End Notched Flexure Test Research Articles

Mode II fracture toughness of asymmetric metal-composite adhesive joints

The paper presents an experimental investigation of the mode II fracture toughness behavior of dissimilar metal-composite adhesive joints using the end-notched flexure (ENF) test. The adhesive joint under study consists of a thin titanium sheet joined with a thin CFRP laminate and is envisioned tobe applied in the hybrid laminar flow control system of future aircraft. Four different industrial technologies for the manufacturing of the joint areevaluated; co-bonding with and without adhesive and secondary bonding using either a thermoset or a thermoplastic composite. The vacuum-assisted resin transfer molding (VARTM) technique is employed for the manufacturing of the titanium-CFRP joint. After manufacturing, the joint is stiffened from its both sides with two aluminum backing beams to prevent large deformations during the subsequent ENF tests. Towards the fracture toughness determination from the experimental data, an analytical model recently reported by the authors is applied; that model considers the bending-extension coupling of each sub-laminate of the joint as well as the effect of the manufacturing-induced residual thermal stresses. The load-displacement behaviors, failure patterns, and fracture toughness performances for each of the four manufacturing options (MO) investigated are presented and compared.

Fracture envelope estimation of a structural adhesive by dedicated fracture tests

Cohesive zone modelling (CZM) is widespread for the strength analysis of bonded joints. The fracture toughness (GC) is required to use CZM. A scarcely studied mixed-mode test is the Asymmetric Tapered Double-Cantilever Beam (ATDCB), which merges a Tapered Double-Cantilever Beam (TDCB) adherend with a Double-Cantilever Beam (DCB) adherend. This work addresses the ATDCB test to estimate the fracture envelope of a structural adhesive. TDCB and End-Notched Flexure (ENF) tests were also performed to acquire the tensile (GIC) and shear fracture toughness (GIIC), respectively. Numerically, mixed-mode CZM laws were constructed based on the obtained data, and the results were compared with experiments, to validate the CZM laws and the mixed mode propagation criterion. As a result, the best damage propagation criterion for mixed mode was estimated and validated.

Characterization of cohesive model and bridging laws in mode I and II fracture in nano composite laminates

The cohesive model and traction-separation curves have brought a considerable possibility for researchers and fracture engineers to assess and simulate failure in composite laminates. Reliable determination of the traction–separation laws is very pivotal to the success of this approach in finite element methods. The objective of this paper is assessment of nanoparticles effect on bridging laws, cohesive mechanism and traction-separation parameters of nanocomposites mode I and II fracture. To do this analyzing of the experimental data from double cantilever beam, and end notched flexure tests including construction of the R-curves (energy release rate versus crack length), reconstruction of these curves in terms of the pre-crack tip opening and sliding displacement, and calculation of the corresponding bridging and traction-separation laws through the J-integral approach were carried out. For the calculation of the energy release rate in Mode I, three corresponding data reduction schemes namely Corrected Beam Theory, Experimental Compliance Method and Modified Compliance Calibration are utilized, while Compliance Calibration Method, Corrected Beam Theory and Compliance-Based Beam and II fracture are applied for that of mode II. The main concern of this research is introduction of critical parameters of two modified models to simulate mode I and II fracture. Adding 0.43 wt% nanoparticles to composite DCB samples leads to increase of 116%, 68% and 70% in GI,0 calculated by CBT, ECM and MCC respectively, and a 72% increase in GI,ss is measured by CBT, while this value for ECM and MCC is 110% and 48%. Adding 0.2 wt% nanoparticles to the composite samples results in 86% (50 MPa) increase in critical stress in mode II fracture calculated by method CBBM. This method presented the lowest value for critical displacement, fluctuated between 0.08-.11 mm in mode II.

A novel experimental procedure to determine the cohesive law in ENF tests

This study presents a novel method to determine the mode II cohesive law of unidirectional carbon fiber reinforced polymer laminates, employing the End Notched Flexure (ENF) test.The fracture toughness in mode II (G) and crack shear displacement (Δt) are determined based on a compliance variation data reduction method (BTBR) by processing the global load displacement curve, without monitoring the crack length and the crack shear displacement during the test. The procedure is validated by finite element analysis including cohesive zone modeling. The results of the method are compared to those obtained with the Direct Method, where the fracture toughness is determined based on the rotations of the load introduction points and the crack shear displacement is directly measured. It is concluded that the proposed data reduction scheme is suitable to obtain the mode II cohesive law using only the load and displacement data obtained from the testing machine, without any external displacement measurement technique and without any assumption of the form of the cohesive law.

