End-notched Flexure Specimens Research Articles

Direct determination of a new mode-dependent cohesive zone model to simulate metal-to-metal adhesive joints

ABSTRACTIn this paper, a new mode-dependent cohesive zone model for the simulation of metal to metal adhesive joints is directly determined. Three consecutive steps have been taken into account for this end. First, double cantilever beam (DCB) and end-notched flexure (ENF) specimens are utilized for the direct experimental extraction of the traction-separation laws (TSLs) for adhesive bonded joints subjected to pure mode I and mode II, respectively. Next, the results are implemented to obtain the relative cohesive zone parameters for defining the simplified Park-Paulino-Roesler cohesive zone model (S-PPR CZM). Finally, mixed-mode characteristics parameters are derived for an arbitrary mode-mixity ratio based on pure mode TSLs. The model is further implemented in ABAQUS® commercial software to be verified against the experimental results of pure mode loadings which leads to the direct extraction of TSLs. The experiments conducted on the strength of single lap joint (SLJ) and scarf joint (SJ) specimens, commonly tested for mixed-mode loading, confirm the accuracy of the developed mixed-mode S-PPR model for different mode-mixity conditions.

Analysis of the fracture process zone and effective crack length in the adhesively bonded end-notched flexure specimen

ABSTRACTThe mode II fracture of adhesive joints is well-known to involve large fracture process zones. Their effect in fracture energy measurements can be taken into account by the effective crack length approach. Moreover, fracture process zones can be simulated by cohesive zone models, which are increasingly used for structural analysis of adhesive joints. This paper aimed at evaluating the influence of the traction-separation law on the fracture process zone and on the effective crack length in end-notched flexure tests. Novel analytical cohesive zone models were developed for the bilinear and trapezoidal traction-separation laws. The latter were shown to affect significantly the energy dissipation rate versus effective crack length curve prior to crack initiation. Therefore, this effect seems to provide a simple approach for evaluating approximate traction-separation laws. The models here developed are easy to apply and provide simple approximate expressions useful for specimen selection.

On the use of thermographic technique to assess the fatigue performance of bonded joints

The thermographic technique is based on the heat released during dynamic loading cycles, correlating the temperature variation on specimen's surface with the applied stress amplitude. An abrupt temperature variation under a specific stress level reveals the presence of an irreversible phenomenon inside the material, which could be associated with fatigue damage process. In this work, the fatigue performance of bonded joints is assessed by carrying out fatigue tests in a servo-hydraulic machine monitored continuously by a thermographic acquisition system, with the utilization of end notched flexure specimens composed by two steel plates of ASTM A36, bonded together with epoxy adhesive. The results confirm that the thermographic technique is quite promising as an alternative approach to estimate the bonded joint fatigue strength.

Evaluation of mode II fatigue disbonding using Central Cut Plies specimen and distributed strain sensing technology

ABSTRACTThe lack of a widely-accepted test standard for characterizing the mode II fatigue disbond growth behavior of adhesively bonded interfaces is a challenge to the research community in terms of producing consistent and repeatable results. Typically, researchers apply the End Notch Flexure specimen, which is already used for static delamination studies. However, the needs for static and fatigue disbond growth characterization are not the same, resulting in some undesirable effects in such specimen. This study looks at a particular mode II test configuration known as the Central Cut Plies (CCP) specimen. A critical evaluation of the suitability of this specimen, including the influence of geometry, disbond measurement approaches and the stability of the disbond growth is carried out through a combination of numerical and experimental investigations. A distributed strain sensing system based on Rayleigh Backscattering provided a surface strain profile from which disbond growth rate data was obtained. A finite element model was used to verify the experimental results and determine the disbond length from the strain profiles. Results of this evaluation have shown that the CCP specimen is a promising specimen configuration for characterizing fatigue disbond growth; however, it also presents several challenges that require consideration in its application.

