End-notched Flexure Specimens Research Articles

Design analysis of end notch flexure sandwich specimen with honeycomb core

Laminate beam theory based design analysis of the sandwich End Notch Flexure (ENF) test specimen is presented. The analysis is specifically considering specimens with honeycomb core, prone to in-plane compression failure of the core, other undesirable failure modes such as face indentation and core shear failure are analyzed. In addition, crack stability and energy dissipation due to frictional sliding between the crack surfaces are analyzed. Parametric analysis of ENF specimens with a range of face and core thickness, is presented to guide the design. In-plane compression failure of the core, but shorten the range of crack lengths were stable growth occurs. A thicker face will also increase the critical load for crack propagation and by so elevate the risk for indentation and core shear failures. Increased core density is beneficial for preventing indentation and core shear failures, but may increase the risk for in-plane compression failure of the core. The thickness of the core does not strongly influence the critical load for crack propagation. A thicker core increases frictional energy loss, but reduces the risk for core shear failure. A thicker core, however, makes the ENF specimen more prone to in-plane compression failure. ENF sandwich specimens with foam and honeycomb core were tested. The test results support the design analysis.

Determination of the shear traction-separation law of adhesive layers using the end-notched flexure specimen

Cohesive zone modelling is increasingly applied to predict the strength of structural adhesive joints. A method is here proposed to determine the bondline shear traction-separation law needed for cohesive zone models. The method combines an effective crack length approach derived from a beam model and an existing J-integral analysis of the end-notched flexure specimen. The resulting work done by the tractions versus separation curves can be easily processed with direct or mixed direct-inverse approaches. Application to some published experimental results demonstrates the potential of the method while highlighting the importance of adherend material and specimen geometry selection.

Mode II fracture energy characterization of brittle adhesives using compliance calibration method

AbstractThe end‐notched flexure (ENF) test is widely used for measuring the Mode II critical strain energy release rate of adhesively bonded joints (ABJs). Unstable crack growth in ENF joints with brittle adhesives is a common phenomenon. Classic data reduction methods like the direct beam theory (DBT) and the compliance‐based beam method (CBBM) usually result in unacceptable scatter when crack grows unstable. In this study, the application of a compliance calibration method (CCM) for ENF adhesive joints with a brittle adhesive is experimentally investigated. For this purpose, ENF specimens were manufactured and tested. Different data reduction methods were considered for treating the results. Afterwards, the obtained fracture energies were used as an input parameter in a finite element (FE) analysis with a cohesive zone model to evaluate the validity of the experimental data. It is shown that the fracture loads obtained by the CCM have the best agreement with the experimental ones comparing with the other data reduction approaches. To study the effect of geometry on the CCM results, ENF specimens with different adhesive thicknesses, substrate thicknesses and span lengths were also considered in this study, and some general conclusions are made about the geometrical parameters effect on the Mode II fracture energy.

On the extent of fracture toughness transfer from 1D/2D nanomodified epoxy matrices to glass fibre composites

In this study, the effects of adding nanofillers to an epoxy resin (EP) used as a matrix in glass fibre-reinforced plastic (GFRP) composites have been investigated. Both 1D and 2D nanofillers were used, specifically (1) carbon nanotubes (CNTs), (2) few-layer graphene nanoplatelets (GNPs), as well as hybrid combinations of (3) CNTs and boron nitride nanosheets, and (4) GNPs and boron nitride nanotubes (BNNTs). Tensile tests have shown improvements in the transverse stiffness normal to the fibre direction of up to about 25% for the GFRPs using the ‘EP + CNT’ and the ‘EP + BNNT + GNP’ matrices, compared to the composites with the unmodified epoxy (‘EP’). Mode I and mode II fracture toughness tests were conducted using double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. In the quasi-static mode I tests, the values of the initiation interlaminar fracture toughness, G_{text{IC}}^{text{C}} , of the GFRP composites showed that the transfer of matrix toughness to the corresponding GFRP composite is greatest for the GFRP composite with the GNPs in the matrix. Here, a coefficient of toughness transfer (CTT), defined as the ratio of mode I initiation interlaminar toughness for the composite to the bulk polymer matrix toughness, of 0.68 was recorded. The highest absolute values of the mode I interlaminar fracture toughness at crack initiation were achieved for the GFRP composites with the epoxy matrix modified with the hybrid combinations of nanofillers. The highest value of the CTT during steady-state crack propagation was ~ 2 for all the different types of GFRPs. Fractographic analysis of the composite surfaces from the DCB and ENF specimens showed that failure was by a combination of cohesive (through the matrix) and interfacial (along the fibre/matrix interface) modes, depending on the type of nanofillers used.

