An Ultrasonic Study on Anelasticity in Metals

Ultrasonic waves are highly sensitive to microstructural variations in materials and have been used extensively to investigate anharmonic effects in various metals and alloys[1–3]. A major focus of these studies is on the higher order elastic constants and their relation to the microstructure of the material. Ultrasonic techniques have also proven quite useful for characterizing the stress state of a material [4–6]. Recently, while using the magnetoacoustic (MAC) method to investigate the residual stress in various steel samples, a time dependent change in the results was observed. It became apparent that the measurements were exhibiting anelastic effects due to some intrinsic properties of the samples.</p

ELASTIC CONSTANTS OF A STRESSED LAYER FROM SURFACE ACOUSTIC WAVE MEASUREMENTS

It is well known that the propagation of both bulk and surface acoustic waves (SAWs) is affected by the presence of static stresses, a phenomenon known as the acoustoelastic effect. Ultrasonic measurements of velocity therefore depend on the stresses within the material, as well the elastic constants and the density. Although the effect of stress on the velocity is small, many ultrasonic methods are sufficiently accurate to detect the changes involved. When inverting such measurements to obtain elastic constants, it is desirable to take the effect of stress into account. Similarly, when using ultrasonic methods to measure stress, it is necessary to have accurate values for the elastic constants. In practice, the material parameters of the ‘natural’, unstressed state are often either completely unknown or not known with sufficient accuracy (as is usually the case for residual stresses), or else cannot be assumed to be equal to bulk values (as in the case of layered materials). This is a major distinction between situations involving residual as opposed to applied stress, since a reference state of some description is always available in the latter case

Groupwork and computers: Task type and children's performance

In the majority of British primary schools, children use the computer in groups of two or three. This is partly due to the lack of resources, that is, most classrooms still only have one or two computers to share between around 30 children. Groupwork on computers is also justified for pedagogic reasons. Previous research comparing children working in groups or alone has revealed an advantage for children working in groups although the work undertaken has generally been problem solving tasks. This investigation examined the performance of pairs and individual 6-year old children on a drill and practice program using a pre-test, intervention, post-test design. In comparison to the problem solving evidence, individuals were found to have a significant advantage over pairs during the computer-based task.link_to_subscribed_fulltex