Fracture Toughness Of Composite Laminates Research Articles

Machine learning-based determination of Mode II translaminar fracture toughness of composite laminates from simple V-notched shear tests

This paper presents a novel method for measuring the translaminar crack resistance curve of composite laminates under Mode II shear loading. A machine learning (ML)-based approach is utilized to extract the inapparent information of the crack resistance curve from the translaminar shear strength measurements obtained from simple V-notched shear tests. The entire campaign is built on the framework of the Finite Fracture Mechanics (FFM) combined with Finite Element Method (FEM). Special emphasis is made on the nonlinear mechanical behavior of composites under shear stress since the original FFM models are designed for quasi-brittle materials. With the well-trained recurrent neural network model, the Mode II R-curve of composite laminate can be obtained with un-notched and V-notched shear strength values as inputs. Experiments were conducted on carbon fiber-reinforced composites to validate the accuracy of the R-curve obtained by the proposed approach and that by the traditional compact shear test. The successful implementation of the method suggests a more convenient and low-cost way of obtaining this important damage-related parameter for composites.

Effect of electrospun PA66 nanofibrous mat thickness on mode-II fracture toughness using acoustic emission (AE) with data clustering technique

Considering the importance of improving the fracture toughness of composite laminates, in this research, by embedding electrospun nanofibers of polyamide 66 between the layers, the fracture toughness of prepared samples was evaluated in Mode-II loading and compared to the reference state. It was demonstrated that adding polyamide 66 nanofibers of two distinct thicknesses, 110 and 140 μm increased the fracture toughness in mode-II by 84 % and 47 %, respectively. Thus, there is an optimum value for thickness of nanofibers to have maximum enhancement of fracture toughness. Additionally, the acoustic emission method was used to examine the key features that increased fracture toughness. Using acoustic emission features and hierarchical model clustering, it was shown that fiber/matrix debonding is the primary damage mechanism lowering the mode-II fracture toughness. Macro pictures and SEM micrographs were used to observe the failure mechanisms on the sample surfaces.

Is Graphene Always Effective in Reinforcing Composites? The Case of Highly Graphene-Modified Thermoplastic Nanofibers and Their Unfortunate Application in CFRP Laminates.

Graphene (G) can effectively enhance polymers' and polymer composites' electric, thermal, and mechanical properties. Nanofibrous mats have been demonstrated to significantly increase the interlaminar fracture toughness of composite laminates, hindering delamination and, consequently, making such materials safer and more sustainable thanks to increased service life. In the present paper, poly(ethylene oxide) (PEO), polycaprolactone (PCL), and Nylon 66 nanofibers, plain or reinforced with G, were integrated into epoxy-matrix Carbon Fiber Reinforced Polymers (CFRPs) to evaluate the effect of polymers and polymers + G on the laminate mechanical properties. The main aim of this work is to compare the reinforcing action of the different nanofibers (polyether, polyester, and polyamide) and to disclose the effect of G addition. The polymers were chosen considering their thermal properties and, consequently, their mechanism of action against delamination. PEO and PCL, displaying a low melting temperature, melt, and mix during the curing cycle, act via matrix toughening; in this context, they are also used as tools to deploy G specifically in the interlaminar region when melting and mixing with epoxy resin. The high extent of modification stems from an attempt to deploy it in the interlaminar layer, thus diluting further in the resin. In contrast, Nylon 66 does not melt and maintain the nanostructure, allowing laminate toughening via nanofiber bridging. The flexural properties of the nanomodifed CFRPs were determined via a three-point bending (3PB) test, while delamination behavior in Mode I and Mode II was carried out using Double Cantilever Beam (DCB) and End-Notched Flexture (ENF) tests, respectively. The lack of a positive contribution of G in this context is an interesting point to raise in the field of nanoreinforced CFRP.

