Interlaminar Fracture Toughness Of Composites Research Articles

The effect of resin properties on toughness translation in interleaved composites

Interleaving is a well-recognized method for enhancing the interlaminar fracture toughness of composite materials. However, theoretical toughness is often not realized since cracks tend to propagate through regions of lower toughness. While most studies have focused on the effect of interleaf properties, e.g., thickness and interleaf resin properties, on composite interlaminar fracture toughness, the effect of matrix resin properties on interleaved composite toughness has been overlooked. Recently, we hypothesized that there is a relationship between toughness translation and the ratio of matrix to interleaf resin toughness. In this work, we use additive manufacturing to test this hypothesis with a range of resins in interleaved composite. Toughness is quantified via mode I delamination resistance. Our results confirm our hypothesis that the ratio of matrix to interleaf resin fracture toughness, i.e., the ratio of [Formula: see text], is directly correlated to the degree of toughness translation. More specifically, a ratio of approximately 0.5 is required for effective interleaving; below 0.5, the toughness translation is significantly below the theoretical value. This approach has important implications for the design and fabrication of tough composites via choice of resins.

Enhanced interfacial and mechanical properties of glass fabric/epoxy composites via grafting silanized carbon nanotubes onto the fibers

AbstractFor enhancing the interfacial adhesion of glass fabric (GF)/epoxy composites, silanized multi‐walled carbon nanotubes (MWCNTs) were used to treat GF surface, where carboxyl MWCNTs were functionalized by silane coupling agents based on the covalent or non‐covalent bonding, followed by the vacuum infusion process of laminated composites. A detailed study was employed to confirm the effects of silanized MWCNTs uniform dispersion and interfacial properties of resultant composites. Making full use of suitable silane coupling agents (γ‐aminopropyltriethoxysilane) treatment, the uniform MWCNTs dispersion and strong fiber/matrix interfacial adhesion could be achieved. Compared with the control sample, flexural and interlaminar shear strengths of resultant composites were enhanced by about 24% and 22%, respectively, indicating superior mechanical strength for composites; flexural modulus and storage modulus in the glassy state were improved by about 36% and 68%, respectively, suggesting higher stiffness for composites; the work of fracture (Wf) under interlaminar shear tests was raised by about 188%, revealing better interlaminar fracture toughness of composites. In addition, glass transition temperature of resultant composites also increased. This work provides a guiding strategy for manufacturing fiber‐reinforced composites requiring adjustable strength, stiffness, and toughness.Highlights Silane coupling agent was grafted on MWCNTs by covalent/non‐covalent bonding. Active groups of silanized MWCNTs induced crosslinking reactions with matrix. Silanized MWCNTs dispersion and glass fiber/matrix adhesion were improved. Strengthening/toughening mechanisms of laminated composites were analyzed.

Non‐destructive self‐healing design for carbon fiber/epoxy composites

AbstractSelf‐healing composites are promising for engineering, but self‐healing structures or components usually induce reduction in the composite's mechanical properties. In this paper, core‐shell nanofibers which accommodating healing agent in the core and incorporating carbon nanotubes (CNTs) in the shell were produced to develop non‐destructive self‐healing carbon fiber/epoxy (CF/EP) composites. The epoxy resin and curing agent were encapsulated in CNTs‐reinforced polyacrylonitrile (PAN) core‐shell electrospinning nanofiber mats and the mats were placed on carbon fibers to fabricate CF/EP composite laminates by the vacuum‐assisted resin transfer molding (VARTM) process. The results indicated that the flexural strength, flexural modulus, tensile strength, tensile modulus, and Mode II interlaminar fracture toughness of composites with suitable self‐healing components increased by 49%, 44%, 8%, 13%, and 43%, respectively, compared to the pristine CF/EP. The healing flexural strength of the composite with 1 wt% CNTs recovered up to 93.57%, observed 24 h after the bending fracture, which is higher than the initial strength of CF/EP composites without self‐healing components. The research indicates that self‐healing composites can be developed without mechanical properties degrade and even achieve stronger mechanical properties.

