1,721,173 research outputs found
Killer notches: The effect of as-built surface roughness on fatigue failure in AlSi10Mg produced by laser powder bed fusion
Full text linkAdditive manufacturing allows the production of complex components for critical applications such as in the aerospace industry. Laser powder bed fusion is the most widely used form of additive manufacturing and good progress has been made in improved material quality in recent years. Despite the progress, fatigue properties are sometimes still problematic and this requires further investigation. The fatigue properties of additively manufactured metals depend on a variety of factors including surface roughness, microstructure, porosity and residual stress, amongst others. In this work the role of surface roughness in particular is evaluated using micro computed tomography (microCT) scans before and after fatigue tests. The crack locations are identified in scans after fatigue testing and correlated with surface features prior to fatigue tests by careful alignment of CT images. In this way notches on the surface which act as “killer notches” are measured and compared with other defects (both roughness notches and pores) in the vicinity. Direct evidence is thereby provided for specific features acting as killer defects, studied with varying surface topographies depending on build orientation. A statistical analysis using stress intensity factor and fatigue test results of the same samples directly validate the effect of the notches, in comparison to other similar notches across the sample. This is the first notch-based surface roughness evaluation method reported using X-ray tomography, showing promise as analytical methodology. In addition, the experimental campaign shows for the first time a direct correlation of fatigue strength with surface roughness using different typical as-built surfaces. This work lays the foundation for improved non-destructive testing, predictive modelling and overall improvement and management of the performance of additively manufactured parts based on surface features and surface characterization
The effects of microporosity in struts of gyroid lattice structures produced by laser powder bed fusion
Full text linkAdditive manufacturing by laser powder bed fusion allows the production of complex parts including lattice structures in popular metal alloys such as Ti6Al4V. Lattice structures are a class of meta-materials which hold many advantages such as the possibility for the production of lightweight parts with tailored mechanical and other properties: these have many potential applications in aerospace and medical fields. The laser powder bed fusion process can result in microporosity inside the produced material, which can affect the mechanical performance of these types of materials. In this work, different typical microporosity distributions are induced in manufactured gyroid lattice structure samples and the mechanical performance is tested by both static compression and compression-compression fatigue. X-ray tomography was used to validate the microporosity distributions and samples were tested in stress-relieved state and hot isostatic pressed state. In particular, it is found that small amounts of keyhole mode microporosity of ~0.2% make no difference while lack of fusion is critical, especially when this results in inefficient HIP pore closure. The results highlight the effect of microporosity on the mechanical performance of these materials and the results add to the knowledge base and trustworthiness of these materials. © 2018 The Author
Orientation-dependent fatigue assessment of Ti6Al4V manufactured by L-PBF: Size of surface features and shielding effect
Full text linkThe fatigue behaviour of as-built parts produced by means of Laser-Powder Bed Fusion process (L-PBF) is primarily influenced by the presence of stress raisers on the surface, whose morphology strongly depends on the relative orientation between the surface and the build direction. This study aims to shed light into the factors representing the surface morphology that correlate with the fatigue performance of L-PBF Ti6Al4V specimens manufactured in four different orientations. A Fracture Mechanics-based model based on measurable roughness parameters was employed for the prediction of the fatigue properties in both the finite life and endurance limit regions. The fatigue model considers an initial equivalent defect corresponding to the roughness parameter Rv,max. In addition, it includes a geometric factor F accounting for the shielding present at the roots of the micro-notches which was calculated starting from profile roughness parameters determined by X-ray Computer Tomography (XCT) scans. The predicted stress–life curves show that the adoption of the maximum profile depth Rv,max and the shielding factor F yields precise life predictions considering the effect of the surface orientation
The out-of-field phenomenon and leadership for wellbeing: Understanding concerns for teachers, students and education partnerships
No full textThis research shows that the out-of-field teaching phenomenon tilts teachers’ dispositions toward a pattern of observable challenges for their wellbeing, confidence, feelings of belonging and self-efficacy. This paper unveils how the out-of-field phenomenon influences not only teachers’ wellbeing but also students’ wellbeing and school leaders’ effectiveness. This cross-national phenomenological research involves a developed and a developing country. The results capture lived experiences of leaders, teachers, and stakeholders linked to the out-of-field teaching phenomenon across metropolitan and rural schools at primary and secondary levels. Findings indicate the interwoven complexities of the phenomenon, and leadership's understanding, awareness and engagement improves leadership for wellbeing in the out-of-field context. The paper concludes with an emphasis on the need for noticing teachers’ wellbeing with a context-conscious enactment of micro-education policies to address specific wellbeing needs.No Full Tex
