Discrete Stiffness Tailoring for Improved Buckling Performance (original) (raw)

Buckling and strength analysis of panels with discrete stiffness tailoring

Composite Structures, 2019

Continuous variation of stiffness across flat plates has been shown, theoretically, to improve buckling performance by up to 60%. However, steered fibre manufacturing methods cannot achieve the minimum radius of curvature required for improvement whilst maintaining a high deposition rate. An alternative concept, Discrete Stiffness Tailoring (DST), which varies stiffness within a ply through discrete changes of angle, is compatible with high rate deposition methods such as Advanced Tape Laying. Through the simple example of redistribution of the material in a quasi-isotropic [±45/90/0]2S laminate whilst maintaining ply percentages, DST is shown both experimentally and theoretically to improve buckling stress by at least 15% with no indication of failure in regions of discrete angle change (seams). However, the reduced tensile strength of seams obtained by virtual and experimental testing means that increased buckling performance in the principle load direction needs to be balanced against loss of transverse strength.

Laminate stiffness tailoring for improved buckling performance

Thin-walled Structures, 2021

This article presents an algorithm to tailor bending stiffness properties for double angle-ply laminates. A database of orthotropic double angle-ply laminates is derived for non-crimp fabric configurations, highlighting the severity of this manufacturing constraint compared to the use of single unidirectional layers. The significance of the new algorithm is demonstrated through the development of a range of new laminate designs, all with matched isotropic bending stiffness properties, allowing the isolated effects of axial stiffness to be studied, in this case to improve the critical length at which the transition from local to overall mode instability occurs in thin walled columns. Many of the non-crimp fabric designs can be tapered in thickness, through ply terminations, without introducing undesirable thermo-mechanical coupling behaviour. These configurations can now be matched to either bending or extensional stiffness of equivalent balanced and symmetric laminate designs, which are shown to occupy only specific, ply number dependent regions within the design space. This is demonstrated for typical aircraft components, to identify configurations with improved buckling performance.

Buckling behavior of variable-stiffness composite laminates manufactured by the tow-drop method

Composite Structures, 2016

The current investigation deals with the buckling behavior of variable-stiffness composite panels manufactured by the automated fiber placement (AFP) process. In order to minimize the occurrence of AFP-inherent defects as gaps and overlaps, the so-called tow-drop method was adopted. Compressionbuckling tests were performed on large panels containing gaps or overlaps under simply-supported boundary conditions. The specific responses of the out-of-plane deflections, which were tracked by four laser sensors focused on the axial centerline of the panels during compression loading, were explained by the measured initial geometric curvatures, which were characteristic of variable-stiffness panels. The tracking of the in-plane strains using sixteen strain gauges located strategically on the panels confirmed that the presence of gaps and overlaps does not affect the symmetry of variable-stiffness panels. Finally, it was established that the tow-drop method significantly improved the structural performance in terms of the pre-buckling stiffness, buckling load, and the failure load while keeping minimal geometric disturbances.

Variable-stiffness composite panels: Defect tolerance under in-plane tensile loading

Composites Part A: Applied Science and Manufacturing, 2014

Automated Fiber Placement is being extensively used in the production of major composite components for the aircraft industry. This technology enables the production of tow-steered panels, which have been proven to greatly improve the structural efficiency of composites by means of in-plane stiffness variation and load redistribution. However, traditional straight-fiber architectures are still preferred. One of the reasons behind this is related to the uncertainties, as a result of process-induced defects, in the mechanical performance of the laminates. This experimental work investigates the effect of the fiber angle discontinuities between different tow courses in a ply on the un-notched and open-hole tensile strength of the laminate. The influence of several manufacturing parameters are studied in detail. The results reveal that 'ply staggering' and '0% gap coverage' is an effective combination in reducing the influence of defects in these laminates.