Examination of Tensile Strength and End Notch Flexure Properties of Unidirectional Basalt Fabric Reinforced Polymer Composites With and Without Addition of Biochar Filler

The effect of biochar filler (carbon particle) on the mechanical properties over the inter-laminar surfaces of basalt unidirectional fabric/polyester resin is going to be scrutinized using Mode II delamination method. The ENF (end notch flexure) specimen is fabricated using hand layup method. Here basalt fabric is synthesized using unsaturated polymer resin with the help of methyl ethyl ketone peroxide and cobalt naphthenate as a catalyst. Both specimens are manufactured with and without biochar conditions. The laminate length and width will be customized as per the required dimensions of the ASTM standard. The ENF specimens are tested using the universal testing machine and the strain energy released rate and displacement of each specimen are been measured. The biochar based laminated composite has a resistant towards the fracture developed on laminated composite during testing due to the addition of carbon particles.

An end notched flexure specimen model for mode-II fracture of high-temperature superconductor tapes with thermal effect

End notched flexure (ENF) experiment using three-point bending beam specimen is a very convenient test configuration to evaluate the mode-II fracture toughness of composites. High-temperature superconductor (HTS) tapes are multi-layered composite that have been widely used in magnets and cables. HTS tapes are manufactured at room temperature ([Formula: see text][Formula: see text]300 K) but are used in the liquid-nitrogen temperature (77 K) environment. The large temperature change leads to large shear stress which may cause delamination of HTS tapes. In this paper, a closed-form expression of energy release rate (and therefore the stress intensity factor) for mode-II fracture of HTS tapes with thermal effect is developed based on the end notched flexure specimen. Results show that HTS tapes in working condition are prone to delaminating. Increasing the thickness of Hastelloy layer can reduce the energy release rate of mode-II cracking thus increase the interface strength. In the design of HTS tapes, the silver and copper layers are used to enhance the stretch of the HTS tapes. While silver and copper layers will increase the energy release rate for mode-II crack. This fact indicates that thicker silver and copper layers are not always good from the fracture mechanics point of view. The results of thermomechanical model and fracture criterion model of this research are useful for determining mode-II fracture toughness of superconducting composites for high-temperature applications.

Mode II interlaminar fracture toughness of woven E-glass/epoxy composites in the presence of mat interleaves

Composite laminates are widely used as primary load bearing structures in various industries. Using a thin layer of chopped strand mat between sub-laminates of a thick structural composite laminate would prevent crack development across the whole laminate thickness and reduce the risk of catastrophic failure in the structure. In this research, end-notched flexure (ENF) specimens were used to examine the impact of the mat layer between woven layers on the mode II interlaminar fracture toughness of glass/epoxy composites. ENF specimens are fabricated using hand lay-up method assisted by vacuum bagging technique. Experimental results revealed that the existence of a mat interlayer resulted in a reduction of interlaminar fracture toughness of the delamination initiation and propagation by 59% and 35%, respectively, compared to one without mat interlayer. Moreover, results showed that in the ENF specimens without interleaving of the mat, the dominant mechanism of increased fracture toughness is not fiber bridging and delamination propagates in fiber-matrix interface in these specimens.

Investigation of mode II interlaminar fracture toughness of lignocellulosic laminated specimens – an experimental and numerical approach

Mode II interlaminar fracture toughness of lignocellulosic laminate composite materials made of beech veneer with urea formaldehyde resin and rye flour was investigated using the end notched flexure specimens (ENF). In order to determine the critical value of the strain energy release rate GIIc associated with the interlaminar crack onset, the experimental data analysis was conducted by means of a mixed mode bending device in conjunction with a compliance calibration approach for data reduction. The interlaminar crack propagation under pure mode-II loading was also simulated by finite element analysis using general contact in Abaqus/Explicit, with Virtual Crack Closure Technique (VCCT) as fracture criterion. Good agreements between the load-displacement curves obtained through the use of finite element analyses and the experimental testing results were obtained.

