Ply Drops Research Articles

A machine learning assisted preliminary design methodology for bolted composite joints in large structures

Abstract Damage initiation hotspots around features, such as bolts and ply drops, must be investigated during the preliminary design phase of large composite structures, such as composite airframes. A global-local modelling approach is commonly employed to perform this investigation, whereby a global low-fidelity model is used to drive high-fidelity local models around the features of interest. However, this methodology is slow, repetitive and expert-dependent. In this investigation, we address these issues by applying machine learning techniques to this global-local modelling framework and demonstrate the time-saving benefit when predicting damage initiation of bolted composite joints. Feature engineering of model inputs and outputs, and appropriate customisation of machine learning methods enables damage initiation prediction. Special consideration is given to the boundary conditions that must be varied to simulate the response of the bolted composite joints. Results show over three orders of magnitude time-saving benefit and satisfactory accuracy of the proposed methodology. This indicates its potential to be developed further into a rapid design and optimisation tool.

Automatic modeling and optimization of tapered laminates with ply drops

Ply-drop (PD) is the termination of specific plies for laminated composite structures to obtain continuous thickness changes. It brings flexibility to the design of tapered composite laminates. However, as a structural defect, ply drops could have an impact on performance. Considering the impact of ply drop during stacking sequence design can provide more accurate performance analysis, but this will bring challenges in modeling and optimization. To consider the PD impact and achieve convenience in optimization, this paper proposes a high-fidelity finite element automatic modelling method of tapered laminates and corresponding optimization framework. By parameterizing the PD information and defining the basic elements and nodes of start stacking surface of the structure, the entire finite element model is layer-wisely constructed and controllable. Subsequently, based on the genetic algorithm framework, a repair strategy and its genetic operations are proposed to ensure that the design variables satisfy the ply-drop design guidelines. And a detailed optimization process is provided. Finally, the strength and deflection performance optimization problem of a tapered laminate with PD from 28 layers to 16 layers under three-point bending test is introduced for illustration of the proposed automatic modeling and optimization method. Comparisons between simulation results and experimental data of the obtained optimization solution verify the effectiveness of the proposed modeling and optimization method.

Investigation of a secondary bonded pultrusion composite laminate containing a ply-drop, Part 2: A novel two-dimensional hybrid fracture discrete element for producing debond failure

This is the second of two papers in which a novel numerical method to predict the debond failure of a secondary bonded pultrusion laminate is presented. In Part 1 of this study, experimental work was described which is used here in the development, calibration and validation of the numerical model. The structure investigated in this work may be represented by a bonded composite laminate with external ply-drops (PDs).In this part of the study, a fully coupled mixed mode cohesive zone model is developed which relies upon a traction-separation relation and the virtual crack closure technique to obtain the mode mixity. These are used to develop a hybrid fracture discrete element (HFDE). Fracture toughness tests, as well as tests on PD specimens are used to calibrate the model. Based on these tests, run-arrest fracture criteria are defined. The model is validated by comparing test results for two other PD specimen types to those obtained with the HFDE and finite element analyses.

Determination of delamination-related material properties and sensitivities in a ply drop composite using a semi-numerical approach

The aims of this paper are to investigate damage initiation and propagation in the vicinity of a ply drop focusing on delamination, to extract delamination-related material properties without conducting classical single and mixed mode fracture tests, and to analyze the sensitivities of delamination evolution with respect to these material properties. Quasi-static tensile tests were conducted on the coupon level for a UD composite with a ply drop. A combined numerical/experimental approach was followed to identify the material properties associated with delamination evolution. For that purpose, a finite element model was created using the cohesive zone model. The delamination-related properties were characterized by fitting the numerical model to the experiments by calibrating the cohesive zone parameters such that the global coupon response and the delamination extent for different load levels were in good agreement. Based on the identified parameter set, a sensitivity analysis was carried out to raise awareness of the influence of material properties on damage evolution. Damage around the ply drop was governed by mode II delamination. Several sensitivities were found to be non-linear with respect to a variation of input parameters. The methodology can be used in future work focusing on different ply drop configurations, and the results so far can serve as a database for benchmark simulations of delamination evolution in the vicinity of ply drops.