Effect of unstable crack growth on mode II interlaminar fracture toughness of a thermoplastic PEEK composite

An end notched flexure (ENF) test was used to study the effect of growth stability on mode II interlaminar fracture toughness (GIIC) of a carbon/PEEK composite. The instability of the ENF test was used to induce both stable and unstable crack growth with a static loading. Two techniques were used to evaluate mode II fracture toughness: the traditional compliance method and the infra-red thermography technique. The compliance technique has the advantage of being simple and it has already been confirmed for standard tests. Nevertheless its results may be inaccurate for unstable crack growth propagation. The infra-red thermography technique enables fracture toughness to be accurately measured for non-standard tests in which the crack growth may be unstable.Both methods are complementary and this study came to the conclusion that GIIC is sensitive to crack growth velocity. Fracture toughness showed a low value for crack growth start and unstable propagation, and a high value for stable crack growth. The transition between a ductile fracture mode (for low crack growth velocity) and a brittle fracture mode (for high crack growth velocity) explained the important variability in GIIC values.

Direct injection molding and mechanical properties of high strength steel/composite hybrids

Metal polymer hybrid technology is a promising way of automotive weight reduction and safety improvement. In this study, high strength steel and glass-fiber reinforced PA66 composite were integrated by direct injection molding process. The mechanical properties and failure behavior of steel/composite hybrids under tension, bending, mode I and II fracture loads were investigated experimentally, theoretically and numerically. The properties of the interface between steel and composite were measured using double cantilever beam and end-notched flexure tests for the first time. The analytical model and finite element simulation considering the interface mechanical behavior were proposed and validated. They can be utilized to predict the mechanical properties and progressive failure process of hybrid material cost-effectively and accurately. It is found that the cracks initiate at the interface, and then propagate through composite until the hybrid material loses load-carrying capacity completely. The steel dominates the load-bearing capacity, and the interface dominates the failure process. The results obtained in this study offer a scientific guidance and theoretical basis for the engineering application of metal polymer hybrid materials.

Development of a 3D reinforcement by tufting in carbon fiber/epoxy composites

Since the early development, the sectors where the composite materials are being used have been growing gradually. Nowadays, these materials have extensive application in the structural components in the aerospace, defense, transportation, civil, and energy industries. The composites, especially carbon fabric/epoxy resin, have excellent in-plane properties. However, the susceptibility to delamination from out-of-plane loads, due to the lack of fibers oriented through the thickness, is still one of the weaknesses of this kind of materials. The introduction of reinforcement through the thickness has the potential to increase out-of-plane properties of composite materials. This can be accomplished by different reinforcement methods, such as Z-pinning, stitching, and tufting. This work aimed to develop and implement a simplified method of through-the-thickness reinforcement, based on tufting reinforcement. The experimental methodology was developed in a CNC router machine where glass fiber and polyamide were used as the main reinforcement materials in different square patterns (5 × 5 and 7 × 7) and applied in laminated carbon/epoxy composites manufactured by vacuum resin transfer molding process. Then, efficiency of the reinforcement was evaluated by means of mechanical testing, i.e., tensile and end-notched flexure testing in order to evaluate the ability to improve the interlaminar fracture strength. Results showed that the presence of the reinforcements provided around 27% increase in the delamination resistance compared to the non-reinforced composites in the thickness direction.

