End-notched Flexure Research Articles

Influence of crystallinity on interlaminar fracture toughness and impact properties of polyphenylene sulfide/carbon fiber laminates

The crystallization behavior of thermoplastic matrix is considered as an important property which can highly affect for the mechanical and thermal properties of laminated composite structures. This manuscript aims to evaluate the influence of different degrees of crystallinity induced in polyphenylene sulfide (PPS) / carbon fiber laminates, processed by hot compression molding. In the present study, the morphology, thermal and mechanical properties (compression, interlaminar shear stress, impact and end notch flexure) of the processed laminates were evaluated. From the mechanical testing results obtained, an increase in the in plane compressive and interlaminar shear strengths respectively, as well as impact resistance is observed for composite coupons processed with lower cooling rates. Based on end-notch flexure (ENF) tests, the mode-II fracture toughness is also strongly dependent on the degree of crystallinity of the thermoplastic matrix, as lower cooling rates yield stronger interfacial bonds. In this way, the present study demonstrates that the mechanical properties of thermoplastic laminates can be optimized by taking into account the matrix cooling rate during processing cycle.

A comparison of surface preparation techniques for wind turbine field repairs

Wind turbine service lifetimes can exceed 20 years. This extreme operational demand necessitates reliable and consistent field repairs; however, the effects of current wind turbine field repair surface preparation techniques are not well-documented and are thus not well-understood. This leads to reliance on technician experience rather than scientific data for repair procedures, which causes variability in the quality of repairs. Solvent wiping is a common procedure for contaminant removal after damaged material is removed from a wind turbine blade, but it was unknown if solvent wiping reduced the surface energy and consequently the likelihood of a durable repair. The goal of this study was to quantify the effects that various surface preparation techniques have on the overall strength and reliability of wind turbine repairs. Type of reagent-grade solvent, fiber direction, matrix, and adhesive were varied for contact angle, lap shear, Double Cantilever Beam (DCB), and End-Notch Flexure (ENF) testing. The type of solvent used did not affect surface energy, maximum lap shear stress, or modes I and II fracture toughness values. However matrix-adhesive combinations significantly affected maximum lap shear stress and fracture toughness values.

Mixed-mode I/II fracture criteria for adhesively-bonded pultruded GFRP/steel joint

The pure mode-I, pure mode-II and the mixed-mode I/II fracture behavior of the adhesively-bonded pultruded GFRP/steel joint were experimentally investigated by using double cantilever beam (DCB) test, and four-point end-notched flexure (4ENF) test combined with four-point single leg bending (4SLB) test, respectively. The extended global method (EGM) was employed to obtain closed-form analytical solutions of mode-II fracture toughness for the 4ENF specimen and mixed-mode I/II fracture toughness components for the 4SLB specimen. Combined with the mode-I fracture toughness obtained by the experimental compliance method (ECM), the power law criteria in initiation and propagation processes were developed for adhesively-bonded pultruded GFRP/steel joint. Validation on the developed mixed-mode fracture criteria was performed through employing it to predict the load-displacement responses of the 4SLB specimens combined with VCCT. Finally, the mixed-mode fracture criteria of the adhesively-bonded GFRP/steel joint were compared to those of the adhesively-bonded GFRP/GFRP joint for quantifying their difference.

Moisture Absorption Effects on Mode II Delamination of Carbon/Epoxy Composites.

It is necessary to consider the influence of moisture damage on the interlaminar fracture toughness for composite structures that are used for outdoor applications. However, the studies on the progressive variation of the fracture toughness as a function of moisture content M (%) is rather limited. In this regard, this study focuses on the characterization of mode II delamination of carbon/epoxy composites conditioned at 70 °C/85% relative humidity (RH). End-notched flexure test is conducted for specimens aged at various moisture absorption levels. Experimental results reveal that mode II fracture toughness degrades with the moisture content, with a maximum of 23% decrement. A residual property model is used to predict the variation of the fracture toughness with the moisture content. Through numerical simulations, it is found that the approaches used to estimate the lamina and cohesive properties are suitable to obtain reliable simulation results. In addition, the damage initiation is noticed during the early loading stage; however, the complete damage is only observed when the numerical peak load is achieved. Results from the present research could serve as guidelines to predict the residual properties and simulate the mode II delamination behavior under moisture attack.

