Fracture Toughness Of Laminates Research Articles

In-situ monitoring of crack growth and fracture behavior in composite laminates using embedded sensors of rGO coated fabrics and GnP paper

Graphene-based nanomaterials have found significant interest in the development of embedded sensors to monitor the fracture behavior in composite structures. In this work, in-situ crack propagation and fracture behavior within a glass fiber reinforced polymer composite (GFRP) was monitored using embedded reduced graphene oxide (rGO) coated fabrics and highly conductive graphene nanoplatelet (GnP) paper. All laminates were fabricated using the resin infusion process. The piezoresistive performance of both types of laminates was evaluated using in-plane and out-of-plane mechanical tests. The effect of GnP paper thickness (50/150/240 µm) and loading rate was also evaluated using tensile and Mode-I fracture loadings. Piezoresistivity of the sensors was reduced by increasing loading rate during tensile tests. All laminates with GnP paper exhibited poor mechanical performance under tensile loading. The laminates with 50 µm GnP paper showed highest sensitivity under Mode-I loading. In comparison to pristine laminates, the interlaminar fracture toughness of laminates with 50 µm GnP paper was reduced by 70%. Furthermore, laminates with rGO coated fabrics demonstrated stable crack propagation under Mode-I loading as compared to GnP based laminates. The fractured surfaces were analyzed using scanning electron microscopy to investigate the underlying fracture mechanisms of the sensors in the composite laminates.

Measuring the interlaminar fracture toughness of thin carbon fiber/polyamide6 composites using adhesively bonded stiffeners

The present work investigates the interlaminar fracture toughness of a thin unidirectional carbon fiber-reinforced/polyamide6 (CF/PA6) composite laminate. Aluminum (Al) stiffening beams are adhesively bonded on both sides of the thin laminate to avoid undesirable large deformations during the subsequent double cantilever beam tests. Experimentation with various surface treatment techniques and different adhesives is presented, from which it is concluded that the mode I fracture toughness of the CF/PA6 laminate is always higher than the fracture toughness of the adhesives used. Consequently, the crack initiates at the Al–composite interface. A finite element (FE) model is proposed, and quantitative results for the fracture toughness of the laminate are provided (e.g., GIc,ini=2.1N/mm). Overall, the study highlights the challenges in the adhesive joining of stiffening beams to CF/PA6 surfaces, ways to improve adhesion, and the FE simulation as an alternative to determining the interlaminar fracture toughness of such thin laminates.

The fibre kinking fracture toughness of laminated composites under combined compression and shear

Compact compression specimens with off-axis fibres are employed in this paper, to investigate the effect of in-plane shear stress on the fibre kinking compressive fracture toughness of laminates. The strain distribution on the surface of the specimen was analysed using the digital image correlation method, and the damage process was monitored with thermal imaging. It was noted that the in-plane shear stress introduced by off-axis angle fibres caused an increase in the values of the R-curves when the off-axis angle was 10° or more. The microscopic morphology reveals that as the off-axis angle of the fibres increases, there is an increasing splitting of large fibre bundles, followed by the fibre compression failure within these bundles. This multi-step damage process may be responsible for the increase in energy dissipation.

Fracture Toughness of Hydrogel Laminates: Experiments, Theory, and Modeling

Abstract Possessing enhanced mechanical durability and multiple novel functions, hydrogel laminates have found wide applications in diverse areas, including stretchable and bio-integrated electronics, soft robotics, tissue engineering, and biomedical devices. In the aforementioned scenarios, hydrogels are often required to sustain large deformation without mechanical failure over a long time. Compared to the fast movement in functions design, the failure mechanism of hydrogel laminates has been much less explored and researched, as well as laminates’ fracture toughness—a key parameter characterizing their fracture behavior. To address this largely unexplored issue, this article further studies the fracture toughness of hydrogel laminates both experimentally and theoretically. A kind of modified pure-shear test suitable for measuring the fracture toughness of hydrogel laminates is proposed, which is then applied to testing a PAAm-PAA laminate’s toughness. Through theoretical analysis and numerical modeling, the experimentally observed enhancement in the fracture toughness of PAAm-PAA laminates is explained—the fracture toughness of the laminates covers the energy required for both the crack and concomitant interfacial delamination to propagate, and the theoretical predictions agree well with the experimental results. The results from this study provide quantitative guidance for understanding the fracture behavior of hydrogel laminates.

