Fatigue crack growth and related damage mechanisms were investigated experimentally in a hybrid laminate consisting of carbon fibre-reinforced epoxy and an aluminium alloy. During the fatigue cycling, the strain within a defined area in the vicinity of the crack was measured directly using embedded fibre-optic Bragg grating sensors. The progress of delamination damage was monitored by means of an in situ ultrasonic C-scanning technique. A simplified empirical model has been applied to determine an effective stress intensity factor applied to the crack tip in the alloy material. Fatigue crack growth rate in the hybrid laminate, characterised using the effective stress intensity factor, was demonstrated to be in reasonable agreement with the data for the monolithic aluminium alloy, showing that the methods used to partition the load were reliable. The paper thus demonstrates a viable technology for in situ strain measurements, offers a new insight into the damage mechanisms that prevail in hybrid laminates, and validates simplified procedures for estimating crack growth.

A complete and detailed analysis of the microstructural development during ageing in an 8090 (Al-2.3Li-1.2Cu-1Mg-0.1Zr) alloy, an 8090/20wt%SiCp MMC, an Al-1.5Li-Cu-Mg MMC and an Al-Cu-Mg MMC (all with similar Cu and Mg contents) has been performed. Volume fractions of all precipitates relevant for precipitation strengthening of the alloys (δ' phase, S' phase and GPB zones) have been determined using a recently derived method based on differential scanning calorimetry (DSC). The volume fractions have subsequently been successfully fitted using a novel model for transformation kinetics. The sizes of these precipitates have been analysed using newly derived expressions consistent with the latter model. As a result of dislocation generation around misfitting SiC particles the volume fractions of both GPB zones and S' phase depend strongly on the presence of these particles. Also the amount of Li present in the alloys influences the volume fractions of the phases significantly. The sizes of S' are similar for the four alloys.

Hot rolled Al-6Li-1Cu-1Mg-0 center dot 2Mn (at.-%) (Al-1 center dot 6Li-2 center dot 2Cu-0 center dot 9Mg-0 center dot 4Mn, wt-%) and Al-6Li-1Cu-1Mg-0 center dot 03Zr (at.-%) (Al-1 center dot 6Li-2 center dot 3Cu-1Mg-0 center dot 1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (MAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150 degrees C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190 degrees C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S ''), (c) S phase and (d) delta' spherical particles rich in Li. The Zr containing alloy also contains beta' (Al3Zr) precipitates and composite beta'/delta' particles. The beta' precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.

Age forming of lower wing skin structures for civil airframes requires an alloy with good age formability and mechanical properties (yield strength, ultimate tensile strength, fatigue resistance, toughness). Using property modelling and general metallurgical understanding, a series of Al-Cu-Mg-Li (Mn, Zr, Sc) alloys have been designed. After artificial ageing representative of age-forming several of the newly designed alloys have yield strength, fatigue crack growth resistance and toughness that are at least comparable to the incumbent damage tolerant material for such applications, viz. 2024-T351. Coarse grain structure and high Li content are seen to be associated with good fatigue resistance but reduced formability, and an optimum balance needs to be sought.

Fatigue crack closure has been studied in an aluminium alloy via microtomography. Gallium wetting has been used to visualise grain structure, along with electron backscattered diffraction assessment of grain orientations and crack trajectories. Through this synthesis of techniques, a detailed direct assessment of microstructure and closure micromechanisms has been achieved.

A novel computational modelling technique has been developed for the prediction of crack growth in load bearing orthopaedic alloys subjected to fatigue loading. Elastic-plastic fracture mechanics has been used to define a three-dimensional fracture model, which explicitly models the opening, sliding and tearing process. This model consists of 3D nonlinear spring elements implemented in conjunction with a brittle material failure function, which is defined by the fracture energy for each nonlinear spring element. Thus, the fracture energy criterion is implicit in the brittle material failure function to search for crack initiation and crack development automatically. A degradation function is employed to reduce interfacial fracture properties corresponding to the number of cycles; thus fatigue lifetime can be predicted. Unlike other failure modelling methods, this model predicts the failure load, crack path and residual stiffness directly without assuming any pre-flaw condition. As an example, fatigue of a cobalt based alloy (CoCrMo) femoral stem is simulated. Experimental fatigue data was obtained from four point bending tests. The finite element model simulated a fully embedded implant with a constant point load. Comparison between the model and mechanical test results showed good agreement in fatigue crack growth rate.

In this contribution, the validity of a number of key quench factor analysis (QFA) assumptions is discussed. It is shown that the incorporation of a square root dependency of yield strength on precipitate volume fraction provides a sounder physical basis for quench factor modelling. Peak-aged strength/hardness prediction accuracies are not affected, but C-curve positions are. It is also demonstrated that transformation kinetics are described more correctly by a modified Starink–Zahra equation than by a Johnson–Mehl–Avrami–Kolmogorov type equation, yielding better prediction accuracies when a physically realistic Avrami exponent of 1.5 or greater is used. Finally, a regular solution model is introduced to quantify the influence of the solute solubility temperature-dependency on the minimum strength. These improvements are all implemented within the framework of classical QFA.

