Department of Mechanical and Mechatronic Engineering

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Adaptive digital image correlation using neural networks
(Stellenbosch : Stellenbosch University, 2023-03) Atkinson, Devan James; Becker, Thorsten Hermann; Neaves, Melody; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: Subset size selection is crucial to the accuracy and precision of digital image correlation (DIC) measured displacements. Increasing the subset size improves noise suppression (reducing random errors) at the cost of spatial resolution (ability to accurately measure complex displacement fields). The tradition of global correlation parameter assignment is suboptimal because the speckle pattern quality and displacement field complexity can vary spatially. Dynamic subset selection (DSS), which assigns location specific optimal subset sizes, is challenging because the metrological performance of correlation is dictated by complex interactions between correlation parameters (subset size and shape function) and image set properties (noise, speckle pattern and displacement field complexity). This dissertation uses an open-source DIC framework to investigate the potential of artificial neural networks (ANNs) for error prediction and DSS, prior to the DIC process, from purely image information. ANNs are capable of modelling complex relationships within noisy, incomplete data without imposing fixed relationships, inspiring their recent resurgence for DIC applications. Despite the plethora of open-source DIC algorithms available, none offer spatially and temporally independent assignment of correlation parameters. Subsequently, a modular, open-source DIC framework capable of such flexibility is developed. This framework is predominantly consistent with current state-of-the-art practices and performs on par with well-established open-source and commercial DIC algorithms. Drawing direct links between the well-documented theory of DIC and its nuanced practical implementation, bridges this gap in literature which has acted as a barrier to newcomers intending to develop the capabilities of DIC. This framework, implemented in 117 and 202 lines of MATLAB code for 2D and stereo DIC, respectively, is attractive as a starting point to further the capabilities of DIC. The feed-forward ANN developed using this DIC framework, predicts random errors based on the speckle pattern quality (contained within a subset) and standard deviation of image noise more accurately and precisely than established theoretical derivations. A DSS framework is developed which uses this ANN to appoint subset sizes, based on the local speckle pattern, that offer random errors consistent with a stipulated threshold value. Appropriate selection of the random error threshold offers a favourable compromise between noise suppression and spatial resolution for up to moderate displacement gradients. Consequently, in the presence of varying speckle pattern quality this framework outperforms the traditional approach of trialand- error global subset size selection for the same mean subset size. Speckle pattern characteristics outside the training scope reveal the generalisability limitations of the DSS method, and associated ANN, as it performs on par with the traditional global subset size approach, motivating the need to broaden its training scope. Investigation of convolutional neural networks for dynamic shape function selection is initiated, showing they are capable of quantifying displacement field complexity between image pairs to guide spatially and temporally independent shape function assignment. The dissertation reveals that ANNs are an attractive approach to model the correlation parameter assignment. Furthermore, such models facilitate dynamic correlation parameter assignment from purely image information such that they can operate as a pre-process to DIC.
Chemical etching investigation on surface finish and fatigue behaviour of laser powder bed fusion produced Ti-6Al- 4V.
(Stellenbosch : Stellenbosch University, 2021-12) Malcolm, J. S.; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: An experimental methodology which implemented the practice of chemical etching was developed with the aim of reducing the surface roughness of laser powder bed fusion (LPBF) produced Ti-6AI-4V and thereby, improving the high cycle fatigue (HCF) performance. Hydrofluoric-nitric acid solutions, HF-HNO3, were used in the chemical etching experiments. The concentration of the HNO3 acid was fixed at 20 % while the concentration of the HF acid was varied to alter the solution concentration. The surface roughness of the LPBF produced Ti-6AI-4V samples were measured using a skidded contact profiler instrument. The subsequent chemical etching investigations revealed that an equivalent reduction in surface roughness can be achieved using either a low 2 % HF acid, or a high 12 % HF acid etching solution concentration. The solution temperature investigation demonstrated that the mass removal rate of a chemical etching process can be increased with raising solution temperature, albeit at the expense of process control. From the insights obtained from the chemical etching investigations, the recommended solution to reduce the surface roughness is a 2 % - 4 % HF acid concentration in conjunction with a solution temperature that is below room temperature. Etching durations of 60 minutes and longer allow for the minimum surface roughness that chemical etching as-built (AB) LPBF samples can be achieved. The internal pores in LPBF produced samples become exposed at the surface during the chemical etching process which limits the average surface roughness, Ra, from reducing to values significantly lower than Ra ≈ 2 μm. The HCF testing conducted was tension-tension cyclic uniaxial testing with a stress ratio, R, of R = 0.1. The LPBF produced Ti-6Al-4V fatigue samples were separated into four groups: the AB samples and then the three remaining sample groups which were chemically etched in a 4%HF20%HNO3 solution for 5, 15 and 90 minutes, respectively. The fatigue samples which received the 90 minute etch had both the lowest average surface roughness, Ra = 1.84 ± 0.03 μm, and the largest improvement in fatigue performance with reference to the AB sample’s fatigue behaviour. All the fatigue failed samples had crack initiation sites at the surface of the sample. The fabrication process-induced surface roughness acted as crack initiation sites for the AB and 5 minute etch sample groups. Chemical etching caused the controlling defect responsible for fatigue crack initiation to change from surface roughness to internal defects which were brought to the surface by the chemical etching mechanism. The opened subsurface defects acted as stress raisers which caused the fatigue crack initiations for the 15 and 90 minute etch sample groups.
