Showing papers by "Peter Davies published in 2001"

The relative importance of shading and nutrients on algal production in subtropical streams

Abstract: 1. We used artificial substrata in forested and open streams in South-East Queensland, Australia, to determine the relative importance of shading from riparian vegetation and of nutrients on periphyton growth, and whether nitrogen and/or phosphorus limited algal productivity. 2. Nutrient-diffusing substrata consisting of agar enriched with N, P and N + P, and controls without nutrients, were deployed in duplicate at 15 sites in headwater streams with riparian canopy cover ranging from 0 to 88%. 3. Shading was the over-riding factor controlling periphyton biomass accrual on the artificial substrata, with nutrients playing a relatively minor role. Microscopic examination of periphyton scrapings taken after 7 weeks revealed that diatoms dominated on the artificial substrata in shaded streams, whereas filamentous green algae dominated the algal assemblage in the more open canopy streams. 4. Whilst nutrients had little effect on the accrual of algal biomass compared with riparian shading, there was evidence that nitrogen, and not phosphorus, stimulated periphyton production in streams with sufficient light.

Identifying the natural flow regime and the relationship with riparian vegetation for two contrasting western Australian rivers

Abstract: The natural flow regime and the relationship between flows and riparian vegetation are described for sites on both the Blackwood River in south-western Australia and the Ord River in north-western Australia. Analysis of long-term flow data showed the historic mean monthly river discharge for the Blackwood River is strongly seasonal and highly predictable with generally low variability each month. The Ord River showed a strong seasonality of flows with about 92% of the (total) yearly flow occurring between December and March. Flow variability was very high (e.g. coefficient of variation >100% for all months) but highly predictable, with this mostly attributed to low but constant dry-season flows. Water depth, duration of flood events and the number of flood events per year show a significant correlation with aspects of the riparian vegetation within experimental vegetation plots. Results highlight the strong relationship between floristics, life form structure and population dynamics with stream hydrology. On the Blackwood River, species richness and cover of shrubs reduced with increased duration and frequency of flooding, while cover of exotic species and annual herbs increased with increased flooding. Germination of tree seedlings was not influenced by flood regime but size class of tree species increased with flooding frequency. On the Ord River, species richness was not influenced by flooding regime. However, cover of perennial grasses increased with flooding frequency whilst cover of shrubs decreased. There was no relationship between flooding and seedling establishment whilst tree size class decreased with increased flooding. The methods described here can be used to compare the response of different components of the riparian vegetation to different fluvial regimes (e.g. because of impoundment and abstraction). This technique can be expanded for the management of riparian zones and planning rehabilitation programmes. It may also be useful for improving the ecological knowledge base for setting environmental flows in regulated systems. Copyright © 2001 John Wiley & Sons, Ltd.

Sea Water Aging of Glass Reinforced Composites: Shear Behaviour and Damage Modelling:

Abstract: This paper presents results from a study of the wet aging of four thermoset resins and their [0°/90°] stitched glass fibre reinforced composites. The matrix resins are orthophthalic polyester, isophthalic polyester, vinyl ester and epoxy. Resins and composites were aged for 18 months, under three immersion conditions: 20°C sea water, 50°C sea water and 50°C distilled water. Tensile tests, on resins and at 45° to fibre direction of composites, both before and after aging enable the influence of matrix resin and aging medium on weight changes and matrix dominated property degradation to be evaluated. This has enabled a unique dataset to be obtained. Alarge part of the shear property loss after aging is recovered after drying. An original application of damage mechanics parameters is used to quantify the changes in composite shear behaviour, in order to provide a more complete representation of the inelastic response.

Mode II delamination

Abstract: Publisher Summary This chapter elaborates the different aspects of the mode II delamination. Introducing shear loads into composite materials has presented major problems to the composite industry. Tests to determine shear moduli and strengths abound and all are open to criticism. Shear loading of cracked specimens involves many of the difficulties associated with these shear tests, but adds others, such as friction between sliding crack faces, instabilities in some of the specimen geometries, and non-linear behavior. When a crack propagates by the relative sliding of two crack faces, the friction between these faces can have a significant effect on the measured fracture toughness. It is advisable to perform both methods of data analysis—the beam theory and compliance calibration—because differences between the two can give an indication of the presence of fracture mechanisms that may invalidate some of the test data. Large differences between propagation values for the two methods may reflect multiple crack propagation. The recommendation in the ESIS protocol is to include either a thin film of PTFE or a pencil lead between the two crack faces to minimize friction.

Mode I delamination

Abstract: Publisher Summary This chapter discusses the test results for Mode I delamination resistance or fracture toughness, mostly from round robin tests. It also presents the developments leading to the early Mode I draft procedures for fiber-reinforced laminates using the DCB-specimen The chapter summarizes the round robin tests organized within ESIS TC4. The experimental compliance methods are based on the assumption of a certain type of functional dependence of the compliance on the delamination length. By plotting the measured specimen compliance or a quantity derived from the compliance ( sometimes including suitable corrections) versus the delamination length, the assumed functional dependence is represented in the chapter. A linear dependence between the respective quantities representing compliance and delamination length is obtained and the slope of a linear fit through the data points yields the proportionality constant. The experimental compliance methods are based on the assumption of a certain type of functional dependence of the compliance on the delamination length.

Delamination fracture of continuous fibre composites: Mixed-mode fracture

Abstract: Publisher Summary This chapter elaborates the delamination fracture of continuous fiber composites. A delamination in a composite structure is unlikely to experience pure mode I or mode II loading. The loading is more likely to be a combination of modes I, II, and III. Of the possible combinations, combined mode I and mode II has received the most attention and is termed as “mixed-mode I/II”. It is recommended that a polytetrafluoroethylene (PTFE) film insert is located at the mid-thickness of the laminate during manufacture to act as a crack starter. The maximum allowable thickness of this film is 13 μm, which is consistent with the double cantilever beam (DCB) and the ELS test protocols. Altering the position of the film to a location not at the mid-thickness has been used to intentionally alter the mixed-mode ratio. The chapter shows that that stable crack growth will be obtained from the asymmetric double cantilever beam (ADCB) test, provided that the ratio of the crack length, a, to free length, LF, be >0.41. The relevant crack length will be that of the extended crack after precracking. It has been observed that unstable crack growth can occur when testing directly from an insert, even if the condition (a/LF)>0.41 is met.

Movers and stayers: determinants of post-16 educational choice

Abstract: The authors develop an economic analysis of educational decision-making to investigate the determinants of post-16 educational choice in England and Wales. Concentrating upon the decision of whether to stay at their current school, they examine the nature of the costs and benefits of moving. They conclude that the existence of search and switching costs, together with the similarity of provision amongst providers in local schooling markets, should lead to a high degree of consumer inertia. The results from the questionnaire provide support for this proposition. Their data enables a detailed examination of the decision-making process. They show that the decision-maker is best viewed as a composite of student and parent, and that parents are still an active partner in decision-making. Movers predominately reject the A-level pathway, in favour of the more diverse curriculum available at their local further education college. However, they also find evidence that switching costs differ between movers...