Browsing by Subject "mechanical engineering"

Now showing 1 - 5 of 5
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    Experimental and numerical study on the effect of strain rate to ductile damage
    (2009) Bowden, Andrew Scott; Cloete, Trevor
    Ductile fracture modelling is extensively used in the automotive, aerospace, aluminium and steel industries. However, these models are often only validated in a limited region of stress states, for example tensile failure by void growth but not shear. In addition, the predictions generally do not include strain rate or temperature effects. Quasistatic tests are often used in calibration, even though many applications such as automotive accidents and ballistic impact operate in the dynamic range. Thus the aims of this thesis were to develop a system to test the damage properties of materials at both quasistatic (≈ 1 s−1 ) and dynamic (> 1×103 s −1 ) strain rates, and then to determine the influence of strain rate to ductile fracture. From the literature the Bai-Wierzbicki damage model was identified as being applicable to the widest range of loading conditions. Thus tests to calibrate this failure locus were conducted on sheet specimens with notches cut into each to introduce non-axial stresses, resulting in a range of loading conditions. This testing procedure involved experimental testing combined with finite element analysis (FEA) to determine the stress and strain state at the position of fracture initiation. All specimens used material from the same sheet of mild steel. To break the dynamic specimens a tensile split Hopkinson pressure bar, or TSHB, was optimized and built. Hopkinson bars are the standard method of conducting high strain rate characterisation tests, however, there is no universal design to examine tensile deformation. The apparatus built used a tubular striker and produced a square input pulse with low noise as desired. Sheet specimens were glued into slotted sections of threaded bar, which in turn screwed into the split Hopkinson bars. This method was successful as in every case the specimens broke before the epoxy. FEA modelling techniques were optimized to minimize computation time. The most important was the use of infinite elements to simulate the bars which, when calibrated, were found to be the ideal method of modelling split Hopkinson bars. Ultimately it was found that strain rate does influence ductile damage. The dynamic specimens failed at a lower strain than the quasistatic equivalents. This indicates that, at high strain rates, fracture strain decreases with strain rate. In contrast, in the quasistatic range strain rate tends to decrease displacement to fracture and thus it is proposed that at quasistatic strain rates, fracture strain increases with strain rate. It is speculated that the degree that strain rate influences ductile fracture is related to the Lode angle, which is a measure of the third deviatoric stress invariant.
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    Growth in a computational planning pipeline for treatment in patient-specific coarctation of the aorta
    (2024) Hampwaye, Nasonkwe; Ngoepe, Malebogo
    Coarctation of the Aorta (CoA) is a Congenital Heart Disease (CHD) that is present at birth and is usually detected in the early years of the child. In the individualized treatment of a CoA patient, a non-severe case which initially exhibits no symptoms, and thus no treatment is necessary, could potentially turn severe due to the growth of the baby. Growth is characteristic of living structures including the aorta and is understood to occur in response to a given mechanica
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    Multiscale modelling of sutures in a high-performing biological protective structure: the turtle shell
    (2022) Alheit, Benjamin; Reddy, Batmanathan; Bargmann, Swantje
    Many natural protective structures, such as alligator armour, turtle shells, and the skulls of many animals including humans, contain networks of sutures; those are, soft tissue that bonds adjacent stiff plates typically made of bone. Such protective structures ought to withstand large loads associated with predator attacks. If one considers the optimization process of evolution and the ubiquity of suture networks in natural protective structures, it is reasonable to hypothesize that sutures improve the mechanical behaviour of protective structures during predator attacks. However, the effect of sutures in such loading scenarios is not well understood. We address this by using computational models of turtle shells where special attention is paid to the influence of the network of sutures. Additionally, we elucidate the structure-function relationship using parametric studies varying the suture geometry. Computational experiments are carried out at the suture scale to elucidate its mechanical behaviour and at the shell scale to elucidate the effect that sutures have on the shell. Among other insights, we show that: the compliance of the shell during small deformations can be increased by increasing the height of the interlocking bone protrusions and suture thickness; the bone plates interlock for sufficiently large deformations of sutures with sufficiently long protrusions; suture geometry can be used to tailor stress-wave propagation; and the presence of sutures can reduce the maximum strain energy density, a key indicator for a material failure, during a predator attack by 31 times. The work presented paves the way for the inclusion of sutures in biomimetic protective structures such as helmets and body armour. Computational solid mechanics aspects include multiscale modelling, model order reduction, and finite strain constitutive modelling aspects, such as viscoelasticity, hyperelasticity, and anisotropy.