Development of titanium alloys for laser powder bed fusion with the use of d-optimal design
(Stellenbosch : Stellenbosch University, 2023-03) Barnard, Christoffel Jacobus; Becker, Thorsten Hermann ; Ter Haar, Gerrit; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: Titanium is a costly and difficult material to manufacture, therefore it has primarily been used in high-value applications. The excellent material properties of titanium are what makes the material difficult to manufacture using traditional manufacturing methods. In the growing industry of additive manufacturing titanium has been adopted as one of the primary materials. The ability to manufacture intricate parts with little waste being very suited to eliminate the problems of titanium manufacturing. Titanium and its alloys have been extensively used in the expensive laser powder bed fusion process, primarily the Ti6Al4V alloy. The manufacturing process parameters for this alloy have thus been thoroughly optimised. The inherent cost of this process and of titanium make it expensive to develop new alloys for this manufacturing process. This research study investigates the use of D-optimal design experiments to determine the optimal process parameters for the TC11 and Ti6246 α+β titanium alloys and to construct a mathematical model that predicts the porosity of the alloys depending on the process parameters. A D-optimal design experiment was designed with the use of Design Expert software to find the optimal process parameters and to determine the terms of the mathematical model. Promising process parameter combinations from this experiment was then used to manufacture tensile samples to investigate the mechanical properties of the as-manufactured alloys and to compare them to the wrought alloys. The Ti6Al4V alloy was also manufactured as another way to determine the efficacy of the D-optimal approach. The Ti6Al4V alloy was manufactured with densities ranging from 97 to 100 %. The mathematical model was able to predict the porosity of samples with an overall error of 0.13 %. The TC11 alloy was manufactured with densities ranging from 97 to 100 %. The mathematical model was found to be able to predict the porosity of samples with an overall error of 0.513%. The mechanical properties of the TC11 tensile samples were superior to the wrought alloy in tensile strength and had similar elongation. Tensile strengths and elongation of 1511 MPa and 16.8 % were found. The Ti6246 alloy was manufactured with densities ranging between 96.5 to 100 %. The mathematical model was able to predict the porosity of samples with a mean error of 0.129 % up to 1.95 %. Tensile samples were manufactured with tensile strengths superior to that of the wrought alloy, but with very low ductility for moderate and high VED levels (167 and 250 J/mm3). Tensile samples manufactured at low VED level showed superior elongation and tensile strength compared to the wrought alloy, but with a much lower yield strength.
Evaluating measurement techniques: establishing a testing framework for residual stress in selective laser melted Ti-6Al-4V
(Stellenbosch : Stellenbosch University, 2017-12) Anderson, Lucas Steven; Becker, Thorsten Hermann ; Westraadt, Johan; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Residual stress (RS) plays an important role in the mechanical performance of components. Due to the manufacturing process involved in Selective Laser Melting (SLM), high RS is generated within the produced components. These stresses can increase component failure rates either during the manufacturing phase or in service. An investigation was performed into the capabilities of various stress measurement techniques for the application of measuring the RS distribution in SLM produced Ti-6Al-4V (Ti64). This investigation will be used as the basis for creating a testing framework for further studies involving the RS distribution in SLM produced Ti64. The stress measurement techniques were identified and reviewed with respect to the following: stress scale measurable, the stress tensor produced, measurement type, measurement penetration into SLM produced Ti64 and the achievable stress resolution in Ti64. Three techniques were selected for further evaluation, namely: neutron diffraction (ND), X-ray diffraction (XRD) and stress relaxation coupled with Digital Image Correlation (DIC). SLM produced Ti64 specimens built with nine combinations of build layer thickness and exposure strategy were used as test specimens. ND was used to resolve the macro-stress distribution along a plane running through the depth of the tested specimens and XRD was used to measure both near surface stress and, combined with electro-polishing, the stress distribution through individual build layers. The development of a technique – using focused ion beam (FIB) micro-milling and DIC displacement mapping – for the measurement of the residual stress at the layer scale, was also initiated. ND was capable of performing volumetric stress distribution measurements through the full depth of the specimens. Long testing durations and limited accessibility limits its application to RS measurements in SLM produced Ti64. A reduced analysis domain should be used in future testing to allow for more stress orientations to be scanned. The XRD technique, coupled with electro-polishing, was capable of resolving the in-plane stress distribution through individual build layers. The use of the sin2ψ method simplifies the calculation of the stress components. Care should be taken when interpreting the results obtained at the surface as high surface roughness can lead to erroneous stress results. Due to equipment failure the FIB-DIC technique could not be investigated fully. A validation test showed that the technique was able to resolve the in-plane strain components resulting from stress relaxation to a depth of ~20 μm. Further work on this method will include testing on SLM produced Ti64 specimens. The influence of build layer thickness and exposure strategy on RS was also investigated. An increase in the build layer thickness resulted in a decrease in the stress component magnitude and gradient at both the component scale and at the layer scale. The exposure strategy influences the homogeneity of the stress distribution. A uni-directional exposure strategy produces an approximately uni-axial stress distribution at the component scale, whereas the use of two or more laser vector directions results in an approximately bi-axial stress distribution at the component scale. The stress distribution at the layer scale remains uni-axial regardless of the exposure strategy used.
