316 research outputs found
Modelling Skin-Stringer Separation in a Post-Buckled Composite Stiffened Panel through Single Stringer Specimens
No full textThe collapse of composite stiffened panels in compression is complex and involves the interactions of many different failure modes. However, when the panel is allowed to enter the post-buckling regime, skin-stringer separation due to the interaction of the post-buckling deformations with the skin-stringer interface is often the critical failure mode. The accurate prediction of skin-stringer separation in these types of panels is therefore crucial for their design. Due to the high computational costs associated with modelling damage in a large multistringer panel and the manufacturing and testing costs associated with testing such a panel, single stringer specimens that accurately represent the behaviour of critical regions in these panels have been proposed in literature. In this thesis, transversely loaded single stringer specimens are designed and verified and are used to predict skin-stringer separation in the critical regions of a specific multistringer panel. A specific composite stiffened panel is considered that contains a skin of a tape material and four co-cured stringers of a fabric material. At the intersections of the skin and stringers, resin-rich noodle regions are created. A buckling analysis is first performed on the panel to obtain its post-buckling deformations. Based on these, two types of regions where skin-stringer separation may occur are identified; a mode I dominated skin-stringer separation at the minima of the buckling waves and a combined mode II and mode III skin-stringer separation at the inflection points of the buckling waves. A seven point bending specimen is designed based on the panel deformations near the former critical region, while a four point twisting specimen is designed based on the deformations near the latter region. Finally, detailed damage models are set up for the single stringer specimens to predict skin-stringer separation and the models are verified with the multistringer panel. The analyses showed that a mode I dominated skin-stringer separation is critical for this panel. The detailed analysis of the four point twisting specimen showed that no damage occurred at the inflection point. To predict the skin-stringer separation at the minimum, a verified seven point bending specimen has successfully been obtained. To do so, it was important to accurately simulate the deformation of the skin at the stringer flange edge, since this proved to be driving for the initiation and propagation of skin-stringer separation. Next to that, a trilinear cohesive law was used to capture the R-curve effect in the tape/fabric interfaces. Finally, a first assessment of the noodle region behaviour showed that damage in this region will probably happen at the location of maximum bending. Recommendations for future research have been provided.Aerospace Engineerin
Effect of Composite Stiffened Panel Design on Skin-Stringer Separation in Postbuckling
No full textTo design aeronautical composite multi-stringer panels that can safely operate in a postbuckled state, it is important to identify the parameters that can influence the different modes in which skin-stringer separation might occur. A methodology is under development to study the interaction between the skin-stringer separation and the postbuckling deformation using the building block approach and single-stringer specimens. In particular, the methodology can identify whether the skin-stringer separation occurs due to bending or twisting, so that these two possible modes can be studied separately. For bending, a simple criterion that can predict the location of initiation is presented. This procedure has the potential to reduce the overall development cost and allows the investigation of the design parameters that influence the skin-stringer separation.Virtual/online event due to COVID-19Aerospace Structures & Computational Mechanic
Study of Skin-Stringer Separation in Postbuckled Composite Aeronautical Panels
No full textAeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology to assess and improve the capabilities of stiffened composite structures subjected to postbuckling deformations.Two regions are identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deformation and the region of maximum twisting. Both regions are studied using a finite element model of a representative single-stringer specimen. For the region of maximum deformation, a seven-point bending configuration is used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting is approximated using an edge crack torsion configuration, with two supports and two loading points. These two configurations are studied by changing the positions of the supports and the loading points. An optimization procedure is carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the four-stringer panel. The optimal configurations are applied to a finite element model of a single-stringer specimen including cohesive elements to simulate damage initiation. The types of damage initiation that occur in these configurations are compared to the global/local analysis of the two identified regions of the four-stringer panel. Furthermore, the effect of the position of the supports and loading points on damage initiation is investigated. The edge crack torsion configuration that is found to be the best at approximating the twisting deformation of the panel also shows the most similarity in damage characteristics with respect to the four-stringer panel.Aerospace Engineerin
Study of Skin-Stringer Separation in Postbuckled Composite Aeronautical Structures
No full textAeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology based on analysis and experiments to assess and improve the strength, life, and damage tolerance of stiffened composite structures subjected to postbuckling deformations. Two regions were identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deflection and the region of maximum twisting. Both regions have been studied using a finite element model of a representative single-stringer specimen. For the region of maximum deflection, a seven-point bending configuration was used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting was studied using an edge crack torsion configuration, with two supports and two loading points. These two configurations were studied by changing the positions of the supports and the loading points. An optimization procedure was carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the fourstringer panel.Aerospace Structures & Computational Mechanic
