Thin-ply Composites Research Articles

Shape recovery simulation of viscoelastic thin-ply composite coilable booms

Coilable booms made of thin-ply composites provide mass and volume efficiency for deploying large structures in space. The recovered shape of the booms after being stowed in the coiled configuration is critical to the deployed stiffness and shape precision of the spacecraft, but is heavily influenced by the viscoelasticity of the polymer composites. A numerical study of coiling, stowing, uncoiling and shape recovery of a viscoelastic thin-ply composite coilable boom is presented in this paper. The formulation of two micromechanics-based viscoelastic shell models and their numerical implementation in structural simulations are presented. The shape recovery of a composite boom coiled for two years, replicating the typical timeline for deployable structures used in solar sail missions, has been simulated. The demonstrated modeling and simulation capability is useful for designing deployable spacecraft structures.

Hydrogen permeability of thin-ply composites after mechanical loading

Hydrogen is a sustainable alternative to conventional fuels, and it may be obtained with near zero carbon footprint. However, hydrogen storage remains a key challenge, and the use of composite tanks has gained significant interest over the last few years. In addition, thin-ply composites promote fibre damage by delaying matrix microcracking and free edge delamination. In this work, the H2 permeation/diffusion performance of virgin and mechanically loaded thin cross-ply laminates is studied. In addition, Scanning Electron Microscopy (SEM) is used to identify defects and micro-damage in the laminates and explain the experimental values. The study shows that the hydrogen (H2) barrier performances of thin-ply composites are lower than conventional metallic systems. Obtained permeability values, however, resulted well below the allowable limits for most combinations of temperature and pressure and remain unaffected despite the application of high tensile strains showing that permeation is not accelerated.

Strain rate sensitivity of the in-plane mechanical properties of two UHMWPE thin ply composites

The mechanical characterisation of two recent grades of Ultra High Molecular Weight PolyEthylene (UHMWPE) fibres composite is proposed. Used in bulletproof vests, they share the same type of fibres, while composed of different matrices (respectively polyurethane and rubber). The in-plane characterisation is performed through tensile tests on thin layer samples at 0°of the fibres and in-plane shear, in both quasi-static and dynamic conditions. Digital Image Correlation is used for each test to accurately measure strains and evaluate its homogeneity. To study the strain rate effect, various experimental methods were adapted: universal press, drop tower, Split Hopkinson Tensile Bars (SHTB). A significant increase of stiffness with strain rate was observed for both materials At 0°, it transcribed into an increase of 60% for the Young’s modulus, and 40% on failure stress. The in-plane shear modulus was multiplied by two and even eight depending on the considered grade.

Efficient method to predict the strain rate sensitivity of an UHMWPE thin ply composite

Understanding the mechanical behaviour of materials used in ballistic application when subjected to dynamic loading is crucial. However, performing mechanical tests at high strain rates on materials such as Ultra High Molecular Weight PolyEthylene (UHMWPE) is complex. This article addresses this issue by presenting an evaluation of the strain rate sensitivity of an UHMWPE thin ply composite using Dynamic Mechanical Temperature Analysis (DMTA). By applying Time Temperature Superposition Principle (TTSP), a master curve is obtained in the frequency-domain. Additionally, a viscoelastic model is determined to convert to time-domain, enabling the calculation of the elastic modulus over a wide range of strain rates. The dynamic elastic modulus is twice as high as the quasi-static one. The obtained results are similar to those measured by conventional experiments. Therefore this study presents a reliable, quick and cost efficient method to evaluate the strain rate sensitivity of the mechanical behaviour of such materials. Moreover, it allows to reach higher strain rates than conventional tests and reach strain rates involved in ballistic impacts which cannot be obtained using conventional tests.