Buckling strength improvements for Fibre Metal Laminates using thin-ply tailoring

Composite Structures

The buckling response and load carrying capacity of thin-walled open cross-section profiles made of Fiber Metal Laminates, subjected to static axial compression loading are considered. These include thin-walled Z-shape and channel cross-section profiles adopting a 3/2 FML lay-up design, made of 3 aluminium layers. The objective of the investigation is the comparison of standard thickness Fibre Reinforced Plastic layers 2 versus thin-ply material technology. Whilst thin ply designs differ only by the layer thickness, they offer an exponential increase in stacking sequence design freedoms, allowing detrimental coupling effects to be eliminated. The benefit of different hybrid materials are also considered. The comparisons involve semi-analytical and finite element methods, which are validated against experimental investigations.

Thermomechanical buckling and postbuckling responses of composite panels with skewed stiffeners

Finite Elements in Analysis and Design, 1997

The results of a detailed study of the buckling and postbuckling responses of composite panels with skewed stiffeners are presented. The panels are subjected to applied edge displacements and temperature changes. A first-order shear-deformation geometrically nonlinear shallow-shell theory that includes the effects of laminated anisotropic material behavior is used to model each section of the stiffeners and the skin. A mixed formulation is used in the analysis with the fundamental unknowns consisting of the generalized displacements and the stress resultants of the panel. The nonlinear displacements, strain energy, transverse shear stresses, transverse shear strain energy density, and their hierarchical sensitivity coefficients are evaluated. The hierarchical sensitivity coefficients measure the sensitivity of the buckling and postbuckling responses to variations in three sets of interrelated parameters; namely, the panel stiffnesses; the effective material properties of the individual layers; and the constituent material parameters (fibers, matrix, interface and interphase). Numerical results are presented for rectangular panels with open section I-stiffeners, subjected to edge shortening and uniform temperature change. The results show the effects of variations in the material properties of the skin and the stiffener on the buckling and postbuckling respon,;es of the panel, as well as on the sensitivity coefficients. © Elsevier Science B.V.

Shear Design of Slender Composite Panels: Post-Buckling Tension Field Action

5th International Conference

The potential of glass fibre reinforced polymers (GFRP) as structural shear panels, i.e. slender elements loaded primarily in in-plane shear, is far from being exploited. Current design proposals only consider the resistance up to elastic panel buckling, leading to a relatively low economic competitiveness of such structural elements. This paper reports on a comprehensive research project on the structural behaviour of Vierendeel frames made of GFRP U-profiles, braced with riveted or bolted GFRP panels of variable thickness, particularly targeting their post-buckling (i.e. post-critical) shear resistance. The contribution discusses the results of full-scale tests and evaluates the influence of the different types of connections on ultimate loads and general bearing behaviour of the test specimens. It reports on findings from extensive non-linear FE analysis and compares them to the full-scale test results as well as to predictions based on existing proposals for the post-critical shear resistance of slender steel panels, i.e. tension field approaches. It further draws conclusions on the appropriate structural design method for determining the post-critical resistance of slender GFRP panels loaded in in-plane shear and identifies future research needs.

Design and fabrication of variable stiffness composite laminates with enhanced dynamic behavior

Proceedings of the 21st European Conference on Composite Materials, 2024

Design of variable stiffness (VS) laminates, featuring curvilinear fiber paths, requires efficient methodologies to attain structural performance levels beyond those of conventional constant stiffness (CS) composites. However, optimization and fabrication of VS laminates pose significant challenges. This problem is addressed in the present study, where lamination parameters (LPs) and spectral Chebyshev approaches are incorporated to obtain a multi-step design framework for symmetric and balanced composites. Initially, master points were defined to attain optimal LP distributions, which were subsequently used to retrieve the discrete fiber angles. Then, to obtain the continuous fiber paths, streamline and equidistant fiber path planning methods were implemented. For modal testing, laminate samples with the optimal fiber paths were fabricated via the automated fiber placement technique. The results show that the designed VS panels provide up to 8% performance improvement over optimal CS laminates. The experimental fundamental frequencies were found to be around 20% lower than the simulation results on average.