Buckling and postbuckling behavior of laminated composite stringer stiffened curved panels under axial compression: Experiments and design guidelines

Abstract

It is well known [1] that non-closely stiffened panels can have considerable postbuckling reserve strength, enabling them to carry loads significantly in excess of their initial buckling load. If appropriately designed, their load carrying capacity will even appreciably exceed that corresponding to an equivalent weight unstiffened shell (i.e. a shell of identical radius and thicker skin and which is also more sensitive to geometrical imperfections). In these shells, initial buckling of their panels takes place in a local mode, i.e. skin buckling between stiffeners, and not in an overall mode, i.e., an Euler or wide column mode. The design of aerospace structures places great emphasis on exploiting the behavior and on mass minimization of such panels to reduce lifecycle costs. An optimum (minimum mass) design approach based on initial buckling, stress or strain, and stiffness constraints, typically yields an idealized structural configuration characterized by almost equal critical loads for local and overall buckling. This, of course, results in little postbuckling strength capacity and susceptibility to premature failure. However, an alternative optimum design approach can be imposed to achieve lower mass designs for a given loading by requiring the initial local buckling to occur considerably below the design load and allowing for the response characteristics known to exist in postbuckled panels [2] ,i.e. capability to carry loads higher than their initial buckling load. To meet the requirements of low structurally weight, advanced lightweight laminated composite elements are increasingly being introduced into new designs of modern aerospace structures for enhancing both their structural efficiency and performance. In recognition of the numerous advantages that composites offer, there is a steady growth in replacement of metallic components by composite ones in marine structures, ground transportation, robotics, sports and other fields of engineering. Many theoretical and experimental studies have been performed on buckling and postbuckling behavior of flat stiffened composite panels (see for example Refs.3-8). Recently, a wide body of description and detailed data on buckling and postbuckling tests has been compiled [9] (see chaps.1214). However, studies on cylindrical composite shells and curved stiffened composite panels are still quite scarce (see for example Refs.10-15). Most of them have been discussed in detail in Ref. 9 (see chap. 14). In light of the above discussion, it has been suggested that permitting postbuckling to take place under ultimate load of fuselage structures, i.e. alleviation of design constraints, may provide a means for meeting the objectives for the design of next generation aircraft, where the demand is reduction of weight without prejudice to cost and structural life (see paper Vision 2020 of the European Community). This approach has been undertaken in an experimental study (Improved POstbuckling SImulation for Design of Fibre COmposite Stiffened Fuselage Structures POSICOSS project) as a part of an ongoing effort on design of low cost low weight airborne structures initiated by the 5 European Initiative Program. It was aimed at supporting the development of improved, fast and reliable procedures for analysis and simulation of postbuckling behavior of fiber composite stiffened panel of future generation fuselage structures and their design. Within the POSICOSS project, the Technion performed a long test series, on curved laminated composite stringer stiffened panels under axial compression, shear load introduced by torsion and combined axial compression and shear. The buckling and postbuckling behavior of these panels was recorded till their final collapse. The first part of this test series, dealing with panels PSC1-PSC9 was summarized in Ref. 16. The results of the tests with panels BOX1-BOX4, which deal with two identical panels, combined together by two flat non-stiffened aluminum panels, to form a torsion box, thus enabling application of shear tractions, through introduction of torsion, and combined axial compression and shear, were reported in [17]. The present manuscript is aimed at describing the buckling and postbuckling behavior, test results and relevant numerical studies of another four panels, AXIAL1-AXIAL4, having J type stringers of the configurations shown in Fig. 1.