Buckling and strength analysis of panels with discrete stiffness tailoring

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 strengthThis work was supported by the UK EPSRC ADAPT research project (grant number EP/N024508/1) which is gratefully acknowledged. Richard Butler is supported by a Royal Academy of Engineering and GKN Aerospace Research Chair. Lucie Culliford’s PhD studentship is 50% funded by GKN Aerospace

High fidelity analysis to predict failure in T-joints

Composite T-joints and similar skin-stiffener joints have a central role in aerospace structures. However, their accurate numerical analysis is still a great challenge due to their complex geometry and stress state during loading. In this work, an experimentally validated high-fidelity finite element model is developed to simulate the failure of a T-joint subjected to tensile loading. For the first time in literature, the model accounts for the shape of the manufactured filler, ply thickness variability in the laminate, stress gradient across the ply thickness, thermal stresses and in-situ mechanical properties. A new phenomenon called “filler effective ply thickness” is introduced to address the increased strength of the filler when embodied in a laminate. With this method, the model predicted the location and the failure initiation load within 5% accuracy of the experimental specimens, whereas conventional approaches using unidirectional strength significantly underpredicted the strength of the joint

Optimum fibre-steering of composite plates for buckling and manufacturability

This paper reveals the potential weight saving in creating lightweight composite panels using the novel Continous Tow Shearing technique. The method provides a unique capability to simultaneously manipulate fibre angles and local thicknesses, whilst achieving defect-free, high curvature fibre paths. Considering the buckling of a typical aircraft wing skin, the nonlinear optimisation problem presented, shows theoretical designs with significant weights savings if compared to panels made from conventional straight fibre angles (38%) or steered using Automated Fibre Placement techniques (20%). The implications due to constraints on panelstrength, damage tolerance and manufacturing costs are then considered

Novel filler materials for composite out-of-plane joints

In the manufacture of the out-of-plane joints of composite stiffened panels, such as the connection between skin and T, I, omega shaped stiffeners, a filler material is needed to fill the void between the flanges, web and skin. The most common filler is a rolled unidirectional prepreg tape (“noodle”), which is not only expensive to manufacture, but also has low strength that can lead to premature failure of the loaded joint. In this work, eight novel filler concepts are introduced and experimentally validated against the baseline noodle via T-joint tensile tests. Polyamide nonwoven interleaved joints increase the damage tolerance of the structure and nonwoven nanofibres increase the failure initiation load. 3D printed fillers have lower strength but demonstrate the possibility of thermoplastic-thermoset hybrid structures. Fillers made of chopped prepreg match the strength of the baseline noodle and can serve as a low cost replacement. Another low cost, resin infused braided concept has lower strength, but its counterpart using multiple individual braids has the same strength as the unidirectional noodle. Moreover, the latter concept shows that different resin systems can be cured together without causing a knockdown in strength, and can serve as a basis for a range of novel applications

Novel filler materials for composite out-of-plane joints

In the manufacture of the out-of-plane joints of composite stiffened panels, such as the connection between skin and T, I, omega shaped stiffeners, a filler material is needed to fill the void between the flanges, web and skin. The most common filler is a rolled unidirectional prepreg tape (“noodle”), which is not only expensive to manufacture, but also has low strength that can lead to premature failure of the loaded joint. In this work, eight novel filler concepts are introduced and experimentally validated against the baseline noodle via T-joint tensile tests. Polyamide nonwoven interleaved joints increase the damage tolerance of the structure and nonwoven nanofibres increase the failure initiation load. 3D printed fillers have lower strength but demonstrate the possibility of thermoplastic-thermoset hybrid structures. Fillers made of chopped prepreg match the strength of the baseline noodle and can serve as a low cost replacement. Another low cost, resin infused braided concept has lower strength, but its counterpart using multiple individual braids has the same strength as the unidirectional noodle. Moreover, the latter concept shows that different resin systems can be cured together without causing a knockdown in strength, and can serve as a basis for a range of novel applications

Compressive strength of delaminated aerospace composites

An efficient analytical model is described which predicts the value of compressive strain below which buckle-driven propagation of delaminations in aerospace composites will not occur. An extension of this efficient strip model which accounts for propagation transverse to the direction of applied compression is derived. In order to provide validation for the strip model a number of laminates were artificially delaminated producing a range of thin anisotropic sub-laminates made up of 0°, ±45° and 90° plies that displayed varied buckling and delamination propagation phenomena. These laminates were subsequently subject to experimental compression testing and nonlinear finite element analysis (FEA) using cohesive elements. Comparison of strip model results with those from experiments indicates that the model can conservatively predict the strain at which propagation occurs to within 10 per cent of experimental values provided (i) the thin-film assumption made in the modelling methodology holds and (ii) full elastic coupling effects do not play a significant role in the post-buckling of the sub-laminate. With such provision, the model was more accurate and produced fewer non-conservative results than FEA. The accuracy and efficiency of the model make it well suited to application in optimum ply-stacking algorithms to maximize laminate strength.</jats:p

Buckling and strength analysis of panels with discrete stiffness tailoring

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. </p

Minimum-mass optimisation for high-rate manufacture of damage tolerant and unbuckled composite components

A significant increase in the rate of composite manufacture is needed to meet demand for short-range commercial aircraft. The enabling automated manufacturing processes can, however, induce undesirable process features such as wrinkles. Additionally, the potential for Barely Visible Impact Damage has resulted in widespread use of overly-conservative strain allowables which has led to overweight aircraft structures. Two new constraints are presented which enable formability and damage tolerance to be incorporated into a two-stage minimum-mass optimisation framework for performance and manufacturability. An efficient, approximate method is presented for determining a conservative lower bound on the strain required to propagate a single, circular delamination, given a general through-thickness position and an upper bound on delamination size. A Compatibility Index is used to predict the propensity for wrinkles to occur during a forming manufacturing process. Optimised stacking sequences for two benchmark design problems; a flat plate and blade-stiffened panel, are obtained subject to minimum formability, damage tolerance and buckling constraints alongside common industry design rules. The damage tolerance and formability constraints are met for a diverse set of design requirements, without increasing mass or reducing buckling load, thereby demonstrating that components may be optimised for manufacture using high-rate processes without detriment to performance.</p

A plate model for compressive strength prediction of delaminated composites

Damage tolerance is of critical importance to laminated composite structures. In this paper, we present a new semi-analytical method for predicting the strain at which delamination propagation will initiate following sublaminate buckling. The method uses a numerical strip model to determine the thin-film buckling strain of an anisotropic sub-laminate created by delamination, before evaluating the strain energy release rate for delamination propagation. The formulation assumes that all energy is available for propagation in a peeling mode (Mode I); avoiding an approximate mixed-mode criterion. Results are compared with twelve experimentally obtained propagations strains, covering a variety of laminates each containing a circular PTFE delamination. Comparison shows agreement to within 12% for balanced sublaminate tests in which delamination propagation occurred before intra-ply cracking. The method can be used to significantly improve the damage tolerance of laminates, opening up new opportunities for structural efficiency using elastic tailoring, non-standard ply angles and material optimisation.</p