Experimental and numerical investigations on the mode I delamination growth behavior of reinforced laminates via a ZPI (Z-pin pre-hole insertion) technique

This paper proposes a low-in-plane damage ZPI (Z-pin pre-hole insertion) process. The delamination growth behavior and the influence of Z-pin insertion density on the mode I interlaminar fracture toughness of laminate was explored experimentally and numerically. Meanwhile, Z-pin pull-out and shear tests were conducted in order to provide accurate data for the DCB simulation test. What’s more, an infinite-focus microscope was used to scan the specimen’s defect caused by the ZPI process. It can be found that the ZPI process can lead to less Z-pin inclination, fibre breaking, and crack initiation at the bonded interface. The DCB test results show that the load-displacement curve can be distinguished into four stages for the pinned specimen. In addition, a specimen pinned with a higher Z-pin volume fraction to more suitable for increasing the G IC. The simulation results show that the simulated maximum load and the G IC of the specimen are 286.63 N and 940.06 J·m−2, with a 0.56% and 10.10% error from the experimental result, indicating that the proposed FE model is validated by comparing the predictions with the experimental results. These results can provide some guidance for fabricating the reinforced laminates via the ZPI process.

Simultaneously improve the mode II interlaminar fracture toughness, flexural properties, and impact strength of CFRP composites with short aramid fiber interlaminar toughening

AbstractThis work mainly focuses on the mode II interlaminar fracture behavior of carbon fiber composite that was interlaminar toughened by short aramid fiber modified epoxy resin. The short fibers were first dispersed in the epoxy matrix and then in the laminate interlayer area during the lamination process. The ENF test evaluated the Mode II interlaminar fracture toughness of laminates. The results show that mode II interlaminar fracture toughness increases with the addition of short fibers. When the fiber content is 10 g/m2, the toughening effect is the best at 2437.72 J/m2, which is about 61.8% higher than the untoughened sample. The toughening mechanism of CFRP laminates by adding short fibers was observed by mode II fracture surface SEM images. In addition, flexural properties and Charpy impact properties of short fiber modified resin interlayer toughened laminates were also studied. The results showed that with this method, flexural properties and impact strength were improved to a certain extent, proving that this method is an effective interlaminar toughening method.

Strengthening of composite T-joints using Ø 0.11 mm Z-pins via an ultrasound-guided insertion process

This paper investigates the effect of Ø 0.11 mm Z-pinning on the structural properties of the composite T-joints via the ultrasound-guided insertion process. The structural properties of the joints are determined by the stiffener web pull-off testing. The experimental results show that carbon Z-pins exhibit a better improvement effect than steel Z-pins due to the interface between carbon Z-pins and laminate owning higher pullout strength. When the skin-flange area of the joints is inserted with 0.2 vol% of Ø 0.11 mm carbon Z-pins, the peak load and total energy absorption capacity can be improved by 72.4% and 436.5%, respectively. With the Ø 0.11 mm Z-pinning, the tensile strength decreased by only 2% when the volume content reached 0.2%. The improvements to the T-joint structural properties are explained by the enhancement mechanism of Ø 0.11 mm Z-pinning on mixed mode I/II interlaminar fracture toughness of laminates. Finally, the Ø 0.11 mm Z-pinned T-joint model is established to verify the experimental results.

Can a 3D woven GFRP composite really provide better impact resistance compared to a 2D woven GFRP composite at all of the thicknesses?

The large majority of previous studies about comparison of impact response of 2D and 3D woven composites focused on thin laminates but not thicker ones. Thus, the aim of this study is to determine whether thicker 3D laminates exhibit better impact resistance compared to 2D ones or not. For this purpose, firstly several tensile tests were performed on the composite specimens in nine different thicknesses. In fact, the longitudinal tensile modulus extracted from tensile tests was used to determine delamination threshold load of them. Then, 2D and 3D laminates with an identical thickness were tested by drop weight tower under several energies to specify the perforation and penetration energy thresholds. Finally, 2D and 3D composites with nine different thicknesses were subjected to impact loading to investigate their impact behavior. The experimental results revealed that the relationship between thickness and delamination threshold load of 2D woven E-glass/unsaturated polyester composites can be fitted with a power regression function. The extracted empirical function was also validated with the analytical results obtained from a simple method based on mode II fracture toughness of laminates. Contrary to what is normally believed, the value of energy absorption in 2D specimens has no dependency on their thickness. In addition, analysis of results in terms of energy absorption, damage area and damage initiation load indicated that thicker 2D woven fabrics exhibit higher impact resistance compared to 3D ones unlike thinner laminates.

