Fibre Wrinkling Research Articles

An accurate forming model for capturing the nonlinear material behaviour of multilayered binder-stabilised fabrics and predicting fibre wrinkling

Prefabricated stacks of binder-stabilised dry fabrics are used to increase reinforcement deposition rates in the manufacturing of large composite structures such as wind turbine blades. To ensure optimal part quality, accurate simulation tools may be used to avoid costly trial-and-error experiments. This paper introduces a new modelling framework for predicting wrinkle defects that arise from forming thick binder-stabilised stacks of quasi-unidirectional non-crimp fabric. In this context, quasi-unidirectional non-crimp fabrics are unidirectional rovings stabilised with a backing layer. The non-constant bending stiffness of the fabric is modelled using an asymmetric compressive modulus, while a mixed-mode cohesive formulation with user-defined damage evolution is used for modelling the initial non-constant elastic binder interface stiffness and interface degradation. The model is verified against cantilever bending tests of single fabric layers and transverse shear experiments of bindered fabric layers. Comparison with experimental results on the forming of preforms shows that the model accurately predicts wrinkle location and geometry during the forming of thick binder-stabilised preforms. The error between the wrinkle geometry predicted by the new modelling approach and the experimentally measured wrinkle geometry is reduced by 35% to 94% compared to wrinkles predicted by conventional modelling approaches. The modelling framework is freely available at https://doi.org/10.5281/zenodo.10077428.

A modified bias-extension test method for the characterisation of intra-ply shear deformability of hybrid metal-composite laminates

The bias-extension test is one of the test methods to characterise the intra-ply shear behaviour of continuous fibre reinforced composites including fabrics and unidirectional (UD) materials. For the determination of the major mechanical properties of metals, often a uniaxial tensile test is used. Combination of these two methods for the shear deformation of hybrid metal-composite laminates is proposed comparing the method for cross-plied unidirectional prepregs and woven fabric prepregs. The effects of material constituent, shear rate, preheat temperature and normal pressure on the intra-ply shear behaviour are investigated. The results indicate that the material constituents and the frictional responses depending on processing parameters play a critical role in the shear characterisation of the hybrid laminate. The shear angle measurement at four typical strains demonstrates that the support of metal layers improves the shear deformability by delaying the onset of fibre wrinkling. This modified intra-ply shear test contributes to a better understanding of the process design for wet (uncured) hybrid metal-composite laminate manufacturing.

Manufacture of long spars: Laminate design, experimental trials and non-destructive evaluation

Although an enabler of high-rate manufacture, automated forming of components from flat laminates can result in fibre wrinkling. For the first time, fibre length is demonstrated to be a key driver of wrinkling defects when forming a C-spar with a central recess, at industrial length scales. Three such spars, of equal in-plane stiffness, were manufactured using Single Diaphragm Forming. Two contained standard ply angles (0°, 90° or ±45°) including 6 m long 0° fibres but had different stacking sequences, one shown to be formable in short spar trials, the other un-formable. A third spar also had a formable sequence but contained non-standard ply angles (angles other than 0°, 90° or ±45°) so that maximum fibre length was less than 0.6 m. Wrinkles occurred in both standard angle spars but did not occur in the non-standard angle case, demonstrating that use of formable, non-standard angle laminates can reduce scrappage of automatically formed aerospace parts, leading to significant time and cost savings in production.

Experimental analysis of the effects of wrinkles in the radius of curvature of L-shaped carbon-epoxy specimens on unfolding failure

An experimental study has been carried out to describe the evolution of the failure load relative to the maximum misalignment angle of curved carbon–epoxy laminates with out-of-plane wrinkle defects for a large range of misalignments. L-angle specimens with different levels of fibre wrinkling were manufactured and tested in four-point bending tests. Digital Image Correlation (DIC) and infrared thermography were used to monitor the tests. They allowed the stress concentrations induced by the wrinkles to be identified, together with their influences on the failure load of the sample. In most cases, increases in wrinkling levels led to decreases in failure loads.

Vacuum-bagged composite laminate forming processes: Predicting thickness deviation in complex shapes

This paper focuses on the manufacturing of composite laminates using vacuum-bag processes. When forming complex shapes, such as corners, the laminate thickness may deviate from the nominal thickness obtained for flat plates. This is due to two phenomena that occur in corners: (i) because of the geometry, the available consolidation pressure differs from the expected pressure; and (ii) friction may prevent adequate conformation of the laminate to the mould. The thickness deviation is associated with defects (porosity, dry or resin-rich areas or fibre wrinkling). We propose an analytical model to describe these two phenomena, which relies only on two geometric ratio: radius to flange length and thickness to radius, and two material properties: bulk factor and inter ply friction coefficient. The model estimation was compared to an extensive experimental database including a variety of configurations: male or female tools and various flange lengths, weavings and corner radii.