Determining effective interface fracture properties of 3D fiber reinforced foam core sandwich structures

Foam core sandwich composites are finding a wider use in aerospace, automotive, and construction applications. These structures present unique challenges in terms of material failure and interaction and are sensitive to damage and imperfections introduced during manufacturing. An emerging class of 3D fiber reinforced foam core aims to replace monolithic foams used in sandwich structure cores particularly in demanding high-performance aerospace applications. This research is focused on investigating the development of testing methods capable of measuring the effective interface fracture properties between the facesheet and the core in 3D fiber reinforced foam cores. Double cantilever beam and end-notched flexure specimens are developed to evaluate the mode I and mode II fracture properties of a 3D fiber reinforced foam core. The design, development, and initial failure of a mode I interface fracture test for 3D fiber reinforced foam cores are presented. The digital image correlation results on the failed tests allowed for a different approach to be utilized in designing a new bonded double cantilever beam specimen for testing the mode I fracture of a 3D fiber reinforced foam core sandwich structure that resulted in a successful interface fracture test. The bonded DCB specimens exhibited relatively smooth crack propagation and produced GIc values similar to honeycomb sandwich structures and significantly higher than comparable foam structures.

Mode I and Mode II Delamination of Flax/Epoxy Composite Laminate

In recent decades, natural fibres are getting their attention as reinforcement in composite materials. This is because natural fibres are environmental friendly. However, delamination is commonly recognised as one of the earliest failures in composite laminates. The objective of the present work is to investigate mode I and mode II delamination behaviour of flax fabrics reinforced epoxy composite. The delamination characterisation was carried out using double cantilever beam (DCB) and three point end notched flexure (ENF) tests. The fracture toughness were calculated using experimental calibration method (ECM). Results showed that the average fracture toughness was 485 N/m and 962 N/m, respectively. Finally, through scanning electron micrographs, it was observed that the ply/ply debonding and fibre/matrix debonding were the major fracture mechanisms in DCB specimen. As for ENF specimen, shear fracture dominated the energy dissipation process.

Fatigue of adhesively bonded epoxy-AA6061T651 joints

Purpose Adhesively bonded joints are gaining importance in the structural joining processes competing against welding and bolting processes. However, long-term behaviour of adhesively bonded joints is still an open question. Due to the increasing interest in adhesively bonded joints, mainly in the transports industry, there is a need to deep the knowledge about the fatigue behaviour of adhesive joints with metallic substrates allowing the development of reliable joints to resist cyclic loadings. The paper aims to discuss these issues. Design/methodology/approach An experimental research aiming at characterizing the fatigue behaviour of adhesively bonded aluminium substrates is presented in this paper, covering both fatigue crack propagation and global S-N behaviours. Double cantilever beam (DCB), end notch flexure (ENF) and double lap joints (DLJ) specimens built using the AA6061T651 substrate and epoxy adhesive were used to evaluate the pure modes I and II fatigue crack propagation rates and the S-N fatigue behaviours. Findings DCB and ENF specimens allowed the formulation of pure modes I and II fatigue crack propagation laws including the propagation thresholds. DLJs showed higher static shear strength than recommended by the manufacturer for aluminium substrates, but fatigue resistance of the DLJs was lower than suggested by the manufacturer. The fatigue damage process in the DLJs was dominated by a fatigue crack initiation process. Originality/value A consistent fatigue research on adhesively bonded aluminium substrates is presented covering in the same study aspects of fatigue crack propagation and fatigue crack initiation. Data reduction schemes involving both numerical and analytical procedures were followed. Proposed work constitutes a rigorous basis for future fatigue prediction models developments.

