Hopkinson bar testing of cellular materials

Cellular materials are often used as impact/blast attenuators due to their capacity to absorb kinetic energy when compressed to large strains. For such applications, three key material properties are the crushing stress, plateau stress and densification strain. The difficulties associated with obtaining these mechanical properties from dynamic/impact tests are outlined. The results of an experimental investigation of the quasi-static and dynamic mechanical properties of two types of cellular materials are reported.The dynamic tests were carried out using Hopkinson pressure bars. Experimentally determined propagation coefficients are employed to represent both dispersion and attenuation effects as stress waves travel along the bars. Propagation coefficients were determined for 20 mm and 40 mm diameter viscoelastic PMMA pressure bars and for elastic Magnesium pressure bars. The use of the elementary wave theory is shown to give satisfactory results for frequencies of up to approximately 15 kHz, 8 kHz and 30 kHz for the 20 mm and 40 mm diameter PMMA bars and the 23 mm diameter Magnesium bars respectively. The use of low impedance, viscoelastic pressure bars is shown to be preferable for testing low density, low strength materials.The quasi-static and dynamic compressive properties of balsa wood, Rohacell-51WF and Rohacell-110WF foams are investigated along all three principal directions. The dynamic properties were investigated by performing Split Hopkinson Pressure Bar (SHPB) and Direct Impact (DI) tests. In general, the crushing stress, the plateau stress and the densification strain remain constant with increasing strain rate of the SHPB tests. However, a dynamic enhancement of the crushing stress and plateau stress was revealed for balsa wood and Rohacell-51WF. In contrast, the plateau stresses of the Rohacell-110WF specimens are lower for SHPB than quasi-static tests. From the DI tests, it is shown that compaction waves have negligible effect on the stresses during dynamic compaction of along and across the grain balsa wood at impact speeds between approximately 20-100 m/s. Alternatively, the proximal end stresses of both Rohacell-51WF and 110WF foams increase with increasing impact velocity, following the quadratic trend predicted by 'shock theory'. This indicates that compaction waves are important for the case of Rohacell foam, even at low impact velocities.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Through-thickness compression testing and theory of carbon fibre composite materials

This study investigates the through-thickness behaviour of carbon/epoxy laminates. A through-thickness compression test regime was conducted utilising three specimen designs, which are waisted, hollow cylindrical and cubic specimens. An assessment and comparison of each specimen is given regarding their advantages and disadvantages in characterising the through-thickness response of [+45/-45/90/0]s quasi-isotropic AS4/8552 carbon/epoxy laminates. A finite element (FE) study of the three specimens is presented which results in specimen geometries that provided a macroscopically uniform stress response throughout the gauge length whilst also minimising other features such as stress concentrations. Further to the final geometries being presented, the method of manufacture for the laminate and machining processes for each of the specimens is given. A mesoscopic FE study is presented relating to the free-edge effects induced by through-thickness loading in quasi-isotropic laminates. The results presented show that free-edge effects will be present in the test specimens and will have a larger overall impact on the hollow cylindrical specimen. The free-edge effects also increase the stress concentrations present in the corners of the waisted and cubic specimens. Characteristic stress strain curves are presented for each specimen with strain data taken from post yield strain gauges attached to the specimens. The extracted initial Young's modulus Ez and Poisson's ratios vzx and vzy show a small variation between specimens. The strength values for the three specimens vary greatly with the waisted specimen being the strongest and cylindrical specimen the weakest, indicating that the chosen specimen geometry dominates failure. The experimental data will be used for test case 12 in the Second World Wide Failure Exercise (WWFE-II). A study is presented to predict the effective elastic properties of Z-pinned laminates. The materials under consideration are UD and [0/90]s cross-ply AS4/3501-6 carbon/epoxy laminates. Estimates on the effective properties are provided by two FE approaches and two analytical bounding approaches; namely Voigt and Reuss bounds and Walpole's bounding theory. The two FE approaches are based on extreme assumptions about the in-plane fibre volume fraction in the presence of Z-pins and provide a tight range of values in which the real result should lie. Furthermore, whilst the bounding methods are simple and in the case of Young's moduli produce very wide bounds the selection of the suitable bound result can lead to a good estimate in comparison with the FE data. Typically the best bounding method result for each elastic property is within 10% of the FE predictions.EThOS - Electronic Theses Online ServiceQinetiQGBUnited Kingdo

Altered Regional and Circuit Resting-State Activity Associated with Unilateral Hearing Loss

The deprivation of sensory input after hearing damage results in functional reorganization of the brain including cross-modal plasticity in the sensory cortex and changes in cognitive processing. However, it remains unclear whether partial deprivation from unilateral auditory loss (UHL) would similarly affect the neural circuitry of cognitive processes in addition to the functional organization of sensory cortex. Here, we used resting-state functional magnetic resonance imaging to investigate intrinsic activity in 34 participants with UHL from acoustic neuroma in comparison with 22 matched normal controls. In sensory regions, we found decreased regional homogeneity (ReHo) in the bilateral calcarine cortices in UHL. However, there was an increase of ReHo in the right anterior insular cortex (rAI), the key node of cognitive control network (CCN) and multimodal sensory integration, as well as in the left parahippocampal cortex (lPHC), a key node in the default mode network (DMN). Moreover, seed-based resting–state functional connectivity analysis showed an enhanced relationship between rAI and several key regions of the DMN. Meanwhile, lPHC showed more negative relationship with components in the CCN and greater positive relationship in the DMN. Such reorganizations of functional connectivity within the DMN and between the DMN and CCN were confirmed by a graph theory analysis. These results suggest that unilateral sensory input damage not only alters the activity of the sensory areas but also reshapes the regional and circuit functional organization of the cognitive control network

Biomechanical comparison of screw-based zoning of PHILOS and Fx proximal humerus plates

Background Treatment of proximal humerus fractures with locking plates is associated with complications. We aimed to compare the biomechanical effects of removing screws and blade of a fixed angle locking plate and hybrid blade plate, on a two-part fracture model. Methods Forty-five synthetic humeri were divided into nine groups where four were implanted with a hybrid blade plate and the remaining with locking plate, to treat a two-part surgical neck fracture. Plates’ head screws and blades were divided into zones based on their distance from fracture site. Two groups acted as a control for each plate and the remaining seven had either a vacant zone or blade swapped with screws. For elastic cantilever bending, humeral head was fixed and the shaft was displaced 5 mm in extension, flexion, valgus and varus direction. Specimens were further loaded in varus direction to investigate their plastic behaviour. Results In both plates, removal of inferomedial screws or blade led to a significantly larger drop in varus construct stiffness than other zones. In blade plate, insertion of screws in place of blade significantly increased the mean extension, flexion valgus and varus bending stiffness (24.458%/16.623%/19.493%/14.137%). In locking plate, removal of screw zones proximal to the inferomedial screws reduced extension and flexion bending stiffness by 26–33%. Conclusions Although medial support improved varus stability, two inferomedial screws were more effective than blade. Proximal screws are important for extension and flexion. Mechanical consequences of screw removal should be considered when deciding the number and choice of screws and blade in clinic