Design of an element test specimen for fatigue delamination growth of a thick laminate

Following the observation of multiple delaminations growing from a tunnelling crack in the spar cap of a wind turbine rotor blade subjected to cyclic loading, an element test specimen is proposed. Design of the element test specimen to study fatigue delamination of a thick laminate is presented. Complexities inherent to a full structure e.g., curvatures, ply drops, manufacturing defects, etc. are removed to study the fatigue delamination damage mechanism. The element test specimen is made of unidirectional layers with two embedded artificial delamination cracks connected by a tunnelling crack. In this paper, analytical and numerical (2D) models are used to design the element test specimen.

Design and manufacture of a Type V composite pressure vessel using automated fibre placement

Automated fibre placement (AFP) can improve composite pressure vessel design by enabling selective reinforcement and allowing a wider range of fibre orientations than the traditional filament winding process. This paper presents the development of an AFP Type V composite pressure vessel with a design working pressure of 4.22 MPa made using a novel collapsible mandrel. The design process is presented along with a thickness control strategy for the AFP parts using ply drops. The tank was manufactured, and the thickness was validated using 3D scanning. The masses of the AFP parts were also measured to within 2.23% of the predicted values. The finished tank was hydrostatically pressure tested to understand the leakage behaviour. Defects and difficulties encountered in this prototype are discussed and opportunities for future work and improvements have been identified.

Design of Artificial Neural Networks for Damage Estimation of Composite Laminates: Application to Delamination Failures in Ply Drops

This work presents a data-driven approach based on Artificial Neural Networks (ANN), that benefits from parametric non-linear finite element analyses, in order to provide a “cheaper” numerical alternative to the more expensive experimental testing of advanced composite laminates. The chosen subject of study is the damage evolution and associated delamination failures of ply drops. As such, physical-based modeling of these phenomena are used as training data for obtaining the most suitable ANN capable of estimating the damage evolution using only the prescribed initial conditions (i.e., laminate geometry, material orientation, applied loading). Additionally, this work aims at providing first-hand experience into the procedure that leads to obtaining such ANN. In order to do so, we detail each of the required steps, such as choosing a network architecture, defining a custom loss function, as well as deciding on the better learning parameters. Each stage on this process is guided by intuitive statistical analyses and simple criteria, such as selecting the “simpler” model over whenever a more complex one “improves” on the performance but only by a narrow margin. Indeed, for every step, multiple numerical experiences are provided so that the reader can also get a deeper understanding on the inner-workings of these ANN models. With this goal in mind, we employ dimensionality reduction techniques (PCA and t-SNE) to also propose a geometrical visualization of the nonlinear transformations performed by the ANN. Finally, additional tests regarding the network ability to generalize to unseen data showed that the optimal well-trained ANN is accurate and robust enough for near real-time predictions of the various damage evolution patterns, and outperforms other data-driven methods under comparison.

Delamination suppression in tapered unidirectional laminates with multiple ply drops using a tape scarfing technique

Composite laminate thickness tapering is essential for weight efficient structures. This is achieved by terminating specific plies, but these can in turn act as sites for delamination initiation. One area of importance is the spacing between adjacent ply drops, which can have a significant impact on the delamination stress. In this work, a novel tape scarfing method that applies a tapered profile to dropped ply ends was used on unidirectional tapered laminate specimens with multiple ply drops. The effect of ply drop spacing on the delamination stress of both conventional and scarfed plies in tension was studied experimentally. The results showed that the scarfed ply drop configurations can completely suppress delamination in certain configurations. The scarfed ply drops also retained higher performance with smaller ply drop spacings. This indicates that in using scarfed plies, conventional laminate design rules could be relaxed and the weight of the structure could be further reduced. The underlying failure mechanisms were also investigated using both analytical and numerical methods. A simple stress-based formula for estimating the delamination stress of scarfed plies was introduced and shown to be consistent with both experimental and numerical results, which could be used (and further developed) to make tapered laminate design easier.

