Browsing by Author "Govender, Reuben Ashley"

Now showing 1 - 6 of 6
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    Open Access
    Characterisation of Glass Fibre Polypropylene and GFPP based Fibre Metal Laminates at high strain rates
    (2011-12) Govender, Reuben Ashley; Langdon, Genevieive; Nurick, Gerald
    Fibre reinforced polymers (FRP) are finding increasing use in structures subjected to high rate loading such as blast or impact. Proper design of such structures requires thorough characterisation of the material behaviour over a range of loading rates from quasi-static to impact. This thesis investigated the quasi-static and impact response of Glass Fibre Polypropylene (GFPP) in compression, bending and delamination. The bending and delamination response of Fibre Metal Laminates (FMLs) based on GFPP and aluminium was also investigated at quasi-static and impact rates. High strain rate (5x10^2 to 10^3 /s) compression tests were conducted on GFPP using a compressive Split Hopkinson Pressure Bar (SHPB) and a Direct Impact Hopkinson Pressure Bar (DIHPB), in the through-thickness and in-plane directions. In both loading directions, the peak stress of GFPP increased linearly with the logarithm of strain rate. For in-plane loading, the failure modes were dominated by localised fibre buckling and kink bands, leading to delamination. The through thickness loading produced macroscopic shear and spreading failure modes. However, both of these failure modes are linked to in-ply fibre failures, due to through thickness compression causing transverse tensile strain. Previous studies of similar materials have not explicitly stated the link between through thickness compression and fibre failure associated with transverse tensile strain. A novel test rig was developed for Three Point bend testing at impact rates. The specimen was supported at the outer points on a rigid impacter and accelerated towards a single output Hopkinson Pressure Bar (HPB), which impacted the specimen at its midspan. Previous impact bend test rigs based on HPBs were limited to testing specimens with deflections to failure up to approximately 1mm, whereas the rig implemented herein measured deflections up to approximately 10 mm. This configuration permits the output HPB to be chosen purely on the magnitude of the expected impact force, which resulted in superior force resolution to configurations used in other studies. The HPB Impact Bend rig was used to test GFPP and aluminium-GFPP FML specimens, at impact velocities ranging from 5 to 12 m/s. The flexural strength of GFPP increased with strain rate, while the flexural response of the FML specimens was relatively insensitive to strain rate. v Several candidate delamination test geometries were investigated at quasi-static displacement rates (1 mm/min), and the Single Leg Bend (SLB) test was identified as suitable for adaptation to higher rate testing. Single Leg Bend delamination tests of both GFPP and FML specimens were performed using the HPB Impact Bend rig, at impact velocities of 6 to 8 m=s. The shape of the force displacement response for the high rate testswas markedly different from the quasi-static tests, for both the GFPP and FML specimens. Finite element (FE) simulation of the quasi-static and impact rate SLB tests on GFPP indicated that the difference was probably due to the interaction of flexural vibrations and stress waves in the specimen and the impacter cross member. The experimental results and FE analysis suggest that the delamination fracture toughness of GFPP decreases slightly as strain rate increases. High rate delamination tests on FML specimens resulted in unstable crack growth.
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    Open Access
    Design, build and test of a planar biaxial tensile testing machine for skin and other collagenous soft tissues
    (2022) Caine, Jonathan Andrew; Govender, Reuben Ashley
    This investigation forms part of a larger study, investigating the mechanical properties of skin and other soft membrane tissue. The focus of this project was the design and construction of a planar biaxial tensile testing machine, suitable for ultimately testing skin and other soft membrane tissues. The mechanical properties of skin and other collagenous soft tissues are of interest to a wide range of researchers and professionals. Interests include medical device design, research into the change in skin's mechanical properties due to disease, and skin's material characterisation for computational simulation. None of the laboratories at the University of Cape Town (UCT) were capable of performing planar biaxial tensile tests on soft tissue prior to this project. The machine was designed as a stand-alone, portable and accurate tensile tester with two independent and orthogonal testing axes. The machine was designed to be operated using GRBL software: an open-source motion control system for CNC milling. Two 50 N tensile load cells (K-S2M, Hottinger Baldwin Messtechnik, Darmstadt, Germany) were used for load measurement and a non-contact Digital Image Correlation (DIC) system (Dantec, Denmark) was used for three-dimensional surface displacement measurement. Commissioning testing was done on a silicone elastomer (Smooth-On, Dragon Skin 10) which was used as a skin surrogate. The commissioning testing did not seek to characterise the surrogate material, but to verify that the machine operated as intended. As testing of biological specimens will involve non-trivial specimen preparation and mounting, this is left to future research. Initially, a single axis of the custom machine was used to perform uniaxial tensile tests on the surrogate material. Specimens of the same surrogate material were tensile tested using a calibrated Instron (Norwood, MA, USA) tensile testing machine at the Centre for Materials Engineering (CME) at UCT for comparison. Twenty specimens were tested on each machine: five specimens per cross-head displacement rate: 10, 50, 120 and 400 mm/min. Sources of error were identified and addressed through minor design and procedure changes. Further uniaxial testing on the revised first axis, and the newly built second axis, was performed at 50 mm/min and 400 mm/min. These results compared favourably to the results from the Instron tensile testing machine. Planar biaxial tensile tests were performed on cruciform specimens of the surrogate silicone elastomer. Biaxial testing was performed at four different cross-head displacement rates (10, 50, 120 and 400 mm/min) combined with five different loading ratios between the axes (1:1, 1:0.75, 1:0.5, 1:0.25, 1:0). A total of 30 specimens were tested. The performance of the machine was critically assessed and the areas of difficulty encountered during testing were: handling and gripping the highly pliable specimens, consistency in the boundary conditions of the specimen, inter specimen variability, and alignment issues. The testing demonstrated that the machine functions as intended, and meets the requirements to be used in the testing of skin and other membrane tissue in the future.
