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Liyong Tong - One of the best experts on this subject based on the ideXlab platform.

  • bending effect of through thickness reinforcement rods on mode ii delamination toughness of enf specimen elastic and rigid perfectly plastic analyses
    Composites Part A-applied Science and Manufacturing, 2007
    Co-Authors: Liyong Tong
    Abstract:

    Abstract In this paper, a new simple metallic z-rod model is proposed to study the bending effect of the metallic z-rods on mode II delamination toughness of laminated composites. A new transverse shear force–deformation relationship for a metallic z-rod is obtained by using the classical beam theory and modeling its surrounding matrix as linearly elastic, rigid–perfectly plastic or linearly elastic–perfectly plastic springs. The bridging traction provided by a metallic z-rod to the mode II delamination toughness is assumed to be only the shear force carried by a z-rod created by the relative slippage between two substrate beams in an End-Notched Flexure (ENF) specimen, whereas the longitudinal sliding friction is assumed to make negligible contribution to the bridging traction. Mode II strain energy release rate (SERR) is employed to evaluate the influence of the metallic z-rods on the interlaminar fracture toughness of End-Notched Flexure (ENF) specimens. A parametric study of ENF specimens reinforced with the z-rods is conducted to demonstrate the effect of the new bridging mechanism by the metallic z-rods on the mode II delamination toughness.

  • Bending effect of through-thickness reinforcement rods on mode II delamination toughness of ENF specimen: Elastic and rigid–perfectly plastic analyses ☆
    Composites Part A: Applied Science and Manufacturing, 2007
    Co-Authors: Liyong Tong, Xiannian Sun
    Abstract:

    Abstract In this paper, a new simple metallic z-rod model is proposed to study the bending effect of the metallic z-rods on mode II delamination toughness of laminated composites. A new transverse shear force–deformation relationship for a metallic z-rod is obtained by using the classical beam theory and modeling its surrounding matrix as linearly elastic, rigid–perfectly plastic or linearly elastic–perfectly plastic springs. The bridging traction provided by a metallic z-rod to the mode II delamination toughness is assumed to be only the shear force carried by a z-rod created by the relative slippage between two substrate beams in an End-Notched Flexure (ENF) specimen, whereas the longitudinal sliding friction is assumed to make negligible contribution to the bridging traction. Mode II strain energy release rate (SERR) is employed to evaluate the influence of the metallic z-rods on the interlaminar fracture toughness of End-Notched Flexure (ENF) specimens. A parametric study of ENF specimens reinforced with the z-rods is conducted to demonstrate the effect of the new bridging mechanism by the metallic z-rods on the mode II delamination toughness.

A B De Morais - One of the best experts on this subject based on the ideXlab platform.

  • Determination of the shear traction-separation law of adhesive layers using the End-Notched Flexure specimen
    Engineering Fracture Mechanics, 2020
    Co-Authors: A B De Morais
    Abstract:

    Abstract 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.

  • Analysis of the fracture process zone and effective crack length in the adhesively bonded End-Notched Flexure specimen
    The Journal of Adhesion, 2018
    Co-Authors: A B De Morais
    Abstract:

    ABSTRACTThe mode II fracture of adhesive joints is well-known to involve large fracture process zones. Their effect in fracture energy measurements can be taken into account by the effective crack length approach. Moreover, fracture process zones can be simulated by cohesive zone models, which are increasingly used for structural analysis of adhesive joints. This paper aimed at evaluating the influence of the traction-separation law on the fracture process zone and on the effective crack length in End-Notched Flexure tests. Novel analytical cohesive zone models were developed for the bilinear and trapezoidal traction-separation laws. The latter were shown to affect significantly the energy dissipation rate versus effective crack length curve prior to crack initiation. Therefore, this effect seems to provide a simple approach for evaluating approximate traction-separation laws. The models here developed are easy to apply and provide simple approximate expressions useful for specimen selection.

  • Elastic-plastic analysis of the adhesively bonded End-Notched Flexure specimen
    Engineering Fracture Mechanics, 2018
    Co-Authors: A B De Morais
    Abstract:

    Abstract The End-Notched Flexure (ENF) specimen has often been employed to characterise the mode II fracture of adhesive joints. This paper reports a beam model for ENF specimens with metal adherends and typical bondline thickness values. The initial formulation adopting adhesive linear elasticity predicted accurately the shear stress distribution, the compliance and the strain-energy release rate. The adhesive elastic-perfectly plastic behaviour subsequently introduced generated a plastic zone whose size could be predicted by a closed-form expression. The model provides a basis for selecting specimen geometries that enable accurate fracture energy measurements from a corrected beam theory with effective crack length scheme.

