End-notched Flexure Research Articles

Experimental estimation and numerical validation of cohesive zone parameters in hydroxyl functionalized MWCNTs‐reinforced CFRP under pure mode II loading

AbstractThe present study concerns the interface mode II fracture of hydroxyl functionalized multi‐walled carbon nanotubes (MWCNTs) reinforced carbon fiber epoxy composites. The well‐known cohesive zone model (CZM) is employed for investigating crack propagation. The major difficulty in applying the CZM is the parameter identification. In this work, the CZM's strength and energy parameters are determined reliably from the Short beam shear (SBS) and End‐notched flexure (ENF) tests, respectively, for pristine and three MWCNTs variations—0.1 wt.%, 0.2 wt.%, and 0.3 wt.%. The derived CZM parameters from the tests are imputed to a bilinear cohesive law to simulate the global load vs. displacement responses, which correlate well with the experimental findings. It is observed that the addition of 0.2 wt.% of MWCNTs increases the cohesive strength and fracture energy up to 14.14% and 17.78%, respectively, when compared to the pristine composites. Finally, the field emission scanning electron microscopy (FESEM) analysis is carried out to investigate the fracture mechanisms.Highlights Cohesive properties under mode II are estimated using standard fracture tests. The homogenization technique is used to calculate the effective properties of CFRP. The maximum % error among the experimental and the numerical peak load is 3.01%. A higher density of hackles with an increase of MWCNTs up to 0.2 wt.%.

Increase of shear fracture energy of adhesive joints using double-double laminates

ABSTRACT Double – Double (DD) is a new family of laminates that holds the potential to completely overhaul the design of composite structures. These laminates are not only lighter and more robust than the classical QUAD laminates, but they also accelerate manufacturing processes and reduce costs. However, the application of DD laminates as substrates in adhesive joints has not been studied so far. In this work, end-notched flexure (ENF) tests with composite substrates are performed in both QUAD and DD configurations. The main goal is to evaluate the maximum load and shear fracture energy for both laminates. Additionally, the displacement fields are measured through the digital image correlation (DIC) analysis. Experimental results showed that DD laminates can significantly improve the shear fracture energy of adhesive joints and also delay the crack propagation over the test. The findings presented herein could play a crucial role in driving significant alterations in the design procedures of adhesive joints with composite materials, highlighting the DD advantages.

Effects of carbon/glass nonwoven interleaving veils and their areal density on opening and shearing mode interlaminar fracture toughness of glass epoxy composites

The study explores the impact of Carbon and Glass nonwoven interleaving veils with varying areal density on delamination resistance in unidirectional (UD) Glass epoxy composites under Opening (Mode-I) and Shearing (Mode-II) mode loadings. Commercially available non-woven Carbon and Glass interleaves with areal densities of 15, 20, 25, and 30 g/m2 were used to manufacture the interleaved Glass epoxy laminates. Double Cantilever Beam (DCB) and End Notch Flexural (ENF) specimens were used to determine the mode-I and mode-II fracture energies. The results reveal that, during mode-I initiation, interleaved laminates exhibit a lower interlaminar fracture toughness (IFT) except for the Carbon fiber interleaved laminates of areal density 30 g/m2. However, during propagation, all veils interleaved laminates presented significant improvement in IFT ranging from 5 to 25 % accompanied by substantial fiber bridging. In case of mode-II test, the laminates with Carbon and Glass veils exhibit enhanced IFT (60–182 %) with increasing areal density compared with the non-interleaved Glass laminates. Optical and scanning electron micrographs (SEM) provide evidence of fracture mechanisms involved in the interleaved and non-interleaved composite laminates.