Fatigue crack growth rate at material and geometry transitions in glass-epoxy composites

An element test specimen with ply drops, intended to be representative of a composite structure of varying thickness such as the main laminate in a wind turbine rotor blade structure, is used to investigate the fatigue damage initiating from a ply drop under cyclic tension–tension loading. The focus is to measure the growth rate of delamination cracks propagating from a thin towards a thicker section. Several delaminations initiate from tunneling cracks - cracks between the ply drops and resin reach areas - after very few load cycles. All except one delamination crack propagate for a number of cycles but eventually stop growing. The only delamination crack that continued to grow has the characteristic that its growth rate increases as it propagates to thicker sections of the element specimen. The experimental findings are supported by finite element results.

Effect of saw-tooth ply drops on the mechanical performance of tapered composite laminates

Automated Fibre Placement (AFP) is a manufacturing technique to produce large, high quality composite parts, where preimpregnated carbon fibre tapes are laid side-by-side to generate the composite preform. Thickness changes within a component are realised through internal ply terminations, with the tapes being cut perpendicular to the fibre direction. In plies laid up at an angle to the taper direction, the AFP tape cuts create saw-tooth shaped ply drop tips. This increases the size of resin rich zones and enlarges the region where stress concentrations and cracks can develop, which amplifies the risk of failure. This study investigates the effect of simulated AFP saw-tooth ply drop tips, created by hand layup, on the mechanical properties and failure behaviour of carbon fibre/epoxy composites and compares its results to reference broad goods layups. The stiffness was found to be unaffected and the strength was reduced by ~ 10%. The failure mode was governed by delamination for the saw-tooth ply drop tips, whereas the reference specimens failed by rupture of the unidirectional fibres. A finite element modelling technique was used to select the layup to be tested. The models were then refined in the light of experimental results, to accurately predict the failure and explain the failure mode transition.

Porosity detection and localization during composite cure inside an autoclave using ultrasonic inspection

Composite materials offer unique benefits in aerospace applications such as increased strength-to-weight ratio and improved fatigue properties. They are increasingly being used in major commercial aircraft programs. However, current processing methods can lead to defects in composite parts, which must be detected using post-manufacturing inspection methods. Porosity (i.e., pores, voids) is a critical defect from the cure process that is detrimental to performance of the composite part. It is therefore necessary to understand and eliminate the formation of porosity defects. At NASA Langley Research Center, an in-situ cure monitoring system was developed to detect porosity defects as they form in real-time inside an autoclave. A capability to directly detect and localize porosity within a composite during cure did not exist before.This study is focused on an elevated-temperature ultrasonic inspection system to detect porosity defects in composites during autoclave cure. The ultrasonic inspection system operated inside an autoclave within an enclosure cooled by intermittent liquid nitrogen (LN2) injections. A high-temperature, 2.25 MHz, transducer transmitted ultrasonic waves through the tool plate and into the composite part and measured the amplitude and time of flight of the reflected waves with a 1 mm × 1 mm step size/areal resolution. Porosity was observed via the ultrasonic reflections, which experienced increased attenuation in regions of high porosity. Distinct regions of increased porosity were present due to uneven pressure across the panel, which was driven by an intentional misfit between the flat caul plate and the tapered composite panel with ply drops. The results were validated by post-cure ultrasonic inspection and micrographs. The in-situ inspection system was able to successfully provide porosity detection and localization in the tapered ply-drop panel. The successful results indicate the promise of this system for future implementation in the manufacturing of composite structures for aerospace applications.

Morphology of ply drops in thermoplastic composite materials manufactured using laser-assisted tape placement

Mechanical properties of composite laminates can be tuned by either tailoring ply orientations or by using ply drops to change thickness profiles. The latter cause resin-rich pockets to form in thermoset composites that are autoclave cured after lay-down. This paper focuses on the morphology of ply drop regions with carbon fibre thermoplastic composites manufactured using in-situ consolidation with laser-assisted tape placement. Our work shows that when laying a 0° ply (parallel to the ply drop) over a ply drop then the ply readily conforms to the shape of the ply drop, eliminating voids. However, when a 90° ply covers a ply drop, the processing direction affects behaviour. When the ply ascends a ply drop, a void is created. On the other hand, when the ply descends a ply drop, the substrate changes its shape due to combined laser heat and roller pressure, leading to significantly smaller voids.

