mode-II Fracture Research Articles

Experimental and XIGA-CZM based Mode-II and mixed-mode interlaminar fracture model for unidirectional aerospace-grade composites

This article presents a fracture model for evaluation of Mode-II and Mixed-Mode (Mode-I + II) interlaminar fracture properties of unidirectional laminated composites. Mode-II and Mixed-Mode (Mode-I + II) interlaminar fracture tests have been carried out on End-Notched Flexure and Mixed-Mode bending specimens, respectively. The non pre-craked specimens are tested under pure Mode-II and Mixed-Mode loading conditions for the estimation of energy required to initiate crack growth from the natural crack tip. Pre-cracked specimens have been tested under pure Mode-II loading conditions to investigate the nature of delamination propagation from the sharp crack tip. A new set of interfacial fracture properties has been evaluated for AS4/914 laminates. An expression has been developed to correlate the experimentally obtained energy release rate (ERR) and equivalent crack growth (Δa) using a compliance based beam method. The curve-fitted expression between ERR and Δa is further introduced to the cohesive zone model for numerical modelling. Fractured surfaces at the final fracture load point in non pre-cracked specimens during Mode-II fracture tests are examined using scanning electron microscopy technique. Numerical simulations of crack initiation and propagation for all fracture tests have been performed using a combined formulation of fracture mechanics and damage mechanics. The introduction of experimentally obtained interlaminar interfacial properties into a combined formulation of extended isogeometric analysis and cohesive zone model shows a good agreement with experimentally obtained results.

A quantitative assessment of the damage mechanisms of CFRP laminates interleaved by PA66 electrospun nanofibers using acoustic emission

Interleaving composite laminates with nanofibrous mat is one of the most reliable methods for increasing interlaminar fracture toughness. The present study seeks to find out how the damage mechanisms of carbon fiber reinforced polymers (CFRPs), subjected to the mode-I and mode-II fracture tests, are affected while those are modified by interleaved Polyamide 66 (PA66) electrospun layers. For this goal, acoustic emission (AE) and scanning electron microscope (SEM) techniques were used for assessing the damage mechanisms. The mode-I test results showed that adding nanofibers could decrease matrix cracking, fiber breakage, and fiber/matrix debonding by 92%, 27%, and 87%, respectively. The AE demonstrated that no fiber breakage occurred during mode-II loading in both non-modified and nanomodified specimens which was validated by SEM images. On the other hand, the two other damage modes, i.e. matrix cracking and fiber/matrix debonding, decreased about 97% in the nanomodified laminates.

Boosting Inter-ply Fracture Toughness Data on Carbon Nanotube-Engineered Carbon Composites for Prognostics

In order to build predictive analytic for engineering materials, large data is required for machine learning (ML). Gathering such a data can be demanding due to the challenges involved in producing specialty specimen and conducting ample experiments. Additionally, numerical simulations require efforts. Smaller datasets are still viable, however, they need to be boosted systematically for ML. A newly developed, knowledge-based data boosting (KBDB) process, named COMPOSITES, helps in logically enhancing the dataset size without further experimentation or detailed simulation. This process and its successful usage are discussed in this paper, using a combination of mode-I and mode-II inter-ply fracture toughness (IPFT) data on carbon nanotube (CNT) engineered carbon fiber reinforced polymer (CFRP) composites. The amount of CNT added to strengthen the mid-ply interface of CFRP vs the improvement in IPFT is studied. A simpler way of combining mode-I and mode-II values of IPFT to predict delamination resistance is presented. Every step of the 10-step KBDB process, its significance and implementation are explained and the results presented. The KBDB helped in not only adding a number of data points reliably, but also in finding boundaries and limitations of the augmented dataset. Such an authentically boosted dataset is vital for successful ML.

Influence of crystallinity on interlaminar fracture toughness and impact properties of polyphenylene sulfide/carbon fiber laminates

The crystallization behavior of thermoplastic matrix is considered as an important property which can highly affect for the mechanical and thermal properties of laminated composite structures. This manuscript aims to evaluate the influence of different degrees of crystallinity induced in polyphenylene sulfide (PPS) / carbon fiber laminates, processed by hot compression molding. In the present study, the morphology, thermal and mechanical properties (compression, interlaminar shear stress, impact and end notch flexure) of the processed laminates were evaluated. From the mechanical testing results obtained, an increase in the in plane compressive and interlaminar shear strengths respectively, as well as impact resistance is observed for composite coupons processed with lower cooling rates. Based on end-notch flexure (ENF) tests, the mode-II fracture toughness is also strongly dependent on the degree of crystallinity of the thermoplastic matrix, as lower cooling rates yield stronger interfacial bonds. In this way, the present study demonstrates that the mechanical properties of thermoplastic laminates can be optimized by taking into account the matrix cooling rate during processing cycle.