Experimental characterization and numerical study on the interlaminar fracture toughness of carbon fibre reinforced polymer laminates reinforced with carbon nanotubes

AbstractThe present study is focused on the nanoreinforcement of carbon fiber reinforced polymer laminates to evaluate its effect on delamination growth in double cantilever beam specimen. Multi‐walled carbon nanotubes were sprayed on the laminate interface using air‐brush spraying technique. Three nanofiller contents of 0.05 wt.%, 0.1 wt.%, and 0.5 wt.% were examined to evaluate the mode I fracture toughness (GIC) in a double cantilever beam specimen and the results were compared with the base laminate (0.0 % carbon nanotubes). The average mode I fracture toughness of composite laminates increased from 180.6 J/m2 to 270.92 J/m2 due to the addition of 0.05 wt.% multi‐walled carbon nanotubes, and to 268.27 J/m2 for 0.1 wt.% multi‐walled carbon nanotubes. Mechanisms for the increase in GIC of the nanofiller reinforced laminates are discussed based on the experimental results and microscopic study of fractured specimen surfaces from scanning electron microscopy. Energy dissipation from carbon nanotube bridging ahead of the crack front is the major toughening mechanism resulting in improved fracture toughness. Numerical simulations of delamination growth were performed as well by using virtual crack closure technique analysis in Abaqus. The R‐curve behaviour from the experiments is implemented in the finite element modeling of delamination growth to validate the effect of nanoreinforcements.

Determination of fibre tension fracture toughness of composite laminates at high loading rate

The dynamic fracture toughness of composite laminates in fibre tension failure mode is characterized here with compact tension (CT) sample, at loading rate of 25 m/s using bi-directional electromagnetic Hopkinson bars. Digital image correlation (DIC) with high-speed imaging is employed for obtaining the full-field strain fields. A dynamic J-integral method is proposed to analyse the dynamic fracture toughness, the kinetic energy and inertia effects involved in dynamic loading and crack propagation process has been analysed and calibrated. The fracture toughness of fibre tension failure at high loading rate is found to be about 25% lower than that in quasi-static load regime, and the R-curve effect under high loading rate is not significant than that in quasi-static load regime. It is suggested from X-ray CT and SEM images that the fibre pull-out length and size of failure process zone is reduced with increase in loading rate.

The electrical conductivity and interlaminar fracture toughness of composite laminate interleaved with silver-plated nylon mesh

A composite laminate with high conductivity and high mode I fracture toughness was prepared by using a silver-plated nylon mesh as the interleaf. Through in-depth comparison with the silver-plated non-woven veil interleaved composites, the mechanisms of the conductivity improvement and interlaminar toughening were discussed. The conductivities along the fiber direction, in transverse to fiber direction and through thickness direction reached 739 S/cm, 734 S/cm, and 5.4 S/cm, respectively, which increased by 196%, 344,500%, and 4326%, respectively, and are much higher than these of the other modified systems. At the same time, the mode I interlaminar fracture toughness increased by 112%. Mechanism study shows that the conductivities along the fiber direction and in transverse to fiber direction are greatly affected by the interleaf conductivity, while the conductivity through thickness direction is little affected. The toughening mechanism of the plain or silver-plated nylon mesh is much different from that of the non-woven veils. Both of the composites interleaved with the plain or silver-plated nylon mesh have higher mode I interlaminar fracture toughness, but less improved mode II toughness. The effect of silver-plating on the mode I and mode II fracture toughness was studied.

Tuning interlaminar fracture toughness of fine z-pin reinforced polymer composite

This paper aims to improve the interlaminar fracture toughness of carbon fiber reinforced polymer (CFRP) composites by implanting fine z-pins with the minimum damage on in-plane fibers. Z-pins with diameters as small as 0.1 mm and 0.2 mm were prepared by using carbon fiber tows with different mechanical properties. The effect of the mechanical property of carbon fiber pin on the interlaminar fracture toughness of composite laminate was investigated to reveal the enhancement mechanism. The results show that fine z-pins significantly improve the interlaminar fracture toughness and simultaneously are favorable for maintaining high retentions of in-plane mechanical properties of composite laminate. Compared with control sample, the propagation GIC values of CFRPs implanted with 0.2 mm and 0.1 mm CCF800 z-pins increase by 276 % and 541 % respectively at a low pin volume fraction of 0.16 vol%. Both z-pin pull-out and z-pin fracture failure behaviors can be observed for these fine z-pin reinforced composites. The higher tensile properties of z-pins and better transverse shear resistance tend to result in the failure of z-pin pull-out. With the decrease of z-pin diameter, the probability of z-pin fracture failure becomes greater, and correspondingly stronger pinning effect and larger failure load are achieved.

