End Notched Flexure Test Research Articles

A new technique to measure shear fracture toughness of adhesives using tensile load

The end notched flexure (ENF) test is the most common method for measuring the mode II fracture energy of adhesives which provides reliable results but has some inherent problems. The shear fracture energy calculated using ENF for brittle adhesives is often based on a single data point (corresponding to the fracture load) obtained from the test where the crack grows catastrophically and the results are significantly sensitive to pre-crack tip conditions. The aim of the current study is to introduce a new pure mode II fracture test suitable for measuring the shear fracture energy of adhesives. A new technique for measuring the mode II critical strain energy release rate (SERR) of brittle adhesives is proposed based on the central cut ply (CCP) configuration. Fracture tests were carried out on adhesively bonded steel CCP specimens. It is shown that the crack growth in CCP joints is more stable, and the failure develops in multiple steps that generates more experimental data point during the crack propagation. The mode II fracture energy of a brittle epoxy-based adhesive is extracted using CCP method and is compared with the shear fracture energy obtained from the ENF tests. The effect of adhesive thickness on the obtained fracture energies is also analyzed. The CCP technique can be used to find both the crack initiation and crack propagation toughness values whereas the ENF method only provides the crack initiation fracture energy.

Shear traction‐separation laws of European beech under mode II loading by 3D digital image correlation

An experimental and numerical study on mode II fracture behaviour of European beech (Fagus sylvatica L.) in the RL and TL crack propagation systems is performed. It is a hardwood species that has attracted increasing interest for structural use in Europe in recent years. Three-point end notched flexure tests are performed. The R-curves of both crack propagation systems are obtained, from which the critical strain energy release rate (GIIc) is derived by applying the compliance-based beam method. This data reduction scheme avoids crack length monitoring during its propagation, which is an advantage in wood. Using a direct method, the shear traction‐separation laws in mode II loading are determined. Full field displacements around the crack tip are monitored by 3D digital image correlation technique, and the crack tip shear displacements are analysed. The proposed method is numerically validated by finite element analysis. Cohesive zone models are developed implementing a shear traction–separation law with exponential damage evolution zone and the average value of the experimental elastic and fracture properties. The numerical results for the different properties including upper and lower limits represent well the experimental data.

Determination of the resistance curve of solid wood by measuring two load–deflection compliances in the double cantilever beam and three-point end-notched flexure tests

Double cantilever beam and three-point end-notched flexure tests of solid wood were performed, and the Mode-I and Mode-II resistance curves were characterised by a novel data reduction method proposed in this study. In the proposed method, the load–deflection compliance along the loading line and that at another point in the test sample were measured; the resistance curve was determined using these compliances. Additionally, the resistance curves were compared with those determined from four conventional data reduction methods. The resistance curves determined from the proposed method were often consistent with those determined using the other methods. The proposed method enables an easy and simple determination of the resistance curve while reducing the disadvantages in conventional data reduction methods; therefore, it is a promising candidate to characterise the resistance curve of solid wood.

Inverse identification of material constants of various cohesive laws for delamination of composites using experimental results

There are several methods for modeling delamination in composite structures, among which the cohesive zone model (CZM) is one of the best. Various CZMs are implemented through specific constitutive equations. There are several material parameters in the formulation of the CZMs which have a prominent effect on the model’s accuracy. In the present study, an inverse methodology is proposed for obtaining the material constants of various traction-separation laws of the delamination phenomenon in composite materials. Linear, exponential, and a tabular form of cohesive law are utilized in this research to model the delamination phenomenon. Firstly, through some numerical examples, the robustness of the proposed inverse algorithm is assessed, and the effect of measurement error and initial guesses on the performance of the algorithm is studied. Then, two experiments, i.e., the double cantilever beam (DCB) and the end notch flexure (ENF) tests, are performed on specimens made from composite fiberglass. The inverse algorithm is then used to obtain the material constants of the cohesive law, and the results are compared with those obtained from standard tests. It is shown that in comparison with the standard test techniques, the proposed inverse algorithm requires fewer experiments to obtain the material constants while providing reliable results.