In-situ X-ray computed tomography of mode I delamination in carbon-epoxy composites: The effect of the interface ply orientation

Test methods determining interlaminar fracture toughness of composites are typically performed in conjunction with in-situ surface imaging techniques to detect the crack initiation and monitor its propagation. However, these methods struggle to visualise the 3D evolution of the crack front. In such tests, initiation detection is based on force-displacement curve features, such as the non-linearity point, or edge observations. This paper reports the implementation of a methodology for in-situ 3D characterisation of the crack front in a double cantilever beam test using microfocus X-ray computed tomography. The crack initiation and propagation inside the bulk were determined through crack segmentation in the in-situ tomograms, allowing quantification of the fracture toughness without the need for artificially imposed initiation criteria such as the non-linearity. Two different delamination interfaces were investigated: 0°//0° and +45°//−45°. These two interfaces can be viewed as two extremes in terms of delamination behaviour. The results of the proposed methodology are compared against those measured via the “conventional” edge microscopy coupled with digital image correlation.

Composite interlaminar fracture toughness imparted by electrospun PPO veils and interleaf particles: A mechanistical comparison

Thermoset composite toughening by interleaf materials receives increasing attention as requirements for high toughness become essential for primary structures. Here, polyphenylene oxide (PPO) electrospun veils were used for carbon fiber epoxy composite interleaf toughening. Their Mode I, Mode II interlaminar fracture toughness (GIC and GIIC) were measured as a function of veil loading levels. 10 wt% veil loading rendered the best toughening performance, reached to 530 J/m2 and 1400 J/m2 in GIC and GIIC, respectively, and it yielded further increase of 140% and 30% compared to the corresponding PPO particles. Scanning electron microscopy (SEM) and 3-D laser microscopic imaging analysis (3D-LMIA) on fracture surfaces revealed energy absorbing mechanisms such as bridging and plastic deformation. The continuous network structure imparted by the veils accounted for the additional toughening mechanisms, i.e. the deformation, breakage and pull-out of PPO fibers, that were responsible for the outstanding fracture toughness.

Production and characterization of cellulose nanocrystals of different allomorphs from oil palm empty fruit bunches for enhancing composite interlaminar fracture toughness

The effect of different alkaline treatments during the extraction of α-cellulose from oil palm empty fruit bunches on the properties of subsequently extracted cellulose nanocrystals are reported. Sodium hydroxide in an alkaline process step results in a change of native cellulose from type I to II. Treatment with potassium hydroxide maintains the native allomorph of cellulose I. Compared to commercial cellulose nanocrystals (CNCs), oil palm empty fruit bunch (OPEFB) CNCs type I materials have higher aspect ratios, higher crystallinity index, but lower charge, and thermal stability compared to commercial CNCs. OPEFB CNCs type II have a higher aspect ratio, but lower charge than both types of CNC. Type I CNCs exhibit intermediate charge, but higher thermal stability which might make them more suitable for processing with thermoplastic polymers. A full map of physical properties, comparing the oil palm based CNCs to both commercial and lab-based materials is presented to better understand how they might be exploited in composite applications. It is demonstrated that aligned electrospun cellulose acetate butyrate nanofibrous networks containing these CNCs can enhance the mode-II interlaminar fracture toughness of a carbon fibre-epoxy laminate.

The enhanced interlaminar toughness of carbon fiber reinforced poly(ether ether ketone) composites modified by carbon nanotubes

• PEEK prepregs were modified with CNTs by a simple powder slurry method. • Laminates directly manufactured by prepregs have enhanced interlaminar toughness. • The G Ic and G IIc with 0.5 wt% CNTs ware improved by 78.8% and 90.5%, respectively. • The flexural properties and interlaminar shear strength were enhanced by CNTs. The interlaminar toughness of poly(ether ether ketone)(PEEK) composites has received extensive attention. In this study, carbon fiber reinforced PEEK prepregs were manufactured by the powder slurry method. Carbon nanotubes(CNTs) were introduced into prepregs during the manufacturing procedure. The double cantilever beam(DCB) and end notched flexure(ENF) specimens were prepared to evaluate the interlaminar fracture toughness of the composites. The results reveal that by the addition of 0.5 wt% CNTs, the mode I and mode II interlaminar fracture toughness of PEEK composites were improved by 78.8% and 90.5%, respectively. The flexural properties and interlaminar shear strength (ILSS) were also enhanced by CNTs. This paper provides a facile method for the direct preparation of high interlaminar toughness prepregs.

Effects of the Interlayer Toughening Agent Structure on the Flow Behavior during the z-RTM Process.

