ENF Test Research Articles

Progressive damage analysis of green composite laminates subjected to in-plane crashworthiness

In this paper the initiation and propagation of delamination in thin-walled green composite structures were studied. In particular, laminates of flax and hemp fibers with epoxy resin were investigated from experimental and numerical points of view, using the finite element code LS-DYNA. First, an experimental campaign was conducted using DCB and 4ENF tests to obtain the values of the interlaminar fracture modes, required for the cohesive fracture modeling. In addition, in-plane crashworthiness tests were performed on composite plates to analyze the damage of the specimens, under both quasi-static and dynamic conditions. A trigger mechanism was considered to help delamination to start and to ensure the most progressive crushing possible. The predominant damage mode was splaying, but some ply fragmentation in the middle layers and some buckling phenomena were also observed. In general, more stable progressive crushing was observed under dynamic conditions than under quasi-static ones, even if a reduction in terms of absorbed energy was obtained by moving to the dynamic condition. The numerical predictions showed a good agreement with the experimental reference results, validating the current framework for crashworthiness analysis of green composite structures.

Increasing the Interlayer Fracture Toughness of Polymer Fabric Composites Using Local 3D-Reinforcement (Felting)

Introduction. One of the reasons for undesirable delamination of polymer composites with fabric reinforcement is low transverse shear properties. It is known that the reinforcement of polymer fabric composites in the Z direction reduces the sensitivity to delamination and increases the viscosity of interlayer fracture. Various methods of three-dimensional reinforcement of polymer fabric composites are proposed in the literature. However, they complicate the manufacturing process of the structure. The problem is solved by the method of three-dimensional reinforcement proposed in this article — felting. This is a local reinforcement of the composite in the Z direction with minimal production changes. The degree of Z-reinforcement is determined by the felting density, i.e., the number of needle punches per 1 cm2 of the fabric package. The work is aimed at evaluating the effect of felting on the interlayer crack resistance of a composite material.Materials and Methods. The interlayer fracture toughness GIIc was determined on a cross-woven fiberglass with felting of 10 cm-2. The material was impregnated with Etal-370 resin and Etal-45 hardener. Experiments according to ASTM D7905M–14 and GOST 33685–2015 standards were carried out on an Instron 5900R test machine. The stress state at the crack tip was analyzed with regard to the nonlocal strength theory in the ANSYS Workbench program (option “static strength analysis”). The finite element method (FEM) was used.Results. The “load — displacement” curves were considered for the samples. Values GIIc were calculated. The results of ENF tests for felting density of 0 cm–2 and 10 cm–2 were summarized. Control samples and felting samples were compared. In the latter case, GIIс turned out to be ~33% higher. The stress state at the crack tip was calculated under DCB and ENF loading. The dependences of maximum normal and shear stresses, as well as displacements, were visualized in the form of graphs and color charts. To get the calculated “load — displacement” dependences using FEM, the reverse method of obtaining transverse shear constants was used. DCB loading showed that felting provided increasing the rupture strength in the Z direction to ~18%, by 39 to 46 MPa, and in the planes XZ— to ~16%, by 77 to 89 MPa.Discussion and Conclusion. Felting as a method of local three-dimensional reinforcement enhances the interlayer crack resistance of polymer fabric composites. It provides reducing the area of stratifications after local impacts during the operation of structures. Flexible felting technology makes it possible to create zones with an arbitrary impact density, increasing fracture toughness only in the required places of structures. The FEM analysis of the stress state at the crack tip within the framework of the nonlocal strength theory has shown that in strength calculations, the stratification crack can be considered as a stress concentrator.