Influence of the Hybrid Combination of Multiwalled Carbon Nanotubes and Graphene Oxide on Interlaminar Mechanical Properties of Carbon Fiber/Epoxy Laminates

An effective strategy to improve the mode I and mode II interlaminar fracture toughness (G IC and G IIC ) of unidirectional carbon fiber/epoxy (CF/E) laminates using a hybrid combination of multiwalled carbon nanotubes (MWCNTs) and graphene oxide (GO) is reported. Double cantilever beam (DCB) and end notched flexure (ENF) tests were conducted to evaluate the G IC and G IIC of the CF/E laminates fabricated with sprayed MWCNTs, GO and MWCNTs/GO hybrid. Scanning electron microscopy was employed to observe the fracture surfaces of tested DCB and ENF specimens. Experimental results showed the positive effect on the G IC and G IIC by 17% and 14% improvements on CF/E laminates with 0.25 wt.% MWCNTs/GO hybrid content compared to the neat CF/E. Also, the interlaminar shear strength value was increased for MWCNTs/GO-CF/E laminates. A synergetic effect between MWCNTs and GO resulted in improved interlaminar mechanical properties of CF/E laminates made by prepregs.

Damage evaluation of laminated composite material using a new acoustic emission Lamb-based and finite element techniques

In this paper, a very promising procedure is proposed to evaluate delamination using Acoustic Emission (AE) technique in composite laminates. First, a new procedure was developed to decompose the fundamental Lamb wave modes in small size specimens. The damage mechanisms in End Notched Flexure (ENF) in woven and unidirectional specimens were then discriminated using Fuzzy Clustering Method (FCM). Afterwards, the crack-arrest phenomenon was examined in each specimen. After that, experimental and Cohesive Zone Modeling (CZM) techniques were conducted to characterize the delamination using ENF specimens. The results showed how, it is possible to successfully decrease the effect of propagating media such as attenuation of AE signals using the new proposed methodology. As a final point, the results of this study could lead to efficiently distinguishing different damages in laminated composite using AE Lamb-based technique.

Distributed optical sensing in composite laminates

An optical fiber–based full-field strain measurement technique was used to investigate delamination growth in laminated composites. An experimental setup to load the test samples under idealized modes of delamination was used to investigate the ability to capture the shape and location of the delamination front. It is envisioned that the demonstrated approach has significant field applications in controlled laboratory settings where delaminations have to be located accurately. Furthermore, the ability of this measurement system to provide full-field strain measurements at any given pre-implanted location through the thickness overcomes the surface strain measurements obtained by digital image correlation. In order to demonstrate the technique, distributed fiber optic sensing is used to monitor the propagation of delaminations under pure mode I and II loading. Optical fibers were embedded one ply from the crack plane of both double cantilever beam and end notch flexure specimens. To establish a repeatable fabrication methodology, manufacturing techniques for embedding the optical fibers during the laminate layup process were established. Specimens with and without embedded fibers were tested to verify the fibers did not affect measured fracture toughness values. Crack lengths measured with the optical fibers compared well with true crack lengths, and measured strain distributions compared well with results from finite element analysis.

Application of adhesively bonded single lap joints for fracture assessment of adhesive materials

ABSTRACTIn this study, a method has been proposed to obtain the failure envelope of brittle adhesives using the experimental failure loads of precracked single lap joints (SLJs). The proposed technique is based on the principles of linear elastic fracture mechanics (LEFM), on J-integral relations, and on results of a numerical analysis. Compared to the previous approaches, the introduced experimental method has some advantages such as low manufacturing costs and simpler test procedure. The proposed method can also provide a wide range of mode mix ratios without the need of an additional apparatus. The fracture envelope obtained from the proposed method was then verified by performing some fracture tests including double cantilever beam (DCB), end-notched flexure (ENF), and single leg bending (SLB) specimens. Good correlation was seen between the fracture envelopes of the proposed method and the ones obtained from the fracture mechanics experiments.