In this paper, interlayer toughening composites were prepared by the z-directional injection RTM process (z-RTM), which has the advantage of increasing the interlaminar toughness and shortening the filling time and completely impregnating the fibers. The nonwoven fabrics and dot matrix structure material were used as ex situ interlayer toughening agents. The effect of the interlayer toughening agent structure on the resin flow behavior during the z-RTM process was investigated. The macro-flowing and micro-infiltration behaviors of the resin inside the preforms were deduced. The permeability of the fabric preforms with different toughening agents was investigated. The results show that the introduction of the nonwoven structure toughening agent makes the macro flow slow, and the flow front more uniform. The toughening agent with a dot matrix structure promotes the resin macro flow in the preforms, and shortens the injection time. The z-directional permeability of the preform with a dot matrix structural toughening agent is one order of magnitude lower than that of the non-toughened preform, while being higher than the preform toughened by the nonwoven fabric preforms, which is helpful for the further applicability of the z-RTM process. Furthermore, the mode II interlaminar fracture toughness of composites was evaluated.

Experimental evaluation of mode I fracture toughness of dissimilar-material joints with thermal residual stresses

The mode I interlaminar fracture toughness of dissimilar-material joints with thermal residual stresses is experimentally evaluated. The double cantilever beam (DCB) test is commonly used to assess the interlaminar fracture toughness of composites but is inadequate when dissimilar materials are joined owing to the mixed mode, which arises from differences in the bending stiffness and the thermal residual stresses formed during manufacturing because of differences in the linear expansion coefficient of the adherends. To address these difficulties, a new test, named the constant-load DCB test, was developed to evaluate the mode I interlaminar fracture toughness. In this study, a testing machine, based on the theoretical method previously established to derive the mode I interlaminar fracture toughness of dissimilar-material joints, is manufactured, and the fracture toughness is experimentally determined. As a result, good agreement between the theoretical value and experimental results is obtained, and the mode mixity ratio is significantly decreased compared with that of the general DCB test.

Alternating salt‐fog spray aging of stitched woven composites: Mode I interlaminar fracture toughness

AbstractThe experimental investigation was focused on the effect of salt‐fog corrosion on the interlaminar facture behavior of stitched woven composites with various stitching patterns. To this aim, four kinds of stitched woven composites were exposed to alternating salt‐fog and hot‐air environmental conditions according to GJB 150.11A‐2009 standard, for 2 and 4 cycles. In particular, an un‐stitched woven composite was used as a control. Double cantilever beam (DCB) tests were carried out in order to evaluate the interlaminar fracture toughness of woven composites during the aging process, and a scanning electron microscope (SEM) was employed to reveal the interlaminar damage mechanisms. The results provide evidence that the stitching method can significantly improve the interlaminar fracture toughness of woven composites. The decrease of Mode I interlaminar fracture toughness for stitched woven composites is more serious as the salt‐fog spray aging cycles are increased. The stitching density has a significant effect on Mode I interlaminar fracture properties of the stitched woven composite both before and after the salt‐fog spray aging treatment. The effect of salt‐fog spray aging became more significant with an increase of the stitching density, while the interlaminar fracture toughness of the composites increased significantly.

WITHDRAWN: Experimental assessment of the effect of fibre distribution of interleaved veils on the interlaminar fracture toughness of composites

WITHDRAWN: Experimental assessment of the effect of fibre distribution of interleaved veils on the interlaminar fracture toughness of composites

Investigation of toughening effect of biomass‐derived SiCB on basalt fiber‐reinforced unsaturated polyester composites

AbstractFiber‐reinforced polymer composites are outstanding lightweight structural materials, so that they are widely used in automotive, marine, medical, defense, and aerospace fields. However, when subjected to structural loads, they are prone to failure due to their inherent brittleness and the susceptibility to delamination, which can lead to the loss of the load‐bearing capacity of composite materials. This article proposes an effective solution of innovatively using the silicon carbon black, a kind of sustainable green biomass material derived from rice husks, to toughen the composites. When the content of silicon carbon black (SiCB) added to the composites is 1.5 wt%, the maximum values of flexural strength, bending energy absorption, and the mode II interlaminar fracture toughness values are 333 MPa, 6.10 J, and 1591 J/m2, respectively, corresponding to the improvement percentage of 138%, 120%, and 216%. The addition of SiCB significantly improved the bending properties and the mode II interlaminar fracture toughness of composites. The toughening mechanisms of SiCB were studied based on the fracture surface morphologies of the composites.

