Multidirectional Laminates Research Articles

Why not Designing Multidirectional Laminates with In-Plane Strength Design Sheets Applying the UD Criteria of TSAI–WU and Cuntze? Part1: Analytical Foundation

Why not Designing Multidirectional Laminates with In-Plane Strength Design Sheets Applying the UD Criteria of TSAI–WU and Cuntze? Part1: Analytical Foundation

Mode I fatigue delamination growth behavior and model for multidirectional laminates with the same overall stiffness

In order to isolate the influence of ply orientation on the mode I fatigue delamination propagation behavior of carbon fiber reinforced composite laminates, multidirectional laminates with the same overall stiffness are designed while three different interfaces (0//0, 45//45, and 90//90), which can avoid the coupling effect of remote plies. Test results show that the fatigue delamination behavior is obviously affected by the interface angle. In addition, a novel and simple method for determining the fatigue delamination resistance is proposed. The measured fatigue delamination resistance is lower than the quasi-static one for all interfaces. The specimen with 45//45 interface exhibits more significant delamination resistance, whereas the delamination resistances of specimens with 0/0 and 90//90 interfaces are similar, which is consistent with the phenomenon of more bridging fibers accompanying in the delamination process of 45//45 interface. In order to consider the effect of fiber bridging and reduce the dispersion of the data, experimental data are further processed using a normalized model considering the effect of fiber bridging. The normalized results can be characterized by a single curve, suggesting that the normalized model is effective in describing the fatigue delamination behavior in the presence of fiber bridging.

A multi-linear constitutive relation considering the temperature effect on quasi-static mode I delamination in UD/MD laminates

In this study, a multi-linear constitutive relation taking into account temperature and fiber bridging is proposed for characterizing delamination behavior in composite laminates under various temperature conditions. An approach combining analytical solution and J-integral is also established for determining the cohesive parameters in the multi-linear constitutive relation. To validate the proposed constitutive relation, mode I quasi-static delamination experiments of unidirectional (UD) and multidirectional (MD) carbon/bismaleimide laminates are carried out at 25 ℃ (room temperature), 80 °C and 130 ℃. The experimental results show that the increasing temperature resulted in a monotonic increase in the fracture toughness of the UD laminates while affect the fracture toughness of MD laminates slightly. A FE model is established with the implementation of the proposed multi-linear constitutive relation using UMAT subroutine. Good agreements between the experimental and simulated results demonstrate the validity of the proposed constitutive relation, with the relative difference of peak load between predicted and experimental values less than 8.2 % and the relative difference of initial and steady-state fracture toughness between predicted and tested results less than 15 %. This study provides the possibility to numerically study the temperature effect on the delamination behavior of laminates and has promising applications in the damage tolerance design of composite structures.

Effect of fiber orientation of adjacent plies on the mode I delamination fracture of carbon fiber reinforced polymer multidirectional laminates

AbstractThe extensive use of multidirectional composite laminates requires to understand the delamination behavior at interfaces between plies with different fiber orientations. In this study, double cantilever beam testing was performed to investigate the mode I interlaminar fracture of carbon fiber reinforced polymer laminates with different central interfaces (0//0, 0//+45 and +45//−45). X‐ray microtomography revealed the curved crack front shape that was incorporated to calculate fracture toughness. The fracture toughness varies with the crack length in a typical delamination resistance curve, which can be quantified by an empirical equation. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate, compared to constant toughness. It was found that the delamination mechanism is independent of central interfaces. However, compared to unidirectional laminates, multidirectional laminates are less resistant to crack initiation, but more resistant to propagation with higher toughness and shorter fiber bridging zone.Highlights Mode I interlaminar fracture of CFRP laminates with different central interfaces was investigated experimentally and numerically. Curved crack front shape was incorporated to calculate fracture toughness. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate. Multidirectional laminates are less resistant to crack initiation but more resistant to propagation with higher toughness and shorter fiber bridging zone.

Multi-scale analysis of ultimate bearing capacity in composite hull joints: failure mechanism and numerical simulation

ABSTRACT Progressive failure analysis method is the main method to analyse the ultimate bearing capacity and failure mode of composite structures. Multi-scale method can effectively reduce the dependence of damage mode and stiffness degradation analysis on material parameters. Based on the multi-scale method, from micro-scale to mesoscale and then to macro-scale, the equivalent modulus and equivalent strength of fibre bundles, single-layer laminates and multi-directional laminates were predicted respectively, and the prediction method of ultimate bearing capacity of large-scale composite structures from material level to structure level was constructed. The mechanical properties of standard composite materials and the ultimate bearing capacity of full-scale composite joints were tested respectively, and the ultimate bearing capacity and failure mode of L-joints were predicted by numerical method. The results show that the numerical simulation method based on multi-scale can accurately predict the ultimate bearing capacity and failure mode of large-scale joints of the hull.