Characterization and comparison of thin ply IM7/8552 composites processed by automated tape placement and hand layup

Manufacturing of large composite structures is labor-intensive and challenging. Traditional processing methods can be replaced by automated approaches such as Automated Tape Placement (ATP). The focal point of this effort is on thin-ply thermoset prepreg materials which have shown better resistance to damage tolerance under applied stress when compared to standard ply-thickness materials in recent times. However, composites processed by thin-ply materials require more placement iterations to achieve the same net volume of coverage as the standard ply-thickness materials. This can increase the probability of placement defects such as gaps and overlaps, which may degrade the mechanical performance of the composite. Modifications to the existing ATP system are made to address the issues of placement defects, which resulted in improved accuracy for thin ply placement. Unidirectional and quasi-isotropic panels are fabricated from 12’’ wide unidirectional prepreg sheet with the conventional hand layup (CHL) method for the baseline property comparison with panels fabricated by the ATP process using a ¼’’ wide slit tape. This paper investigates the effects of processing defects and material variability on the mechanical properties such as tensile, Open hole tension and Bearing tests of thin ply composites. In addition, the microstructure of the fabricated thin-ply composite panels is analysed using ultrasonic C-scans and confocal microscopy to examine the processing and material quality.

Micro-mechanical analysis of transverse tensile in-situ effect of thin-ply composites considering interlaminar resin region

The mechanical property of the thin-ply composite are improved, which has more uniform microstructure and fewer manufacturing defects compared with the standard ply composite. Based on the micromechanics theory, an RVE analysis method is proposed considering the interlaminar resin region in this paper. The influence of the interlaminar resin region on the RVE model of thin-ply composites is studied. The initiation and propagation of cracks are discussed in the thin-ply laminate under transverse tensile, and the in-situ transverse strength of 90° ply is predicted with different thicknesses. The results show that considering the interlaminar resin region has a great influence on the in-situ strength prediction results. Reducing the ply thickness can change the fracture mechanism and improve the transverse in-situ strength of the material.

Progressive damage and residual strength of open-hole thin-ply CFRP laminates under tensile fatigue loading

Thin-ply composites are promising structural materials because of their excellent damage resistance and fatigue properties, as well as the advantage of improved design freedom in composite structures. In this study, open-hole tensile fatigue tests were performed on carbon-fiber-reinforced polymer laminates using thin-ply prepregs to investigate the effects of varying ply thickness. Fatigue tests were performed on three types of quasi-isotropic laminates with different ply thicknesses (thin-ply, intermediate-ply, and thick-ply) at several stresses. Fatigue damage growth around a circular hole was examined by evaluating stiffness degradation and internal damage using X-ray radiography. Additionally, the fatigued specimens were subjected to static tensile tests to evaluate the effect of fatigue damage on the residual strength after fatigue tests. The thin-ply laminates can suppress fatigue damage growth and retain the residual strength properties under high-cycle fatigue at high stress level, thus validating their superiority over thick-ply laminates.

Thin ply composite materials with embedded functional elements for cryogenic environments

• Use of thin plies in fiber reinforced composites to reduce microcracking due to thermal cycling. • Embedding of metallic functional elements into fiber composites. • Damage due to metallic elements embedded in fiber reinforced composites after thermal cycling. Fiber reinforced polymer composites are popular in aerospace applications due to stiffness-to-weight advantages for large structures. However, microcracking presents a limitation to the application of composites at cryogenic temperatures. Moreover, the effects of embedded functional elements are unclear at these temperatures. This study proposes thin-ply composites as a damage mitigation strategy for cryogenic environments. Thermal cycling and subsequent mechanical testing demonstrate the effectiveness of this strategy and provides guidelines for the integration of embedded elements.

Enhancement of tensile strength of tapered laminates by utilizing thin-ply composites

Tapered CFRP laminates formed by dropping off plies are extensively used on modern aircraft due to the special design requirement for certain components. Thin-ply composites have recently been proven to have good damage tolerance in experimental and numerical studies. The use of thin-ply composites in tapered laminates is expected to achieve a higher taper ratio and enhance the design flexibility, which has yet to be investigated. This paper studied the mechanical behavior of tapered laminates without belt plies with different taper ratios and ply thickness under the loading-unloading tensile test. The damage behavior of tested specimens was observed through microscope imaging after the unloading process. The experimental results indicate that tapered laminates utilizing thin-ply composites have excellent mechanical properties and damage-resistant capability. And it was found that the use of thin-ply composites in tapered laminates could allow for a wider range of taper ratios. This allows for more selectivity in the design methods of tapered composite structures and enables lightweight.

Effect of Embedded Thin-Plies on the Charpy Impact Properties of CFRP Composites.