Effect of process parameters on mode-II interlaminar fracture toughness and fractographic features of automated fibre placement prepreg laminates

The present study concerns the quantitive assessment of the influence of process parameters on the interlaminar properties of carbon fiber reinforced epoxy composites. Mode II interlaminar fracture toughness is evaluated based on the ENF test. Scanning electron microscopy images are used to evaluate the change in microstructures. The results indicate that the process parameters have a direct influence on the microstructural features and fracture toughness of laminates. Specifically, the elevation of layup speed leads to the increase of interlaminar porosity which results in a decline of fracture toughness. The moderate pressure could sufficiently evacuate air bubbles during the deposition which to a certain extent decreases the porosity; however, the excessive pressure squeezes resin out from the interlaminar region, which decreases the fracture toughness. Furthermore, the excessive temperature could both increase porosity and result in a thinner resin layer due to the change in resin flowability. In which case, the fracture toughness would be significantly decreased.

Interlaminar Fracture Behavior of Carbon Fiber/Polyimide Composites Toughened by Interleaving Thermoplastic Polyimide Fiber Veils

Carbon fiber reinforced thermosetting polyimide (CF/TSPI) composites were interleaved with thermally stable thermoplastic polyimide (TPPI) fiber veils in order to improve the interlaminar fracture toughness without sacrificing the heat resistance. Both of the mode I and mode II interlaminar fracture toughness (GIC and GIIC) for the untoughened laminate and TPPI fiber veils interleaved laminates were characterized by the double cantilever beam (DCB) test and end notch flexure (ENF) test, respectively. It is found that the TPPI fiber veils interleaved laminates exhibit extremely increased fracture toughness than the untoughened one. Moreover, the areal density of TPPI greatly affected the fracture toughness of laminates. A maximum improvement up to 179% and 132% on GIC and GIIC is obtained for 15 gsm fiber veils interleaved laminate, which contributes to the existence of bicontinuous TPPI/TSPI structure in the interlayer according to the fractography analysis. The interlaminar fracture behavior at elevated temperatures for 15 gsm fiber veils interleaved laminate were also investigated. The results indicated that the introduction of thermally stable TPPI fiber veils could enhance the fracture toughness of CF/TSPI composites by exceeding 200% as compared to the untoughened one even as tested at 250 °C.

An experimental analysis of the end-notched flexure composite laminates beams with elastic couplings

This paper presents an experimental study of the composite beams with elastic couplings subjected to the end-notched flexure (ENF) test. The object of research were carbon/epoxy composite laminates with specific stacking sequences exhibiting the bending–extension and the bending–twisting couplings. In addition, multidirectional laminates with different delamination interfaces were tested. Influence of elastic coupling phenomena on behavior of laminates subjected to the bending moment as well as on fracture toughness and delamination surfaces after the ENF tests was discussed. All experiments were performed according to the ASTM D7905 Standard. Determination of crack initiation was supported by the acoustic emission technique. Data analysis of the ENF test was performed by using both the direct beam theory and the corrected beam theory. Results obtained by using the classical data reduction schemes were compared with the compliance-based beam method (CBBM). Greater values of the fracture toughness obtained by using the non-standardized methods can be explained by the effect of the fracture process zone which was taken into account in the CBBM calculation scheme. Delamination surfaces of coupled laminates after the ENF tests were investigated by using the scanning electron microscopy. Results revealed that the effect of elastic couplings might influenced on intense bridging phenomena, more extensive fracture process zone and contribution of the mode III in total delamination resistance. Nevertheless, application of the standardized ENF procedure to determination of the fracture toughness of laminates exhibiting elastic couplings is still possible, provided assistance of additional data reduction schemes and techniques or numerical analysis.

