Faculty of Science
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Item Maintenance policies based on time-dependent repair cost limits(Academy of Science of South Africa (ASSAf), 2006-11) Beichelt, F.; Nkadimeng, R.M.; Yadavalli, S.S.This paper considers two replacement policies for systems which, during their useful life, are subject to deterioration. Strategy 1: after a failure, the repair cost is estimated. If the repair cost exceeds a given limit, the system is not repaired, but replaced with a new one. So far, this policy has been analysed only for constant repair cost limits. This paper investigates the effect of applying time-dependent repair cost limits on the long-run maintenance cost rate. Examples show that, compared to the application of constant repair cost limits, a reduction of the maintenance cost rate between 5% and 10% can be expected. Strategy 2: the system is replaced as soon as the total repair costs arising during its running time exceed a given limit. Compared to the economic lifetime method, which is based on the average repair cost development and that requires the same data input, maintenance cost savings up to 20% could be achieved.Item An Acheulean handaxe from Gladysvale Cave site, Gauteng, South Africa.(Academy of Science of South Africa (ASSAf), 2006-03) Hall, G.; Pickering, R.; Lacruz, R.; Hancox, J.; Berger, L.R.; Schmid, P.WE DESCRIBE A SINGLE HANDAXE FROM fossiliferous breccias at Gladysvale Cave, South Africa. The artefact is the only known tool so far discovered during the controlled excavations conducted at this site over the last decade, and was recovered from decalcified sediments near the stratigraphic interface of two breccia units, making it difficult to assign it with confidence to either. The morphology of the handaxe indicates a middle-late Acheulean industry, and preliminary electron spin resonance and palaeomagnetic dating suggest an age of greater than 780 000 years.Item A partial skull of Paranthropus robustus from Cooper's Cave, South Africa.(Academy of Science of South Africa (ASSAf), 2008-03) Berger, L.R.; Kuhn, B.F.; Steininger, C.A partial hominin skull (COB 101) was identified in the fossil collections of the Transvaal Museum, Pretoria, attributed to the Cooper's Cave site in South Africa. The find represents the most complete hominin specimen recovered from localities at this site to date. COB 101 comprises the supraorbital, zygomatic, infraorbital and nasoalveolar regions of the right side, and the right upper third premolar. The specimen has undergone post-depositional distortion that resulted in the flattening of the facial structures. Here we describe and compare COB 101 with other hominin material from Africa and find that this specimen shares numerous diagnostic features with Paranthropus robustus. The discovery of COB 101 augments the number of specimens attributed to this species from other South African sites and other Cooper's Cave localities.Item THE VISIBILITY AND INVISIBILITY OF HERDERS' KRAALS IN SOUTHERN AFRICA, WITH REFERENCE TO A POSSIBLE EARLY CONTACT PERIOD KHOEKHOE KRAAL AT KFS 5, WESTERN CAPE(Africa Magna Verlag, Frankfurt M., 2006) Fauvelle-Aymar, F-X; Sadr, K; Bon, F; Gronenborn, DThe Europeans who landed on the shores of the South African Cape from the late 15th century onwards encountered local herders whom they later referred to as the Hottentots (now known as the Khoekhoe). There are written references to the settlements and livestock of these pastoralists, but archaeologists have not had much success in discovering any such sites. This absence of archaeological evidence for recent Khoekhoe kraals has been interpreted by some scholars as an indication for a general archaeological invisibility of nomadic pastoralist sites. This article reports on the archaeology of an extensive, low density surface spread of artefacts, KFS 5 (Western Cape), which possibly represents a Khoekhoe kraal dating to the time of the first contact with Europeans. Data are compared to other archaeological evidence of cattle pens in southern Africa and the issues of the visibility of prehistoric and historic kraals are re-addressedItem Diffusion and precipitation in alloys(Institute of Metals, 1947) Nabarro, F.R.N.A general review is given of recent work on diffusion and precipitation in alloys, with special emphasis on the theoretical mechanism of diffusion, and on the factors governing the shape and size of the precipitate. The effects of internal stresses on diffusion and precipitation, and of precipitation on the mechanical properties, are discussed.Item Dislocation theory and transient creep(Physical Society, 1948) Mott, N.F.; Nabarro, F.R.N.The purpose of this paper is to put forward certain advances in the theory of dislocations, and in particular to discuss their application to the theory of transient creep, in the sense in which the term is used by Andrade (1911, 1914, 1932) and by Orowan (1947).Item Mechanical effects of carbon in iron(Physical Society, 1948) Nabarro, F.R.N.Snoek has shown that when carbon atoms move from one possible set of interstitial sites in the lattice of a-iron to another set they cause shear strains. Cottrell has shown that the stress around a dislocation may be relieved by the migration of solute atoms in its neighbourhood, and that the dislocation is then bound to its present position. By combining these theories with the usual theory of age-hardening it is possible to explain the existence of a yeield point, quench aging, strain aging, delayed yield and blue brittleness as consequences of the presence of carbon in iron. A rough quantitative theory of the time-aging effects is given, and shown to agree with experiment. The need for further experiment is emphasized.Item Laszlo's papers on tessellated stresses: a review(Institute of Metals, 1947-10) Nabarro, F.R.N.Four recent papers by Laszlo on "Tessellated Stresses, Parts I-IV" published in the Journal of the Iron and Steel Institute, are reviewed in detail. The present review contains no new results.Item Nuclear ferromagnetism(Physical Society, 1947) Frohlich, H; Nabarro, F.R.N.It is shown that at sufficiently low temperatures metals become ferromagnetic owing to an orientation of the nuclear spins. The domain structure of such ferromagnetics is analogous to that of ordinary ferromagnetics.Item Dislocations in a simple cubic lattice(Physical Society, 1947) Nabarro, F.R.N.The properties of dislocations are calculated by an approximate method due to Peierls. The width of a dislocation is small, displacements comparable with the interatomic distance being confined to a few atoms. The shear stress required to move a dislocation in an otherwise perfect latttice is of the order of a thousandth of the "theoretical" shear strength. The energy and effective mass of a single dislocation increase logarithmically with the size of the specimen.