Browsing by Author "Govender, Reuben"

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    Open Access
    Design and implementation of a high strain Town rate biaxial tension test for elastomeric materials and biological soft tissue
    (2020) Graham, Aaron; Govender, Reuben; Cloete Trevor
    The mechanical properties of biological tissues are of increasing research interest to disciplines as varied as designers of protective equipment, medical researchers and even forensic Finite Element Analysis (FEA). The mechanical properties of biological tissue such as skin are relatively well known at low strain rates and strains, but there is a paucity of data on the high rate, high strain behaviour of skin - particularly under biaxial tension. Biaxial tensile loading mimics in vivo conditions more closely than uniaxial loading [1, 2], and is necessary in order to characterise a hyper-elastic material model[3]. Furthermore, biaxial loading allows one to detect the anisotropy of the sample without introducing noise from inter-sample variability - unlike uniaxial tensile testing. This work develops a high strain rate bulge test device capable of testing soft tissue or polymer membranes at high strain rates. The load history as well as the full field displacement data is captured via a pressure transducer and high speed 3D Digital Image Correlation (DIC). Strain rates ranging from 0.26s −1 to 827s −1 are reliably achieved and measured. Higher strain rates of up to 2500s −1 are achieved, but are poorly measured due to equipment limitations of the high speed cameras used. The strain rates achieved had some variability, but were significantly more consistent than those achieved by high rate biaxial tension tests found in the literature. In addition to control of the apex strain rate, the bi-axial strain ratio is controlled via the geometry of the specimen fixture. This allowed for strain ratios of up to 2 to be achieved at the apex 1 . When testing anisotropic membranes, the use of full field 3D DIC allowed for accurate and efficient detection of the principal axis of anisotropy in the material. No skin is tested, but instead three types of polydimethylsiloxane (PDMS, ”silicone') skin simulant are tested. These simulants were chosen to fully encapsulate the range of mechanical behaviour expected from skin - they were chosen to have stiffness's, strain hardening exponents and degrees of anisotropy significantly above or below the behaviour exhibited by skin. This ensured that the device was validated over a wider range of conditions than expected when testing skin. A novel approach to specimen fixation and speckling for silicone membranes is developed, as well as a fibre reinforced skin simulant that closely mimics the rate hardening and anisotropic behaviour of skin. In addition to bulge tests, uniaxial tensile tests are conducted on the various simulant materials in order to characterise their low strain rate behaviour. The composite skin simulant is characterised using a modified version of the anisotropic skin model developed by Weiss et al (1996) [4], and the pure silicone membranes are characterised using the Ogden hyper-elastic model.
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    Design, manufacture and commissioning of a low pressure quasistatic bulge tester for skin and membrane tissue
    (2020) Curry, Andrew Michael; Govender, Reuben; Nurick, Gerald
    The material properties of skin are of great importance to a variety of fields such as dermatology and reconstructive surgery. Relatively little infrastructure and expertise exists locally in South Africa for testing biological tissue. The difficulties of testing the material properties of skin are the non-uniformity and anisotropy across specimen location and subjects. This anisotropy is most commonly measured by tensile testing of samples cut in different orientations. However, the individual samples at different orientations would be extracted from slightly different locations on the same subject, which will naturally vary in response. Bulge testing is a method of determining response to tension in different directions at the same location, by applying biaxial tension. It uses a positive pressure applied to a peripherally clamped specimen to deform the specimen in a balloon type manner. In this project, bulge testing apparatus was designed and built for the purpose of testing skin and membrane tissue, under biaxial tension. The testing apparatus consists of a syringe pump to control inflation of a specimen, which is clamped in an inflation chamber. Digital Image Correlation (DIC) was used to capture the 3D deformation fields of the specimen, and hence infer the strain fields. To simplify commissioning testing, a commercial silicone elastomer suited for skin prosthetics, was used to manufacture specimens for uniaxial and bulge experimental testing. Two types of bulge specimens were manufactured, standard round specimens and elliptical specimens. The round specimens were used to compare their material response to uniaxial tests and the elliptical bulge specimens were used to simulate the anisotropic response of skin. The method of analysis used in this project is based on using DIC and curvature calculations at multiple points to calculate membrane stresses in principal directions. The method of calculating principal curvatures from DIC is adapted from the work by Machado et al. [1] that calculated Gaussian curvature using the first and second fundamental forms of a surface. In total 18 round, 6 elliptical and 10 uniaxial specimens were tested and the material properties were found to vary slightly between each specimen. The spread in data between the uniaxial and bulge tests was found to be very similar with the bulge data showing 10 % spread at 1.2 stretch and constant 8 % spread above 1.2 stretch and the uniaxial data showing increasing spread from 7 % to 15 %. The curvature results showed very clear principal directions of curvature for the elliptical specimens. This demonstrated that the method used in this project is capable of clearly extracting the orientations of stiffer fibre directions of skin and other collagenous tissue.
