End-notched Flexure Research Articles

Surfactant assisted dispersion of MWCNT's in epoxy nanocomposites and adhesion with aluminum

This article presents an experimental investigation into the adhesion between aluminum and epoxy nanocomposites reinforced with multi-walled carbon nanotubes (MWCNT's). The nanotubes are dispersed in epoxy chemically with the aid of a surfactant, rather than mechanically via high shear mixing or ultrasonication. Four MWCNT weight fractions are considered viz. 0%, 0.1%, 0.5% and 1%. The adhesion with aluminum is tested via end-notched flexure tests conducted on specimens consisting of Aluminum strips adhered together with various epoxy nanocomposite glues. The best results are obtained for 1% MWCNT, where the tests show a notable increase in adhesion, evidenced by an intact bond despite considerable plastic deformation of Aluminum. However, the peak load capacity is seen to be not enhanced. The higher adhesion with 1% MWCNT addition is seen to successfully suppress the brittle debonding failures even at very high levels of adherend plasticity. For this weight fraction the overall response is highly ductile involving shearing of the glue and is desirable for engineering applications. Despite promising results, the surfactant itself is seen to be not very effective as a dispersing agent for the epoxy resin considered here.

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 carbon nanotubes/epoxy film-interleaved carbon fiber composites

In recent years, the demand for composite materials in various industries such as aerospace, defense, and automotive industries has increased significantly. Furthermore, the mechanical properties of carbon fiber-reinforced plastic (CFRP) composites have been improved significantly. However, the out-of-plane (thickness direction) mechanical properties of CFRP laminates are considerably inferior to their in-plane mechanical properties. Owing to their unique properties, carbon nanotubes (CNTs) have been used to improve the mechanical properties of CFRP composites. However, it is challenging to disperse high concentrations of CNTs in the polymer resin matrices of such composites. In this study, a high concentration of CNTs was incorporated in CFRP laminates in the form of CNT/epoxy films by using ultrasonication and three-roll milling. Furthermore, unidirectional and plain woven prepregs were used for reinforcement. The mode II interlaminar fracture toughnesses of the CNT/epoxy film-interleaved CFRP composites with different CNT concentrations were measured by carrying out their end-notched flexure (ENF) tests. Optical micrographs were obtained to observe the microstructures of the specimens with different CNT loadings. The fracture surfaces obtained after the ENF tests were examined using a scanning electron microscope to investigate the toughening mechanism of the composites.

On the fracture toughness of metal-composite adhesive joints with bending-extension coupling and residual thermal stresses effect

The present work presents results from an analysis of the mode I and II interfacial fracture behavior of a new adhesive joint between thin titanium and CFRP adherents, that is destined for application in the future aircraft's leading edge. The joint is stiffened with two aluminum beams to prevent large deformations during testing. An engineering approach for the design of fracture toughness tests for the present joint is followed. The vacuum-assisted resin transfer molding technique is employed for the manufacturing of specimens subsequently tested using the double cantilever beam (DCB) and the end-notched flexure (ENF) configurations. Towards the fracture toughness determination from the experimental data, a new analytical model that considers the bending-extension coupling induced by the presence of the aluminum beams and the manufacturing-induced residual thermal stresses is applied. The analytical fracture toughness predictions are validated by two-dimensional finite element analyses. Crack propagation analyses based on the virtual crack closure technique and the cohesive zone modeling are performed to capture the experimental behavior and extract useful fracture toughness properties of the joint. A comparison of various literature's data reduction schemes reveals the level of error if factors such as the bending-extension coupling and/or the residual thermal stresses are ignored.

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.

