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Stephen R. Hallett - One of the best experts on this subject based on the ideXlab platform.

  • numerical analysis of high velocity oblique impacts and residual tensile strength of carbon epoxy laminates
    Composite Structures, 2021
    Co-Authors: Ashwin R. Kristnama, Michael R Wisnom, Stephen R. Hallett
    Abstract:

    Abstract This paper presents prediction of the high velocity, oblique impact response and quasi-static residual tensile strength of thin [45/90/−45/0]2s carbon/epoxy laminates using finite element (FE) models. A High-Fidelity Finite Element Method (Hi-FEM) with an automated unit cell meshing technique was employed. The predicted impact damage, characterised by the extent of Fibre Failure and delamination area, was validated against results from gas-gun tests for a range of impact velocities. The numerical results captured the trend of increasing impact damage with impact energy as observed from the tests. Changes in projectile orientation before impact were shown to increase the extent of Fibre Failure at high impact energies, up by 38% in edge impact cases. The residual tensile strength of the impacted laminates was then investigated, where the numerical results for edge-impacted laminates agreed with the test data within 8%. On the other hand, the residual strength modelling results of centre-impacted laminates were found to be unconservative, mainly due to the extent of Fibre Failure predicted during impact. Machined notches were also studied for their residual tensile strength in comparison to impact induced damage. The predicted strength of edge-notched laminates was found to be in close agreement with the experimental results for edge-impacted laminates, differing by an average of 9%.

  • Numerical analysis of high velocity, oblique impacts and residual tensile strength of carbon/epoxy laminates
    Composite Structures, 2021
    Co-Authors: Ashwin R. Kristnama, Michael R Wisnom, Stephen R. Hallett
    Abstract:

    Abstract This paper presents prediction of the high velocity, oblique impact response and quasi-static residual tensile strength of thin [45/90/−45/0]2s carbon/epoxy laminates using finite element (FE) models. A High-Fidelity Finite Element Method (Hi-FEM) with an automated unit cell meshing technique was employed. The predicted impact damage, characterised by the extent of Fibre Failure and delamination area, was validated against results from gas-gun tests for a range of impact velocities. The numerical results captured the trend of increasing impact damage with impact energy as observed from the tests. Changes in projectile orientation before impact were shown to increase the extent of Fibre Failure at high impact energies, up by 38% in edge impact cases. The residual tensile strength of the impacted laminates was then investigated, where the numerical results for edge-impacted laminates agreed with the test data within 8%. On the other hand, the residual strength modelling results of centre-impacted laminates were found to be unconservative, mainly due to the extent of Fibre Failure predicted during impact. Machined notches were also studied for their residual tensile strength in comparison to impact induced damage. The predicted strength of edge-notched laminates was found to be in close agreement with the experimental results for edge-impacted laminates, differing by an average of 9%.

  • compressive Failure of laminates containing an embedded wrinkle experimental and numerical study
    Composites Part A-applied Science and Manufacturing, 2015
    Co-Authors: Supratik Mukhopadhyay, Mike I Jones, Stephen R. Hallett
    Abstract:

    An experimental and numerical study has been carried out to understand and predict the compressive Failure performance of quasi-isotropic carbon–epoxy laminates with out-of-plane wrinkle defects. Test coupons with artificially induced Fibre-wrinkling of varied severity were manufactured and tested. The wrinkles were seen to significantly reduce the pristine compressive strength of the laminates. High-speed video of the gauge section was taken during the test, which showed extensive damage localisation in the wrinkle region. 3D finite element (FE) simulations were carried out in Abaqus/Explicit with continuum damage and cohesive zone models incorporated to predict Failure. The FE analyses captured the locations of damage and Failure stress levels very well for a range of different wrinkle configurations. At lower wrinkle severities, the analyses predicted a Failure mode of compressive Fibre-Failure, which changed to delamination at higher wrinkle angles. This was confirmed by the tests.

