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  • Journal article
    Yu X, Azor A, Sharp DJ, Mazdak Get al., 2020,

    Mechanisms of tensile failure of cerebrospinal fluid in blast traumatic brain injury

    , Extreme Mechanics Letters, Vol: 38, Pages: 1-9, ISSN: 2352-4316

    Mechanisms of blast-induced Traumatic Brain Injury (BTBI), particularly those linked to the primary pressure wave, are still not fully understood. One possible BTBI mechanism is cavitation in the cerebrospinal fluid (CSF) caused by CSF tensile failure, which is likely to increase strain and strain rate in the brain tissue near the CSF. Blast loading of the head can generate rarefaction (expansion) waves and rapid head motion, which both can produce tensile forces in the CSF. However, it is not clear which of these mechanisms is more likely to cause CSF tensile failure. In this study, we used a high-fidelity 3-dimensional computational model of the human head to test whether the CSF tensile failure increases brain deformation near the brain/CSF boundary and to determine the key failure mechanisms. We exposed the head model to a frontal blast wave and predicted strain and strain rate distribution in the cortex. We found that CSF tensile failure significantly increased strain and strain rate in the cortex. We then studied whether the rapid head motion or the rarefaction wave causes strain and strain rate concentration in cortex. We isolated these two effects by conducting simulations with pure head motion loading (i.e. prescribing the skull velocity but eliminating the pressure wave) and pure blast wave loading (i.e. eliminating head motion by fixing the skull base). Our results showed that the strain increase in the cortex was mainly caused by head motion. In contrast, strain rate increase was caused by both rapid head motion and rarefaction waves, but head motion had a stronger effect on elevating strain rate. Our results show that rapid motion of the head produced by blast wave is the key mechanism for CSF tensile failure and subsequent concentration of strain and strain rate in cortex. This finding suggests that mitigation of rapid head motion caused by blast loading needs to be addressed in the design of protective equipment in order to prevent the tensile failure

  • Journal article
    Yu X, Ghajari M, 2019,

    An assessment of blast modelling techniques for injury biomechanics research

    , International Journal for Numerical Methods in Biomedical Engineering, Vol: 35, Pages: 1-15, ISSN: 1069-8299

    Blast-induced Traumatic Brain Injury (TBI) has been affecting combatants and civilians. The blast pressure wave is thought to have a significant contribution to blast related TBI. Due to the limitations and difficulties of conducting blast tests on surrogates, computational modelling has been used as a key method for exploring this field. However, the blast wave modelling methods reported in current literature have drawbacks. They either cannot generate the desirable blast pressure wave history, or they are unable to accurately simulate the blast wave/structure interaction. In addition, boundary conditions, which can have significant effects on model predictions, have not been described adequately. Here, we critically assess the commonly used methods for simulating blast wave propagation in air (open-field blast) and its interaction with the human body. We investigate the predicted blast wave time history, blast wave transmission and the effects of various boundary conditions in 3 dimensional (3D) models of blast prediction. We propose a suitable meshing topology, which enables accurate prediction of blast wave propagation and interaction with the human head and significantly decreases the computational cost in 3D simulations. Finally, we predict strain and strain rate in the human brain during blast wave exposure and show the influence of the blast wave modelling methods on the brain response. The findings presented here can serve as guidelines for accurately modelling blast wave generation and interaction with the human body for injury biomechanics studies and design of prevention systems.

  • Journal article
    Whyte T, Stuart C, Mallory A, Ghajari M, Plant D, Siegmund GP, Cripton PAet al., 2019,

    A review of impact testing methods for headgear in sports: considerations for improved prevention of head injury through research and standards