Mode I and II Interlaminar Fracture in Laminated Composites: A Size Effect Study

This work investigates the mode I and II interlaminar fracturing behavior of laminated composites and the related size effects. Fracture tests on geometrically scaled double cantilever beam (DCB) and end notch flexure (ENF) specimens were conducted. The results show a significant difference between the mode I and mode II fracturing behaviors. The strength of the DCB specimens scales according to the linear elastic fracture mechanics (LEFM), whereas ENF specimens show a different behavior. For ENF tests, small specimens exhibit a pronounced pseudoductility. In contrast, larger specimens behave in a more brittle way, with the size effect on nominal strength closer to that predicted by LEFM. This transition from quasi-ductile to brittle behavior is associated with the size of the fracture process zone (FPZ), which is not negligible compared with the specimen size. For the size range investigated in this study, the nonlinear effects of the FPZ can lead to an underestimation of the fracture energy by as much as 55%. Both the mode I and II test data can be captured very accurately by the Bažant’s type II size effect law (SEL).

R-curve behaviour of the mixed-mode I/II delamination in carbon/epoxy laminates with unidirectional and multidirectional interfaces

To successfully predict the delamination behaviour of the laminated composite structures with fibre bridging, the R-curve has to be studied. This work experimentally investigates the complete R-curve behaviour of the unidirectional and multidirectional carbon/epoxy composite laminates. The delamination tests use the double cantilever beam (DCB), mixed-mode bending (MMB) and end-notched flexure (ENF) specimens for mode I, mixed-mode I/II and mode II loading, respectively. The test results show that the interfacial ply and mode mixity (φ-ratio) have significant influences on the initial fracture toughness, steady-state fracture toughness and fibre bridging length. The ratio between the steady-state fracture toughness and its initial value is approximately same for both interfaces, which indicates a similar enhanced effect of the fibre bridging on the fracture toughness. The Benzeggagh-Kenane (B-K) criterion is capable of representing the relation between the fracture toughness and the φ-ratio. Based on the DCB and MMB test results, the predicted values of the mode II fracture toughness via the B-K criterion are very close to the experimental ones, which illustrates the possibility of determining the mode II fracture toughness without executing the mode II delamination tests. Furthermore, a semi-empirical expression is proposed, which can well predict the mixed-mode I/II delamination behaviour.

Determination of Mode II Interlaminar Fracture Toughness of CFRP Composites Using Numerical Simulations

This paper deals with a numerical simulation of the interlaminar fracture toughness of woven carbon fibre reinforced polymer. Composite materials are increasingly used for their unique properties in many branches of engineering. They are also used for flexible components such as springs, couplings, etc. The strength of these parts must be determined not only in terms of their intralaminar properties but also in terms of their interlaminar properties. This paper provides a methodology for determining the main parameters for Mode II interlaminar fracture toughness using numerical simulation. End Notch Flexure (ENF) specimens were created for fitting fracture toughness parameters of the laminate according to ASTM standards. Three point bending ENF tests were carried out on a Zwick/Roell Z050 machine. The numerical simulation was created in Siemens Simcenter 12.0 using NX Nastran nonlinear solver. The results from the numerical simulation correspond to those from the experimental test with an accuracy of 4%.

Effect of notch length and pre-crack size on mode II fracture energy of brittle adhesives

In this study, the effects of notch length and pre-crack size on mode II fracture energy of brittle adhesives are investigated. These effects are not considered by the direct beam theory (DBT) and the compliance-based beam method (CBBM) for the end-notched flexure (ENF) specimens. The sum of the lengths of the notch and the pre-crack are known as the pre-crack size in common data reduction schemes. In the current work, experiments were performed on ENF adhesive joints using digital image correlation (DIC) technique. Then, the fracture energy was obtained experimentally based on the J-integral method, the classic beam theory, and the compliance-based beam method. Numerical analyses were carried out to measure the stress distribution ahead of the pre-crack tip. Based on the obtained results, it is shown that there is a specific pre-crack length and notch size in which the pure mode II condition in an ENF test is obtained. Other combinations of the notch to pre-crake size ratio result in mixed mode conditions where the obtained fracture energy significantly changes.

Experimental and numerical analysis on the cohesive behavior of an automotive adhesive improved by MWCNT subjected to mode I and II loadings

This paper presents the effect of multi-walled carbon nanotubes (MWCNT) incorporation in the fracture and damage behavior of an epoxy adhesive. In this regard, the fracture energies and cohesive behavior of the neat and reinforced adhesives are obtained by testing Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) specimens in mode I and mode II, respectively. The cohesive behavior is studied by extracting traction-separation law (TSL) corresponding to each and every adhesive using direct method. In order to validate the extracted TSL, an experimental test and numerical simulation on Single Lap Joint (SLJ) specimens are conducted. The good agreement between experimental and numerical load-displacement responses proves the accuracy and precision of TSLs. The results revealed that in pure modes, the fracture toughness of the epoxy adhesive is improved by adding MWCNTs up to a critical %wt. Afterwards, incorporating excess additive would weaken the adhesive. Similarly, load-displacement responses of SLJs indicates that the highest improvement of the joint energy would be achieved using the same critical %wt and result in an enhancement of over 80%.