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    Open Access
    Domes and Crosses: Exploiting synergies in two methodologies for biaxial tensile testing of membrane tissues
    (2023) Pons, Daniel; Govender, Reuben Ashley
    Biaxial tensile testing is the preferred method for mechanically testing membranous tissue as it can capture the tissue load response more holistically than uniaxial methods. There are two dominant approaches within the field of biaxial tensile testing: planar and bulge. Both methods can induce a state of biaxial tension within a specimen and both have their advantages and disadvantages. Bulge testing has the benefit of imposing a simple boundary condition on the tissue, making it quick and easy to set up. Planar Biaxial Tensile (PBT) testing is very sensitive to specimen preparation and requires non-trivial gripping systems. Some knowledge of the direction of maximum stiffness, prior to specimen mounting, is necessary for PBT to yield useful data. However, literature suggests that PBT is the more rigorous of the methods when it comes to collecting data to fully characterise a material model for membrane tissues. This study used the ease of bulge testing to determine the mean fibre axis of the tissue which informed the angle of PBT specimen excision. This was a rapid, non-destructive and creative method to avoid otherwise highly expensive imaging approaches to determine mean fibre direction. Further work was also done to develop a method of accurately determining specimen thickness for very thin tissues using a creative histological technique. By using a block of cutting medium to shape the membrane during processing steps, all four of the loaded edges of the tissue could be sectioned simultaneously for thickness measurement. Finally, the study served to develop a membrane tissue test protocol for further research using the in-house built biaxial tensile machines.
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    An experimental and theoretical study on the effect of strain rate on ductile damage
    (2016) Weyer, Matthew; Cloete, Trevor; Govender, Reuben Ashley
    Simulation of fracture in ductile materials is a challenging problem, since it typically occurs at length scales that are orders of magnitude smaller than that of the structures in which the fracture is occurring and, hence, difficult to resolve . One approach is to avoid modelling the micro-mechanics of ductile fracture by describing the macroscopic effects of fracture using damage parameters. Damage in metals can be defined as a measure of discontinuous deformation of a body. Many numerical models include some measure of damage to predict when a material will fracture under certain conditions, however there is little consensus as to what measures and parameters will accurately predict the onset of fracture. Most notably, the effect of strain rate at the point of fracture is significant and must be taken into account. The literature indicates that in the quasistatic regime where inertial effects are negligible, an increase in strain rate increases the strain at fracture. However, the research conducted in this dissertation suggests the opposite is true. The aim of this research is to conduct further high strain rate ductile damage experiments so as to extend the available data set, and develop a pragmatic damage model to relate the plastic strain at fracture to material parameters such as triaxiality, lode angle and strain rate in a specimen, which is verified using experiments performed under various loading conditions and strain rates.
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    Numerical investigation of dispersion in Hopkinson Pressure Bar
    (2005) Govender, Reuben Ashley; Cloete, T J; Nurick, Gerald N
    The Hopkinson Pressure Bar (HPB) is used as a load-time or displacement-time transducer in impact or blast experiments. The Split Hopkinson Pressure Bar (SHPB) is the accepted form of material testing for strain rates between 102 S-I and 104 S-I. Explicit Finite Element Analysis (FEA) codes are increasingly used to model HPB experiments numerically, due to the complicated boundary conditions imposed by tensile and shear SHPB experiments. However, most publications on numerical modelling of HPB experiments have focussed on the response of the specimen and have paid very little attention to the modelling of the stress wave propagation in the cylindrical bars. This dissertation focuses on the numerical modelling of stress wave propagation in HPBs.
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    Real-time measurement of biaxial tensions using digital image correlation methods
    (2022) Kyd, Haemish; Govender, Reuben Ashley; Nicolls, Frederick
    The mechanical properties of biological materials need to be measured for various applications. A means of inducing biaxial tensions in samples like these is with an inflation or bulge test. Normally the material under test would be measured with displacement gauges, however, under these conditions, where the specimen is soft and further, where the measurement cycle cannot be reliably paused, a contactless real-time measurement system is necessary to obtain reliable deformation data. Digital Image Correlation (DIC) is one such method. Pioneered in the 1980s the field has developed from basic 2D displacement measurements to very sophisticated full field 3D displacement measurement systems. The question becomes can the current state of the field, as well as the advances in modern technology, be leveraged to create a useable 3D DIC measurement system that is: • Useable in a real-time context. • Portable enough to be able to run these experiments wherever the experiment apparatus is located. • Cost effective enough to reduce the barrier to entry that the current commercial options present. To this end off-the-shelf components were acquired to form the technology base of the system. The open-source DICe framework, which enabled the necessary level of access to the underlying code base, was implemented on an NVIDIA Jetson Nano single board computer. Synchronised, stereo image acquisition was implemented via an Arducam 12 MP camera system. A stepper motor controlled linear drive was used to experimentally investigate accuracy and speed of the DIC system, for both rigid body motion and deforming targets. A thorough review of the concepts involved in DIC is undertaken followed by a detailed description of the design and build of the system. Ultimately a set of experiments are executed that show that, within a set of important constraints, it is indeed possible to run 3D DIC in real-time with off the shelf, cost effective components.