Experimental study on toughening UV pre-curing adhesives for electrically conductive applications

Epoxy adhesives are not conductive materials, usually requiring the use of fillers to create electrical bridging between the joint's substrates. This work aims at the implementation of UV fixation, instead of clamping jigs, on doped joints to ensure enough contact bridging pressure during curing. As such, an UV pre-curable adhesive was mechanically characterised in terms of strength – bulk tensile test and TAST (shear) –, and fracture toughness – DCB (mode I) and ENF (mode II) tests –, in the neat state (reference). Afterwards, for the doped configurations – 5%v/v conductive spheres (Sph), 15%v/v aluminium (Al) and 10%v/v graphite (C) – only mode I and II tests were considered accessing the filler's toughening effect. The electrical conductivity of the real joints was assessed through electric discharge machining (EDM). In the end, the Sph's were deemed necessary for bond thickness control. And the Al (mode I +50 % and II +61 %) and C (mode I +25 % and II +42 %) powders presented improvements in toughening the adhesive. However, UV-fixation failed to provide enough clamping pressure, during curing, to maintain the electrical bridging necessary for EDM.

Experimental investigation of stacking interface on mode II interlaminar behaviour of self-healable vitrimeric CFRP

With the extensive research on ester-exchanged vitrimer, laminates utilizing vitrimer as matrix have garnered increasing attention due to their healability for achieving long-term durability of the composite structure. However, current research on the healing of mode II delamination for vitrimeric CFRP (vCFRP) laminates remains notably limited. In this study, the ENF tests were implemented to acquire load–displacement curves of eight typical vCFRP laminates with various interfaces. The results indicated a pronounced impact of the stacking interface on the fracture toughness values, the R-curves, fiber bridging effect, and crack growth behaviours. The continuous 0° fiber hindered the upward migration for the laminates with 0°//θ° interface, while the discontinuous θ1° fabric had a lower in-plane strength and were more prone to cracking for the laminates with θ1°//θ2° interface. Subsequently, the healing efficiencies of specimens were characterized. The healed laminates undergone three shear delamination damages could still achieve a substantial recovery of about 76.33 %.

Investigation of mechanical properties and self‐healing efficiency of carbon fibers composites infused with unsaturated polyester vitrimer

AbstractUnsaturated polyester resins (UPR) are widely in composite manufacturing, particularly in the energy, wind, and construction sectors. When these composites sustain damage, defects or cracks, several methodologies are employed to heal and repair them to extend their lifespan while reducing environmental impacts. In this work, the use of vitrimeric UPR (UPRv) as a matrix for carbon fiber composite, was investigated in terms of healing efficiency by mechanical and thermal properties. Flexural and interlaminar shear strength tests revealed healing efficiencies exceeding 40% and 90%, respectively. These results were attributed to the transesterification exchange occurring within the crosslinked networks of UPRv. As described in our previous work, the incorporation of “Covalent Adaptable Networks (CANs)” enables the conversion of commercial thermoset (UPR) into vitrimeric resins, thereby providing reprocessing and healing capabilities. Furthermore, a comparison between pristine and healed composites with Mode I DCB and Mode II ENF tests, confirm how the introduction of vitrimer in composite facilitates the manufacturing of a self‐healing product, with a significant recovery of the original properties.Highlights Application of a previous synthetized vitrimeric unsaturated polyester resin (UPRv)—fabrication of CF's composites reinforced with UPRv—investigation of the self‐healing property of the developed laminates—potential application as green and sustainable composites.

Fatigue crack growth characterization of composite-to-steel bonded interface using ENF and 4ENF tests

In this paper, mode II fatigue crack growth properties of the composite-to-steel interface are characterised through different test configurations, namely ENF and 4ENF tests. Different loading types including force control and displacement control methods are compared. An innovative shear strain based method is proposed for monitoring the mode II crack growth at the bi-material interface through Digital Image Correlation (DIC). A 3D finite element model with Virtual Crack Closure Technique (VCCT) is built and used for obtaining the strain energy release rate (SERR) to investigate the effect of geometrical nonlinearity, friction at the interface and steel yielding, as well as to verify the mode mixity. The results show that the standard 3-point bending ENF specimen can be unstable under force control and sweeps narrow SERR range by a single test under displacement control. The 4-point bending 4ENF test shows stable crack propagation and clear SERR developing trend. More pronounced geometrical nonlinearity and friction effect exist for 4ENF test which can be considered when interpreting the Paris curves by a nonlinear finite element model.

