In Situ Thermal Inspection of Automated Fiber Placement Operations for Tow and Ply Defect Detection

The advent of Automated Fiber Placement (AFP) systems have aided the rapid manufacturing of composite aerospace structures. One of the challenges that AFP systems pose is the uniformity of the deposited prepreg tape layers, which complicates detection of laps, gaps, overlaps and twists. The current detection method used in industry involves halting fabrication and performing a time consuming, visual inspection of each tape layer. Typical AFP systems use a quartz lamp to heat the base layer to make the surface tacky as it deposits another tape layer. The innovation proposed in this paper is to use the preheated base layer as a through-transmission heat source for inspecting the newly added tape layer in situ using a thermographic camera mounted on to the AFP hardware. Such a system would not only increase manufacturing throughput by reducing inspection times, but it would also aid in process development for new structural designs or material systems by providing data on as-built parts. To this end, a small thermal camera was mounted onto an AFP robotic research platform at NASA, and thermal data was collected during typical and experimental layup operations. The data was post processed to reveal defects such as tow overlap/gap, wrinkling, and peel-up. Defects that would have been impossible to detect visually were also discovered in the data, such as poor/loss of adhesion between plies and the effects of vacuum debulking. This paper will cover the results of our experiments, and the plans for future versions of this inspection system

Characterising, understanding and predicting the performance of structural power composites

Dramatic improvements in power generation, energy storage, system integration and light-weighting are needed to meet increasingly stringent carbon emissions targets for future aircraft and road vehicles. The electrification of transport could significantly reduce direct CO2 emissions; however, battery energy and power density limitations pose a major technological barrier. The introduction of multifunctional structural power composites (SPCs), which simultaneously provide mechanical load-bearing and electrochemical energy storage, offers new possibilities. By replacing conventional materials with SPCs, electrical performance requirements could be relaxed, and vehicle mass could be reduced; however, for SPCs to outperform monofunctional systems, significant performance and reliability improvements are still required. The use of computational models to support experimental efforts has so far been overlooked, despite wide recognition of the benefits of such a combined approach. The aim of this work was to develop predictive finite element models for structural supercapacitor composites (SSCs), and use them to investigate their mechanical, electrical, and electrochemical behaviour. A unit cell modelling technique was used to generate realistic mesoscale models of the complex microstructure of SSCs. The effects of composite manufacturing processes on the final performance of SSCs were investigated through characterisation and modelling of compaction and manufacturing defects. Numerical predictions of the elastic properties of SSCs were evaluated against data from the literature; and the presence of defects was shown to significantly degrade performance. Motivated by the large series resistance of SSCs, direct conduction models were developed to better understand electrical charge transport. Based on investigations of various current collector geometries, design strategies for the mitigation of resistive losses were proposed. To enable analysis of the combined mechanical-electrochemical behaviour of SSCs, an ion transport user element subroutine was developed but could not be validated. Overall, this work demonstrates that substantial improvements in the mechanical and electrical properties of SSCs are possible through control of the composite microstructure. The models developed in this work provide guidance for the optimisation of manufacturing processes and the design of new SSC architectures, and underpin the future certification and deployment of these emerging materials.Open Acces

Multi-Channel Ground-Penetrating Radar for the Continuous Quantification of Snow and Firn Density, Depth, and Accumulation

