Interlaminar fracture toughness and associated fracture behaviour of bead-filled epoxy/glass fibre hybrid composites

To investigate enhancement of matrix-dominated properties (such as interlaminar fracture toughness) of a composite laminate, two different bead-filled epoxies were used as matrices for the bead-filled epoxy/glass fibre hybrid composites. The plane strain fracture toughness of two different bead-filled epoxies have been measured using compact tension specimens. Significant increases in toughness were observed. Based on these results the interlaminar fracture toughness and fracture behaviour of hybrid composites, fabricated using bead-filled epoxy matrices, have been investigated using double cantilever beam and end notch flexure specimens for Mode I and Mode II tests, respectively. The hybrid composites based on carbon bead-filled matrix shows an increase in both G _IC initiation and G _IIC values as compared to a glass fibre reinforced plastic laminate with unmodified epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15% and 20%. However, the unmodified epoxy glass-fibre composite shows a higher G _IC propagation value than that of hybrid composites, due to fibre bridging, which is less pronounced in the hybrids as the presence of the beads results in a matrix-rich interply region.     

Fracture behaviour of model toughened composites under Mode I and Mode II delaminations

The fracture behaviour of two toughened epoxy composite systems was investigated using various microscopy techniques. The Mode I delamination fracture toughness,G _IC, Mode II delamination fracture toughness;G _IIC, and compression after impact (CAI) strength of these model composite systems were also measured. Under Mode I fracture, it was found that these composites exhibit nearly identical toughening mechanisms to those of the rubber-modified neat resins. The composites differ primarily in having smaller damage zones than the neat resin equivalents. Under Mode II fracture, the typical hackles were found to initiate from inside the resin-rich interlaminar region due to the presence of the toughener particles. The CAI strength, based on the present study as well as the work conducted by others, appeared to be related to, but not necessarily strongly dependent on, the interlaminarG _IC andG _IIC, the thickness of the interlaminar resin-rich region, and the type of the interlaminar toughener particles. Approaches for improving theG _IC,G _IIC, and CAI strength of high-performance toughened composites are discussed.     

Comparison of Interlaminar Fracture Toughness in Unidirectional and Woven Roving Marine Composites

This paper investigates the effect of fibre lay-up and matrix toughness on mode I and mode II interlaminar fracture toughness (G_Ic and G_IIc) of marine composites. Unidirectional and woven roving fibres were used as reinforcements. Two vinyl ester resins with different toughness were used as matrices. Results from both modes showed toughness variation that is consistent with matrix toughness. Values of G_Ic were not significantly influenced by fibre lay-up except at peak load points in the woven roving/brittle-matrix composite. Each peak load point, caused by interlocked bridging fibres, signified the onset of unstable crack growth. For unidirectional specimens, crack growth was stable and G_Ic statistically more reliable than woven roving specimens, which gave fewer G_Ic values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, G_IIc was higher in woven roving composites. This was due to fibre bridging, perpendicular to the crack growth direction, which encouraged stable crack growth and increased energy absorption. Mode II R-curves were obtained for the woven roving specimens. These R-curves provide additional information useful for characterising delamination resistance. The paper concludes that composites with woven roving fibres show similar mode I delamination characteristics to the unidirectional composites; but their mode II delamination characteristics, after crack initiation, are quite different.     

Interlaminar fracture properties of weft-knitted/woven fabric interply hybrid composite materials

An experimental study has been undertaken to characterize the delamination behavior and tensile properties of interply hybrid laminated composites reinforced by interlock weft-knitted and woven glass fiber preform fabrics. The hybrid composites, comprising the alternate layers of interlock and uniweave fabrics, were compared to interlock knitted (only) and uniweave (only) composites with respect to delamination and tensile performances. Mode-I double cantilever beam and mode-II end-notched flexure tests were carried out to assess the interlaminar fracture toughness using aluminum-strip stiffened specimens. The mode-I and mode-II interlaminar fracture toughness values, G _IC and G _IIC, for the hybrid composite were about three and two times higher than that for the uniweave composite, respectively. The tensile strength and modulus of the hybrid composite were 315 MPa and 12.8 GPa in the wale direction, respectively, demonstrating that the strength and modulus were found to be slightly lower than those of the uniweave composite, and significantly improved in comparison with the interlock knitted composites.     