Effect of Resin Layer Thickness on Mode II Delamination Growth Property of CFRTP Laminates under Static Loadings

Carbon Fibre Reinforced Thermoplastics (CFRTP) are expected to be used in various fields for the point of their superior mechanical properties. CFRP laminates with continuous fibres tend to be damaged by microcracks in the layer and interlaminar delamination. Especially, it is necessary to evaluate the mode II delamination growth property, which is correlated with compression after impact (CAI) strength. It is reported that CF/Epoxy laminates with a thicker interlaminar resin layer show higher toughness. By applying an extra thick interlaminar resin layer to CFRTP in which thermoplastic resin with relatively higher fracture toughness is used for the matrix, CFRTP with higher interlaminar fracture toughness can be developed. In this study, the mode II delamination growth property of CFRTP laminates under static loading was evaluated for the specimens with various layer thicknesses of polyamide (PA) resin in the middle layer of the laminates. Their moldability and damage propagation properties were evaluated by three-point bending tests and end notched flexure (ENF) tests. CF/PA laminated composites with a thicker PA layer showed superior mode II delamination growth property under static loading since they had more ductile fracture due to a thicker PA layer.

Experimental and Numerical Investigation of Mode I and Mode II Interlaminar Behavior of Ultra-Thin Chopped Carbon Fiber Tape-Reinforced Thermoplastics

This manuscript describes an experimental and numerical study conducted to investigate two representative cases of delamination in ultra-thin chopped carbon fiber tape-reinforced thermoplastics (UT-CTT): delamination in a double cantilever beam (DCB, mode I) and delamination in an end-notched flexure (ENF, mode II). In examples of mode I delamination, unstable crack growth resulted in sawtooth-like load–displacement curves after the initial stage of increasing load, while some specimens displaying mode II delamination exhibited stable crack propagation. The crack surfaces of DCB and ENF UT-CTT specimens were observed by a three-dimensional measurement macroscope and a scanning electron microscope. The values of mode I and mode II interlaminar fracture toughness were obtained based on linear elastic fracture mechanics and beam theory in reference to JIS K 7086 standard, respectively. Furthermore, numerical analysis utilizing the surface-based cohesive zone model based on the triangular traction–separation law was used to predict the delamination behavior in UT-CTT. The minimum and maximum values of critical strain energy release rate were used to define the range of load–displacement curves corresponding to unstable crack propagation under mode I, while the parameters including interlaminar shear strength and friction were taken into account for the predictions of delamination propagation under mode II. The numerically predicted load–displacement curves showed good correlation with the theoretical and experimental results. The research provides interlaminar mechanical properties for damage modeling and numerical simulation method to express fracture behaviors of UT-CTT structures.

The influence of mode II test configuration on the cohesive law of bonded joints

This paper aims to discuss the effect mode II type tests have on the cohesive law of bonded joints. For this reason, an objective inverse method has been developed to extract the cohesive law for experimental load-displacement data using an analytical procedure to represent the fracture process zone of mode II tests. The method is implemented on the more popular and standardized mode II tests: End-Notched Flexure and End-Load Split. The analysis of the experimental data shows that the cohesive law is independent of the test type. In addition, the obtained fracture toughness calculated by integrating the cohesive law shows good agreement with the J-integral method.

Development of cohesive zone models for the prediction of damage and failure of glass/steel adhesive joints

The use of mild steel/tempered glass adhesive joints has increased rapidly over recent years. Cohesive zone modelling (CZM) is used extensively for the numerical analysis and failure prediction of adhesive joints. The bonding to the glass surface is generally weaker than the bonding to metal substrates, and therefore the development of cohesive laws by testing on different substrates generally leads to overoptimistic and non-conservative predictions. However, the interface characterisation using standardised methods for glass/steel joints is complicated due to the relatively low strength of the glass substrate leading to premature failure. This paper presents modifications proposed for the Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests bonded with dissimilar glass/steel adherends used to extract traction-separation laws in fracture modes I and II. For this relatively small coupon size, an in-house glass heat strengthening process was developed. The cohesive laws were validated by comparing the numerical predictions for two different adhesives with experimental test data for double lap shear joints subjected to four different load cases.

