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Pores are common defects generated during fabrication, which restrict the application of carbon/carbon (C/C) composites. To quantitatively understand the effects of pores on mechanical strength, this paper proposes a representative volume element model of unidirectional (UD) C/C composites based on the finite element method. The Hashin criterion and exponential degraded rule are used as the failure initiation and evolution of pyrolytic carbon matrices, respectively. Interfacial zones are characterized using the cohesive constitutive. At the same time, periodic boundary conditions are employed to study transverse tensile, compressive, and shear deformations of UD C/C composites. Predicted results are compared with the experimental results, which shows that the proposed model can effectively simulate the transverse mechanical behaviors of UD C/C composites. Based on this model, the effects of microstructural parameters including porosity, pore locations, the distance between two pores, pore clustering, and pore shapes on the mechanical strength are investigated. The results show that porosity markedly reduces the strength as porosity increases. When the porosity increases from 4.59% to 12.5%, the transverse tensile, compressive, and shear strengths decrease by 35.91%, 37.52%, and 30.76%, respectively. Pore locations, the distance between two pores, and pore clustering have little effect on the shear strength of UD C/C composites. For pore shapes, irregular pores more easily lead to stress concentration and matrix failure, which greatly depresses the bearing capacity of UD C/C composites. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Pores are common defects generated during fabrication, which restrict the application of carbon/carbon (C/C) composites. To quantitatively understand the effects of pores on mechanical strength, this paper proposes a representative volume element model of unidirectional (UD) C/C composites based on the finite element method. The Hashin criterion and exponential degraded rule are used as the failure initiation and evolution of pyrolytic carbon matrices, respectively. Interfacial zones are characterized using the cohesive constitutive. At the same time, periodic boundary conditions are employed to study transverse tensile, compressive, and shear deformations of UD C/C composites. Predicted results are compared with the experimental results, which shows that the proposed model can effectively simulate the transverse mechanical behaviors of UD C/C composites. Based on this model, the effects of microstructural parameters including porosity, pore locations, the distance between two pores, pore clustering, and pore shapes on the mechanical strength are investigated. The results show that porosity markedly reduces the strength as porosity increases. When the porosity increases from 4.59% to 12.5%, the transverse tensile, compressive, and shear strengths decrease by 35.91%, 37.52%, and 30.76%, respectively. Pore locations, the distance between two pores, and pore clustering have little effect on the shear strength of UD C/C composites. For pore shapes, irregular pores more easily lead to stress concentration and matrix failure, which greatly depresses the bearing capacity of UD C/C composites.

Despite recent advancements in using machine learning (ML) techniques to establish the microstructure-property linkage for composites’ representative volume elements (RVEs), challenges persist in effectively characterising the effect of microstructural randomness on material properties. This complexity arises from the difficulty of expressing randomness as definitive variables and its intertwined relations with other factors, such as material constituents. Such complexities result in limitations in generalising ML models across different material constituents. Conventional solutions to these challenges usually necessitate large datasets, which require considerable computational resources, for an accurate and generalisable ML models to be trained. This paper presents an innovative approach to tackling these challenges by integrating a high-accuracy convolutional neural network (CNN) with a novel microstructure-factored constitutive model (MCM). The MCM, rooted from classic empirical constitutive modelling, effectively segregates the microstructural and constituting material effects, extending the generalisability and thus significantly enhancing the efficacy of the CNN. This new approach enabled a CNN trained on the transverse stress-strain curves of one set of material constituents (CF/PEEK at 270 °C) to be generalised for the rapid prediction of various sets of material constituents at different temperatures, unseen by the CNN during training, with an average mean absolute percentage error around 3 %.

In this study, we compare the reinforcing efficiency of pineapple leaf fiber (PALF) and cultivated flax fiber in unidirectional poly(butylene succinate) composites. Flax, known for robust mechanical properties, is contrasted with PALF, a less studied but potentially sustainable alternative. Short fibers (6 mm) were incorporated at 10 and 20% wt. levels. After two-roll mill mixing, uniaxially aligned prepreg sheets were compression molded into composites. At 10 wt.%, PALF and flax exhibited virtually the same stress–strain curve. Interestingly, PALF excelled at 20 wt.%, defying its inherently lower tensile properties compared to flax. PALF/PBS reached 70.7 MPa flexural strength, 2.0 GPa flexural modulus, and 107.3 °C heat distortion temperature. Comparable values for flax/PBS were 57.8 MPa, 1.7 GPa, and 103.7 °C. X-ray pole figures indicated similar matrix orientations in both composites. An analysis of extracted fibers revealed differences in breakage behavior. This study highlights the potential of PALF as a sustainable reinforcement option. Encouraging the use of PALF in high-performance bio-composites aligns with environmental goals. [ABSTRACT FROM AUTHOR]