Acoustic Emission Analysis of Mode II Interlaminar Fracture Toughness of 3D Reinforced CFRP

The use of composites in industry is increasing due to their ability to replace traditional materials. Carbon fiber-reinforced polymers offer a favorable strength-to-weight ratio, making them advantageous in numerous applications. Delamination is a common failure mode for composite materials, making it a crucial factor in ensuring material safety during service life. While fiber orientation in composites is designed for specific directional reinforcement, out-of-plane loads are often neglected, posing a critical challenge. Implementing through-thickness reinforcement, such as tufting, can enhance out-of-plane resistance, enabling more accurate structural designs. Non-destructive testing methods, particularly acoustic emission, play a significant role in ensuring component safety by detecting early damage and flaws. This study focused on monitoring mode II interlaminar fracture toughness and end-notched flexure (ENF), using acoustic emissions to compare the performance of samples with different through-thickness reinforcements against that of nonreinforced samples. The research analyzed acoustic emission patterns during testing, revealing a strong correlation with failure stages and the resistance induced by reinforcements. This approach provided valuable insights into damage characterization, supported by fractography analysis, especially concerning the final stages of failure due to damage, and the effects of different thread reinforcements. Acoustic emission proved crucial for real-time monitoring, enabling informed decisions to be made regarding component repair and lifespan extension in composite materials.

Influence of co-deposition modification on mode I and II interlaminar toughness of epoxy laminates interleaved with waste textiles using experimental measurement and finite element technique

Interlaminar delamination has been a persistent problem that restricts the ability of fiber reinforced laminated composites under applied load. The influence of polyethyleneimine-catechol (PEI-CCh) co-deposition modification on the mode I (GIC) and II interlaminar fracture toughness (GIIC) of plain glass fiber fabric reinforced epoxy composites (PGFRECs) were discussed using double cantilever beam (DCB) and end notch flexural (ENF) tests. It was found that interfacial properties of PET/epoxy and PET/cotton/epoxy interfaces were substantially improved after co-deposited modification, which laid a solid foundation for enhancing the resistance to interlaminar delamination. GIC and GIIC in platform of crack propagation greatly increased with the introduction of co-deposition layer. The fracture surfaces of delaminated failure specimens were observed by scanning electron microscope. The higher propagation fracture toughness for co-deposition modified PGFRECs is due to the anchoring effect of functional layer combining waste fabrics with epoxy matrix to provide favorable stress transfer channel prior to macroscopic crack formation. In regard to the delamination crack, new crack generation and bridge fiber failure mainly dominated the mode I crack extension process, while zigzag sections were particularly common for mode II fracture specimens.

Investigation of fracture characteristics of titanium/CFRP hybrid composites through experimental and numerical methods

In this study, the delamination resistance of carbon fiber reinforced polymers (CFRP) consolidated with titanium alloy at the interface between the metal and composite was investigated experimentally and numerically. End-notched flexure (ENF) tests were performed to assess the fracture toughness (GIIC) for Mode II crack expansion of Ti6Al4V titanium alloy/CFRP composite parts. The EFN test is applied to Ti6Al4V-carbon fiber/low melt poly (aryl ether ketone) (CF/LM-PAEK) and Ti6Al4V-carbon fiber/poly (ether ketone ketone) (CF/PEKK) composites with the [0°]24 stacking sequence of unidirectional (UD) fibers. Experimental results indicate that the LM-PAEK composites exhibited Mode II strain energy release rate values 27.64 % higher than those of the PEKK composites. The finite element simulation by LS‐DYNA shows good correlations with the experimental results, with an average error of 5.44 % for the PEKK and 10.58 % for the LM-PAEK, respectively.

Analytical Modeling for Asymmetric Four-Point Bend End-Notched Flexure Delamination Testing of Composite Laminates Considering Friction

Analytical Modeling for Asymmetric Four-Point Bend End-Notched Flexure Delamination Testing of Composite Laminates Considering Friction

Improved interlayer performance of short carbon fiber reinforced composites with bio-inspired structured interfaces