Mode I delamination behaviour of short fibre reinforced carbon fibre/epoxy composites following environmental conditioning

Carbon fibre laminates based on Newport Adhesives NCT-301 uniaxial pre preg were prepared for Mode I DCB testing. Polyamide and Polyolefin web materials supplied by Spunfab Ltd and Toyobos’ Zylon HM fibres were applied to the interlaminar region to enhance fracture toughness. Three of each specimen type were “treated”, subjected to immersion in 95 °C water for a period of 600 h. Two “untreated” control specimens of each type were aged at 21 °C and 40-95% relative humidity. Specimen weight gains in the order of 2% were found to occur in all immersed samples. Fracture toughness increases were evidenced in all treated samples with the exception of the Polyamide Web sample. Average G_Ic increases in treated samples were 250–300%. Untreated samples were found to have exclusively interlaminar crack propagation. Untreated samples containing Polyamide or Polyolefin Web reinforcement displayed increased fracture toughnesses. These toughness increases were attributed to improved interlaminar bonding.     

Characterization of transverse tensile, interlaminar shear and interlaminate fracture in CF/EP laminates with 10 wt% and 20 wt% silica nanoparticles in matrix resins

The transverse tensile properties, interlaminar shear strength (ILSS) and mode I and mode II interlaminar fracture toughness of carbon fibre/epoxy (CF/EP) laminates with 10 wt% and 20 wt% silica nanoparticles in matrix were investigated, and the influences of silica nanoparticle on those properties of CF/EP laminates were characterized. The transverse tensile properties and mode I interlaminar fracture toughness (G_IC) increased with an increase in nanosilica concentration in the matrix resins. However, ILSS and the mode II interlaminar fracture toughness (G_IIC) decreased with increasing nanosilica concentration, especially for the higher nanosilica concentration (20 wt%). The reduced G_IIC value is attributed to two main competing mechanisms; one is the formation of zipper-like pattern associated with matrix microcracks aligned 45° ahead of the crack tip, while the other is the shear failure of matrix. The ratio of G_IIC/G_IC decreased with the concentration of silica nanoparticles, comparable with similar CF/EP laminates with dispersed CNTs in matrix. Fractographic studies showed that interfacial failure between carbon fibre and epoxy resin occurred in the neat epoxy laminate, whereas a combination of interfacial failure and matrix failure occurred in the nanosilica-modified epoxy laminates, especially those with a higher nanosilica concentration (20 wt%).     

Improving the interlaminar properties of polymer composites using a situ accumulation method to construct the multi-scale reinforcement of carbon nanofibers/carbon fibers

A novel method was developed to realize the situ accumulation of carbon nanofibers (CNFs) in the carbon fiber reinforced polymer composites (CFRPs) to construct the multi-scale reinforcement for improving the interlaminar properties. In this method, the prepreg was sealed by the nanomicroporous nylon membrane, and the excess resin was extracted from the prepreg by the vacuum-assisted method. It was found that the use of nylon membrane resulted in effective CNFs accumulation, especially in the interlayer by scanning electron microscopy. Short-beam strength tests and the end-notched flexure tests were conducted respectively to evaluate the interlaminar properties of CFRPs under shear loading. The results indicated that the interlaminar shear strength (ILSS) and the Mode II interlaminar fracture toughness (G_IIC) of CFRPs made by the filtering membrane-assisted method remarkably increased compared with those prepared without using filtering membrane.     

Damage Resistance of Composites Based on Glass Fibre Reinforced Low Styrene Emission Resins for Marine Applications

Composites based on glass fiber reinforced low styrene emission polyester resins have been widely used over the last 10 years, in order to meet increasingly strict safety regulations, particularly in the pleasure boat industry. Previous studies of their mechanical properties suggested that although these resins are generally more brittle than traditional orthophthalic polyester resins this did not adversely affect the properties commonly used for quality control (short beam shear and tensile failure strength of mat reinforced composites). In the present paper results from a more detailed study of damage behaviour are presented. Tests include fracture toughness (K _Ic ) tests on resins, fibre/matrix interface energy, detection of composite damage initiation in tension by acoustic emission, composite delamnation (G _Ic and G _IIc ), and low energy impact. Overall the results indicate that the low failure strain of low styrene emission resins results in significantly lower composite damage resistance.     

Mode II interlaminar fracture properties of Moso bamboo

Bamboo is a kind of biological composites reinforced by unidirectional long fiber. Once there exists crack, the propagation of delamination is controlled by the interlaminar fracture toughness instead of by strength. In this paper, the end notched flexure (ENF) beam specimen was used to test the Mode II interlaminar fracture toughness G_IIC along grain of Moso bamboo internode and the fracture surface was analyzed. The results were obtained that the Mode II interlaminar fracture toughness G_IIC calculated by the experiment parameter substitution method was more accurate and the value was 1303.18 J/m² (coefficient of variation = 8.96%) which was about three times higher than the value of Mode I interlaminar fracture toughness; the crack propagation of Mode II interlaminar fracture was mainly self-similar cracking, but the fracture surface was rougher. Ground tissue in the zone of Mode II crack propagation was characterized by hackle shearing deformation. The SEM photos showed that ground tissue separated from fiber along middle lamella under shear stress and as the increasing of the dislocation of upper and lower layer, the thin-walled ground tissue would fracture transversely by tension, while to thick-walled fiber cell, only middle lamella and primary wall were torn then debonded, fragments remained.     