The Coulomb friction law is incorporated into the traction separation law, so a friction cohesive zone model (FCZM) combining sliding friction, interfacial debonding, and the coupling of the debonding and friction is developed. The proposed interface model is used to describe the interfacial mechanical responses of fiber/resin composite. To verify this model, the finite element model of microdroplet is built, then the calculated results of the finite element model are compared with the microdroplet test. To analyze the effect of interface friction on the macroscopic strength and stiffness of unidirectional composites, the FCZM is applied to representative volume element (RVE), and the mechanical behaviors of composites is simulated by finite element analysis of RVE, then the results are compared with the test in reference [1, 2]. It is found that the interfacial debonding is accelerated and the macroscopic strength is reduced due to the friction. [ABSTRACT FROM AUTHOR]

Unidirectional (UD) composites with 0° fibre orientation have high axial strength along the direction of fibres. Hence, the axial compressive testing of 0° UD composites is extremely difficult due to associated issues like misalignment, buckling, non-uniform gripping and non-uniform stress distribution. These issues can intensify during fatigue testing, especially at higher load levels. The data available in the literature on the compressive response of unidirectional fibre reinforced composites is scarce. Many researchers have attempted designing a standard testing fixture for compressive testing of the fibre reinforced polymers (FRPs). Two most common fixtures are Boeing-modified ASTM D695 end loading fixture and ASTM D6641 combined loading compression (CLC) fixture. However, these fixtures are only feasible for compressive testing of the rectangular composite laminates. There is no standard fixture for compressive testing of cylindrical pultruded composites. This paper presents a methodology for testing the compressive strength of pultruded cylindrical composites. A fixture is designed and fabricated to conduct the tests. Different misalignment, gripping and stress concentration issues that could occur during the compression testing are addressed and a corresponding solution is provided. Unidirectional (UD) carbon/epoxy pultruded rods are used as test specimens. The test data of 21 specimens is presented and the failure modes are analysed using scanning electron microscope (SEM) images. [ABSTRACT FROM AUTHOR]

A representative volume element (RVE) with 11 uniform fiber packings is studied. The effective modulus of RVE corresponding to the different packing types is evaluated using the finite-element modelling and a semianalytical multistep rule of mixtures. The equivalent modulus of the RVE depends on the packing type. The elongated triangular packing type was closest to the random packing type. All the packing types displayed a strong transverse isotropy with less than a 1% variation in equivalent modulus in the two transverse directions. The multistep rule of mixtures could allow for changes in the moduli due to the different packing types. However, this method consistently underpredicted the equivalent modulus compared with full FE results. The effective modulus for the random packing closest to that of the elongated triangular packing type. The stress distribution in the interface varied strongly with packing type and loading direction. The sparsest packing type, i.e., the truncated hexagonal one led to both the highest equivalent modulus and the highest stress concentrations in the interface. The densest packing type, i.e., the triangular packing, led to the lowest modulus and lowest radial stress concentrations. [ABSTRACT FROM AUTHOR]

This research delves into the failure mechanisms exhibited by unidirectional Carbon Fiber Reinforced Polymer (CFRP) composites under longitudinal compression, while considering the interplay of three types of manufacturing-induced defects: initial waviness of fibres, void volume fraction, and void size. The study employs 3D high-fidelity intact representative volume element (RVE) models, incorporating the initial waviness of fibres and voids based on Micro-CT imaging. A novel algorithm is proposed to generate more accurate 3D void shapes, departing from conventional circular or triangular approximations. The results highlight the substantial influence of the initial waviness angle on the reduction of predicted compressive stiffness and strength. The volume and size of voids play a significant role in determining damage initiation within the composites. The failure mechanisms of the composite under the coupled effects of initial waviness of fibres and voids are discussed, exhibiting reasonable agreement with experimental observations. [ABSTRACT FROM AUTHOR]

In this study, we compare the reinforcing efficiency of pineapple leaf fiber (PALF) and cultivated flax fiber in unidirectional poly(butylene succinate) composites. Flax, known for robust mechanical properties, is contrasted with PALF, a less studied but potentially sustainable alternative. Short fibers (6 mm) were incorporated at 10 and 20% wt. levels. After two-roll mill mixing, uniaxially aligned prepreg sheets were compression molded into composites. At 10 wt.%, PALF and flax exhibited virtually the same stress–strain curve. Interestingly, PALF excelled at 20 wt.%, defying its inherently lower tensile properties compared to flax. PALF/PBS reached 70.7 MPa flexural strength, 2.0 GPa flexural modulus, and 107.3 °C heat distortion temperature. Comparable values for flax/PBS were 57.8 MPa, 1.7 GPa, and 103.7 °C. X-ray pole figures indicated similar matrix orientations in both composites. An analysis of extracted fibers revealed differences in breakage behavior. This study highlights the potential of PALF as a sustainable reinforcement option. Encouraging the use of PALF in high-performance bio-composites aligns with environmental goals.

The mechanical properties of composites are traditionally measured using numerical and experimental approaches, which impede the innovation of materials due to the cost, time, or effort involved. This study for the first time develops a machine learning-assisted model, which aims at predicting the transverse mechanical properties of unidirectional (UD) carbon fiber reinforced polymer (CFRP) composites with microvoids. To this end, the stochastic microstructures are generated by random sequential expansion (RSE) and hard-core model. And the transverse elastic modulus, transverse tensile strength and transverse compressive strength are computed by micromechanics-based finite element (FE) method. Then, the reduced order representation of microstructures is determined using 2-point spatial correlations and principal component analysis (PCA). Finally, a genetic algorithm (GA) optimized back propagation (BP) neural network is implemented to capture the potential nonlinear relationship between microstructure and transverse mechanical properties. The presented data-driven techniques can reproduce the FE simulation results with an R-value of 0.89 or greater. The excellent agreement between the predicted results and test datasets verifies the successful application of data science methodologies in elucidating the microstructure-property linkages of UD-CFRP composites with microvoids, and thus provides a promising tool for accelerating the smart design and optimization of composites.

