VIBRATIONS OF PRESTRESSED VARIABLE STIFFNESS COMPOSITE AEROSPACE STRUCTURES BY RITZ APPROACH

With the introduction of the variable stiffness concept, the design space for highperformance lightweight composite structures has expanded significantly. A larger design space, in particular, allows designers to find more effective solutions with higher overall stiffness and fundamental frequency when considering prestressed dynamically excited aerospace components. In this context, an efficient and versatile Ritz method for the transient analysis of prestressed variable stiffness laminated doubly-curved shell structures is presented. The considered theoretical framework is the first-order shear deformation theory without further assumptions on the shallowness or on the thinness of the structure. A rational Bézier surface representation is adopted for the description of the shell allowing general orthogonal surfaces to be represented. General stacking sequences are considered and the unknown displacement field is approximated by Legendre orthogonal polynomials. Stiffened variable angle tow shell structures are modelled as an assembly of shell-like domains and penalty techniques are used to enforce the displacement continuity of the assembled multidomain structure and the kinematical boundary conditions. For the transient analysis of prestressed variable stiffness structures, classical Rayleigh damping is considered and solutions are obtained through the Newmark integration scheme. The proposed approach is validated by comparison with literature and finite elements results and original solutions are presented for prestressed free and forced vibrations of VS stiffened shell structure, proving the ability of the present method in dealing with the analysis of complex aerospace structures.

IMPACT DAMAGE DETECTION IN COMPOSITE AEROSPACE STRUCTURES BY MULTI-RESOLUTION NDE INSPECTIONS

Assessing the health of aerospace structures and understanding the underlying mechanics that govern composite strength constitute a main focus of research in the area of aerospace design and airworthiness certification. Impact damage is one of the major threats to composite aerospace structures for its frequency of occurrence, complexity and minimum external visibility. While non-destructive evaluation (NDE) provides a variety of solutions to inspect the subsurface and internal components of structures non-invasively, a gap exists between the mechanics of damage formation, growth and tolerance, and the inspectability of the structure. This study is focused on the quantitative correlation between impact damage mechanics and ultrasonic NDE inspections, where damage severity, mode interaction and progression are identified in real-scale composite panels of complex geometry, representative of commercial aircraft, impacted to reproduce different damage types at the skin-to-stringer interface and the stringer cap. High resolution X-ray CT scanning and conventional ultrasonic scanning (UT) have been used to map the damage state and identify relevant impact damage features. Ultrasonic guided wave (UGW) scanning was then employed as a rapid in-situ inspection technique to not only detect damage but also provide quantitative information about damage severity and mode. The correlation of multi-resolution multi-dimensional NDE data promises new insights on damage studies and solutions to damage detection and prognosis through viable NDE inspections.

Co-cured manufacturing of multi-cell composite box beam using vacuum assisted resin transfer molding

Co-curing holds great promise to minimize assembly weight, time, and cost for stiffened aerospace structures, which are conventionally fabricated separately and then integrated either through mechanical fastening or adhesive bonding—also known as secondary bonding. This study presented a low-cost co-curing process using VARTM to fabricate stiffened shells, particularly composite box beams. The experimental investigation was performed and the co-curing process was improved by scrutinizing the critical process parameters, such as foam strength and coating, and curing cycle. This work was also intended to present the demonstration of the proposed co-curing method and its comparison with the conventional secondary bonding technique for three-cell carbon fiber-reinforced polymer (CFRP) composite box beams. Fiber volume fraction measurements were carried out to the specimens extracted from the various section of the co-cured part, namely top skin, web, and bottom skin and as a result, around 60% of fiber volume fraction was measured, which was in good agreement with the results obtained from optical microscopy-based image analysis. Structural-level four-point bending test results showed that the weight normalized maximum and the ultimate load of the part increased by 44% and 45% with the use of the co-curing process, respectively. The improved mechanical properties indicated that stronger structural integration can be achieved by integrally curing structures. SEM micrographs revealed a favorable fiber-matrix interface, bolstering the superior integration of the co-cured part. These findings suggest that the low-cost co-curing process can be a potential candidate for the fabrication of stiffened aerospace structures, such as composite box beams.