In this study, different configurations of epoxy composite laminates that contained thin plies were prepared and characterised for sudden impact load bearing applications. The primary aim of this investigation was to develop a hybrid epoxy-based thin ply composite for aerospace and automotive applications that would be tolerant of high impacts. The impact properties of the selected configurations were investigated both experimentally and numerically under low-velocity Charpy impact loading conditions. Furthermore, any damage to the laminates was evaluated with an emphasis on the identification of dominant damage mechanisms and locations. This included a comparison between the laminates that were made from traditional plies and the thin ply laminates in terms of their absorbed energy and failure modes. The results revealed that the integration of thin plies into normal ply had a major effect on the amount of absorbed energy under flatwise conditions: up to 8.7 J at a cut-off angle of 90°. However, edgewise conditions produced a maximum observed energy of 10.0 J for the thin plies that were surrounded by normal plies (Plate 3). The damage assessments showed the increased damage resistance of the hybrid thin ply composites due to their uniform stress distribution. The traditional ply composites incurred large deformations from the impact loads. Moreover, it was noted that delamination formed in the middle regions of the traditional plies. The FEM model analysis revealed that it was capable of accurately predicting the absorbed energy for different configurations of composites, which were prepared and analysed experimentally. Both the experimental and numerical values were very similar to each other. These impact damage assessments improved the thin ply composites so that they could be used as working materials for applications that are prone to high loads, such as the aerospace, defence, automotive and structural industries.

Fatigue behaviour and failure mechanism of the thin/thick-ply hybrid laminated composite bolted joints

Thin-ply composites have attracted increasing attention from researchers due to their higher damage tolerance and delamination suppression. This paper presents a detailed investigation of the fatigue behaviour and damage mechanism of symmetrical thin/thick hybrid design laminate-titanium joints under tension-tension loading. Laminates with four different configurations, thick, thick-thin-thick, thin-thick-thin and thin, were designed. According to the quasi-static bearing limit of the thick-ply laminates joints, fatigue testing of each hybrid joint was carried out at two load levels for a certain number of cycles. The deformation and damage of the hybrid joint were characterized by scanning electron microscopy (SEM). With thick plies and the addition of a number of thin plies, the hybrid design laminates can significantly improve the fatigue performance. Stacking thin-ply on the outermost layers of the laminate is more meaningful for double-lap joints than the use of thin-ply joints in the middle. Under an identical number of cycles, the deformation around a hole in the thin-ply laminate is only approximately 50% of that of the thick ply. The damage is delayed, and the propagation of inter- and intralaminar cracks is suppressed in the thin-ply laminates, resulting in the best fatigue resistance and less delamination under the two fatigue loads.

Viscoelastic modeling and characterization of thin-ply composite laminates

Thin-ply composite laminates are promising materials for deployable spacecraft structures owing to their low mass and high folding curvature. However, the deployment force and the shape accuracy of the deployed configuration after a long duration of stowage is adversely affected by the inherent viscoelastic behavior of the polymer matrix. This paper presents a computational and experimental investigation into the effective viscoelastic response of thin-ply composite laminates made of woven carbon fabrics and an epoxy matrix intended for deployable spacecraft applications. A two-scale homogenization approach is utilized to model thin-ply composites as viscoelastic plates represented with relaxation ABD matrices. Micrographic analysis and relaxation tests are conducted to obtain the geometric and material characteristics that are incorporated into the model. The bending relaxation behavior is experimentally characterized using the recently developed column bending test methodology to compare against model predictions. The relaxation response is studied in two different orientations. The effects of ply phase arrangement, laminate thickness, fiber volume fraction, specimen width, and coordinate rotation are investigated.

Effective bending modulus of thin-ply composites with non-uniform fibre spacing

Composite materials are gaining increasing interest for design purposes in many industries such as aeronautical and automotive, not only because of their reduced weight, but also due to their enhanced mechanical properties. The heterogeneity inherent in the nature of composites arises not only because of the mismatch in fibre/matrix properties but also due to the fibre arrangement within the matrix. Previous studies have shown that classical homogeneous elasticity models can either underestimate or overestimate stiffness properties for small ply thicknesses. As such, both positive (stiffer) and negative (more compliant) size effects become increasingly prevalent with decreasing ply thickness, when considering regular distributions of fibres within a matrix material. Current work addresses the effects of irregular fibre spacing on size effects and identifies a characteristic length that can be used to represent the heterogeneity in an equivalent homogeneous micropolar model. Results show that, based on the type of irregular fibre spacing, both kinds of positive and negative size effect arise in a composite ply. As a result, the bending characteristic length embedded in micropolar theory can be defined even for the difficult case whereby fibres are stiffer than matrix depending on the nature of the irregular fibre spacing.