  • a finite element based statistical model for progressive tensile Fibre Failure in composite laminates
    Composites Part B-engineering, 2013
    Co-Authors: Stephen R. Hallett, Michael R Wisnom
    Abstract:

    Abstract This paper presents a two-parameter integrated Weibull model that is implemented in a finite element analysis to capture the well known “size effect” in composite materials. It can thus account for the effect of randomly distributed defects on Fibre dominated tensile Failure in a general loading situation. Here it is combined with cohesive interface elements to include the interaction with delamination. As a verification of the modelling method, scaled four-point bend tests from previous research were simulated. The models gave excellent results for both the strains to Failure and the final Failure modes and captured the observed size effect which cannot be represented by conventional progressive damage models.

  • experimental study of damage propagation in over height compact tension tests
    Composites Part A-applied Science and Manufacturing, 2009
    Co-Authors: Stephen R. Hallett, Navid Zobeiry, Reza Vaziri, Anoush Poursartip
    Abstract:

    This paper describes an experimental investigation into progressive damage development in notched Fibre-reinforced composites laminates. The Over-height Compact Tension (OCT) test captures the behaviour of laminates typical of large structures and permits the stable formation of a process zone ahead of the crack tip. This allows a study of the influence of sub-critical damage on progression of Fibre Failure. A range of lay-ups have been tested using dispersed and blocked plies in the thickness direction. The load vs. pin opening displacement (POD) curve is used to characterise the progressive Failure of specimens. A number of interrupted tests were performed for each lay-up to capture the sub-critical damage process before the onset of Fibre fracture. Results show that dispersed plies promote Fibre Failure and crack growth whilst blocked plies promote a larger amount of splitting and delamination which in turn causes a larger process zone and ultimately a tougher laminate.

Michael R Wisnom - One of the best experts on this subject based on the ideXlab platform.

  • numerical analysis of high velocity oblique impacts and residual tensile strength of carbon epoxy laminates
    Composite Structures, 2021
    Co-Authors: Ashwin R. Kristnama, Michael R Wisnom, Stephen R. Hallett
    Abstract:

    Abstract This paper presents prediction of the high velocity, oblique impact response and quasi-static residual tensile strength of thin [45/90/−45/0]2s carbon/epoxy laminates using finite element (FE) models. A High-Fidelity Finite Element Method (Hi-FEM) with an automated unit cell meshing technique was employed. The predicted impact damage, characterised by the extent of Fibre Failure and delamination area, was validated against results from gas-gun tests for a range of impact velocities. The numerical results captured the trend of increasing impact damage with impact energy as observed from the tests. Changes in projectile orientation before impact were shown to increase the extent of Fibre Failure at high impact energies, up by 38% in edge impact cases. The residual tensile strength of the impacted laminates was then investigated, where the numerical results for edge-impacted laminates agreed with the test data within 8%. On the other hand, the residual strength modelling results of centre-impacted laminates were found to be unconservative, mainly due to the extent of Fibre Failure predicted during impact. Machined notches were also studied for their residual tensile strength in comparison to impact induced damage. The predicted strength of edge-notched laminates was found to be in close agreement with the experimental results for edge-impacted laminates, differing by an average of 9%.

  • Numerical analysis of high velocity, oblique impacts and residual tensile strength of carbon/epoxy laminates
    Composite Structures, 2021
    Co-Authors: Ashwin R. Kristnama, Michael R Wisnom, Stephen R. Hallett
    Abstract:

    Abstract This paper presents prediction of the high velocity, oblique impact response and quasi-static residual tensile strength of thin [45/90/−45/0]2s carbon/epoxy laminates using finite element (FE) models. A High-Fidelity Finite Element Method (Hi-FEM) with an automated unit cell meshing technique was employed. The predicted impact damage, characterised by the extent of Fibre Failure and delamination area, was validated against results from gas-gun tests for a range of impact velocities. The numerical results captured the trend of increasing impact damage with impact energy as observed from the tests. Changes in projectile orientation before impact were shown to increase the extent of Fibre Failure at high impact energies, up by 38% in edge impact cases. The residual tensile strength of the impacted laminates was then investigated, where the numerical results for edge-impacted laminates agreed with the test data within 8%. On the other hand, the residual strength modelling results of centre-impacted laminates were found to be unconservative, mainly due to the extent of Fibre Failure predicted during impact. Machined notches were also studied for their residual tensile strength in comparison to impact induced damage. The predicted strength of edge-notched laminates was found to be in close agreement with the experimental results for edge-impacted laminates, differing by an average of 9%.

