End Notched Flexure Test Research Articles

Mode-II fracture behavior evaluation for adhesively bonded pultruded GFRP/steel joint using four-point bending test

The pure mode-II fracture behavior of an adhesively bonded glass fiber reinforced polymer (GFRP)/steel joint was experimentally investigated using a four-point end-notched flexure (4ENF) test. Cracking of the 4ENF specimen initiated and propagated in the adhesive-steel surface. The dominant failure mode observed in all specimens was adhesive-steel interface failure. The compliance of the 4ENF specimen was determined with the rigid joint model and finite element method. The extended global method was employed to obtain a simple analytical solution of the mode-II energy release rate, and the corresponding results were compared with that determined by the virtual crack closure technique (VCCT). The full R-curve for the pure mode-II fracture behavior of an adhesively bonded pultruded GFRP/steel joint was obtained. Then, the VCCT was used to determine the load–displacement curves of the 4ENF specimens for validating the accuracy and applicability of the pure mode-II fracture criteria of the adhesively bonded pultruded GFRP/steel joint.

On mode II fracture toughness of sintered silver based on end-notch flexure (ENF) test considering various sintering parameters

Shearing fracture toughness of sintered silver (Ag) is an important parameter to reflect its shearing resistance. In this paper, mode Ⅱ fracture toughness of sintered Ag is investigated based on end-notched flexure (ENF) test. Various sintering conditions are adopted to study the effect of sintering parameters on mode Ⅱ fracture toughness of sintered Ag. The results show that the fracture toughness of sintered Ag increases rapidly with the increase of sintering temperature and holding time. Three shearing crack types are confirmed, i.e., interface delamination, tunneling cracking and cohesive cracking, and the cracking type varies from “interface delamination” or “tunneling cracking” to “cohesive cracking” with the increase of sintering temperature and holding time. An empirical equation is proposed to predict the mode II fracture toughness of sintered Ag. Through statistics investigation on microstructure evolution, it is found that the shearing fracture toughness heightens with enlargement of the Ag particle size, increase of the particle shape form factor, and decrease of the porosity. This study provides an alternative method to rigorously evaluate the shearing fracture toughness of die-attach materials in electronics packaging.

Effect of material nonlinearity on the toughness evaluation in quasi-static mode II interlaminar fracture toughness tests of composite laminates

In mode II interlaminar fracture toughness tests of composite laminates, the fracture toughness evaluated based on linear elastic fracture mechanics deviates from the true toughness due to the material nonlinearity of the matrix. In this study, an elastoplastic finite-element analysis was performed on a new quasi-static mode II interlaminar fracture toughness test, i.e., the doubly end-notched tension (DENT) test, to investigate the difference between the perceived energy release rate and the J-integral. The effect of material nonlinearity was compared with other quasi-static mode II interlaminar fracture toughness tests: three- and four-point bend end-notched flexure tests. Analysis of the DENT showed that for a given J-integral loading, the size of the plastic zone at the delamination crack tip was the same regardless of the crack length. The difference between the perceived energy release rate and J-integral in the DENT test with respect to the plastic zone size was independent of the crack length, and was smaller compared with that of flexure-type tests. Furthermore, the compliance calibration method was applied to the elastoplastic analysis results of the DENT test to reduce the evaluation difference of the energy release rate.

Theory of dynamic mode-II delamination in end-notched flexure tests

The dynamic mode-II energy release rate of the end-notched flexure (ENF) test with applied time-dependent displacement is derived for the first time with the effect of vibration included. A dynamic Euler-Bernoulli beam theory is employed together with a deflection condition to simulate contact. To investigate the dynamic effect and the relative dynamic contribution from each vibration mode, a dynamic factor and a spatial factor are defined. It is found that the contribution of the ith vibration mode is dependent on the spatial factor (which is a function of the delamination length and the total length of the ENF specimen) and that certain vibration modes are dominant (depending on the delamination-length ratio). In addition, for a given spatial factor, there may be a certain vibration mode with a zero contribution to the ERR. The developed theory is verified against results from finite-element-method simulation for two cases of ENF tests and they are in excellent agreement. This work now allows the loading rate-dependent mode-II delamination toughness of layered materials to be determined using ENF tests. In addition, it provides understanding of the structural dynamic response in the presence of mode-II delamination and can guide the design of structures to mitigate the vibration-driven delamination.