Influence of surface roughness on the mode II fracture toughness and fatigue resistance of bonded composite-to-steel joints

Hybrid bi-material concepts in engineering structures where fibre-polymer composites are used together with steel structural members have potential to reduce material usage, extend fatigue life and improve structural reliability. The bond performance of the bi-material composite-to-steel interface is of crucial importance and highly relies on the surface preparation quality of the steel element. This paper investigates the influence of steel surface roughness on the mode II fracture toughness and fatigue crack growth behaviour of the bonded composite-to-steel joints. Glass fibre composite and mild structural steel material is considered directly bonded in a wet lay-up process. 4-point end notch flexure (4ENF) tests are conducted on specimens with the steel plates prepared with low, medium and high roughness, respectively. In addition, morphology of the fracture surfaces are characterised by a 3D profilemeter and the friction coefficient is measured by a tribometer for each roughness level. Results show that in quasi-static fracture, fibre bridging is dependent on the surface roughness. A roughness level with Sq ≥ 22 µm of the steel surface can promote significant fibre bridging thus improved fracture toughness due to enlarged effective bonding area. Under cyclic loading, no fiber bridging is observed across all roughness levels tested. However, the Paris curve parameter C is significantly affected by the roughness level, which decreases by approximately 100 times as the steel surface roughness increases from the low level of Sq = 5 µm to high level of Sq = 22 µm. The m parameter of the Paris curve remains fairly constant across all the roughness levels tested.

Mode II and mode III delamination of carbon fiber/epoxy composite laminates subjected to a four-point bending mechanism

Accurate determination of mode III interlaminar fracture toughness is paramount in composite materials due to its critical role in edge delamination, which nonetheless remains a significant challenge encountered. As such, this study focused on the investigation of mode II and III interlaminar fracture behavior of carbon fiber (CF)/epoxy composite laminates using four-end notched flexure (4ENF) tests and four-point bending plate (4PBP) tests, respectively. In particular, a cohesive zone model was employed for the simulation of the delamination process via finite element analysis (FEA). The mode II fracture toughness of CF/epoxy composites was determined to be 1.41 N/mm in experimental work. Additionally, experimental data in relation to force-displacement curves were in good agreement with numerical simulation results, which validated this simulation approach to successfully capture the mechanical response of composite laminates. In a similar manner, mode III delamination fracture toughness for CF/epoxy composites was numerically estimated to be 2.1 N/mm. Microscopic analysis indicated shear cusps were observed in both mode II and III specimens, as opposed to existing flakes discovered in mode III specimens only. Overall, this research enlightens a simple and effective way to estimate pure mode III fracture toughness and corresponding delamination behavior with respect to crack initiation and propagation.

Fractographic Investigation of Cryogenic Temperature Mode-II Delamination Behavior of Filament Wound CFRP Laminates with Varied Resin Systems

This study investigates the mode-II delamination performance of filament-wound unidirectional composites with different types of epoxies as their matrix phase under room and cryogenic temperatures. A typical vacuum infusion resin, an aerospace-grade cold-curing resin and crack-resistant toughened resin systems were wet-wound with 12K carbon fiber tows to manufacture the composite samples. Test samples with a (0)16 ply sequence were tested according to ASTM D7905-19. The tested samples were investigated via microscopic analysis to assess the failure mechanisms associated with varying the matrix material and temperature. ENF tests at room temperature were found to be susceptible to the inherent variance in the fiber architectures along with resin-viscosity-driven fiber wetting. Cryogenic conditions induce a shift in the mode-II delamination behavior from a rather complex failure mechanism to a consistent fiber/matrix debonding mode with diminishing GIIc values except for the toughened resin system. The provided comprehensive fractographic analysis enables an understanding of the various causes of fracture, which determines the laminate performance. The combined evaluation of the distinctive damage modes reported in this study provides guidance on the conventional wet-winding process, which still remains a volumetrically dominant and viable option for cryogenic applications, particularly for vessels with limited operational durations like sounding rockets.