Effect of Ply Drop in Aerospace Composite Structures

In most of the aerospace laminated composite structures, thickness variation is achieved by introducing the ply drops at the appropriate locations. Ply drop means the resin rich regions created due to abrupt ending of individual plies within the set of plies. This research is focused on understanding and quantifying the effect of these ply drop regions on the mechanical performance of the aerospace composite structures. This is achieved here by designing the appropriate coupons (with and without ply drops) and analyzing them using finite element analysis. Some typical designs of coupons were manufactured using aerospace grade carbon composite materials, and then tested under four-point bend, cantilever and short beam shear tests to check and validate the effect that was seen in the analysis. It is concluded here that allowable failure strains are different for with and without ply drop cases by a significant amount.

Bilevel Programming Weight Minimization of Composite Flying-Wing Aircraft with Curvilinear Spars and Ribs

This paper studies weight minimization of a composite flying-wing aircraft using bioinspired arbitrarily shaped spars and ribs, known as SpaRibs, for the internal structural layout design. A generalized mathematical model is developed to parameterize the shape of SpaRibs using a four-node nonuniform rational basis spline curve. Additionally, for reducing weight, composite structures and lift redistribution using multiple control surfaces are considered. A stacking sequence table is used to parameterize the wing laminate configuration with ply drops for flexible wing design. For the optimization problem including topology as well as discrete and size design variables, a bilevel programming optimization is employed by using a previously developed parallel particle swarm optimization and a gradient-based optimization, respectively, for the upper- and lower-level optimization problems. The upper-level optimization problem is employed to satisfy such design requirements as the flutter constraints for achieving a specific body freedom flutter mode. The lower-level optimization is used to optimize the control surface rotations to minimize the wing root bending moment, which is considered to be a surrogate for wing weight. Different internal structural layouts and flutter constraints are studied for comparison. Optimization studies show that using SpaRibs for aircraft wing box internal structural layout design can reduce the wing weight by 21.8% as compared to the base model and by 7.3% as compared to the wing box with traditional straight spars and ribs. The additional specific flutter result constraints lead to a 4.2% increase in the wing weight for aircraft design with SpaRibs as compared to the one obtained only with the lower bound on the flutter speed.

Optimal design, manufacturing and testing of non-conventional laminates

Composite materials are finding increasing application, for example in commercial aircraft. Traditionally fiber angles are restricted to 0°,±45° and 90°. The current work exploits the possibility of using multiple ’non-conventional’ laminates where either fiber steering (‘variable stiffness’), ply drops (‘variable thickness’), or a combination of both is used. This leads to varying mechanical properties which means the load is being redistributed, increasing the overall buckling load. A flat panel of 400×600 mm loaded in uni-axial compression is optimized in the current work. As a benchmark a conventional laminate is used. The non-conventional laminates are 15% lighter to emphasize the possible weight savings. Only using variable stiffness or variable thickness is experimentally shown to not be sufficient to match the buckling load of the benchmark panel. However, using a combination of both, a 10% increase in the buckling load was found for a panel that is 15% lighter. This highlights the potential of non-conventional laminates.

A level-set-based strategy for thickness optimization of blended composite structures

An approach is presented for the thickness optimization of stiffened composite skins, which guarantees the continuity (blending) of plies over all individual panels. To fulfill design guidelines with respect to symmetry, covering ply, disorientation, percentage rule, balance, and contiguity of the layup, first a stacking sequence table is generated. Next, a level-set gradient-based method is introduced for the global optimization of the location of ply drops. The method aims at turning the discrete optimization associated with the integer number of plies into a continuous problem. It gives the optimum thickness distribution over the structure in relation to a specific stacking sequence table. The developed method is verified by application to the well-known 18-panel Horseshoe Problem. Subsequently, the proposed method is applied to the optimization of a composite stiffened skin of a wing torsion box. The problem objective is mass minimization and the constraint is local buckling.