Displacement Rate Effects on the Mode II Shear Delamination Behavior of Carbon Fiber/Epoxy Composites

This paper studies the influence of displacement rate on mode II delamination of unidirectional carbon/epoxy composites. End-notched flexure test is performed at displacement rates of 1, 10, 100 and 500 mm/min. Experimental results reveal that the mode II fracture toughness GIIC increases with the displacement, with a maximum increment of 45% at 100 mm/min. In addition, scanning electron micrographs depict that fiber/matrix interface debonding is the major damage mechanism at 1 mm/min. At higher speeds, significant matrix-dominated shear cusps are observed contributing to higher GIIC. Besides, it is demonstrated that the proposed rate-dependent model is able to fit the experimental data from the current study and the open literature generally well. The mode II fracture toughness measured from the experiment or deduced from the proposed model can be used in the cohesive element model to predict failure. Good agreement is found between the experimental and numerical results, with a maximum difference of 10%. The numerical analyses indicate crack jump occurs suddenly after the peak load is attained, which leads to the unstable crack propagation seen in the experiment.

Study on interlaminar performance of CNTs/epoxy film enhanced GFRP under low-temperature cycle

The laminates of unidirectional (UD) Glass Fiber Reinforced Plastic (GFRP) were prepared by inserting Toner/epoxy film (as a comparison) and CNTs/epoxy film between layers to optimize the interlaminar performance. The effect of temperature variation on the interlaminar properties of materials has yet to be identified in practical applications. To explore the impact of low-temperature cycle (21 cycles between −58 °C and RT) on the specimens and the strengthening mechanism of the two epoxy films, interlaminar shear strength (ILSS) test, end notch flexure (ENF) test and double cantilever beam (DCB) test were prepared and carried out. The results showed that the interlaminar fracture toughness of GFRP embedded with the CNTs/epoxy film was superior to that of GFRP without film and embedded with the Toner/epoxy film in modes I (based on DCB) and II (based on ENF). The interlaminar shear strength was greatly enhanced by the interlaminar integration of CNTs/epoxy film. The impact of low-temperature cycle on the interlaminar performance of CNTs/epoxy enhanced GFRP was not significant, and the effect on the interlaminar properties of CNTs/epoxy enhanced GFRP was minimal compared with the other two kinds of specimens. The microstructure of the interlaminar strengthening interface was further studied by scanning electron microscopy (SEM).

Interlaminar Fracture Behavior of Carbon Fiber/Polyimide Composites Toughened by Interleaving Thermoplastic Polyimide Fiber Veils

Carbon fiber reinforced thermosetting polyimide (CF/TSPI) composites were interleaved with thermally stable thermoplastic polyimide (TPPI) fiber veils in order to improve the interlaminar fracture toughness without sacrificing the heat resistance. Both of the mode I and mode II interlaminar fracture toughness (GIC and GIIC) for the untoughened laminate and TPPI fiber veils interleaved laminates were characterized by the double cantilever beam (DCB) test and end notch flexure (ENF) test, respectively. It is found that the TPPI fiber veils interleaved laminates exhibit extremely increased fracture toughness than the untoughened one. Moreover, the areal density of TPPI greatly affected the fracture toughness of laminates. A maximum improvement up to 179% and 132% on GIC and GIIC is obtained for 15 gsm fiber veils interleaved laminate, which contributes to the existence of bicontinuous TPPI/TSPI structure in the interlayer according to the fractography analysis. The interlaminar fracture behavior at elevated temperatures for 15 gsm fiber veils interleaved laminate were also investigated. The results indicated that the introduction of thermally stable TPPI fiber veils could enhance the fracture toughness of CF/TSPI composites by exceeding 200% as compared to the untoughened one even as tested at 250 °C.

Analysis of the influence of ply-orientation in delamination progression in composites laminates using the Serial/Parallel Mixing Theory

In this work, an analysis on the influence of fibre-orientation in the progression of delamination was carried out. The study was focused on a cracked beam according to an End Notch Flexure test configuration. The Serial/Parallel Mixing Theory was used as the composite constitutive law along with a continuum damage model to simulate delamination. This mathematical approach was implemented in a custom-made FEM code and applied to model the delamination progression. The model was validated with two sets of experimental results from the literature involving 0° fibre-oriented laminates. After validation, a beam made of plies oriented between 0° and 90° was analysed. The model predicts the loss of stiffness and strength due to the increment of fibre angle. In addition, the stress concentration phenomenon at the crack tip and delamination onset was properly represented. The difference observed in the progression of delamination along the width of the beam highlights the need for a three-dimensional representation to model fibre-oriented beams.

