1,721,019 research outputs found
Analytical Solution for the Displacement Field in Composite Single-Lap Shear Joints With Zero-Thickness Interfaces
No full textAn analytical model for predicting the displacement field in composite single-lap shear joints with zero-thickness interfaces is developed and verified with numerical simulations. This two-dimensional model imposes no restrictions on the composite layup or the dimensions of the adherends. It closely aligns with the displacement field predicted by numerical simulations, provided that the assumptions of small deformations and plane strain are satisfied. Small discrepancies are observed near the overlap region because the stress-free boundary condition at the overlap ends is not satisfied exactly. Consequently, the joint stiffness is slightly overestimated compared to the numerical simulations. Nonetheless, the analytical model can serve as a useful tool for providing input for more detailed analyses of single-lap shear joints, for example for determining the interlaminar stress field at the interface between the two adherends
Sensitivity analysis for buckling characterisation using the vibration correlation technique
No full textThe Vibration Correlation Technique (VCT) is a non-destructive method to predict buckling loads for imperfection-sensitive structures. While successfully used to validate numerical models and predict experimental buckling loads, recommendations for defining the VCT experiment are scarce. Here, its sensitivity towards the number of load steps and the maximum load level measured is studied, and an uncertainty quantification of the measured frequency affecting the VCT prediction is performed First, a series of finite element (FE) models representing nominally identical cylinders, and validated by buckling experiments, are used to perform a sensitivity study. When no frequency deviations are introduced in the FE results, a positive correlation between the VCT predictions and the maximum load used for measurements is found, the number of load steps used being only relevant in reducing the errors. Introducing frequency deviations deterred the predictions correlation with the maximum load, while using more load steps reduced this influence. Second, a sensitivity study based on experimental data confirmed most of the trends previously observed using the FE results, the exception being a poor prediction sensitivity as a function of the maximum load, owing to several cylinders for which the VCT method gave predictions that progressively decreased with increasing the load.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerospace Structures & Computational Mechanic
Design and manufacture of conical shell structures using pregreg laminates
No full textThe design and manufacture of unstiffened composite conical structures is very challenging, as the variation of the fiber orientations, lay-up and the geometry of the ply pieces have a significant influence on the thickness imperfections and ply angle deviations imprinted to the final part. This paper deals with the manufacture of laminated composite cones through the prepeg/autoclave process. The cones are designed to undergo repetitive buckling tests without accumulating permanent damage. The aim is to define a process that allows the control of fiber angle deviations and the removal of thickness imperfections generated from gaps and overlaps between ply pieces. Ultrasonic scan measurements are used to proof the effectiveness of the proposed method
Numerical characterization of imperfection sensitive composite structures
No full textThe need to take the imperfection sensitivity of thin-walled unstiffened cylinders into account during the early design phases motivated the development of the NASA SP-8007 guideline, which brings an empirically based lower-bound curve that gives the knock-down factor (KDF) as a function of the cylinder’s radius and thickness. This guideline is still used also for composites, using correction. The conservativeness of these guidelines has been proved both stochastically by Arbocz, Starnes Jr., Hilburger, et al.; and deterministically by Geier, Hühne, Degenhardt, et al. A deterministic method: single perturbation load approach; developed by Hühne, showed that after some radial load (called P1) the imperfection sensitivity decreases, i.e. the buckling load remains nearly constant even increasing the radial load. Such physical benefit is not taken into account by the NASA SP-8007 guideline. This study, in the context of DESICOS, investigates the buckling mechanism of composite benchmark cylinder, developed by Zimmermann, being extreme-case laminates with high and low imperfection sensitivity. The explanation why the buckling load is constant after P1 is given and a physical meaning for the P1 value is proposed. These findings pave the way for the development of semi-analytical approaches for calculating P1. By applying the P1 value in the analysis, one can obtain less conservative KDFs which have shown to be closer to test results, allowing cost savings for the space industry
Exploring the constancy of the global buckling load after a critical geometric imperfection level in thin-walled cylindrical shells for less conservative knock-down factors
No full textSome of the knock-down factors applied in design of rocket launcher structures are based on design recommendations which rely on lower-bound curves from experimental data. The best known example is the NASA guideline SP 8007, published in 1965 and revised in 1968, which is applied for cylindrical structures in the space industry. This guideline is based on test data, computational methods and resources from the 1930–1960's. At that time the application of less empirical methods for the design of actual cylindrical shells could not count with the current computational power, and the available methods led to quite large discrepancies between experiments and test observations. Significant improvement on the available analyses approaches and manufacturing techniques since 1960's have not been taken into account in design processes using the NASA SP-8007, and many authors have recognized that for the current standards this guideline is leading to conservative structures. Another aspect for attention regarding application of the NASA SP-8007 for composite shells is that it does not consider the laminate stacking sequence. Moreover, physical observations regarding how does the imperfection sensitivity of unstiffened cylindrical shells change with the presence of an induced geometric imperfection have also suggested that the current applied design rules are too conservative. This conservativeness is confirmed by many tests carried out recently. This study presents an overview of the problem and a detailed description of the physical observations regarding the buckling mechanism of the thin shells under consideration. It is discussed how these observations can be used for less conservative, laminate dependent, knock-down factors accounting for geometric imperfections. The single perturbation load approach is studied in detail and a physically based definition for the minimum perturbation load (P1) is given, paving the way for the development of semi-analytical methods to calculate this minimum perturbation load