Effect of fatigue loading rate on lifespan and temperature of tailored blank C/PPS thermoplastic composite

This article reports an experimental investigation of the effects of the loading frequency on the temperature change, fatigue behaviour, and failure mechanisms of carbon-fibre-fabric-reinforced polyphenylenesulphide (PPS) laminates, the thicknesses of which were varied by ply drops. The experiments specifically considered two ply drop configurations and fatigue loading frequencies of 0.5–15 Hz. Fractographic examination revealed the presence of loading-frequency-based surface fracture features in the tested specimens. With increasing loading frequency, the local temperature also increases significantly, reaching as high as above 110 °C, accompanied by more than one order of magnitude decrease in the fatigue life. For a surface temperature of up to 38 °C, there was no specific relationship with the fatigue life. However, further increase of the surface temperature up to and beyond 75 °C was accompanied by significant reduction of the fatigue life. An analytic relationship between the load rate and the local temperature was derived and used to define limits for the fatigue testing of tailored blank structures.

Ultrasonic Analytic-Signal Responses From Polymer-Matrix Composite Laminates.

Ultrasound has been used to inspect composite laminates since their invention but only recently has the response from the internal plies themselves been considered of interest. This paper uses modeling techniques to make sense of the fluctuating and interfering reflections from the resin layers between plies, providing clues to the underlying inhomogeneities in the structure. It shows how the analytic signal, analyzed in terms of instantaneous amplitude, phase, and frequency, allows 3-D characterization of the microstructure. It is found that, under certain conditions, the phase becomes locked to the interfaces between plies and that the first and last plies have characteristically different instantaneous frequencies. This allows the thin resin layers between plies to be tracked through various features and anomalies found in real composite components (ply drops, tape gaps, tape overlaps, and out-of-plane wrinkles), giving crucial information about conformance to design of as-manufactured components. Other types of defects such as delaminations are also considered. Supporting evidence is provided from experimental ultrasonic data acquired from real composite specimens and compared with X-ray computed tomography images and microsections.

Optimisation of ply drop order in variable stiffness laminates

Modern composite structures offer multiple avenues of optimising performance. One avenue is to optimise a single stacking sequence over the structure leading to constant stiffness designs. Another avenue is to allow the stacking sequence to vary over the structure leading to variable stiffness laminates. This may be achieved either by dropping plies or by steering the fibres. When using ply drops to optimise the thickness distribution two different set of decisions are involved: the selection of ply drop boundaries, and the selection of the ply drop order. In this paper, the fibre angle distribution, the ply drop boundaries, and the ply drop order are simultaneously optimised. The optimisation of fibre angle distribution lends itself easily to gradient based methods. The ply drop boundary optimisation is formulated using topology optimisation techniques and is thus solvable using gradient based methods as well. The ply drop order optimisation requires discrete variables and is hence approached using an evolutionary algorithm based on stacking sequence tables. In this paper an efficient multi-step algorithm is developed to combine the optimisation of all aspects of variable stiffness laminates. The results indicate that significantly improved designs may be obtained by including the ply drop order in the optimisation.

A hybrid embedded cohesive element method for predicting matrix cracking in composites

The complex architecture of many fibre-reinforced composites makes the generation of finite element meshes a labour-intensive process. The embedded element method, which allows the matrix and fibre reinforcement to be meshed separately, offers a computationally efficient approach to reduce the time and cost of meshing. In this paper we present a new approach of introducing cohesive elements into the matrix domain to enable the prediction of matrix cracking using the embedded element method. To validate this approach, experiments were carried out using a modified Double Cantilever Beam with ply drops, with the results being compared with model predictions. Crack deflection was observed at the ply drop region, due to the differences in stiffness, strength and toughness at the bi-material interface. The new modelling technique yields accurate predictions of the failure process in composites, including fracture loads and crack deflection path.