Glass-fibre-reinforced composites with enhanced mechanical and electrical properties - Benefits and limitations of a nanoparticle modified matrix

Nanoparticles and especially carbon nanotubes (CNTs) provide a high potential for the modification of polymers. They are very effective fillers regarding mechanical properties, especially toughness. Furthermore, they allow the implication of functional properties, which are connected to their electrical conductivity, into polymeric matrices. In the present paper, different nanoparticles, as fumed silica and carbon black, were used to optimise the epoxy matrix system of a glass-fibre-reinforced composite. Their nanometre-size enables their application as particle-reinforcement in FRPs produced by the resin-transfer-moulding method (RTM), without being filtered by the glass-fibre bundles. Additionally, an electrical field was applied during curing, in order to enhance orientation of the nanofillers in z-direction. The interlaminar shear strengths of the nanoparticle modified composites were significantly improved (+16%) by adding only 0.3 wt.% of CNTs. The interlaminar toughness G_Ic and G_IIc was not affected in a comparable manner. The laminates containing carbon nanotubes exhibited a relatively high electrical conductivity at very low filler contents, which allows the implication of functional properties, such as stress-strain monitoring and damage detection.     

The interlaminar fracture behaviour and toughening mechanisms of new carbon fibre-reinforced bismaleimide composites

The interlaminar fracture behaviour of carbon fibre-reinforced bismaleimide (BMI) composites prepared by using a new modified BMI matrix has been investigated by various methods. Laminates of three typical stacking sequences were evaluated. Double cantilever beam, end-notch flexure and edge-delamination tension tests were conducted under conventional conditions and in a scanning electron microscope. The strain energy release rates in Mode I and Mode II, G_Ic and G_IIc, as well as the total strain energy release rate, G_mc, have been determined and found to be higher than those for laminates with an epoxy matrix. Dynamic delamination propagation was also studied. The toughening mechanisms are discussed.     

Mode I interlaminar fracture toughness properties of advanced textile fibreglass composites

The Mode I interlaminar fracture toughness properties of vinyl ester-based composites reinforced with fibreglass manufactured by the advanced textile technologies of braiding, knitting, stitching and through-the-thickness weaving are assessed in comparison to a variety of traditional composites made from fibreglass such as unidirectional or woven rovings. The interlaminar fracture toughness (G_Ic) of braided and knitted composites are higher than traditional composites by factors of more than two and four, respectively. Toughening in these textile composites was caused by extensive crack branching as the interlaminar crack was forced to follow a tortuous path through the complex fibre architectures. The G_Ic values of the composites reinforced in the through-thickness direction by weaving or stitching were higher than traditional composites by factors of nearly two and three, respectively, with the main toughening mechanism being crack bridging by the through-thickness binder yarns/stitches. A review of Mode I interlaminar fracture data collected from papers shows that advanced textile techniques are capable of manufacturing composites with substantially improved delamination resistance.     

Toughness properties of a three-dimensional carbon-epoxy composite

The three-dimensional (3D) orthogonal interlocked fabric contains through-the-thickness rein-forcement in order to enhance the interlaminar fracture toughness of the composite. The interlaminar fracture toughness of a carbon-epoxy orthogonal interlocked fabric composite was experimentally determined by use of the recently developed tabbed double cantilever beam specimen. The data reduction methods applicable to these tests and materials and the interpretation of the results were discussed. The results of critical strain energy release rate,G _Ic, were compared to those of a two-dimensional (2D) laminate having the same in-plane structure. The energy-dissipating crack propagation processes were described. The in-plane fracture toughness of the 3D fabric was experimentally measured and compared to that of the 2D laminate. The through-the-thickness fibres were found to create a ten-fold increase in interlaminar toughness, and a 25% improvement in the in-plane fracture toughness.     

Hygrothermal effects on the shear properties of carbon fiber/epoxy composites

The environmental factors, such as humidity and temperature, can limit the applications of composites by deteriorating the mechanical properties over a period of time. Environmental factors play an important role during the manufacture step and during composite’s life cycle. The degradation of composites due to environmental effects is mainly caused by chemical and/or physical damages in the polymer matrix, loss of adhesion at the fiber/matrix interface, and/or reduction of fiber strength and stiffness. Composite’s degradation can be measure by shear tests because shear failure is a matrix dominated property. In this work, the influence of moisture in shear properties of carbon fiber/epoxy composites (laminates [0/0]_s and [0/90]_s) have been investigated. The interlaminar shear strength (ILSS) was measured by using the short beam shear test, and Iosipescu shear strength and modulus (G ₁₂) have been determinated by using the Iosipescu test. Results for laminates [0/0]_s and [0/90]_s, after hygrothermal conditioning, exhibited a reduction of 21% and 18% on the interlaminar shear strenght, respectively, when compared to the unconditioned samples. Shear modulus follows the same trend. A reduction of 14.1 and 17.6% was found for [0/0]_s and [0/90]_s, respectively, when compared to the unconditioned samples. Microstructural observations of the fracture surfaces by optical and scanning electron microscopies showed typical damage mechanisms for laminates [0/0]_s and [0/90]_s.     