An investigation on the effect of polyvinyl butyral interlayer forms on the fracture toughness of glass reinforced phenolic laminates

Abstract In the recent years, thermoplastic interlayers are employed to improve the fracture toughness of laminated composites. In this research, PolyvinylButyral (PVB) was utilized as a flexible thermoplastic interlayer in forms of electrospun nanoweb, solution, film and powder to evaluate the effect on interlayer shape on the fracture toughness of glass-phenolic laminates. DCB and ENF tests were conducted to study the fracture toughness under modes I and II conditions. Among all the specimens, those modified with 30 μm electrospun PVB, with 60% increase in the mode II fracture toughness, showed the highest performance. Those consist of PVB solution (3 phr), with an increase of 50% in mode I fracture toughness, ranked the second. In the mode II fracture toughness test, for the specimens modified with the nanoweb, solution and film interlayers, and the value of GIIc increased by 38%, 24% and 18%, respectively. Samples containing PVB powder showed a decrease of 22% in the property.

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.

Improvement of interlaminar fracture toughness of phenolic laminates interleaved with electrospun polyvinyl butyral nanofibers

Among different types of composites, laminated composites are those may be delaminated under applied loads. Phenolic resin/Glass fiber composite is one of the most widely used composites, however, it is susceptible to delamination very likely that it will delaminate. In this research, polyvinyl butyral (PVB) electrospun nanowebs have been used as the interlayer to toughen phenolic resin/glass fiber composites. In order to evaluate the effect of main factors on the fracture toughness of the laminates, interlayer dimensional parameters such as interlayer thickness (30, 90 and 150 µm), nanofiber diameter (200 and 500 nm) and nanofiber orientation (random and aligned) were investigated in Mode I and Mode II fractures. The surface response method (RSM) was used to find the optimal diameter of PVB electrospun nanofibers. The results showed that the thickness of the interlayer is the most effective factor in improving the composite toughness in Mode I and Mode II. The thickness of 30 μm showed the best result in Mode I and Mode II and increased GIc and GIIc values by 70% and 38%, respectively. Also, with decreasing the nanofibers diameter and increasing the collision level, the amount of energy absorbed from the crack increases and results in to improvement of the fracture toughness. Nanofiber alignment showed less effect on the composite fracture compared to other factors.

Notch failure in laminated composites under opening mode: The Virtual Isotropic Material Concept

Experimental and theoretical fracture investigations are performed on laminated glass/epoxy composite specimens with different numbers of ply and lay-up configurations weakened by central U-shaped notches of various tip radii and loaded under pure opening mode. Since the main purpose of this research is to evaluate the load-carrying capacity (LCC) of the U-notched glass/epoxy composite specimens, it is experimentally recorded by the testing machine and the failure behavior of the specimens is carefully observed. To predict the experimentally obtained LCCs or the last-ply-failure (LPF) loads theoretically, a novel concept, called Virtual Isotropic Material Concept (VIMC), is proposed and linked to two well-known stress-based brittle fracture models in the context of the linear-elastic notch fracture mechanics (LENFM), namely the maximum tangential stress (MTS) and the mean stress (MS) criteria. It is shown that both VIMC-MTS and VIMC-MS combined criteria can predict the experimental results well without the need for ply-by-aply failure analysis, the first-ply-failure (FPF) prediction, and the progressive damage modeling (PDM) etc. Thanks to both VIMC and LENFM, the LPF prediction is direct, rapid, and convenient, because only two distinct fracture parameters, namely the ultimate tensile strength and the trans-laminar fracture toughness of laminate are necessary for LPF prediction.

Prediction of the net-tension strength of single bolt joint using the 0° ply strength and fracture toughness

This paper puts forward a new method to predict the net-tension strength of single blot joint based on the Finite Fracture Mechanics (FFM) model. Instead of using the tensile strength and fracture toughness of the laminates, only the properties of the 0° ply are used to obtain the failure stress and critical crack length by considering the relation between mode I fracture toughness of laminates and that of 0° ply. While the tensile strength of the 0° ply can be easily obtained through uniaxial tensile tests, the size effect law of double edge notched specimens is incorporated for the determination of the crack resistance curve of the ply. After validating the proposed method by the experimental results of central-notched IM7/8552 laminates, a 3D finite element model is established and the secondary bending is taken into consideration to find the maximum stress distribution of 0° ply around the bolt hole of the single bolt joint. It is concluded that the predicted strength by the semi-analytical method suits well with the experimental results. The secondary bending and the crack resistance curve, which can be seen as improvements of the Finite Fracture Mechanics model, have an influence on the predicted value.

