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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.10, #3)
Optimization of Embedded Optical Sensor Location in Composite Repairs by George Tsamasphyros; Nikos Furnarakis; George Kanderakis; Zaira Marioli-Riga (pp. 129-140).
Optical fibers were embedded in a bonded composite patch in order to detect the strain field variations of a load bearing structure. The study concentrated on a classical cracked metallic structure repaired with this ‘smart’ patch and using finite element analysis. Six different laminates constituted the model of the composite patch, a layered structure with three-dimensional elements. Each laminate is assumed to have different mechanical properties, according to the case under any specific study, in order to simulate different stacking sequence or material used. A resin rich ‘eye’ pocket has also been modeled in order to simulate the exact form of the resin area produced during the manufacturing process. The patch is bonded over a cracked aluminum sheet through a small adhesive layer placed in between. External loads were applied only on the metal structure, as in a real repair case. The primary loading axis of the metal was assumed to be parallel to the direction of the optical fibers. The different nature of the materials that form the composite patch generated complex mechanical interactions between the fibers and the surrounding material, resulting in a complicated stress field along the optical fiber sensor, which affects the structural integrity of both the patch and the repair. Different optical fiber positions were considered, moving towards the horizontal and vertical dimensions of the patch, as well as different patch architectures (single and double patch configurations), with the hope of studying their effect on the structural integrity of the patch.
Keywords: smart composite patching; embedded brag grating sensing
Computational Analysis and Optimization for Smart Patching Repairs by George J. Tsamasphyros; Nikos K. Furnarakis; George N. Kanderakis; Zaira P. Marioli-Riga (pp. 141-148).
A classical cracked metallic structure, repaired with a ‘smart’ bonded composite patch with embedded optical fibers (to detect the strain field variations of the loaded structure), has been studied here-in. Finite element analysis was used, where-in the composite patch was modeled as a layered structure with three-dimensional elements constituting six different laminae. Each lamina is assumed to have different mechanical properties, according to the studied case, in order to simulate different stacking sequence. A resin rich ‘eye’ pocket has also been modeled in order to simulate the exact form of resin area produced during the manufacturing process. The patch is bonded over a cracked aluminum sheet through a small adhesive layer placed in between. External loads were applied only on the metal structure, as in a real repair case. The primary loading axis of the metal was assumed to be parallel to the direction of the optical fibers. Due to the different nature of the materials that form the composite patch, complex mechanical interactions between the fibers and the surrounding material occur, resulting in a complicated strain field along the optical fiber sensor. This affects the structural integrity of both the patch and the repair. Different optical fiber layer positions were considered, to study their effect on the resulting strain field and the structural integrity of the patch. Analysis concluded that the best available embedding position of an optical fiber in a laminated patch coincides to the one predicted as neutral surface, according to Rose's analytical equations.
Keywords: smart patching repairs; embedded optical fibers; composite patch; laminated patch
Thermal Analysis by Numerical Methods of Debonding Effects near the Crack Tip under Composite Repairs by G. J. Tsamasphyros; G. N. Kanderakis; Z. P. Marioli-Riga (pp. 149-158).
Composite patch repair of metallic structures has become a rapidly grown technology in the aerospace field due to the demand for significant increases in the useful life of both military and civilian aircraft. This has led to significant advances overall in the repair technology of cracked metallic structures. Adhesively bonded composite reinforcements offer remarkable advantages such as mechanical efficiency, repair time, cost reduction, high structural integrity, repair inspectability, damage tolerance to further causes of future strains, anticorrosion and antifretting properties. However, because of the different nature and properties of the materials that form a repair (metals, composites, adhesives), side-effects may occur: debonding due to high stress concentration in the vicinity of the crack, thermal residual stresses because of different thermal expansion coefficients of the adherents, etc. In this paper a three-dimensional finite elements analysis of the area around a patch repaired crack of a typical aircraft fuselage is performed, taking into account both the properties and the geometry of the involved materials. Examined in this case are 2024-T3 aluminum alloy as base material, FM-73 as the adhesive system and F4/5521 boron/epoxy prepreg as the patch material. Through the thickness stresses near the crack tip and along the patch edges with and without temperature effects are calculated and debonding near the crack tip is examined. Finally, the calculated results are compared with existing theories.
Keywords: delamination; buckling; patch repair; growth behaviour
Mechanical Characterizations of the Dispersion U3Si2-Al Fuel Plate with Sandwich Structure by Xi-Shu Wang; Yong Xu (pp. 159-167).
This paper is to assess effect of the dispersion U3Si2-Al fuel plate with sandwich structure on its mechanical properties and to assess the quality of this fuel plate by the suitable processing because the optimum performance of fuel plate affects directly the safety and reliability of a power reactor. For this purpose, the mechanical properties of the fuel plate with sandwich structure were described that the relationship between the strength and the sandwich microstructure of dispersion U3Si2-Al fuel plate based on the experimental investigations and the fracture analysis of SEM images. These results shown that it can be improved that the mechanical properties of dispersion U3Si2-Al fuel plate with sandwich structure by the optimizing clad material, U3Si2-Al powder composite contents and process of rolling as well as the optimizing the heat treatment methods.
Keywords: sandwich structure; fuel plate; mechanical properties; fracture; SEM
Residual Stress Assessment in Thin Angle Ply Tubes by A. S. Kaddour; S. T. S. Al-Hassani; M. J. Hinton (pp. 169-188).
This preliminary study aims to investigate the residual stresses developed in hot cured thin-walled angle-ply filament wound tubes made of E-glass/epoxy, Kevlar/epoxy and carbon/epoxy materials. The residual stresses were estimated from change in geometry of these tubes when axially slitted at ambient temperature. Three basic deformation modes; namely opening up, closing-in and twisting, were observed and these depended on the winding angle, material and wall thickness. The residual stresses were also determined from hoop and axial strain gauges mounted on both the inner and outer surfaces at various locations around the tube. The stresses were compared with theoretical prediction based upon a linear thermo-elastic analysis. Both the predicted and measured values were found to increase with increasing hoop stiffness but there was a large discrepancy between the predicted and measured data, reaching a factor of 5 for the thinnest case. When compared with predicted failure stresses, the experimentally determined stresses were some 15% of the computed compressive strength.
Keywords: angle ply; kevlar; carbon; glass composite rings; slitting; residual thermal stress release; twisting