Fatigue crack growth rate threshold of laser powder bed Fusion Ti-6AI-4V
(Stellenbosch University, 2021-12) Dhansay, N. M.; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Typically, producing Ti-6Al-4V through the laser powder bed fusion (LPBF) technique, results in the material having large residual stresses and martensitic microstructure. These stresses and microstructure have been shown to result in Ti-6Al-4V having poor fatigue properties. However, the insight into the fatigue failure mechanisms caused by the residual stress and microstructure has been limited. LPBF is one of many additive manufacturing (AM) techniques in which parts are built in a layer- wise manner with the use of powdered material and consolidated through high power laser melting. This allows for complex geometries and previously impossible geometries to be manufactured with minimal material wastage. Many industries are aware of the potential in this manufacturing technique and have shown interest in it becoming a viable option for the manufacturing of some of their components. In particular, the use of LPBF produced Ti-6Al-4V is of interest to the aerospace and biomedical industries. This is because Ti-6Al-4V is already well established in many existing industries. For LPBF produced Ti-6Al-4V to be a viable option in industry, researchers need to have an adequate understanding of the material and the implications for its mechanical properties. Fatigue property investigations have largely focused on the fatigue life approach (crack initiation) and Paris regime (region II, crack propagation). However, in recent years, the near-threshold regime (region I, crack propagation) has become of interest, albeit limited in approach. The consensus within literature shows that the large tensile residual stresses, martensitic microstructure and porosity results in poor fatigue properties. Unfortunately, the insight into the fatigue fracture mechanisms brought about by the residual stress and microstructure is not yet well established. Furthermore, the near-threshold fatigue crack growth rate regime (FCGR) experiences crack closure mechanisms which result in premature near-threshold values. The majority of the near-threshold investigations on LPBF produced Ti-6Al-4V do not account for the crack closure mechanisms and therefore produce premature results. As a result of the low crack growth rates achieved in the near-threshold regime, a window into observing the fatigue crack initiation mechanisms is obtained. More specifically, how the fatigue crack initiation mechanisms are influenced by residual stress, martensitic microstructure and changing microstructural morphology. Literature has shown that porosity acts as crack initiation sites and reduce the fatigue life of a component. Furthermore, surface and near-surface porosity have been shown to have a more severe impact on fatigue life than internal porosity. It is through using the near-threshold FCGR approach in which one can calculate the allowable pore size under operational loads. Using the load-shedding technique to obtain near-threshold FCGRs, the results showed anisotropic behaviour dependent on residual stress levels and R-ratios. The fatigue fracture mechanisms were predominantly governed by transgranular quasi-cleavage mechanisms. Furthermore, the fracture is directed by PBG morphology which results in anisotropic crack- closure effects. In addition, primary α lath orientation is governed by the crystallographic texture of the PBG which influences the mechanisms of crack propagation. With the increase in grain size, the presence of β improves the near-threshold FCGRs in the duplex anneal (DA) condition due to the superior plastic flow abilities more than in the as-fabricated and stress relief conditions. This study investigates the near-threshold FCGRs of LPBG produced Ti-6Al-4V in the as- fabricated, stress relief and bi-modal conditions in three build orientations. In addition, crack closure mechanisms are accounted for by implementing variable R-ratio testing. This research presents the influencing mechanisms of residual stress and microstructure on fatigue behaviour.
Feasibility of additive manufacturing for patient-specific knee replacements.
(2021-12) Nortje, B. D.; Van der Merwe, J. H.; Becker, Thorsten Hermann
ENGLISH ABSTRACT: Osteoarthritis causes the degradation of the articular cartilage of the knee. This results in a loss of function of the joint. With a total knee replacement surgery, these articular surfaces are removed and replaced with artificial components. The ability to integrate patient-specific implant components can improve the results of a total knee replacement. The focus of this research is to investigate the feasibility of manufacturing methods required for a femoral knee replacement component for patient-specific implants. Through understanding what has been accomplished in literature, a multi-directional pin-on-flat wear tester is developed. This machine is based on the motion that is observed by the knee articulation in specific. A cross-shear ratio is designed to achieve similar wear characteristics to that seen in a knee replacement. This development is validated through the use of Co-Cr-Mo samples. The developed and implemented machine is utilised to perform an assessment of laser powder bed fusion, a powder bed based additive manufacturing technique, for use in patient-specific femoral knee replacements. Focus is placed on the use of Ti-6Al-4V and Co-Cr-Mo; where the latter is used for a benchmark to indicate the relative wear properties of the former. The average wear rates that resulted after 3 x 106 cycles were 2.58 mg/MC and 2.63 mg/MC for Co-Cr-Mo and Ti-6Al-4V, respectively. Additive manufacturing provides the ability to manufacture a near net shape component that is suited for the patient’s geometry. Orthopaedic surgeons consider the full and natural functioning of the replaced joint to be an indication of the operation’s success. The unsatisfactory performance of an implant can be attributed to the incorrect tension of the ligaments surround- ing the joint; this is often found to be a result of geometric intolerances of the standard implant sizes in relation to the patient’s natural joint. This can be rectified through the use of additive manufacturing which provides the ability to manufacture components of high geometric tolerances. A feasibility analysis is done with regards to cost and suitability of man- ufacturing processes for patient-specific femoral knee replacements. Additive manufacturing is compared to CNC machining to assess the feasibility of pur- suing a patient-specific femoral knee replacement that is manufactured from Ti-6Al-4V. The device that was developed illustrated satisfactory performance in ac- cordance to what is expected from literature. Through these tests, Ti-6Al-4V displayed suitable wear properties for use in articulating joints. The study places focus on the knee joint, but can be adapted for many other articulating joints with lower loading. It can then be shown that additive manufacturing may not be a feasible option for manufacturing based on cost, but may prove beneficial for reasons such as geometry complexities, surface finish, accuracy and extent to which the surfaces can be altered.
