer coating) come into the contact. The presence of such stress concentration zone can lead to the destruction of the optical fiber. One of the options for eliminating the established stress concentration is related to the design of the geometry in the fiber input-output zone in accordance with the properties of the optimal geometries nearby the singular points. In the present problem, such geometry addition can be formed from epoxy binder at the stage of the polymer composite formation. In this case for exclusion of singular solutions the coupling angles to a fiber and material surfaces must be zero. This geometry will correspond to the geometry of the additional region, which has a quarter-circle shape in the cross section (it is indicated by dashed lines in Fig. 4a). The radius of this circle is equal to 0.82 mm. For this variant, the stress intensity distribution over the surface of the quartz fiber is shown in Fig. 5. These results demonstrate the effectiveness of stress concentration elimination. Also, it should be noted that, due to the small diameter of the optical fiber, the technological implementation of the proposed solution is quite complex. 16 20 24 0 8