    , Journal of Biomechanical Engineering, Vol: 141, ISSN: 0148-0731

    Standards for sports headgear were introduced as far back as the 1960s and many have remained substantially unchanged to present day. Since this time, headgear has virtually eliminated catastrophic head injuries such as skull fractures and changed the landscape of head injuries in sports. Mild traumatic brain injury (mTBI) is now a prevalent concern and the effectiveness of headgear in mitigating mTBI is inconclusive for most sports. Given that most current headgear standards are confined to attenuating linear head mechanics and recent brain injury studies have underscored the importance of angular mechanics in the genesis of mTBI, new or expanded standards are needed to foster headgear development and assess headgear performance that addresses all types of sport-related head and brain injuries. The aim of this review is to provide a basis for developing new sports headgear impact tests for standards by summarizing and critiquing: 1) impact testing procedures currently codified in published headgear standards for sports and 2) new or proposed headgear impact test procedures in published literature and/or relevant conferences. Research areas identified as needing further knowledge to support standards test development include defining sports-specific head impact conditions, establishing injury and age appropriate headgear assessment criteria, and the development of headgear specific head and neck surrogates for at-risk populations.

  • Journal article
    Siegkas P, Sharp D, Ghajari M, 2019,

    The traumatic brain injury mitigation effects of a new viscoelastic add-on liner

    , Scientific Reports, Vol: 9, Pages: 1-10, ISSN: 2045-2322

    Traumatic brain injury (TBI) affects millions of people worldwide with significant personal and social consequences. New materials and methods offer opportunities for improving designs of TBI prevention systems, such as helmets. We combined empirical impact tests and computational modelling to test the effectiveness of new viscoelastic add-on components in decreasing biomechanical forces within the brain during helmeted head impacts. Motorcycle helmets with and without the viscoelastic components were fitted on a head/neck assembly and were tested under oblique impact to replicate realistic accident conditions. Translational and rotational accelerations were measured during the tests. The inclusion of components reduced peak accelerations, with a significant effect for frontal impacts and a marginal effect for side and rear impacts. The head accelerations were then applied on a computational model of TBI to predict strain and strain-rate across the brain. The presence of viscoelastic components in the helmet decreased strain and strain-rate for frontal impacts at low impact speeds. The effect was less pronounced for front impact at high speeds and for side and rear impacts. This work shows the potential of the viscoelastic add-on components as lightweight and cost-effective solutions for enhancing helmet protection and decreasing strain and strain-rate across the brain during head impacts.

  • Journal article
    Wang H, Kow J, Raske N, De Boer G, Ghajari M, Hewson RW, Alazmani A, Culmer Pet al., 2017,

    Robust and high-performance soft inductive tactile sensors based on the Eddy-current effect

    , Sensors and Actuators A: Physical, Vol: 271, Pages: 44-52, ISSN: 0924-4247

    Tactile sensors are essential for robotic systems to interact safely and effectively with the external world, they also play a vital role in some smart healthcare systems. Despite advances in areas including materials/composites, electronics and fabrication techniques, it remains challenging to develop low cost, high performance, durable, robust, soft tactile sensors for real-world applications. This paper presents the first Soft Inductive Tactile Sensor (SITS) which exploits an inductance-transducer mechanism based on the eddy-current effect. SITSs measure the inductance variation caused by changes in AC magnetic field coupling between coils and conductive films. Design methodologies for SITSs are discussed by drawing on the underlying physics and computational models, which are used to develop a range of SITS prototypes. An exemplar prototype achieves a state-of-the-art resolution of 0.82 mN with a measurement range over 15 N. Further tests demonstrate that SITSs have low hysteresis, good repeatability, wide bandwidth, and an ability to operate in harsh environments. Moreover, they can be readily fabricated in a durable form and their design is inherently extensible as highlighted by a 4 × 4 SITS array prototype. These outcomes show the potential of SITS systems to further advance tactile sensing solutions for integration into demanding real-world applications.

  • Journal article
    de Boer G, Raske N, Wang H, Kow J, Alazmani A, Ghajari M, Culmer P, Hewson Ret al., 2017,

    Design optimisation of a magnetic field based soft tactile sensor

    , Sensors, Vol: 17, Pages: 1-20, ISSN: 1424-8220

    This paper investigates the design optimisation of a magnetic field based soft tactile sensor, comprised of a magnet and Hall effect module separated by an elastomer. The aim was to minimise sensitivity of the output force with respect to the input magnetic field; this was achieved by varying the geometry and material properties. Finite element simulations determined the magnetic field and structural behaviour under load. Genetic programming produced phenomenological expressions describing these responses. Optimisation studies constrained by a measurable force and stable loading conditions were conducted; these produced Pareto sets of designs from which the optimal sensor characteristics were selected. The optimisation demonstrated a compromise between sensitivity and the measurable force, a fabricated version of the optimised sensor validated the improvements made using this methodology. The approach presented can be applied in general for optimising soft tactile sensor designs over a range of applications and sensing modes.

