ENF Test Research Articles

Equivalent energy release rate and crack stability in the mixed-mode I/II ENFR test configuration

Crack propagation performance in the ENFR (End Notched Flexure test with roller) test configuration is analyzed. The test configuration is based on the ENF test with a roller introduced between the two surfaces of the crack in order to get mixed-mode. Based on the reliability of the linear criterion, an equivalent energy release rate is proposed taking into account the interaction between the two modes and II. The stability of the crack propagation is defined by means of the derivative of the proposed equivalent energy release rate. Experimental tests with specimens of F593/T300 carbon/epoxy unidirectional composite have been carried out to assess the proposed equivalent energy release rate and to evaluate the crack stability condition. Finally, some recommendations for performing tests in the ENFR configuration under optimum conditions are summarized.

Interlaminar Crack Propagation Analysis of ENF Specimens Based on the Cohesive Zone Model

Based on classical laminated plate theory and cohesive zone model, a theoretical model of general delamination cracked laminates is established for crack propagation of pure mode Ⅱ ENF specimen. Compared with the conventional beam theory, the proposed model fully consideres the softening process of the cohesive zone and introduced the nonlinear behavior of ENF specimens before failure. The predicted failure load is smaller than the beam theory and closer to the experimental data in literature. Compared with the beam theory with only fracture toughness considered, the proposed model can simultaneously analyze the influence of interface strength, fracture toughness and initial interface stiffness on the load-displacement curves in ENF tests. The results show that the interface strength mainly affects the mechanical behavior of specimens before failure, but has no influence on crack propagation. The fracture toughness is the main parameter affecting crack propagation, and the initial interface stiffness only affects the linear elastic loading section. The length of cohesive zone increases with the increase of fracture toughness and decreases with the increase of interface strength. The effect of interface strength on the length of cohesive zone is more obvious than fracture toughness. When the tip of the adhesive zone reaches the half-strength of the specimen, the length of the adhesive zone decreases to a certain extent.

The analysis of mode decomposition in interlaminar fracture tests of asymmetric specimens

Interlaminar failure tests are usually carried out in specimens where crack arms are symmetric. Asymmetric Double Cantiliver Beam (ADCB) is an exception where both crack arms have different thickness in order to get mixed mode I/II. The asymmetry in ADCB specimens could be applied also to te case of End Notched Flexure Tests, obtaining also mixed mode I/II. The decomposition proposed by Williams is not valid in the case of ADCB tests. On the other hand, articles related to asymmetric ENF tests including the mode decompositiion have not been found. The present study deals with the analsys of the mode decomposition in the case of specimens with asymmetric arms. The asymmetry may be related to the different thickness of the crack arms, to different materials or a combination of both factors. The results obtained will be compared with the results of mode decompsition that have been published in the case of ADCB tests.

Effect of process parameters on mode-II interlaminar fracture toughness and fractographic features of automated fibre placement prepreg laminates

The present study concerns the quantitive assessment of the influence of process parameters on the interlaminar properties of carbon fiber reinforced epoxy composites. Mode II interlaminar fracture toughness is evaluated based on the ENF test. Scanning electron microscopy images are used to evaluate the change in microstructures. The results indicate that the process parameters have a direct influence on the microstructural features and fracture toughness of laminates. Specifically, the elevation of layup speed leads to the increase of interlaminar porosity which results in a decline of fracture toughness. The moderate pressure could sufficiently evacuate air bubbles during the deposition which to a certain extent decreases the porosity; however, the excessive pressure squeezes resin out from the interlaminar region, which decreases the fracture toughness. Furthermore, the excessive temperature could both increase porosity and result in a thinner resin layer due to the change in resin flowability. In which case, the fracture toughness would be significantly decreased.

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.