Correction of the large displacement effect on determination of mode I interlaminar fracture toughness of composite

Large displacement has a considerable effect on determination of mode I interlaminar fracture toughness of composite. It is found that the mode I interlaminar fracture toughness of composite double cantilever beam (DCB) with piano hinge under large displacement is greatly underestimated by using the data reduction methods in the ASTM D5528 and ISO 15024 standards. The nonlinear J-integral, nonlinear beam theory, and finite element method are used to study the effect of large displacement on the determination of the interlaminar fracture toughness of composite. These standards are improved by this paper. Accurate and consistent fracture toughness can be obtained by using the J-integral method and the corrected ASTM/ISO data reduction methods.

Experimental study towards inter-laminar fracture toughness of different fibre reinforced polymer composites

Throughout the years, fibre reinforced polymer (FRP) composites have been developed as new valuable materials which are suitable for a number of engineering applications due to its high performance such as high strength and superior characteristics. However, FRP composites are susceptible to the damage such as inter-laminar delamination, which tend to greatly reduce the stiffness of the composite structure. The aims of this study are to compare and evaluate the Mode I inter-laminar fracture toughness of the natural FRP composites and the synthetic FRP composites. Samples from flax, glass and carbon fibres were prepared using resin infusion technique. The Mode I inter-laminar fracture toughness of these composites stress were tested under tensile loading by using a Double Cantilever Beam (DCB) test in accordance to the ASTM standard. The results showed that the fracture toughness of the natural FRP composites increases with delamination length. Whereas, for the synthetic FRP composites, the fracture toughness was scattered, in which it was slightly increased within the first few delamination length, gradually decreased and increased again with respect to the delamination length.

Synergistic mode II delamination toughening of composites using multi-scale carbon-based reinforcements

This paper presents an investigation into the synergistic improvements to the mode II interlaminar fracture toughness of composites using multi-scale carbon reinforcements spanning the nano to millimetre length scales. Varying volume fractions of carbon nanofibres (CNFs) and millimetre-long carbon fibre z-pins were used to increase the mode II delamination resistance of multi-scale reinforced composites. The results reveal that adding CNFs into the epoxy matrix and transversely inserting z-pins creates a synergistic, rather than additive, improvement to the mode II fracture toughness. The magnitude of the synergistic toughening effect depends on the volume contents of both the nanofibres and z-pins, with the measured synergistic improvement to the mode II toughness being up to 26% (i.e. greater than the expected additive toughening contribution from each filler type). A finite element (FE) numerical model has been developed to accurately predict the mode II fracture properties and the synergistic toughening effect exhibited by the multi-scale reinforced epoxy composites.

Effect of nano-OMMTs on mode I and mode II fracture toughness of continuous glass fibre reinforced polypropylene composites

The influence of nano-organic montmorillonites (nano-OMMTs) on mode I and mode II interlaminar fracture toughness of continuous glass fibre reinforced ammonium polyphosphate polypropylene (CGF/PP/APP) composites was experimentally studied in this paper. Nano-OMMTs modified CGF/PP/APP unidirectional prepreg composites were fabricated by hot-melt impregnation process. To investigate the effect of nano-OMMTs on the interlaminar fracture toughness of composites, double cantilever beam (DCB) test and end-notched flexure (ENF) test were conducted to obtain the mode I initiation interlaminar fracture toughness (GIC init.), mode I propagation interlaminar fracture toughness (GIC prop.), and mode II initiation interlaminar fracture toughness (GII C init.) of samples, respectively. Through comparison against to the neat CGF/PP/APP composites, with the addition of 6.5 wt% nano-OMMTs into PP matrix, the GIC init., GIC prop., and GII C init. of composites can be enhanced from 0.502 kJ/m2, 0.746 kJ/m2 and 0.623 kJ/m2 to 0.712 kJ/m2, 1.119 kJ/m2, and 1.195 kJ/m2, respectively. Finally, it was found that nano-OMMTs in matrix can act as “pins”, which could absorb energy in the forms of nano-particles pullout and enhance the interlaminar fracture toughness values in DCB and ENF tests.