Composite materials based on magnesium–lithium (MgLi) and magnesium–yttrium (MgY) matrices reinforced with unidirectional carbon fibers were prepared using the gas pressure infiltration method. Two types of carbon fibers were used, high-strength PAN-based T300 fibers and high-modulus pitch-based Granoc fibers. The PAN-based carbon fibers have an internal turbostratic structure composed of crystallites. The pitch-based carbon fibers have a longitudinally aligned graphite crystal structure. The internal carbon fiber structure is crucial in the context of the interfacial reaction with the alloying element. There are various mechanisms of bonding to carbon fibers in the case of magnesium–lithium and magnesium–yttrium alloys. This paper presents the use of the DMA method for the characterization of the role of alloying elements in the quality of interfacial bonding and the influence on the complex modulus at increasingly elevated temperatures (50–250 °C). The complex modulus values of the composites with T300 fibers were in the range of 118–136 GPa. The complex modulus values of the composites with Granoc fibers were in the range of 198–236 GPa. The damping capacity of magnesium-based unidirectionally aligned carbon fiber composites is related to the quality of the interfacial bonding. [ABSTRACT FROM AUTHOR]

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume. [ABSTRACT FROM AUTHOR]

An interval-based method is presented to evaluate the uncertainty in the computed mechanical properties and the failure assessment of composite unidirectional (UD) laminates. The method was applied to two composite laminates: a carbon/epoxy and a glass/epoxy. The mechanical properties of the UD lamina were derived using simplified micromechanical equations. An uncertainty level of ±5% was assumed for the input properties of the constituents. The global minimum and maximum values of the properties were computed using an interval branch-and-bound algorithm. Interval arithmetic operations were used to evaluate the uncertainty in the Hashin-type failure criteria in a closed form. Using the closed-form uncertainties of intervals and sets of stresses obtained by finite element analysis, the uncertainty in the failure assessment was quantified for the two composite laminates. For the assumed uncertainty level of ±5%, the computed uncertainty for the mechanical properties ranges from 6.64% to 10.63% for the carbon/epoxy material and from 6.72% to 12.28% for the glass/epoxy material. For evaluating the uncertainty effect on the efficiency of failure criteria, a probability of failure function, which employs interval boundaries, was defined and proved capable of evaluating the whole spectrum of stresses. [ABSTRACT FROM AUTHOR]

In this article, mode II fracture toughness (GIIc{G}_{\text{IIc}}) of unidirectional E-glass/vinyl ester composites subjected to sulfuric acid aging is studied at two different temperatures (25 and 90°C). Specimens were manufactured using the hand lay-up method with the [0]20{{[}0]}_{20} stacking sequence. To study the effects of environmental conditions, samples were exposed to 30 wt% sulfuric acid at room temperature (25°C) for 0, 1, 2, 4, and 8 weeks. Some samples were also placed in the same solution but at 90°C and for 3, 6, 9, and 12 days to determine the interlaminar fracture toughness at different aging conditions. Fracture tests were conducted using end notched flexure (ENF) specimens according to ASTM D7905. The results obtained at 25°C showed that mode II fracture toughness increases for the first 2 weeks of aging and then it decreases for the last 8 weeks. It was also found that the flexural modulus changes with the same trend. Based on the results of the specimens aged at 90°C, a sharp drop in fracture toughness and flexural modulus with a significant decrease in maximum load have been observed due to the aging. Finite element simulations were performed using the cohesive zone model (CZM) to predict the global response of the tested beams.

Laser-assisted thermoplastic composite automated placement method has been applied to the molding of composites due to its advantages of fast molding speed and high heating efficiency. This study focuses on designing a robot for laser-assisted thermoplastic composite automated fiber placement, employing a modular approach to create a versatile fiber placement head. Control functions, such as electrical circuits, a computer interface, and tension control, were developed. Hybrid laminates, combining GF/PP unidirectional and braided composites, were manufactured by independently adjusting laser power (275 W–400 W) and compression force (150 N–400 N). Bonding properties were analyzed using wedge peel tests and microscopic detection, revealing superior toughness in laser-assisted laminates compared to autoclave counterparts. Altering laser power and compression force impacted polypropylene resin distribution, influencing hybrid interlayer peel strength. The inclusion of braided materials and elevated mold temperature proved effective in minimizing lamination warpage in GF/PP laminates. • The modular laser-assisted head for automated fiber placement is designed for rapid bonding of thermoplastic composites. • The peel strength of hybrid laminates made by laser-assisted placement was superior to those made by autoclave, which had better toughness. • Adding braided material increases laminate stiffness, reducing warpage during automated thermoplastic composite placement. [ABSTRACT FROM AUTHOR]

Replacing weaving and stitching used in textile-based reinforcements by using short fibers as a binder was discussed based on the tensile and compressive properties of [0], [± 45], and [0/90] laminates. A slight decrease in the longitudinal properties of the [0] composites was accompanied by a significant reduction of the properties of variability. In the case of [± 45] laminates, the presence of the short flax fiber shows a positive effect on the tensile, shear, and compressive properties. The mechanical properties of [0/90] laminates have shown a limited effect of the short fibers, with a slight decrease of both modulus and strength.