Enhanced Electrical Conductivity and Interlaminar Fracture Toughness of CF/EP Composites via Interleaving Conductive Thermoplastic Films

The poor electrical conductivity and interlaminar fracture toughness of carbon fiber reinforced epoxy (CF/EP) composites could reduce their reliability for aerospace applications. In this work, multi-walled carbon nanotubes (MWCNTs) doped polyethersulfone (PES) conductive thermoplastic films (CTFs) were prepared by solution casting method. The as-obtained composites were then interleaved into CF/EP prepregs to yield composite laminates. The effects of CTFs on the electrical conductivity and interlaminar fracture toughness of the as-prepared CF/EP composites were investigated. The results suggested an increase in conductivity of laminates by 104% and 84% in the transverse (Y) and through-thickness (Z) directions, respectively. The Mode I and Mode II interlaminar fracture toughness values were evaluated by double cantilever beam (DCB) and end-notched flexure (ENF) testing. The data revealed that the introduction of CTFs with 10wt% MWCNTs led to enhancement in Mode I and Mode II interlaminar fracture toughness of Composite laminates by 206% and 47%, respectively. The failure mechanism was investigated through microstructural and morphological changes in the composites studied by scanning electron microscopy (SEM). This work provides valuable guidance for the simultaneous enhancement of electrical conductivity and interlaminar fracture toughness of CF/EP composites.

Study on mode-I fracture toughness of composite laminates with curved plies applied by automated fiber placement

Interlaminar fracture toughness is an important parameter to characterize the delamination performance of carbon fiber-reinforced epoxy laminates. In this study, the mode-I fracture toughness of interlaminar interfaces with different local ply angles were studied by experiments and numerical simulations. A set of double cantilever beam specimens of T800 carbon fiber/EH104 epoxy were prepared according to the ASTM-D5528–01 standard, where four specimens had straight ply interfaces (0°//0°, 15°//−15°, 30°//−30°, and 45°//−45°) and two specimens had curved ply interfaces (curvature radius: 300 and 500 mm). The mode-I fracture toughness was characterized using a data reduction scheme based on the modified beam theory. The results revealed that the mode-I fracture toughness values for the specimens with a curved ply interface were significantly higher than those for specimens with a straight ply interface.

Experimental and Numerical Assessment of Fibre Bridging Toughening Effects on the Compressive Behaviour of Delaminated Composite Plates.

Increasing the Mode I inter-laminar fracture toughness of composite laminates can contribute to slowing down delamination growth phenomena, which can be considered one of the most critical damage mechanisms in composite structures. Actually, the Mode I interlaminar fracture toughness (GIc) in fibre-reinforced composite materials has been found to considerably increase with the crack length when the fibre bridging phenomenon takes place. Hence, in this paper, the fibre bridging phenomenon has been considered as a natural toughening mechanism able to replace embedded metallic or composite reinforcements, currently used to increase tolerance to inter-laminar damage. An experimental/numerical study on the influence of delamination growth on the compressive behaviour of fibre-reinforced composites characterised by high sensitivity to the fibre bridging phenomenon has been performed. Coupons, made of material systems characterised by a variable toughness related to a high sensitivity to the fibre bridging phenomenon and containing artificial through-the-width delaminations, were subjected to a compressive mechanical test and compared to coupons made of standard material system with constant toughness. Out-of-plane displacements and strains were monitored during the compression test by means of strain gauges and digital image correlation to assess the influence of fibre bridging on delamination buckling, delamination growth and on the global buckling of the specimens, including buckling shape changes. Experimental data were combined with a numerical study, performed by means of a virtual crack closure technique based procedure, named SMart Time XB – Fibre Bridging (SMXB-FB), able to mimic the crack bridging effect on the toughness properties of the material system. The combination of numerical results and experimental data has allowed the deformations and the buckling shape changes to be correlated to the onset and evolution of damage and, hence, contributes to improving the knowledge on the interaction of the failure mechanisms in the investigated composite specimens.

Is there a ply thickness effect on the mode I intralaminar fracture toughness of composite laminates?

The apparent crack resistance curves associated with longitudinal tensile failure of carbon fibre reinforced epoxy composite systems with ply thicknesses of 0.075 mm, 0.134 mm and 0.268 mm are determined experimentally from the size effect law of geometrically similar double edge notch tension cross-ply specimens. An increase in notched strength as a function of the ply thickness and a corresponding increase of the measured intralaminar fracture toughness is observed. This increase is shown to be a consequence of the appearance of split cracks in the thicker 0° plies in the vicinity of the notches. The numerical models developed demonstrate that if the notch blunting mechanisms are properly represented, the laminate strength is well predicted for a constant value of the ply intralaminar fracture toughness. This supports the hypothesis that these mechanisms are responsible for the higher strength of the thicker plies, and that the intralaminar fracture toughness of the 0° plies should not be scaled with the ply thickness.