Cure path dependency of static and dynamic mode II interlaminar fracture toughness of interlayer toughened composite laminates

Faster cure cycles are desirable to achieve higher production rates for thermoset prepreg composites. This work studies the effect of cure cycle on the static and dynamic Mode II interlaminar fracture toughness (GIIc) of an interlayer toughened thermoset prepreg composite, Toray T800SC/3900-2 B. Unidirectional laminates with different interlayer microstructures were manufactured to the same degree of cure using cure cycles with varying heating rates (0.1–7.0 °C/min) and cure temperatures (120–200 °C). Initiation and propagation GIIc were measured for a range of loading rates (0.0083–2871 mm/s) using non-precracked (NPC) and precracked (PC) end-notched flexure (ENF) tests, respectively. Results show that initiation toughness is not sensitive to curing conditions, but static propagation toughness is. Furthermore, the cure dependence of propagation GIIc decreases with increasing loading rate. This was found to be due to the delamination migration produced when loaded at different rates. Specifically, it was found that the interlayer microstructure and loading rate both influence the formation of interlaminar microcracks and the subsequent rate at which the crack migrates from the toughened interlayer to the more brittle intralayer. In laminates made with high heating rates or high cure temperatures, the crack mainly remains in the toughened interlayer during Mode II loading regardless of loading rate, and there is no noticeable reduction of propagation GIIc. This study suggests that it may be possible to develop faster cure cycles than the one currently recommended for T800SC/3900-2 B laminates, yet retain the same interlaminar fracture toughness.

Strain rate effect of mode II interlaminar fracture toughness on the impact response of a thermoplastic PEEK composite

Recent advancements in composite production and processing are making thermoplastics a viable option in a wider array of aerospace applications. In particular, Carbon Fibre Reinforced Plastics (CFRP) with thermoplastic resin are believed to have better damage tolerance properties than thermosets. However, few studies have been conducted regarding the numerical modelling of the behaviour of such materials submitted to low energy impacts. Here the Discrete Ply Model (DPM), that predicts the failure of laminated composites with the help of cohesive elements, is used to compare thermosetting and thermoplastics impact damage tolerances. The DPM is improved to take into account the strain rate effect of the fracture toughness (FT) in mode II of interlaminar interfaces. First, the End Notched Flexure (ENF) test that induces unstable crack growth is used both to experimentally measure the value of FT in mode II for high speed crack growth and to identify the strain rate effect used in the model. Second, the DPM is then used to simulate impact tests for various stacking sequences ([452, −452, 02, 902]2S, [02, 452, 902, −452]2S, [02, 302, 902, −302]2S and [902, −452, 02, 452]2S) and impact energy levels (10, 20 and 30 J). Good correlations with experiment are observed in terms of force/displacement curves and delaminated areas. The numerical model correctly describes the asymmetry of the delaminated interfaces and the propagation of groups of interfaces located near the mid-thickness of the laminated plates. Finally, the damage associated with a 30 J impact is compared for the carbon/PEEK of this study and classical carbon/epoxy plates using numerical simulations (DPM). No significant difference has been found. The results corroborate those obtained in previous studies showing the relatively low value of FT in mode II, using an ENF test and infrared thermography (IRT). This article therefore questions the apparent superiority of carbon/PEEK laminated composites over carbon/epoxy laminated composites in terms of impact damage tolerance.

Numerical analysis of the delamination in CFRP laminates: VCCT and XFEM assessment

This document develops a critical analysis of the capabilities offered by well-known numerical approaches such as eXtended Finite Element Method (XFEM) and Virtual Crack Closure Technique (VCCT) to predict delamination in composite materials. Despite several computational analyses having been performed so far, the study of the adequacy of using different modelling approaches in the delamination of composites is still limited. This paper addresses this matter, confronting the advantages and disadvantages offered by VCCT, a well-established numerical approach, and XFEM, a promising and relatively novel modelling technique. For this purpose, the delamination of carbon fibre reinforced polymer (CFRP) laminates is investigated with the simulation of three common tests: Double Cantilever Beam (DCB), End-Notch Flexure (ENF) and Mixed-Mode Bending (MMB). Numerical results are validated with experimental data, taken from other publications, for both modelling approaches analysed. Consistency is maintained for all finite element (FE) simulations carried out in this work to draw meaningful comparisons between XFEM and VCCT. Several interesting conclusions are extracted from this work. For instance, VCCT simulations overall have high accuracy and low computational time, while XFEM shows high capabilities to predict Mode I fracture.

Nonlinear XFEM Modeling of Mode II Delamination in PPS/Glass Unidirectional Composites with Uncertain Fracture Properties.