Mixed-mode I/II fracture criteria for adhesively-bonded pultruded GFRP/steel joint

The pure mode-I, pure mode-II and the mixed-mode I/II fracture behavior of the adhesively-bonded pultruded GFRP/steel joint were experimentally investigated by using double cantilever beam (DCB) test, and four-point end-notched flexure (4ENF) test combined with four-point single leg bending (4SLB) test, respectively. The extended global method (EGM) was employed to obtain closed-form analytical solutions of mode-II fracture toughness for the 4ENF specimen and mixed-mode I/II fracture toughness components for the 4SLB specimen. Combined with the mode-I fracture toughness obtained by the experimental compliance method (ECM), the power law criteria in initiation and propagation processes were developed for adhesively-bonded pultruded GFRP/steel joint. Validation on the developed mixed-mode fracture criteria was performed through employing it to predict the load-displacement responses of the 4SLB specimens combined with VCCT. Finally, the mixed-mode fracture criteria of the adhesively-bonded GFRP/steel joint were compared to those of the adhesively-bonded GFRP/GFRP joint for quantifying their difference.

Fracture toughness analysis of HCCD specimens of Longmaxi shale subjected to mixed mode I-II loading

Longmaxi shale exhibits strong anisotropy due to its bedding planes induced by diagenesis. The anisotropic properties of shale have a significant effect on the stability control of geotechnical engineering. To investigate the influence of anisotropy of shale on its mixed mode fracture toughness, we performed Brazilian disc splitting tests on hollow center cracked disc shale specimens with different bedding and crack inclination angles (α and β, respectively). In this study, the fracture toughness of Longmaxi shale was calculated by the finite element method combined with either the J-integral or crack tip-opening displacement method. The results show that the dimensionless stress intensity factors and β values for pure mode-I and mode-II fractures both show anisotropic behavior by varying α. For all combinations of mode-I and mode-II loading, the fracture toughness also exhibits anisotropy by varying α; the mode-I fracture toughness decreases with increasing α when β > 30°, and the anisotropy is negligible for β = 0°. The anisotropy for mode-II fracture toughness is weak when β is small or large, whereas the anisotropy is significant when β = 45° and 60°. The effect of α on mode-I fracture toughness is greater than that on mode-II fracture toughness when β is small or large. Further, a comparison of four mixed-mode fracture criteria with the experimental results shows that the maximum tangential stress criterion for a transversely isotropic body can predict the mixed-mode fracture behavior of Longmaxi shale well. At the same time, it is suggested that the role of the T-stress should be considered to obtain more accurate prediction results.

Toughening Behavior of Carbon/Epoxy Laminates Interleaved by PSF/PVDF Composite Nanofibers

This paper presents an investigation on fracture behavior of carbon/epoxy composite laminates interleaved with electrospun nanofibers. Three different mats were manufactured and interleaved, using only polyvinylidene fluoride (PVDF), only polysulfone (PSF), and their combination. Mode-I and Mode-II fracture mechanics tests were conducted on virgin and nanomodified samples, and the results showed that PVDF and PSF nanofibers enhance the Mode-I critical energy release rate (GIC) by 66% and 51%, respectively, while using a combination of the two registered a 78% increment. The same phenomenon occurred under Mode-II loading. SEM micrographs were taken, to investigate the toughening mechanisms provided by the nanofibers.

Experimental and numerical failure analyses of dissimilar material joints between aluminium and thermoplastic

In the automotive industries, joining of dissimilar materials has extensively attracted interest, because combinations of various lightweight materials in car bodies becomes more significant in order to achieve the most optimum design. On the other hand, several techniques for hybrid assembly have been continuously developed. In this work, aluminium sheets A5052 were joined with PBT-30GFR using two different methods, namely, hot pressing and adhesive bond. Then, resulted load carrying capacities were evaluated by means of single-lap joint (SLJ) specimens and occurred failure mechanisms were examined. Effects of surface treatments for aluminium samples on the strength of SLJ samples were also studied. Furthermore, FE simulations of single-lap shear tests were performed, in which cohesive zone (CZ) model was applied for representing the separation and subsequent failure behaviour of SLJ specimens. The parameters of the traction-separation law were individually determined for each joining techniques by mode-I and mode-II fracture toughness tests. Afterwards, force-displacement curves, maximum loads and failure occurrences of SLJ specimens with different overlap lengths were predicted. It was found that the proposed FE models and CZ parameters could correctly describe the experimental results of both joining methods.