Delamination assessment of a composite laminate interleaved with electrospun TPU fibers under Mode-I and Mode-II loading

The critical strain energy release rate (G C ) and the traction-separation law of a composite laminate interleaved with electrospun thermoplastic polyurethane (TPU) fibers were investigated under mode I and mode II loading. Pristine unidirectional prepreg carbon-fiber/epoxy as well as interleaved with TPU membrane in the midplane were manufactured and tested by conducting the Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests. The Compliance-based beam method (CBBM), which is based on the equivalent crack length concept, was used as a data reduction scheme to determine G c through the test. The load-displacement curve in terms of the maximum load was compared between the specimens with and without interleaved membrane. The improvement in delamination resistance and the development of the bridging zones at the crack tip were evaluated between specimens under mode I and mode II. The traction-separation law was only determined for the specimens with interleaved membrane under mode I and mode II loading. The resulting maximum strength and G c for the interlayer-modified specimens were used in the finite element models (FEMs). Finite element analyses were performed in terms of load-displacement for mode I and mode II loading, and compared with experimental results of the laminate with TPU interleaved membrane embedded in epoxy. The delamination resistance was improved in composite laminates interleaved with the TPU fibers membrane in both modes I and II. In mode II, the improvement in delamination resistance is associated with the plastic deformations at the TPU fibers interlayer. An effective bridging zone was observed only in mode I for both the pristine and interleaved laminates. Good agreement between the FE curves and the experiments in mode I and mode II was observed. The use of electrospun TPU fibers is advantageous compared with other thermoplastic fibers since its melting point is higher than most composite curing temperature, which originates the bridging zone that was observed in the experiments, and the electrospun membranes does not require a dispersion on the resin as the particle toughening method.

Cohesive zone failure modeling of polymeric adhesives used in ceramic/metal armor

In this paper, a trilinear cohesive zone modeling approach available in the explicit nonlinear finite element software LS-DYNA is used to model the dynamic impact failure of an adhesive layer. This approach is an improvement over simpler cohesive zone models presented in the literature. The model is validated for the SikaForceTM6 7752-L60 polyurethane adhesive using force–displacement curves of double cantilever beam and end-notched flexure tests extracted from the literature. The trilinear cohesive zone model was then implemented to simulate the behavior of the adhesive bonding a ceramic alumina tile to an aluminum backing. Simulations were performed to explore the effect of the adhesive layer thickness, manufacturing defects (air bubbles simulated through deleted elements), and strain rates. It was found that: (1) a thicker adhesive layer decreased the ceramic/metal armor performance for a single hit, but resulted in a reduction of the damage area of the top ceramic tile in simulations; (2) an increase in the amount of defects resulted in greater depth of penetration and increased delays to stop the projectile, resulting in a reduction of simulated armor performance; and (3) including strain rate effects in the model resulted in predictions of a reduced depth of penetration and an increase in the damage region of the interlayer after the impact event for all of the simulated impact velocities, thereby predicting a decreased performance for multi-hit impact conditions of the armor system.

Mode II fracture characterisation of a honeycomb/carbon-epoxy sandwich panel using the asymmetric end-notched flexure test

The objective of this work is to determine the fracture energy of a honeycomb/carbon-epoxy sandwich panel under mode II loading using the Asymmetric End-Notched Flexure (AENF) test. Experimental tests and numerical analyses aiming to validate all the procedures were performed. An equivalent crack length data reduction scheme was developed in order to simplify the experimental determination of the Resistance-curves ( R-curves) using exclusively the load-displacement ( P- δ) data. This strategy makes unnecessary the crack length monitoring during its propagation, which is hard to perform in mode II fracture tests. A mixed-mode I+II trapezoidal cohesive zone model was used for validating the proposed data reduction method. It was concluded that this methodology constitutes a suitable procedure for mode II fracture characterisation of debonding failure in composite sandwich panels.

The mode II interlaminar fracture toughness and healing efficiency of repeatable self-healing composite

Hollow glass fibers containing self-healing resin (SR) and two types of microcapsules (HGFs-Ms) that can repeatedly heal the crack at the same place were manufactured. The repeatable self-healing carbon-epoxy composites (RSH-CFRPs) were made by incorporating the HGFs-Ms and microcapsules containing hardener into the mid-plane. The effects of the diameter and length of HGFs-Ms, the content of SR and the diameter of the microcapsules containing SR inside HGFs-Ms on the mode II interlaminar fracture toughness, 1st and 2nd healing efficiencies of the RSH-CFRPs were studied by performing end-notched flexure tests. The optimal combination of the above parameters was obtained which could result in 69.77% improvement in the fracture toughness compared to virgin CFRPs and could accomplish 100% healing efficiency at the same place twice. The toughening and healing mechanisms of the RSH-CFRPs were deeply analyzed, which proved the effectiveness of the self-healing method proposed in this study.