Effect of nano-OMMTs on mode I and mode II fracture toughness of continuous glass fibre reinforced polypropylene composites

The influence of nano-organic montmorillonites (nano-OMMTs) on mode I and mode II interlaminar fracture toughness of continuous glass fibre reinforced ammonium polyphosphate polypropylene (CGF/PP/APP) composites was experimentally studied in this paper. Nano-OMMTs modified CGF/PP/APP unidirectional prepreg composites were fabricated by hot-melt impregnation process. To investigate the effect of nano-OMMTs on the interlaminar fracture toughness of composites, double cantilever beam (DCB) test and end-notched flexure (ENF) test were conducted to obtain the mode I initiation interlaminar fracture toughness (GIC init.), mode I propagation interlaminar fracture toughness (GIC prop.), and mode II initiation interlaminar fracture toughness (GII C init.) of samples, respectively. Through comparison against to the neat CGF/PP/APP composites, with the addition of 6.5 wt% nano-OMMTs into PP matrix, the GIC init., GIC prop., and GII C init. of composites can be enhanced from 0.502 kJ/m2, 0.746 kJ/m2 and 0.623 kJ/m2 to 0.712 kJ/m2, 1.119 kJ/m2, and 1.195 kJ/m2, respectively. Finally, it was found that nano-OMMTs in matrix can act as “pins”, which could absorb energy in the forms of nano-particles pullout and enhance the interlaminar fracture toughness values in DCB and ENF tests.

Interlaminar properties of GFRP laminates toughened by CNTs buckypaper interlayer

The interlaminar property is essential to the application of fiber reinforced composites for the overall performance. For the purpose of enhancing the interlaminar properties, the unidirectional Glass Fibre Reinforced Plastics (GFRP) laminates were fabricated by adding carbon nanotubes (CNTs) buckypaper to the mid-plane layers under different curing pressures (1 MPa, 2 MPa, 3 MPa). In order to study the enhancement mechanism of the CNTs buckypaper, a series of experiments were designed and conducted based on double cantilever beam (DCB), end notch flexure (ENF) and interlaminar shear strength (ILSS) tests followed by scanning electron microscope (SEM) examination to study the surfaces of the specimens. It was observed that the modes I and II interlaminar fracture toughness of GFRP embedded with CNTs buckypaper were relatively higher than those without CNTs buckypaper. Considerable improvements in interlaminar shear strength of the laminates were obtained with the integration of the CNTs buckypaper between the interlayers. The micro-structure of the enhanced mixed interface of fiber-resin-CNTs responsible for interlaminar strengthening was further studied with the help of SEM. It was found that the higher curing pressure could increase the inter-diffusion of the resin and CNTs offering a stronger interfacial adhesion. The interlaminar characteristics of the specimens cured at 2 MPa pressure were found to be relatively superior with optimal interlaminar fracture toughness and interlaminar shear strength.

Fracture behavior of adhesively bonded carbon fabric composite plates with nano materials filled polymer matrix under DCB, ENF and SLS tests

In our earlier paper [Srivastava et al. Eng. Fract Mech, 180 (2017)] we found that mode I, mode II fracture toughness of carbon nanotubes (CNTs) filled woven carbon fabric composites increases with the addition of nanofillers. In order to prove the effect of nanofillers on adhesively bonded joints of woven carbon fabric laminates, the present study is conducted under double cantilever beam (DCB), end notched flexural (ENF) and single lap shear (SLS) tests. Graphene nanoplates (GnPs) filled epoxy resin, and carbon blacks (CBs) filled epoxy resin, and pure epoxy resin adhesives were used to prepare the DCB, ENF and single lap adhesively bonded joint specimen from two similar woven carbon fabric composite plates to measure them. Fracture toughness (mode I mode II) and shear strength of adhesively bonded joints were determined.The results show that the adhesively bonded woven carbon fabric composite joints with GnPs-filled epoxy resin and CBs-filled epoxy resin adhesives give better fracture toughness mode I, mode II and shear strength compare to pure epoxy resin adhesively bonded composite specimen. Mode I value is lower than mode II value due to the sharp propagation of cracks under tensile stress. Scanning electron microscopy (SEM) was utilized to monitor the distribution of nanofillers within the adhesively bonded texture to examine the effect of fracture surfaces in the bonded DCB, ENF and single lap shear joint specimens respectively.

Mode II fracture toughness of carbon–epoxy bonded joints with femtosecond laser treated surfaces

Mode II fracture toughness of carbon–epoxy bonded joints considering laser treated surfaces was evaluated using the End-Notched Flexure (ENF) test. Four different surface preparations were performed using distinct processing conditions before bonding the specimen arms. The Resistance-curves of the fracture tests were obtained considering an equivalent crack length based procedure. The influence of the different surface treatments on the mode II fracture behaviour was assessed comparing the average values of the fracture toughness. It was verified that a laser treatment employing two interfering laser beams, which produces a complex multi-scale surface morphology free of contaminants, is the best surface preparation regarding higher values of the mode II fracture energy. A cohesive zone analysis was used aiming to validate the followed procedures. The excellent agreement obtained between numerical and experimental results reveals that the used techniques are sound.