Acoustic emission with machine learning in fracture of composites: preliminary study

In this paper, preliminary studies on the failure analysis of hybrid composite materials utilizing acoustic emission and machine learning are presented. The main purpose of this study was to analyze the possibilities of using machine learning techniques as a way to better cluster the data obtained from acoustic emission. In this paper, we focus on data preparation, feature extraction (Laplacian score), determination of cluster number (Caliński–Harabasz, Silhouette, and Davies–Bouldin), and testing three clustering techniques, namely K-means, fuzzy C-means, and spectral clustering. The dataset was obtained by testing fiber metal laminates—composites consisting of metal and composite layers. Two experimental tests were realized on pre-cracked rectangular specimens—one with loading in mode I and one with loading in mode II (DCB—double cantilever beam and ENF—end-notch flexural test). Elastic waves were recorded during these tests via an acoustic emission system. Preliminary studies show that the proposed method can be used successfully to cluster data obtained in this way. The obtained dataset was split into 3 clusters (for the ENF test) and 5 clusters (DCB test). In the next stages of the research campaign, based on the presented results, we intend to change the approach to semi-supervised by running additional single-cause damage tests to enhance the achieved results and enable easier damage recognition.

Experimental evaluation on the synergistic effect of PC/ABS/DGEBA blend in fracture toughness enhancement of CFRP composites at cryogenic temperatures

AbstractThe current study confirms that modified carbon fiber reinforced polymer (CFRP) composites have higher fracture toughness than unmodified CFRP composites achieved by exploiting the synergistic effect of a polycarbonate (PC)/acrylonitrile butadiene styrene (ABS) blend in toughening the diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The CFRP composite specimens are tested at near cryogenic temperatures using TMA, DMA, and microcrack analysis to determine the best‐suited concentration of ABS in the PC/ABS blend. TMA and DMA results, as well as microcrack analysis at cryogenic temperatures (CT), confirm that the blend 90/10 is effective in reducing the brittle nature of DGEBA resin and increasing bond strength, resulting in the fracture toughness enhancement of CFRP specimens at CT. Further investigation of 90/10 modified CFRP (90/10 m‐CFRP) and unmodified CFRP specimens using Mode II fracture using ENF test and SEM analysis reveal significant reduction in brittle characteristics of matrix with increase in elongation at failure and fracture surface morphologies confirm nano web‐like structures bridging the CF layers, proving to improve fiber/matrix bond strength. This study concludes the effectiveness of hybrid PC/ABS blend in synergistically‐modifying DGEBA resin for improved fracture toughness of CFRP laminates across a wide temperature range (−150°C to 150°C).

A comparative numerical-experimental investigation on the tensile behaviour of bonded, rivetted and hybrid composite joints configurations

This paper deals with the development of accurate numerical models to predict the tensile mechanical behaviour of composite joints. The problems of damage initiation and propagation in composite joints for an economic solution with minimized oversizing is addressed. Different types of joints (Single Lap Joint, Single Strap Joint, Joggle Joint) and couplings (bonded, rivetted, hybrid) have been combined to find out the most suitable and reliable joining technique depending on the design constraints. On the basis of preliminary DCB and ENF tests, the characteristics of the adhesive material (stress limits and fracture energies) have been determined. These data have been employed for the numerical simulations on bonded and hybrid (bonded/rivetted) joints. The introduced numerical models, taking into account geometrical non-linearity and progression of damage in composite plies, have been preliminary validated by ad-hoc experimental tests to demonstrate their effectiveness in simulating the mechanical behaviour of general composite joints.