Fracture mechanics-based fatigue life assessment of additively manufactured Ti-6Al-4V
(Stellenbosch : Stellenbosch University, 2024-02) Macallister, N; Becker, Thorsten Hermann ; Blaine, DC; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: This dissertation presents a study on fracture mechanics-based fatigue life assessment for Additively Manufacturing (AM). The mature laser powder bed fusion (LPBF) process with the Ti-6Al-4V alloy in particular is selected for study, as it is well suited to the South African context with regard to economic climate, strong AM relationships and abundant mineral titanium reserves available. Furthermore, the Ti-6Al-4V alloy is a staple of aerospace, automotive and biomedical industries which are amongst the largest promoters for using AM technology, and for whom fatigue characterisation remains a prevalent topic as many end-use applications are intended for cyclic loading. Though significant research in fatigue behaviour exists, the conundrum of reliably certifying fatigue life in AM parts persists. This problem stems from the complex relation between AM print parameters, build orientation, surface roughness, inherent defects, residual stresses, meso- and microstructure; and establishing reportable fatigue strength baseline values required by industry. Moreover, as the AM environment promises saving in cost and time, full fatigue testing schemas are undesirable. As such, alternate damage-tolerant methods are becoming increasingly popular, where adopting fracture mechanics-based frameworks accompanied by limited or non-destructive testing could aid in certification. For this purpose, the dissertation first presents a novel version of the fatigue predictive NASGRO model where parameters are established that are unique to LPBF produced Ti-6Al-4V meso- and microstructures. In establishing these parameters for LPBF produced Ti-6Al-4V, the influence of process inherent microstructure, residual stress, and orientational dependant meso- structure is considered through examining near-threshold in combination with steady-state fatigue crack growth rates. The analysis shows that the descriptors of material constraint are sensitive to build orientation and microstructure. Furthermore, the effect of residual stresses is observed to not be severe. In this a clear effect of build orientation and meso- and microstructure is established for selecting NASGRO model parameters. Secondly, the proposed NASGRO formulation is translated into a comprehensive novel damage-tolerant fracture mechanics-based model to estimate fatigue life. Non-uniform defect populations, typical of AM material, in terms of size, shape and location are captured through X-ray tomography and surface profilometry and used as inputs modelled as equivalent crack lengths. The fatigue strength estimations are shown to be sensitive to fatigue crack growth rate threshold parameters and short crack growth mechanic descriptions. Furthermore, by introducing multiple crack initiations, the fatigue estimates are shown as distributions and are sensitive to defect number. Finally, sub-size specimen testing is investigated as a potentially elegant solution to accompany fatigue life assessments for threshold validation. Where results show inconsistent near-threshold fatigue behaviour linked to the microstructure. In this, considering unique meso- and microstructural features of LPBF produced Ti-6Al-4V, the domain and suitability in using sub-size specimens for fatigue crack growth rate threshold testing is discussed. Overall, this dissertation walks the path required in establishing reliable damage tolerant fatigue life estimation approaches for LPBF produced Ti-6Al-4V. Providing fundamental insights into interactions of fracture mechanic mechanisms and descriptions necessary for reliably modelling fatigue behaviour, therefore contributing to the developing frameworks and philosophies in AM to help in certification of fatigue performance of LPBF produced Ti-6Al-4V components.