To improve implant biocompatibility, we developed a simple cost-effective thermal surface treatment allowing an increase in the oxide layer thickness of a titanium (Ti) alloy used in orthopaedic implants. The goal of this study was to test in vitro the reaction of osteoblasts to the developed surface treatment and to compare it to the osteoblast reaction to two other surface treatments currently used in the practice of implant surgery. Quantification of osteoblast gene expression on a large scale was used in this study. The kinetics of gene expression over 120 h was followed for 58 genes to quantify the effect of the developed surface treatment. Twenty eight genes were further selected to compare the effects of surface treatments on osteoblasts. Based on the genes studied, we could propose a general pathway for the cell reaction according to the surface treatments used: (1) metal ion release changes the time course of gene expression in the FAK pathway; (2) once the accumulation of metal ions released from the Ti surface exceeds a threshold value, cell growth is diminished and apoptosis may be activated; (3) PTK up-regulation is also induced by metal ion release; (4) the expression of Bcl-2 family and Bax may suggest that metal ions induce apoptosis. The developed treatment seems to increase the Ti–6Al–4V biocompatibility as highlighted by the lower impact of this treatment by the different pathways studied, on the lower inflammatory reaction that could be induced, as well as by the lower induced osteoblast apoptosis compared to the two other surface treatments.

The suitability of age forming for the shaping of damage tolerant structures is investigated by formulating and testing new alloy-age forming combinations. The alloy formulation process is driven initially by modelling of strength and semi-quantitative understanding of other microstructure-property relations. Using this a range of Al-Cu-Mg-Li-(Zr-Mn) based alloys predicted to provide yield strengths in aged condition comparable with incumbent 2024-T351 alloy for lower wing skins are selected. It is shown that several of these new alloys after artificial aging representative of age-forming have proof strength (PS), fatigue crack growth resistance (FCGR) and toughness that are comparable or better than 2024-T351. UTS to PS ratios of the new alloys are lower than 2024-T351.

The stress distribution within the polyethylene insert of a total knee joint replacement is dependent on the kinematics, which in turn are dependent on the design of the articulating surfaces, the relative position of the components and the tension of the surrounding soft tissues. Implicit finite element analysis techniques have been used previously to examine the polyethylene stresses. However, these have essentially been static analyses and hence ignored the influence of the kinematics. The aim of this work was to use an explicit finite element approach to simulate both the kinematics and the internal stresses within a single analysis. A simulation of a total knee joint replacement subjected to a single gait cycle within a knee wear simulator was performed and the results were compared with experimental data.The predicted kinematics were in close agreement with the experimental data. Various solution-dependent parameters were found to have little influence on the predicted kinematics. The predicted stresses were found to be dependent on the mesh density. This study has shown that an explicit finite element approach is capable of predicting the kinematics and the stresses within a single analysis at relatively low computational cost.

Hardness data for cast, homogenised, solutionised and subsequently peak aged 6082 samples that had undergone a range of homogenizing and quenching treatments can be modelled well using a recently derived model. Hardness increases with homogenizing temperature and time for all quenched conditions. In contrast with extruded 6082 alloys, the homogenizing condition has little effect on quench sensitivity, even though the density of dispersoids, which act as nucleation sites for non-hardening precipitates, decreases markedly on increasing homogenizing temperature.

The purpose of the present work was to examine the effect of different Ti–6Al–4V surface treatments on osteoblasts behaviour. Previous work in this laboratory has demonstrated that an ageing treatment reduces metal ion release from this alloy compared to standard passivation procedures. In this study, human osteosarcoma MG-63 were used in short-term in vitro tests to assay for cell viability and cell proliferation at 12, 24 and 72h while SaOS-2 were used in long-term in vitro tests to assay for osteonectin, osteopontin, osteocalcin gene expression, total protein amount (TP), alkaline phosphatase activity (ALP) and fibronectin production (FN) for 1–4 weeks. Epifluorescence microscopy was used to observe SaOS-2 cell morphology. After 24h, there was no difference in MG-63 cell viability/proliferation or in SaOS-2 cell morphology between the different surface treatments. For the long-term tests, the aged Ti–6Al–4V induced significantly higher cell proliferation than the control Ti–6Al–4V at 72h. At week 1, no difference in the osteonectin, osteopontin, and osteocalcin gene expression was found between samples. The peak of ALP activity appeared earlier at week 2 for the control surface compared with the passivated and aged surfaces. The early increase in ALP activity for the control sample could be a compensatory effect of decreased osteoblasts proliferation. There was no difference in the expression of FN for the different surface treatments. Our present results showed that the different surface treatments, which induced different metal ion release kinetics and surface properties, influenced the cell proliferation and ALP activity of osteoblast cells. Aluminium ions release kinetics as well as presence of vanadium ions may play a major role in influencing the osteoblasts behaviour in the present study.

Various reports in the literature have highlighted the effects of particle distribution on the fatigue behaviour of particulate reinforced metal matrix composites (PMMCs), although few attempts have been made at modelling such effects. A micromechanical understanding of the effects of clustering on short crack growth behaviour in Al–SiCp composites has been achieved via finite element modelling. Comparison of preliminary models with the literature has shown that shielding/anti-shielding effects were significantly affected by the relative sizes of the particle and the overall model such that, when edge effects were removed, a crack was predicted to be accelerated rather than decelerated as it propagated through closely spaced pairs of particles. Consistent differences were identified between models with homogeneous versus clustered particle arrangements in terms of crack path morphologies and local crack–tip stress intensity fluctuations. Furthermore, predicted influences of clustering on growth rates in the numerical models were found to be consistent with previous experimental results (i.e. growth rates rose with increased clustering), demonstrating that load transfer effects associated with changes in particle distribution may play a direct role in controlling the growth of short cracks in these materials.