Composite materials based on magnesium–lithium (MgLi) and magnesium–yttrium (MgY) matrices reinforced with unidirectional carbon fibers were prepared using the gas pressure infiltration method. Two types of carbon fibers were used, high-strength PAN-based T300 fibers and high-modulus pitch-based Granoc fibers. The PAN-based carbon fibers have an internal turbostratic structure composed of crystallites. The pitch-based carbon fibers have a longitudinally aligned graphite crystal structure. The internal carbon fiber structure is crucial in the context of the interfacial reaction with the alloying element. There are various mechanisms of bonding to carbon fibers in the case of magnesium–lithium and magnesium–yttrium alloys. This paper presents the use of the DMA method for the characterization of the role of alloying elements in the quality of interfacial bonding and the influence on the complex modulus at increasingly elevated temperatures (50–250 °C). The complex modulus values of the composites with T300 fibers were in the range of 118–136 GPa. The complex modulus values of the composites with Granoc fibers were in the range of 198–236 GPa. The damping capacity of magnesium-based unidirectionally aligned carbon fiber composites is related to the quality of the interfacial bonding.

Polymer‐matrix composites are widely used in various industries due to their high specific strength and specific stiffness. However, the void formation is inevitable as a by‐product during manufacturing processes, which may have negative effects on its mechanical properties. The purposes of this paper are to quantitatively evaluate the influence of voids on the anisotropic elastic properties of composites and to provide corresponding theoretical prediction models. Firstly, three‐dimensional representative volume elements (RVE) of fiber‐reinforced composite materials with different fiber contents (36.37%, 45.47%, 50.92%) and different void contents (1%, 3%, 5%, 7%) are established. To obtain the elastic properties in different directions, various periodic boundary conditions are applied to the RVE models and corresponding subroutines are developed by ABAQUS‐PYTHON. Secondly, a series of theoretical models are proposed to quickly predict the anisotropic elastic properties of unidirectional fiber/epoxy composites containing random‐sized void defects, which agree well with the finite element simulation results. Especially, the proposed models have concise expressions, which require only a few parameters to be input, and hence they are convenient for engineering application. Both theoretical and numerical results show that void defects have an obvious influence on the transverse modulus, major Poisson's ratio, and the out‐of‐plane shear modulus. When the void volume reaches 7%, all of the properties mentioned above decrease by more than 10% for the FRPs studied. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new algorithm to generate distinctive and similar microstructures for FRP RVEs. • A CNN for accurate properties prediction by learning random microstructures. • An MCM segregating microstructural and material effects on RVE effective properties. • Rapid prediction integrating CNN and MCM for random RVEs with various materials. Despite recent advancements in using machine learning (ML) techniques to establish the microstructure-property linkage for composites' representative volume elements (RVEs), challenges persist in effectively characterising the effect of microstructural randomness on material properties. This complexity arises from the difficulty of expressing randomness as definitive variables and its intertwined relations with other factors, such as material constituents. Such complexities result in limitations in generalising ML models across different material constituents. Conventional solutions to these challenges usually necessitate large datasets, which require considerable computational resources, for an accurate and generalisable ML models to be trained. This paper presents an innovative approach to tackling these challenges by integrating a high-accuracy convolutional neural network (CNN) with a novel microstructure-factored constitutive model (MCM). The MCM, rooted from classic empirical constitutive modelling, effectively segregates the microstructural and constituting material effects, extending the generalisability and thus significantly enhancing the efficacy of the CNN. This new approach enabled a CNN trained on the transverse stress-strain curves of one set of material constituents (CF/PEEK at 270 °C) to be generalised for the rapid prediction of various sets of material constituents at different temperatures, unseen by the CNN during training, with an average mean absolute percentage error around 3 %. [ABSTRACT FROM AUTHOR]

• Laser induced shockwaves are used for in-fiber direction characterization at high-strain rates; • Back-face velocity analysis, sound speed evaluation, and damage observations are performed; • The speed of sound along the fibers is evaluated at 10.19 km/s for HSC4 and 10.96 km/s for UTS17. • Microscopic images of the back-face revealed matrix flexural cracks and microcracks; Composite materials are of paramount interest in the aerospace industry. To evaluate their survivability when subjected to high-strain rate loading, laser-generated shockwaves are employed. Back-face velocities are analyzed for the characterization of shockwave propagation in the fiber direction of unidirectional carbon-fiber composites, and back-face microscopic inspection is used for damage observation. Challenges such as reflective surface requirements and limited sample thickness were addressed by gold-coating the back face and using thin bonded composite stripes. Experimental investigations revealed a linear relationship between back-face velocity and density of power and provided in-plane characteristics at high-strain rate regimes: microscopic inspections unveiled flexural cracks and microcracks in the matrix; speed of sound evaluation provided comparable values to ultrasonic measurements on similar composites. [ABSTRACT FROM AUTHOR]

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume.