Architected cellular materials: a review on their mechanical properties towards fatigue-tolerant design and fabrication
Full text linkAdditive manufacturing of industrially-relevant high-performance parts and products is today a reality, especially for metal additive manufacturing technologies. The design complexity that is now possible makes it particularly useful to improve product performance in a variety of applications. Metal additive manufacturing is especially well matured and is being used for production of end-use mission-critical parts. The next level of this development includes the use of intentionally designed porous metals - architected cellular or lattice structures. Cellular structures can be designed or tailored for specific mechanical or other performance characteristics and have numerous advantages due to their large surface area, low mass, regular repeated structure and open interconnected pore spaces. This is considered particularly useful for medical implants and for lightweight automotive and aerospace components, which are the main industry drivers at present. Architected cellular structures behave similar to open cell foams, which have found many other industrial applications to date, such as sandwich panels for impact absorption, radiators for thermal management, filters or catalyst materials, sound insulation, amongst others. The advantage of additively manufactured cellular structures is the precise control of the micro-architecture which becomes possible. The huge potential of these porous architected cellular materials manufactured by additive manufacturing is currently limited by concerns over their structural integrity. This is a valid concern, when considering the complexity of the manufacturing process, and the only recent maturation of metal additive manufacturing technologies. Many potential manufacturing errors can occur, which have so far resulted in a widely disparate set of results in the literature for these types of structures, with especially poor fatigue properties often found. These have improved over the years, matching the maturation and improvement of the metal additive manufacturing processes. As the causes of errors and effects of these on mechanical properties are now better understood, many of the underlying issues can be removed or mitigated. This makes additively manufactured cellular structures a highly valid option for disruptive new and improved industrial products. This review paper discusses the progress to date in the improvement of the fatigue performance of cellular structures manufactured by additive manufacturing, especially metal-based, providing insights and a glimpse to the future for fatigue-tolerant additively manufactured architected cellular materials. © 2021 The Author(s
A comparison of fatigue analysis methods for L-PBF net-shape surfaces in Ti6Al4V parts
Full text linkThe fatigue performance of additive manufacturing components is strongly limited by the surface topology, in particular considering the effect of the surface orientation. In the present study, Ti6Al4V laser powder bed fusion (L-PBF) net-shape specimens were printed considering four critical orientations to investigate and compare HCF fatigue properties and two different fatigue assessment methods.
Detailed X-ray Computer Tomography (XCT) allowed us to carry out numerical simulations of the 4-Point Bending test samples, by adopting the critical distance method (TCD method). Endurance limit predictions based on the most critical valleys were performed by TCD and compared to predictions made with a Fracture Mechanics model that relies on simple profile roughness parameters.
Comparison of the methods show that TCD inherently includes the shielding effect and it can better account for isolated features, while FM is more rapid and conservative
An improved experimental setup for high-resolution vacuum ultraviolet laser spectroscopy
Full text linkCITATION: Steenkamp, C. M., Du Plessis, A. & Rohwer, E. G. 2005. An improved experimental setup for high-resolution vacuum ultraviolet laser spectroscopy. South African Journal of Science, 101(5-6):272-275.The original publication is available at https://journals.co.zaThe experimental setup for vacuum ultraviolet (VUV) laser spectroscopy at Stellenbosch University's Laser Research Institute, has been improved by the addition of a VUV monochromator. The VUV source has been characterized over the extended wavelength range 144.2-157.7 nm, and simultaneous measurement of laser-induced fluorescence and absorption spectra has been achieved. We discuss current investigations of carbon monoxide molecules and complexes, facilitated by the narrow bandwidth of the VUV light and the low temperature collision-free conditions in a supersonic jet.https://journals.co.za/content/sajsci/101/5-6/EJC96405Publisher's versio
High-resolution vacuum ultraviolet laser spectroscopy of molecules in a free supersonic jet : in search of rare CO isotopomers and CO-Ar van der Waals molecules
Full text linkCITATION: Steinmann, C. M., Du Plessis, A. & Rohwer, E. G. 2005. High-resolution vacuum ultraviolet laser spectroscopy of molecules in a free supersonic jet : in search of rare CO isotopomers and CO-Ar van der Waals molecules. South African Journal of Science, 101(1-2):87-88.The original publication is available at https://journals.co.zaTunable vacuum ultraviolet radiation (in the range 142.7-146.7 nm) from a novel laser source was used to probe the electronic excitation spectrum of cold carbon monoxide molecules in a supersonic noble gas jet. Rotationally resolved spectra of 12C16O,13C16O, as well as the rare but astronomically important 12C18O and 12C17O isotopomers, were recorded. Evidence was obtained for the formation of CO-containing van der Waals complexes in the jet.https://journals.co.za/content/sajsci/101/1-2/EJC96347Publisher's versio
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