Manufacturing characteristics of the continuous tow shearing method for manufacturing of variable angle tow composites

Automated fibre placement (AFP) is well-known as a cutting-edge technology for manufacturing variable angle tow (VAT) composites with tailored fibre paths. However, its process-induced defects prevent the wide application of VAT composite structures. As an alternative manufacturing method, the continuous tow shearing (CTS) technique, utilising the ability to shear dry tows, has been developed. It was shown that CTS could significantly reduce process-induced defects such as fibre wrinkling, resin rich areas and fibre discontinuities. In this paper, its manufacturing characteristics such as material characteristics, layup accuracy, and thickness variation are investigated experimentally.

Continuous tow shearing for manufacturing variable angle tow composites

Automated fibre placement (AFP) is the key technology for manufacturing variable angle tow (VAT) composites with tailored fibre paths. Although it was developed in the 1980s, much research still focuses on the minimization of the process-induced defects because the fundamental principle has not been changed. In this work, a novel fibre placement technique using the shear deformation characteristic of dry tow has been developed, which was named ‘Continuous Tow Shearing (CTS)’. Through the prototype tests, it is shown that CTS could significantly reduce process-induced defects such as fibre wrinkling, resin rich areas and fibre discontinuity. Also, an intuitive notation for defining a curved tow path using piecewise quadratic Bezier curve is developed.

The Influence of Processing Parameters on the Void Content and Compressive Strength of Carbon Fibre-Epoxy Laminates

This research addresses the influence of various processing parameters on the post cure quality of carbon fibre composites. Four processing parameters were investigated in the study, in terms of their impact on void content and overall compressive strength. The first parameter distinguishes between laminates cured in a vacuum oven and those cured in an autoclave under high positive pressure. The second parameter describes the impact on voids of differing fibre architectures, comparing a unidirectional fibre structure to that of woven cloth. Thirdly, the influence of compaction during manufacture is analysed and lastly, variation in cure temperature was tested to determine its effect on final laminate quality. The quality of the cured laminate samples was assessed from visual inspection, and in terms of compressive strength and void fraction calculated by Micro-CT X-ray Tomography. The results show that autoclave-cured samples feature significant quality improvements in terms of void fraction and compressive strength when compared to oven-cured samples. Unidirectional laminates incur higher sensitivity to void inclusion than cloth laminates due to the influence of fibre wrinkling. Compaction has no effect on laminate strength; it does however reduce variability in certain cases. Temperature affects different fibre structures in different ways, these being highly dependent on curing method. Finally, it was discerned that curing by autoclave was the dominant processing parameter. Thus, regardless of other manufacturing techniques, the autoclave samples featured almost zero voids and were consequently of the highest quality.

Automated analysis and advanced defect characterisation from ultrasonic scans of composites

With the rapidly escalating usage of composite materials, not only in military aircraft but in civil airliners as well, production NDT throughput is already stretched to its limit internationally. NDT data analysis is set to become the bottleneck preventing the required rise in production rates of composite civil aircraft in the next few years. Thus there is an urgent requirement for rapid, automated analysis of up to a Terabyte of data per airliner, escalating to over 200 Terabytes per year - worldwide. The primary aim of automated analysis is to release operators from the time-consuming analysis of all scans and focus operator attention on non-compliant structures. A secondary aim is to provide three-dimensional quantitative information that lightens the operator's decision-making burden. Through advanced characterisation methods, NDT also has the potential to provide crucial feedback to control the composite production process, increase production yield and decrease costs. Current analysis methods for ultrasonic scans produce through-thickness average parameters, which provide little useful information to assist the stress analysis for defects, or the production process. Three-dimensional characterisation of defects can increase yield by informing the concession/disposition process for defects. For future process control, information is required about the 3D distribution of material properties in the structures on the production line, providing comprehensive long-term trend analysis. As well as demonstrating a new rapid automated analysis method for large-area ultrasonic scans, this paper proposes new ultrasonic methods for generating quantitative 3D profiles of porosity, resin layer thickness, ply spacing and fibre orientation. From these it is possible to determine ply stacking sequence and measure in-plane fibre waviness and out-of-plane fibre wrinkling. Armed with these new tools there is the potential for solving the NDT data analysis bottleneck for composite aircraft. Examples are given of the use of these tools on carbon-fibre composite structures. Stacking sequence has been determined in structures up to 18 mm thick, and out-of-plane wrinkling measured in 35 mm-thick structures, although penetration depends on the measurement parameter, material quality and the ply spacing. The tools are software based and are applied through post-processing of full-waveform data acquired using one of several suitable ultrasonic acquisition systems. These include commercially available phased array systems.

Rapid phased array ultrasonic imaging of large area composite aerospace structures

There has been an increase in the use of composite materials in the aerospace industry which is driving a need for new NDT techniques that can rapidly scan large structures and provide quantitative data on the material integrity. In many applications there are common requirements for the ultrasonic inspection of composites for porosity, delaminations, foreign body contamination and fibre wrinkling. Traditional methods of ultrasonic inspection require the use of a single-point probe or a multiplexed group of probes. NDT Solutions has developed new inspection techniques based on ultrasonic arrays housed within a rubber coupled wheel sensor. The wheel can be manually applied or scanned over the structure with an automated scanning system. Applications for the technology include real-time imaging and reporting of impact damage, porosity sizing, delaminations and fibre wrinkles. Examples and case studies from a range of aerospace applications are given, including inspection of small and large area composite structures.