An interval-based method is presented to evaluate the uncertainty in the computed mechanical properties and the failure assessment of composite unidirectional (UD) laminates. The method was applied to two composite laminates: a carbon/epoxy and a glass/epoxy. The mechanical properties of the UD lamina were derived using simplified micromechanical equations. An uncertainty level of ±5% was assumed for the input properties of the constituents. The global minimum and maximum values of the properties were computed using an interval branch-and-bound algorithm. Interval arithmetic operations were used to evaluate the uncertainty in the Hashin-type failure criteria in a closed form. Using the closed-form uncertainties of intervals and sets of stresses obtained by finite element analysis, the uncertainty in the failure assessment was quantified for the two composite laminates. For the assumed uncertainty level of ±5%, the computed uncertainty for the mechanical properties ranges from 6.64% to 10.63% for the carbon/epoxy material and from 6.72% to 12.28% for the glass/epoxy material. For evaluating the uncertainty effect on the efficiency of failure criteria, a probability of failure function, which employs interval boundaries, was defined and proved capable of evaluating the whole spectrum of stresses.

This paper proposes a micromechanical approach aimed at identifying the response of unidirectional fuzzy fiber composites undergoing inelastic fields. Fuzzy fibers are reinforcement fibers coated with radially aligned straight or wavy carbon nanotubes grown through chemical deposition process (PVD or CVD). Due to this nature, the composite with fuzzy fibers is described by three scales: i) the microscale consisting of carbon nanotubes and their surrounding matrix, ii) the mesoscale containing the fiber, the nanocomposite and the matrix, and iii) the macroscale related to the overall fuzzy fiber composite. The developed framework considers for the mesoscopic scale an analytical formulation, based on the composite cylinders assemblage (CCA) method, combining the principles of the Transformation Field Analysis (TFA) technique. A numerical example that includes comparisons with full field homogenization strategies confirms the accuracy of the framework to predict the overall response, as well as the average local fields of the constituents. [ABSTRACT FROM AUTHOR]

A new constant life diagram (CLD) model featuring asymmetric bilinear constant-life curves was proposed to better describe the longitudinal fatigue behavior of unidirectional laminae (UD) under a wide range of stress ratios. This model is able to predict S-N curves with satisfactory accuracy not only in tension-tension (T-T) fatigue mode but also in tensioncompression (T-C) and compression-compression (C-C) modes, whereas the conventional Goodman CLD model shows inferior performance especially in T-C and C-C modes. Apart from static tension and compression tests, high- and low-cycle fatigue tests at two stress ratios corresponding to T-T and C-C modes should be performed to determine the parameters in the proposed model. Fatigue test data of several different GFRP UDs at various stress ratios were utilized to validate the proposed model, and the S-N curves predicted by the proposed model agreed well with the experimental results. Compared with the Goodman CLD model, the proposed CLD model demonstrates an enhanced predictive capability without losing its simplicity. [ABSTRACT FROM AUTHOR]

This paper proposes a 0° tensile strength prediction model for unidirectional composites with a low interfacial strength or low matrix yield stress applied to carbon fiber reinforced polypropylene. First, a critical dimension of broken fiber cluster in the presence of splitting is formulated using the principles of fracture mechanics. Based on the geometry of fiber packing, a relation between the rate of fiber breakage and the probability distribution of the cluster dimension is derived. Lastly, the assumption that the cross-sectional area separated by splitting sustains no load is used as a discounting factor to reduce the tensile stress in the unidirectional composites and the stress–strain relation is then predicted. The discounting factor is calculated by comparing the probability distribution for the cluster dimensions estimated from the rate of fiber breakage with that of the critical cluster dimension, both of which are functions of fiber stress. When the interfacial toughness related to the critical cluster dimension is estimated from the single-fiber pull-out test and substituted into this model, the predicted tensile strengths are in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Replacing weaving and stitching used in textile-based reinforcements by using short fibers as a binder was discussed based on the tensile and compressive properties of [0], [± 45], and [0/90] laminates. A slight decrease in the longitudinal properties of the [0] composites was accompanied by a significant reduction of the properties of variability. In the case of [± 45] laminates, the presence of the short flax fiber shows a positive effect on the tensile, shear, and compressive properties. The mechanical properties of [0/90] laminates have shown a limited effect of the short fibers, with a slight decrease of both modulus and strength. [ABSTRACT FROM AUTHOR]

The longitudinal and transverse strength of three unidirectional thermoplastic prepreg systems: carbon fiber/polypropylene (CF/PP), polyamide 6 (CF/PA6), and polyphenylene sulfide (CF/PPS) are studied and analytical formulas are proposed for the estimation of matrix and fiber/matrix interface properties from composites properties. Since the matrices are semi-crystalline thermoplastics, the influence of cooling rate on the strength is statistically evaluated. While the 0º tensile strength is found to be independent of the cooling rate, the 90º tensile strength is strongly influenced by the matrix type and cooling rate. The matrix modulus increases as the cooling rate is decreased; the degree of crystallinity also increases. The matrix residual stress, interfacial shear strength, and mode II interlaminar fracture toughness are also found to depend on the cooling rate, with the trends different for different matrices. [ABSTRACT FROM AUTHOR]

