Unidirectional Composite Laminates Research Articles

Characterization of multidirectional carbon-nanotube-yarn/bismaleimide laminates under tensile loading

Unidirectional, cross-ply, and quasi-isotropic composite laminates are made from continuous carbon nanotube (CNT) yarns with bismaleimide (BMI) resin. The laminates are highly graphitic and have low resin content. Elastic modulus and strength of CNT/BMI laminates and IM7/8552 carbon-epoxy laminates are measured using a scaled-down tensile test method. For CNT/BMI laminates, the variation in the measured tensile modulus is high and the laminates fail in a more gradual manner than IM7/8552 laminates. Microscopy of the failed specimens indicates that intra-yarn splitting is a common feature in all CNT/BMI laminates tested. The results of this investigation will inform the development of CNT yarn reinforced composites for structural applications.

Multiaxial failure behavior and fatigue life prediction of unidirectional composite laminates

Complex coupling mechanisms exist between longitudinal and shear stress on the multiaxial fatigue behavior of composites. A thorough reanalysis of matrix shear failure mechanism under multiaxial fatigue loading is presented. Longitudinal single splitting fracture were analyzed and validated as an independent failure mode under multiaxial loading by analogy to the failure mechanism of natural composites. A new multiaxial fatigue life prediction model was established by double projection on the critical and subcritical fracture planes. Experiments on unidirectional tubular specimens were carried out under combined tension-torsion loadings to separate and identify the multiaxial fatigue failure behavior of composites. The proposed model has been verified using unidirectional laminates subjected to cyclic tension-torsion and off-axis loading, and the results showed that the predicted life is in good agreement with the experimental ones. Applications of three-dimensional digital image correlation (3D-DIC) and microscopy regarding verification in multiaxial damage mechanism are also addressed.

Detailed Finite Element Models for the Simulation of the Laser Shock Wave Response of 3D Woven Composites

This paper presents a numerical study on the laser shock wave propagation in a 3D woven carbon-fiber-reinforced polymer (CFRP) material by means of detailed and homogenized finite element (FE) models. The aim of this study is to numerically characterize the shock wave response of the 3D woven CFRP in terms of back-face velocity profiles and the induced damage, and to investigate whether the detailed FE models could be effectively replaced by homogenized FE models. The 3D woven geometry was designed using the TexGen 3.13.1 software, while the numerical analyses were executed using the R11.0.0 LS-Dyna explicit FE software. A high-strain-rate behavior was considered for the matrix. The fiber bundles in the detailed models were modeled as a high-fiber-content unidirectional composite laminate, with its mechanical properties calculated by micromechanical equations. A progressive damage material model was applied to both the fiber bundles of the detailed model and the homogenized models. The results of the detailed model reveal a considerable effect of the material’s architecture on the shock wave propagation and sensitivity of the back-face velocity profile to the spot location. Consequently, the homogenized model is not capable of accurately simulating the shock wave response of the 3D woven composite. Moreover, the detailed model predicts matrix cracking in the resin-rich areas and in the bundles with high accuracy, as well as fiber failure. On the contrary, the homogenized model predicts matrix cracking in the same areas and no fiber failure.

Investigation on interlayer debonding behavior of unidirectional composite laminates under cyclical hygrothermal aging duration effects

AbstractThe composite interlamination is the key part of load transfer and plays a decisive role in the overall mechanical performance of composite structures. The interlaminar property is predominantly influenced by the matrix resin which is easily affected by the hygrothermal environment. This paper delved into the effect of hygrothermal aging duration on the interlayer debonding behavior of unidirectional (UD) laminates. The pre‐cracked specimens were treated by hygrothermal aging with a period of 0, 15, 30, 60, 90, and 120 days. The double cantilever beam (DCB) tests were employed to evaluate the interlaminar property. The effect of hygrothermal aging on load–displacement responses, R‐curves, crack growth speed, and fracture surface topography was analyzed. The results revealed that the load‐bearing capacity of aged specimens presented a reduction of 18.6%–29.0% compared with unaged ones. The interlaminar fracture toughness of specimens subjected to 15 days of treatment decreased significantly compared with that of unaged specimens due to the increase of water absorption caused by hygrothermal aging. With the increase in hygrothermal aging time, the interlaminar fracture toughness peaked at 60 days because of the post‐curing effect, followed by a declining trend influenced by the dominant hygrothermal aging effect. The fiber bridging of aged samples was increasingly obvious, became the most prominent at 60 days, and then gradually reduced. The fiber bridging diminished the crack propagation speed, hindered interlaminar debonding, and contributed to the interlaminar toughening.Highlights The samples underwent hygrothermal aging for 0, 15, 30, 60, 90, and 120 days. Load ability of aged samples was at least 18.6% lower than that of unaged ones. R‐curves rose before the crack length of 79 mm and then tended to stabilize. Fracture toughness first decreased, then increased and reduced with aging time. Fiber bridging could hinder crack growth and improve the interlayer property.