Hygrothermal ageing effects on mode I fatigue delamination in multidirectional composite laminates

Ageing is known to have significantly detrimental effect on mode I fatigue delamination growth (FDG) in unidirectional (UD) composite laminates. However, composite structures are usually designed with multidirectional (MD) layups, which raises the question that is it enough to only conducted fatigue delamination experiments on specimens with a UD layup. The aim of this study is therefore to explore mode I FDG in MD composite laminates with 45//45 interface after different ageing, i.e. at 70 °C 85 % relative humidity (RH) and immersion in 70 °C water bath. Fatigue delamination experiments were conducted at stress ratios R = 0.1 and 0.5. The fatigue data, interpreted via Paris-type fatigue laws, demonstrated that: (1) the change of ageing severity has no influence on mode I FDG in MD composite laminates; (2) FDG remains the same in composite laminates after different ageing, regardless of layups. In all cases, the same master resistance curves can be obtained to determine the intrinsic mode I fatigue delamination resistance of UD and MD composite laminates after different ageing. The physical reasons for these findings were discussed based on the moisture content analysis, Fourier transform infrared (FTIR) analysis, dynamic mechanical thermal analysis (DMTA), and fractographic examinations. It was found that material degradation and delamination mechanisms remain the same for UD and MD layups, as well as for 85 %RH and water bath conditioning.

Bioabsorbable Composite Laminates of Poly‐Lactic Acid Reinforced with Surface‐Modified Mg Wires for Orthopedic Implant Applications

Unidirectional and multidirectional laminates of Mg‐wire‐reinforced poly‐lactic‐acid–matrix composites are manufactured by an improved compression molding strategy that allows excellent control on the position and orientation of the wires. Two different types of Mg wires, with and without surface modification by continuous plasma electrolytic oxidation, are used, the former to improve the interfacial strength and to reduce the degradation rate of Mg wires in biological environments. The mechanical properties of the constituents as well as of composites are measured before and after in vitro degradation by immersion in simulated body fluids and the corresponding deformation, fracture, and degradation mechanisms are analyzed in detail. It is found that the presence of the Mg wires improves the mechanical behavior in tension and compression of unidirectional composites in the longitudinal direction (close to cortical bone) and that quasi‐isotropic laminates with tailored properties can be designed from the data of unidirectional composites.

Translaminar-envelope ply for CFRP mechanical enhancement

Pursuing the trend of ply-level innovation in the field of high-performance CFRP composites, a pseudo-woven layup configuration has been developed by interlacing a thin-ply prepreg in the translaminar-envelope manner space-orthogonally inside a homogenized 4-ply sub-laminate to acquire mechanical superiorities for the global multidirectional laminate. This evolved layup structure possesses the stiffening feedbacks on in-plane principal properties by the locally-regulated 0° ply amount, and exerts the multi-interface synergy on shear-driven interlaminar fractures by the long-acting binding action. Further, the high robustness of structural performances has been revealed by resistance and tolerance against severe low-velocity impact damage, achieving up to 60 % decrease in the projected delamination area and over 60 % increase in the post-impact compressive strength. The translaminar-envelope layup configuration is expected to enhance the service capability of CFRP laminates.

Bottom-up stochastic multiscale model for the mechanical behavior of multidirectional composite laminates with microvoids

This study for the first time develops a coherent and efficient bottom-up stochastic multiscale modeling approach, which aims at revealing the impact of microvoids on the mechanical properties and damage mechanisms of multidirectional (MD) carbon fiber-reinforced polymer (CFRP) composites. To this end, void defects within the composites are characterized using X-ray micro-computed tomography to construct a high-fidelity representative defective volume (RDV). Subsequently, the influence of voids on the microscale failure modes and macroscopic mechanical properties are analyzed by using micromechanics-based finite element (FE) method. Finally, a stochastic multiscale analysis method is developed to predict the mechanical response of MD CFRP laminates and validated by tensile and three-point bending experiments. The results indicate that the proposed framework can accurately predict the mechanical behavior and damage mechanisms of MD CFRP composites. This framework is believed to be a promising method to provide valuable guidance for reliability design and optimization of CFRP composites.