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    Development of image based crack measurements, to investigate delamination in different weave Fibre Reinforced Polymers
    (2019) Harnekar, Abraar; Govender, Reuben
    This project investigates the delamination dependence of fibre reinforced polymers, of different weave patterns, using an image-based crack measurement method. Glass fibre reinforce polymer (FRP) with three different weave pattern namely Unidirectional, Plain weave and Twill weave patterns were manufactured using the infusion process. Waterjet cutting was used to cut the panels to produce the test specimens. The Double Cantilever Beam (DCB) test was used to measure the Mode I fracture toughness, following the standardised test method ASTM D5528. DCB tests requires two hinge blocks to be bonded to the specimens and is conducted using the Zwick machine which actively measures force and opening displacement. In order to calculate the fracture toughness from a DCB test, the crack length must be measured. An image-based crack measurement method was developed, using still images that were extracted from a digital video of the DCB experiment. The image-base method involved scripting a MATLAB file to detect the specimen surface as edges. The specimen was painted white and the test had a black background. This caused a sharp change in intensity which made the specimen edge easier to detect. The detection algorithm only catered for accuracy and not speed. A series of tests were conducted to verify the detection algorithm, of which included designing an Ultrasound Wedge device. The Wedge device was used to emulate a DCB tests in a static position whereby an Ultrasound Thickness Tester was used to obtain and verify the position of the crack tip obtained by the algorithm. DCB tests showed that the Twill weave specimens had the greatest resistance to delamination, while the Unidirectional weave offered the least resistance to delamination. The Plain weave pattern was inconclusive due to the large variation between the Plain weave specimens.
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    Rapid acceleration of legged robots: a pneumatic approach
    (2021) Van Zyl, Joshua; Patel, Amir; Govender, Reuben
    For robotics to be useful to the public in a multifaceted manner, they need to be both legged and agile. The legged constraint arises as many environments and systems in our world are tailored to ablebodied adults. Therefore, a practically useful robot would need to have the same morphology for maximum efficacy. For robots to be useful in these environments, they need to perform at least as well as humans, therefore presenting the agility constraint. These requirements have been out of reach of the field until recently. The aim of this thesis was to design a planar monopod robot for rapid acceleration manoeuvres, that could later be expanded to a planar quadruped robot. This was achieved through a hybrid electric and pneumatic actuation system. To this end, modelling schemes for the pneumatic cylinder were investigated and verified with physical experiments. This was done to develop accurate models of the pneumatic system that were later used in simulation to aid in the design of the platform. The design of the platform was aided through the use of Simulink to conduct iterative testing and multivariate evaluations using Monte Carlo simulation methods. Once the topology of the leg was set, the detail design was conducted in Solidworks and validated with its built in simulation functions. In addition to the mechanical design of the platform, a specialist boom was designed. The design needed to compensate for the forces the robot exerts on the boom as well as the material constraints on the boom. This resulted in the development of a cable-stayed, four bar mechanism boom system. An embedded operating system was created to control the robot and take in and fuse sensor inputs. This was run using multiple sensors, sub-controllers and microcontrollers. Sensor fusion for the system was done using a Kalman Filter to improve readings and estimate unmeasured states of the robot. This Kalman Filter took LiDAR and accelerometer readings as inputs to the system to produce a subcentimetre accurate position measure for the system. Finally, the completed platform was validated using fixed-body forward hopping tests. These tests showed a significant degree of similarity to the simulated results and therefore validated the design process.
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    Support Bath Deposition for Embedded 3D Printing
    (2020-12-30) Masters, Jessica Siobhan Shelagh; Govender, Reuben
    In recent years, the prinKng of som constructs has been aided by the development of embedded 3D printing. A notable form of embedded 3D printing known as freeform reversible embedding of suspended hydrogels (FRESH) was developed and published by Hinton et al in 2015. In FRESH, the print material is deposited into a gel-like support bath which is removed once the print material has set . One of the primary limitations in embedded 3D printing is the size of the support bath. This project aimed to solve this issue through the development of a gel deposition system. A variety of parameters, including nozzle shape and size, the motion of the nozzle and the temperature of the support gel were considered. A peristaltic pump was developed in order to deliver gel to the deposition nozzle. The various components for the gel deposition system were designed, manufactured and assembled, and the effects of motion and temperature on the quality of the support bath and print material were tested. The results of these tests were qualitatively assessed. The testing outcomes indicated that, with further development, the deposition could feasibly be used to deliver support bath material in embedded 3D printing.
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    The Design and Construction of a Bulge Testing Device Platform for Human Skin Tissue Applications
    (2020) Fischer, Dustin; Govender, Reuben
    Limited standard mechanical testing practises and stress-strain data are available for anisotropic human skin tissue in biaxial loading configurations to suitably represent skin in vivo. Inconsistencies in mechanical and physical properties in the literature due to numerous physiological factors have restricted development of biaxial testing equipment in laboratories to ad hoc research solutions having limited modifiability and parametric control. This project aims to develop a biaxial tensile testing device and testing platform which can be used in a research laboratory setting to provide a springboard to expediate mechanical skin tissue testing. The device can be easily reconfigured to accommodate a range of bulge pressures, while being driven via a 10bar compressed air supply. Based on simplified modelling of skin as an elastomer, mechanical and pneumatic resistivecapacitive pressure vessel models are developed. These are used respectively to initially specify a modifiable piston-cylinder bulge testing apparatus, and to design a customisable discrete proportional-integral closed-loop feedback pressurisation rate control system and software control environment. Pressure-time histories were successfully collected and stored on a dedicated computer for silicone sheet samples of 50mm diameter, as a surrogate for skin, that were tested using the platform to maximum pressures of about 200 kPa, at rates set between 2 20 kPa/s. The efficacy of the rate control system was affected by resolution of discrete pressurisation components that were used. The described platform is currently suitable for controlled and measured bulge pressurisation of elastomers. It is recommended to extend facility of the current platform by integrating 3D imaging and measurement technologies, to evaluate deformation of bulged anisotropic skin tissue and map inhomogeneous stress-strain fields for complex tensile stress-strain evaluations.