The effect of rolling process on the mechanical and electrical properties of CNTs-enhanced GFRP

In this paper, the mechanical properties of glass fiber reinforced plastic (GFRP) laminates are enhanced by improving the preparation process and toughening with the nano-fillers. The carbon nanotubes (CNTs) solution was prepared and sprayed into GFRP laminates as an interlaminar enhancement layer. To explore the effect of different rolling reduction rates (1.5%, 3%, 6%) and different rolling patterns (cold rolling and hot rolling) on CNTs-enhanced GFRP, the three-point bending test, the end-notch flexure (ENF) test, and the tensile test were designed and carried out. Results revealed that the optimal rolling reduction rate was 3%, and the mechanical properties of GFRP laminates enhanced by the hot rolling process were found to be relatively superior to the cold rolling process. Simultaneously, the CNTs interlayer could significantly improve the mechanical properties of GFRP, i.e., flexural strength, mode II interlaminar fracture toughness and tensile strength enhancing with 69%, 56% and 50%, respectively. The results of the Scanning Electron Microscope (SEM) exhibited that the hot rolling pressure could accelerate the mutual diffusion of the resin and CNTs, effectively forming a fiber/resin/CNTs strengthening interface. In addition, the rolling process could cause the partial disconnection of conductive network paths of CNTs film, resulting in an increase of the resistance change rate.

Experimental Identification of Frictional Effects on Interlaminar Toughness of Composite Laminates in 4ENF Test

BackgroundUnderstanding and characterizing delamination in composite laminates is fundamental for analysing their structural integrity, since operational loads may promote the propagation of interlaminar defects. Propagation often occurs in mode II and is driven by shear stress. However, the methods used to characterise this propagation mode are affected by frictional effects between crack surfaces.ObjectiveThis work aims to build up an experimental method to identify the effect of friction in a 4 points End Notched Flexure test, which does not require the use of analytical or numerical models of the specimen.MethodsThis goal is achieved by performing a series of loading–unloading cycles before the delamination test, which helps to calibrate an analytical expression that estimates the energy dissipated by friction, and the analytical expression is then inserted into the formulation of the Irwin-Kies equation. Experimental validation is carried out considering tests on different materials and different friction conditions between the crack surfaces, as well as validation by means of a virtual experiment being performed for comparison with an analytical model presented in literature. The novelty of this method lies in the fact that it does not require the development of any analytical or numerical model of the specimen and consequently no calibration between models and the experiment is required.ResultsTests on composite specimens show good results, the friction contribution estimated by the method is comparable with those presented in the literature. Moreover, the virtual experiment shows that there is a good match between the results obtained using this method and those obtained using an analytical model presented in the literature.ConclusionsThis method seems to provide satisfactory results for both real and virtual experiments, moreover, the procedure is relatively simple, making it a suitable method for the evaluation of frictional effects in the 4 point End Notched Flexure test.

A new method for the identification of cohesive laws under pure loading modes

In this work, a new and simple methodology is proposed to identify the cohesive law of composite materials submitted to pure mode I and II loading. This methodology combines the experimental measured crack opening displacement and corresponding strain energy release rate with numerical simulation, using finite element method including cohesive zone modelling. The proposed procedure was tested and validated numerically, considering the determination of cohesive laws with different shapes for pure mode I and II loading. This was accomplished using the double cantilever beam (mode I) and end-notched flexure (mode II) tests. It was verified that the proposed methodology points towards the unicity of the identified solution and reproduces well the cohesive laws used as input.

Use of high spatial resolution distributed optical fiber to monitor the crack propagation of an adhesively bonded joint during ENF and DCB tests

Similarly to other industrial areas, there is a strong interest for the use of bonded FRP (Fiber Reinforced Polymers) repair or reinforcement for steel structures in the case of offshore applications. However, the reliability of the adhesively bonded (FRP) shall stand as high as steel renewal, this requires additional developments, in particular, a complete understanding of the repair mechanical strength which depends on material and interfacial properties. Fracture mechanics is an interesting approach to assess the risk to undergo interlaminar fracture or steel to adhesive interfacial disbonding failure. The experimental determination of the required design values for this an approach (critical toughness) are generally obtained using common tests such as Double Cantilever Beam (DCB), End Notched Flexure (ENF) or Mixed Mode Bending (MMB) tests. These tests require a precise crack length monitoring that is currently carried out using visual observation or Digital Image Correlation (DIC) on the flank of the sample. This may induce error in crack length measurement especially if the crack doesn't remain straight during the test. The paper presents a study of crack front monitoring by a distributed optical fiber as an alternative to the standard techniques to monitor crack front and to determine the critical toughness in mode I and II through respectively, DCB (Double Cantilever Beam) and ENF (End Notched Flexure) tests. Firstly, the issues related to the use of this continuous optical fiber are raised (insertion, precision resolution, measurement noise, exploitation methodologies). Then, some experimental investigations on ENF and DCB tests are presented and analyzed using the proposed methodology to monitor crack propagation using the optical fiber strain measurement. The obtained results are compared, focusing on the proper determination of the critical toughness of the adhesive. These results show that an optical fiber bonded on the surface of the sample can be used to measure and follow the crack propagation during the test which simplifies and adds precision to the standardize critical toughness computation method.

