Alan Crosky

Precipitation and solute distribution in an interrupted-aged Al–Mg–Si–Cu alloy

Quantitative analysis of the precipitate species and solute distribution was carried out on Al–Mg–Si–Cu alloy 6061 aged to peak hardness using a conventional T6 heat treatment and the so-called T6I6 heat treatments. In this latter, a dwell period at reduced temperature (65°C) is introduced into the T6 ageing cycle (at 177°C or 150°C) which modifies the microstructure and results in the simultaneous improvement of both tensile properties and fracture toughness. Analysis of three-dimensional atom probe data reveals that the superior mechanical properties of the T6I6/177 temper are achieved by a combined effect of a greater consumption of solute atoms by precipitates, an increased number density of fine precipitates and the presence of greater fractions of the effective strengthening precipitates in the final microstructure. Three types of precipitates were found to be characteristic of the peak aged conditions: β′′ precipitates, Guinier–Preston zones and Mg–Si(–Cu) co-clusters. The composition of the strengthening precipitates was found to vary over a wide range for the different heat treatment schedules, corresponding to a variation in the number density of stable nuclei, without any accompanying change in their morphology. All precipitates were found to contain substantial quantities of aluminium. The results also indicate that the strengthening precipitates are preferentially formed from Si-rich nuclei that contain Cu atoms, as opposed to Cu-free nuclei.

Failure of rockbolts in underground mines in Australia

Over the last 15 years there has been an increasing incidence of failure in rockbolts used in underground mines in Australia. Failures have also been observed in the United Kingdom where Australian Technology rockbolting is also used. Most of the failures in the United Kingdom were found to be initiated by corrosion pits, but in Australia, the fractures were considered likely to be due to stress corrosion cracking (SCC). This paper reports a metallurgical study of 44 failed rockbolts from four different underground mines in Australia. The study confirmed that failure was generally due to SCC and showed that this was usually initiated by bending of the bolts that occurred due to lateral movement of the rock strata. It also showed that many of the failed bolts had very low toughness with Charpy impact values of 4–7 Joules.

Secondary Precipitation in Aluminium Alloys & Its Role in Modern Heat Treatment

This paper reviews some of the practical outcomes of exploiting secondary precipitation during the heat treatment of aluminium alloys and discusses current understanding of this phenomenon. Recent studies have utilised the techniques of positron annihilation spectroscopy (PAS), 3D atom probe (3DAP) as well as the more traditional transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) to investigate early precipitation events that occur during secondary ageing at low temperatures (~20-65°C). This work has confirmed that clustering and GP zone formation can modify the nature and distribution of precipitates that form when ageing is subsequently resumed at more elevated temperatures. Prospects for achieving further improvements in heat treatment schedules and alloy compositions to take greater advantage of secondary precipitation are also considered.

Cement-Implant Interface Contamination: Possible Reason of Inferior Clinical Outcomes for Rough Surface Cemented Stems

Background: Shape-closed cemented implants rely on a stronger bond and have displayed inferior clinical outcomes when compared to force-closed designs. Implant contamination such as saline, bone marrow and blood prior to cement application has the potential to affect the cement-implant bond. The consequences of implant contamination were investigated in this study. Methods: Fifty Titanium alloy (Ti-6Al-4V) dowels were separated into ten groups based on surface roughness and contaminant, and then cemented in polyvinyl chloride tubes. Push-out testing was performed at 1mm per minute. The roughness of the dowel surface was measured before and after the testing. The dowel surface and cement mantel were analyzed using a Scanning Electron Microscopy (SEM) to determine the distribution and characteristics of any debris and contaminants on the surface. Results: Contaminants largely decreased stem-cement interfacial shear strength, especially for rough surfaces. Saline produced the greatest decrease, followed by blood. The effect of bone marrow was less pronounced and similar to that of oil. Increasing surface roughness increased the interfacial bonding strength, even with contaminants. There was a non-significant increase in mean bonding strength for smooth surfaces with bone marrow and oil contamination. SEM showed that contaminants influence the interfacial bond by different mechanisms. More debris was found on rough samples following testing. Conclusions: The results of this study underscore the importance of keeping an implant free from contamination, and suggest if contamination does occur, a saline rinse may further decrease the stability of an implant. The deleterious effects of contamination on rough surface cement bonding were considerable, and indicate that contamination at the time of surgery may, in part, contribute to inferior clinical outcomes for rough surfaced cemented stems.