  • Conference paper
    Ghajari M, Hellyer PJ, Sharp DJ, 2017,

    Predicting the location of chronic traumatic encephalopathy pathology

    , 2017 IRCOBI Conference, Publisher: International Research Council on Biomechanics of Injury (IRCOBI), Pages: 699-700, ISSN: 2235-3151

    Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease linked to head impacts. Its distinctive neuropathologic feature is deposition of tau proteins in sulcal depths and in perivascular regions. Previous work has investigated pathological and clinical features of CTE, and here the authors report recent work on exploring the link between strain and strain rate distribution within the brain and location of CTE pathology. The authors used a high fidelity finite element (FE) model of traumatic brain injury (TBI) to test the hypothesis that strain and strain rate produced by head impacts are greatest in sulci, where neuropathology is prominently seen in CTE. The authors also analyzed diffusion tensor imaging (DTI) data from a large cohort of TBI patients to provide converging evidence from empirical neuroimaging data for the model’s prediction.

  • Journal article
    Ghajari M, Hellyer P, Sharp D, 2017,

    Computational modelling of traumatic brain injury predicts the location of chronic traumatic encephalopathy pathology

    , Brain, Vol: 140, Pages: 333-343, ISSN: 0006-8950

    Traumatic brain injury can lead to the neurodegenerative disease chronic traumatic encephalopathy. This condition has a clear neuropathological definition but the relationship between the initial head impact and the pattern of progressive brain pathology is poorly understood. We test the hypothesis that mechanical strain and strain rate are greatest in sulci, where neuropathology is prominently seen in chronic traumatic encephalopathy, and whether human neuroimaging observations converge with computational predictions. Three distinct types of injury were simulated. Chronic traumatic encephalopathy can occur after sporting injuries, so we studied a helmet-to-helmet impact in an American football game. In addition, we investigated an occipital head impact due to a fall from ground level and a helmeted head impact in a road traffic accident involving a motorcycle and a car. A high fidelity 3D computational model of brain injury biomechanics was developed and the contours of strain and strain rate at the grey matter–white matter boundary were mapped. Diffusion tensor imaging abnormalities in a cohort of 97 traumatic brain injury patients were also mapped at the grey matter–white matter boundary. Fifty-one healthy subjects served as controls. The computational models predicted large strain most prominent at the depths of sulci. The volume fraction of sulcal regions exceeding brain injury thresholds were significantly larger than that of gyral regions. Strain and strain rates were highest for the road traffic accident and sporting injury. Strain was greater in the sulci for all injury types, but strain rate was greater only in the road traffic and sporting injuries. Diffusion tensor imaging showed converging imaging abnormalities within sulcal regions with a significant decrease in fractional anisotropy in the patient group compared to controls within the sulci. Our results show that brain tissue deformation induced by head impact loading is greatest in sulcal

  • Journal article
    Wang H, de Boer G, Kow J, Ghajari M, Alazmani A, Hewson R, Culmer Pet al., 2017,

    A low-cost soft tactile sensing array using 3D hall sensors

    , Procedia Engineering, Vol: 168, Pages: 650-653, ISSN: 1877-7058

    Tactile sensors are essential for robotic systems to safely interact with the external world and to precisely manipulate objects. Existing tactile sensors are typically either expensive or limited by poor performance, and most are not mechanically compliant. This work presents MagTrix, a soft tactile sensor array based on four 3D Hall sensors with corresponding permanent magnets. MagTrix has the capability to precisely measure triaxis force (1 mN resolution) and to determine contact area. In summary, the presented tactile sensor is robust, low-cost, high-performance and easily customizable to be integrated into a range of robotic and healthcare applications.

