Mode-II interlaminar fracture of composite materials in the presence of randomly distributed defects

International Journal of Fracture - A Correction to this paper has been published: https://doi.org/10.1007/s10704-020-00492-w     

Mode I and Mode II interlaminar fracture toughness of composite laminates interleaved with electrospun nanofibre veils

In this study, the effects of interleaved nanofibre veils on the Mode I and Mode II interlaminar fracture toughness (ILFT) of autoclave cured unidirectional carbon/epoxy composite laminates were investigated. Various electrospun nanofibre veils consisting of a range of different polymer types, fibre diameters and veil architectures were placed in the laminate mid-planes, which were subsequently subjected to double cantilever beam and end-notch flexure tests. It was found that the polymer type and veil areal weight were the most important factors contributing to laminate performance. A 4.5 g/m² PA66 veil provided the best all-round performance with fracture toughness improvements of 156% and 69% for Mode I and Mode II, respectively.     

Evaluation and analysis of interlaminar fracture toughness of bead filled matrix hybrid composite materials using orthotropic fracture mechanics

Evaluation of Mode I interlaminar fracture toughness for unidirectional hybrid composites fabricated with a bead filled epoxies was carried out. The two important fracture toughness parameters, G_IC and K_IC values of hybrid composites, were reviewed in accordance with the orthotropic fracture model. The deviation of measured G_IC and K_IC values from predicted values were explained based on the critical review of the basic assumption of orthotropic fracture model and characteristic material properties of hybrid composites. It can be said that, basically, the orthotropic fracture model can be used for evaluation of hybrid composite materials. However, careful analysis for G_IC and K_IC values which were derived from different source and some correction factor for K_IC values are necessary.     

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.     

Mode III interlaminar fracture of carbon/epoxy laminates using a four-point bending plate test

A new four-point bending plate (4PBP) test was used for characterising the mode III interlaminar fracture of carbon/epoxy laminates. The specimen has a cross-ply lay-up and two edge delaminations whose propagation becomes visible at the edges. Although the test setup is very simple, determination of the mode III critical strain energy release rate G_IIIc requires finite element analyses (FEA). The virtual crack closure technique with an assumed initiation region was first proposed for computing G_IIIc. This scheme was subsequently validated by crack growth simulations with a cohesive zone model. The results showed an average G_IIIc = 1550 J/m², which is significantly higher than the G_IIIc = 850–1100 J/m² and G_IIc = 800 J/m² measured in previous studies.     

A sandwich three-point bend specimen for testing mode-I interlaminar fracture toughness for fiber-reinforced composite materials

A sandwich three-point bend specimen has recently been proposed to test mode-I interlaminar fracture toughness for fiber-reinforced composite materials. The test composite consist of a thin layer bonded by two lateral reusable steel bars (Sohn et al. 1995). Some time earlier this specimen configuration was used to test fracture toughness of adhesives (Zdaniewsk et al. 1987). However, formulae for analysing its fracture mechanics parameters such as stress intensity factor and energy release rate can not be found in the literature. The lack of adequate formulae may explain why suitable quantitative analysis using this specimen configuration has not been achieved. In this paper, a simple and effective homogenisation method is used to change the bi-material system, which represents the specimen, into single uniform test material. This physical homogenisation is carried out by geometric change of the cross section of lateral steel parts based on equal deflection rigidity. For the transformed specimen configuration of single uniform material, the corresponding stress intensity factor solution from handbooks is available. Two formulae of stress intensity factor for the sandwich three-point bend specimen are given as upper limit and lower limit respectively, they are plotted with varying elastic tensile modulus mismatch. Then the relation between stress intensity factor and energy release rate, with special consideration of orthotropy of the tested composite material, is used to derive its energy release rate. The specimen and its formulae can also be applied to test other materials such as wood, welded joints (Burstow and Ainsworth, 1995), as well as to test dynamic fracture toughness. This revised version was published online in July 2006 with corrections to the Cover Date.     

Numerical analysis of the Edge Crack Torsion test for mode III interlaminar fracture of composite laminates

A detailed numerical analysis of the Edge Crack Torsion test was performed to verify its adequacy for the mode III interlaminar fracture characterization of composite laminates. A new data reduction scheme based on specimen compliance was proposed. Three-dimensional finite element analyses including a cohesive damage model were performed in order to evaluate the accuracy of perceived G_IIIc values obtained with the proposed method. Despite some degree of crack length dependency of perceived G_IIIc, acceptable errors could be achieved within a certain crack length range, which is thus recommended for experimental tests.     

