1,721,183 research outputs found
Application of uniform design in optimisation of three stage ageing of Al-Cu-Mg alloys
No full textThe hardness variations and precipitation behaviour during the three stage ageing of three Al-Cu-Mg alloys were investigated using micro-hardness testing and differential scanning calorimetry (DSC). To facilitate the determination of an optimised heat treatment, a uniform design method was employed by which the influences of three stage interrupted ageing on the hardness and precipitation can be elucidated with a limited number of experiments. It is found that optimised heat treatment with maximum hardness can be achieved for non-stretched materials by applying the method of three stage ageing, but there is no obvious effect for stretched T351 materials. A long initial Stage I and a suitable Stage II ageing time are beneficial for hardness, and the Stage II ageing at 25C shows more beneficial effect than ageing at 65C. The hardness increase in Stage I and Stage III ageing is closely related to the S phase content
Influence of grain structure and slip planarity on fatigue crack growth in low alloying artificially aged 2xxx aluminium alloys
Full text linkThe fatigue crack growth behaviour under constant amplitude loading of three low alloying artificially aged 2xxx aluminium alloys with distinct microstructures is analysed. Fatigue crack growth tests show a correlation between fatigue performance and the occurrence of crack closure. Fractography and fracture surface measurements show that rougher surfaces give higher closure levels suggesting a dominating influence of RICC in these alloys. The relationship between the crack path and the microstructure, i.e. grain structure and slip planarity, is assessed semi-quantitatively. A criterion to evaluate the propensity for slip band formation is derived and reasonable correlation is found between the fatigue fracture behaviour of the three alloys and this criterion. <br/
Modelling of fracture toughness in high strength 7xxx aluminium alloys
No full textThe relationship between plane strain fracture toughness, KIc, tensile properties and the microstructure of two high strength 7xxx alloys has been examined. Two formulations of the 7449 composition were examined, specifically, one containing Zr as the dispersoid forming element, whilst the other contained Mn. Fractography revealed coarse voiding at intermetallics, fine tensile voiding at dispersoids and a combined intergranular/transgranular shear fracture mode. In the Zr containing alloy in particular, the observed increase in toughness with overageing is accompanied by a change in fracture mode from predominantly intergranular/transgranular shear failure to coarse voiding. A new fracture toughness model has been derived based on the microstructurally dependent work hardening factor, KA, introduced in Ashby’s theory of work hardening. This model predicts a linear relationship between KIc and KA^0.85/?ys^0.35 which is shown to be consistent with the experimental data
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Analytical modelling of the influence of local mixed mode displacements on roughness induced crack closure
No full textApplication of crack closure analytical modelling to predict the behavior of complex fatigue crack growth properties is limited partly due to restricted two-dimensional modelling approaches. An analytical model of roughness induced crack closure (RICC) is developed in the present paper considering a three-dimensional twisted and kinked crack path. Residual shear deformations at asperities in the crack wake affect the crack opening generating closure. These residual shear deformations are explicitly formulated from residual plastic deformation. The respective influence of the in-plane and out-of-plane deformation on RICC is discussed. The crack twisting is found to be a less significant effect on the RICC than the crack kinking, for similar deviation angles. However, these out-of-plane crack deflections are found to be non negligible particularly at low tilting angles. This analytical model is used to predict the closure stress intensity factor for a range of 2xxx aluminium alloys. Predictions are compared to experimental results obtained from low ?K fatigue crack growth tests. Experimental results show that fatigue performance scale with closure level and roughness of the failed fatigue specimen surfaces. Comparison of estimated and measured closure stress intensity factor show similar trends. <br/
Analytical and finite element modelling of roughness induced crack closure
No full textCrack closure is an established component of fatigue understanding, however, significant confusion remains in both experimental determination and micromechanical modelling. Analytical and finite element models of roughness induced crack closure (RICC) are developed in the present paper. A novel interpretation of RICC is explored where shear strains causing asperity contact arise explicitly from residual plastic deformation in the wake of a propagating crack, in a manner that is essentially analogous to plasticity induced crack closure (PICC). The analytical model estimates the crack opening along a simple deflected crack path, defined by a crack deflection angle and length, and residual shear at each asperity interfering with this opening to generate closure. The results from the analytical model are compared to finite element modelling and experimental results, with insight being provided into a controlling influence of the ratio of asperity size to plastic zone size on closure levels
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