Damage-mechanics mesoscale modeling of composite laminates considering diffuse and discrete ply damages: Effects of ply thickness

Thin-ply composites are gaining significant attention owing to their high damage resistance and their ability to enhance design flexibility in a composite structure. Herein we proposed a damage mechanics model considering diffuse and discrete ply damages in thin-ply composite laminates for mesoscale simulation. To investigate the effects of ply thickness on damage evolution, loading and unloading tests were conducted using cross-ply, angle-ply, and quasi-isotropic laminates with several ply thicknesses. The damage evolution was quantitatively evaluated using continuum damage mechanics. The evolution laws of damage were formulated considering the ply thickness dependency. A damage simulation using the proposed model demonstrated that the retardation and suppression effects of thin-ply were captured by the proposed model. The simulation results also validated that the stiffness degradation and failure strength of laminates with three different ply thicknesses were predicted with good accuracy by considering both diffuse and discrete damages.

Fatigue and fatigue after impact behaviour of Thin- and Thick-Ply composites observed by computed tomography

This study investigates the influence of load ratio and impact damage on the fatigue behaviour of high-performance carbon fibre reinforced polymers (CFRP) with areal fibre weights between 30 gsm and 360 gsm. For undamaged samples, the ultimate tensile and compressive strength, as well as the fatigue properties, are evaluated with regard to their layer thicknesses. The fatigue tests were performed under tension-tension (R=0.1), tension-compression (R=-0.5) and compression-compression (R=10) regime. The results are illustrated as a constant-life diagram, and a piecewise linear interpolation examines a first prediction. The results show that static and fatigue performance improves with decreasing layer thickness. Particularly under tension-compression loading, significant improvements are observed, due to the suppression of matrix cracks and delaminations with thinner layers. In addition, the effect of low-energy impact on the fatigue behaviour of Thin- and Thick-Ply laminates is investigated. The tests demonstrate that although the delamination area is larger, Thin-Ply laminates can sustain higher stresses and still reach the same number of load cycles in contrast to Thick-Ply laminates. Computed tomography measurements visualize 3-dimensional the damage progression after various cycles and prove that the Thin-Ply composites show no increase in the damaged area during fatigue. The interlaminar stress at the delamination is not sufficient for expansion. In contrast, in the case of thicker layers, the damage growths progressively throughout the whole sample with increasing number of cycles.

Mechanical Properties of Thin-Ply Composites Based on Acoustic Emission Technology.

Compared with standard-ply composites, thin-ply composites exhibit a superior mechanical performance under various operating conditions due to their positive size effects. Thin-ply laminate failure modes, including matrix initial damage (MID), matrix failure (MF), and fiber failure (FF), have been distinguished through a systematic acoustic emission (AE) signals analysis combined with scanning electron microscopy (SEM). First, the characteristic frequencies of various failure modes are identified based on unidirectional laminates ([90] 68 and [0] 68). Then, according to the identified frequencies corresponding to distinctive damage modes, four lay-up sequences (02[[90m/0m]ns]02, m = 1, 2, 4, 8, n × m = 16) with a constant total thickness are designed, and the effects of the number of identical plies in the laminate thickness on the damage evolution characteristics and the damage process under uniaxial tension loads are dynamically monitored. The obtained results indicate that the characteristic frequency ranges for MID, MF, and FF are identified as 0–85 kHz, 165–260 kHz, and 261–304 kHz, respectively. The thickness of identical plies has a significant effect on onset damage. With the decrease of the number of identical plies (i.e., m in the stacking sequences), the thin-ply laminates exhibit the initiation of damage suppression effects and crack propagation resistance.