Analysis of transverse thermal conductivity for mesophase pitch-based carbon fibers and the through-thickness heat conduction of their composites

A quantitative analysis method for the transverse thermal conductivity (TC) of carbon fiber is developed, which consists of three steps including TC and morphology characterization of unidirectional composite laminate, fiber contour extraction, and finite element inverse analysis. Two different pitch-based carbon fibers with folded-radial and onion-skin microstructure are characterized, and the influences of fiber volume fraction and microstructure on the heat conduction of their composites are investigated. The equivalent transverse TCs of TC–HC–800 and PCF-1 carbon fibers are measured to be 9.27 and 2.87 W m−1 K−1, respectively. The through-thickness TC of unidirectional composite exhibits rapid growth with the increase in fiber volume fraction. The finite element analysis reveals that more continuous heat conduction paths are formed with the increase in fiber volume fraction. Benefited from the bigger graphitization degree, larger cross-sectional area, and bigger aspect ratio, TC–HC–800 unidirectional composite shows higher through-thickness TC than PCF-1 composite at the same fiber volume fraction.

Finite-volume micromechanics-based multiscale analysis of composite structural model accounting for elastoplastic and ductile damage mechanisms

A hierarchical homogenization framework is proposed for nonlinear elastic-plastic and damage simulation of a unidirectional composite laminate. Proper scale transition rules between the macro- and the micro-scale variables were introduced, wherein the finite-element and finite-volume methods are employed for predicting the structural and material point response, respectively. Specifically, at the macroscopic scale, the ABAQUS/CAE structural analysis provides macroscopic strains for the lower-scale unit cell simulation (top-down) at each load increment. Reversely, at the microscopic scale, the unit cell problem is modelled using extended parametric finite-volume direct averaging micromechanics (FVDAM) through a Meta-UMAT subroutine that links the global finite element solver and the constitutive laws (UMAT subroutines). Homogenization of stresses and effective stiffness tensor obtained from the unit cell predictions is passed back to the ABAQUS/CAE for the higher-scale finite-element structural model computation (bottom-up). The stresses and strains at the global and local scales can be tracked during the incremental loading on the macroscopic structures through continuous homogenization and localization. The predictive capabilities of the proposed technique are verified vis-à-vis the experimental response of the Boron/Aluminum (B/Al) laminate obtained from off-axis tensile and shear loading available in the literature.

Mode I fracture toughness with fiber bridging of unidirectional composite laminates under cryogenic temperature

Carbon fiber reinforced polymer (CFRP) composites have widely been used in the aerospace field owing to their excellent mechanical properties. However, composite structures often fail under complex space environmental conditions. In particular, the fiber bridging and cryogenic temperature may greatly change the fracture toughness of CFRP composites. Herein, the effects of cryogenic temperature and fiber bridging on the fracture toughness of unidirectional composite laminates were experimentally examined by double cantilever beam (DCB) testing at different temperatures (room temperature, −50 °C, −70 °C, −100 °C, −150 °C, and −180 °C). Different calculation methods of fracture toughness were used and the results were compared. A method without measuring the delamination length at low temperatures was used and the results were consistent with other methods. The average difference in fracture toughness values was less than 5 %. The fiber-bridging microstructures of composites under cryogenic temperatures were examined by confocal microscopy along with the geometric morphology and parameters of surface roughness. The results suggested changes in the matrix failure mechanism at −50 °C, resulting in maximum fracture toughness, which is 26.45 % higher than that at room temperature. A semi-analytic approach was then proposed to express the bridge stress, and a bilinear cohesive zone model was established to simulate the delamination of composite laminates at low temperatures. Overall, the numerical data agreed well with the experimental results (peak force maximum error of 17 %), suggesting the usefulness of the proposed method for the prediction of interlamination properties and structural damage design of composite structures at cryogenic temperature.