Elastic-plastic analysis of the adhesively bonded end-notched flexure specimen

The end-notched flexure (ENF) specimen has often been employed to characterise the mode II fracture of adhesive joints. This paper reports a beam model for ENF specimens with metal adherends and typical bondline thickness values. The initial formulation adopting adhesive linear elasticity predicted accurately the shear stress distribution, the compliance and the strain-energy release rate. The adhesive elastic-perfectly plastic behaviour subsequently introduced generated a plastic zone whose size could be predicted by a closed-form expression. The model provides a basis for selecting specimen geometries that enable accurate fracture energy measurements from a corrected beam theory with effective crack length scheme.

Mixed mode fracture characterization of GFRP-concrete bonded interface using four-point asymmetric end-notched flexure test

A combined analytical and experimental study using an asymmetric end-notched flexure specimen under four-point bend loading is conducted to characterize the fracture of glass fiber reinforced polymer (GFRP)-concrete bonded interface. Both the classic composite (rigid joint) and interface deformable (flexible joint) bi-layer beam models are used to calculate the compliance and energy release rate (ERR) of the proposed four-point asymmetric end-notched flexure (4-AENF) specimen. The validity and accuracy of the models are obtained by comparison with the numerical finite element results. The results show that the flexible joint model predicts more accurately the compliance and ERR compared with those of rigid joint model in 4-AENF specimen due to the attribute of crack tip deformation. Moreover, the calculated ERR by the flexible joint model can be reduced to that of the rigid joint one when the specimen is properly sized. Then, the designed 4-AENF specimens are utilized to characterize the fracture toughness of GFRP-concrete bonded interface. To overcome the obstacle of low tensile strength and cracking behavior of concrete and prevent the premature fracture of concrete substrate before debonding of bonded interface takes place, a reduced section scheme is adopted and the steel bars are used to reinforce the concrete substrate beams. An aluminum beam with different thickness is bonded to the thin GFRP layer so as to change the stiffness of the composite GFRP/aluminum substrate, resulting in different fracture mode mixities. The fracture toughness values of GFRP-concrete bonded interface under three different mode ratios are obtained. The proposed 4-AENF specimen and data reduction procedures for interface fracture toughness evaluation can be used to effectively characterize mixed mode and mode-II dominated fracture of hybrid material bonded interface.

Appropriate shape of cohesive zone model for delamination propagation in ENF specimens with R-curve effects

Shape and parameters of a cohesive zone model (CZM) significantly affect the finite element modeling of delamination propagation in composites. The influence of various forms of interface laws on the delamination initiation and propagation behavior of end-notched flexure (ENF) specimens with R-curve effects is addressed. Four different traction-separation laws have been considered whose shapes are bilinear, linear-exponential, trapezoidal, and trilinear. The trilinear CZM used in this study is produced by superposing of two bilinear CZMs. All CZMs have the same values of the maximum traction and of the associated fracture energy, and are implemented in the ABAQUS/Standard. Numerical results are assessed with the experimental load-displacement responses of glass/epoxy ENF specimens available in the literature. Results show that the trapezoidal and trilinear CZMs simulate the delamination propagation in ENF specimens more accurately than others. At the same time, the superposed trilinear traction-separation law considers the length of fracture process zone and predicts both the nonlinearity point and maximum point of load-displacement curve more accurately than the trapezoidal one.

Mode II interlaminar fracture toughness of CF/EP composite containing microencapsulated healing resins

The effects of microcapsules containing healing resins on the Mode II interlaminar fracture toughness (GIIC) of carbon fibre/epoxy composite laminates were investigated using end-notched flexure (ENF) specimens. The results indicate that the Mode II interlaminar fracture toughness of the original CF/EP laminates was improved with the incorporation of a dual-component microencapsulated self-healant. It was observed that the microcapsules induced toughening via two mechanism: i) larger plastic deformation due to a thicker resin-rich interlaminar region and ii) microcapsules in tempering hackle formation. When subjected to healing after release of the microencapsulated resins, the dual-component healant provided the laminates with 63% healing efficiency in Mode II interlaminar fracture toughness. Observation of fracture surfaces using scanning electron microscopy also revealed that the healing efficiency varied directly with the concentration of healant and subsequently the formation of a dense interlaminar region after the original fracture.