Failure of the threaded region of rockbolts in underground coal mines

Abstract HSAC 840 steel rockbolts were examined, all of which had failed in service through the threaded region exterior to the rock face by brittle fracture transverse to the bolt axis. The bolts had been in service in an Australian coal mine in which the groundwater had a pH of 7·5–8·5 and a relatively high bicarbonate level of 515–2200 mg l−1. All failures had occurred within the region comprising the domed washer and the end plate. The majority of the bolts showed a small thumbnail-shaped discoloured region 1.5-4.5 mm in length at the fracture origin, as is characteristic of stress corrosion cracking (SCC) in rockbolts. In half of these bolts, SCC had initiated immediately at the root of the thread, but in the other half of the bolts, SCC had initiated from a small pre-existing crack that had been produced in the thread as a result of a prior bending load.

A Study Wear Behaviour of Induction Hardened 4140 and Carburised 8617H Steels on 1040 Steel

This study was undertaken to evaluate the likely effect on the wear rate of changing the pinion material in a rack and pinion steering box from carburised SAE-AISI grade 8617 H steel to induction hardened SAE-AISI grade 4140 steel of the same hardness. Accordingly, pin on disc wear tests were conducted using carburised 8617 H pins and through hardened 4140 pins. The surface hardness of the pins was approximately 60 HRC for both materials. The discs were made of SAE-AISI grade 1040 steel through hardened to a hardness of 45 HRC. The results showed that both the pins and the discs wore more rapidly when the tests were carried out with 4140 pins. The study indicates that the wear rate would be increased on both the rack and the pinion if the pinion material were changed from case hardened 8617 H to induction hardened 4140.

Laboratory-based coupon testing for the understanding of SCC in rockbolts

Coupon testing has been used extensively to understand the effects of general corrosion on engineering materials within their operational environment. This study examined the use of various coupon specimens to examine stress corrosion cracking (SCC) of rockbolts in the underground mining environment. The standard ASTM G39 bend beam SCC specimen, as well as a pin-loaded slotted specimen, was used in this investigation. It was identified that the critical stress threshold for HSAC 840 rockbolt steel is approximately 580 MPa, slightly lower than the material’s yield strength. It was shown that the higher toughness HSAC 840 grade rockbolt steel performed better than AISI 1355 grade rockbolt steel, emphasising the importance of steel chemistry. It was also found that galvanising improved SCC resistance but shot peening reduced SCC resistance.

Failure of a fairground ride

On 22 February 1997, one of the arms of an “Octopus” amusement ride, operating at the Rylestone Show in New South Wales (NSW), Australia, became detached from the central pylon, allowing the passenger carriage at the end of the arm to fall to the ground. This accident resulted in the death of one of the three young occupants. The Octopus ride is shown after the accident in Fig. 1. The ride had eight arms that rotated along with the central pylon while also moving up and down by pivoting through an axle attached to the central pylon. The axle pivoted in two bearings (one at each end of the axle) fitted to housings attached to the central pylon. To allow disassembly of the arms when the ride was transported from town to town, a cast steel bearing cap was fitted to the top of each housing. The cap was hinged to the housing at the inboard end and bolted to the housing by a retaining bolt at the outboard end.Detachment of the arm was found to have occurred as a result of fracture of the bearing cap and the retaining bolt, which then allowed the axle to move out of its housing. The ride was first registered in 1956 (the year that registration became compulsory in NSW) but had been in service for some time prior to that and may have been built as early as 1939.[1] This paper presents the findings of a metallurgical analysis of the failure.

Unsupervised Deep Learning for Data-Driven Reliability and Risk Analysis of Engineered Systems

Reliability and risk analysis concerns analyzing and predicting the state transition of engineered systems given historic information. With an increasing volume of available data, conventional machine learning algorithms may fail to capture hidden patterns behind the reliability engineering problems due to the curse of dimensionality. Autoencoders, a class of deep learning frameworks, have been reviewed in this study. They are a powerful tool in extracting hidden representations and producing a robust reconstruction for further predicting tasks. We have developed autoencoders and applied them on four data sets. Results showed that autoencoders can not only classify the states at a good accuracy, but also help to discover the failure mechanism.