Advanced Materials Research Vols. 41-42

Abstract: The effects of coating/substrate modulus mismatch and margin geometry on contact damage in bi-layer systems were investigated. Following an earlier study, convex specimens having curvature of 12 mm inner coating diameter and 1mm thick brittle layer on a polymeric and dental composite support bases were produced. Sample coating geometry at the margins was varied by grinding the edge of the glass shells in various shapes. The specimens were tested by applying single cycle load at the specimen’s axis of symmetry using flat indenter of low elastic modulus. The effects of margin geometry and support layer modulus on radial crack initiation and damage evolution was examined, with particular attention paid to the relevance of such damage to lifetimelimiting failures of all- ceramic dental crowns. Finite element modeling was used to evaluate stress distribution in the glass coating. Experimental trends interrupted with peak maximum principal stresses at the margins. The results of this study illustrate that the fracture behaviour of brittle layered structures is not dominated by certain variables. It is demonstrated that critical loads for initiation of radial cracks are sensitive to support layer modulus as well as margin geometry. Support layer modulus plays an important role in crack propagation and subsequent damage patterns, especially at specimen side walls.

Abstract: Ca-containing anodic alumina (CAA) has been successfully prepared by anodizing Al film in an alkali solution at a constant voltage and subsequently electro-depositing calcium salts on and into anodic alumina. This paper investigated the anodic behavior of Al, deposition behavior of calcium salt, and microstructure of CAA. The results show that the anodic behavior of Al in Na3PO4 electrolyte can be described as three continuous stages as of initial growth of the compact barrier layer, formation of porous alumina and further development of its pores and columnar holes. It is also found that cell voltage of electro-deposition process plays an important role at the deposition behavior of calcium slat and the microstructure of Ca-containing anodic alumina (CAA). The higher the cell voltage is, the faster the deposition rate, and the more calcium being deposited at the surface of anodic alumina and into the columnar holes or at the walls of the holes of anodic alumina. It is expected that Ca-containing anodic alumina films are promising substrates for fabricating functional bio-coatings for prosthetic applications.

Abstract: Dynamic properties of Nanostructured Metal Matrix Composites (NMMCs) materials consisting of nanograined aluminum matrix with nano or micro-sized ceramics particulates are calculated using Ansys/LS-DYNA in this paper. There are three conditions for the mechanics analysis involved the target with 10 vol. % SiC, 30 vol. % SiC and 50 vol. % SiC. It is assumed that the SiC particles uniformly distributed in the matrix. According to the analysis, the strength of NMMCs material is reduced, and the deformation of target is weakened by the vol. % of SiC increasing.

Abstract: Contact pressure of porous Al2O3 probed by nanoindentation was investigated by dimensional analysis with special attention paid to scaling effects in the mechanical behavior. It was found that, for sample containing small grains and interconnected pores, the contact pressure is manifest dominated by bonding strength of the porous alumina. Whereas the samples with coarse grain and various porous structures exhibit higher contact pressures and smaller residual deformations, which can be attributed to the mechanical response of the solid-phase under current limited peak loads.

Abstract: Ca-containing anodic alumina (CAA) has been successfully prepared by anodizing Al film in an alkali solution at a constant voltage and subsequently electro-depositing calcium salts on and into porous anodic alumina. This paper studied the induction ability of Ca-containing anodic alumina (CAA) for calcium phosphates salts by immersing CAA in a simulated body fluid (SBF). The morphologies and compositions of the inductive coating are studied in depth using SEM and XRD. The results show that the porous Ca-containing anodic alumina (CAA) exhibits good induction ability of calcium phosphates in SBF. The Ca/P atomic ratio of the inductive coating on CAA after 7 days immersion in SBF is of 1.68 and the inductive coating on CAA is apatite. Consequently apatite /Al2O3 (anodic) composite has been obtained after apatite inductively formed on Ca-containing anodic alumina. Tafel polarization test indicates that apatite /Al2O3 composite has good anti-corrosion ability in simulated body environment. Therefore, Ca-containing anodic alumina films are promising substrates for fabricating functional coatings and its inductively formed apatite/Al2O3 composite is a promising material for hard tissue repair applications.

