Design, manufacturing, and testing of a variable stiffness composite cylinder

Variable stiffness composite structures take full advantages of composite’s design ability. An enlarged design space will make the structure’s performance more excellent. Through an optimal design of a variable stiffness cylinder, the buckling capacity of the cylinder will be increased as compared with its constant stiffness counterpart. In this paper, variable stiffness composite cylinders sustaining combined loadings are considered, and the optimization is conducted based on the multi-objective optimization method. The results indicate that variable stiffness cylinder’s loading capacity is increased significantly as compared with the constant stiffness, especially when an inhomogeneous loading is considered.

An efficient D-vine copula-based coupling uncertainty analysis for variable-stiffness composites

Effect of structural parameters on design of variable-stiffness composite cylinders made by fiber steering

For Variable Stiffness (VS) composites with steered curvilinear tow paths, the fiber orientation angle varies continuously throughout the laminate, and is not required to be straight, parallel and uniform within each ply as in conventional composite laminates. Hence, the thermal properties (conduction), as well as the structural stiffness and strength, vary as functions of location in the laminate, and the associated composite structure is often called a “variable stiffness” composite structure. The steered fibers lead not only to the alteration of mechanical load paths, but also to the alteration of thermal paths that may result in favorable temperature distributions within the laminate and improve the laminate performance. Evaluation of VS laminate performance under thermal loading is the focus of this chapter. Thermal performance evaluations require experimental and numerical analysis of VS laminates under different processing and loading conditions. One of the advantages of using composite materials in many applications is the tailoring capability of the laminate, not only during the design phase but also for manufacturing. Heat transfer through variable conduction and chemical reaction (degree of cure) occurring during manufacturing (curing) plays an important role in the final thermal and mechanical performance, and shape of composite structures. Three case studies are presented in this chapter to evaluate the thermal performance of VS laminates prior to and after manufacturing. The first case study is a numerical analysis that investigates the effect of variable conductivity within the VS laminate on the temperature and degree of cure distribution during its cure cycle. The second case study is a numerical analysis that investigates the transient thermal performance of a rectangular VS composite laminate. Variable thermal conductivity will affect the temperature profile and results are compared to a unidirectional composite. The effect of fiber steering on transient time and steady state solutions is compared to the effect of unidirectional fibers. The third case study is an experimental one that was conducted to evaluate the thermal performance of two variable stiffness panels fabricated using an Advanced Fiber Placement (AFP) Machine.

Automated fibre placement (AFP) machines are able to place simultaneously several bundles of fibres, called tows, on a surface. Using AFP machines, it is also possible to manufacture composite laminates with fibres placed in curvilinear paths. The fibre orientations and stiffness properties of these laminates are spatially varied and hence they are called variable stiffness (VS) laminates in contrast to the traditional laminates with straight fibres which are called constant stiffness (CS) laminates. Past research has shown that the structural performance of laminated composite structures can be improved by spatial tailoring of laminate stiffness. One of the widely used structural components in aerospace industry are cylindrical shells. In this thesis, a computationally efficient framework was developed for circumferential laminate stiffness tailoring of unstiffened and longitudinally stiffened cylindrical shells with arbitrary cross sections to maximise the buckling capacity with consideration of strength constraints. In addition, the AFP manufacturing constraint on the maximum curvature of curvilinear fibre paths was considered. This constraint was applied to avoid wrinkling of the fibres placed inside the turn of a curved tow. The aforementioned framework utilised computationally efficient analysis and optimisation tools.

The design space for high-performance lightweight composite structures has grown considerably since the advent of the variable stiffness concept. In fact, variable stiffness composites have been found to improve buckling performance and dynamic stability, and to tune the structure’s dynamic response by tailoring structural stiffness. Thus, in order to exploit this wider design space, efficient linear analysis tools have an important role in preliminary design of variable stiffness structures, enabling designers to find more effective solutions when considering prestressed dynamically excited aerospace components. Considering this, a multi-domain Ritz method for eigenfrequency, transient and dynamic instability analysis of prestressed variable stiffness laminated doubly-curved shell structures is presented. Working within the first-order shear deformation theory, Sanders–Koiter shell kinematics allow general orthogonal surfaces to be modelled without further assumptions on the shallowness or on the thinness of the structure. The efficiency of the proposed Ritz method is granted by using Legendre orthogonal polynomials as displacement trial functions, while flexibility in modelling and design is given by penalty techniques that allow stiffened variable angle tow shell structures to be modelled as an assembly of shell-like domains. The proposed approach is verified by comparison with published benchmark results and finite element solutions. Original solutions are presented for a prestressed stiffened variable angle tow shell structure, which show great accuracy with an order of magnitude fewer variables when compared to standard finite element procedures, proving the reliability and efficiency of the present method in dealing with the dynamic analysis of multi-part aerospace structures.