Mixed-mode I/II fracture behavior for adhesively-bonded pultruded GFRP joint under four-point bending

The experimental investigation of mixed-mode I/II fracture behavior for the adhesively-bonded pultruded GFRP joint was performed with the four-point single leg bending (4SLB) test. Cracks initiated and propagated in mat layer of the pultruded GFRP laminate for 4SLB specimens with three different thicknesses of upper adherend. The compliances of 4SLB specimens were experimentally and numerically determined. The extended global method was used for providing the simple closed-form solutions of the mixed-mode I/II fracture toughness in initiation and propagation stages, which was compared with the obtained results using virtual crack closure technique (VCCT). Combined with the pure mode-I and mode-II fracture toughness determined by using double cantilever beam (DCB) test, four-point end-notched flexure (4ENF) test, respectively, the mixed-mode fracture criteria of adhesively-bonded pultruded GFRP joint were constructed. Then, the load–displacement curves of 4SLB specimens obtained by VCCT were compared to experimental results for validating the accuracy and applicability of the developed fracture criteria of adhesive GFRP joint.

Enhancing the fracture toughness of carbon fibre/epoxy composites by interleaving hybrid meltable/non-meltable thermoplastic veils

Interlaying thermoplastic veils into carbon fibre/epoxy composites has proved to significantly increase the interlaminar fracture toughness. The main toughening mechanism is thermoplastic fibre bridging for the non-meltable veils and matrix toughening for the meltable veils. Herein, to take advantage of different toughening mechanisms, hybrid meltable/non-meltable thermoplastic veils were used to interlay two types of aerospace-grade composites produced from unidirectional (UD) prepregs and resin transfer moulding of non-crimp carbon fibre fabrics (NCF). The mode-I and mode-II fracture behaviour of the interleaved laminates were investigated. The experimental results demonstrated outstanding toughening performance of the hybrid veils for the mode-I fracture behaviour of the UD laminates and for both of the mode-I and mode-II fracture behaviour of the NCF laminates, resulting from the combination of different toughening mechanisms. For example, the maximum increases in the mode-I and mode-II fracture energies of the NCF laminates were observed to be 273% and 206%, respectively.

Significantly enhanced structural integrity of adhesively bonded PPS and PEEK composite joints by rapidly UV-irradiating the substrates

A high-power UV-irradiation technique was proposed for the surface treatment of PPS and PEEK composites, aiming to achieve good adhesion with epoxy adhesives. The composite substrates were rapidly UV-irradiated for a duration of between 2–30s, and then bonded using an aerospace film adhesive to produce joints. Tensile lap-shear strength and mode-I and mode-II fracture energies of the adhesive joints were investigated. It was observed that the application of a short-time UV-irradiation to the substrates transformed the failure mode of the specimens from adhesion failure to substrate damage in all cases. This consequently resulted in remarkable improvements in the mechanical and fracture performance of the adhesive joints. For example, the lap-shear strength increased from 11.8MPa to 31.7MPa upon UV-irradiating the PPS composites for 3s, and from 8.3MPa to 37.3MPa by applying a 5s UV-irradiation to the PEEK composites. Moreover, the mode-I and mode-II fracture energies significantly increased from ∼50J/m2 to ∼1500J/m2 and from <300J/m2 to ∼7000J/m2, respectively for both of the adhesively bonded PEEK and PPS composite joints.

Mode-I, Mode-II, and Mode-III Stress Intensity Factor Estimation of Regular- and Irregular-Shaped Surface Cracks in Circular Pipes