Influence of geometric and material parameters on the mode II interlaminar fatigue/fracture characterization of CFRP laminates

The objective of this work is to perform fatigue/fracture characterization of carbon-epoxy laminates under pure mode II loading. The End-Notched flexure (ENF) test was employed to execute quasi-static and fatigue/fracture tests aiming to determine the mode II fracture energy, the fatigue lives and the modified Paris law parameters representative of this material. These Paris law coefficients were used as input in a high-cycle fatigue cohesive zone model that was validated against experimental fatigue lives. Afterwards, the numerical cohesive zone model was employed to assess the influence of several geometrical and material parameters on the mode II interlaminar fatigue/fracture behaviour of Carbon-Fibre Reinforced Plastic (CFRP) laminates. Important conclusions were drawn concerning the fatigue performance of these materials under pure mode II loading.

Fabrication and Thermo-Electro and Mechanical Properties Evaluation of Helical Multiwall Carbon Nanotube-Carbon Fiber/Epoxy Composite Laminates.

The development of advanced composite materials has taken center stage because of its advantages over traditional materials. Recently, carbon-based advanced additives have shown promising results in the development of advanced polymer composites. The inter- and intra-laminar fracture toughness in modes I and II, along with the thermal and electrical conductivities, were investigated. The HMWCNTs/epoxy composite was prepared using a multi-dispersion method, followed by uniform coating at the mid-layers of the CF/E prepregs interface using the spray coating technique. Analysis methods, such as double cantilever beam (DCB) and end notched flexure (ENF) tests, were carried out to study the mode I and II fracture toughness. The surface morphology of the composite was analyzed using field emission scanning electron microscopy (FESEM). The DCB test showed that the fracture toughness of the 0.2 wt.% and 0.4 wt.% HMWCNT composite laminates was improved by 39.15% and 115.05%, respectively, compared with the control sample. Furthermore, the ENF test showed that the mode II interlaminar fracture toughness for the composite laminate increased by 50.88% and 190%, respectively. The FESEM morphology results confirmed the HMWCNTs bridging at the fracture zones of the CF/E composite and the improved interlaminar fracture toughness. The thermogravimetric analysis (TGA) results demonstrated a strong intermolecular bonding between the epoxy and HMWCNTs, resulting in an improved thermal stability. Moreover, the differential scanning calorimetry (DSC) results confirmed that the addition of HMWCNT shifted the Tg to a higher temperature. An electrical conductivity study demonstrated that a higher CNT concentration in the composite laminate resulted in a higher conductivity improvement. This study confirmed that the demonstrated dispersion technique could create composite laminates with a strong interfacial bond interaction between the epoxy and HMWCNT, and thus improve their properties.

Improving the delamination fatigue resistance of composites by 3D woven metal and composite Z-filaments

Metallic and composite z-filaments have been reported as effective solutions for improving the interlaminar fatigue resistance of fibre-polymer laminates, which is important to many applications that demand high delamination fatigue endurance. Here, we examine the mechanisms of three different z-filaments made from metallic and fibre composite materials and to quantify their effectiveness in improving the modes I and II interlaminar fatigue resistance. Interlaminar fatigue experiments were conducted under cyclic displacement control conditions using the Double Cantilever Beam (DCB) and End Notch Flexure (ENF) test methods. Results show that, despite the metallic z-filaments having much greater ductility than carbon fibre z-filament, they are less effective in raising the interlaminar fatigue resistance of laminated composites. Of the two metallic filaments, steel z-filaments are more effective at resisting the initiation and growth of fatigue cracks under modes I and II interlaminar cyclic loads than copper filaments. The different extents of improvements of the three filaments are attributed to the differences in their crack bridging efficiency under interlaminar fatigue loading.

Inverse analysis on mode II adhesive properties of PP film in fiber metal laminate using hybrid Levenberg–Marquardt methods

Fiber metal laminate constants of the metallic sheet and the fiber-reinforced composite sheet. FMLs can undergo delamination under transverse loads. Therefore, their nominal stress and energy release rate (ERR) should be experimentally derived to quantitatively evaluate their adhesive properties. However, the adhesive properties obtained experimentally contain some unavoidable errors. In this study, we propose an inverse analysis method to more accurately predict the mode II nominal stress and the ERR of a polypropylene film. First, an end-notched flexure test was conducted to obtain the mode II ERR, and a cohesive zone model was proposed to predict the delamination of the specimens. Next, we proposed the hybrid Levenberg–Marquardt methods to predict the mode II nominal stress and the ERR of the PP film. The methods were compared using the root mean square error to confirm whether each method escaped the local minimum and plateau regions, and the computational resources.