Mode II fracture of wood: Comparison between End-Notched Flexure and Compact Shear testing

End-Notched Flexure (ENF) and Compact Shear (CS) tests were performed on Australian Radiata Pine to quantify Mode II fracture properties in wood. Capabilities and limitations of each fracture test are compared with respect to ease of testing, underlying reduction schemes to determine Mode II fracture toughness (or strain-energy release rate), stable crack growth and mixed mode behaviour during crack growth. In particular, different reduction schemes are presented with and without the need for cumbersome crack length measurements. In contrast to CS tests, it is found that ENF tests yield stable and reliable Mode II fracture with negligible mixed-mode behaviour. Considering resistance curves, crack length measurements are only needed to determine initial fracture energy values associated with crack length values up to 20 mm while compliance-based beam theory can be used to calculate steady-state fracture energies by means of an equivalent crack length that only requires global load–displacement test data.

Experiments on the mode II fracture toughness in ENF tests of CFRP curved beams

The purpose of this study is to establish end-notched flexure (ENF) tests of curved beam specimens cut from tanks or cylinders. To provide data reduction methods for experiments, a compliance-based beam method (CBBM) for curved beams in ENF tests is proposed based on the previously established Euler–Bernoulli curved beam theory formulations of ENF tests. ENF tests were conducted using curved beam specimens with three different curvature radii, and the results were in good agreement with the curved beam theory. In addition, the fracture toughness values obtained from the proposed CBBM for curved beams were validated using three-dimensional finite element (FE) analysis. The results revealed that the FE analysis with the measured fracture toughness agreed well with the experimental load–displacement curves, verifying the validity of the obtained fracture toughness values for curved beams.

Experimental and numerical study on open-hole tension/compression properties of carbon-fiber-reinforced thermoplastic laminates

The stress-strain responses and damage initiation/propagation mechanisms of T700G/LM-PAEK, an open-hole carbon-fiber-reinforced thermoplastic were investigated experimentally and numerically. To obtain the mechanical properties necessary for numerical simulations, uniaxial tensile/compressive, double cantilever beam, and end notched flexure tests were conducted. T700G/LM-PAEK was found to have comparable or higher Young’s modulus, strength, and interlaminar fracture toughness relative to thermoset CFRPs with carbon fiber of a similar grade. These superior mechanical properties are mainly attributable to the higher toughness and ductility of the thermoplastic resin. The interfacial fracture toughnesses were evaluated by finite element analysis with the cohesive zone model to determine the interlaminar fracture toughnesses for crack initiation and propagation. Based on the above experimental and numerical results, the stress strain response and damage evolution of open-hole specimens were analyzed by a quasi-3D extended finite element method (XFEM) and compared with the experimental results. The computational model with the elastoplastic constitutive law provided an accurate prediction of the stress-strain response in both open-hole tension and compression (OHT and OHC, respectively), suggesting that the elastoplastic constitutive law should be considered in XFEM to guarantee the accuracy of strength prediction for both OHT and OHC. The OHT model showed that the Weibull criterion was satisfied without any delamination at the failure strain, corresponding to the brittle failure mode due to fiber breakage. For the OHC simulation, the damage initiation of 0°-ply kinking was observed at 88% of the peak stress. These predicted damage mechanisms agreed reasonably well with the experimental observations.

A Numerical Investigation of Delamination Response of CNT/Epoxy Film Interleaved Composite

In this study, numerical modeling through the cohesive zone theory was performed to simulate the end notch flexure (ENF) test with same conditions of the experimental results of previous study that investigated the effect of a carbon nanotube (CNT)/epoxy film in carbon fiber reinforced polymer (CFRP) composite through the mode II interlaminar fracture toughness of a non-interleaved, epoxy film interleaved, CNT/epoxy film interleaved CFRP laminate specimen. The effect of the presence of CNT/epoxy film interleave on a composite laminate was modeled. The influence of the interleave cohesive parameters was studied to link the parameters to the material strength and energy release rate. Cohesive parameter identification was performed by matching the initial loading and the damage evolution phase by dividing the cohesive zones into cohesive front and remaining cohesive zones. This is because, when modeling with a single cohesive zone, the critical load point that causes delamination or the curve after load drop do not match the experimental values. Results showed that the divided cohesive zone model is in good agreement with the experimental results and that there is a clear relationship between the cohesive energy of the interface and CNT/epoxy film parameters.