Abstract: Investigations have been conducted on the fatigue behavior of Ti-6Al-4V alloy with the bimodal microstructure in air at room temperature, in vacuum at room temperature and in vacuum at low temperature (100K), respectively. The results show that the fatigue life of this alloy is longer in vacuum and/or at low temperature than that in air at room temperature. The combination of vacuum and low temperature can much improve the fatigue properties of this alloy because of their purifying effect on fatigue cracks propagation. SEM observation of fatigue fracture surface indicates that all the fatigue cracks initiate in the sample surface when Nf ≦106 cycles. The above three testing conditions lead to different modes of fatigue crack propagation and therefore much different morphology of fatigue fracture.

Abstract: The influence of loading rate on the tensile fracture of polystyrene-polyisoprenepolystyrene (SIS) and polystyrene-poly(ethylene-co-butylene)-polystyrene (SEBS) has been investigated. The tensile strength of SIS initially increased with increasing strain rate, eventually reaching a plateau at elevated strain rates. In contrast, the tensile strength of SEBS was relatively unaffected by strain rate. The fracture surfaces of the tensile test specimens were examined by scanning electron microscopy. The fracture surface morphologies indicated that fracture initiated via cavitation, followed subsequently by void coalescence and catastrophic fracture. For both materials there was no qualitatively obvious change in fracture surface morphology with increasing strain rate. The results indicate that the ultimate strength of styrenic thermoplastic elastomers is governed by the nature of the dominant failure mechanism at the molecular scale; when chain scission dominates, the tensile strength is independent of the strain rate, but when chain pull-out dominates, the tensile strength increases with increasing strain rate.

Abstract: High pressure die-casting (HPDC) is widely used as a cost-effective way to massproduce metal components that are required to have close dimensional tolerances and smooth surface finishes. Approximately 50%, by mass, of the aluminium castings produced worldwide are made by this manufacturing route. However, HPDC components are relatively porous compared with other types of castings and so cannot usually be conventionally heat treated to improve mechanical properties. This follows because during solution treatment (e.g. at 540°C for 8h), the pores expand, resulting in unacceptable surface blisters, distortion and poor mechanical properties. Recent work within the CSIRO Light Metals Flagship has revealed a heat treatment procedure by which the problems of blistering and distortion can be avoided [1]. As a result, large improvements in strength have been achieved, as compared with the as-cast condition. One uncertainty is the behaviour of heat treated HPDCs under cyclic stress and this paper investigates the fatigue properties of a common high pressure die-casting alloy, A380 (Al-8.5Si-3.5Cu). Comparisons are made between as-cast, T4 and T6 conditions. Fatigue strength is highest for the alloy aged to a T6 temper and ratios of fatigue strength to tensile strength for the as-cast, T4 and T6 conditions are constant at a value of approximately 0.6, which is particularly high for aluminium alloys.

Abstract: A section of gas pipeline containing dormant stress corrosion cracks was removed from service and pressure cycled, and the crack growth from fatigue was measured. Crack growth was able to be conservatively calculated by BS7910. Parts of the pipeline section had composite repair sleeves placed over it in order to compare fatigue crack growth of sleeved and unsleeved cracks. Sleeved cracks consistently showed less crack growth than unsleeved cracks; this is believed to be due to reduced hoop stresses in the pipe under the composite repair sleeve and reduced crack opening. A simple model of the sleeve repair was developed which was consistent with the measured strains in the pipe. The application pressure of the sleeve repair affects the amount of stress reduction in the pipe and the amount of crack growth experienced. Two possible methods of repair of SCC affected pipelines were validated by this work.

Abstract: Two practical methods have been used to estimate thermal shock stress intensity factors (TS-SIF`s) associated with a flexibly restrained cracked plate. The first method used a traditional mechanical weight function (MWF) methodology in conjunction with the MWF derived for an equivalent freely restrained cracked component. The second method used a traditional MWF methodology in conjunction with the MWF derived for an equivalent rigidly restrained cracked component. Fracture mechanic finite element analyses were also undertaken to construct a comparative benchmark for the TS-SIF`s associated with a flexibly restrained cracked plate. The results of this comparison demonstrate the errors associated with these MWF methods to be acceptable, if and only if, the crack size is small, the crack component aspect ratio is large or the flexible boundary restraint stiffness are deemed negligible or near rigid. In addition, the curves derived with these MWF methods do not intersect and there is no obvious transition to distinguish which MWF method would be more appropriate for a given a particular flexible boundary restraint configuration. In effect, this means that these MWF methods cannot be confidently used to evaluate the TS-SIF`s associated with a flexibly restrained cracked component.