Improvement of mode-II interlaminar fracture toughness of carbon textile composites with modified matrix of thermoplastic and thermoset epoxy-addition of glass fibers

Interlaminar fracture toughness of a material with a layered structure such as CFRP is poor and cracks between the layers easily lead to fatal result. Interlaminar fracture toughness still plays an important role in damage propagation of CFRP. In this study, mode II inter-lamina fracture toughness of CFRP was investigated by ENF test for clarifying effect on weight-average molecular weight of matrix and matrix modification with glass fiber. Micro glass fibers whose diameter was 0.5 μm with 0.3wt% of weight content were added into thermoplastic epoxy resin and thermoset epoxy resin, respectively, for the matrix of the textile carbon fiber composite prepared by our laboratory. State of the fracture surface was also changed accompanying with the change of the weight-average molecular weight of the matrix. In these results, Mode II interlaminar fracture toughness of the composite was improved by preventing the crack propagation with fiber bridging of added glass fibers on the interface regardless of the types of matrix of thermoplastic and thermoset epoxy resin.

Effect of Paper or Silver Nanowires-Loaded Paper Interleaves on the Electrical Conductivity and Interlaminar Fracture Toughness of Composites

The effect of plant-fiber paper or silver nanowires-loaded paper interleaves on the electrical conductivity and interlaminar fracture toughness of composites was studied. Highly conductive paper was prepared by surface-loaded silver nanowires. The percolation threshold appeared at about 0.4 g/m2. The surface resistivity reached 2.3 Ω/sq when the areal density of silver nanowires was 0.95 g/m2. After interleaving the conductive papers in the composite interlayers, in-plane electrical conductivity perpendicular to the fiber direction was increased by 171 times and conductivity through thickness direction was increased by 2.81 times. However, Mode I and Mode II interlaminar fracture toughness decreased by 67.3% and 66.9%, respectively. Microscopic analysis showed that the improvement of conductivity was attributable to the formation of an electrical conducting network of silver nanowires which played a role in electrical connection of carbon fiber plies and the interleaving layers. However, the density of the highly packed flat plant fibers impeded the infiltration of resin. The parallel distribution of flat fibers to the carbon plies, and poor resin-fiber interface made the interlaminar fracture occur mainly at the interface of plant fibers and resin inside the interleaves, resulting in a decline of the interlaminar fracture toughness. The surface-loading of silver nanowires further impeded the infiltration of resin in the densely packed plant fibers, resulting in further decline of the fracture toughness.

The temperature effects on the fracture toughness of carbon fiber/RTM-6 laminates processed by VARTM

The increasing use of composite in the aircraft industry has raised the interest for a better understanding of the failure process in these materials, which can be also influenced by the manufacturing process of the laminate. Some materials used in vacuum assisted resin transfer molding process have been studied in the open literature but very few data have been published for resin transfer molding-6 epoxy based laminates, in particular studies showing the influence of the temperature on the interlaminar fracture behavior of this type of laminates. The aim of this article is to investigate the interlaminar fracture behavior of resin transfer molding-6 based carbon composite laminates manufactured by vacuum assisted resin transfer molding subjected to Modes I and II at 25℃ and 80℃. The results show the influence of the temperature on the interlaminar fracture toughness of composites and provide a database to design composite aerostructures subjected to temperatures commonly experienced in civil aviation. The fracture aspects of the tested laminates were also investigated and directly related to the trend in results found for the fracture toughness values.

Comprehensive Study on Using VTBN Reactive Oligomer for Rubber Toughening of Epoxy Resin and Composite

ABSTRACTWhile vinyl-terminated butadiene acrylonitrile is frequently used for the toughening of vinylester and polyester, very limited research has been conducted on modification of epoxy with this oligomer. Herein, the effect of vinyl-terminated butadiene acrylonitrile addition to epoxy in bulk and glass reinforced composite is systematically investigated. Thermo-physical behavior and mechanical characteristics of the samples are determined. To interpret the test results, the void content of reinforced samples is measured and fracture surface of the specimens is investigated. It is found that vinyl-terminated butadiene acrylonitrile improves the toughness with slight negligible effects on other characteristics. Incorporation of 15 phr of vinyl-terminated butadiene acrylonitrile increases the KIC of epoxy from 0.6 MPa to 2.3 MPam0.5. Similarly, addition of 15 phr vinyl-terminated butadiene acrylonitrile leads to 67% enhancement in the interlaminar fracture toughness of composites. The toughening mechanisms and toughness transfer from bulk to composite are discussed.