Damage tolerance evaluation of a C/C-SiC composite body flap of a re-entry vehicle

This paper deals with the delamination propagation in Ceramic Matrix Composites (CMC) for spacecraft applications. Presently, Ceramic Matrix Composites (CMC) are increasingly adopted for application where heat resistance and structural stiffness requirements need to be fulfilled at the same time. In particular, in aerospace applications, where the lightweight requirement is of primary concern, CMC materials seem to be the most fascinating solution for effective and performant design. One of the possible limits in the use of such materials for aerospace applications is related to the onset and evolution of damages, in particular by delamination under service loading conditions. Indeed, commonly used rules and regulations on delamination propagation, which are well-established for carbon fibre reinforced plastic, are not straight applicable for CMC materials due to the specific nature of this ceramic materials. Moreover, methodologies for fracture toughness evaluation are still in a preliminary stage for these materials.In this work, a preliminary CMC material model calibration for Cohesive elements numerically has been performed at coupon level (DCB and ENF tests) and validated by experimental literature data. Secondly, a sensitivity analysis on delamination position and size has been carried out on a C/C-SiC body flap for a reusable re-entry vehicle.To comply with a step by step gradually increased model complexity-based damage tolerance approach, preliminary analyses have been performed on plates with embedded circular delamination, comparing the delamination onset stress level with respect to the stress state corresponding to the most critical body flap service load.

Enhanced mode II fracture toughness of secondary bonded joints using tailored sacrificial cracks inside the adhesive

The mode II fracture toughness of secondary bonded joints can be improved by creating tailored sacrificial cracks inside the adhesive. To this end, we inserted a PTFE film inside the adhesive bondline during the bonding process to create sacrificial cracks inside the adhesive. We demonstrated the efficiency of this technique through ENF tests that characterize mode II fracture toughness of adhesive-bonded CFRP adherends. We ascribed the improvement in toughness to the reduction of the strain concentration at the crack tip, which delayed the crack propagation and thus improved joint initiation fracture toughness, GIIi, and the maximum load capacity, Pmax. Moreover, after crack propagation, sacrificial cracks arrested the crack propagation at the upper interface, grew secondary backward cracks at the lower interface, and created adhesive ligaments. These three damage mechanisms dissipated more energy during propagation, which improved the propagation fracture toughness, GIIc. The improvement rates depend on the sacrificial crack width and the gap between two successive cracks reaching 96%, 98%, and 25% for GIIi, GIIc and Pmax, respectively, for a 2 mm sacrificial crack width and 5 mm gap. Our approach works well for both thin and thick adhesives and is a simple technique to substantially enhance the toughness of secondary bonded joints.

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.

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.

A new interface element connecting 3D finite elements with non-coincident nodes to simulate delamination in composite laminates

Composite failure phenomena remain complex and lead to over-size structures in many industries, involving long and costly experimental campaigns. Numerical approaches are a good alternative to decrease sizing costs. The Discrete Ply Model developed in Institut Clément Ader over these last ten years shows good results in simulation of impact and compression after impact of composite laminates, ply drop-offs, open hole tensile tests… But the present approach leads to certain limitations for complex stacking sequences and induces unwanted characteristic lengths between consecutive plies. A user element for delamination calculation is developed that allows to define an interface corresponding to the overlapping zone between the two volume elements of upper and lower plies, whereas this overlapping zone is not directly defined by nodes of the mesh (volume elements have non-coincident nodes). The present paper details the new interface element implementation, its validation from DCB and ENF test simulations, and then its improvement is discussed.

