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

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

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

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• Conference paper
  de Boer GN, Wang H, Ghajari M, Alazmani A, Hewson R, Culmer Pet al., 2016,
  , 17th Annual Conference, TAROS 2016, Publisher: Springer International Publishing, Pages: 65-74, ISSN: 0302-9743
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• Journal article
  Sharif-Khodaei Z, Ghajari M, Aliabadi MH, 2016,
  , 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.

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

  MULTIPLE IMPACT PERFORMANCE OF COMPOSITE FUSELAGE PANEL

  , 20th International Conference on Composite Materials (ICCM), Publisher: AALBORG UNIV PRESS
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• Journal article
  Thiene M, Ghajari M, Galvanetto U, Aliabadi MHet al., 2014,
  , 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.

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

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• 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.

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  • Citations: 1
• Book
  Childs PRN, Bull AMJ, Ghajari M, 2013,

  Helmet Performance and Design

  , Publisher: DEG, ISBN: 978-0-9572298-3-9
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• 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
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• Journal article
  Ghajari M, Sharif-Khodaei Z, Aliabadi MH, Apicella Aet al., 2013,
  , SMART MATERIALS AND STRUCTURES, Vol: 22, ISSN: 0964-1726
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  • Citations: 98
• Conference paper
  Ghajari M, Caserta GD, Galvanetto U, 2013,

  The Impact Attenuation Test of Motorcycle Helmet Standards

  , First International Conference on Helmet Performance and Design
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• Journal article
  Ghajari M, Peldschus S, Galvanetto U, Iannucci Let al., 2013,
  , ACCIDENT ANALYSIS AND PREVENTION, Vol: 50, Pages: 263-271, ISSN: 0001-4575
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  • Citations: 43
• Conference paper
  Khodaei ZS, Ghajari M, Aliabadi MH, Apicella Aet al., 2013,
  , 11th International Conference on Fracture and Damage Mechanics, Publisher: TRANS TECH PUBLICATIONS LTD, Pages: 581-+, ISSN: 1013-9826
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  • Citations: 10
• Conference paper
  Ghajari M, Khodaei ZS, Aliabadi MH, 2013,
  , 11th International Conference on Fracture and Damage Mechanics, Publisher: TRANS TECH PUBLICATIONS LTD, Pages: 565-568, ISSN: 1013-9826
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  • Citations: 3
• Journal article
  Sharif-Khodaei Z, Ghajari M, Aliabadi MH, 2012,
  , SMART MATERIALS AND STRUCTURES, Vol: 21, ISSN: 0964-1726
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  • Citations: 122
• Conference paper
  Ghajari M, Khodaei ZS, Aliabadi MH, 2012,
  , 10th International Conference on Fracture and Damage Mechanics (FDM 2011), Publisher: TRANS TECH PUBLICATIONS LTD, Pages: 767-770, ISSN: 1013-9826
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  • Citations: 21

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