The effectiveness of studying inter-laminar delamination in laminated composites with the help of thickness-stretch shell elements which utilize a 3-D material model sub-routine as compared to the traditional plane-stress shell elements has been investigated using a non-linear finite element solver (LS-DYNA®). A strain-rate-dependent micro-mechanical material model using ply-level progressive failure criteria has been used to simulate the initiation and propagation of delamination. A methodology of assigning physical significance to the choice of damage parameters has been presented. The numerical delamination growth has been qualitatively analyzed against the experimental C-scan images for multiple impact events on different composite plates. [ABSTRACT FROM AUTHOR]

A method making it possible to calculate the density and matrix volume fraction of two-component and hybrid composites reinforced with unidirectional rovings of carbon fibers (CF) and ultrahigh-molecular-weight polyethylene (UHMWPE) fibers is presented. Experimentally found constants of the rovings were used in calculation. The values of calculated density and matrix volume fraction were compared with an actual data of samples. The samples were obtained by resin impregnation of the rovings at free air. The impregnation was implemented simultaneously with laying of the rovings. This procedure does not permit saving the prespecified sample volume without volume control. The effects of volume deviations on the actual density of sample and the actual volume fraction of matrix were investigated on the samples with uncontrolled and controlled volume. The roving volume fractions and concentrations of hybrid and two-component composites were specified as the basic data of calculations. The actual densities of samples, as well as the value of matrix volume fractions, were compared with the calculated ones. The experimental data are in good agreement with the calculated data. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Punch shear in unidirectional composites is induced by transverse shear loading that progressively perforates the laminate within a narrow shear annulus. At lower micromechanical length scales, punch shear loading creates unique micromechanical damage mechanisms dominated by transverse fiber shear failure, fiber–matrix interphase debonding and large inelastic deformation and cracking of the matrix. A new punch shear experimental method has been developed to test unidirectional S glass/DER353 epoxy composite ribbons at sub-millimeter length scale. The experimental data consist of a statistical measurement of the continuum response (load-deformation and punch shear strength) and the characterization of micromechanical damage modes. A simplified 2D micromechanical finite element model incorporating Weibull fiber strength distribution has been developed and correlated with the experimental data. The 2D micromechanical finite element model can simulate the punch shear failure of the ribbon incorporating mixed mode fiber fracture, and fiber–matrix debonding mechanisms using zero thickness cohesive elements. Results from stochastic simulations of punch shear experiments show that an equivalent 2D micromechanical finite element model can predict the micromechanical damage mechanisms and the statistical distribution of punch shear strength of the continuum with favorable correlation with the experiments. This paper presents a combined experimental and computational approach in simulating the stochastic non-linear progressive punch shear behavior of unidirectional composites for the first time in the literature. [ABSTRACT FROM AUTHOR]

Punch shear in unidirectional composites is induced by transverse shear loading that progressively perforates the laminate within a narrow shear annulus. At lower micromechanical length scales, punch shear loading creates unique micromechanical damage mechanisms dominated by transverse fiber shear failure, fiber–matrix interphase debonding and large inelastic deformation and cracking of the matrix. A new punch shear experimental method has been developed to test unidirectional S glass/DER353 epoxy composite ribbons at sub-millimeter length scale. The experimental data consist of a statistical measurement of the continuum response (load-deformation and punch shear strength) and the characterization of micromechanical damage modes. A simplified 2D micromechanical finite element model incorporating Weibull fiber strength distribution has been developed and correlated with the experimental data. The 2D micromechanical finite element model can simulate the punch shear failure of the ribbon incorporating mixed mode fiber fracture, and fiber–matrix debonding mechanisms using zero thickness cohesive elements. Results from stochastic simulations of punch shear experiments show that an equivalent 2D micromechanical finite element model can predict the micromechanical damage mechanisms and the statistical distribution of punch shear strength of the continuum with favorable correlation with the experiments. This paper presents a combined experimental and computational approach in simulating the stochastic non-linear progressive punch shear behavior of unidirectional composites for the first time in the literature. [ABSTRACT FROM AUTHOR]

Abstract Loading-unloading tests on unidirectional HTS40/PA6 carbon/polyamide laminates are performed in this study to identify the nature of off-axis nonlinear deformation of fibrous composites under tension and compression. Experimental results reveal the involved residual strain (plastic strain) and hysteresis loop during loading and unloading, and their dependence on stress level, fiber orientation, and tension or compression mode. The hysteresis behavior is assumed to be induced by a component of strain, which is considered as anelastic strain. This strain is recoverable like elastic deformation but dissipates work like plastic deformation. An approach is developed to predict the tension-compression asymmetry in plastic strain and anelastic strain, as well as the dependence of asymmetry on stress level and fiber orientation. The proposed approach is a modification of a strength differential model that considers the nonlinear deformation completely as plastic strain for monotonic tests. Predicted off-axis loading-unloading responses of unidirectional laminates are compared with the experimental results. The modified model is found to be effective for adequately describing the tension-compression asymmetry not only in plastic strain but also in complex nonlinear hysteresis behavior. [ABSTRACT FROM AUTHOR]