Effect of Heating Process on Flexural Strength and Toughness of CFRTP Molded by Multi-layer Press -Investigation on Effect of Residual Stress-

This study investigated the effect of differences in heating process of each mold due to the molding with multi-layer press on changing in mechanical properties of CFRTP. CFRTP specimens molded with 10 pages multi-layer were prepared for 3 point bending tests and DCB tests, ENF tests in order to investigate the different of mechanical properties. The change in the mechanical properties of CFRTP molded by multi-layer press were investigated considering residual stress occurred by differences in heating process of each mold. Average of flexural strength, mode-I interlaminar fracture toughness and mode-II interlaminar fracture toughness of the specimen molded at 5th page were decreased compared with that of 1st page. Assuming the fiber as a rigid and considering the thermal shrinkage of resin, compressive residual stress should be occurred in the fiber since free shrinkage of the resin was constrained. It was considered that the reduction of flexural strength with the failure close to the compression surface was caused due to this additional compressive residual stress. The tensile residual stress also should be occurred in the resin due to constraint of free shrinkage of the resin. The observed reduction of toughness in the resin was also occurred for the procedure of crack propagation. The reduction rate of mode-I interlaminar fracture toughness was much remarkable than that of mode-II interlaminar fracture toughness. Tensile residual stress in the resin was dominant since the mode-I fracture is a crack opening mode.

Improving mode II fracture toughness of secondary bonded joints using laser patterning of adherends

We improve mode II fracture toughness of secondary bonded joints based on the laser treatment of adherend surfaces. We applied CO2 Laser treatment to adherend surface alternatively with low and high energy, resulting in a repeated pattern of rough and smooth surfaces. We used ENF test to characterize mode II fracture toughness, GII, of the bonded CFRP substrates. The results demonstrated that our proposed pattern arrested the crack propagation and triggered new damage mechanisms, such as adhesive cracking, adhesive failure and crack migration to the other interface. These additional mechanisms, some of them were non-local in nature, resulted in higher energy dissipation during propagation and, improved GII compared to joints with uniformly-treated adherends. The improvement in GII was dependent on the pattern morphology and reached up to 60%. The proposed strategy worked well for both thin and thick adhesives, and thus could be applied in aerospace and civil structures.

An investigation on the effect of polyvinyl butyral interlayer forms on the fracture toughness of glass reinforced phenolic laminates

Abstract In the recent years, thermoplastic interlayers are employed to improve the fracture toughness of laminated composites. In this research, PolyvinylButyral (PVB) was utilized as a flexible thermoplastic interlayer in forms of electrospun nanoweb, solution, film and powder to evaluate the effect on interlayer shape on the fracture toughness of glass-phenolic laminates. DCB and ENF tests were conducted to study the fracture toughness under modes I and II conditions. Among all the specimens, those modified with 30 μm electrospun PVB, with 60% increase in the mode II fracture toughness, showed the highest performance. Those consist of PVB solution (3 phr), with an increase of 50% in mode I fracture toughness, ranked the second. In the mode II fracture toughness test, for the specimens modified with the nanoweb, solution and film interlayers, and the value of GIIc increased by 38%, 24% and 18%, respectively. Samples containing PVB powder showed a decrease of 22% in the property.

Experimental-numerical analysis of failure of adhesively bonded lap joints under transverse impact and different temperatures

Cohesive failure, energy absorption, and post-load-bearing capacity of the adhesive bonded lap joints were studied through experimental–numerical comparative analysis under four different temperatures (-30 °C, room temperature (RT), 50 °C, and 80 °C) and four different transverse impact (parallel to the normal direction of the bonded surfaces) energies (4, 8, 12, and 16 J). A bilinear cohesive zone model was used for simulations, DIC (digital image correction) based DCB and ENF tests were conducted to obtain load-displacement curves, and then, R-curves as well as the cohesive laws of the bonded joints were induced through the direct modeling method. The CZM based simulation results showed good agreement with the experimental results on both the impact damage and post-load-bearing capacity of the adhesive joints and showed that both the absorbed energy and maximum post-bearing load did not significantly change with increasing temperature except at 80 °C, which is near the glass-transition temperature (Tg).