Stress intensity factor (SIF) of surface cracks (semi-circular, semi-elliptical, and irregular shapes) located on a circular pipe (hollow) has been determined using numerical method. The irregular-shaped crack geometry was considered in the present work in order to simulate the cracks that are generally developed during service condition. The interaction behavior of multiple cracks on SIF calculation has also been investigated. Crack aspect ratios [a/c; “a” is the crack depth and where “c” is semi-major axis of elliptical crack] ranging between 0.6 and 1.0 and crack depth ratios [(a/d); where “d” diameter of the pipe] ranging between 0.1 and 0.4 were considered. Irregular-shaped cracks were modeled by keeping the crack depth (a) constant. For a regular profile semi-elliptic crack [(a/c) = 0.6], higher SIF was observed at crack middle region [(S/S0) = 0]. In contrast, higher SIF was observed at crack surface region [(S/S0) = ± 1] for semi-circular crack. Asymmetric spreading of SIF was noticed because of added effect of mode-II and mode-III fracture. In addition, the leading mode of fracture at short crack depth [(a/d) ≤ 0.1] was mode-II (in-plane shear) and mode-III (out of plane shear) fractures whereas at deep crack depths [(a/d) ≥ 0.4], the leading mode of fracture was mode-I fracture which is crack opening mode. Marginal influence of crack aspect ratio was noticed at crack surface region [(S/S0) = ± 1] and the effect is significant at crack middle region [(S/S0) = 0]. For a constant crack depth, higher SIF was obtained at the middle region [(S/S0) = 0] compared to regular profile crack. When comparing the observations of regular semi-elliptic crack, higher influence of mode-II and mode-III fracture was observed for irregular-shaped semi-circular crack which may leads to additional crack growth. Thus, approximating the standard crack geometry may over/under predict the SIF values. It is also inferred that the presence of adjacent cracks significantly affects the SIF of middle crack due to interaction effect. The interaction behavior was observed to be more pronounced when multiple cracks are located at closer interval. The interaction effect reduces gradually when the distance between crack centers increases. The improved SIF solutions obtained with consideration of additional fracture modes will be useful during the residual life determination.

Mode-II fatigue response of AS4/8552 carbon /epoxy composite laminates interleaved by electrospun nanofibers

Abstract The advantages of applying electrospun nylon 6,6 as a toughening agent in epoxy-based composite laminates have been considered already under quasi-static mode-I and mode-II fracture loads. In the present study, the fatigue behavior of unidirectional carbon/epoxy laminates interleaved by nylon 6,6 is investigated under mode-II loading. To this aim, a 50-μm nylon 6,6 nanofibrous mat was put between mid-layers of a carbon/epoxy laminate, and then quasi-static and cyclic loadings were applied on End Notched Flexure (ENF) samples to investigate the onset of delamination growth and crack propagation rate (da/dN) under different ratios of GIIin/GIIC. The results showed that the mode-II fracture toughness increased about 161% under quasi-static loading tests. Additionally, the fatigue test results proved that the crack propagation was 14–27 times slower in the modified sample (with the same GIImax) because of the bridging between composite layers caused by nanofibers. Different fractography techniques including optical microscopy, microscopic 3D surface scanning device, and scanning electron microscopy (SEM) were also used to explore further the toughening mechanism.

The influence of interlayer/epoxy adhesion on the mode-I and mode-II fracture response of carbon fibre/epoxy composites interleaved with thermoplastic veils

The compatibility between the majority of thermoplastic veils (TPVs) and epoxies is typically poor, owing to the inherently low surface energies of thermoplastics. This tends to largely affect the toughening performance of TPVs as interlayer materials of carbon fibre/epoxy composites. The traditional methods for surface activation of thermoplastics, such as corona discharge, plasma treatment and acid etches, are not applicable to TPVs as they could cause significant damage to the thermoplastic fibres with nano-/micro-scale diameters. Herein, a UV-irradiation technique was proposed to active the surfaces of polyphenylene-sulfide (PPS) veils, that effectively improved their adhesion with epoxies. Consequently, the effects of an improved veil/epoxy adhesion on the mode-I and mode-II fracture behaviour and corresponding fracture mechanisms of the interleaved laminates were investigated. It was found that an improved veil/epoxy adhesion significantly enhanced the toughening performance of the PPS veils for the laminates manufactured by resin transfer moulding of non-crimp fabrics, by introducing additional carbon fibre delamination and significant PPS fibre damage during the fracture process. In contrast, the increased level of veil/epoxy adhesion inhibited PPS fibre bridging during the fracture process of the laminates produced from unidirectional prepregs, and caused considerable adverse effects on the fracture performance.

The influence of interlayer/epoxy adhesion on the mode-I and mode-II fracture response of carbon fibre/epoxy composites interleaved with thermoplastic veils

The influence of interlayer/epoxy adhesion on the mode-I and mode-II fracture response of carbon fibre/epoxy composites interleaved with thermoplastic veils

Fracture Mechanism of the Formation with a Natural Flaw Under a Single-Tooth Cutting