Experimental study on effects of asymmetrical stacking sequence on carbon fiber/epoxy filament wound specimens in DCB, ENF, and MMB tests

This study focused on the delamination fracture behavior of asymmetrical stacking sequences of CFRP laminates subjected to different loading directions in double cantilever beam (DCB), end-notched flexure (ENF), and mixed-mode bending (MMB) tests. CFRP laminates with three different stacking sequences mimicking the winding angles of filament wound structures were considered. As the stacking sequences were asymmetrical, flipped and non-flipped specimens were tested to examine the effects on different test loading conditions. According to the results, each stacking sequence provided a different load–displacement response and fracture toughness. The flipped and non-flipped specimens did not appear to affect the fracture toughness in the DCB test. However, significant effects were observed in the ENF and MMB tests. Observations at the edges of the specimens indicated the dependency of the delamination behavior on the specimen stacking sequence. The delamination surfaces were further analyzed using a scanning electron microscope (SEM). SEM images revealed several delamination mechanisms that contributed to the load–displacement response and fracture toughness of the laminates.

The effect of sizing optimization on the interface between high strength steel and fiber reinforced composite

This paper aims to examine fiber type and fiber orientation’s effects on the interface bonding between steel and fiber-reinforced composites. To this end, fracture loads for modes I and II were experimentally determined. Three different composites were used: glass fibers/epoxy (GFRP), carbon fibers/epoxy (CFRP), and Kevlar fibers/epoxy (KFRP). Seven different fabric orientations were examined: 0°, 15°, 30°, 45°, 60°, 75°, and 90°. End-notched flexure (ENF) and Double cantilever beam (DCB) tests were utilized to determine modes I and II fracture toughness, respectively. Results showed that fiber orientations and fiber types have significantly affected the interface bonding between the steel and fiber-reinforced composite. For both modes I and II tests, the CFRP/steel interface exhibited the highest toughness when comparing the different tested fabric types. However, when comparing the different tested fabric orientations for GFRP/Steel, the 0° GFRP/Steel interface had the maximum toughness for modes I and II tests. All steel-composite specimens tested have shown matrix, debonding, fiber breakage, delamination, and fiber kinking using the scanning electron microscopic technique.

Effect of interfacial fibre orientation and PPS veil density on delamination resistance of 5HS woven CFRP laminates under mode II loading

This paper presents an experimental study on the effect of interfacial fibre orientation and interleaved thermoplastic veil on Mode II interlaminar fracture toughness of 5-harness satin woven carbon fibre reinforced polymer composite laminates. Three-point End-Notched Flexure tests were carried out to determine delamination resistance, GIIC, of specimens with five fibre orientation biases and two veil densities at the midplane. Results show that delamination resistance of 5-harness satin woven laminates depends on the layup configurations at the midplane with 90/45 fibre orientation bias exhibiting the greatest resistance. The delamination resistance enhancement from polyphenylene sulfide (PPS) veil interleaves is also fibre orientation dependent but a further increase of the veil density from 10 gm−2 to 20 gm−2 offers little extra benefit. Fracture surface morphologies were examined under SEM to understand the failure mechanism and fracture process of the woven laminate under the combined effects of the interfacial fibre orientation and the veil density. Fibre orientation relative to the delamination path, surface texture misfit, and veil density are the three main contributors identified for the variation of delamination resistance of 5HS woven laminates.

The mode II delamination toughness of arc unidirectional fiber-reinforced composites with ENF test

To obtain the mode II delamination toughness (the energy release rate GIIC) of arc unidirectional fiber-reinforced composites, the end notched flexure (ENF) test with arc specimen was used to study. In terms of theory, the formulas of compliance and GIIC for Arc-ENF specimen were derived by using energy method and Irwin-Kies formula, and the crack length correction and friction coefficient were respectively introduced to correct these formulas, with the verification by the numerical simulation. As the ENF test, the compliance curve of the Arc-ENF specimen was recorded under the condition of the constant loading speed, and the crack tip position was determined by the strain diagram of the specimen in 1-3 plane with the optical strain gauge. Finally, the friction coefficient and the GIIC of the Arc-ENF specimen in the ENF test were determined using the corrected formulas, the numerical simulation method and the ENF test data.