Fiber fracture behavior of high-pressure resin transfer molded (HP-RTM) carbon/epoxy multidirectional laminates under compression-compression fatigue loading

The influence of different stress levels on the fatigue life and fracture behavior of laminated composites have been evaluated under compression-compression (C–C) loading conditions. Carbon/epoxy multidirectional (MD) laminate specimen was designed with eight layers [0°, ± 45°, 0°]s of unidirectional plies. The preforms were molded by a high-pressure resin transfer molding (HP-RTM) process. As part of the study, a static compression test was performed. The C–C fatigue loads were applied at −260 MPa, −280 MPa, −300 MPa, and −320 MP compression stress until failure or two million cycle loadings. The fatigue limit has been obtained at −265 MPa stress by stress-cycle number (S-N) curve analysis, which is 51.4 % of static compression strength. The damage behavior was identified after C–C fatigue tests. By using optical and scanning electron microscopy, fiber fracture, matrix cracking, and fiber/matrix interface debonding were observed. Under the C–C fatigue loading conditions, fiber fracture modes were characterized by rough cross-sections, shear, and line-shaped edges.

Reducing stress concentrations in static and fatigue tensile tests on unidirectional composite materials: A review

The longitudinal tensile strength is an important basic property of unidirectional (UD) composites or plies that often governs failure in multidirectional laminates. The standard tensile testing methods almost always result in sample failure near the grip. Finite element analyses revealed the presence of multiaxial stress concentrations, including longitudinal, transverse, and shear stresses in the tabbed section of the standards’ recommended design. There are still unknowns and uncertainties about the causes of these stress concentrations and ways to eliminate them. A major challenge is obtaining acceptable failure within the gauge section and yielding the highest tensile strength of UD composites. This paper reviews the different methods for performing quasi-static and fatigue tensile tests on UD composites. The primary sources of stress concentrations and the parameters that affect them are reviewed using the available experimental and modeling investigations. We survey the effects of the different specimen and end tab designs as well as test setups on quasi-static and fatigue loading. Specific proposals are made for each of the discussed parameters for more reliable results.

Efficient modelling of progressive damage due to quasi-static indentation on multidirectional laminates by a mesh orientation independent kinematically enriched continuum damage model

This work proposes a computationally efficient high-fidelity modelling framework for analysis of damage in composites subjected to quasi-static indentation. A ply-by-ply modelling approach is adopted, and a new semi-discrete continuum damage model is used for intra-ply cracking that allows for cracks to grow independent of the ply mesh pattern. This feature greatly simplifies the meshing effort since the requirement of a ply-oriented mesh and imposition of tie constraints at mesh-mismatched ply interfaces is eliminated. The model is further enhanced to realise kinematic interactions between the cracked and uncracked material domains at the constitutive level. Inter-ply delamination is simulated using cohesive elements. Through a challenging problem of static indentation on a quasi-isotropic laminate, it is shown that the model can capture solution-dependent multiple discrete ply cracks and detailed crack-delamination interaction, but with reduced computational time and complexity, compared to a reference model with ply-oriented mesh and pre-seeded cracks at known locations.

Characterizing compressive failure mechanisms and their transitions in woven composites under on and off-axis loading

The compressive failure of fiber-reinforced composites is complex, occurring via multiple mechanisms depending on the loading angle with the fiber directions. This work is aimed at the detailed characterization and understanding of this dependence in a 2 × 2 epoxy/carbon twill woven composite. Reported are the results of a variety of on-axis and off-axis compressive tests with loading angles ranging from 0°to 45°, carried out as per ASTM standards. The tests show that for angles from 0°to 10°, no appreciable change in compressive strength occurs. Failure in the load aligned tows is dominated by out of plane shear failure and out of plane fiber kinking. The behavior is very brittle, consisting of a sharp peak load immediately dropping to a non-zero residual stress plateau. Extensive micro-cracking in the transverse tow is also observed. Between 10°and 25°many transitions occur, including increased ductility, pre-peak non-linearity, increasing width of the out-of-plane fiber kink band, more gradual post-peak load decrease, and an increase in the residual stress. The lowest strength occurs at 45°where the behavior is completely ductile and failure occurs by in-plane fiber kink band formation. A correlation between the in-plane shear stress and kink band width appears to exist. The maximum stress based strength envelope theory captures this loading angle dependence of strength. The findings are crucial for designing multi-directional woven composite laminates subjected to compressive loading, and for the validation of computational models.