The process of a single-tooth cutting formation is taken as the representative to analyze the fracture mechanism of the formation with natural flaws under the PDC drill bit. The modeling of the stress field distribution of the formation under a single tooth is proposed first. Subsequently, the modeling of the stress intensity factor K, T-stress and the initiation angle $$\theta $$ of the flaw under two far-field compressive loads in two cases are presented based on the mode-II fracture. Finally, the results of theoretical calculations and simulations are given and discussed. Based on the analysis undertaken, it can be concluded that the confinement ratio $$\lambda $$ applying on the flaw changes nonlinearly with the variation in $$\alpha +\varphi $$ . The relationship between the dimensionless ratio $$\beta _{{x}}/\beta _{{y}}$$ of T-stress and the rotational polar angle $$\varphi $$ is a parabola with an upward opening. When the direction of the cutting force is consistent with the flaw, the stress intensity factor K and the initiation angle $$\theta $$ at the tip of the flaw reach the maximum. In addition, the angle between the cutting force and the flaw weakens the T-stress and inhibits the crack initiation and propagation of the flaw. The results of numerical simulations show good agreement with theoretical calculations.

Examination of the block shear fracture test method to measure the Mode-II critical stress intensity factor

Block shear fracture tests were conducted using specimens of western hemlock to determine the properties of the fracture mechanics under the Mode-II condition. In the test, a fixture standardised as the block shear test of solid wood in the Japanese Industrial Association (JIS Z2101-2009) was used. The specimen configuration, location of the crack, and initial crack length were varied, and the effects of these conditions were examined. The Mode-II critical stress intensity factor was effectively obtained when the initial crack was introduced at the opposite location to that where the shear load was applied using a specimen with a cubic configuration. Additionally, the introduction of the additional crack length due to the fracture process zone ahead of the crack tip was also effective in the block shear fracture test and other Mode-II fracture tests.

Experimental and analytical investigation of bond behavior in glass fiber-reinforced composites based on gypsum and cement matrices

In this paper, an innovative composite, formed by a glass fiber net embedded in a gypsum-based matrix and aimed at strengthening masonry structures, is proposed. Experimental and analytical investigations on composite systems with gypsum- and cement-based matrixes, both coupled with a glass fiber net, are illustrated. Bond tests between brick and composite systems were carried out on different bond lengths. The gypsum-based composite (FRGM) showed higher peak loads and a more brittle behavior compared to the response of the standard FRCM system. In the framework of mode-II fracture mechanics, analytical modeling of the tested behavior was undertaken based on local cohesive laws derived from experimental evidence. Both short- and long-bond lengths were modeled, and the results well agree with the experimental behavior of the cement-based system.

Mode-II fracture of nanostitched para-aramid/phenolic nanoprepreg composites by end-notched flexure

The mode-II interlaminar fracture toughness considering the end-notched flexure method of nanostitched para-aramid/phenolic composite structures was investigated. The fracture toughness (GIIC) of the nanostitched and stitched composites exhibited a slight increase as compared to the pristine sample. Hence, the nanostitching enhanced the fracture toughness of the para-aramid/phenolic composite structures. Although the type of stitch fiber was not effective, the fabric interlacement frequency, notably prepreg Twaron nanostitched yarn and basket nanoprepreg biaxial interlaced fabric was of critical importance. The principle mechanism for raising the GIIC in the nanostitched composite structure was the interlayer resin fracture particularly as a form of slight shear hackle marks. Cracks grew around the inter- and intrayarn boundaries where the resin was fractured half way around each yarn cross-section. This is called a “zigzag crack path,” and microcracks moved to the through-the-thickness of the composite where nanostitching arrested the crack growth and suppressed delamination in the stitching zone. At the blunt crack tip, carbon nanotubes in the phenolic resin and multiple filament bundles probably diminished the stress clustering via friction/debonding/pull-out/sliding or stick-slip. Thus, nanostitched para-aramid/phenolic composite structures demonstrated better damage tolerance behavior considering the neat structure.

Toughening phenolic composite laminates by interleaving hybrid pyrolytic carbon/polyvinyl butyral nanomat

This paper is granted with a novel method for improving the toughness of phenolic composite laminates. This method employs electrospun hybrid nanofibers containing polyvinyl butyral (PVB) and pyrolytic carbon (PyC), which are interleaved between mid-layers of the laminate. Mode-II fracture test is used for studying the toughening behavior of modified laminates. The results suggest shortage of pure PVB nanofibers in causing a substantial increase in the fracture toughness of laminates (about 7% increase). However, incorporation of PyC particles to PVB nanofibers can enhance the fracture toughness up to about 24%. Scanning electron microscope images reveal the toughening mechanism provided by each type of nanofibrous mat. Thermal gravimetric analysis was also applied for studying the role of PyC particles in the thermal stability of PVB nanofibers.