Investigating the transient angle in a unidirectional composite in tension and compression

In unidirectional composite laminates, when there is a slight deviation between fiber and load-applying direction, the failure mode in tensile and compression will be changed from fiber breakage and buckling, respectively, and will be turned to matrix cracking. The transient angle is the angle of misalignment between fiber and load-applying direction, in which the strength properties of the composite will sharply and considerably decrease due to the change of failure mode. In this paper, the transient angle has been investigated experimentally in fiber-reinforced glass epoxy unidirectional composites. To determine the transient angle, certain unidirectional composite specimens with specific digression angles from the load-applying direction were manufactured due to standards, and tensile and compression tests were performed on them. Results demonstrate that the tensile transient angle is between 4° and 8°, and the compressive transient angle is between 8° and 12°. Moreover, experimental results were compared with analytical calculations using Hashin’s failure criteria. Investigations showed that Hashin failure criteria can adequately predict the amount of load causing failure in unidirectional composite laminates before the transient angle. Still, by approaching this angle, Hashin’s failure criteria can’t correctly predict the load causing failure, and error will increase gradually as it passes the transient angle. This error will occur after the transient angle because Hashin’s Failure Criteria can’t predict changes in failure modes in unidirectional composite laminates.

Gamma-ray irradiation to achieve high tensile performance of unidirectional CNT yarn laminates

Continuous carbon nanotube (CNT) yarn fabricated from a floating catalyst chemical vapor deposition (FCCVD) method is treated under gamma-ray irradiation to enhance the mechanical properties of the CNT yarn and its unidirectional composite laminates. Gammy-ray doses varying from 50 kGy to 1200 kGy are used to irradiate CNT yarns and their microstructures, tensile properties and surface characterizations are studied. The graphitic structure change is not clear from the transmission electron microscopy, however, the specific tensile strength and modulus of yarn vary slightly within 10 % as the dose increased. This modulus trend coincides with mesoscopic distinct element modeling (mDEM) simulation results. Surface characterization shows additional oxygen functional groups and smaller contact angles after irradiation. Interestingly, the specific tensile properties of composite laminates also increase relative to the yarns, and the unidirectional laminate from CNT yarn treated with the optimal dose of 700 kGy achieves specific strength and modulus as high as 1.89 GPa/gcm−3 and 258 GPa/gcm−3, respectively, which are 30.9 % and 37 % increases compared to the control laminate. The results indicate that radiation-induced crosslinking among the CNTs and the formation of surface-active sites leads to enhanced load transfer in the yarns and promote CNT/resin interfacial bonding.

Application of the effective crack length method to model delamination of unidirectional composite laminates under Mode II shear loadings

Damage modelling of composite material delamination is an intense field of research to understand the complex composite failure behaviour and predict the residual strength of damaged structures. One of the widely employed methods to simulate delamination is the cohesive zone modelling (CZM) technique. To successfully utilize this approach, accurate characterisation of the interlaminar fracture toughness is crucial, while the composite delamination is dominated by Mode I and Mode II fracture in most cases. Numerous studies have been conducted on composite materials delamination under Mode I loadings using double cantilever beam (DCB) tests. Accordingly, the ASTM standard testing procedure and data reduction scheme to obtain Mode I fracture toughness (GIc) have been well established and widely accepted. However, it is still challenging to characterize the composite Mode II delamination resistance due to the susceptibility problems inherent to the existing testing methods and the lack of robust data reduction schemes to accurately identify the initial crack tip and monitor the crack growth. This study attempts to find a reliable solution to obtain the Mode II fracture toughness (GIIC) and an effective modelling strategy to simulate Mode II delamination in laminated composites. First, we reviewed the existing testing set-ups and surveyed the data reduction schemes conventionally used to obtain GIIC. The advantages and drawbacks of the three most used test methods, and particularly the standard end-notched flexure (ENF) test (ASTM D7905/D7905M) and the end-loaded split (ELS) test (ISO 15114:2014) were examined. Second, the advantages of the effective-crack-length-based data reduction schemes against the classical data reduction schemes were empirically studied with the ENF tests conducted on G40-800/5276-1 carbon-fibre reinforced composite laminate coupons. Although many studies have studied the accuracy of the effective-crack-length-based approaches from different aspects, there is a lack of direct comparison between the numerical results and experimental data. Our modelling study demonstrates that among the five data reduction schemes that are examined, the compliance calibration method (CCM) required by ASTM D7905 yields the most conservative GIIC values, which is suitable for establishing material property for a design purpose. The compliance-based beam method (CBBM) may generate the least conservative GIIC values, which are deemed to be the most accurate ones for modelling and simulation validation purposes. Finite element (FE) modelling of the ENF tests using the CZM technique was carried out and comprehensive parametric studies were conducted to achieve an efficient and robust strategy for delamination modelling of composite laminates under Mode II shear loadings.