Functionally stiff lattice structure for bone reconstruction
(Stellenbosch : Stellenbosch University, 2023-03) Heynemann, Marli; Van der Merwe, Johan; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH SUMMARY: The treatment of segmental bone loss with reconstructive procedures based on bone grafting is associated with the limitation of donor bone availability and complications at the harvest site, introducing a second surgical site on a patient, increased operating times and increased opportunity of infection. Endoprosthesis, used as a bone substitution, is an alternative reconstructive method which reduces the pain experienced after the procedure, operating time and healing period before a patient may resume load-bearing activities. However, unoptimised titanium implants used for bone substitution have higher stiffness properties than the surrounding bone. The incompatibility in stiffness between the implant and the surrounding bone causes stress shielding, which leads to bone resorption at the interface between the implant and the bone. The reduction in bone mass surrounding the implant leads to complications such as implant loosening, bone fractures and possibly implant failure, which leads to further corrective surgeries affecting the patient’s quality of life. This resulted in the investigation of using a lattice structure to reduce the apparent stiffness of an implant to match, or mimic, the stiffness of bone to reduce the stress shielding phenomenon. A body-centred cuboid unit cell was identified and investigated to match the longitudinal and transverse stiffness bone when repeated in a Ti-6Al-4V (ELI) lattice structure. An analytical model was used to design a body-centred cuboid unit cell with an expected longitudinal stiffness of 17.9 GPa and a transverse stiffness of 10.1 GPa, similar to human cortical bone's average longitudinal and transverse stiffness. The stiffness of the lattice structure was verified by performing compression tests on lattice test specimens according to ISO 13314. The measured elastic modulus of the lattice structure was 14.7 GPa in the longitudinal direction and 8.4 GPa in the transverse direction. The average longitudinal stiffness of bone varies between 14 GPa – 21.8 GPa; consequently, the unit cell stiffness could match the longitudinal stiffness of cortical bone. The measured transverse stiffness of the unit cell also falls within the range of the average transverse stiffness of cortical bone between 7.7 GPa and 12.5 GPa. Based on the compression test findings, the proposed lattice structure could mimic the anisotropic stiffness of bone. Therefore, implementing the proposed unit cell in the design of titanium bone substitution implants could reduce the stress shielding phenomenon. This would increase the success rate of titanium endoprostheses and improve a patient’s quality of life by reducing the effects of the complications associated with stress shielding.
High-temperature mechanical property characterisation of additively manufactured Inconel 718
(Stellenbosch : Stellenbosch University, 2024-02) Blackwell, MA; Neaves, Melody; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: As aerospace technology advances and the need for more advanced and intricate parts rises, traditional manufacturing methods such as casting fall short. Additive manufacturing techniques, such as laser-powder bed fusion (LPBF), have shown promising results in creating the intricate parts required. Near full density parts have been achieved, making these parts viable for use; however, the additive manufacturing process results in a unique microstructure not found in their wrought counterparts. The high temperature performance of the LPBF parts is not well documented. These characteristics are crucial, given the demanding operating conditions that these aerospace parts will encounter. Inconel 718 (IN718) is one of the most widely used alloys for high temperature aerospace applications. This research investigates the high temperature mechanical properties of LPBF produced IN718 with a specific focus on identifying whether LPBF IN718 exhibits a similar rapid strength drop-off above 650 ˚C as the wrought material. Specimens were produced using a Concept Laser Mlab 200R Laser Cusing machine and heat treated according to one of two heat treatment schemes, one of which was chosen due to its potential to improve high temperature strength, with the other more standard heat treatment being chosen for comparison. Specimen quality and heat treatment effects were confirmed through density testing and microstructural analysis. Mechanical testing was performed using a Gleeble 3800 thermomechanical simulator which resistively heats the specimen, creating a temperature gradient across the gauge region. This allows for properties to be extracted over a range of temperatures from a single specimen. Displacement and full-field thermal data were simultaneously captured using a stereo digital image correlation system and infrared camera. These datasets were both temporally and spatially synchronised using a common triggering system and calibration procedure, respectively. This data was subsequently processed to extract temperature dependant mechanical properties; namely, Young’s modulus, Poisson’s ratio, yield strength, ultimate tensile strength and percentage elongation at failure. These properties were compared to those of both wrought and LPBF produced IN718 found in literature. It was found that the LPBF produced IN718 specimens exhibited a similar strength drop-off above 650 ˚C. In addition, the data showed that the LPBF IN718 specimens subjected to the first heat treatment scheme achieved strengths comparable to that of the wrought material and were able to achieve the minimum strength requirements defined in Aerospace Material Standard (AMS) 5663, while the specimens subject to the second, more standard, heat treatment scheme were unable to meet the necessary strength requirements. None of the LPBF specimens were able to meet the elongation requirements dictated by AMS5663 at 650 ˚C. Significant anisotropy of the material properties were observed depending on build orientation.
Laser powder bed fusion produced Ti-6AI-4V: microstructural transformations and changes in deformation behaviour through thermal treatments.
(Stellenbosch : Stellenbosch University, 2021-12) Ter Haar, G.; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The current 4th Industrial Revolution has brought about enormous potential for social-economic development in South Africa. Additive manufacturing, a key technology of the 4th Industrial Revolution, and titanium, an abundantly mined but under-developed mineral commodity, collectively inspired industrial embracing of titanium-powder additive manufacturing, and specifically, laser powder bed fusion produced Ti-6Al-4V. Manufactured part quality concerns and the limited understanding of the innovative process-structure-property interactions impede its industry acceptance. Through developing fundamental insights into interactions between the microstructure and thermal treatments, mechanical performance can be improved, tailored, and optimised. Since conventional process routes cannot be applied to additively manufactured components, novel thermal treatments need to be developed. This dissertation documents experimental research into thermal treatments of laser powder bed fusion produced Ti-6Al-4V. By developing novel insights into the material’s unique metallurgical response to thermal treatments, tensile behaviour of the material is improved. The research investigates three temperature regions for thermal treatments post-fabrication and one in-situ approach. Thermal treatments between 750 – 960 °C are used to develop insight into the microstructure’s large-scale morphological transformation. Thermal treatments at 960 °C achieve fragmentation and grain globularisation. This is followed by quenching to attain a superior bi-modal microstructure. Thermal treatments at temperatures below 650 °C are used to develop insight into the initial stages of martensite decomposition phase transformation and material stress relief. Microhardness and tensile properties revealed material embrittlement. Fine precipitates in the microstructure were identified using high-resolution transmission electron microscopy. Based on these findings, two theories for the cause of material embrittlement based on two possible transformation routes of the initial stages of martensite decomposition are proposed. Mechanical properties of laser powder bed fusion produced Ti-6Al-4V depend on build orientation. Although the unique columnar-shaped and textured prior-β microstructure is identified as a probable cause of deformation anisotropy, limited insight into the cause of anisotropic deformation exists. Thermal treatments above 975 °C are used to globularise the columnar prior-β grain morphology. Microstructural anisotropy between two orientations is quantified using electron backscatter diffraction maps, and the influence of microstructure on deformation and crack initiation is studied. Insight into the deformation behaviour and the identified relation between prior-β crystallographic texture and α-lath morphological texture is used to formulate a theory of the probable cause of material deformation anisotropy. The study lastly investigates a novel approach to thermal treatments by using high-energy process parameters to achieve in-situ heating. An iterative approach for part optimisation using non-default process parameters is undertaken. While findings indicate that an improved microstructural and residual stress state can be achieved, detrimental effects of part oxidation and part-edge bulging are also observed.