A priority of ice sheet surface mass balance (SMB) prediction is ascertaining the surface density and annual snow accumulation. These forcing data are inputs for firn density models and can be used to inform remotely sensed ice sheet surface processes and to assess Regional Climate Model (RCM) skill. The Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) retrieved 16 shallow firn cores and dug 42 snow pits along the Western percolation zone of the Greenland Ice Sheet (GrIS) during May and June of 2016 and 2017. I deployed and maintained a multi-channel 500 MHz ground-penetrating radar in a multi-offset configuration throughout the two traverse campaigns. The multi-channel radar technique accurately and independently estimates density, depth, and annual snow accumulation -- between the firn core and snow pit sites -- by horizon velocity analysis of common midpoint radar reflections from the snow and shallow firn. I analyzed a 45 km section of the traverse in a high accumulation zone, known as the GreenTrACS Core 15 Western Spur. Deviations in surface density up to +- 15 kg/m3 from the transect mean correlate with surface elevation and surface slope angle. Spatial variation in mean annual accumulation of ~0.175 m w.e. ɑ-1 occurs across a trough in the surface topography ~5 km wide. The reported variability of density and accumulation demonstrates that RCMs must be down-scaled to resolutions within 5 km to assess subtle yet significant contributions to the GrIS SMB

Efficient Design Optimization Methodology for Manufacturable Variable Stiffness Laminated Composite Structures

Because of their superior mechanical and environmental properties compared to traditional metals, fiber-reinforced composite materials have earned a widespread acceptance for different structural applications. The tailoring potential of composites to achieve high specific stiffness and strength has promoted them as promising candidates for constructing lightweight structures. From that aspect, designers have tackled the problem of designing composite laminates, which is inherently challenging due to the presence of non-linear, non-convex, and multi-dimensional optimization problems with discrete and continuous design variables. However, despite their increased usage, the possible improvements that can be achieved by composite laminates have not been fully exploited. With the introduction of new manufacturing technologies such as advanced fiber placement, engineers now have the capability to harness the full potential of nonconventional variable stiffness composite laminates using in-plane fiber steering. This can be a blessing as well as a curse for the designer, where the additional improvements can be attained at the expense of an increased complexity of the design problem. To circumvent this difficulty, this research aims to develop appropriate design tools to help unlock the advancements achieved by nonconventional variable stiffness laminates. The purpose is to adopt an efficient design optimization methodology to abandon the traditional usage of straight fiber composite laminates in the favor of exploring the structural improvements that can be achieved by steered laminated composite structures, subject to manufacturing constraints and industry design guidelines. This represents a remarkable step in the development of energy-efficient light-weight structures and in their certification. The complexity of the optimization problem imposes the need for an efficient multi-level optimization approach to achieve a global optimum design. In this work, the importance of including a design-manufacturing mesh is demonstrated in each optimization step of the multi-level optimization framework. In the first step (Stiffness Optimization), a theoretical optimum stiffness distribution parameterized in terms of lamination parameters is achieved that accounts for optimum structural performance while maintaining smoothness and robustness. The design-manufacturing mesh allows the spatial stiffness distribution to be expressed as a B-spline or NURBS surface defined by the control points of the design-manufacturing mesh. The fiber angle distribution is then obtained in the second optimization step (Stacking Sequence Retrieval) to match the optimum stiffness properties from the first optimization step while accounting for the maximum steering constraint and laminate design guidelines to attain manufacturability and feasibility. A bilinear sine angle variation is presented to obtain smooth fiber angle distributions, and the maximum steering constraint is derived to guarantee a certain degree of manufacturability at the second optimization step. Using the design-manufacturing mesh, a constant curvature arc solution is developed in the third optimization step (Fiber Path Construction) to generate manufacturable fiber paths with piecewise constant curvature arcs that match the optimal fiber orientation angles from the second optimization step while locally satisfying the maximum curvature constraint. To minimize gaps and overlaps obtained due to fiber steering, a design-for-manufacturing tool is developed to generate tow-by-tow descriptions of the steered plies in the form of manufacturing boundaries for the AFP machine with optimized cut and restart positions. The design of cylindrical shells under bending with a specified cutout is chosen as an aerospace application to demonstrate the effectiveness of using nonconventional variable stiffness laminates compared to traditional conventional laminates. The presence of the cutout in the cylindrical shell imposes severe stress concentrations yielding a need to use variable stiffness laminates that have continuously varying fiber orientation angles to redistribute the stresses and obtain a structurally optimal design. A design-manufacturing mesh was introduced to perform the buckling load optimization, where both circumferential and longitudinal stiffness variations were considered to physically understand the importance of the stiffness tailoring mechanism in efficient load redistribution and local reinforcements around the regions of the cutouts. The multi-level optimization framework is utilized to obtain a manufacturable fiber-steered laminate that improves the buckling load significantly. The design-for-manufacturing tool developed then generates the tow-level information in the form of exported AFP boundaries. The designed cylindrical shell is imported into CATIA V5® for composite design programming to demonstrate the applicability of the design-for-manufacturing tool developed