Evaluation of non-linear buckling loads of geometrically imperfect composite cylinders and cones with the Ritz method
No full textA semi-analytical model to predict the non-linear behavior of unstiffened cylinders and cones considering initial geometric imperfections and various loads and boundary conditions is presented. The formulation is developed using the Classical Laminated Plate Theory (CLFT) and Donnell's equations, solving for the complete displacement field. The non-linear static problem is solved using a modified Newton-Raphson algorithm with line-search. A numerical integration scheme for the non-linear matrices is proposed and details regarding the implementation of the proposed method are given. Two methods to include measured imperfections into the analyses are presented and for one method the effect of using different approximation levels for the imperfection field on the non-linear response is investigated, and a minimum approximation accuracy that should be used is determined. The semi-analytical results are verified using finite elements and previous models from the literature. The implemented routines are distributed on-line and are based on a matrix notation simply applicable to other problems. (C) 2014 Elsevier Ltd. All rights reserved.European Community's Seventh Framework Programme (FP7) [n282522
The single perturbation load approach applied to imperfection sensitive conical composite structures
No full textThe importance of taking into account geometric imperfections for cylindrical and conical thin-walled
structures prone to buckling had been already recognized by the
fi
rst authors dealing with new
formulations. Nowadays, the analysts still use empirically based lower-bound methods such as the NASA
SP-8007 guideline to calculate the required knock-down factors (KDFs), which does include important
mechanical properties of laminated composite materials, such as the stacking sequence. New design
approaches that allow taking full advantage of composite materials are required.
The single perturbation load approach (SPLA), a new deterministic approach
fi
rst proposed by
Hühne, will be investigated with unstiffened composite conical structures varying the geometry, lamina
and layup. The SPLA's capability for predicting KDF is compared with the NASA approach. The SPLA was
applied to the geometrically perfect structures and to the structure with geometric imperfections of two
types, mid-surface imperfections and thickness imperfections. The study contributes to the European
Union (EU) project DESICOS, whose aim is to develop less conservative design guidelines for
imperfection sensitive thin-walled structures
Future structural stability design for composite space and airframe structures
No full textSpace and aircraft industry demands for reduced development and operating costs. Structural weight reduction by exploitation of structural reserves in composite
space and aerospace structures contributes to this aim, however, it requires accurate and experimentally validated stability analysis. Currently, the potential of
composite light weight structures, which are prone to buckling, is not fully exploited as appropriate guidelines in the field of aerospace and space applications do
not exist. This paper deals with the state-of-the-art advances and challenges related to coupled stability analysis of composite structures whichshow very complex
stability behaviour. Two types of thin-walled light weight structures endangered by buckling will be considered; imperfection tolerant and imperfection sensitive
structures. For both groups improved design guidelines for composites structures are still under development. This paper gives a short state-of-the-art and presents
proposals for future design guidelines
Geometric imperfections and lower-bound methods used to calculate knock-down factors for composite cylindrical shells
No full textThe important role of geometric imperfections on the decrease of the buckling load for thin-walled cylinders had been recognized already by the first authors investigating the theoretical approaches on this topic. However, there are currently no closed-form solutions to take imperfections into account already during the early design phases, forcing the analysts to use lower-bound methods to calculate the required knock-down factors (KDF). Lower-bound methods such as the empirical NASA SP-8007 guideline are commonly used in the aerospace and space industries, while the approaches based on the Reduced Stiffness Method (RSM) have been used mostly in the civil engineering field. Since 1970s a considerable number of experimental and numerical investigations have been conducted to develop new stochastic and deterministic methods for calculating less conservative KDFs. Among the deterministic approaches, the single perturbation load approach (SPLA), proposed by Hühne, will be further investigated for axially compressed fiber composite cylindrical shells and compared with four other methods commonly used to create geometric imperfections: linear buckling mode-shaped, geometric dimples, axisymmetric imperfections and measured geometric imperfections from test articles. The finite element method using static analysis with artificial damping is used to simulate the displacement controlled compression tests up to the post-buckled range of loading. The implementation of each method is explained in details and the different KDFs obtained are compared. The study is part of the European Union (EU) project DESICOS, whose aim is to combine stochastic and deterministic approaches to develop less conservative guidelines for the design of imperfection sensitive structures
A semi-analytical approach for linear and non-linear analysis of unstiffened laminated composite cylinders and cones under axial, torsion and pressure loads
No full textA semi-abalytical approach for linear and non-linear analysis of unstiffened laminated compsite cylinder
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