Measuring fracture properties using digital image and volume correlation: decomposing the J-integral for mixed-mode parameters
(Stellenbosch : Stellenbosch University, 2017-12) Molteno, Matthew Robert; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The degradation of material properties over time is one of the core constituents of structural integrity monitoring. For this purpose, the materials’ resistance to brittle fracture is the most widely used property in the determination of safe operation and predicting component life. Measuring such properties by conventional methods presents challenges: large numbers of samples are needed, accounting for mixed-mode loading. Non-contact measurements by digital image correlation shows promise in extracting such parameters from in situ loaded structures, accounting for their complex geometry and loading. This thesis aims to develop a framework that allows for the extraction of mixed-mode brittle fracture properties, using three-dimensional (3D) image correlation techniques: multi-camera Digital Image Correlation (stereo-DIC), and Digital Volume Correlation (DVC). The 𝐽-integral method was chosen for obtaining the fracture resistance, with the decomposition method used for determining separate 𝐽𝐼−𝐼𝐼𝐼 components (opening, sliding and shearing: modes I, II and III), and stress intensity factors (𝐾𝐼−𝐼𝐼𝐼), as both methods are resilient to crack tip and displacement errors. The first stage of development is a finite element based method for the removal of experimental displacement artifacts from DIC or DVC results, intended as a pre-processor for the 𝐽-integral. Subsequently, a theoretical derivation is provided to link the two volumetric forms: the volume and the path-area integrals. This results in a proposed ‘hybrid’ integral, which benefits from the resilience to crack-front positional errors of the path-area approach, and resilience to random displacement noise of the volume integral. Initial testing of the 𝐽-integral based decomposition method was on the surface using stereo-DIC and the Arcan fixture to induce mixed-mode loading. The results are verified with mixed-mode fracture toughness values measured from PMMA and compared to literature and ASTM 1820 tests. The typical image correlation errors close to fracture are quantified by a methodology of applying the 𝐽-integral to analytical crack tip fields to which displacement errors are added artificially. It was found that the 𝐽-integral is most prone to DIC errors under anti-symmetric (mode II/III) loading. Testing in the volume used X-ray computed tomography to acquire images and DVC for displacement maps. The measurements were verified on two configurations: a SENT polyurethane composite specimen (mode I), and a shear loaded inclined notch in Magnesium alloy-WE43 (mixed-mode). The 𝐽-integral was verified against values from finite element fields resulting again in larger errors in mixed-loading. Decomposition of the volume integral requires an approach to separate the anti-symmetric 𝐽-integral. Two extensions are proposed: the first using a ratio derived from mode II/III Williams series formulas, and the second using the interaction integral. Both approaches are verified on DVC displacements. This thesis finds that 𝐽 based decomposition offers a versatile method to extract 𝐾𝐼−𝐼𝐼𝐼 values from non-standard 3D geometries and loadings. However, the pre-processing of fields to minimise errors is essential when mode II-III displacement fields are prominent. Although it is beyond the scope of this thesis, the hope is that this work will assist in the adoption of full-field measurement techniques as a standard testing practice in structural integrity assessments, and lead to better informed maintenance and inspection schedules.
Measuring mechanical properties using digital image correlation: extracting tensile and fracture properties from a single sample.