3D TEXTILE PREFORMS AND COMPOSITES FOR AIRCRAFT STRCUTURES: A REVIEW

Over the last decades, the development of 3D textile composites has been driven the structures developed to overcome disadvantages of 2D laminates such as the needs of reducing fabrication cost, increasing through-thickness mechanical properties, and improving impact damage tolerance. 3D woven, stitched, knitted and braided preforms have been used as composites reinforcement for these types of composites. In this paper, advantages and disadvantages of each of them have been comprehensively discussed. The fabric architects and their specification in particular stitched preforms and their deformation mode for aerospace applications have been reviewed. Exact insight into various types of damage in textile preforms and composite that have the potential to adversely affect the performance of composite structure along with their inspection using NDT techniques have been elaborated. The research review reported in this paper can be very valuable to researchers to release the 3D composite behaviour under different loading conditions and also to get familiar with the manufacture of high quality textile composite for aircraft structures

Biophysics of archaeal chromatin studied at the single molecule level

DNA compaction is a universal requirement across the three domains of life. The proteins responsible for DNA compaction in archaea and eukaryotes are structurally distinct from those found in bacteria. Archaea and eukaryotes share a conserved protein fold for DNA compaction: the histone fold. This implies that the nucleosome, which is the fundamental building block of eukaryotic chromatin, evolved before the archaeal and eukaryotic domains of life separated. Despite their low sequence homology, recent structural studies indicate that the canonical nucleosome core can be formed by archaeal histones. However, the archaeal nucleosome might assemble into a continuous, solenoid-like, fibre called a ‘hypernucleosome’, which is unlike the discrete, regularly-spaced, nucleosomes formed in eukaryotes. I have used a combination of single molecule approaches (magnetic tweezers and total internal reflection fluorescence microscopy) to investigate the mechanics of DNA binding, wrapping and compaction by the abundant histone protein (‘A3’) from the hyperthermophilic archaeon, Methanocaldococcus jannaschii. I have found that assembly of the histones on the DNA is concentration dependent, and that the stability under moderate load of the resulting fibre is improved by increasing histone concentration. I have found that negative supercoiling also confers stability to the hypernucleosome structure, and that the chromatin fibre shows a sharp buckling transition between positive and negative supercoiling regimes. Furthermore, I have demonstrated that the hypernucleosome can be disrupted by force, within the physiological range, with the fibre reverting to the worm-like chain model for naked DNA under sufficient load. I also propose a model for the force-extension of archaeal chromatin based upon these observations. Significantly, I have proposed a binding mechanism that may enable processing enzymes to access the DNA by shunting or displacing nucleosomes, without the need for active chromatin remodelling proteins - which have not yet been identified or characterised in the archaeal domain