  • Mode II fatigue delamination of carbon/epoxy laminates using the End-Notched Flexure test
    Composite Structures, 2015
    Co-Authors: J A Sousa, A. B. Pereira, A.p. Martins, A B De Morais
    Abstract:

    The fatigue delamination behaviour of carbon/epoxy laminates was investigated through End-Notched Flexure tests carried out under displacement control. Fatigue tests were conducted at different stress ratios from pre-cracks generated in initial quasi-static tests, which provided fracture toughness values. Crack propagation rates were obtained by a new approach that employed the effective crack method and considered propagation increments of small strain-energy release rate variation. The present results were found to give a more realistic view of the fatigue delamination behaviour by taking either a recently defined strain-energy release rate range or the stress intensity factor range as the main fatigue driving parameter. The analysis took into account the typical power law fits, the influence of the stress ratio and the indication of a possible fatigue threshold. Finally, quasi-static tests conducted from the fatigue pre-crack showed significantly lower initiation critical strain-energy release rates than the ones previously measured.

  • Application of the effective crack method to mode I and mode II interlaminar fracture of carbon/epoxy unidirectional laminates
    Composites Part A: Applied Science and Manufacturing, 2007
    Co-Authors: A B De Morais, António B. Pereira
    Abstract:

    This paper describes an experimental study involving double cantilever beam (DCB), End-Notched Flexure (ENF) and four-point End-Notched Flexure (4ENF) tests on carbon/epoxy unidirectional specimens. The main purpose was to evaluate the so-called effective crack method (ECM), which avoids operator crack position monitoring. Extensive fibre bridging in DCB tests made the ECM inappropriate, as shown in subsequent finite element analyses. On the other hand, the ECM gave very consistent results from ENF tests, despite the usually unstable initiation. The present results confirmed some dependence of perceived initiation toughness values on ENF specimen geometry predicted in recent numerical studies. Differences between results of ENF and 4ENF tests could be explained by friction effects in the latter.

Barry D. Davidson - One of the best experts on this subject based on the ideXlab platform.

  • Standardization of the End-Notched Flexure Test for Mode II Interlaminar Fracture Toughness Determination of Unidirectional Laminated Composites
    Journal of Testing and Evaluation, 2015
    Co-Authors: Barry D. Davidson
    Abstract:

    Research is described that supports the approach used in the new ASTM International standard for the determination of the mode II interlaminar fracture toughness of unidirectional polymer matrix composites. Reasons for choosing the End-Notched Flexure (ENF) test over other candidate methods are first presented. This is followed by results from a combined numerical and experimental study that led to a compliance calibration method being chosen for data reduction, as well as two subsequent studies to determine the delamination lengths used for the calibration tests. The development of a method of creating static mode II precracks is described, and validation studies are presented. Other considerations that went into finalizing the test standard, including the determination of span length, range of allowable thicknesses, method of locating the delamination tip after precracking, and load levels to be used during compliance calibration, are then discussed. Results from an interlaboratory study are presented, which illustrates that the variation in test results that are observed in the ENF test using the standardized test method are quite small by historical standards. This indicates that the new standard is appropriate for general use and provides a welcome complement to the existing ASTM International standards for determination of the mode I and mixed-mode I-II interlaminar fracture toughness.

  • Evaluation of Precracking Methods for the End-Notched Flexure Test
    AIAA Journal, 2007
    Co-Authors: Carlos L. Perez, Barry D. Davidson
    Abstract:

    *† Results are presented from a study to evaluate and develop static mode II precracking methods for use with the End-Notched Flexure test. Precracking followed by fracture toughness testing was performed on specimens from two different materials. Precracking was performed using both the four-point bend and conventional three-point bend endnotched Flexure geometries. All testing was performed in the latter geometry. There was no difference in the precracks created by the two geometries. However, delamination toughness was observed to strongly depend on the amount of dynamic crack advance that occurs during the precracking process. This was hypothesized to be due to the faster crack speeds that are associated with larger amounts of advance. To address this, various precracking geometries were evaluated in order to determine those that would produce precracks that were essentially straight, perpendicular to the direction of crack advance, and of sufficient length to produce a precracked toughness at or near the minimum value that occurs with increasing precrack length. This results in a recommended geometry for use with the endnotched Flexure test that is appropriate for both precracking and testing, and which therefore allows for non-precracked and precracked toughnesses to be obtained from the same test specimen.