The weak layer interfaces of 3D-printed short carbon fiber (SCF) reinforced polymer composites have remained an issue due to planar layer printing by traditional 3D printers. Recently, multi-axis 3D printing technology which can realize non-planar layer printing has been developed. This study’s aim was to evaluate and compare the bonding performance of non-planar interfaces produced by multi-axis 3D printing with that of planar interfaces. The tested non-planar interfaces were designed as bio-inspired structured interfaces (BISIs) based on microstructural interfacial elements in biological materials. The standard specimens with the 0°/90° and 0° infill line directions were printed by a robotic arm multi-axis 3D printer. Double cantilever beam (DCB) and end-notched flexure (ENF) tests were conducted to obtain Mode Ⅰ and Mode Ⅱ interlaminar toughness of SCF-reinforced composites. Test results showed that the critical energy release rates of the integrally formed BISI were significantly improved compared with the planar interface (PLAI) for both Mode I and Mode II delamination. In particular, the BISI with 0° infill line direction exhibited the greatest increase in critical energy release rate, and the damaged areas were spatially swept through the curved interfaces of the BISI with different infill line directions by scanning electron microscopy (SEM) and computed tomography (CT), which showed that the higher critical energy release rate was always accompanied with a larger damaged area. In addition, the tensile and flexural properties of 0°-infilled PLAI and BISI specimens were also measured. This work provides an in-depth investigation of the PLAI and BISI properties of SCF-reinforced composites, demonstrating the potential benefits of integrally formed BISI by multi-axis 3D printing and fostering new perspectives to enhance layer interfaces of 3D printed composites.

Automated Quantification of Interlaminar Delaminations in Carbon-Fiber-Reinforced Polymers via High-Resolution Ultrasonic Testing.

This article presents a method of ultrasonic testing (UT) that detects and quantifies interlaminar delaminations in CFRP composites with high resolution in terms of both spatial resolution in the planar dimension and depth into the laminate. Unidirectional and woven CFRP laminates were fabricated for this study, with a PTFE film inserted at various depths throughout the laminate to act as intentional crack initiation sites. All samples were mechanically tested via a three-point, end-notched flexure (ENF) test, followed by a quantification of the extent of the induced interlaminar delaminations using UT and X-ray computed tomography (CT). UT analysis for unidirectional CFRP samples was able to show a clear contrast between the delaminated area and the non-delaminated area. UT analysis of the woven CFRP samples yielded comparable results but required finer tuning of analysis parameters due to the interlocking woven fabric. CT results revealed a significant contrast between the crack and composite; thus, fine geometrical features of the crack front could be observed. UT and CT measurements were then compared, revealing an average difference of 1.09% in the delamination area, with UT overestimating as compared to CT. A UT depth study was also performed to automatically locate the interlaminar delamination at different depths throughout the components, with the delamination being predicted within one lamina interface for all samples. These results demonstrate UT's ability to accurately detect and quantify the extent and location of interlaminar delaminations due to bending.

Mode II Delamination under Static and Fatigue Loading of Adhesive Joints in Composite Materials Exposed to Saline Environment.

This study investigates the fatigue delamination behavior of adhesive joints in epoxy carbon composite materials under Mode II fracture loading. The joints were characterized using the End-Notched Flexure (ENF) test, comprising adhesive joints formed by bonding two unidirectional carbon fiber epoxy matrix laminates with epoxy adhesive. These joints were subjected to different exposure periods (1, 2, 4, and 12 weeks) in a saline environment. Prior to dynamic fatigue testing, critical Mode II energy release rate values were determined through quasi-static tests, serving as a reference for subsequent fatigue characterization. This study aimed to comprehend how exposure duration to a saline environment affected the initial stage of fatigue delamination growth and employed a probabilistic model based on the Weibull distribution to analyze the experimental data. The results, gathered over a two-year experimental program, revealed varying behaviors in adhesive joint resistance to delamination based on exposure duration. A noteworthy reduction in fatigue strength capacity was observed, with fracture energies for infinite fatigue life reaching approximately 20% of their static loading capacity. This study sheds light on the deterioration of adhesive joints when exposed to a saline environment.

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.