Strength prediction of adhesively bonded ferrite pillar–tin bronze plate under axial shear loading

With the fast development of electronic, automotive and aerospace engineering in recent years, ferrite material has been widely used in devices including inductor, voltage transformer, filter and choke coil, etc. The proper characterisation on the mechanical capacity of the connection between ferrite and traditional metals has become a key issue for both industrial and academic fields. This work focused on the mechanical performance as well as fracture behaviour of adhesively bonded ferrite–tin bronze plate (FTBP), subjected to axial shear loading through experimental and numerical approaches. In the process, a new set of Arcan testing methods was developed for mechanical parameter determination of high flow epoxy adhesives. The material parameters of the epoxy adhesive connecting the ferrite pillar and bronze were experimentally determined. Curing mould was designed for the manufacture of the selected adhesive with high flowability in dumbbell tensile testing and Arcan testing under 0° and 90° loading directions. Quasi-static shear loading test was then conducted on bonded FTBP with a specially designed jig, and the failure surface was studied through optical microscopy and scanning electron microscopy (SEM) observations. Finite element (FE) modelling was carried out to simulate the loading process up to failure, where the crack propagation in the adhesive layer was modelled using cohesive zone model (CZM) with a bilinear traction-separation response. The experimentally measured and numerically simulated results of the adhesively bonded FTBP were compared with each other, proving the validity of the strength prediction approach developed in this work.Abbreviation: FTBP: Ferrite - Tin Bronze Plate; SEM: Scanning Electron Microscope; FE: Finite Element; CZM: Cohesive Zone Model; CIR: Cold In-place Recycling; DIC: Digital Image Correlation; DCB: Double Cantilever Beam; ENF: End-notched Flexure; PTFE: Polytetrafluoroethylene; CTOD: Crack Tip Opening Displacement; SDEG: Scalar Stiffness Degradation Variable; DOF: Degree of Freedom.

Numerical-Experimental Correlation of Interlaminar Damage Growth in Composite Structures: Setting Cohesive Zone Model Parameters

Composite laminates are characterized by high mechanical in-plane properties while experiencing, on the contrary, a poor out-of-plane response. The composite laminates, indeed, are often highly vulnerable to interlaminar damages, also called “delaminations.” One of the main techniques used for the numerical prediction of interlaminar damage onset and growth is the cohesive zone model (CZM). However, this approach is characterised by uncertainties in the definition of the parameters needed for the implementation of the cohesive behaviour in the numerical software. To overcome this issue, in the present paper, a numerical-experimental procedure for the calibration of material parameters governing the mechanical behaviour of CZM based on cohesive surface and cohesive element approaches is presented. Indeed, by comparing the results obtained from the double cantilever beam (DCB) and end-notched flexure (ENF) experimental tests with the corresponding numerical results, it has been possible to accurately calibrate the parameters of the numerical models needed to simulate the delamination growth phenomenon at coupon level.

Impact Improvement of Tape Carbon Fiber Composite Modified by Submicron Glass Fiber

It is well known that thermoplastic composite is vulnerable to impact fracture. Submicron glass fiber (sGF) was used to modify the matrix of chopped tape carbon fiber reinforced polypropylene composite. The impact resistance improved 20% and 7.4% coressponding to the dimeter sGF of 0.28 and 0.69 µm used in modified-composite. To shed light upon the mechanism of this improvement, the internal damage statement of post-impact specimens was observed by the CT scanner. The results pointed out that the increase of the impact resistance was due to the enlargement of delamination area under impact load. The micro droplet test and end notch flexure test suggest that the decrease of Mode II fracture toughness in modified-composite comes from narrowing the difference between the interfacial shear strength (IFSS) and the bending strength of matrix thanks to significant improving of IFSS with the addition of sGF while the flexural strength remains the unchanged. Consequently, the failure mode changed from debonding fiber/matrix in unmodified composite into brittle matrix failure in modified composite, resulting in the decrease of the Mode II interlaminar fracture toughness and the enlargement of delamination area. The stress transfer test also indicates that the modified composites is prone to the brittle matrix failure.

A Numerical and Experimental Study of Adhesively-Bonded Polyethylene Pipelines.