Strength and fracture toughness of carbon fibre polyester composites

Fracturing of carbon fibre/polyester composites has been studied by means of mechanical testing and scanning electron microscopy. Carbon fibres were surface-treated in several ways so as to vary the interlaminar shear strength of the composites, and the effect of this variation on the work of fracture was determined by means of Charpy V-notch impact tests and slow three-point bend tests on notched specimens of triangular cross-section. The effect of moisture on the fracture toughness was also studied by measuring toughness and interlaminar shear strength after exposure to steam. Improvement of the fibre/resin bond results, as expected, in an increase in the brittleness of composites and it appears that a purely mechanical bond, such as might be obtained by acid-etching the fibre surface, is less proof against deterioration in humid atmospheres than a chemical bond, such as can be obtained by the use of coupling agents. Estimates of the magnitude of various contributions to the fracture toughness show that in carbon-fibre-reinforced resins the effect of increasing the stiffness or load-bearing ability of the matrix and the work done against friction in pulling broken fibres out of the matrix contribute approximately one fifth and four fifths, respectively, of the total work of fracture.     

Mechanical properties of carbon fiber/fullerene-dispersed epoxy composites

This study examined the effect of fullerene dispersion on the mechanical properties of carbon-fiber reinforced epoxy matrix composites (CFRPs). Mechanical properties such as tension, compression, open-hole compression, comparession after impact (CAI), binding, short beam shear, and interlaminar fracture toughness were evaluated for [0]₈, [90]₁₆, [45/0/?45/90]_2S laminates. Tension and compression strengths increased 2–12% by dispersing 0.5% of fullerene into the matrix resin. Furthermore, interlaminar fracture toughness of the composite was improved by about 60%. It was revealed that a small amount of fullerene (0.1–1 wt.%) increased the failure strain of epoxy resin itself, thereby improving the CFRP strength.     

Mode II fracture mechanics properties of wood measured by the asymmetric four-point bending test using a single-edge-notched specimen

Using a single-edge-notched specimen of spruce, an asymmetric four-point bending test was conducted to obtain the mode II fracture toughness G_IIc and critical stress intensity factor K_IIc, and the test method was numerically and experimentally analyzed. A three-point bend end-notched flexure test was also conducted and the results were compared with those of the asymmetric four-point bending tests. The crack length had a small influence on the load/loading-line displacement relationship in the asymmetric four-point bending test, so it was difficult to determine the value of G_IIc, which requires the measurement of loading-line displacement. In contrast, the value of K_IIc obtained by two tests was similar when the initial crack length ranged from 0.7 to 0.85 times the depth of the specimen. These results show that the asymmetric four-point bending test is a promising means of determining K_IIc.     

Mode I and mode II fracture toughness of E-glass non-crimp fabric/carbon nanotube (CNT) modified polymer based composites

In this study, mode I and mode II interlaminar fracture toughness, and interlaminar shear strength of E-glass non-crimp fabric/carbon nanotube modified polymer matrix composites were investigated. The matrix resin containing 0.1 wt.% of amino functionalized multi walled carbon nanotubes were prepared, utilizing the 3-roll milling technique. Composite laminates were manufactured via vacuum assisted resin transfer molding process. Carbon nanotube modified laminates were found to exhibit 8% and 11% higher mode II interlaminar fracture toughness and interlaminar shear strength values, respectively, as compared to the base laminates. However, no significant improvement was observed for mode I interlaminar fracture toughness values. Furthermore, Optical microscopy and scanning electron microscopy were utilized to monitor the distribution of carbon nanotubes within the composite microstructure and to examine the fracture surfaces of the failed specimens, respectively.     

A practical testing approach to determine mode II fracture energy GIIF for concrete

Continuing the experiments on the double-edge notched specimens on which the mode II fracture toughness K _IIc of concrete was measured, a practical testing approach to determine mode II fracture energy G _IIF is studied using the same geometry.     

Influence of water-cement ratio on micro-cracking of ordinary concrete

The influence of the water/cement ratio on fracture toughness of ordinary concrete has been investigated. The stress intensity factorK _IIc and fracture energyJ _IIc has been tested (Mode II, shearing). The concrete structure was examined by SEM and the influence of water/cement ratio on concrete cracking has been established.