  • Journal article
    Ghajari M, Farajzadeh Khosroshahi S, 2016,

    Optimization of the chin bar of a composite-shell helmet to mitigate the upper neck force

    , Applied Composite Materials, Vol: 24, Pages: 931-944, ISSN: 1573-4897

    The chin bar of motorcyclefull-face helmets is the most likely region of the helmet tosustain impactsduring accidents, with alarge percentageof these impacts leadingto basilar skull fracture. Currently, helmet chin bars are designed to mitigate the peak acceleration at the centre of gravityof isolated headforms, as required by standards, but they are not designed to mitigate the neck force, which is probably the cause of basilar skull fracture, a type of head injury that can lead to fatalities. Here we test whether it is possible to increase the protection of helmet chin bars while meeting standard requirements. Fibre-reinforced composite shells are commonly used in helmets due to their lightweight and energy absorption characteristics. We optimize the ply orientation of a chin bar made of fibre-reinforced composite layersfor reduction of the neck force in a dummy modelusing a computational approach. We use thefinite element model of a human head/neck surrogateandmeasure the neck axial force, which has been shown to be correlated withthe risk of basilar skull fracture. The results show that by varying the orientationof the chin bar plies, thus keeping the helmetmass constant, the neck axial force can be reduced by approximately 30%while ensuring that the helmet complies withtheimpact attenuation requirementsprescribed in helmet standards.

  • Journal article
    Wang H, de Boer G, Kow J, Alazmani A, Ghajari M, Hewson R, Culmer Pet al., 2016,

    Design methodology for magnetic field-based soft tri-axis tactile sensors

    , Sensors, Vol: 16, Pages: 1-20, ISSN: 1424-8220

    Tactile sensors are essential if robots are to safely interact with the external world and to dexterously manipulate objects. Current tactile sensors have limitations restricting their use, notably being too fragile or having limited performance. Magnetic field-based soft tactile sensors offer a potential improvement, being durable, low cost, accurate and high bandwidth, but they are relatively undeveloped because of the complexities involved in design and calibration. This paper presents a general design methodology for magnetic field-based three-axis soft tactile sensors, enabling researchers to easily develop specific tactile sensors for a variety of applications. All aspects (design, fabrication, calibration and evaluation) of the development of tri-axis soft tactile sensors are presented and discussed. A moving least square approach is used to decouple and convert the magnetic field signal to force output to eliminate non-linearity and cross-talk effects. A case study of a tactile sensor prototype, MagOne, was developed. This achieved a resolution of 1.42 mN in normal force measurement (0.71 mN in shear force), good output repeatability and has a maximum hysteresis error of 3.4%. These results outperform comparable sensors reported previously, highlighting the efficacy of our methodology for sensor design.

  • Conference paper
    de Boer GN, Wang H, Ghajari M, Alazmani A, Hewson R, Culmer Pet al., 2016,

    Force and Topography Reconstruction Using GP and MOR for the TACTIP Soft Sensor System

    , 17th Annual Conference, TAROS 2016, Publisher: Springer International Publishing, Pages: 65-74, ISSN: 0302-9743
  • Journal article
    Sharif-Khodaei Z, Ghajari M, Aliabadi MH, 2016,

    Impact damage detection in composite plates using a self-diagnostic electro-mechanical impedance-based structural health monitoring system

    , Journal of Multiscale Modelling, Vol: 6, ISSN: 1756-9737

    In this work, application of the electro-mechanical impedance (EMI) method in structural health monitoring as a damage detection technique has been investigated. A damage metric based on the real and imaginary parts of the impedance measures is introduced. Numerical and experimental tests are carried out to investigate the applicability of the method for various types of damage, such as debonding between the transducers and the plate, faulty sensors and impact damage in composite plates. The effect of several parameters, such as environmental effects, frequency sweep, severity of damage, location of damage, etc., on the damage metric has been reported.

  • Conference paper
    Psarras S, Ghajari M, Robinson P, 2015,

    Multiple impact performance of composite fuselage panel

    This research investigates the post-impact behaviour of composite fuselage panels subjected to multi-site low-velocity impacts. Large curved stiffened panels (1.2m x 0.8m, with composite skins/stiffeners and aluminum frames) of two different skin thicknesses were subjected to sequential drop-weight impacts at locations previously determined to be critical in finite element (FE) simulations. After assessment of the impact damage each panel was tested in compression. High speed video, strain gauges, Digital Image Correlation and acoustic emission were used to monitor the failure development and to provide data for comparison with the FE simulations. The FE models, which were based on a mesomechanical approach, showed a good agreement with both the impact damage and the subsequent compression performance.