A micro-mechanical composite material model is developed to simulate the behavior of unidirectional composites under impact loading conditions in the nonlinear finite element solver (LS-DYNA®). The nonlinear strain rate and pressure dependency in the composite material model is accounted by the resin, which uses previously developed state variable viscoplastic equations. These equations have been originally developed for metals; however, these are modified to account for the significant contributions of hydrostatic stresses typically observed in polymers. The material model also uses a continuum damage mechanics (CDM) based failure model to incorporate the progressive post-failure behavior. A set of Weibull distribution functions are used to quantify this behavior, and a methodology of assigning physical significance to the choice of damage/softening parameters used in these functions is presented. The impact response of composite laminate plates has been simulated and compared to the experiments. It has been observed that the predicted results compare favorably to the experiments. [ABSTRACT FROM AUTHOR]

An approach to predict intralaminar failure mechanisms in laminate fibre-reinforced composites, at finite strains, is proposed. Transverse matrix cracking is described by a smeared crack model, activated by an invariant-based failure criterion, where an embedded cohesive crack is considered in the continuum description. A deformation gradient decomposition based on homogenisation considerations is developed for a more objective finite strain kinematic description of transverse cracks. The local equilibrium problem is established in the reference configuration to naturally include re-orientation of the fracture plane, and the employed cohesive law is able to deal with non-monotonic loading. Moreover, a continuum damage model formulated in terms of the Green–Lagrange strain tensor is used for longitudinal failure. The simulation of an open-hole multi-directional specimen subjected to tension illustrates the ability of the proposed constitutive equations to capture experimental observations. • The finite strain kinematic description of transverse cracks is based on homogenisation considerations. • Re-orientation of the fracture plane with ongoing deformation is taken into account. • The cohesive law can deal with non-monotonic loading. • Longitudinal failure is addressed with a continuum damage model. • Simulations of single elements and laminated composites illustrate the features of the model. [ABSTRACT FROM AUTHOR]

Puck criterion, a transverse failure criterion for composites, has more applications in engineering, but the empirical nature of its mathematical expression and the complexity of its parameters hinder its further development. In this work, a failure function is proposed based on the Puck hypothesis. According to the parameters of composites in special states, two modes of transverse tension and compression are distinguished, the pending coefficients in the failure function are derived, and then a transverse failure criterion for composites is formulated. The proposed criterion was evaluated experimentally in terms of envelope curves and off-axis strength using the existing data in the literature, and was compared with several other criteria. The results show that the proposed criterion is in good agreement with the experimental data, and its prediction accuracy ranks high among several criteria, demonstrating the rationality of the proposed criterion. Additionally, the effect of material parameters on the proposed criterion is analyzed and the results show that changes in transverse tensile and compressive strength can change the strengthening effect of transverse compression against in-plane shear, while changes in in-plane shear strength cannot change the strengthening effect. This work provides a correction method for the Puck criterion, which has a more rational mathematical expression with fewer parameters and can be used in the design of engineering composite structures, which has significant design reference value and theoretical significance. [Display omitted] [ABSTRACT FROM AUTHOR]

Machining operations are performed on composite parts to obtain the final geometry. However, machining composites is challenging due to their low machinability and high cost. Numerical modelling of machining presents a valuable tool for cost reduction and a better understanding of the cutting process. Meshfree methods are an attractive choice to model machining problems due to their capability in modelling large deformations. This work presents an explicit meshfree model for orthogonal cutting of unidirectional composites based on the element-free Galerkin (EFG) Method. Advantages of the proposed model include: simple and automated preprocessing, advanced material modelling and ability to model high-speed machining. Workpiece material is modelled as orthotropic Kirchhoff material with a choice of three failure criteria: maximum stress, Hashin and LaRC02. Frictional contact calculations are performed based on central differencing, therefore avoiding the use of penalty parameters. Validation of the EFG model is conducted by comparing cutting forces against orthogonal cutting experiments on GFRP samples using a vertical milling machine. It is found that while the numerical cutting forces are in good agreement with experimental ones, the numerical thrust forces are significantly under-estimated. Analysis of failure showed that chip is formed along the fibre direction in the studied range. [ABSTRACT FROM AUTHOR]

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The aim of this paper is to analyze the relationship between the tensile properties of flax and jute fibres and the mechanical performance of associated grafted polypropylene (PP-MAPP) biocomposites. Firstly, we analyzed the differences of the morphological and mechanical properties of these two types of fibres. Flax fibres demonstrated better mechanical properties. Secondly, polypropylene matrix unidirectional (UD) composites were manufactured and tensile tested. We noticed a strong correlation between the experimental Young's modulus and the estimated modulus with the rule of mixtures. Finally, short fibre PP-MAPP composites were extruded and injected and their tensile behavior was analyzed. Despite the lower tensile properties of the jute fibres, the PP-MAPP/jute composite exhibited better mechanical performances. A detailed analysis of the microstructure was conducted; it highlighted the preponderant role of fibre orientation on the mechanical properties of injected composites. [ABSTRACT FROM AUTHOR]