Initiation and propagation of cracks in composite materials can severely affect their global mechanical properties. Due to the lower strength of the interlaminar bonding compared to fibers and the matrix, delamination between plies is known to be one of the most common failure modes in these materials. It is therefore deemed necessary to gain more insight into this type of failure to guide the design of composite structures towards ensuring their robustness and reliability during service. In this work, delamination of interlaminar bonding in composite end-notched flexure (ENF) samples was modeled using a newly developed stochastic 3D extended finite element method (XFEM). The proposed numerical scheme, which also incorporates the cohesive zone model, was used to characterize the mode II delamination results obtained from ENF testing on polyphenylene sulfide (PPS)/glass unidirectional (UD) composites. The nonrepeatable material responses, often seen during fracture testing of UD composites, were well captured with the current numerical model, demonstrating its capacity to predict the stochastic fracture properties of composites under mode II loading conditions.

Mixed-mode I/II fracture behavior for adhesively-bonded pultruded GFRP joint under four-point bending

The experimental investigation of mixed-mode I/II fracture behavior for the adhesively-bonded pultruded GFRP joint was performed with the four-point single leg bending (4SLB) test. Cracks initiated and propagated in mat layer of the pultruded GFRP laminate for 4SLB specimens with three different thicknesses of upper adherend. The compliances of 4SLB specimens were experimentally and numerically determined. The extended global method was used for providing the simple closed-form solutions of the mixed-mode I/II fracture toughness in initiation and propagation stages, which was compared with the obtained results using virtual crack closure technique (VCCT). Combined with the pure mode-I and mode-II fracture toughness determined by using double cantilever beam (DCB) test, four-point end-notched flexure (4ENF) test, respectively, the mixed-mode fracture criteria of adhesively-bonded pultruded GFRP joint were constructed. Then, the load–displacement curves of 4SLB specimens obtained by VCCT were compared to experimental results for validating the accuracy and applicability of the developed fracture criteria of adhesive GFRP joint.

An experimental analysis of the end-notched flexure composite laminates beams with elastic couplings

This paper presents an experimental study of the composite beams with elastic couplings subjected to the end-notched flexure (ENF) test. The object of research were carbon/epoxy composite laminates with specific stacking sequences exhibiting the bending–extension and the bending–twisting couplings. In addition, multidirectional laminates with different delamination interfaces were tested. Influence of elastic coupling phenomena on behavior of laminates subjected to the bending moment as well as on fracture toughness and delamination surfaces after the ENF tests was discussed. All experiments were performed according to the ASTM D7905 Standard. Determination of crack initiation was supported by the acoustic emission technique. Data analysis of the ENF test was performed by using both the direct beam theory and the corrected beam theory. Results obtained by using the classical data reduction schemes were compared with the compliance-based beam method (CBBM). Greater values of the fracture toughness obtained by using the non-standardized methods can be explained by the effect of the fracture process zone which was taken into account in the CBBM calculation scheme. Delamination surfaces of coupled laminates after the ENF tests were investigated by using the scanning electron microscopy. Results revealed that the effect of elastic couplings might influenced on intense bridging phenomena, more extensive fracture process zone and contribution of the mode III in total delamination resistance. Nevertheless, application of the standardized ENF procedure to determination of the fracture toughness of laminates exhibiting elastic couplings is still possible, provided assistance of additional data reduction schemes and techniques or numerical analysis.

MODELLING OF CRACK DEVELOPMENT PROCESSES IN COMPOSITE ELEMENTS BASED ON VIRTUAL CRACK CLOSURE TECHNIQUE AND COHESIVE ZONE MODEL

Fiber composites based on polymer matrices are promising structural materials that meet high requirements for strength, reliability, durability, and hardness. Therefore, composite materials are widely used as structural materials for aerospace products. The problems associated with the destruction of fiber composites were relevant at all stages of technology development. A variety of reinforcing fibers and polymer binders, as well as reinforcement schemes, allow directional control of strength, stiffness, level of working temperatures and other properties of polymer composite materials. This article discusses a methodology for experimental determination of the mechanical properties of carbon-based fiber-reinforced polymer composite materials, including the determination of the interlayer fracture toughness under loading under separation conditions using the doublecantilever beam method (DCB) and the fracture toughness under transverse shear conditions using the ENF (End-Notched Flexure) method and interlayer strength. The test results of samples of polymer composite materials with a carbon reinforcing filler with different surface densities are presented. The experimental data were used to identify the parameters of the VCCT (Virtual Crack Closure Technique) and CZM (Cohesive Zone Model) closure models used to describe the development of cracks in the composites under consideration. It was found that the parameters determining the strength of layered composites are such characteristics as interlayer strength and crack resistance. It was found that the decrease in the strength of individual layers of the composite does not always affect the current stress state of the entire structure, which is often difficult to detect experimentally, but can significantly affect the further behavior of the object under study provided that the crack develops further.