  • Influences of Friction, Geometric Nonlinearities, and Fixture Compliance on Experimentally Observed Toughnesses from Three and Four-point Bend End-Notched Flexure Tests
    Journal of Composite Materials, 2007
    Co-Authors: Barry D. Davidson, Xuekun Sun, Anthony J. Vinciquerra
    Abstract:

    Three and four-point bend End-Notched Flexure tests, comprising a number of different test geometries, are performed on two different graphite/epoxy composites, and the toughnesses are obtained by a compliance calibration method of data reduction. The coefficient of friction along the crack plane and the flexural modulus of each material are then determined experimentally and used with nonlinear finite-element analyses to simulate these same test configurations. By using the mean experimentally observed critical load, these simulations are used to obtain the materials' toughnesses by three different methods. The first uses a previously developed `direct energy balance approach,' which is assumed to produce the `true' toughness. The second is by a simulated compliance calibration procedure, which is used to obtain the perceived toughness for an infinitely stiff fixture. In the third approach, experimentally determined fixture compliances, as a function of the test geometry, specimen, and crack length, are ...

  • Geometry and Data Reduction Recommendations for a Standardized End Notched Flexure Test for Unidirectional Composites
    Journal of ASTM International, 2006
    Co-Authors: Barry D. Davidson, Xuekun Sun
    Abstract:

    Recommendations are made for choosing a test geometry and data reduction method for a standardized End-Notched Flexure test for unidirectional fibrous laminated polymeric composites with glass or graphite reinforcements. To this end, the accuracies of a variety of commonly used data reduction methods are evaluated over a range of geometries. This is done using a combination of numerical and experimental approaches. It is determined that the compliance calibration method of data reduction, with an associated expression for compliance, C, of the form C=A+ma3, provides the best combination of accuracy and usability of all those data reduction methods considered. Here, A and m are curve fitting parameters and a is the crack length, and the expression for compliance is obtained prior to the fracture test by a series of compliance tests at different crack lengths to a load approximately equal to 50 % of that required for fracture. When this approach is used along with the associated recommendations for slenderness ratio of the specimen and roller diameters of the fixture, it is shown that the toughness that is extracted from the test will be within 3 % of its true value for materials with coefficients of friction, μ, along the interface containing a preimplanted insert of 0.0≤μ≤0.5. All materials for which experimental data are available show μ to be within this range. As such, this compliance calibration method is recommended as the sole data reduction method for the standardized test.

  • Numerical evaluation of the effects of friction and geometric nonlinearities on the energy release rate in three- and four-point bend End-Notched Flexure tests
    Engineering Fracture Mechanics, 2006
    Co-Authors: Xuekun Sun, Barry D. Davidson
    Abstract:

    Abstract Nonlinear finite element analyses are used to examine the effects of friction and geometric nonlinearities on the energy release rate in three- and four-point bend End-Notched Flexure tests. Energy release rates are first determined by a recently developed direct energy balance approach. It is shown that the finite diameter loading rollers that are typically used in practical test set-ups cause both tests to be inherently nonlinear. The effect of these nonlinearities on the energy release rate is shown to be larger in the four point than the three point test and to increase with increasing roller diameter, increasing coefficient of friction along the crack plane, and decreasing supporting span length. For the four point test, the effect of these nonlinearities is also shown to increase with increasing ratio of inner to outer span length. Next, energy release rates at the onset of crack advance are determined by a simulated compliance calibration technique. This “perceived toughness” is compared with predictions of the “true toughness” given by the direct energy balance approach at the same load. It is shown that perceived toughnesses from this simulated compliance calibration procedure are larger than previously reported results that were obtained in a similar fashion using linear theory. In addition, the perceived toughness is shown to strongly depend upon the range used for fitting the load versus deflection data to obtain compliance. These findings are used to make some general recommendations regarding use of the two test methods and their associated data reduction techniques.

D. E. Kretschmann - One of the best experts on this subject based on the ideXlab platform.

  • Effect of varying dimensions on tapered End-Notched Flexure shear specimen
    Wood Science and Technology, 1995
    Co-Authors: D. E. Kretschmann
    Abstract:

    An effort is underway by the International Union of Testing and Research Laboratories for Materials and Structures (RILEM) Technical Committee 133, Fracture of Timber, to investigate options for a standardized test method to establish Mode II fracture mechanics properties in wood. In this study, varying size and thickness effects on the performance of the RILEM proposed tapered End-Notched flexture (TENF) shear specimen were studied. Three sizes and thicknesses of Sitka spruce (Picea sitchensis) specimens with a tangential-longitudinal orientation and density between 400 and 500 kg/m^3 were tested. For the range of dimensions investigated, no evidence was found of a size or thickness effect on fracture energy.

Hiroshi Yoshihara - One of the best experts on this subject based on the ideXlab platform.