Viscoelastic-damage interface model formulation with friction to simulate the delamination growth in mode II shear

In this paper a formulation of a viscoelastic-damage interface model with friction in mode-II is presented. The cohesive constitutive law contains elastic and damage regimes. It has been assumed that the shear stress in the elastic regime follows the viscoelastic properties of the matrix material. The three element Voigt model has been used for the formulation of relaxation modulus of the material. Damage evolution proceeds according to the bilinear cohesive constitutive law combined with friction stress consideration. Combination of damage and friction is based on the presumption that the damaged area, related to an integration point, can be dismembered into the un-cracked area with the cohesive damage and cracked area with friction. Samples of a one element model have been presented to see the effect of parameters on the cohesive constitutive law. A comparison between the predicted results with available results of end-notched flexure specimens in the literature is also presented to verify the model. Transverse crack tension specimens are also simulated for different applied displacement velocities.

Interlaminar and Intralaminar Fracture Behavior of Carbon Fiber Reinforced Polymer Composites

Interlaminar and intralaminar fracture behavior of carbon fiber reinforced composites have been experimentally studied. Unidirectional, woven reinforcement and thermoplastic and thermoset polymer matrix laminates have been characterized using double cantilever beam (DCB) and end notch flexure (ENF) specimens for Mode-I and Mode-II fracture toughness, respectively and compact tension (CT) specimens for intralaminar fracture. AS4/PEEK, AS4/8552 and AGP193PW/8552 laminates have been characterized in this study. The fracture toughness determined from the experimental data could be related to the constituents and reinforcements. It has been observed between the two UD laminates, AS4/PEEK exhibit higher fracture resistance under both interlaminar and intralaminar fracture. Woven reinforcement is found to show higher mode-II interlaminar fracture toughness.

Studies on effect of pre-crack length variation on Inter-laminar fracture toughness of a Glass Epoxy laminated composite

Laminates of fiber reinforced polymer composites are good in in-plane properties and inherently weak in through thickness direction. To address this through thickness properties, the inter-laminar fracture toughness (GIc and GIIc) of a unidirectional (UD) Glass epoxy composite laminates were subjected to Mode-I and Mode-II loadings. Experiments were conducted using Double cantilever beam (DCB) and End notch flexure (ENF) specimens with varying pre-crack lengths. Mode I energy release rate (GIc) were also evaluated with modified beam and modified compliance theories. The experimental results reveal that, GIc fracture toughness increases with increasing in pre-crack length, where as in GIIc the effect of increase in pre-crack length exhibits reduced fracture toughness.

Application of the direct method for cohesive law estimation applied to the strength prediction of double-lap joints

Currently, significant efforts are being made in the design of aircraft and aeronautical applications to reduce weight and improve reliability. Thus, adhesive bonding techniques have been largely employed, which also enables the combined use of steel with lighter materials such as aluminium or high strength composites. 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. 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.

Measurement of interlaminar fracture properties of composites using the J-integral method

This study explored the use of J-integral approach for characterizing mode I, mode II and mixed-mode I/II interlaminar fracture toughness of composites materials. Delamination tests were conducted to measure the fracture toughness and the resistance curve (R-curve) for double cantilever beam, end-notched flexure and mixed-mode bending specimens. The J-integral approach and the well-established ASTM standard methods based on LEFM were compared in this work. The results obtained from both methods are in very good agreement. However, the J-integral method has the advantages of being applicable to materials with large fracture process zone (e.g., composites with fiber bridging) and provides a simpler experimental procedure with fewer inputs. More importantly, the presented method avoids the ambiguity in visual measurements of delamination length required by the ASTM standard methods. In this study, an image-processing program based on Hough transform was developed to obtain the rotation angles of the specimen. The method provides good accuracy and has lower requirements for image resolution, light and material surface compared to previous methods.