The Effect of Transverse Shear in Symmetric and Asymmetric End Notch Flexure Tests-Analytical and Numerical Modeling.

This paper focuses on the effects of transverse shear and root rotations in both symmetric and asymmetrical end-notched flexure (AENF) interlaminar fracture toughness tests. A theoretical model is developed, whereas the test specimen is subdivided into four regions joined by a rigid interface. The differential equations for the deflection and rotations of each region are solved within both the Euler–Bernoulli simple beam theory (SBT) and the more refined Timoshenko beam theory (TBT). A concise analytical equation is derived for the AENF deflection profile, compliance, and transverse shearing forces as a function of the specimen geometry, stacking sequence, delamination length, and fixture span. Modeling results are compared with numerical finite element analyses, obtaining a very good agreement. Performed analyses suggest that even in the case of symmetrical and unidirectional laminates considered as pure mode II fracture, a complex compression/tension and bending moment state is present, as well as a slight contribution of anti-planar shear at the vicinity of the crack tip.

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

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

Mode-II fatigue response of AS4/8552 carbon /epoxy composite laminates interleaved by electrospun nanofibers

Abstract The advantages of applying electrospun nylon 6,6 as a toughening agent in epoxy-based composite laminates have been considered already under quasi-static mode-I and mode-II fracture loads. In the present study, the fatigue behavior of unidirectional carbon/epoxy laminates interleaved by nylon 6,6 is investigated under mode-II loading. To this aim, a 50-μm nylon 6,6 nanofibrous mat was put between mid-layers of a carbon/epoxy laminate, and then quasi-static and cyclic loadings were applied on End Notched Flexure (ENF) samples to investigate the onset of delamination growth and crack propagation rate (da/dN) under different ratios of GIIin/GIIC. The results showed that the mode-II fracture toughness increased about 161% under quasi-static loading tests. Additionally, the fatigue test results proved that the crack propagation was 14–27 times slower in the modified sample (with the same GIImax) because of the bridging between composite layers caused by nanofibers. Different fractography techniques including optical microscopy, microscopic 3D surface scanning device, and scanning electron microscopy (SEM) were also used to explore further the toughening mechanism.

A cohesive zone model to predict fatigue-driven delamination in composites

A cohesive zone model is proposed to analyse composite delamination propagation under high-cycle fatigue loading. A new method to compute the strain energy release rate at any point within the element fatigue life cycle range is presented. The proposed scheme is based on the J-integral method evaluated at an estimated position of the crack-tip within the element, instead of the element integration points. Furthermore, a new fatigue damage evolution law is proposed to account for the unwanted quasi-static damage during the element fatigue degradation process. The model prediction capabilities were verified against experimental data available in the literature and theoretical solutions using a double cantilever beam configuration for mode I loading, four-point end-notched flexure configuration for mode II loading, and mixed-mode bending configuration for mixed-mode loading. The simulations were performed at both constant and variable amplitude loading. The numerical predictions obtained using the proposed model correlated very well with literature’s experimental data.

Interfacial Fracture Toughness Assessment of a New Titanium–CFRP Adhesive Joint: An Experimental Comparative Study

Adhesive joints between dissimilar layers of metals and composites are increasingly used by different industries, as they promise significant weight savings and, consequently, a reduction in energy consumption and pollutant emissions. In the present work, the interfacial fracture behavior of a new titanium–carbon fiber reinforced plastic (CFRP) adhesive joint is experimentally investigated using the double cantilever beam (DCB) and end-notched flexure (ENF) test configurations. A potential application of this joint is in future large passenger aircraft wings. Four characteristic industry relevant manufacturing approaches are proposed: co-bonding with/without adhesive and secondary bonding using thermoset/thermoplastic CFRP. For all of them, the vacuum-assisted resin transfer molding (VARTM) technique is utilized. To prevent titanium yielding during testing, two aluminum backing beams are adhesively bonded onto the primary joint. A data reduction scheme recently proposed by the authors, which considers effects such as bending–extension coupling and manufacturing-induced residual thermal stresses, is utilized for determination of the fracture toughness of the joint. The load–displacement responses, fracture behaviors during testing, and fracture toughness performances of the four manufacturing options (MOs) under consideration are presented and compared.