Specimen designs for accurate tensile testing of unidirectional composite laminates

Tensile testing of unidirectional composites based on standardized test methods nearly always leads to premature failure in the end tab region. The material near the tab section is under a complex loading state, including longitudinal, transverse, and shear stress components. The present work examines different conventional designs for tensile testing along with novel ones to find the method that minimizes the geometric discontinuity and yields the highest failure strain. Finite element (FE) models are utilized to predict the stress concentrations for all end tab designs with representation of the actual grip components. The simulation reveals that the stress concentration near the edge of the end tabs can vary significantly with end tab design. FE models are used to further optimize the geometry of end tabs. The experimental results reveal that specimens with novel arrow-shaped tabs and continuous tabs yield the highest failure strain, and hence are best at avoiding premature failure.

Analysis of unidirectional polyethylene-glass fiber-reinforced phenolic polymer composite laminate

Analysis of unidirectional polyethylene-glass fiber-reinforced phenolic polymer composite laminate

Automated woven background removal for enhanced infrared thermographic inspection of fabric composites

Infrared thermography is a promising non-destructive testing methodology for the inspection of composite materials. While there are many accounts in literature that report an excellent performance on unidirectional composite laminates, this is not the case for woven fabric composites. Their inspection is typically characterised by the presence of an imprint of the weave pattern in the thermographic recordings, which is most-often found to be disturbing for reliable anomaly detection. In this contribution, it is proposed to evaluate the thermographic data in ????-space in order to separate the weave pattern from the defect information. To this end, an algorithm is introduced to automatically perform this decomposition. The efficiency of the proposed methodology is demonstrated for several weave patterns with different defect cases.

On temperature-dependent fiber bridging in mode I delamination of unidirectional composite laminates

On temperature-dependent fiber bridging in mode I delamination of unidirectional composite laminates

Scalable High Tensile Modulus Composite Laminates Using Continuous Carbon Nanotube Yarns for Aerospace Applications.

An approach is established for fabricating high-strength and high-stiffness composite laminates with continuous carbon nanotube (CNT) yarns for scaled-up mechanical tests and potential aerospace structure applications. Continuous CNT yarns with up to 80% degree of nanotube alignment and a unique self-assembled graphitic CNT packing result in their specific tensile strengths of 1.77 ± 0.07 N/tex and an apparent specific modulus of 92.6 ± 3.2 N/tex. Unidirectional CNT yarn reinforced composite laminates with a CNT concentration of greater than 80 wt % and minimal microscale voids are fabricated using filament winding and aerospace-grade resin matrices. A specific tensile strength of up to 1.71 GPa/(g cm-3) and specific modulus of 256 GPa/(g cm-3) are realized; the specific modulus exceeds current state-of-the-art unidirectional carbon fiber composite laminates. The specific modulus of the laminates is 2.76 times greater than the specific modulus of the constituent CNT yarns, a phenomenon not observed in carbon fiber reinforced composites. The results demonstrate an effective approach for fabricating high-strength CNT yarns into composites for applications that require specific tensile modulus properties that are significantly beyond state-of-the-art carbon fiber composites and potentially open an unexplored performance region in the Ashby chart for composite material applications.

Study of Hybrid Composite Joints with Thin-Ply-Reinforced Adherends.

It has been demonstrated that a possible solution to reducing delamination in a unidirectional composite laminate lies in the replacement of conventional carbon-fibre-reinforced polymer layers with optimized thin-ply layers, thus creating hybrid laminates. This leads to an increase in the transverse tensile strength of the hybrid composite laminate. This study investigates the performance of a hybrid composite laminate reinforced by thin plies used as adherends in bonded single lap joints. Two different composites with the commercial references Texipreg HS 160 T700 and NTPT-TP415 were used as the conventional composite and thin-ply material, respectively. Three configurations were considered in this study: two reference single lap joints with a conventional composite or thin ply used as the adherends and a hybrid single lap. The joints were quasi-statically loaded and recorded with a high-speed camera, allowing for the determination of damage initiation sites. Numerical models of the joints were also created, allowing for a better understanding of the underlying failure mechanisms and the identification of the damage initiation sites. The results show a significant increase in tensile strength for the hybrid joints compared to the conventional ones as a result of changes in the damage initiation sites and the level of delamination present in the joint.