  • a novel technique to accurately measure the Fibre Failure strain in composite laminates under a combined in plane tension and shear stress state
    21st International Conference on Composite Materials ICCM 2017, 2017
    Co-Authors: Meisam Jalalvand, Mohammad Fotouhi, Mun Choong Leong, Michael R Wisnom
    Abstract:

    A simple and accurate test method is presented to investigate the influence of shear stresses on tensile Failure of unidirectional carbon Fibre/epoxy. Glass/carbon hybrid laminates are used to eliminate stress concentrations and avoid premature Failure at the end-tabs. Free-edge delamination is suppressed using thin-plies. Angle-ply carbon/epoxy laminates have been designed to achieve combinations of tensile and shear stresses. It is shown that in-plane shear does not significantly affect the Fibre Failure strains.

  • a finite element based statistical model for progressive tensile Fibre Failure in composite laminates
    Composites Part B-engineering, 2013
    Co-Authors: Stephen R. Hallett, Michael R Wisnom
    Abstract:

    Abstract This paper presents a two-parameter integrated Weibull model that is implemented in a finite element analysis to capture the well known “size effect” in composite materials. It can thus account for the effect of randomly distributed defects on Fibre dominated tensile Failure in a general loading situation. Here it is combined with cohesive interface elements to include the interaction with delamination. As a verification of the modelling method, scaled four-point bend tests from previous research were simulated. The models gave excellent results for both the strains to Failure and the final Failure modes and captured the observed size effect which cannot be represented by conventional progressive damage models.

  • modelling the interaction between matrix cracks and delamination damage in scaled quasi isotropic specimens
    Composites Science and Technology, 2008
    Co-Authors: Stephen R. Hallett, W G Jiang, Bijoysri Khan, Michael R Wisnom
    Abstract:

    A series of tensile tests on scaled quasi-isotropic laminates have been carried out and modelled using finite element analysis to predict Failure. Observations during testing and examination of the failed test specimens showed significant influence of matrix cracking and delamination on the final Failure. Initially the virtual crack closure technique (VCCT) was used to determine the applied load that would cause free edge delamination. Experimental results showed that Failure occurred at loads lower than those predicted. Matrix cracks, observed in the testing, were introduced into the model and interface elements were used to model the delamination development. This approach gave good correlation to the behaviour observed in the tests. Even in the specimens apparently dominated by Fibre Failure, delamination was shown to be significant.

Ashwin R. Kristnama - One of the best experts on this subject based on the ideXlab platform.

  • numerical analysis of high velocity oblique impacts and residual tensile strength of carbon epoxy laminates
    Composite Structures, 2021
    Co-Authors: Ashwin R. Kristnama, Michael R Wisnom, Stephen R. Hallett
    Abstract:

    Abstract This paper presents prediction of the high velocity, oblique impact response and quasi-static residual tensile strength of thin [45/90/−45/0]2s carbon/epoxy laminates using finite element (FE) models. A High-Fidelity Finite Element Method (Hi-FEM) with an automated unit cell meshing technique was employed. The predicted impact damage, characterised by the extent of Fibre Failure and delamination area, was validated against results from gas-gun tests for a range of impact velocities. The numerical results captured the trend of increasing impact damage with impact energy as observed from the tests. Changes in projectile orientation before impact were shown to increase the extent of Fibre Failure at high impact energies, up by 38% in edge impact cases. The residual tensile strength of the impacted laminates was then investigated, where the numerical results for edge-impacted laminates agreed with the test data within 8%. On the other hand, the residual strength modelling results of centre-impacted laminates were found to be unconservative, mainly due to the extent of Fibre Failure predicted during impact. Machined notches were also studied for their residual tensile strength in comparison to impact induced damage. The predicted strength of edge-notched laminates was found to be in close agreement with the experimental results for edge-impacted laminates, differing by an average of 9%.