Mode I and mode II fracture behavior in nano‐engineered long fiber reinforced composites

AbstractDelamination still remains one of the common concerns in continuous fibers reinforced polymer composites. In the literature, the incorporation of nanoconstituents into constitutive composites plies has been suggested with the aim of improving interface toughness. In this study, vertically aligned carbon nanotubes forests (VACNTs) are transferred at composites interfaces, and mode I (DCB tests) and mode II (ENF tests) fracture tests carried out on composite samples. Microscopic analysis of reference and nano‐engineered composites help to understand fracture behavior and toughness. While the crack path in ENF samples is observed at interlaminar VACNTs‐resin interfaces, the crack path in DCB samples is intralaminar, located within the unidirectional carbon fiber plies. These microscopic observations give explanation for unstable crack propagation in nano‐engineered ENF samples and the unchanged toughness from the nano‐engineered to the reference zone in DCB samples.

Fracture mechanical stability of DCB and 4ENF tests complemented with linear springs

In this work the stability of the DCB and 4ENF fracture specimens is analyzed using some beam theories and the compliance calibration method. The main goal is that the systems are complemented with linear springs and it is investigated how the springs influence the stability of crack propagation from the theoretical standpoint. Large number of experiments were carried out including crack initiation tests providing the experimental load displacement responses, the compliance datasets and the qualitative observation of crack initiation. Apart from the analytical method the compliance calibration method was employed to evaluate the experimental data and to determine the fit curves of the compliance datasets, the R-curves and finally the bifurcation curves or in other words the critical displacement and crack length relationships including several additional spring stiffnesses for both test configurations. The results indicate that - as expected - the smaller the spring stiffness is the worst the stability of the DCB specimen will be, leading to the possibility of destabilization. On the contrary the analysis and experiments confirm that the 4ENF test cannot be destabilized independently of the stiffness of additional springs.

Interlaminar performance of 3D CNTs/carbon black film enhanced GFRP under low-temperature cycling

Unidirectional Glass Fiber Reinforced Plastic (GFRP) laminates were prepared by inserting three-dimensional (3D) CNTs/carbon black film or CNTs/graphene oxide (GO) film into the laminates to improve the interlaminar performance. The effects of low-temperature cycles (35 cycles between −58 °C and room temperature (RT)) on the samples were investigated, and the damage caused to the reinforcement layer was directly detected by comparing the interlaminar images. By comparing the experimental results of the double cantilever beam (DCB) and end notch bending (ENF) test, the optimising effect of the new composite film (CNTs/carbon black film) on the interlaminar fracture toughness was verified. The results showed that the interlaminar fracture toughness of CNTs/GO film and CNTs/carbon black film in modes I (ENF test) and II (DCB test) was similar and superior to that of CNTs film. Regarding the effect of the low-temperature cycles on the samples, the change of CNTs/carbon black film and CNTs/GO film is also lower than that of CNTs film. Finally, the matrix interface and failure mechanisms of the interlaminar reinforcement were investigated by scanning electron microscope (SEM).

Effect of Flame Treatment on Bonding Performance of GF/EP Pultrusion Sheets Used for VARI Process

This paper presents an easy and low-cost flame treatment method to improve the bonding performance of GF/EP (Glass Fiber-Reinforced Epoxy) pultrusion plates, which are using widely for large size wind blades. In order to explore the effect of flame treatment on the bonding performance of the precast GF/EP pultruded sheet vs. the infusion plate, the GF/EP pultruded sheets were treated with different flame treatment cycles and were embedded in the fiber fabrics during the vacuum-assisted resin infusion process (VARI). The bonding shear strengths were measured by tensile shear tests. It is found that after 1, 3, 5, and 7 flame treatments, the tensile shear strength between the GF/EP pultrusion plate and infusion plate increased by 8.0%, 13.3%, 22.44%, and -2.1%, respectively. This indicates that the maximum tensile shear strength can be obtained after five times of flame treatment. In addition, DCB and ENF tests were also adopted to characterize the fracture toughness of the bonding interface with the optimal flame treatment. It is found that the optimal treatment gives increments of 21.84% and 78.36% for G I C and G II C, respectively. Finally, the surficial topography of the flame-treated GF/EP pultruded sheets were characterized by optical microscopy, SEM, contact angle test, FTIR, and XPS. The results show that flame treatment plays an impact on the interfacial performance through the combination of physical meshing locking and chemical bonding mechanism. Proper flame treatment would remove the weak boundary layer and mold release agent on the surface of the GF/EP pultruded sheet, etch the bonding surface and improve the oxygen-containing polar groups, such as C-O and O-C=O, to improve the surface roughness and surface tension coefficient of pultruded sheet to enhance the bonding performance. Excessive flame treatment destroys the integrity of epoxy matrix on bonding surface which results into the exposure of the glass fiber, and the carbonization of release agent and resin on the surface loosen the surficial structure, which reduces the bonding properties.