(Stellenbosch : Stellenbosch University, 2017-12) Huchzermeyer, Richard Lynn; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Multiple material properties are required to perform structural integrity assessments and reliability estimates on in-service equipment. Conventional material characterization testing approaches do not cater towards the testing of inservice equipment, and therefore ‘near-non-destructive testing’ approaches, in particular the small punch test (SPT), are preferred. The SPT, while capable of determining multiple material properties from a single small sample, does have limitations both in terms the complexity in analysing the resulting data, and the accuracy of the measured properties. These limitations may be addressed through full-field surface displacement analysis techniques facilitated by digital image correlation (DIC). A combined approach to extracting multiple material properties from in-plane (two-dimensional, 2D) surface displacements, measured on a single sample through DIC, has been developed assuming an isotropic linear elastic material. This approach utilizes the virtual fields method (VFM) to obtain Young’s modulus (E) and Poisson’s ratio (v). These tensile stiffness properties (E and v) are in turn input to a non-linear least squares field fitting approach (FF), which is used to obtain the critical stress intensity factor (𝐾𝑓𝑓) associated with a crack or notch in a material. The VFM and FF are applied to two compressively loaded disk shaped sample geometries (containing central notches) as well as an elongated half compact tension sample geometry (W = 25 mm) manufactured from 6 mm thick polymethyl methacrylate (PMMA). The experimental methodology to obtain suitable two-dimensional surface displacement measurements though DIC is described. Furthermore, the implementation of the VFM and the FF is developed on a sample specific basis. Through a comparison to properties determined using standardized ASTM testing, a relative error for the VFM of -1.5 % to 4.6 % in E and 12.9 % to 40.2 % in v is obtained. A concomitant relative error in the FF is determined to be 33 % to 38 % for 𝐾𝑓𝑓. Experimental errors, in particular out of plane rotation, are identified and the limitations of the assumptions made in applying the techniques are examined. Furthermore, the manner in which the error in E and v obtained through the VFM contributes to the associated error in 𝐾𝑓𝑓 identified through the FF is examined. It is found that the FF approach is less sensitive to error in v provided that the error in E is small. The nominally successful combined application of the VFM and the FF to 2D displacement fields measured with DIC on a single sample (assuming a linear elastic isotropic material), strongly motivates for the further development of this approach. The approach could be extended to accommodate out-of-plane deformations measured through DIC and could be developed to extract properties from ductile metallic materials. It is envisioned that this will be addressed in future work, which could lead to the methodology being applied directly to the SPT. A significant first step towards this is presented in this work, which demonstrates the first successful combined application of the VFM and the FF for extracting stiffness and fracture properties from full-field in-plane 2D displacements measured through DIC on a single sample.
Near-beta titanium alloys produced using laser powder-bed fusion.
(Stellenbosch : Stellenbosch University, 2023-03) Rudolph, SM; Becker, Thorsten Hermann ; Ter Haar, GM; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: β titanium alloys are used extensively in the aerospace industry to fulfil a multitude of applications due to their favorable properties, including the exhibition of high strength, workability, corrosion resistance, interchangeable combinations of strength and toughness, and ability to be heat treated over a wide range. However, little to no literature is available on the printability and mechanical performance of β titanium alloys subjected to additive manufacturing (AM), which is largely used for rapid prototyping and the production of complex components. Therefore, in this research, the printability and consequent mechanical performance of two β titanium alloys, viz. Ti-5Al-5V-5Mo-3Cr (Ti-5-5-5-3) and Ti-15Mo-3Nb- 3Al-0.2Si (Beta 21S), are considered whereby laser powder bed fusion is the means of manufacture. The process parameters were optimized experimentally in which the region of experimentation was determined using simulation software. The density was evaluated using the Archimedes’ principle. Pores were analyzed using optical microscopy. Additionally, the resulting microstructures were studied using scanning electron microscopy to further characterize the two alloys. The hardness and tensile properties of the as-built samples were investigated. A strong correlation was found between those reported and that of literature. The results demonstrate performance competitive with traditionally manufactured products, but more complex builds require investigation to alleviate uncertainties regarding the performance thereof. Additionally, the processing windows developed correlate with literature and provide insight into the combinations of process parameters which yield dense parts. The research presented herein reports the methods used to characterize the printability and mechanical performance demonstrated by commercially available β titanium alloys, Ti-5-5-5-3 and Beta 21S, produced by laser powder bed fusion.
Selective laser melting-produced Ti6Al4V: Influence of annealing strategies on crystallographic microstructure and tensile behaviour
(Stellenbosch : Stellenbosch University, 2017-12) Ter Haar, Gerrit Matthys; Becker, Thorsten Hermann ; Blaine, S. C.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The ability to manufacture complex shapes and structures with little material waste, among other advantages, makes the metal additive manufacturing technique of Selective Laser Melting a superior manufacturing technique. The titanium alloy, Ti6Al4V, serves as a great material of choice for this manufacturing technique due to its excellent mechanical properties and its biocompatibility. These factors make Ti6Al4V parts produced through SLM highly applicable and valuable in the biomedical and aerospace industries. Due to limited research and development in the field however, part quality in terms of achievable mechanical properties, residual stress and density has been below standard (such as that achieved by wrought Ti6Al4V parts). The study aimed to gain a fundamental understanding of the influence of annealing strategies on the microstructure of SLM-produced Ti6Al4V to improve and optimise the tensile properties of the material. SLM-produced Ti6Al4V tensile samples were subject to various tailored heat treatment strategies. Analysis of microstructure through optical and electron backscatter diffraction allowed for correlations to be made between the annealing strategies and the microstructure as well as between the printing process and the microstructure. Tensile test results of annealed samples show a decrease in tensile strength with an increase in annealing temperature as well as an increase in ductility and stiffness with an increase in annealing temperature. It was found that the fine martensitic (α’) microstructure of the as-built samples decomposes into a dual-phase (α+β) microstructure at ~800 °C, thereby improving ductility and stiffness. An optimal duplex annealing strategy allows for a bi-lamellar microstructure to be formed which allows for a substantial increase in ductility while maintaining a high material strength.