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    Stable and high order accurate finite difference method for the incompressible laminar boundary layer equations
    (2020) Nchupang, Mojalefa Prince; Malan, Arnaud G; Nordstrom, Jan
    Numerical simulations of incompressible flows are unequivocally important due to their numerous industrial applications. These applications ranges from the large-scale fluid's flow modelling such as aerodynamics [1], atmospheric-ocean modelling [2] to a simple pipe flows in the petroleum industry [3]. This study is devoted to develop a provably stable and high order approximation for the incompressible laminar boundary layer equations. A new set of energystable boundary conditions are derived using the energy method. It is shown that both the weak and strong implementation of these boundary conditions yields an energy estimate. The semidiscrete problem is formulated by discretizing the continuous spatial derivatives using high order finite difference approximations on summation-by-parts form. The boundary conditions are implemented weakly using the simultaneous approximation terms methods. The discrete energy estimate is derived by mimicking the continuous analysis and hence, the numerical approximation is proved to be stable. The accuracy and linear stability of the developed scheme is also validated by solving the celebrated laminar flat plate flow problem. This is done by injecting the Blasius solution into the coefficient matrix as well as weak boundary conditions
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    The effect of the build orientation and heat treatment on the fatigue and fracture properties of a directed energy deposited nickel-based superalloy (Inconel 718)
    (2020) Paul, Mikyle; George, Sarah
    This project focused on determining the effect of build orientation and heat treatment on the fatigue crack growth rate and fracture toughness properties of the nickel-based superalloy, Inconel 718 that has been manufactured by additive manufacturing technique, directed energy deposition (DED). Three different build orientations namely XY, XZ and ZX, in both the as-built and heat-treated conditions were tested through a combination of comparative fatigue crack growth rate (FCGR) and fracture toughness (FT) tests. A heat treatment protocol was applied to the material to relieve residual stress from the building process to homogenise the microstructure and to introduce precipitation hardening to the material for strengthening. This heat treatment protocol consisted of a solution treatment at 1200 °C for 2 hours followed by a water quench. Thereafter an ageing procedure was conducted at 650 °C for 16 hours followed by air cooling. The Paris equation for each build orientation and material condition was determined with the aid of a crack measurement technique that was developed in this project. This technique was designed to significantly improve the measurement accuracy of the fatigue crack during testing. The developed crack measurement system consisted of a digital camera and stereomicroscope that were mounted to the ESH servo-hydraulic testing machine with a rig that was designed and manufactured during this project. The FCGR and FT tests were conducted on compact tension (CT) specimens and the ASTM E647 and ASTM E1820 test standards were used as guidelines to test the specimens respectively. FCGR tests were conducted at a load range of 7.2 kN and a stress ratio of 0.1. The results indicated that there was a significant variation in the fatigue performance with respect to the material's build orientation. The fatigue crack path encountered several deviations during testing, resulting in inconsistent Paris data. FT tests were conducted at a cross-head speed of 2 mm/min (in the range of 0.8 to 2.0 MPa√m/s). The material exhibited an elastic-plastic behaviour and therefore, a J-integral fracture toughness analysis was undertaken. Jmax values (the value of J at the maximum load) were obtained for each build orientation and material condition, and compared. It was seen that the average Jmax values for each build orientation differed, indicating that the build orientation affected the fracture toughness properties of the material. The applied heat treatment protocol homogenised the microstructure of the material and induced precipitation hardening. This was confirmed with the aid of light microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy, where a high concentration of niobium-rich Laves phase was almost completely eliminated by the heat treatment. The positive effects of the heat treatment on the material were noticed during the FCGR tests as the fatigue crack growth resistance and fatigue life of the material increased. The gradient of the Paris curve, m, was seen to increase from approximately 5 to approximately 8. This effect was also noticed during the FT tests where a uniform increase in Jmax of approximately 50 kJ/m2 was recorded for each build orientation. It was concluded that a post manufacture heat treatment is essential for residual stress relief and homogenisation to occur in IN718 manufactured by DED. The material's response to fatigue is not uniform when in the as-built state for all build orientations due to the presence of the niobium-rich Laves phase. The material displays superior mechanical properties after the heat treatment protocol in terms of fatigue crack growth resistance and fracture toughness when compared to the as-built condition. Furthermore, it was also seen that the build parameters such as scanning width and hatch spacing affected both the fatigue and fracture properties of the material and must therefore also be taken into consideration when producing the material. A successful crack measurement technique and apparatus was also developed that greatly increases the accuracy of Paris data obtained. The XZ orientation was seen to possess the most uniform fatigue and fracture properties amongst the three orientations tested in this project in both the as-built and heat-treated conditions.