Analysis of automotive mixed‐adhesive joints weakened by moist conditions: Experimental characterization and numerical simulation using cohesive zone model

AbstractThis paper is mostly concerned with an experimental and numerical study to clarify the behaviour and failure in the mono‐adhesive joints and mixed‐adhesive joints under different environmental conditions (dry [E0], 75.3% relative humidity [E1], 84.2% relative humidity [E2], and submerged in tap water [E3] at 25°C) and different strain rates (1 and 100 mm/min). Experimental investigations are compared with the numerical analysis, which is carried out by bilinear cohesive zone model (CZM). Through this work, degradations of cohesive parameters are calculated by using open‐faced double cantilever beam (DCB) and end notch flexure (ENF) specimens. The experimental data show that the cohesive parameters of Araldite 2015 have negative correlation to moisture content. Although Araldite AV138 parameters experience a decrease in mode I, in mode II, its cohesive parameter increases. First, it is found that the decrease in experimental failure load of mixed‐adhesive joints with regard to the dry condition is recorded as 32.6%, 54%, and 59.1% for E1, E2, and E3 conditions, respectively. Following this further, single lap joint (SLJ) that has non‐uniform moisture content distribution is modelled by engaging specific CZM parameters related to the specific moisture content. The results show a good agreement between experimental and numerical data.

Simultaneous extraction of tensile and shear interactions at interfaces

A simple beam analysis is developed that accounts for the tensile and shear interactions between the surfaces of two beams that have been brought into contact. The generally coupled differential equations for the relative normal and tangential displacements between the interacting surfaces and the normal and shear components of the traction between the two surfaces are decoupled when a balance condition involving the bending stiffness and height of each beam is satisfied. This condition also allows the normal and shear components of the J-integral and the crack tip displacement to be obtained from measurements of the applied loads along with load point displacement and rotation of each beam. As a result, the derivative of each component of the J-integral with respect to its associated crack tip displacement simultaneously provides both the normal and shear traction–separation relations at any value of the mode-mix without invoking any assumption on the particular form. As an application of this method, the normal and shear traction–separation relations for a silicon/epoxy interface were determined over nominal mode-mixes ranging from −53° to 87.5° using non-symmetric end-loaded split (ELS) and end-notched flexure (ENF) specimens. It is found that many of the assumptions made previously in developing traction–separation relations based on potential or damage-based approaches do not apply to this particular interface, which should motivate future theoretical developments.

A finite element study of fatigue crack propagation in single lap bonded joint with process-induced disbond

This paper presents a method for predicting fatigue crack propagation in adhesive bonded composite joints with an initial full-width disbond using finite element analysis and numerical integration of the material's fatigue crack growth rate law. Fatigue tests were conducted on single lap joints. Crack lengths were monitored from four runout corners. In-situ crack growth measurements were performed by ink injection to identify the crack front profile during fatigue loading. The crack growth was modelled using a fracture mechanics criterion considering two different crack propagation patterns. The material's fatigue crack growth rate law was determined experimentally using the standard double cantilever beam and end notch flexure specimens. Using the total strain energy release rate and the two crack scenarios, the numerical model predicted the lower and upper bounds of the measured fatigue crack growth rates of the lap joint.

Fatigue crack growth characterization in adhesive CFRP joints

Adhesive joints find an increasing use in lightweight structures, which is proportional to the evolution of carbon fiber-reinforced plastics (CFRPs). Understanding of fatigue crack growth behavior in adhesive CFRP joints is essential for the efficient maintenance and repair of existing joints and the design of new joints. Here, the fatigue crack growth behavior of adhesive CFRP joints under Mode-I, Mode-II and Mixed-Mode I + II loading conditions is characterized experimentally by means of Mode-I fatigue fracture toughness tests, Mode-II fatigue fracture toughness tests and the Mixed-Mode fatigue lap shear test. For the three different tests, the Double Cantilever Beam (DCB), the End-Notch Flexure (ENF) and the Cracked Lap Shear (CLS) specimens are used, respectively. Crack growth versus number of cycles is reported and modified Paris-laws are derived. The DCB specimens failed in cohesive failure mode while the ENF and CLS specimens in adhesive. The crack growth in the DCB specimens was more stable and showed a smaller scatter among the different specimens than the ENF specimens. Crack propagation with number of cycles in CLS specimen was almost linear. The results reported herein suggest a full experimental characterization of fatigue crack growth behavior of the considered aerospace CFRP/adhesive material system and can be proved very useful in the development and validation of fatigue crack growth simulation models.

