Browsing by Author "Hallett, Stephen R."
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Item Open Access Bridging mechanisms of through-thickness reinforcement in dynamic mode I&II delamination(Elsevier, 2017-04-13) Cui, Hao; Yasaee, Mehdi; Kalwak, Gordon; Pellegrino, Antonio; Partridge, Ivana K.; Hallett, Stephen R.; Allegri, Giuliano; Petrinic, NikZ-pin through-thickness reinforcement is used to improve the impact resistance of composite structures; however, the effect of loading rate on Z-pin behaviour is not well understood. The dynamic response of Z-pins in mode I and II delamination of quasi-isotropic IM7/8552 laminates was characterized experimentally in this work. Z-pinned samples were loaded at both quasi-static and dynamic rates, up to a separation velocity of 12 m/s. The efficiency of Z-pins in mode I delamination decreased with loading rate, which was mainly due to the change in the pin misalignment, the failure surface morphology and to inertia. The Z-pins failed at small displacements in the mode II loading experiments, resulting in much lower energy dissipation in comparison with the mode I case. The total energy dissipation decreased with increasing loading rate, while enhanced interfacial friction due to failed pins may be largely responsible for the higher energy dissipation in quasi-static experiments.Item Open Access 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.Item Open Access Coupon scale Z-pinned IM7/8552 delamination tests under dynamic loading(Elsevier, 2019-08-01) Cui, Hao; Mahadik, Yusuf; Hallett, Stephen R.; Partridge, Ivana K.; Allegri, Giuliano; Ponnusami, Sathiskumar A.; Petrinic, NikDynamic impact onto laminated composite structures can lead to large-scale delamination. This can be mitigated by the introduction of through-thickness reinforcement, such as z-pins. Here, mode I & II and mixed-mode delamination tests have been designed and conducted at high loading rate, for both unpinned and Z-pinned coupons to study the effect of rate of loading. It was found that the Z-pins were not effective in delaying the dynamic crack initiation or resisting the dynamic propagation of delaminations shorter than 5 mm. However, the further growth of cracks was substantially delayed by Z-pinning, especially for the pure mode I and mode I dominated failure modes. On the other hand, the effectiveness of Z-pins in shear tests was relatively modest. The mode I dominated delamination resistance of Z-pinned laminates was found to be sensitive to the loading rate.Item Open Access Dynamic bridging mechanisms of through-thickness reinforced composite laminates in mixed mode delamination(Elsevier, 2017-11-23) Cui, Hao; Yasaee, Mehdi; Hallett, Stephen R.; Partridge, Ivana K.; Allegri, Giuliano; Petrinic, NikDelamination resistance of composite laminates can be improved with through-thickness reinforcement such as Z-pinning. This paper characterises the bridging response of individual carbon fibre/BMI Z-pins in mixed mode delamination at high loading rate using a split Hopkinson bar system. The unstable failure process in quasi-static tests, was also captured with high sampling rate instruments to obtain the complete bridging response. The energy dissipation of the Z-pins were analysed, and it was found that the efficacy of Z-pinning in resisting delamination growth decreased with an increase in mixed mode ratio, with a transition from pull-out to pin rupture occurring. The Z-pin efficacy decreased with loading rate for all mode mix ratios, due to the changing in failure surface with loading rate and rate-dependent frictional sliding.Item Open Access Dynamic bridging response of through-thickness reinforcement in composite laminates(International Committee on Composite Materials, 2017-12-31) Cui, Hao; Melro, António R.; Mahadik, Yusuf; Yasaee, Mehdi; Allegri, Giuliano; Partridge, Ivana K.; Hallett, Stephen R.; Petrinic, NikThe present experimental study aims to extend the understanding of delamination crack bridging mechanisms in Z-pinned laminates subjected to highly dynamic loading conditions. The bridging response of single Z-pins was characterized with both quasi-static and high loading rate. Standard delamination tests of Z-pinned laminates were carried out at varying velocity. The experimental results at both length scales showed that Z-pin efficiency in improving delamination resistance decreases with increasing loading rate.Item Open Access 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.Item Open Access 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.Item Open Access 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.Item Open Access On the delamination self-sensing function of Z-pinned composite laminates(Elsevier, 2016-03-19) Zhang, B.; Allegri, Giuliano; Yasaee, Mehdi; Hallett, Stephen R.; Partridge, Ivana K.This paper investigates for the first time the usage of through-thickness reinforcement for delamination detection in self-sensing composite laminates. Electrically conductive T300/BMI Z-pins are considered in this study. The through-thickness electrical resistance is measured as the delamination self-sensing variable, both for conductive and non-conductive laminates. The Z-pin ends are connected to a resistance measurement circuit via electrodes arranged on the surface of the laminate. The delamination self-sensing function enabled by conductive Z-pins is characterised for Mode I/II delamination bridging, using single Z-pin coupons. Experiment results show that, if the through-thickness reinforced laminate is electrically conductive, the whole Z-pin pull-out process associated with delamination bridging can be monitored. However, for a non-conductive laminate, delamination bridging may not be sensed after the Z-pin is pulled out from one of the surface electrodes. Regardless of the electrical properties of the reinforced laminate, the through-thickness electrical resistance is capable of detecting Mode II bridging, albeit there exists an initial “blind spot” at relatively small lateral deformation. However, the Z-pin rupture can be clearly detected as an abrupt resistance increase. This study paves the way for exploring multi-functional applications of through-thickness reinforcement.Item Open Access Soft body impact on composites: delamination experiments and advanced numerical modelling(Elsevier, 2021-03-20) Selvaraj, Jagan; Kawashita, Luiz F.; Kalwak, Gordon; Hallett, Stephen R.Cohesive interface elements have become commonly used for modelling composites delamination. However, a limitation of this technique is the fine mesh size required. Here, a novel cohesive element formulation is proposed and demonstrated for modelling the numerical cohesive zone with equal fidelity but fewer elements in comparison to a linear cohesive element formulation. The newly proposed formulation has additional degrees of freedom in the form of nodal rotations which when combined with the use of multiple integration points per cohesive element, allows for delamination propagation to be modelled with increased stability. This element formulation is introduced with an adaptive modelling method, termed Adaptive Mesh Segmentation (AMS). To demonstrate its effectiveness under impact loading the new model is applied to a soft body beam bending test. This test, containing a delamination pre-crack, uses inertial constraints and results in a dynamic stress state when impacted by a gelatin cylinder.Item Open Access 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.Item Open Access Suppressing delaminations in composites across a range of loading modes(2017-08-31) M’membe, Beene; Yasaee, Mehdi; Hallett, Stephen R.; Partridge, Ivana K.This study presents two means of achieving high fracture toughness throughout the entire mixed Mode I/II test range, using customised placement of composite and metal Z-pins in hybrid arrays, and novel hybrid metal/composite Z-pins. The study shows that hybrid arrays that contain an equal number of composite and metal Z-pins exhibit a notable increase in the apparent fracture toughness in Mode II compared to 100% composite pins, while maintaining adequate Mode I performance. Hybrid metal/composite Z-pins, which consist of a composite exterior and a metal core have been shown to offer a single Z-pin solution for high fracture toughness under mixed Mode I/II loads without compromising either Mode I or Mode II performance of individual composite or metal Z-pins respectively. The composite exterior of the hybrid Z-pin ensures high resistance to pull-out failure, whilst the metal core guarantees high energy absorption at high mixed mode load angles via plastic deformation.