Browsing by Author "Mohamed, Galal"

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    Cohesive element formulation for z-pin delamination bridging in fibre reinforced laminates
    (Elsevier, 2017-03-26) Mohamed, Galal; Allegri, Giuliano; Yasaee, Mehdi; Hallett, Stephen R.
    Z-pins are an effective method of reinforcing laminated composite materials for resisting the propagation of delamination. In this paper, a novel numerical method combines the classical cohesive finite element (FE) method with a semi-analytical z-pin crack bridging model. Special purpose cohesive elements, in which the generalized traction-displacement characteristics are provided by the semi-analytical model z-pin bridging map, are implemented in macro-scale FE models. This cohesive element offers the flexibility to employ two cohesive laws concurrently for prediction of delamination propagation, for both the pinned and unpinned behaviour. Its efficacy is evaluated by the simulation of double cantilever beam (DCB), mixed-mode bend (MMB), and pure mode II End-Loaded Split (ELS) fracture tests at 2% z-pin areal density. The numerical results in terms of load-deflection predictions agree well with experiments. The different simulations were all performed using a single set of input parameters derived from single z-pin tests with no fitting factors.
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    Dynamic mode II delamination in through thickness reinforced composites
    (Springer, 2016-09-21) Yasaee, Mehdi; Mohamed, Galal; Pellegrino, Antonio; Petrinic, Nik; Hallett, Stephen R.
    Through thickness reinforcement (TTR) technologies have been shown to provide effective delamination resistance for laminated composite materials. The addition of this reinforcement allows for the design of highly damage tolerant composite structures, specifically when subjected to impact events. The aim of this investigation was to understand the delamination resistance of Z-pinned composites when subjected to increasing strain rates. Z-pinned laminated composites were manufactured and tested using three point end notched flexure (3ENF) specimens subjected to increasing loading rates from quasi-static (~0m/s) to high velocity impact (5m/s), using a range of test equipment including drop weight impact tower and a split Hopkinson bar (SHPB). Using a high speed impact camera and frame by frame pixel tracking of the strain rates, delamination velocities as well as the apparent fracture toughness of the Z-pinned laminates were measured and analysed. Experimental results indicate that there is a transition in the failure morphology of the Z-pinned laminates from quasi-static to high strain rates. The fundamental physical mechanisms that generate this transition are discussed.
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    Influence of Z-pin embedded length on the interlaminar traction response of multi-directional composite laminates
    (Elsevier, 2016-11-10) Yasaee, Mehdi; Bigg, Lawrence; Mohamed, Galal; Hallett, Stephen R.
    The work in this paper investigated the performance of composites through-thickness reinforcing Z-pins as a function of their embedded length in pre-preg laminates. Single Z-pins were inserted into multidirectional carbon fibre laminates with increasing thicknesses, corresponding to embedded lengths from 1 mm to 10 mm and tested through a range of mixed mode displacement ratios to investigate their interlaminar bridging traction response. Detailed analysis of the tests revealed a non-linear tangential friction response and its strong dependence on the embedded length of the Z-pin. Using a new power law empirical relationship for the tangential friction force per unit length, a modified Z-pin bridging traction analytical model was proposed, giving good predictions of the full mixed mode bridging mechanics of a CFRP Z-pin in a multidirectional composite laminate of varying thickness. Several characteristics of the model are discussed and their influence on predicting the Z-pin bridging energy response have been analysed.
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    Interaction of Z-pins with multiple mode II delaminations in composite laminates
    (Springer, 2016-05-31) Yasaee, Mehdi; Mohamed, Galal; Hallett, Stephen R.
    The application of Z-pinning is a subject of great interest in the field of through-thickness reinforcement (TTR) of composite laminates. To date, the majority of Z-pin characterisation work has been conducted on fracture coupons containing a single embedded delamination, which is often not representative of real failure of reinforced composite structures in service. In this investigation a test procedure to produce two independent Mode II delaminations was developed to analyse their interaction with a region of Z-pin reinforcement. Initially numerical models were used to optimise the chosen configuration. Experimental results show in detail the response of Z-pins to two independent delaminations. These results highlight the ability of the Z-pins to effectively arrest mode II delaminations at multiple levels through the sample thickness. Additionally they provide a much needed data set for validation and verification of Z-pin numerical modelling tools.
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    Strain rate dependence of mode II delamination resistance in through thickness reinforced laminated composites
    (Elsevier, 2017-05-04) Yasaee, Mehdi; Mohamed, Galal; Pellegrino, Antonio; Petrinic, Nik; Hallett, Stephen R.
    A thorough experimental procedure is presented in which the mode II delamination resistance of a laminated fibre reinforced plastic (FRP) composite with and without Z-pins is characterised when subjected to increasing strain rates. Standard three-point End Notched Flexure (3ENF) specimens were subjected to increasing displacement loading rates from quasi-static (∼0 m/s) to high velocity impact (5 m/s) using a range of test equipment including drop weight impact tower and a Modified Hopkinson Bar apparatus for dynamic three-point bending tests. The procedure outlined uses compliance based approach to calculate the fracture toughness which was shown to produce acceptable values of GIIC for all loading rates. Using detailed high resolution imaging relationships between delamination velocities, apparent fracture toughness, longitudinal and shear strain rates were measured and compared. Confirming behaviours observed in literature, the thermosetting brittle epoxy composite showed minor increase in GIIC with increase in strain rate. However, the Z-pinned specimens showed a significant increase in the apparent GIIC with loading rate. This highlights the need to consider the strain rate dependency of the Z-pinned laminates when designing Z-pinned structures undergoing impact.