Effect of seawater ageing on interlaminar fracture toughness of carbon fiber/epoxy composites containing carbon nanofillers

This work reports the influence of seawater ageing on the mode I and mode II interlaminar fracture toughness ([Formula: see text] and [Formula: see text]) of prepreg-based unidirectional carbon fiber/epoxy laminates containing carbon nanofillers. Double cantilever beam and end notched flexure specimens were fabricated from composite laminates containing multiwalled carbon nanotubes and/or reduced graphene oxide at their middle plane interface. Experimental results showed that the addition of carbon nanofillers moderately increased the [Formula: see text] and [Formula: see text] propagation of composite laminates before and after their immersion in seawater with respect to the reference laminate under dry condition. For double cantilever beam and end notched flexure specimens aged in seawater, it was observed that [Formula: see text] and [Formula: see text] increased by 57% and 13% for specimens with multiwalled carbon nanotube/reduced graphene oxide hybrid combination, 39% and 4% for specimens with multiwalled carbon nanotubes and 53% and 8% for specimens with reduced graphene oxide respectively, as a consequence of the plasticization effect of seawater immersion on the matrix. Fracture surface examination by scanning electron microscopy revealed interlaminar failure associated to mode I and mode II delamination and toughening mechanisms produced by the multiwalled carbon nanotubes and reduced graphene oxide at delaminated regions of composite laminates.

Real-time damage monitoring in trailing edge bondlines of wind turbine blades with triboluminescent sensors

Adhesively bonded joints in wind turbine blades, particularly trailing edge joints, are prone to damage due to high-intensity stress concentrations, missing adhesive, or deficient bonds at the bonding surfaces. If not detected early, cracks or disbonds in the trailing edge might propagate rapidly resulting in catastrophic failure of the whole blade. No dedicated in situ sensor system is, as of now, available for real-time damage monitoring in the critical adhesively bonded joints. This article discusses the use of in situ triboluminescent optical fiber sensors for damage monitoring in adhesively bonded composite joints. Double cantilever beam and three-point bend end-notched flexure samples were fabricated from a thick glass fiber/vinyl ester composite and glued with a two-part methacrylate-based structural adhesive. The in situ triboluminescent optical fiber sensors were embedded into the adhesive layer. Double cantilever beam and three-point bend end-notched flexure tests were performed to determine the mode I and mode II fracture toughness of the adhesive joints, respectively. The results show that the triboluminescent emissions occur at the onset of damage when performing double cantilever beam and three-point bend end-notched flexure fracture tests on adhesively bonded composite panels of different adhesive thicknesses. The triboluminescent response increases with the fracture toughness of adhesive joints. The double cantilever beam and three-point bend end-notched flexure specimens undergo cohesive failure in the adherend thickness. Optical microscopy was utilized to obtain better insights of fracture lines. Results indicate that in situ triboluminescent optical fiber sensors can monitor damage in adhesive in real-time without generating false alarms.

Validation of a direct method to predict the strength of adhesively bonded joints

Cohesive zone models (CZM) are a powerful tool for the design of bonded structures, but they require careful estimation of the cohesive laws for reliable results. This work experimentally evaluates by the J-integral/direct method the tensile and shear CZM laws of three adhesives with distinct ductility. Additionally, by the direct method, the precise shape of the cohesive law in tension and shear of the adhesives is defined. The double-cantilever beam (DCB) and end-notched flexure (ENF) specimens were considered to obtain the tensile and shear CZM laws of the adhesives, respectively. After obtaining the tensile and shear CZM laws, triangular, exponential and trapezoidal CZM laws were built to reproduce their behaviour. Validation of these CZM laws was undertaken with a mixed-mode geometry (double-lap joint) considering the same three adhesives and varying overlap lengths (LO). The strength prediction by this technique revealed accurate predictions for a given CZM law shape, depending on the adhesive ductility, although all CZM law shapes were moderately accurate.