  • Numerical analysis of high velocity, oblique impacts and residual tensile strength of carbon/epoxy laminates
    Composite Structures, 2021
    Co-Authors: Ashwin R. Kristnama, Michael R Wisnom, Stephen R. Hallett
    Abstract:

    Abstract This paper presents prediction of the high velocity, oblique impact response and quasi-static residual tensile strength of thin [45/90/−45/0]2s carbon/epoxy laminates using finite element (FE) models. A High-Fidelity Finite Element Method (Hi-FEM) with an automated unit cell meshing technique was employed. The predicted impact damage, characterised by the extent of Fibre Failure and delamination area, was validated against results from gas-gun tests for a range of impact velocities. The numerical results captured the trend of increasing impact damage with impact energy as observed from the tests. Changes in projectile orientation before impact were shown to increase the extent of Fibre Failure at high impact energies, up by 38% in edge impact cases. The residual tensile strength of the impacted laminates was then investigated, where the numerical results for edge-impacted laminates agreed with the test data within 8%. On the other hand, the residual strength modelling results of centre-impacted laminates were found to be unconservative, mainly due to the extent of Fibre Failure predicted during impact. Machined notches were also studied for their residual tensile strength in comparison to impact induced damage. The predicted strength of edge-notched laminates was found to be in close agreement with the experimental results for edge-impacted laminates, differing by an average of 9%.

Pete Linde - One of the best experts on this subject based on the ideXlab platform.

  • a virtual testing based search for optimum compression after impact strength in thin laminates using ply thickness hybridization and unsymmetrical designs
    Composites Science and Technology, 2020
    Co-Authors: A Sasikuma, Pete Linde, D Trias, J Llobe, I R Coza, A Turo, J. Costa
    Abstract:

    In the quest to improve the compression after impact (CAI) strength of thin laminates, ply-hybrid laminates (where plies of different thicknesses are mixed) have been used in a previous study to mitigate the Fibre Failure and, consequently, improve the CAI strength. In the same study, hybrid laminates were proposed following qualitative design rules. In this paper, we systematically look for hybrid stacking sequences with improved damage tolerance by virtually testing all the laminates in a defined design space. While the laminates in the design space are made of intermediate and thick ply grades, the baseline laminate has only intermediate grade plies. Using an in-house numerical model, we virtually tested, (impact and CAI at two impact energies), all the candidate stacking sequences. The best hybrid laminates considerably improved the CAI strength over the baseline (31% and 40% improvement for the symmetric and unsymmetrical hybrid laminates, respectively). One of the best hybrid laminates was then manufactured and tested experimentally to validate the approach. Through virtual testing, this study demonstrates the benefits of using ply thickness hybrid laminates and the feasibility of optimizing the stacking sequence for impact damage tolerance.

  • effect of ply thickness and ply level hybridization on the compression after impact strength of thin laminates
    Composites Part A-applied Science and Manufacturing, 2019
    Co-Authors: A Sasikuma, Pete Linde, D Trias, N Lanco, J O, J. Costa
    Abstract:

    There is a lack of research available on how thin laminates respond to impact and post impact loads, even though thin structures are used in present-day aircrafts. This experimental paper employs thick, standard and thin uni-directional plies to investigate the effect ply thickness has on thin laminates on their impact and compression after impact (CAI) response. Further, we propose two hybrid laminates where thick or standard plies are mixed with thin plies, respectively, in an effort to improve the CAI strength of thin laminates. Results reveal that, contrary to thick laminates, thin laminates made of only thin plies exhibit extensive Fibre Failure, leading to a considerably reduced CAI strength. Moreover, the hybrid laminate where thick 0° plies are mixed with thin plies improves the CAI strength by 40% over the thin ply baseline laminate. Thus, hybridization with thin laminates appears to be an economic prospective in terms of improving damage tolerance.