A comprehensive study on the fracture behavior modeling of adhesively bonded composite joints using a compliance-based beam method

In this study, the fracture response of adhesively bonded composite joints under different modes of I, II and mixed-mode I/II conditions was characterized by employing the double cantilever beam (DCB), end notched flexure (ENF), and mixed-mode bending (MMB) tests, respectively. In addition, various mechanical tests were executed for determining the mechanical properties of the glass fiber/epoxy composite substrates. The flexural modulus and equivalent crack length relations for the MMB specimen were derived using the compliance-based beam method (CBBM). This method enabled the determination of strain energy release rate (SERR) and fracture energy only using the load-displacement curve. Furthermore, the proposed method simplified the process of obtaining resistance curves (R-curves), as the crack length did not need to be measured during the test and considered the effects of energy loss in the fracture process zone that are neglected in classical methods. The R-curves obtained from the proposed method were compared with the direct beam theory for pure mode I and corrected beam theory for pure mode II and mixed mode loadings. According to this comparison between the classical methods and CBBM, SERR achieved by the proposed method was higher than that obtained by classical methods. The cohesive zone model and corresponding traction-separation laws developed by a direct method were used in conjunction with the finite element method to validate the acquired experimental data. Moreover, the failure patterns occurred during testing was determined after investigating the fracture surfaces under the loading modes of I, II, and mixed I/II.

Enhancing the fracture toughness of laminated composites through loop warp yarns

The delamination tendency is a key factor affecting the practical use of laminated composites. How to improve the interlaminar strength and interlaminar fracture toughness of laminated composite is a focus of research on composite materials. A three-dimensional (3D) weaving technique is employed to weave loop warp yarns (loops) with 3D fiber bundles instead of warp yarns on the basis of plain fabrics. The contribution of loops to the interlayer performance is revealed by double cantilever beam (DCB) tests, end-notch flexure (ENF) tests, flexural tests, and double notch shear (DNS) tests. Test results show that the mode I and mode II interlaminar fracture toughness values of the single-sided-loop two-dimensional (2D) woven laminated composite (SWLC) are 77% and 58.2% higher than the interlaminar fracture toughness values of the general 2D woven laminated composite (GWLC). As companied with the ones of GWLC, flexural strength and modulus of SWLC increase by 73% and 69.1%, and the interlaminar shear strength of SWLC increases by 45.7%. The loop yarns act as a link between the interlaminar matrix and the reinforcing fiber fabric and thus improve the poor interlaminar performance of GWLC. The reported information may provide a reference for the application of the SWLC in practice.

Fracture energy and mechanical properties of toughened epoxy resin with eggshell powder

Eggshells are daily food waste disposed of in landfills, producing environmental issues and an unpleasant odour. Eggshells powder may be suitably applied as a filler in epoxy resins to improve their mechanical properties. The material properties and fracture energy of toughened epoxy with eggshell powder (TEEP) were measured and investigated. Dog-bone specimens were fabricated to investigate the mechanical properties of TEEP (i.e. elastic modulus, Poisson’s ratios and tensile strength) and tested under quasi-static tensile loading. Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests were used to determine the mode I and Mode II fracture energies of TEEP specimens, respectively. For this purpose, oven-dried eggshells were crushed into particles size of 100 µm. The volume fraction of eggshell powder as filler in TEEP is designated as 0%, 2.5%, 5%, and 10%. The epoxy resin system was made by mixing EPIKOTE Resin 828 and Hardener 651 with a mixing ratio of 5:2 by weight. The results showed that the tensile strength, elastic modulus, fracture energy in Mode I and Mode II were optimum at TEEP 5% with an enhancement of 36.8%, 24.9%, 60.3% and 166.3%, respectively compared to that of neat epoxy. This is due to better surface roughness and the ability of eggshell powder as a crack arrestor. However, TEEP with 10% eggshell prone to agglomeration of eggshell powder caused poor bonding with epoxy resin and affect the mechanical properties.