Post-buckling behavior and collapse of Double-Double composite single stringer specimens
No full textThis paper presents the work on six single-stringer specimens manufactured using the card-sliding technique with non-crimp fabrics and adopting a Double-Double (DD) stacking sequence. These specimens, representative of sub-structure level components, are used to investigate post-buckling and failure in aerospace structures. Two specimens maintain a constant thickness cross-section, while four are tapered, two of which incorporate a Teflon insert in the stringer flange. All specimens are tested under compression loading conditions, inducing skin buckling, skin-stringer separation, and eventual collapse. Numerical simulations are validated by experimental results and serve to analyze the specimens behavior and the failure mode. The load versus displacement curves of both experimental tests and Finite Element Method (FEM) analyses are compared, along with the out-of-plane displacement field. Subsequently, the observed failure modes are discussed, focusing on the various mechanisms that occurred and considering the impact of flanges and stiffener tapering. Both the FEM simulations and experimental tests demonstrate good agreement, with the flanges tapering revealing notable results. This offers promising evidence of a viable solution to optimize aeronautical structures and enhance resistance to skin-stringer separation.Aerospace Structures & Computational Mechanic
A Methodology to Investigate Skin-Stringer Separation in Postbuckled Composite Stiffened Panels
No full textA methodology is presented to investigate and improve the strength and damage tolerance of stiffened composite panels used in aerospace structures subjected to postbuckling deformation. These structural panels have the capability to operate in the postbuckling field, but the possible interaction between the postbuckling deformation and the damage initiation and propagation is yet to be fully understood. The developed methodology considers single-stringer specimens representative of stiffened panels to analyze skin-stringer separation. In this paper single-stringer specimens are studied in a four-point twisting configuration in order to investigate the region of maximum twisting, where the separation between the skin and the stiffener can initiate. A new test set-up is presented that recreates the four-point layout that can trigger separation due to twisting. The applied methodology shows that it is possible to mimic the out-of-plane buckling deformation of a large panel and study this numerically and experimentally through a single-stringer specimen.Aerospace Structures & Computational Mechanic
A methodology to reproduce postbuckling in composite panels to study skin stringer separation
No full textA building block pyramid is designed used to evaluate a composite aircraft structure, combining tests and analyses, using stiffened panels, single-stringer specimens and coupons. Failure in these structures can be caused by postbuckling-induced skin-stringer separation, which is complex, involving matrix cracks, fibre bridges, and delamination migrations. Standardized tests covering postbuckling-induced skin-stringer separation are lacking. First, a material characterization is performed on the coupon level. These properties are applied to a stiffened panel model to identify critical postbuckling regions. Then a single-stringer specimen is designed that combines the material complexities of the coupon level and the geometrical complexities of the panel by mimicking the postbuckling shape. Specifically a seven-point bending configuration to study bending-induced separation and a four-point twisting configuration for skin twisting-induced separation. The guidelines for modelling and testing can assist in standardising these test methods.Using composite materials in aircraft structures can reduce weight compared to conventional metals. However, utilizing more of the material's load-carrying capabilities can further reduce weight
Design, analysis and testing of thermoplastic welded stiffened panels to investigate skin-stringer separation in post-buckling
No full textThermoplastic composite three-stringer panels with omega stiffeners and conduction welded joints are designed, analysed and tested until final failure to investigate the performance of the welded joint in post-buckling. The three-stringer panels are designed to be structurally representative of the fuselage demonstrator of the Clean Sky 2 project STUNNING. A simplified model of the fuselage keel section is analysed by finite element analysis, using the virtual crack closure technique to model skin-stringer separation of the welded joint. The post-buckling and skin-stringer separation behaviour of the fuselage section is then adopted as the reference for the design of the three-stringer panels. Two panels are then tested. The test setup utilises digital image correlation to measure the deformation of the panels, and a high-speed camera to capture the final failure mode. The panels failed in post-buckling due to the separation of the middle stringer, with unstable separation growth followed by separation of the outer stringers and then stringer fracture. The numerical analysis of the panels, with geometrical imperfections included, is able to predict the structural behaviour accurately, with only minor differences in buckling shape and separation behaviour.Aerospace Structures & Computational Mechanic
The Trials and Tribulations of an Unpaid, Overworked Writer : The MTV College Stringer Program
No full textii, 72 p.The author describes her experience as the local college stringer for the Kalamazoo/Battle Creek area in Winter 1998
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