Dependence of damage modes on the ply-thickness of ultra-thin-ply composites

Transitions between damage modes have been observed in ultra-thin-ply laminates. Numerical methods can be effectively applied to the design and optimization of the microstructure of ultra-thin ply laminates. It is difficult to precisely predict transitions between complex damage modes of composites, such as fiber breakage, matrix cracking, shear failure and delamination. In this paper, a numerical model was developed to comprehensively consider in-plane damage and delamination. The initiation and in-plane propagation of damage was simulated using a material degradation model (MDM), which was implemented by a user subroutine USDFLD in ABAQUS. A cohesive interaction between the surfaces of adjacent layers was used to simulate delamination. The numerical predictions for the transitions between damage modes in cross-ply and Bouligand thin-ply laminates were verified by comparison with experimental results. The novelty of this study is discovering the dependence of damage modes on the ply-thickness under bending load and revealing the influence of damage competition mechanism on the bending properties of thin-ply composite. The research results could help develop a useful tool for improving microstructure design or elucidating the underlying mechanism for the mechanical behavior of thin-ply microstructures.

Effective bending modulus of thin ply fibre composites with uniform fibre spacing

Cauchy homogenisation methods are based on the assumption that materials show size independent behaviour. For example, the effective bending modulus of a beam is constant for all cross-sectional areas and is equal to the tensile modulus. However, as demonstrated by numerous authors, the stiffness of heterogenous materials under non uniform loading (e.g. bending) can be influenced by size effects, when the microstructural size scale is comparable to the macroscopic one. In other words, an effective elastic modulus can underestimate or overestimate the bending stiffness depending on the specimen size. In this work, the effect of the thickness dimension on the effective bending modulus of composite plies is investigated. The composite is represented by cylindrical fibres uniformly distributed in a uniform matrix. The cases of fibres stiffer and more compliant than the matrix are considered. The results show that the effective bending modulus depends on fibre volume fraction ratio, mismatch of fibre/matrix material properties and the number of fibres through the thickness. The implications for the analysis of bending of thin ply composites are discussed.

Damage progression and failure of single-lap thin-ply laminated composite bolted joints under quasi-static loading

Composite bolted joints are widely used on primary and secondary load-bearing structures of aircrafts. However, investigating the damage progression and failure of composite bolted joints under high bearing loads is challenging due to the geometric, contact and material nonlinearities. In this work, an extensive experimental study has been carried out to investigate and understand the damage evolution and failure of single-lap thin-ply laminated composites bolted joints under quasi-static loading. Quasi-isotropic carbon/epoxy laminates with stacking sequence [45/0/-45/90]4s were selected for fabricating the test specimen. The specimens were observed using X-ray computed tomography (CT) scanning and SEM imaging at different stages of the loading process to evaluate internal damage and deformation characteristics. The results indicate that the bearing failure of composite bolted joints can be interpreted as an accumulated damage process with local compressing, and mainly includes four stages: damage initiation, damage evolution, non-linear softening and catastrophic failure. The major failure modes of the thin-ply laminates are found to be similar to those of traditional thickness composite, including fiber breakage, matrix cracking, delamination, fiber kinking and fiber-matrix splitting. However, the major difference is that the delamination growth in the bearing area in thin-ply composites is suppressed compared with the traditional thickness composites. Therefore, the obtained experimental data provides valuable information for developing mechanism-based failure models of single-lap thin-ply composite bolted joints.

Energy Characteristics and Failure Mechanisms for Textile Spread Tow Thin Ply Thermoplastic Composites under Low-velocity Impact

Carbon composites are deemed suitable for components with complex geometries which require high impact resistance. The broad range of industrial applications requires composite structures to be lighter and not to compromise with their mechanical performance. Non-crimp fabrics with lower fibre areal weight (FAW) are the class of reinforcement material which will only bring the advantages of better mechanical properties but will also offer more longevity to the structures and reduced maintenance costs. This research aims at investigating the low-velocity impact performance of textile spread tow thin ply (100 gsm) composites with an aim to study the load bearing capability, deflection characteristics and energy characteristics. The results are compared with the baseline thick (200 gsm) fibre reinforced composites. There was 19.2 %, 16.6 %, and 6.57 % higher peak load for spread tow thin ply composites when compared to thick ply composites at 25 J., 42 J., and 52 J impact energies respectively. Significantly lower residual deflection (40 % to 76 %) and higher major damage energy (20 % to 33 %) were observed for spread tow thin ply laminates compared to the thick ply variant at different impact energies. At 42 J and 52 J impact energies, the damage index (DI) was 2 times and 4 times higher for Thick ply laminates highlighting extensive damage, which is also observed with detailed failure mechanisms study.