The Effect of Temperature and Specimen Geometry on the Parameters of the "Local Approach" to Cleavage Fracture

The present work investigates the temperature and specimen geometry dependence of the Weibull parameters of a structural steel (BS 4360 Grade 50D). Notched tensile, notched (Charpy-type) four point bend and fracture mechanics specimens have been used to study the effect of specimen geometry. Axisymmetric and two and three-dimensional finite element analyses have been carried out to determine Weibull parameters for the three specimen types at various temperatures. The results have shown that the parameters determined using notched tensile specimens can be regarded as temperature independent. However, the parameters determined using notched bend and fracture mechanics specimens exhibited some dependence on temperature. There were also differences between the Weibull parameters determined from the different specimen geometries. The results show that there is a need to further develop the 'Local Approach' to cleavage fracture to incorporate effects of temperature and plastic constraint. At present, it is recommended that fracture mechanics specimens be used for the determination of Weibull parameters if the results are to be applied to cracked structures

Processes of particle deposition in membrane operation and fabrication

The processes that control particle deposition on surfaces that are of interest in understanding operational aspects of membrane filtration, also hold significance in controlling the morphology of particle deposits as intermediate steps in membrane fabrication. This paper summarizes processes controlling particle deposit morphology. The implications of these processes for understanding membrane fouling by particles and in fabricating membranes are then considered.</jats:p

Transient permeate flux analysis: model validation

An extended model integrating several particle back-transport theories has been proposed by Sethi and Wiesner to predict transient permeate flux as a function of operational parameters for constant permeate flux (CF) and constant transmembrane pressure (CP) cross-flow regimes. Filtration experiments using monodisperse latex suspensions of particles 0.02 μm, 0.12 μm and 0.68 μm in diameter in a cross-flow flat slit membrane were carried out for model validation. In a separate set of filtration experiments with monodisperse latex particles 0.10 μm, 0.43 μm and 1.00 μm in diameter, the transient behavior of permeate flux in CP and CF modes of operation was compared. Significant differences between model calculations and experiments were observed. A slightly more complex structure at the cake-suspension interface was hypothesized to explain these differences. A representation of the cake with a gradually changing solids concentration allowed for better matching of the extended model and experimental results. The CP operation mode yielded a higher specific permeate flux resulting from a thinner cake or a cake with a lower average specific resistance. Experimentally observed differences between performance in CP and CF modes were less sensitive to the size of particles in the suspension than predicted by theory.</jats:p

Application of the Local Approach to Cleavage Fracture to Failure Predictions of Heat Affected Zones

With the increased capabilities of elastic-plastic finite element analyses and hence the possibilities to determine accurately the stress distribution ahead of sharp cracks the use of fracture risk predictions based on a local fracture parameter has recently attracted much interest. To date most investigations in this area concentrated on homogeneous materials avoiding the complications related to the complex situation of welded joints. In this paper results are presented where the Local Approach to cleavage fracture has been applied to the heat affected zone of two structural steel welds. Local cleavage fracture parameters were determined from tests and finite element modelling of notched tensile specimens. In all cases the properties of the weld metal, the parent material and the heat affected zone (HAZ) were determined. A thermal simulation technique was used to produce large enough areas of HAZ microstructures for tensile and notched tensile testing. Subsequent application of the Local Approach to the crack tip situation for the prediction of the dependence of fracture toughness on temperature and the scatter in fracture toughness only produced satisfactory results for a limited number of the situations modelled. Discrepancies between predictions and actual results are thought to be due partly to the differences in FE mesh and stress distribution between notched tensile and pre-cracked bend specimens