Limited Input Benzeggagh and Kenane delamination failure criterion for mixed-mode loaded fiber reinforced composite laminates

Bilinear delamination failure criteria proposed by Reeder for mixed-mode (I–II) fracture of composite laminates has four limitations: require mixed-mode (I–II) fracture data by the more complex MMB test; determination of bilinear constants by two independent curve fits; failure of bilinear equations to intersect at $$\hbox {G}_{\mathrm{I}}^{\mathrm{m}}/\hbox {G}_{\mathrm{II}}^{\mathrm{m}}=1$$; and the data representation by $$\hbox {G}_{\mathrm{I}}^{\mathrm{m}}$$ versus $$\hbox {G}_{\mathrm{II}}^{\mathrm{m}}$$ instead of the conventional $$\hbox {G}_{\mathrm{c}}^{\mathrm{m}}$$ versus $$\hbox {G}_{\mathrm{II}}^{\mathrm{m}}/\hbox {G}_{\mathrm{T}}$$. Some of these limitations were overcome by limited input bilinear criteria (LIBC) proposed by Davidson and Zhao by replacing the MMB test data by DCB, ENF, and SLB (single leg beam) test data. But the LIBC retained the two discontinuous forms of the equations for no specific reason in a continuous material system. This paper eliminates the two equations by a single power law equation that covers the whole domain of fracture as in Benzeggagh and Kenane’s empirical equation. This simplified mixed-mode delamination fracture criterion we call “Limited Input B–K (LIBK) criterion in recognition of the works of Davidson and Zhao, and Benzeggagh and Kenane. The criterion was validated by a large number of test data in the literature.

Delamination behavior and energy release rate evaluation of CFRP/SPCC hybrid laminates under ENF test: Corrected with residual thermal stresses

Delamination behavior and energy release rate on mode II interlaminar fracture toughness are investigated experimentally and theoretically. The calculation method was included with residual thermal stresses and applied to hybrid laminates. Experimental results showed that GIIC value was affected by specimen position where CFRP on top has a higher value compared to SPCC on top. The difference of GIIC value was about 60% higher. On-line monitoring method was applied during the test by using an optical microscope at the outer surface of hybrid laminates while delamination behavior of laminates was observed. CFRP and SPCC inner surfaces were investigated by using a microscope and SEM-EDS analysis. ENF test results revealed that failure exhibits at CFRP-SPCC interface laminates during ENF test with different position (CFRP on top or SPCC on top) because of different delamination behavior. The calculation model conformed experimental results with a good agreement. Prediction of the effect of thermal difference on ENF test of hybrid laminates also presented.

Experimental approach for the study of wood strength at fracture in mode II by using a modified prototype test specimen and confrontation of the results with those of the 4ENF test. Application: Eucalyptus Grandis

Wood is a material used since the beginning of history by Man, It has proved very practical in various fields, and we quote mainly the sector of building and furniture. Known to be anisotropic, cellular and very complex. Its structural properties have a major influence on the mechanical behavior of the material. The interest of this present work is to further enrich our knowledge of this material through the study of its behavior in the face of external stresses that generate the mode II fracture. To do this, the choice of a specimen to study is imperative, knowing that there is no standardized test for the study of this phenomenon; we then began by studying the different prototypes of specimens mentioned in the literature and identified their advantages and disadvantages. We concluded that each test presents many difficulties of use and a great dispersion of the results caused on the one hand, by the nature of the material but mainly by the instability of the propagation of the crack. Based on this state of the art and the various experimental devices available to us, we have developed a test specimen prototype based on a tensile test for the study of the mode II of fracture. To carry out this work, in order to apprehend the studied material, we began by carrying out a complete characterization of the studied essence: Eucalyptus Grandis that includes the two aspects: mechanical and physical.. The dispersive nature of the material, the tests of the feasibility of the prototype developed and the number of notches require the reproduction of the test, hence the large amount of testing. During this test, the wood has adopted a fragile character, which gives it a linear behavior until it reaches a break with a stable propagation of the crack. This has been noticed for all notch lengths. The microscopic analysis that we carried out enabled us to validate the mode of fracture, to do this we studied the fracture facies of each length of notch using the scanning electron microscope. To determine the GII energy mode II initiation fracture toughness, we used the global approach using the convenience method and the theory of IRWIN KIES, we then deduced the KII constraint factor. We wanted to compare the results obtained with those of the literature but this operation can hardly be feasible except for the case, if we compare that order of magnitude because the tests are not done under the same conditions and using even the species. This led us to reproduce the most used test in the research and which is more accurate compared to other experiments in this case the test 4ENF.