Effect of radial stress relaxation on fibre stress in filament winding of thick composites

During filament winding of thick cylinders, fibre wrinkling often occurs which severely decreases compressive strength. To eliminate fibre wrinkling, appropriate processing conditions must be found. Fibre migration and stress relaxation due to resin flow are generally considered the most important factors affecting fibre buckling. Therefore, the effect of stress relaxation on fibre wrinkling during the filament winding process was investigated. To study the stress development during filament winding of thick cylinders, experiments were carried out using graphite/epoxy prepreg tows as well as dry graphite fibre. Cylinders of approximately 12 mm thickness were hoop wound on a 50.8 mm diameter aluminium mandrel. Winding tensions ranged from 13 to 34 N and winding speed was constant. A foil-type pressure sensor was applied on the mandrel to monitor the interface pressure throughout winding and storage of the cylinder. Significant stress relaxation was found to occur during winding with prepreg tow. Mandrel pressure increased over the winding of the first eight layers or so. However, between the winding of one layer and the next, mandrel pressure dropped quickly. Also, it began to decrease after reaching a maximum value. A stress relaxation analysis was carried out to determine the stress in the cylinders during winding. Several parameters were not known a priori and had to be inferred from the data. Stress distributions following winding were calculated for each case. The radial stress in prepreg wound cylinders was found to relax nearly to zero in the inner part of the tubes. Compressive circumferential stresses occurred throughout each of the cylinders. However, they reached greater magnitudes in the dry wound cylinders due to very low radial moduli. No fibre wrinkling was evident in any of the wound cylinders.

Analysis of fibre wrinkling during squeezing flows of fibre-reinforced composites

An analysis of the stability of squeezing flows between flat plates (consolidation flows) of viscous liquids reinforced by continuous fibres is presented. The ideal linear fibre-reinforced fluid model is used to model the composite as an incompressible Newtonian fluid reinforced with inextensible fibres. The development of small fibre wrinkles initially present in the preimpregnated plies is analysed using linear stability theory. It is shown that when the flows are lubricated by resin rich layers, two perturbation modes are possible. In the first mode, the wrinkles are of the same form throughout the thickness of the sample while in the second mode they vary linearly with distance from the platens. In both cases the stability depends on the normal components of the applied stress. If the only traction acting in addition to hydrostatic pressure is that due to the squeezing force then the first perturbation mode is stable. This prediction is in agreement with experimental results.

A linear stability analysis of flows of transversely isotropic non-Newtonian fluids

A linear stability analysis of flows of molten, unidirectionally reinforced composites is presented. The molten composites are modelled using the ideal linear fibre-reinforced fluid model in which the fluid is assumed to be homogeneous and locally transversely isotropic. Two types of plane, steady-state, shearing flow are discussed, one in which the fibres lie along the flow direction (longitudinal flows) and the other in which the fibres lie in the shear planes but perpendicular to the flow direction (transverse flows). Each flow type is analysed using a normal mode linear stability analysis, with perturbations along and transverse to the fibre direction being discussed separately. For both types of flow the transverse mode is always stable. The instability mode along the unperturbed fibre direction causes fibre wrinkling and the stability conditions associated with it are different in longitudinal and transverse flows. In the latter a normal stress difference equivalent to the fibre tension is the stability-determining factor, while in longitudinal Couette flow the difference in shear viscosities is found to be a source of instability.

Muscle fibre deformation: rigor extensibility of raw bovine muscle

Muscle which entered rigor restrained in a stretched condition shortened when freed, resulting in three types of wrinkling—(1) fibre wrinkling, (2) myofibril wrinkling and (3) myofilament wrinkling. The structural consequences of stretching muscle whilst it was in rigor were studied by light and electron microscopy. Rigor extension of contracted or stretched samples in excess of that required to straighten any wrinkles resulted in three types of damage to a high proportion of the sarcomeres. (1) Breaks occurred in the I bands, but not the clean breaks associated with post-rigor muscle. (2) Breaks occurred in the A bands on either side of the M line. (3) Z lines were pulled apart.

Macroscopic wrinkling of muscle fibres, a source of error in objective assessment of meat toughness

Post-rigor shortening of muscle resulting in macroscopic fibre wrinkling was studied in muscle samples which entered rigor restrained in a stretched condition and were subsequently freed. The amount of wrinkling varied with the initial degree of stretch pre rigor and with the muscle treatment post rigor, depending on such variables as (1) whether or not the muscle was heated and (2) if heated, at what stage the restraint was removed. The consequences of fibre wrinkling relative to the objective testing of meat toughness are discussed.