Experimental and numerical evaluation of hydro-thermal ageing's effects on adhesive connections in offshore structures

In offshore structures, the need to replace or strengthen existing metal components arises over time. One possible solution is the use of bonded connections. Nevertheless, the durability of the adhesive connections is highly compromised by the prolonged contact with water.In this paper, an experimental and numerical evaluation of hydro-thermal ageing's effects on polyurethane adhesive connections has been analysed. An efficient semi-analytical procedure is developed for the design or verification of existing structures reinforced by bonded steel elements. The model can evaluate the interfacial shear stress at all loading stages up to the failure of bonded joints exposed to the marine environment. The model is based on Bernoulli beam theory and satisfies the requirements of equilibrium and strain compatibility, allowing for interfacial deformations.Furthermore, the mechanical behaviour of bonded connections subject to accelerated ageing has been experimentally investigated. More in detail, an experimental program including double lap shear and end-notch flexure tests was performed to evaluate the cohesive behaviour in Mode II of a polyurethane structural adhesive under the combined effects of water and temperature during 150 days of exposure. The experimental and numerical investigation shows how the performance of the adhesive joints is influenced by the ageing conditions.

Using cohesive zone models with digital image correlation to obtain a mixed mode I/II fracture envelope of a tough epoxy

This work describes a method in which the digital image correlation (DIC) method and finite element analysis (FEA) were used to create a quasi-static mixed-mode fracture envelope for bonded joints consisting of 2024-T3 Al adherends and a tough structural epoxy adhesive. Symmetric and asymmetric versions of double cantilever beam, single-leg bend, and end-notched flexure tests are used to populate the mixed-mode fracture envelope with results at several mode mixities. Experiments are conducted in a universal testing machine while recording images for subsequent DIC analysis. Finite element analysis is used to implement cohesive zone models (CZMs) of the adhesive fracture and to account for plastic deformation of adherends. Mode I and mode II traction separation laws (TSLs) are determined from a property identification method with a Benzeggagh–Kenane mixed-mode coupling law used to model mixed-mode behavior. FEA results are shown to provide a good agreement to both the crosshead displacement and DIC data. The methods in this paper serve as a potential framework for future calibration of mixed-mode fracture envelopes for joints bonded with very tough adhesives that complicate assessment with traditional data analysis methods.

Pure (I and II) and mixed-mode (I+II) fracture characterisation of unidirectional AS4/Low-melt-PolyaryletherketoneTM polymer composites

This paper reports an experimental and numerical study on the mixed-mode I+II fracture characterisation of unidirectional AS4/low-melt-Polyaryletherketone (AS4/LMPAEKTM) composites. To address the fracture law characterising the material mixed-mode fracture behaviour, a suitable approach is used, simply applying two different loading conditions regarding five mechanical tests and specimen configurations. Scoping to characterise fracture under pure mode I loading and under mixed-mode I+II loading with mode I predominance, the peel test loading condition is used, respectively, for symmetric and asymmetric double-cantilever beam tests (DCB and ADCB). The three-point bending loading condition is used in order to characterise fracture under pure mode II loading by end-notch flexure test (ENF) and mixed-mode I+II with moderate and predominant mode II, respecting symmetric and asymmetric single-leg bending tests (SLB and ASLB). To diminish the influence of crack monitoring ambiguity on the crack length analysis during experiments, data reduction schemes based on the compliance-based beam method (CBBM) were used. Additionally, numerical finite element analyses were conducted in order to validate the data reduction method and the mode partition strategy based on cohesive zone modelling (CZM). Overall, a good agreement between numerical and experimental results was obtained, providing an adequate identification of the energy fracture criterion.

Mode I, mode II and mixed mode I-II delamination of carbon fibre-reinforced polyamide composites 3D-printed by material extrusion

Continuous carbon fibres can greatly improve the properties of 3D-printed polymer parts made by material extrusion. However, like all laminated composites, 3D-printed parts are susceptible to delamination damage. In addition, the printing process does not include a consolidation at high temperatures and pressure, unlike conventional manufacturing methods, which can lead to poor interlayer cohesion. Due to the combination of the susceptibility to delamination and a weak interface, the assessment of the interlaminar properties of 3D-printed parts is critical. This work experimentally investigates the delamination behaviour of carbon fibre-reinforced polyamide laminates under mode I, mode II and mixed mode I-II loading, using the double cantilever beam (DCB), end-loaded split (ELS), end-notched flexure (ENF) and mixed-mode bending (MMB) tests. An interlaminar fracture toughness at crack initiation of 1.5 kJ/m2 was found in mode I, 2.1 (ELS) and 1.8 (ENF) kJ/m2 in mode II, and 1.0 kJ/m2 in mixed mode I-II with GII/Gtotal = 0.5. Several analytical and numerical models are employed to validate the experimental results. Scanning electron microscopy revealed the micro-mechanical origins of the crack in the different loading configurations.