A statistical shape modelling approach towards the design of a temporomandibular joint implant
(Stellenbosch : Stellenbosch University, 2023-11) Lubbe, Inge ; Van der Merwe, Johan; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The anatomic temporomandibular joint (TMJ) functions as a synovial-joint system whose bilateral bony structures includes the temporal bone and mandible. Articulation is facilitated by the articular disc (within the joint capsule) and ligamentous structures while being supported by the masticatory system. Temporomandibular joint (TMJ) implant design involves the collaborative application of anatomic knowledge and engineering concepts. Medical device design research and development specific to TMJ implant systems is seen to mainly focus on implications of existing systems within implant design history, biomaterial implantology research and clinical success rates. Therefore, in this work a TMJ implant design process was defined from shared attributes in existing TMJ implant systems where the boundaries of development form a sequence of design stages and consistent design features. The developed TMJ implant design process was then implemented in the initial development of a mandibular component design. Anatomic modelling was identified as an important part of the modelling framework to investigate as it can be used to define aspects such as the overall geometric design specifications, geometric design parameters and features within the design formation stage, and validation procedures. A cephalometric analysis of measurements associated with design and surgical navigation was performed followed by the development and implementation of validated statistical shape models to evaluate shape variation. The investigated dataset consisted of 80 subjects (40 males, 52.1 ± 5.9 years; 40 females, 56.7 ± 9.4 years). The data quality and normality were confirmed with a series of inter- and intra-examiner error analyses using an intraclass correlation coefficient (ICC) and Quantile-Quantile (Q-Q) plots, respectively. Size-related anatomic variation (bilateral and sex dimorphism) and anatomic measurement parameter correlations were examined in a cephalometric analysis. Significant sex-related differences (parameter correlation). The mandibular component implant design was formulated using the cephalometric measurements as proposed parameter ranges. In addition, the developed statistical shape models (SSMs) were used to evaluate the level of customisation required in the design implementing a cluster analysis. An implant library size of six implants was determined to sufficiently support the population where the implant-mandible fit was used as the evaluation metric. The female and male implant designs resulted in a mean root means square (RMS) error of 0.71 ± 0.13 mm and 0.67 ± 0.24 mm, respectively. These results were determined to be within the allowable bone modification and screw capability limits (clinically acceptable).
Thermal power plant steel creep deformation measurement using digital image correlation
(Stellenbosch : Stellenbosch University, 2020-12) Van Rooyen, Melody; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Following the rapid expansion of populations and industries with a concomitant rise in the demand for electricity in South Africa, increased focus has been directed to the structural health monitoring of critical components of aging thermal power plants. Long-term operation at high temperatures and loads encountered by plant components results in the degradation of material properties through creep exposure. Assessing creep damage using conventional creep testing poses several challenges in terms of large sampled material requirements for manufacturing standard specimen geometries. This is often not possible for service-retrieved material due to the limited availability for destructive sample removal. This dissertation documents a novel experimental technique that employs digital image correlation (DIC) to characterise the creep damage of service-exposed power plant steel through creep deformation measurement across nonuniform temperature and stress fields. Through the full-field property of DIC, multiple properties are measured from single specimens ofX20CrMoV12-1 (X20) piping steel supplied by Eskom in virgin and various stages of service exposure. Initial development of the technique involves a stereo-DIC setup adapted for high temperature deformation measurement.A temperature profile ranging from 550–600°C is applied using resistive heating from a Gleeble thermo mechanical simulator and measured using infrared imaging.Near-uniform stress profiles result from appropriate specimen geometry and region of interest selection. Several temperature-dependent elastic moduli and Poisson’s ratios are measured from single specimens of virgin X20. Creating an extension to accelerated creep tests(shorter-term tests conducted at higher stresses and temperatures than encountered in service)used on virgin and various ex-service X20 states is explored.Creep curves at several temperatures are successfully realised from single specimens and are useful for identifying differences in creep resistance between ex-service levels in line with traditional damage classification through surface cavity density replication. Micro structural quantification using electron microscopy shows subgrain and M23C6precipitate growth as well as Laves phase precipitation in higher damage states of X20. For middle damage classes, it is highlighted that several complementary damage assessment methods are necessary to identify differences in material deterioration. Zener-Hollomon parameters calculated from DIC-measured strains over the temperature profiles serve as suitable damage indicators. Single camera DIC measurement of creep curves across spatially varying stress profiles is achieved with a waisted specimen design in a traditional creep testing setup for medium-term tests. Virgin X20 creep curves at 140–150MPaare used to calibrate baseline parameters for a continuum damage mechanics model. Subsequently, the model is optimised for subgrain and precipitate parameters using the corresponding ex-service X20 creep responses. These micro structural-based parameters serve as comparative damage sensors that agree with traditional cavity and hardness-based methods. This dissertation reveals that a wealth of creep data can be extracted from fewer X20 specimens, which is highly beneficial in characterising material integrity in a supplementary manner to existing methods.Expansion to small samples through small punch creep testing is also initiated. Although beyond the scope of this work, future aspirations are that these approaches will be integrated into life management philosophies to better guide inspection and maintenance strategies.

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