Rate sensitivity of mode II interlaminar fracture toughness in graphite/epoxy and graphite/PEEK composite materials

This paper presents results from an experimental study of the effect of rate on the Mode II interlaminar fracture toughness, G_IIC, in graphite/PEEK (APC-2) and graphite/epoxy (AS4/3501-6) laminates. The end notched flexure test geometry was employed for Mode II experiments which were performed at room temperature over a range of crosshead speeds from 4·2 × 10⁻⁶ to 9¢2 × 10⁻² mc⁻¹. The APC-2 material exhibited ductile crack growth at low rates and brittle crack growth at high rates. The change in fracture mechanism resulted in a decrease in G_IIC from 1·0 to 0·40 kJ m⁻² at the upper range of test rates. The AS4/3501-6 material exhibited brittle crack growth at all rates. The G_IIC values also decreased at higher test rates from 0·46 to 0·06 kJ m⁻².     

Mode III interlaminar fracture of carbon/epoxy laminates using the edge crack torsion (ECT) test

The mode III interlaminar fracture of carbon/epoxy laminates was evaluated with the edge crack torsion (ECT) test. Three-dimensional finite element analyses were performed in order to select two specimen geometries and an experimental data reduction scheme. Test results showed considerable non-linearity before the maximum load point and a significant R-curve effect. These features prevented an accurate definition of the initiation point. Nevertheless, analyses of non-linearity zones showed two likely initiation points corresponding to G_IIIc values between 850 and 1100 J/m² for both specimen geometries. Although any of these values is realistic, the range is too broad, thus showing the limitations of the ECT test and the need for further research.     

High-temperature fracture of composite materials

The high-temperature fracture of composite materials containing strong particles is discussed on the basis of micromechanical models in which the recovery effect by diffusion of atoms is taken into account. The results of the calculations are compared with experimental results on 21Cr-4Ni-9Mn steels which consist of equiaxed grains containing rod-like M₂₃ C₆ carbide particles. The internal stresses acting in the particles and the grains are calculated on the basis of the model. The fracture mode of the material changes from transgranular to grain boundary fracture with increasing temperature, and the elongation of the material increases as a result. These experimental observations can be interpreted in terms of the calculation results of the constant stress model in which non-uniform distribution of plastic strain is assumed, but the calculated results of the constant strain model cannot explain the experimental results.     

Characterization of interlaminar shear failures of graphite/epoxy composite materials

Research undertaken to develop a more fundamental understanding of interlaminar shear failure in laminated graphite/epoxy composites is described. A test method capable of producing a state of pure interlaminar shear stress within a specified region of a composite test specimen was devised. The test method consisted of the four-point flexural testing of a unique test sample constructed of a coupon of Hercules AS4/3501-6, unidirectional, graphite/epoxy material bonded between two strips of steel using a room-temperature-curing epoxy adhesive. The major advantage of the test method is that the interlaminar shear failure of the composite coupon results from an induced state of pure shear stress, rather than from damage resulting from a complex stress state affecting the region of loading as typically occurs in the case of conventional flexural-type shear tests. The resulting shear failures were characterized with respect to fracture surface appearance, mode of failure, and stress state on the failure plane. A specially designed crack detection device was used to determine the site of fracture initiation and the direction of crack propagation.     

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.     

Mode III interlaminar fracture of carbon/epoxy laminates using the Six-Point Edge Crack Torsion (6ECT)

The recently proposed Six-Point Edge Crack Torsion (6ECT) test was used to evaluate the mode III interlaminar fracture of carbon/epoxy laminates. Plate specimens with starter delaminations in 0/0, 0/90 and 0/45 interfaces were tested. Data reduction was performed with an effective crack scheme validated in a previous numerical study. The tests allowed the evaluation of fairly unambiguous initiation G_IIIC values and of subsequent R-curves. Examinations of specimen cross-section showed considerable lengths of pure interlaminar propagation in specimens with starter delaminations in 0/90 and 0/45 interfaces. The latter specimens had the lowest initiation G_IIIC values.     