Application of the effective crack length method to model delamination of unidirectional composite laminates under Mode II shear loadings

Damage modelling of composite material delamination is an intense field of research to understand the complex composite failure behaviour and predict the residual strength of damaged structures. One of the widely employed methods to simulate delamination is the cohesive zone modelling (CZM) technique. To successfully utilize this approach, accurate characterisation of the interlaminar fracture toughness is crucial, while the composite delamination is dominated by Mode I and Mode II fracture in most cases. Numerous studies have been conducted on composite materials delamination under Mode I loadings using double cantilever beam (DCB) tests. Accordingly, the ASTM standard testing procedure and data reduction scheme to obtain Mode I fracture toughness (GIc) have been well established and widely accepted. However, it is still challenging to characterize the composite Mode II delamination resistance due to the susceptibility problems inherent to the existing testing methods and the lack of robust data reduction schemes to accurately identify the initial crack tip and monitor the crack growth. This study attempts to find a reliable solution to obtain the Mode II fracture toughness (GIIC) and an effective modelling strategy to simulate Mode II delamination in laminated composites. First, we reviewed the existing testing set-ups and surveyed the data reduction schemes conventionally used to obtain GIIC. The advantages and drawbacks of the three most used test methods, and particularly the standard end-notched flexure (ENF) test (ASTM D7905/D7905M) and the end-loaded split (ELS) test (ISO 15114:2014) were examined. Second, the advantages of the effective-crack-length-based data reduction schemes against the classical data reduction schemes were empirically studied with the ENF tests conducted on G40-800/5276-1 carbon-fibre reinforced composite laminate coupons. Although many studies have studied the accuracy of the effective-crack-length-based approaches from different aspects, there is a lack of direct comparison between the numerical results and experimental data. Our modelling study demonstrates that among the five data reduction schemes that are examined, the compliance calibration method (CCM) required by ASTM D7905 yields the most conservative GIIC values, which is suitable for establishing material property for a design purpose. The compliance-based beam method (CBBM) may generate the least conservative GIIC values, which are deemed to be the most accurate ones for modelling and simulation validation purposes. Finite element (FE) modelling of the ENF tests using the CZM technique was carried out and comprehensive parametric studies were conducted to achieve an efficient and robust strategy for delamination modelling of composite laminates under Mode II shear loadings.

Inverse identification of cohesive zone parameters for sintered nano-silver joints based on dynamic convolution neural network

The inverse identification of cohesive zone parameters is an important topic in the field of fracture mechanics. In this paper, three data-driven models, including multilayer perceptron (MLP), convolutional neural network (CNN), and dynamic convolutional neural network (DCNN), were presented to predict the cohesive zone parameters of sintered nano-silver end notched flexure (ENF) joints. Based on the construction of experimental and numerical datasets of load versus displacement curves, bilinear cohesive zone model (CZM) parameters of sintered silver joints are adopted as the prediction target. The investigation shows that MLP, CNN, and DCNN are all valid for predicting CZM parameters through load versus displacement curves with reasonable accuracy. However, DCNN has better prediction accuracy and performance than those of CNN and MLP models based on loss analysis, statistical indicator comparison, and K-fold cross-validation. DCNN can be adopted as the suitable surrogate model for CZM parameters inverse identification with high prediction accuracy. Otherwise, DCNN is also not sensitive to the load versus displacement curves data length. However, the computation efficiency of DCNN during the training process is not as high as that of MLP. Those three methods presented in this paper are very hopeful to be adopted for other inverse identifications of CZM parameters for various kinds of adhesive joints.