Dynamic reservoir characterization from overburden time-lapse strains

Accurate reservoir depletion or pressure change patterns are of great value when planning infill drilling programs for field development, as well as when monitoring injection wells and swept/unswept areas. In addition, a precise dynamic geomechanical description of the reservoir and overburden stress state could prevent costly undesired effects on the production infrastructure such as sea floor subsidence, casing shear and well failure. Dynamic characterization of reservoirs, until recently, had only well data to rely on, which apart from the inherent uncertainties (e.g. due to formation damage), provides no direct information on what is taking place between the wells. The advent of time-lapse seismic at the end of the 1990s meant that this gap could be bridged, providing measurements of the changes taking place in the subsurface. In its origins, time-lapse seismic was conceived as a tool to image intra-reservoir fluid movements via the dependency of reflection amplitudes on acoustic impedance, which is affected by fluid saturation changes in the porous reservoir rocks. However, depletion induced velocity changes are also non negligible. Furthermore, the reflectors may undergo deformation and displacement where compaction and subsidence are involved. As a consequence, analysis of amplitude changes is not straightforward, since in most cases, amplitudes have been shifted by a non negligible time difference or time-shift, presenting not only challenges, but also new possibilities. It is in the possibilities of these time-shifts that the present study is based. This research presents a novel method which numerically solves the static field problem in a multilayered heterogeneous media, relating overburden strain to reservoir depletion. It builds up on previous works based on Geertsma type solutions requiring a homogeneous half-space. This technique makes it possible to estimate the reservoir’s stress state, strain and pressure changes from measured overburden strain by considering the earth as a linear filter with reservoir compaction and overburden strain as parameters. However, some a priori knowledge is needed in the form of a rough subsurface model and a preliminary geomechanics simulation in order to approximate the transfer functions as Wiener filters. In this thesis, the Wiener filter concept has been applied to three real North Sea fields. First, to the Elgin field, an HP/HT shallow marine Upper Jurassic sandstone reservoir located in the UK sector of the North Sea. Then, to the Ekofisk and South Arne fields, both compacting chalk reservoirs in the Norwegian and Danish sector of the North Sea respectively. Additionally, by using a synthetic example the method has been validated and compared with a linear inversion approach using a Geertsma type Green’s function achieving higher accuracy. The project involved not only the development and application of the method itself, but the calculation of time-strains from the measured seismic and the construction and implementation of full field geomechanical models

Comprehensive Process Planning Optimization Framework for Automated Fiber Placement

Advanced composite materials came about in 1966 and have since been widely used due to the possibility of superior structural performance while also achieving weight reductions. Such opportunities have led to composite materials being used to fabricate complex components, often in the aerospace sector. Most components, especially in aviation, are on a large scale and are outside the capabilities of traditional composite manufacturing techniques. Traditional manufacturing methods are also labor intensive, time consuming, have a high level of material scrap, and are prone to human error. This has led to the need for innovative manufacturing solutions to withstand the ever-increasing throughput requirement. One driving technique is Automated Fiber Placement (AFP) which is a relatively new manufacturing technique that has rapidly evolved since its commercial inception in the 1980s. AFP gives industries the ability to manufacture large parts with high speed, repeatability, and process quality. However, even with the state-of-the-art machines and process controls, the AFP process is still far from perfected. With the advent of Industry 4.0, many manufacturing sectors have begun the exploration into the use of smart and digital manufacturing with implementation of machine learning. However, AFP manufacturing, and the remaining composite manufacturing sector, has yet to explore the philosophies of the future of manufacturing. Rather, siloed efforts have been enacted to advance each of the technical challenges associated with AFP resulting in an open loop system that is difficult to optimize on a global level. Such efforts also restrict the possibility of increased manufacturing throughput due to systems operating in suboptimal configurations. To overcome this hurdle, an integrated data flow is enacted that combines the design, process planning, manufacturing, and inspection pillars of AFP into a holistic workflow. This is enabled by employing industry level tools combined with efficient and versatile modeling techniques. The models then allow for an informed analysis that considers the combination of multiple AFP lifecycle trades. With this streamlined workflow, multiple optimization algorithms are developed to determine the globally optimal manufacturing plan to generate a structure with AFP. The combination of these methodologies into a common tool creates a comprehensive AFP process planning optimization. The modeling and optimization framework is applied to a doubly curved tool surface case study. Results demonstrate the effectiveness of the presented framework in terms of manufacturing defect reduction and process efficiency