Experimental study of the CTBN effect on mechanical properties and mode I and II fracture toughness of a new epoxy resin

Due to the high strength and adhesiveness of epoxies, these materials are widely used in various industries such as aerospace. But because of their inherent brittleness and poor fracture toughness, they do not have enough resistance against crack propagation. In adhesive joints, maximum stresses near the ends of the overlaps resulted in premature failure of the joints. In this study, for toughening the epoxy, carboxyl-terminated butadiene acrylonitrile (CTBN) as a liquid rubber was used. Blends of a new epoxy resin (R510) and the CTBN in several weight fractions were produced and characterized. In this article, four tests have been done to find the mechanical properties and mode I and II fracture toughness of the adhesives. The tensile and shear tests were performed to achieve Young’s modulus (E), maximum tensile stress (σf), shear modulus (G), maximum shear stress (τf) and Poisson’s ratio (ν) of the adhesives. Also, the double-cantilever beam (DCB) and end-notched flexure (ENF) tests were used to obtain the mode I fracture toughness (GIc) and mode II fracture toughness (GIIc), respectively. The results from each of the DCB and the ENF tests were analyzed by three different methods and compared. Results showed that by adding CTBN to epoxy, E, G, ν, σf and τf are reduced, but toughness, the resistance to crack growth and as a result, the GIc and GIIc increased efficiently. These data can be used to predict the adhesive joint strength and simulating the damage initiation and its evolution until failure with cohesive zone modeling (CZM).

Mode II Interfacial Fracture Toughness of Multi-Walled Carbon Nanotubes Reinforced Nanocomposite Film on Aluminum Substrate.

In this investigation, various loadings of multi-walled carbon nanotubes (MWCNTs) ranging from 0.3–1.0 wt % were incorporated into the epoxy to fabricate the nanocomposites. Nanocomposite film with a thickness of 0.2 mm was deposited on an aluminum substrate through a hot-pressing process. Theoretical expression of the model II strain energy release rate for the film/substrate composite structure was derived. End-notched flexure (ENF) tests were performed to characterize the mode II fracture energy of the composite structure. Experimental results indicate that the elastic modulus, ultimate strength, and mode II fracture energy increase as the MWCNT loading in the nanocomposite increases. In the case of nanocomposite film with 1.0 wt % of MWCNTs, the elastic modulus, ultimate strength, and mode II interfacial fracture toughness are increased by 20.6%, 21.1%, and 54.4%, respectively in comparison with neat epoxy. In addition, the dispersion of MWCNTs in the epoxy-based matrix was investigated using scanning electron microscope (SEM). The SEM images depict that MWCNTs are well dispersed leading to the enhancement of the mechanical properties of the nanocomposite.

A novel strategy to obtain the fracture envelope under mixed-mode I+II loading of composite bonded joints

The objective of this work is to develop a novel strategy to obtain the fracture envelope of composite bonded joints under mixed-mode I+II loading. Two types of tests are proposed aiming to cover all the range in the space GI-GII. For pure mode I and mixed-mode I+II with predominant mode I, double cantilever beam tests type is proposed considering equal and different specimen arms thickness, respectively. Three-point bending tests are suggested to cover the GI-GII space ranging from mode I prevalence, up to pure mode II loading. This kind of tests includes the asymmetric single-leg bending with different specimen arms thickness ratios and the end-notched flexure test for pure mode II fracture characterisation. A novel and suitable methodology to perform loading mode partition was developed and validated through numerical analysis using cohesive zone modelling.

Mixed mode fracture characterization of brittle and semi-brittle adhesives using the SCB specimen

Fracture mechanics and damage mechanics are widely used to predict the mechanical behavior of adhesive joints. The critical energy release rate is one of the most important parameters in these models. In practice, most adhesive joints experience mixed mode loading conditions. Therefore, mixed mode fracture energy measurement plays an important role in adhesive joint failure analysis. A new test method for mixed mode fracture energy measurement of brittle and semi-brittle adhesive joints is presented in this paper using semi-circular bend specimens. A data reduction scheme based on a finite element analysis is suggested and finite element results are validated by digital image correlation strain measurement technique. The fracture envelops obtained from the proposed specimen are compared to double cantilever beam (DCB), single leg bending (SLB) and, end-notched flexure (ENF) test results. Good agreement between the new specimen and the classic energy measurement methods is observed.