Cohesive zone and interfacial thick level set modeling of the dynamic double cantilever beam test of composite laminate

The mode-I interlaminar fracture toughness of composite laminates under different loading rates can be measured by the double cantilever beam (DCB) test. It is observed from the DCB test of a unidirectional PEEK/carbon composite laminate that as the loading rate increases from quasi-static to dynamic range: (1) delamination crack growth exhibits a transition from stable to unstable (“stick/slip”) and back to a stable type; (2) the interlaminar fracture toughness is not constant as the loading rate increases. In this paper, two numerical approaches are used to reproduce the experimental observations: a cohesive zone model (CZM) and the interfacial thick level set (ITLS) model. CZM simulations with rate-independent and rate-dependent cohesive laws are carried out. A new version of the ITLS is introduced with a phenomenological relation between crack speed and energy release rate. The simulation results of the CZM and the ITLS model are compared with the real DCB test data to evaluate the capability of these two types of models. It is found that the used CZM can reproduce rate-dependence of the fracture energy, but not the stick/slip behavior. The ITLS can capture the stick/slip behavior, but needs different parameter sets for different loading rates.

Influence of sprayed multi-walled carbon nanotubes on mode I and mode II interlaminar fracture toughness of carbon fiber/epoxy composites

Unidirectional carbon fiber/epoxy prepregs were sprayed over their surfaces with different multi-walled carbon nanotubes (MWCNTs) concentrations in order to evaluate their effect on interlaminar fracture toughness of composite laminates under modes I and II loading and the influence in fracture mechanisms. To this aim, determination of the mode I and mode II interlaminar fracture toughness of composite laminates reinforced with sprayed MWCNTs were experimentally investigated using double cantilever beam and end notched flexure tests. Composite laminates without MWCNTs were also fabricated as a reference material. MWCNT characterization using Fourier Transform Infrared and Raman spectroscopy was also carried out in order to evaluate their functional groups and structural defects. The results show that the mode I and mode II interlaminar fracture toughness of the composite laminates can be moderately increased as MWCNT concentrations varying from 0.05 to 0.2 wt.%. Scanning electron microscope images of fracture surfaces provide evidence that the MWCNTs bridged and pulled out at the interface of composite laminates. It was also demonstrated that the optimum weight percentage of MWCNTs needed to improve the mode I and mode II interlaminar fracture toughness was 0.05%, due to the fact that higher concentrations promote the agglomeration of MWCNTs weakening the laminate interfaces.

Experimental characterization of interlaminar fracture toughness of composite laminates assembled with three different carbon fiber lamina

In the present work, the fracture resistance of a carbon fiber composite under mode I and mode II loading have been experimentally determined. For the mode I and II, the energy release rate G has been determinate for each material. In some cases, only a single estimation of G was possible due to problems in the propagation such as extensive fiber bridging and loss of planarity of the running crack. The experimental results relative to DCB tests have been analyzed in order to derive statistical trends. Only the samples for which more than three crack advance data points have been collected are considered in the analysis. The G values are those obtained with the compliance calibration method (CC). For ENF test, determination of critical GII, in addition to the value calculated with the relationship given in the prescription EN6034, other two values, the non linear and visual non linear, are also given. The crack propagation resulted to be unstable for all specimens tested and only a single value of GII could be determined.

Strain rate effects on the intralaminar fracture toughness of composite laminates subjected to compressive load

This paper presents an experimental and numerical study focused on the mode-I intralaminar toughness characterization of a woven carbon/epoxy composite loaded in compression and subjected to high strain rates. Simulations for non-standardized Single Edge Notch Bending (SENB) and Double Edge Notch (DEN) specimens were carried out using a continuum damage mechanics based failure model implemented as an user defined material model within ABAQUS software. A Finite Element Model was used in order to produce an optimal specimen for intralaminar fracture toughness tests. A new data reduction scheme based on the numerical evaluation of the strain energy release rate using the J-integral method is proposed to determine the stress intensity factor for composites. The proposed methodology accounts for finite geometry and material anisotropy effects. The dynamic tests were carried out at strain rates of 560s-1,690s-1,770s-1 using an adapted version of the Split Hopkinson Pressure Bar. A high-speed camera was used for monitoring the crack propagation. A Scanning Electron Microscope (SEM) was used to aid the fractographic analyses on the damaged surface of the tested samples searching for the possible failures mechanisms within the material. The experimental results indicated that the composite laminates studied herein are very sensitive to the strain rate effects.