In this work, the elastic response of unidirectional fiber (UD) reinforced composites is studied in a stochastic multiscale way. First, the micro-structure of UD carbon fiber reinforced composites is statistically studied based on SEM images of its cross-section and an algorithm to generate numerical micro-structures with an equivalent random distribution of fibers is developed. In particular, based on the images spatial analysis, the empirical statistical descriptors are considered as dependent variables and represented using the copula framework, allowing generating micro-structure realizations used as Stochastic Volume Elements (SVEs). Second, a stochastic scale transition is conducted through the homogenization of SVEs. With a view to the use of the resulting meso-scale random field in structural stochastic analyzes, the homogenization is performed in two steps in order to respect the statistical content from the micro-meter-long SVEs to the millimeter-long structural finite elements. To this end, the computational homogenization is applied in a hierarchy model: i) Micro-structure generator produces Small SVEs (SSVEs) which are homogenized; ii) Big SVEs (BSVEs) are constructed from the SSVEs. Finally, it is shown on simple illustrative examples that the scatter of the (homogenized) stress distribution in a composite ply can be simulated by means of the developed methodology. [ABSTRACT FROM AUTHOR]

The loading–unloading hysteresis behavior of unidirectional HTS40/PA6 carbon/polyamide laminates under off-axis compression has been examined both experimentally and constitutively in the present study. Monotonic compressive tests are first performed on unidirectional laminates to identify the basic properties of off-axis deformation. Then, five cycles of compressive loading and unloading tests are carried out with a gradually increasing peak stress for each specimen, followed by testing to failure. Significant nonlinear unloading behavior is observed after the material has been loaded into the nonlinear deformation region, and apparent plastic strain is seen after full unloading. The reloading curve also exhibits nonlinearity and conduces to a slightly open hysteresis loop with unloading. An approach is developed to predict the nonlinear hysteresis behavior of unidirectional composites by assuming an anelastic strain component in the nonlinear compressive deformation. The assumption of anelastic strain is a modification of Sun–Chen’s one-parameter plasticity model that considers the nonlinear deformation completely as plastic strain from monotonic tests. The modified model has been validated by the off-axis loading–unloading test results on unidirectional laminates. Results verify that the model accurately captures not only the plastic strain but also the complex nonlinear hysteresis behavior in the observed off-axis loading–unloading stress–strain curves. [ABSTRACT FROM AUTHOR]

The compressive strength of unidirectional composites is strongly influenced by the elastic and strength properties of the fiber and matrix phases, as well as by the local geometrical properties, such as fiber volume fraction, misalignment, and waviness. In the present investigation, two microbuckling criteria are proposed and examined against a large volume of measured data of unidirectional composites taken from the literature. The first criterion is based on the compressive strength formulation using the buckling of Timoshenko’s beam. It contains a single parameter that can be determined according to the best fit to experimental data for various types of polymeric matrix composites. The second criterion is based on buckling-wave propagation analogy using the solution of an eigenvalue problem. Both criteria provide closed-form expressions for the compressive strength of unidirectional composites. We propose modifications of the two criteria by a fitting approach, for a wide range of fiber volume fractions, applied to four classes of unidirectional composite systems. Furthermore, a normalized form of the two models is presented after calibration in order to compare their prediction against experimental data for each of the material systems. The new modified criteria are shown to give a good match to a wide range of unidirectional composite systems. They can be employed as practical compression failure criteria in the analysis and design of laminated structures. [ABSTRACT FROM AUTHOR]

A fatigue damage micro-model is developed for two-phase composite materials. Constitutive law is derived at the level of fibre and matrix and extended for a damaged lamina using the reformulation of Mori-Tanaka model. A new non-dimensional effective local stress based on a failure criterion which is rewritten in term of average stress of fibre and matrix is coupled with a fatigue damage model for prediction of fatigue damage and fatigue life. The proposed model is able to distinguish between damage in fibre and matrix and also stiffness reduction of the matrix and transverse direction during fatigue loading using three different damage variables. The fatigue behaviour of unidirectional carbon/epoxy and glass/epoxy laminates are studied under off-axis loading conditions. Fatigue life of unidirectional composites is predicted at different stress levels, stress ratios and fibre orientations under tension-tension and tension-compression fatigue. The obtained numerical results from the proposed model are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

In this paper, a new damage modelling approach is presented to describe the mechanical response of unidirectional composites under the small strain assumption. The proposed constitutive equations inherits from the phenomenological theories of Continuum Damage Mechanics (CDM) but brings out a micromechanical description of the damaged Representative Volume Element (RVE) while being formulated in a proper thermodynamical framework. The model is provided with an implicit numerical scheme based on the so-called “return mapping algorithm” as well as the formulation of the tangent operator. The identification and the prediction capabilities of the model are validated using experimental data including off-axis tensile tests. Finally, to provide a better understanding of the model, a multi-axial non-proportional simulation is performed and analysed. [ABSTRACT FROM AUTHOR]