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.

Mode II cohesive zone law of porous sintered silver joints with nickel coated multiwall carbon nanotube additive under ENF test

Various contents of nickel coated carbon nanotube added sintered silver joints are fabricated through mechanical alloying. An analytical method is presented to compute the fracture energy of ENF tests incorporating shearing effect and thickness effect, which shows better accuracy compared with traditional method, which are closer to the solutions of compliance based beam method. Mode II fracture energy, mode II fracture toughness and mode II cohesive zone model for different contents of nickel coated carbon nanotube added sintered silver joints are presented based on the data extraction of the ENF tests. With the increase of adhesive layer thickness, the fracture energy and the mode II fracture toughness improve. However, thickness effect is less significant but not negligible comparing with the strengthening effect on the improvement of fracture toughness, which is caused by adding nickel coated multiwall carbon nanotubes. The relation between mode II fracture toughness and porosity is also discussed. For most of the conditions, the mode II fracture toughness is higher than mode I fracture toughness regardless of additive content due to the crack deflection, crack bridging and pull-out mechanisms during the crack propagation except for those high content of additive whose fracture energy is reduced because of aggregation effect.

A semi-analytical method for determining mode-II fracture toughness and bridging law of composite laminates

Accurate determinations of fracture toughness and the bridging law are important for the design and analysis of composite laminates. In this work, based on the Castigliano theorem and the point friction theory, a semi-analytical method for mode-II fracture toughness and bridging law is established. ENF tests on T800 carbon/epoxy laminates are carried out and fracture toughness are obtained. Public data from laminates made by other different materials are also used for verification. It is found the fracture toughness calculated by the semi-analytical method agree well with experimental results, verifying the accuracy of the proposed method for determining fracture toughness. Furthermore, the relative shear displacement at the pre-crack tip is also calculated and its relation with fracture toughness is established, from which bridging law can be determined by J-integral theory. The determined bridging law is integrated into cohesive zone model for delamination growth modelling. Good agreements between numerical and experimental load–displacement curves exhibit, which further verifies the proposed method. Using the proposed method, only experimental load and displacement data are required when determining fracture toughness and bridging law. Problems of unstable crack growth and difficult observation due to the compression of delamination are avoided in a simple and efficient way.

An efficient semi-analytical method to extract the mode II bridging-traction law in ENF tests directly from the experimental load displacement data

Large scale fiber bridging ahead of the crack tip of ENF specimen is one of the most common toughening mechnisms, leading to a significant resistance phenomenon on the fracture toughness. In order to account for the toughening effect of the fiber bridging to accurately model the delamination of composite laminates, a physically semi-analytical method is presented to determine the mode II bridging law with the assumption of linear form directly from the experimental load displacement curve. The method includes the deflection analysis of the ENF specimen based on the beam theory considering the closing force introduced by fiber bridging and the inverse method iteratively determining the bridging law parameters. To validate the proposed method, the identified bridging tractions by the current method are integrated in the traditional cohesive law to simulate the whole delamination propagation process of undirectional laminates with different materials and multidirectional laminates. Good agreements between the numerical results and experimental ones indicate the accuracy and the applicability of the current method. The major advantage of the proposed method is that it reduces the instructmentation needed in the delamination test and it can be adopted as an efficient method to investigate the delamination behaviour of specimens.