A new damage-based failure criterion for nonlinear behavior of fibrous composite materials

In the present paper, a novel combined damage-based failure criterion is being proposed for predicting failure stresses in unidirectional fibrous composite laminas or laminates having a nonlinear material behavior. The present model incorporates the effect of a quantitative damage factor on the final stresses at failure. This is achieved through a new term called the quantitative directional damage-index (QDD-I) which assesses the contribution and effectiveness of damage in each principal material direction on the present failure criterion. From the QDD-I, it is proved that the principal material-direction with a linear or nonlinear stress-strain behavior showed a quantitative damage response on the proposed failure criterion. In a composite lamina, the contribution of fiber-damage and matrix transverse-damage are proved to have minor effects on the failure criterion, while in-plane shear-damage has the major effect. In order to verify the suitability and applicability of the criterion, results are tested using various theoretical and experimental data available from the literature. Furthermore, the model is compared with other failure criteria under both uniaxial and biaxial loading cases from a worldwide comparison, which showed reasonable accuracy and good agreement. Three types of fibrous composite materials are used; Graphite/Epoxy 4617/Modmore-II, Carbon/Epoxy AS4/3501-6, and Boron/Epoxy Narmco 5505.

Research on Mechanical Properties of Domestic T700S Carbon Fibers

Two types of domestic T700S carbon fibers (CF1 and CF2) were investigated in three batches to find out their mechanical properties, including tensile properties of multifilament, tensile and bending properties of composites. The results showed that the mechanical properties of domestic T700S CF are comparable to those of Toray T700S CF with a small discrete coefficient obtained from experiments for the three batches of CF, and that the unidirectional CF2 composite laminates has the biggest tensile strength discrete coefficient, 3.84%. The difference of properties between the two types of fiber composites was analyzed using the optical contact angle measuring device. It was found that CF2 and resin have better wettability. The two types of carbon fibers were examined by the Ø150 mm standard pressure vessel. Temperature (high temperature, low temperature, and temperature impact) tests and aging tests revealed that: high temperature, low temperature, and temperature impact have little effect on the characteristic index of the pressure vessel; after 800h aging, the characteristic index of the pressure vessel decreased by 15.2%.

The torsional rigidity of an orthotropic bar of unidirectional composite laminate part I: Theoretical solution

In 1934, Timoshenko derived and published the torsional rigidity of a rectangular bar of isotropic material by using the membrane analogy. The rigidity depends on the bar’s shear modulus, width and thickness. In 1950, Lekhnitskii followed Timoshenko’s process to derive the torsional rigidity for an orthotropic bar of unidirectional composite laminate, where the rigidity depends on the laminate’s width, the thickness and the shear moduli. In 1990, Tsai, Daniel and Yaniv solved the same case, deriving a quasi-exact solution of torsional rigidity. In the same year, Tsai and Daniel verified their result through multiple experiments. All these rigidities become different when the definitions of thickness and width are swapped. However, they remain identical numerically, lacking mathematic proof for nearly a century. In 2022, Tsai et al. resolved Timoshenko’s case by considering all the conditions and energy minimization criterion. By a completely different approach, the torsional rigidity was derived in a completely different form from that of Timoshenko but numerically identical. Moreover, the solution satisfies the rule of swapping, which makes perfect sense physically. This work applies Tsai’s process to Lekhnitskii’s case. A general solution is derived that satisfies the rule of swapping involving the swapping of shear moduli.

Effect of Aging on Unidirectional Composite Laminate Polyethylene for Body Armor.

The construction of ballistic-resistant body armor is experiencing an increasing use of flexible unidirectional (UD) composite laminates that comprise multiple layers. Each UD layer contains hexagonally packed high-performance fibers with a very low modulus matrix (sometimes referred to as binder resins). Laminates are then made from orthogonal stacks of these layers, and these laminate-based armor packages offer significant performance advantages over standard woven materials. When designing any armor system, the long-term reliability of the armor materials is critical, particularly with regard to stability with exposure to temperature and humidity, as these are known causes of degradation in commonly used body armor materials. To better inform future armor designers, this work investigates the tensile behavior of an ultra-high molar mass polyethylene (UHMMPE) flexible UD laminate that was aged for at least 350 d at two accelerated conditions: 70 °C at 76% relative humidity (RH) and 70 °C in a desiccator. Tensile tests were performed at two different loading rates. The mechanical properties of the material after ageing demonstrated less than 10% degradation in tensile strength, indicating high reliability for armor made from this material.