Mode II fracture toughness of glass–epoxy laminated composites under various heating conditions and strain rates

The high performance of glass fiber reinforced polymer (GFRP) leads this composite to become one of the favourite research topics in engineering. The GFRP is widely used in building structure and other engineering application. In the design of composite structures, the damage-tolerant consideration of composite structure is essential, especially under environmental factor and dynamic effect. Hence, the damage stability of the composite under loadings should be investigated. In this study, the mode II fracture toughness of glass/epoxy laminated composite is evaluated under various strain rates and heating temperatures. The GFRP laminated specimens were fabricated by hand layout method with the heat-treated temperature of 40 °C, 60 °C and 80 °C. The experimental mode II fracture toughness data were obtained from the end-notched flexure (ENF) test method with the strain rate from 1 mm/min to 5 min/min. The experimental results showed that the mode II fracture toughness of the GFRP laminate composite increase with the increase in strain rate. However, the mode II fracture toughness of the GFRP laminate composite is relatively insensitive to heating temperature..

Wood-adhesive bond loaded in mode II: experimental and numerical analysis using elasto-plastic and fracture mechanics models

Abstract The goal of the study was to analyze fracture properties of adhesive bond using a three-point end-notched flexure test and the compliance-based beam method. Critical strain energy release rates (G IIc ) and cohesive laws were obtained for adhesive bonds made of European beech (Fagus sylvatica L.) and adhesives such as EPI, MUF, PRF and PUR. The experiments were assisted with FE analyses employing three different material models of wood: elastic (Elas), symmetric elasto-plastic (EP) and elasto-plastic with different compressive and tensile yield stresses parallel to fiber (EP+). The highest mean G IIc was achieved for PUR (5.40 Nmm−1) and then decreased as follows: 2.33, 1.80, 1.59 Nmm−1 for MUF, EPI, and PRF, respectively. The failure of bondline was brittle and occurred at bondline for EPI, MUF and PRF, and ductile and commonly occurring in wood for PUR adhesive. The FE simulations employing cohesive models agreed well with the experimental findings for all adhesives. FE model with Elas material was found accurate enough for EPI, MUF and PRF adhesives. For PUR adhesive, the model EP+ was found to be the most accurate in prediction of maximal force. The impact of friction between lamellas may be up to 4.2% when varying friction coefficient from 0 to 1. The impact of the grain angle distortion (α) with respect to longitudinal specimen axis showed its high influence on resulting stiffness and maximal force. It was found that three-point end-notched test is suitable for EPI, MUF, and PRF, while it is less appropriate for a bond with PUR adhesive due to notable plastic behavior.

Modelling prediction on the degradation in mode II fracture energy of structural adhesive subjected to hygrothermal ageing

ABSTRACT In this work, the effect of hot-humid environment on the mode II fracture energy (G IIc) of the selected structural adhesive was experimentally evaluated through End-Notched Flexure (ENF) testing as well as corresponding finite element (FE) modelling approach. The ready-cured ENF samples were first immersed in distilled water and 5% NaCl solution under 50°C for different durations, and then removed from the hot-wet environment for mode II loading up to failure. Degradation was observed in the experimentally measured G IIc for adhesives immersed in both aqueous environments, while a further decrease was found in the samples immersed in distilled water, possibly due to the fact that more water was absorbed than saltwater immersion. A two-step FE modelling approach was developed to simulate the water absorption process under-elevated temperature (Step 1) and subsequent mode II loading test after hygrothermal ageing (Step 2) of ENF specimens. User subroutines were developed to accurately simulate the water ingress process in the adhesive layer and properly transfer the moisture distribution data between the two FE models. The feasibility of the established FE modelling was then successfully validated through comparison between the experimental and numerical results.

Enhanced Electrical Conductivity and Interlaminar Fracture Toughness of CF/EP Composites via Interleaving Conductive Thermoplastic Films

The poor electrical conductivity and interlaminar fracture toughness of carbon fiber reinforced epoxy (CF/EP) composites could reduce their reliability for aerospace applications. In this work, multi-walled carbon nanotubes (MWCNTs) doped polyethersulfone (PES) conductive thermoplastic films (CTFs) were prepared by solution casting method. The as-obtained composites were then interleaved into CF/EP prepregs to yield composite laminates. The effects of CTFs on the electrical conductivity and interlaminar fracture toughness of the as-prepared CF/EP composites were investigated. The results suggested an increase in conductivity of laminates by 104% and 84% in the transverse (Y) and through-thickness (Z) directions, respectively. The Mode I and Mode II interlaminar fracture toughness values were evaluated by double cantilever beam (DCB) and end-notched flexure (ENF) testing. The data revealed that the introduction of CTFs with 10wt% MWCNTs led to enhancement in Mode I and Mode II interlaminar fracture toughness of Composite laminates by 206% and 47%, respectively. The failure mechanism was investigated through microstructural and morphological changes in the composites studied by scanning electron microscopy (SEM). This work provides valuable guidance for the simultaneous enhancement of electrical conductivity and interlaminar fracture toughness of CF/EP composites.