Experimental and numerical analysis on mode II fracture toughness of CFRP adhesive joints using a nonlinear cohesive/friction coupled model

In this work, a nonlinear cohesive/friction coupled model was proposed by combining experiments and numerical simulations for mode II fracture analysis of hybrid carbon nanofillers toughened carbon fiber composite bonded joints. First, end-notched flexure tests were performed to investigate the toughening effects of multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) hybrids modified adhesive on mode II fracture toughness of adhesive joints. Testing results show that 0.75 wt% MWCNTs/GNPs hybrid has an improvement of 45% compared to pure adhesive. Secondly, a phenomenological progressive rising friction model was introduced into an exponential cohesive zone model for delamination analysis of adhesive layer under shear and normal compression loadings. Finally, the proposed model was implemented in ABAQUS/Explicit via user subroutine VUMAT and model parameters were determined by fitting experimental results of end-notched flexure tests for adhesive joints.

Interlaminar Mechanical Properties and Toughening Mechanism of Highly Thermally Stable Composite Modified by Polyacrylonitrile Nanofiber Films.

This work concentrated on the interlaminar mechanical properties and toughening mechanism of carbon fiber-reinforced bismaleimide resin (CF/BMI) composites modified by polyacrylonitrile (PAN) nanofiber films. The PAN nanofiber films were prepared by electrospinning. End-notched flexure (ENF) and short-beam strength tests were conducted to assess the mode II fracture toughness (GIIc) and interlaminar shear strength (ILSS). The results showed that the GIIc and ILSS of PAN-modified specimens are 1900.4 J/m2 and 93.1 MPa, which was 21.4% and 5.4% higher than that of the virgin specimens (1565.5 J/m2 and 88.3 MPa), respectively. The scanning electron microscopy (SEM) images of the fracture surface revealed that the PAN nanofiber films toughen the composite on two scales. On the mesoscopic scale, the composite laminates modified by PAN formed a resin-rich layer with high strength and toughness, which made the crack propagate across the layers. At the microscopic scale, the crack propagation between two-dimensional nanofiber films led to constant pull-out and breakage of the nanofibers. As a result, the interlaminar fracture toughness of the composite laminates improved.

High-rate mode II fracture toughness testing of polymer matrix composites using the Transverse Crack Tension (TCT) test

This paper describes the novel high-rate transverse crack tension (TCT) test to determine mode II fracture toughness for high strain rates and compares this to quasi-static fracture toughness obtained with the standardized End Notch Flexure (ENF) test and the modified Transverse Crack Tension (mTCT) test suggested in the literature. The novel, miniaturized mTCT specimen is used to determine the fracture toughness under quasi-static and high-rate loading conditions with an electro-mechanical test machine and a Split Hopkinson Tension bar, respectively. For the high-rate tests, almost no interfering oscillations were observed in the load signal. A novel data reduction scheme allows achieving repeatable results with a very small standard deviation. The fracture toughness for the tested composite at a loading rate of about 5 m/s was found to be at least 50% higher than the one obtained under quasi-static loading.

Effect of surface finish metallization layer on shearing fracture toughness of sintered silver bonded joints

In this paper, the effect of different surface finish metallization layers (Cu, Ni, Au and Ag) on the shearing fracture toughness of sintered Ag bonded joints under various sintering temperatures is investigated based on end-notch flexure (ENF) test. The results show that the shearing fracture toughness for different kinds of specimens all increase with the sintering temperature augment. The sintered Ag/Ag joint shows the highest shearing fracture toughness while the sintered Ag/Ni joint shows the lowest. The average shearing fracture toughness of the Ag/Cu joint is very close to the Ag/Au joint when sintered from 240 °C to 280 °C, whereas the former became much higher when sintered under 300 °C. By statistically analysis of crack propagation paths, interface microstructure characteristics, and interface reaction, it is found that the main factors affecting the discrepancy of the shearing fracture toughness for different metallized layers are the interface connection ratio and contact angle. A higher interface connection ratio and lower contact angle can increase the actual crack resistance and lower down stress concentration at the bonding interface, which leads to the discrepancy of shearing fracture toughness correspondingly. The different inter-diffusion rates for Ag and various metallization layers are also clarified. This study could deepen the understanding of the metallization layer effect on the shearing fracture behaviors of sintered silver bonded joints.