Evaluation for interfacial fracture of fiber-reinforced pyrocarbon matrix composites by using a zero-thickness cohesive approach

An eight-node zero-thickness element is developed to numerically model the interfacial fracture behaviors of carbon fibers reinforced pyrocarbon (C/C) composites. The constitutive parameters of the developed element are characterised on the basis of the DCB test and three-point ENF test with a novel data reduction approach. A bilinear mixed-mode constitutive criterion is adopted to define the damage initiation and evolution of the interface and the viscous regularization strategy is used to deal with the convergence difficulty in the implicit FE solver. The effects of the interfacial stiffness, the element length and the viscous coefficient on the interfacial fracture behaviors of C/C composites are explored. In addition, a set of angle-ply laminate DCB models of C/C composites are simulated and compared with the experimental tests to validate the effectiveness of the developed cohesive element for modelling the interfacial fracture behaviors of C/C composites. The results show that the proposed interface element with the determined constitutive parameters in this paper can accurately predict the interfacial fracture behaviors of C/C composites.

Assessment of CNT-doping and hot-wet storage aging effects on Mode I, II and I/II interlaminar fracture toughness of a UD Graphite/Epoxy material system

The interlaminar fracture toughness of two unidirectional Graphite/Epoxy composite material systems has been experimentally assessed. The systems studied were prepreg composite and prepreg composite treated with carbon nanotubes (CNT). The fracture toughness has been quantified in Mode I, Mode II and Mode I/II by performing DCB, ENF and MMB tests according to relevant ISO and ASTM standards. The effect of aging by storage under hot-wet conditions has been quantified by studying these systems at room temperature without aging and at 70 °C after aging treatment. Experimental data are reported in a 3- or 5-specimen batch mode, indicating non-linear behavior and sensitivity to imperfections in coupons alignment and load application. Moreover, intermediate variables required for the estimation of fracture toughness are presented in order to be used as a reference guide for principal fracture test data evaluation. In the case of the RT systems, measured data have been compared with analytical solutions and finite element model predictions yielding good correlation for DCB and ENF tests and considerable deviation in the case of MMB tests. Main findings include that CNT-doping leads to an increase of fracture toughness in all modes, especially in Mode II, and that aging leads to less variation in measurements for both systems, indicating a more uniform matrix response.

A simple procedure for the determination of the cohesive law in 4-ENF test with consideration of the friction and R-curve effect

The measurement of initiation and propagation fracture toughness is important for characterizing the delamination behavior of composite laminates, especially in the case with distinct R-curve behavior which can be caused by fiber bridging, shear deformation of matrix, etc. Four-Point End Notched Flexure (4-ENF) is one of the widely adopted tests to measure the mode II toughness values, while the friction between the crack surfaces may have a significant impact on the results. In this paper, a simple procedure to determine the cohesive law in 4-ENF test with consideration of the friction and R-curve effect is proposed. Firstly, determining the equivalent crack advance according to the compliance variation of the specimen is performed, without the need for monitoring the crack length during the test. Next, the Energy Release Rate in mode II (GII) and crack sliding displacement (δt) are derived based on the point-friction assumption and beam theory. Finally, the cohesive law is obtained via the J-integral approach. To obtain suitable values of the penalty stiffness and interface strength, the influence of these parameters on the prediction of 4-ENF test is studied. Comparisons of the numerical results and the experimental data show a good correlation. The effect of variation of friction coefficients on derived cohesive stress and initiate energy release rate is discussed as well.