Effect of number of fatigue cycles on fatigue data: CFRP (prepreg and wet‐layup)

AbstractIn carrying out fatigue delamination propagation tests to assess the propagation rate versus a function of the energy release rate, the question arises as to the necessary number of cycles required to properly characterize the behavior. Tests were performed elsewhere on two different carbon/epoxy, multidirectional woven composite laminates containing a delamination using constant amplitude cycling by means of displacement control for various cycle ratios. The aim was to carry out the tests for cycles. Most specimens complied with this requirement. In this study, the fatigue data are analyzed including the first cycles, the first cycles, the first cycles, and cycles. It was seen that although the delamination length versus the cycle number could be quite similar between different cycle ranges, the Paris relation constants could be significantly different. It is suggested to carry out tests for as many cycles as possible.

Effect of hygrothermal aging on compression behavior of CFRP material with different layups

This study investigates the mechanical degradation mechanism and statistical analysis of residual compressive strength of three lay-up laminates under more than one-year seawater environment. Fick and Langmuir model were used to describe the moisture absorption process with a maximum moisture equilibrium content of 3.78% and various characterization methods were used to reveal the microstructural evolution of CFRP during aging. Experimental results demonstrate that the multiple fiber directions in the MD specimens provide additional moisture absorption channels, leading to a stronger wicking effect and a higher equilibrium moisture content. The layups can significantly affect the compression failure mode after long-term aging. The compressive strength of the cross-ply and multidirectional laminates decreased by 50% while that of unidirectional laminates decreased by 28%, and surface cracking is easily observed in the aged specimens. An empirical prediction model based on the Langmuir model and experimental data was proposed and verified by the data from the literature, with at least a 10% improvement in prediction accuracy. Meanwhile the three-parameter Weibull distribution was used to describe the residual compressive strength after hygrothermal aging and tested by Kolmogorov-Smirnov test. It is expected to provide insight for the application of CFRP in seawater environment.

Experimental study of delamination process in elastically coupled laminates with the acoustic emission technique

This paper aims at experimental study of delamination process in multidirectional composite laminate exhibiting elastic couplings by using the acoustic emission (AE) technique. The mode I double cantilever beam (DCB) test was conducted on carbon/epoxy laminate with the bending-twisting elastic couplings according to the ASTM D5528 Standard. Experimental research was supported with visual high resolution registration of propagated crack tip, as well as with measurements of elastic waves generated by developing internal damages by using the acoustic emission set Vallen AMSY-5 equipped with two piezoelectric sensors. Prior to test, values of fundamental Lamb wave modes velocities were estimated and the dispersion curves were calculated. Delamination initiation point was determined by using two different AE criteria. The mode I critical strain energy release rates were calculated by using three different data reduction schemes. Different damage mechanisms during crack propagation process were quantitatively evaluated by fundamental waveform features divided into three characteristic frequency bands groups. In addition, signal energy concentration regions were assessed by using the wavelet transforms. The outcomes revealed various damage mechanisms related with delamination process, even though the matrix cracking found to be the dominating mode of failure.

Rate dependent compressive failure and delamination growth in multidirectional composite laminates

A novel intralaminar model has, for the first time, been applied and validated for the rate-dependent failure of multidirectional carbon/epoxy laminates. Quasi-static compressive failure is evaluated by the growth of intralaminar rate-dependent damage combined with the interaction of cohesive zones for interlaminar delamination. A special feature of the intralaminar model is the homogenised ply response, allowing simultaneous damage-degradation of the polymer matrix combined with the fibres. To model the observed quasi-brittle failure response of the plies under finite deformation, we have used a viscoelastic-viscoplastic matrix combined with damage and isotropic hardening behaviour. Elastic transverse isotropy is used to model the fibre reinforcement of the plies. Standard cohesive surfaces are used to model the initiation and propagation of delamination. Numerical simulations using ABAQUS/Explicit are performed to predict the growth and delamination of intralaminar damage under compression in different laminates with 56 plies of IM7/8552 carbon/epoxy. Predictions of stress versus strain and damage growth are shown to agree well with experimental results for a range of strain rates and stacking sequences.