Phillip W. Duke - One of the best experts on this subject based on the ideXlab platform.

  • Understanding the Thickness Effect on the Tensile Strength Property of Dyneema®HB26 Laminates
    Materials, 2018
    Co-Authors: Lorenzo Iannucci, Stefano Del Rosso, Paul T. Curtis, Dan J. Pope, Phillip W. Duke
    Abstract:

    In this study, an experimental and numerical investigation is presented on the effect of thickness and test rate within the pseudo static regime on the tensile properties of Dyneema®HB26 laminates. A detailed experimental presentation on the tensile testing of different thickness is presented and highlights the commonly seen observation that the tensile strength of a laminate reduces as a function of the specimen thickness. To understand these experimental observations, a constitutive material model of the individual macro fibril is developed and applied to modelling the Fibre and upscaling to the laminate. The modelling strategy is implemented into ls-dyna and used to perform a parameter study on the specimen geometries used in the experimental study. The model assumes that the fibril strength is a function of the amorphous volume within the Fibre and hence fibril. It can be observed that the experimental behaviour can be simulated by modelling the interface between laminate plies and the fibril, and hence Fibre Failure. The weak interfaces from the fibril to the laminate scale make the testing of Fibres and laminates very difficult. Hence, it is proposed that the intrinsic fibril strength should be used as a measure of strength, and the fundamental strength is determined through numerical studies.

  • understanding the thickness effect on the tensile strength property of dyneema hb26 laminates
    Materials, 2018
    Co-Authors: L Iannucci, Stefano Del Rosso, Paul T. Curtis, Dan J. Pope, Phillip W. Duke
    Abstract:

    In this study, an experimental and numerical investigation is presented on the effect of thickness and test rate within the pseudo static regime on the tensile properties of Dyneema®HB26 laminates. A detailed experimental presentation on the tensile testing of different thickness is presented and highlights the commonly seen observation that the tensile strength of a laminate reduces as a function of the specimen thickness. To understand these experimental observations, a constitutive material model of the individual macro fibril is developed and applied to modelling the Fibre and upscaling to the laminate. The modelling strategy is implemented into ls-dyna and used to perform a parameter study on the specimen geometries used in the experimental study. The model assumes that the fibril strength is a function of the amorphous volume within the Fibre and hence fibril. It can be observed that the experimental behaviour can be simulated by modelling the interface between laminate plies and the fibril, and hence Fibre Failure. The weak interfaces from the fibril to the laminate scale make the testing of Fibres and laminates very difficult. Hence, it is proposed that the intrinsic fibril strength should be used as a measure of strength, and the fundamental strength is determined through numerical studies.

  • Understanding the Thickness Effect on the Tensile Strength Property of Dyneema®HB26 Laminates
    MDPI AG, 2018
    Co-Authors: Lorenzo Iannucci, Stefano Del Rosso, Paul T. Curtis, Dan J. Pope, Phillip W. Duke
    Abstract:

    In this study, an experimental and numerical investigation is presented on the effect of thickness and test rate within the pseudo static regime on the tensile properties of Dyneema®HB26 laminates. A detailed experimental presentation on the tensile testing of different thickness is presented and highlights the commonly seen observation that the tensile strength of a laminate reduces as a function of the specimen thickness. To understand these experimental observations, a constitutive material model of the individual macro fibril is developed and applied to modelling the Fibre and upscaling to the laminate. The modelling strategy is implemented into ls-dyna and used to perform a parameter study on the specimen geometries used in the experimental study. The model assumes that the fibril strength is a function of the amorphous volume within the Fibre and hence fibril. It can be observed that the experimental behaviour can be simulated by modelling the interface between laminate plies and the fibril, and hence Fibre Failure. The weak interfaces from the fibril to the laminate scale make the testing of Fibres and laminates very difficult. Hence, it is proposed that the intrinsic fibril strength should be used as a measure of strength, and the fundamental strength is determined through numerical studies