Adhesive bonding of polyethylene gas pipelines is receiving increasing attention as a replacement for traditional electrofusion welding due to its potential to produce rapid and low-cost joints with structural integrity and pressure tight sealing. In this paper a mode-dependent cohesive zone model for the simulation of adhesively bonded medium density polyethylene (MDPE) pipeline joints is directly determined by following three consecutive steps. Firstly, the bulk stress-strain response of the MDPE adherend was obtained via tensile testing to provide a multi-linear numerical approximation to simulate the plastic deformation of the material. Secondly, the mechanical responses of double cantilever beam and end-notched flexure test specimens were utilised for the direct extraction of the energy release rate and cohesive strength of the adhesive in failure mode I and II. Finally, these material properties were used as inputs to develop a finite element model using a cohesive zone model with triangular shape traction separation law. The developed model was successfully validated against experimental tensile lap-shear test results and was able to accurately predict the strength of adhesively-bonded MPDE pipeline joints with a maximum variation of <3%.

Effect of temperature on cohesive modelling of 3M Scotch-Weld™ 7260 B/A epoxy adhesive

ABSTRACTThis work addresses the effect of the test temperature on the cohesive model parameters for the 3M Scotch-Weld™ 7260 B/A epoxy adhesive. It extends a previous experimental work done at room temperature and further develops a previously proposed parameter identification method based on optimization. Double-Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests were conducted at four different temperatures: 20°C, 40°C, 55°C and 70°C. Moreover, Single Lap Joints (SLJ), and bulk specimens’ tensile tests were carried out. Finite element analyses of DCB and ENF tests were performed and optimization algorithms were used to calculate constitutive cohesive model parameters. Three approaches were followed. First, cohesive parameters were derived from bulk tests. Then, optimization with two variables for each mode was conducted, to identify the parameters ei and σu,i (i = I, II) of a triangular traction separation law. In the third, only σu,i, was taken as variable, while ei was derived from the elastic modulus of the bulk adhesive. Finally, the cohesive models were applied to simulate the response of the SLJ, and numerical results were compared with experiments. The two free variables optimization method allowed to obtain the most accurate predictions of the maximum load and SLJ displacement at failure.

Improving Interlaminar Fracture Toughness and Impact Performance of Carbon Fiber/Epoxy Laminated Composite by Using Thermoplastic Fibers.

The effects of thermoplastic polyimide (PI) and polypropylene (PP) fibers and areal density of toughened layer on interlaminar fracture toughness and impact performance of carbon fiber/epoxy (CF/EP) laminated composites were studied. Mode I interlaminar fracture toughness (GIC) was analyzed via double cantilever beam (DCB) tests. When comparing for the toughener type, PI played a positive role in enhancing the mode-I fracture toughness, while PP was not effective due to the less fiber bridge formed during composite curing. The toughening effects of areal density of PI were further investigated by end notched flexure (ENF) testing and low velocity impact testing to better understand the toughening mechanisms. The results revealed that the toughening effect reached its best effectiveness when the areal density of toughened layer was 30 g/m2. Compared with the control group, GIC and GIIC of CF/EP laminated composite were increased by 98.49% and 84.07%, and Fmax and Ee were enhanced by 92.38% and 299.08% under low velocity impact. There is no obvious delamination phenomenon on the surface of laminates after low velocity impact, indicating the improved interlaminar and impact performance of laminated composite.

Fatigue delamination growth of carbon and glass reinforced fiber metal laminates in fracture mode II

Fatigue delamination growth (FDG) rate was investigated in composite laminates with aluminium and carbon/glass fibers. An End Notch Flexure test was used to determine compliance-calibration as well as fatigue and quasi-static interlaminar fracture. Experimental tests results suggest, that fatigue delamination threshold in a metal-composite interface is governed by the epoxy matrix, while the FDG rate is governed by the fibers/matrix cohesion. Deceleration of delamination growth was observed when delamination reached a central point of the specimen. Generally, laminates reinforced by glass fibers are more resistant for delamination growth than reinforced by carbon fibers.