Prediction of Mode I interlaminar fracture toughness of stitched flax fiber composites

This paper aims to propose a simulation procedure to predict the interlaminar fracture toughness of stitched flax fiber composites through a virtual double cantilever beam test. The proposed procedure is constituted of two steps. First, the interlaminar failure of unstitched flax fiber laminate, as the parent laminate, is modeled using cohesive elements with a nonlinear softening law in order to model the large-scale fiber bridging occurred during delamination. The experimental results are used to calibrate the parameters of the cohesive law. Second, two-node beam elements are superposed onto the cohesive interface of the parent laminate at a prescribed stitch density and distribution to model the bridging stitches present in the validation samples. The stitch material behavior and properties are obtained from the tensile test of impregnated stitch fibers. The out-of-plane flax yarn stitching was found to generate a twofold increase in the delamination resistance of the composite laminate at a medium stitch density. The FE analysis results agreed well with the experimental results, where a good fit between the predicted and experimental R-curves was achieved.     

A new jig for mode II interlaminar fracture testing of composite materials under quasi-static and moderately high rates of loading

While it is still debated whether a pure mode II interlaminar fracture can physically exist in composites, several test methods have been proposed for its characterization. Lack of agreement between the results obtained with different test configurations has been attributed to the use of inconsistent data reduction schemes or inadequate correction factors used to correct for, e.g. the large deformations occurring with some tough modern materials.Aim of this work was to design a new jig that could provide an as pure as possible mode II crack initiation in unidirectional composites materials, that would allow a direct determination of fracture toughness, i.e. requiring almost no assumption for data reduction nor side effects correction and could be amenable to being used under impact as well as quasi-static loading conditions.The geometry of the system was designed in order to obtain great compactness, i.e. reduced masses and contained volume, making it usable with drop-weight testing machines, but at the same time enough stiffness to prevent flexural moments from closing or opening the crack faces, so granting the purity of the wanted mode, mode II, of loading. The compactness of the jig plus specimen system and the rigid confinement to which the composite specimen is subjected also grant that quite small displacements and overall deformations are reached at fracture.A static finite element analysis was conducted to optimize the jig geometry and is discussed here. Preliminary numerical and experimental results obtained with moderately high rate tests are also presented. The method employed for data reduction is based on the experimental calibration of the compliance and it is quite straightforward.     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.     

Interlaminar fracture toughness for composite materials

A new equation for the energy release rate for a double cantilever beam specimen is proposed within the framework of the higher order shear deformable plate theory. The interlaminar fracture toughnesses (IFTs) found using the present theory, the ASTM round robin test method and the acoustic emission method are compared for thermoset graphite/epoxy and thermoplastic AS4/PEEK composites. As a result, IFTs by the present theory show agreement within 5% when compared with those found using the ASTM method and it is shown that IFTs found by the acoustic emission method are lower than those found by the ASTM method. It is observed that the IFT of the thermoplastic AS4/PEEK composite is about 10 times larger than that of the thermoset graphite/epoxy composite.     

Characterization of the interlaminar fracture toughness of a laminated carbon/epoxy composite

Delamination between layers is an important problem in applications of fiber reinforced composite laminates. Tests were carried out to determine the interlaminar fracture toughness of AS4/3501-6 (carbon/epoxy) composite laminates using mixed-mode bending tests. Analysis of the test specimens in terms of mode I and mode II energy release rates showed good agreement between methods based on beam equations, compliance measurements, and detailed finite element analyses. The results showed that the critical mode I energy release rate for delamination decreased monotonically with increasing mode II loading. This is in contrast to some results in the literature. Various analytic representations of the mode interaction from the literature were compared, and shown to fit the data with reasonable accuracy.     

Carbon nanotube shear-pressed sheet interleaves for Mode I interlaminar fracture toughness enhancement

Failure of composite laminates is often the result of “secondary” transverse stresses causing delamination. One well known approach to prevent such failure is to incorporate a distinct interleaf material into the interlaminar region in order to increase its fracture toughness and, consequently, its resistance to delamination. In the recent years various carbon nanotube (CNT) interleaves gained much attention. This work presents experimental study of the Mode I progressive fracture of carbon/epoxy composite laminates modified with high volume fraction, aligned, non-functionalized and functionalized CNT interleaves. The interleaves used here are thin solid sheets produced from vertically grown multiwalled CNT arrays by shear pressing method. A dry or resin infused sheet is integrated between prepreg plies prior to the laminate cure. The obtained results show that both dry and pre-infused CNT interleaves significantly, up to two times, increase the critical strain energy release rate of the baseline non-interleaved laminate. Two methods of functionalizing CNTs within the preform are explored: O₂/CF₄ plasma and H₂SO₄/KnO₄ wet chemical treatments. Both methods maintain the high alignment and aspect ratio of the CNTs. Although, functionalization results in no additional G_IC toughening compared to the non-functionalized interleaves, the characteristics of the fracture surfaces are dramatically different.