Effects of testing variables on the measured compressive strength of high-strength (90 MPa) concrete /

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High-strength resorbable brushite bone cement with controlled drug-releasing capabilities

Brushite cements differ from apatite-forming compositions by consuming a lot of water in their setting reaction whereas apatite-forming cements consume little or no water at all. Only such cement systems that consume water during setting can theoretically produce near-zero porosity ceramics. This study aimed to produce such a brushite ceramic and investigated whether near elimination of porosity would prevent a burst release profile of incorporated antibiotics that is common to prior calcium phosphate cement delivery matrices. Through adjustment of the powder technological properties of the powder reactants, that is particle size and particle size distribution, and by adjusting citric acid concentration of the liquid phase to 800 mM, a relative porosity of as low as 11% of the brushite cement matrix could be achieved (a 60% reduction compared to previous studies), resulting in a wet unprecompacted compressive strength of 52 MPa (representing a more than 100% increase to previously reported results) with a workable setting time of 4.5 min of the cement paste. Up to 2 wt.% of vancomycin and ciprofloxacin could be incorporated into the cement system without loss of wet compressive strength. It was found that drug release rates could be controlled by the adjustable relative porosity of the cement system and burst release could be minimized and an almost linear release achieved, but the solubility of the antibiotic (vancomycin > ciprofloxacin) appeared also to be a crucial factor.

Formability of high strength aluminium sheet

An initial review of the subject emphasises the need for improved fuel efficiency in vehicles and the possible role of aluminium in reducing weight. The problems of formability generally in manufacture and of aluminium in particular are discussed in the light of published data. A range of thirteen commercially available sheet aluminium alloys have been compared with respect to mechanical properties as these affect forming processes and behaviour in service. Four alloys were selected for detailed comparison. The formability and strength of these were investigated in terms of underlying mechanisms of deformation as well as the microstructural characteristics of the alloys including texture, particle dispersion, grain size and composition. In overall terms, good combinations of strength and ductility are achievable with alloys of the 2xxx and 6xxx series. Some specific alloys are notably better than others. The strength of formed components is affected by paint baking in the final stages of manufacture. Generally, alloys of the 6xxx family are strengthened while 2xxx and 5xxx become weaker. Some anomalous behaviour exists, however. Work hardening of these alloys appears to show rather abrupt decreases over certain strain ranges which is probably responsible for the relatively low strains at which both diffuse and local necking occur. Using data obtained from extended range tensile tests, the strain distribution in more complex shapes can be successfully modelled using finite element methods.Sheet failure during forming occurs by abrupt shear fracture in many instances. This condition is favoured by states of biaxial tension, surface defects in the form of fine scratches and certain types of crystallographic texture. The measured limit strains of the materials can be understood on the basis of attainment of a critical shear stress for fracture.

The effect of frequency and microstructure on corrosion fatigue crack propagation in high strength aluminium alloys

Fatigue crack growth in high strength aluminium alloy 7150 commercial plate material has been studied in both laboratory air and acidified aqueous salt solution. The aggressive aqueous environment enhanced fatigue crack growth rates by up to an order in magnitude compared to laboratory air. The enhancement in fatigue crack growth rate was accompanied by evidence of embrittlement in the crack path, involving both brittle intergranular and transgranular failure modes. Both the enhancement of fatigue crack growth rates and the extent of intergranular growth modes are dependent on cyclic frequency which, along with the absence of a similar frequency effect in a spray-formed version of the material with a significantly different grain structure, supports a mechanism of grain boundary hydrogen diffusion for intergranular corrosion fatigue crack growth. The convergence of corrosion fatigue crack growth rates at high ΔK in both spray-formed and conventional plate materials coincides with the operation of identical transgranular corrosion fatigue modes dependent on strain-controlled hydrogen diffusion ahead of the crack tip. © 1997 Acta Metallurgica Inc.