Femtosecond laser drill high modulus CFRP multidirectional laminates with a segmented arc-based concentric scanning method

High modulus carbon fiber reinforced plastic multidirectional laminates (HM-CFRP-MDL) are widely employed in the aerospace industry as crucial structural materials for satellites, rockets, and aircraft. However, it is difficult for the femtosecond laser to drill holes on HM-CFRP-MDL with the traditional concentric scanning method (CSM). To improve drilling quality and precision, this study proposes a segmented arc-based concentric scanning method (SAB-CSM) and investigates the interaction mechanisms between femtosecond lasers and CFRP. By adopting the proposed scanning method with optimized parameters, the overall average HAZ width can be decreased to 10 μm in different regions on the surface of the drilled hole, the hole taper can be reduced significantly, and the consistency across different layers can be improved. Compared to CSM, the HAZ width can be decreased by 40.8 % (12.597 μm to 7.464 μm) and 50.3 % (15.585 μm to 7.748 μm) in the regions where the angles between fiber direction and scanning direction are 45° and 90° respectively, and the taper can be reduced by 34.5 % (0.084 to 0.055). Moreover, the SAB-CSM approach effectively decreases quality discrepancies between different layers, and substantially reduces those defects which results in smooth and uniform hole walls, facilitating synergistic removal of multidirectional carbon fibers and resin matrix.

Experimental characterization of compression failure mechanism initiation and growth in notched carbon fiber reinforced composite specimens

This paper describes an experimental investigation to evaluate the compression failure mechanisms of kink banding, splitting, and delaminations under non-standard quasi-static loading of laminated carbon fiber reinforced composites. Tests were performed on double-edge notch compression (DENC) specimens to observe microscopic compressive damage initiation and progression. Acoustic emission results from specimens loaded to failure were used to define ranges of static stress associated with different forms of damage. Subsequent tests were interrupted at these stress intervals and results obtained from multiple inspection techniques provide quantified characterization of failure mechanism initiation and growth as a function of applied load level. Optical images of the exterior surfaces, micrographs of the laminate thickness generated via a grind/polish procedure, and postmortem X-ray computed tomographs provide detailed information on the 3D morphology and evolution of failure mechanisms in the laminate. Key failure characteristics include interior kink bands, surface ply splitting, and delaminations at two different types of interfaces. Kink bands are only present near ultimate failure while splitting and delamination initiate at approximately 50% lower stress levels. The experimental observations provide insight into the critical and subcritical nature of these failure mechanisms and their possible interactions in a multidirectional laminate under compression loading.

Bottom‐up analysis framework for the continuous multidirectional carbon fiber‐reinforced PEEK composite laminates

AbstractThe continuous carbon fiber‐reinforced polyether ether ketone (CCF/PEEK) composites have been widely utilized in aerospace structures due to their excellent properties. However, the main load‐bearing structure in the next‐generation aircraft needs to withstand high temperatures up to 200°C, and the mechanical properties of CCF/PEEK composites show significant time‐/temperature‐dependence due to the viscoelastic nature of the PEEK matrix. Therefore, accurate predictions of the mechanical performance of CCF/PEEK composites under high‐temperature service conditions are the key issue for structural safety and life assessments and have therefore become a recent research focus. To address these challenges, this study established a bottom‐up analysis framework for the CCF/PEEK composites and employed the theoretical model and the finite‐element representative volume element (FE‐RVE) method to investigate the anisotropic viscoelastic properties of CCF/PEEK composites. We first built an anisotropic viscoelastic model of the unidirectional CCF/PEEK composites by integrating the generalized Maxwell into the bridging model. Upon this foundation, we further developed a viscoelastic laminate model capable of accurately predicting the thermoviscoelastic properties of multidirectional laminates. To verify the developed model, different kinds of FE‐RVEs were built and analyzed. Overall, there is good agreement among the model predictions, the FE‐RVE simulations, and the experimental results within a temperature range of 25–250°C. The developed model can be used to design the high‐temperature performance of CCF/PEEK composites with different fiber volume fractions, laminated configurations, or loading directions.Highlights An anisotropic viscoelastic model of the unidirectional (UD) continuous carbon fiber‐reinforced polyether ether ketone (CCF/PEEK) composites is built. The viscoelastic laminate composite model is developed based on the UD CCF/PEEK composites model. The finite element representative volume element (FE‐RVE) simulation with fibers in a hexagonal or random array has better accuracy compared with the experimental results. The FE‐RVE simulation and the experimental results verified the developed model from 25°C to 250°C.