The functional advantages of tailored stiffness, often seen in nature, are also utilized in composite structures. Advancements in the multiaxis tow placement and automated fiber placement (AFP) machines led to the development of variable angle tow (VAT) composites, also referred further as variable stiffness composites (VSC). These composites are shown to effectively enhance the stress distribution and buckling load capacity of structures with greater flexibility on the design space. This review systematically presents the status of recent research on the topic of VSCs. Various manufacturing techniques of VSC are discussed; constraints and the defects associated with the manufacturing processes are enlisted. The review highlights the optimization studies based on the fiber profile and macro-scale stiffness invariants. Several studies existing in the domain of buckling, vibration, and aeroelastic tailoring of angle tow composites are summarized to connect the important aspects of analysis and present a holistic approach for future studies in this area.

: This report describes geometric design tools which encompass the overall process flow for fiber placed and fiber steered structures, to allow parallel considerations for manufacturability and mechanical performance. Several methods for modeling individual tows in a fiber placed part are defined and verified. Analytical methodologies for fiber steering are demonstrated on four test models and three representative aircraft components. Weight savings of up to 30% are attainable for singularly loaded examples. Rangewise continuous layer-angle and layer-thickness contours are demonstrated, with layer thickness and fiber orientation being load-path sensitive. Weight savings on the order of 7% are demonstrated on more realistic examples. Mechanical testing illustrates that fiber steered components follow predictable trends. In particular, data demonstrates no statistically significant variation in compression stiffness or compression strength to steering radii of 46cm. Destructive and nondestructive evaluation experiments reveal higher density occurrences of tow and course edge features, such as overlaps, twists, gaps and fiber waviness.

A design algorithm to model fibre paths for manufacturing of structurally optimised composite laminates

Ply-drop design of non-conventional laminated composites using Bayesian optimization

Automated fiber placement (AFP) is an advanced manufacturing technology for large‐scale carbon fiber reinforced plastics (CFRP) composite parts. During AFP processes, unidirectional CFRP prepreg tapes are laid up on the mold to become laminates by a robotic arm under heating and pressure conditions. However, the efficient monitoring of lay‐up defects during AFP, especially the out‐of‐plane defects including wrinkles, bridging, gaps, and overlaps, needs more advanced solutions, as manual inspection of the lay‐up defects still accounts for a large proportion of real manufacturing processes at present. To tackle this issue, an innovative method to realize efficient and accurate detection of AFP lay‐up defects was developed in this work. First, a bespoke AFP platform containing a 5‐degree‐of‐freedom moving platform, air cylinder, compaction roller, heating gun, and structural‐light camera was constructed for AFP experiments. A defect detection algorithm based on PointNet++, a convolutional neural network (CNN) for 3D point cloud segmentation, was then created. PointNet++ was trained by the dataset containing 3D point clouds of intentionally laid‐up defects, which were created on the AFP platform and captured by the structural‐light camera. Compared to the existing AFP defect inspection methods, such as laser profilometer and thermography, this method has the advantages of low cost and convenience to operate. The completed method was experimentally validated to be capable of precisely tracking the locations of out‐of‐plane AFP defects in less than 1 second. The detection accuracy of the method can achieve above 72% in Intersection over Union (IoU) for the out‐of‐plane defects. Highlights A lay‐up defect detection method for AFP by structural light scanning. 3D point cloud segmentation by PointNet++ was used for defect segmentation. A bespoke AFP platform was constructed for AFP experiments. The method can track the locations of AFP defects in less than 1 second. The method has the advantages of low cost and convenience to operate.

Failure analysis of variable stiffness composite plates using continuum damage mechanics models

Design of variable stiffness composites for maximum fundamental frequency considering manufacturing constraints of tow steering

Multi-level intelligent design of variable angle tow laminates via image-driven method

Variable-stiffness shells are thin composite structures in which the reinforcement direction is a function of its surface co-ordinates. This paper presents a numerical investigation into the buckling and post-buckling of two variable-stiffness cylinders under axial compression. Both shell walls are made from unidirectional carbon fiber slit tapes that are steered to give them a piecewise-continuous fiber-angle variation around the circumference. Dynamic analyses of the complete loading and unloading cycles are computed using a time-integrated finite element model (Abaqus). The numerical results generated herein are compared with test data and are found to be in good agreement, in terms of axial force versus end-shortening and global displacement fields. The analyses provide significant new insight into the mechanisms underpinning collapse behavior of the shells. For example, the development of the initial nonlinear buckle, its dynamic snap-through, and the formation of a post-buckled configuration are clearly visible. One effect elucidated by this investigation is the symmetry-breaking mechanism of the circumferential stiffness variation. In contrast to a constant-stiffness cylinder, in which the total strain energy is invariant to the translation of a dimple of fixed dimensions, the present structures exhibit a single dominant post-buckling mode that are associated with the formation of ‘trapped’ surface dimples. In one case, this dominant mode is found to be stable over a significant amount of further end shortening.