Modelling the nonlinear behaviour and fracture process of AS4/PEKK thermoplastic composite under shear loading

The accurate determination of non-linear shear behaviour and fracture toughness of continuous carbon-fibre/polymer composites remains a considerable challenge. These measurements are often necessary to generate material parameters for advanced computational damage models. In particular, there is a dearth of detailed shear fracture toughness characterisation for thermoplastic composites which are increasingly generating renewed interest within the aerospace and automotive sectors. In this work, carbon fibre (AS4)/thermoplastic Polyetherketoneketone (PEKK) composite V-notched cross-ply specimens were manufactured to investigate their non-linear response under pure shear loading. Both monotonic and cyclic loading were applied to study the shear modulus degradation and progressive failure. For the first time in the reported literature, we use the essential work of fracture approach to measure the shear fracture toughness of continuous fibre reinforced composite laminates. Excellent geometric similarity in the load-displacement curves was observed for ligament-scaled specimens. The laminate fracture toughness was determined by linear regression, of the specific work of fracture values, to zero ligament thickness, and verified with computational models. The matrix intralaminar fracture toughness (ply level fracture toughness), associated with shear loading was determined by the area method. This paper also details the numerical implementation of a new three-dimensional phenomenological model for carbon fibre thermoplastic composites using the measured values, which is able to accurately represent the full non-linear mechanical response and fracture process. The constitutive model includes a new non-linear shear profile, shear modulus degradation and load reversal. It is combined with a smeared crack model for representing ply-level damage initiation and propagation. The model is shown to accurately predict the constitutive response in terms of permanent plastic strain, degraded modulus as well as load reversal. Predictions are also shown to compare favourably with the evolution of damage leading to final fracture.

Rapidly cured out-of-autoclave laminates: Understanding and controlling the effect of voids on laminate fracture toughness

Voids are one of the most significant defects found within composites and have been demonstrated to reduce the performance of composite structures. The understanding of the impact of the size and distribution of voids on laminate properties is still limited because voids have proven difficult to deliberately control. This study aims to understand the mechanisms by which voids are generated within out-of-autoclave cured laminates. In this study, a process of prepreg conditioning was developed to control the level of voids within test laminates. Non-conditioned laminates highlighted signs of void growth (1.5%), while conditioned laminates showed consistently low levels of voids (<0.3%). Mass spectrometry indicated higher levels of aqueous and solvent volatiles within the non-conditioned prepreg. Finally, Mode II fracture testing revealed a 21% improvement in toughness for the non-voided laminates. A model on the effect of voids within the Mode II stress state has also been proposed.

Fracture toughness of laminates reinforced by piezoelectric z-pins

The reinforcing effect of piezoelectric z-pins to Mode-I fracture toughness of laminates is investigated in the present paper on the basis of the piezoelectric fiber pullout model and the double-cantilever-beam (DCB) model. Our results on the effect of electrical loads show that piezoelectric z-pins can significantly improve the fracture toughness of laminates to resist the laminates delamination and the reinforcement effect can be adjusted by applying an external electrical load. Moreover, the influences of material parameters, radius and distribution of z-pins are discussed. This study provides a useful reference to achieve the optimal design of smart laminate composites.

Analytical model to predict multiaxial laminate fracture toughness from 0° ply fracture toughness

The prediction of nominal strength is very important in the design and evaluation of materials especially polymer matrix composites. Various cohesive laws forms are successfully used in predicting the nominal strength of laminated composite structures. For composite structures, fracture toughness is dominated parameter when using cohesive laws to predict their nominal strength. In spite of complex reported models, this study propose an easy simple model to predict the fracture toughness of multidirectional composite laminates using the fracture toughness of the 0° ply ones. This model is mainly based on the geometry of fiber orientation and linear elastic fracture mechanics and uses the fracture toughness of the 0° ply obtained from compact tension test specimens. A good prediction is obtained by comparing the model results with experimental data which are obtained from center-cracked specimens manufactured using different lay-ups orientations and materials. POLYM. ENG. SCI., 54:234–238, 2014. © 2013 Society of Plastics Engineers