  • Journal article
    Thiene M, Ghajari M, Galvanetto U, Aliabadi MHet al., 2014,

    Effects of the transfer function evaluation on the impact force reconstruction with application to composite panels

    , Composite Structures, Vol: 114, Pages: 1-9, ISSN: 0263-8223

    The determination of a reliable transfer function for force reconstruction of impacts on composite panels is addressed in the paper. The reconstruction of the impact force history requires the knowledge of the transfer function which relates the response to the contact force. In this paper, a new method to determine the transfer function of a composite plate, instrumented with surface bonded piezoelectric sensors, is proposed. Impact tests are carried out and the data are used to evaluate the transfer function. The force reconstruction results, obtained by using the new transfer function, are compared with the results obtained with the classic approach. Significant improvements are observed in predicting the force history, particularly when large deflections are present; these are quantifiable as an out of plane displacement of the same order of magnitude as the thickness of the plate. The influence of increasing the impact velocity, with the related increase in the contact force, is also studied. The proposed method provides good results over a range of impact velocities. Multiple impacts were also investigated and the method could correctly reconstruct force histories of consecutive impacts.

  • Journal article
    Ghajari M, Iannucci L, Curtis P, 2014,

    A peridynamic material model for the analysis of dynamic crack propagation in orthotropic media

    , Computer Methods in Applied Mechanics and Engineering, Vol: 276, Pages: 431-452, ISSN: 0045-7825

    A new material model for the dynamic fracture analysis of anisotropic materials has been proposed within the framework of the bond-based peridynamic theory. This model enables predicting complex fracture phenomena such as spontaneous crack nucleation and crack branching, curving and arrest, a capability inherited from the bond-based peridynamic theory. An important feature of the model is that the bond properties, i.e. the stiffness constant and critical stretch, are continuous functions of bond orientation in the principal material axes. This facilitates fracture analysis of anisotropic materials with random orientations, such as polycrystalline microstructures. Elastic and fracture behaviour of the model has been verified through simulating uniaxial tension of a composite plate and fracture of a cortical bone compact tension specimen, and making quantitative comparisons to analytical and experimental data. To further demonstrate the capabilities of the proposed model, dynamic fracture of a polycrystalline microstructure (alumina ceramic) has been simulated. The influence of the grain boundary and grain interior fracture energies on the interacting and competing fracture modes of polycrystalline materials, i.e. intergranular and transgranular fracture, has been studied.

  • Conference paper
    Psarras S, Ghajari M, Robinson P, Iannucci Let al., 2014,

    Performance of composite plates after multi-site impacts

    Sequential multi-site low velocity impacts (LVI) were performed on CFRP composite plates with different thicknesses and the compression after impact (CAI) behaviour was then investigated. A modified CAI rig was designed and manufactured for testing thin composites. A Finite Element (FE) model of the laminate was developed using continuum shell elements. Layers of cohesive elements were inserted between sublaminates in order to model delamination initiation and growth during impacts. An energy-based damage model, developed at Imperial College and implemented into the Abaqus FE system as a user material subroutine, was employed to represent translaminar damage. Finally, the experimentally observed behaviour of the impacted specimens and the accuracy of the FE predictions are discussed.

  • Book
    Childs PRN, Bull AMJ, Ghajari M, 2013,

    Helmet Performance and Design

    , Publisher: DEG, ISBN: 978-0-9572298-3-9
  • Conference paper
    Thiene M, Galvanetto U, Ghajari M, Aliabadi MHet al., 2013,

    A frequency analysis applied to force identification

    , Fifth International Conference on Structural Engineering, Mechanics and Computation
  • Journal article
    Ghajari M, Sharif-Khodaei Z, Aliabadi MH, Apicella Aet al., 2013,

    Identification of impact force for smart composite stiffened panels


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