A generalized plane-strain crack density-based model for evaluating the finite fracture toughness of composite laminates

ABSTRACTThe present study focuses on a developed crack density-based model for evaluating the material properties of an orthotropic composite ply containing a specified matrix-cracking density. Furthermore, more complementary details of this model, including a closed form solution for evaluating the stress fields as well as stiffness degradation of a damaged ply, will be presented. The derived relations will be applied for evaluating the master plot curve, which is applicable for obtaining the finite fracture toughness (Gmc) of laminated composites. The obtained results will be compared with the available experimental results.

On Consideration the Mode I Fracture Response of CFRP Composite Interleaved by Composite Nanofibers

Inherent brittleness of epoxy in composite laminates restricted their application in many industries. Therefore, researchers are trying to decrease this shortage by interleaving electrospun nanofibers for toughening the matrix. In this research the influence of composite nanofibers on mode I fracture toughness of composite laminate is considered. For this aim, Polycaprolactone (PCL) and Nylon 6,6 nanofibers and their composition were interleaved in the mid-plane of unidirectional carbon/epoxy laminates. The results showed that PCL nanofibers did not have a positive effect on mode I energy release rate (GI) and Nylon 6,6 could increase it significantly, while the effect composite nanofibers is more than both of these nanofibers. For understanding more details and the role of nanofibers during the test, the crack path was investigated by optical microscope.

Delamination Behaviour of Rubber-Toughened Carbon Fibre/Epoxy Composites

In this study, core-shell rubber (CSR) and liquid rubber (LR) were used to modify the matrix toughness of unidirectional carbon fibre/epoxy composites. Double cantilever beam (DCB) and end notched flexure (END) tests were performed to evaluate the interlaminar fracture toughness. It was found that LR was identified to be more effective than CSR in improving GICand GIICof the composites, although fracture toughness of the CSR-modified epoxy was better than that of the LR-modified epoxy. SEM observation of post-fracture surfaces of the specimens shows that the degree of plastic deformation of matrix is well related to the rating of fracture toughness of composites for these unmodified and modified composite laminates, and is the key factor controlling the interlaminar fracture toughness of composite laminates. Further, it was confirmed that rigid fibres constrain growth of plastic zone in composites laminates, comparing with toughened bulk epoxy matrix. However, plastic zone is not limited to a single resin layer and it is capable of developing across rigid fibre layers.

Study on the Mode I Interlaminar Fracture Toughness of Multi-Directional Composite Laminates

The double cantilever beam (DCB) test method and the modified beam theory are adopted to investigate the Mode I interlaminar fracture toughness of multi-directional composite laminates. The test procedure was developed by using a stereoscopic microscope to observe the delamination front tip and a testing machine to record the displacement and load data. A dial indicator was used to eliminate the error due to initial clearance in the clamp. A modified beam theory and a compliance calibration method were used to calculate the interlaminar fracture toughness. The Mode I interlaminar fracture toughness of carbon fiber reinforced bismaleimide resin matrix (BMI) composite laminates with four different interface patterns ( 0/0, 45/-45, 0/-45 and 0/90, respectively) was obtained. The results show that the patterns of interface ply angles have an obvious influence on Mode I interlaminar fracture toughness of composite laminates.

An enhanced beam-theory model of the mixed-mode bending (MMB) test—Part I: Literature review and mechanical model

The paper presents a mechanical model of the mixed-mode bending (MMB) test used to assess the mixed-mode interlaminar fracture toughness of composite laminates. The laminated specimen is considered as an assemblage of two sublaminates partly connected by an elastic–brittle interface. The problem is formulated through a set of 36 differential equations, accompanied by suitable boundary conditions. Solution of the problem is achieved by separately considering the two subproblems related to the symmetric and antisymmetric parts of the loads, which for symmetric specimens correspond to fracture modes I and II, respectively. Explicit expressions are determined for the interfacial stresses, internal forces, and displacements.