Influence of environmental effect on thermal and mechanical properties of welded PPS/carbon fiber laminates

This work consists in the evaluation of the influence of the environmental effects (ultraviolet radiation and humidity) on PPS/carbon fiber thermoplastic composite laminates integrated by electrical resistance welding. In order to evaluate how welding and conditioning influence the properties of the composite, the following tests were performed: Dynamic-Mechanical Analyze (DMA) and Thermomechanical Analyze (TMA) to evaluate the working temperature of the material, the Iosipescu test and Interlaminar shear test (ILSS) to evaluate interlaminar fracture strength and the Double Cantilever Beam (DCB) and End Notched Flexure (ENF) were used to analyze interlaminar fracture toughness. Considering the presented results, both the hygrothermal and UV conditioning did not promote such relevant modifications in the laminate structure, except for the DCB evaluation, which showed a reduction of approximately 53% in relation to the GIc of the unconditioned samples. Another factor also analyzed was that welding did not promote significant reductions in the thermal and mechanical properties of the laminates.

Effect of surface micro-pits on mode-II fracture toughness of Ti-6Al-4V/PEEK interface

Herein, the delamination issue of TiGr(TC4/PEEK/Cf) laminate is addressed by investigating the influence of TC4(Ti-6Al-4V) surface micro-pits on mode-II interfacial fracture toughness of TC4/PEEK interface through experimental and finite element modeling. The micro-pits unit cell, unit strip and the end notched flexure (ENF) models are established based on the finite element simulations and the effect of micro-pit size parameters is studied in detail. The results of micro-pits unit cell model reveal that the presence of micro-pits can effectively buffer the interfacial stress concentration under mode-II loading conditions. Furthermore, the micro-pits unit strip model, with different micro-pit sizes, is analyzed to obtain the interface parameters, which are converted and used in the ENF model. Both the unit strip and ENF models conclude that the presence of interfacial micro-pits effectively improves the mode-II fracture toughness. It is worth mentioning that the utilization of converted interface parameters in ENF model avoided the limitation of micro-pit size and reduced the workload. Finally, the experimental and computational ENF results exhibited excellent consistency and confirmed the reliability of the proposed finite element models. The current study provides useful guidelines for the design and manufacturing of high-performance TC4/PEEK interfaces for a wide range of applications.

Fatigue damage of cohesive interfaces in fiber-reinforced polymer composite laminates

The weak interfaces in fiber-reinforced polymer (FRP) composite laminates often dictates the reliability of the composite structures. In this respect, a new cyclic cohesive zone model (CCZM) is introduced for accurate quantitative description of the interlaminar fatigue failure process. The model incorporates the interlaminar damage through the measured fatigue degradation of the strength, stiffness, and critical energy release rate properties. A combined experimental-finite element (FE) approach is employed to establish the residual interlaminar properties. The new CCZM is examined for the case study on the end-notched flexure (ENF) beam specimen of unidirectional carbon fiber-reinforced polymer (CFRP) composite laminate. The calculated applied resultant shear stress at the critical interface material point, adjacent to the pre-existing crack front, fluctuates at (τmax = 30 MPa, R = 0.1). The interlaminar shear-induced damage shows a sigmoidal form of the damage evolution curve. The pre-existing interface crack propagation begins at 301750 cycles, while maintaining an almost straight advancing crack front.

Development of an explicit three-dimensional progressive mixed-mode I+II damage model

A progressive mixed-mode I+II damage model was implemented in an explicit finite element code aiming to simulate damage initiation and growth in solid elements. A bilinear and a smoother linear-polynomial softening laws were implemented in the explicit code. Computational analyses using the bilinear softening law revealed numerical instabilities on the load-displacement curves. Those numerical instabilities were absent when the linear-polynomial softening law was used which points to that law as more appropriate. The explicit based method was compared with an implicit model incorporating cohesive zone analysis. The comparison consisted on the simulation of interlaminar fracture characterization tests in carbon-epoxy laminates. The double cantilever beam, end-notched flexure and single-leg bending fracture tests were numerically simulated to test the performance of the proposed model under mode I, mode II and mixed-mode I+II loading. It was verified that the explicit and implicit models produce results in close agreement and capture well the toughness used as input in the numerical simulations.