Mode II fracture characterization of wood using the Four-Point End-Notched Flexure (4ENF) test

This work addresses wood fracture characterization under mode II loading using the Four-Point End-Notched Flexure test. A suitable data reduction scheme based on specimen compliance, beam theory and equivalent crack model was developed with the aim of solving several disadvantages intrinsic to this fracture test. Following this methodology, crack length measurements in experimental tests required in classical procedures becomes unnecessary. Moreover, the execution of additional tests to measure the elastic properties are also unnecessary. The method was applied to experimental load-displacement curves to obtain the corresponding Resistance-curves. A finite element analysis including cohesive zone modelling was conducted to validate the followed procedure. It was verified that numerical load-displacement curve and Resistance-curve are well representative of the overall trends obtained experimentally. These results demonstrate good performance of the method, which makes the Four-Point End-Notched Flexure test a valuable and appealing alternative to conduct wood mode II fracture characterization.

Response surface methodology optimization of electrospinning process parameters to fabricate aligned polyvinyl butyral nanofibers for interlaminar toughening of phenolic-based composite laminates

In this study and for the first time, aligned nanofibers were produced from low molecular weight polyvinyl butyral. Using response surface methodology, the preparation condition of aligned nanofiber was optimized in terms of nanofiber diameter and its structural stability. Central composite design as a response surface methodology was employed and the effects of process variables and their influence on nanofiber diameter were investigated. Based on a statistical analysis, the use of a model, which was used to determine the nanofiber diameter, proved to be successful because of its low probability value (0.0073) and high correlation coefficient (0.9619). A high-speed cylinder collector was used to fabricate aligned polyvinyl butyral nanofibers. The optimum conditions of 17.5 kV voltage, 10 cm collector distance, 13% solution concentration, and 2100 r/min rotational speed were obtained from experiments. The least diameter of 158.6 nm along with a stable structure was determined for polyvinyl butyral nanofiber prepared under the optimum conditions. An aligned polyvinyl butyral nanoweb was applied on the mid-layer of glass-phenolic laminated composites as an interlaminar reinforcement. The fracture behavior of the laminates was determined by end-notched flexure tests. Excellent toughening property which was observed for the aligned polyvinyl butyral nanofibers caused the mode-II fracture toughness and its maximum force to increase by 25.2 and 40.8%, respectively.

Tunable effect of polyvinyl butyral nanofiber veil on fracture toughness of glass reinforced phenolic composites manufactured with out of autoclave method

This study proves the ability of polyvinyl butyral (PVB) nanofiber to improve the mode I and mode II interlaminar fracture toughness of out of autoclave phenolic-based composites. For investigation fracture behavior of glass-phenolic composites, a mode-I and mode II fracture test were carried out standard UD laminated and nanoweb interleaved specimens. PVB nanoweb with 25 μm thickness was placed in the laminate mid-planes which was subsequently subjected to a double cantilever beam and end-notched flexure tests. It was found that PVB nanofiber thanks to its excellent adhesion property with phenolic resin and unique features of nanoscales, has a tunable effect on improving mode I and mode II interlayer fracture toughness. PVB nanoweb provided the best all-round performance with fracture toughness improvements of 61% and 34.5% mode I and mode II, respectively; while, the maximum bending load has been improved.

Effect of sewage sludge ash filler on mode I and mode II interlaminar fracture toughness of S-glass/epoxy composites

In this study, the influence of sewage sludge ash (SSA) waste particle contents on the mechanical properties and interlaminar fracture toughness for mode I and mode II delamination of S-glass fiber reinforced epoxy composites were investigated. Composite laminate specimens for tensile, flexural double-cantilever beam (DCB) and end-notched flexure (ENF) tests were prepared and tested according to ASTM standards with 5, 10, 15 and 20 wt% SSA filled S-glass/epoxy composites. Properties improvement reasons was explained based on optical and scanning electron microscopy. The highest improvement in tensile and flexural strength was obtained with 10 wt% content of SSA. The highest mode I and mode II interlaminar fracture toughness’s were obtained with 15 wt% content of SSA. The mode I and mode II interlaminar fracture toughness’s improved by 33 and 63.6%, respectively, compared to the composite without SSA.

