Papers by Keyword: Pitting

Abstract: This paper presents a surface hardness improvement methodology of a wind turbine helical gear which is made up on EN36 alloy steel material. Now days the wind turbine gears are frequently failed due to high contact stress, due to sudden impact load, change of wind speed and wind directions. Pitting is a method of surface failure in a gear. So, to avoid the gear failures we need to improve the surface hardness of wind turbine gears by using Nitriding process. There are three modes of hardness test taken from this process to improve the hardness. They are before nitriding, after nitriding & without nitriding. By comparing the hardness test values, Gas Nitriding is the best suitable process to increase the surface hardness of a large size gear. By the method of improving surface hardness of wind turbine gear, the load withstanding capacity of the wind turbine gears are increased, pitting failure may be avoided at a desired level and the power production rate is increased.

Abstract: The failure of a spherical-roller bearing consisting of spherical rollers, outer and inner rings made of 100CrMn6 steel was studied. After a trial test, in which several spherical rollers were completely destroyed and a long continuous radial crack appeared in the outer ring, pitting degradation was observed on all bearing parts. The bearing components were studied (separately) by means of light and electron microscopy. The chemical composition was estimated by means of glow discharge optical emission spectroscopy. X-ray diffraction was utilized for qualitative and quantitative phase analyses. As a representative of mechanical properties, the hardness was also measured.

Abstract: Use of Magnesium alloys as body implants are breaking into a new paradigm of biomedical engineering as they are biocompatible, biodegradable and have mechanical properties close to that of bone. Even though corrosion fatigue (CF) and stress corrosion cracking (SCC) failures are among the most common concerns for metallic implants, CF behaviour of magnesium alloys in physiological environments has received little attention. This article reports the CF results of a common cast magnesium alloy (AZ91D) in modified simulated body fluid (m-SBF). Results showed that there was a remarkable difference in fatigue strength of Mg alloys when tests were performed in m-SBF.

Abstract: The Royal New Zealand Air Force (RNZAF) utilised the split sleeve cold expansion process to increase the fatigue life of fastener holes in the wings of the C130 transport fleet. As part of the validation of the fatigue improvements offered by the process the Defence Technology Agency conducted a series of fatigue tests on cold expanded fastener holes in aluminium 7075-T651, including specimens with corrosion induced after the cold expansion process had been performed. This research conducted an analysis of fatigue crack origins and modelled the stress concentration factors generated as a result of the corrosion pits. These results were used to explain the differing fatigue life and s-n curves produced by corroded and non-corroded fatigue specimens and the location of crack initiation sites around corroded cold expanded fastener holes.

Abstract: Components and systems of military aircraft are regularly subjected to severe operating conditions, which lead to the development of a wide range of failure modes. The Defence Science and Technology Organisations (DSTO) Forensic Engineering and Accident Investigation group investigates such failures for the Australian Defence Force (ADF). Correct diagnosis of these failures has provided the ADF with immediate advice that has contributed to increased aircraft safety, improved operational availability, and significant cost savings. This paper presents a number of case studies of recent fatigue failures which have occurred in Australian Defence aircraft. The case studies include examples of failures which occurred via differing fatigue initiating and driving mechanisms. Details of the forensic investigations relating to each case study are provided and the ensuing remedial actions discussed.

Abstract: The pitting fatigue failure is one of the main gear failure modes, and it has extremely important significance in studying on the influence of dynamic characteristics on pitting fatigue failure. The article has set up increasing-speed test-bed, using photoelectric encoder and NI DAS to measure and collect rotational-speed pulse of driving wheel and driven wheel. Arc-length difference and rotational-speed difference of driving wheel and driven wheel are analyzed to verify gear rattling phenomenon under increasing-speed transmission. And then gear surfaces under certain cycles are observed and analyzed utilizing surface mapping microscope to explore the influence of gear rattling on pitting fatigue failure under increasing-speed transmission. It is shown that there is rotational-speed difference between driving wheel and driven wheel, which indicates that gear rattling phenomenon appears in the meshing process, or more precisely, gear rattling phenomenon appears on both surfaces of the tooth because rotational-speed difference fluctuates bilaterally. In addition, tooth surface contact stress is 221.3Mpa, in theory, if actual contact stress is less than fatigue limit, pitting fatigue failure should not occur. However, through gear surface observation, pitting phenomenon authentically appears with the tendency from micro pitting to destructive pitting. In a word, gear pitting fatigue failure is induced by gear rattling to a certain extent.

Abstract: An influence of depth of the pits resulting from exposure to corrosive medium on the initiation of stress corrosion cracking process of AW 7020M (Rm=430MPa) alloy has been presented in the paper. The samples were subjected to several weeks of exposure to the solution simulating seawater, which resulted in uniform surface corrosion with the pits of depth from 30 to 100 μm. These samples were then subjected to a tensile test to determine their new properties (after the exposure). The tests have shown that the initiation of neck and cracking spot do not always coincide with the location of pits. The aim of this study was to estimate the minimum depth of pitting under the assumed shape, resulting in the initiation of cracking during the tensile test, using numerical simulations with the CAE software. The paper presents simulation results for the samples made of AW-7020 alloy of 3.75 mm thickness.

Abstract: The present paper summarises an attempt of using the so-called Modified Wöhler Curve Method (MWCM) to estimate fatigue damage in pitted cast iron water pipes subjected to in-service variable amplitude multiaxial fatigue loading. In this setting, pits are treated as hemispherical/hyperbolic notches whose depth increases over time due to conventional corrosion processes taking places in buried cast-iron pipes. The validity of such an approach is proven by showing, through a case study, that, under particular circumstances, the combined effect of corrosion and fatigue can remarkably shorten the in-service lifetime of cast-iron pipes as observed in the case study.

Abstract: In this paper,the laser transmission joint strength is studied with semiconductor laser based on the surface pitting of 304 stainless steel and PA66.Unevenly distributed pits are formed on the surface of 304 stainless steel with 10% ferric chloride solution for two hours, which have certain morphology, depth and size. Different laser power, welding speed and welding pressure are used in laser transmission joining 304 stainless steel and PA66,which 304 stainless steel contains pitting corrosion before and after. The tensile strength test of joined samples shows that: the joint strength of pitted 304 stainless steel and PA66 with laser transmission joining is improved at least 135%; the riveted structure formed at the joining interface improve the joint strength.

Abstract: Metallurgical investigations were directed to probe into the incidence of inordinate rusting and pitting in imported AISI 430 grade hot-rolled ferritic stainless steel sheet coils. Visual examination, electron microprobe analyses (EPMA), scanning electron microscopy (SEM) and electrochemical potentiokinetic reactivation (EPR) were concomitantly employed to investigate the problem. Studies revealed that the unprecedented degree of corrosion in ferritic stainless steel coils, during the short span of shipment time, was attributable to the ingress of sea water and its retention within the tight folds/ wraps of the steel coils during their shipment. The abundance of moisture and chloride (from the entrapped saline electrolyte) on the steel surface together with depleted O2 supply within the tight folds are presumed to have created conditions akin to an actively-corroding crevice, by way of passive film instability and its eventual breakdown on the stainless steel surface. As a consequence, the coils are believed to have suffered an accelerated and intensified chloride-induced corrosion attack and damage within the short span of shipment time. The investigations also revealed that the corrosive conditions were further exacerbated by the vulnerability and susceptibility of ferritic stainless steel to intergranular corrosion (IGC) due to its inherent sensitized condition. The paper thus throws light on an unusual precedent of chloride-induced corrosion in ferritic stainless steel and highlights the investigative metallographic work and corrosion failure analysis that led to above revelations.