Experimental Investigation of Mechanically Coupled Composite Specimens in the ENF Test Configuration

This paper reports an experimental study on the mode II interlaminar fracture of carbon/epoxy multidirectional (MD) laminates as well as on composite laminates with specific stacking sequence exhibited mechanical couplings. The aim of the analysis was to determine the mode II critical strain energy release rate (c-SERR, GIIC) and an applicability of the data reduction schemes. Specimens were subjected to the end notched flexure (ENF) test. During the experiment the applied load and the load point displacement were registered. Additionally, the crack onset and all propagation values were observed and marked on the specimens edges. Moreover, the acoustic emission (AE) data were registered. The values of the mode II c-SERR were obtained with assumptions of the Direct Beam Theory (DBT) and Corrected Beam Theory (CBT). The experiment proved that the fracture toughness depends on the specimen delamination interface. For the asymmetric laminates, the mode II c-SERR reached significantly greater values for those specimens where the bending-extension coupling (BE) occurred.

Mode I and mode II delamination of a chopped strand mat E-glass reinforced vinyl ester composite

ABSTRACTThe objective of the present work is to investigate mode I and mode II delamination behaviour of chopped strand mat (CSM) E-glass reinforced vinyl ester (VE) composite. Double cantilever beam and end notched flexure tests were carried out to evaluate the mode I and mode II delamination, respectively. The fracture toughnesses were calculated using the experimental calibration method. Results showed that the average mode I and mode II fracture toughnesses were 185 and 2386 N m−1, respectively. Furthermore, the mode II–mode I ratio for this material was 12.9. This value was the highest when compared with other composite materials from the literature. Finally, through scanning electron micrographs, the dominant failure mechanisms were found to be matrix cracking, fibre debonding and fibre breakage. In addition, shear cusps were observed in mode II specimen, which signified the shearing between the layers.

Mode-II interlaminar fracture and crack-jump phenomenon in CFRP composite laminate materials

An interlaminar crack-jump event is one of the complex failure phenomena in laminated composite structures. This paper examines the mechanics of the interlaminar damage process leading to crack-jump event in CFRP composites under mode-II loading condition. A series of end-notched flexure tests of CFRP composite were conducted to create a standard interface failure, and an unstable interface fracture that led to crack-jump event. FE simulation of the composite tests was created using a new FE model-based construction and CZM theory in combination with a hybrid experimental-computational approach to assess the interface damage and crack-jump events. The FE model of the standard test predicted a short range crack-jump event instead of a gradual interface crack growth, coincided with the load drop in the structural response. A constant value of interface damage dissipation was predicted at the time of fracture for all composite cases. The unstable crack-jump event occurred due to the release of high strain energy in the composite structure while the interface underwent cracking process.

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.

Structural behavior of secondary-bonded composite joints subjected to Mode II fatigue induced delamination

This paper analyses the Mode II fatigue delamination growth onset for secondary bonded joints compared to co-cured joints. The materials used were composed by two carbon fiber reinforced sub-laminates joined by the secondary bonded and co-cured (without adhesive) methods. Mode II fatigue tests were performed using three-point bending End Notched Flexure test setup. The tests were performed under displacement control and a sinusoidal displacement applied at a frequency of 5 Hz with a Rd = 0.1, that mean, δmin = 0.1 δmax. The main objective of this study was to obtain the strain energy release rate (SEER) versus number of cycles (Nf) in order to evaluate the effect of the adhesive on fatigue life of bonded joints. A Closed form solution for 3-ENF setup was used in order to define the displacement amplitudes that were applied in the fatigue test. The results show that the use of adhesive causes a reduction on Mode II fatigue delamination growth onset SERR (Gth) for secondary-bonded compared to co-cured joints. Finally, a scanning electron microscopy (SEM) was used to analyze the fracture surfaces of the Mode II secondary bonded specimens. The fractography images show no crack growth for the specimens tested below or at Gth loading levels, which indicated that the value corresponds to the threshold.