Elastic constants determination of anisotropic materials from phase velocities of acoustic waves generated and detected by lasers.
This paper describes a method which allows the synthesis of acoustic plane waves by means of laser ultrasonic techniques. When a laser line source radiates at the surface of an anisotropic material, intricate effects are observed on signals obtained by transmission experiments. The material characterization from such a set of signals is not a trivial task since it requires correctly selecting the data necessary for the elastic constants determination, by distinguishing them from all the parasitic phenomena. In the present paper, it is shown that a judicious sum of signals obtained for various positions of the line source at the surface of a sample leads to the synthesis of an acoustic plane wave for which the odd phenomena disappear. Moreover, by applying a constant delay between the various signals, acoustic plane waves can be synthesized with varied refracted angles. One of the advantages of this technique is to offer convenient access to the phase velocity, providing an easier determination of the elastic constants. This method is successively applied to recover the stiffness coefficients of a silicon crystal and of a composite material. The good agreement between the so-recovered elastic constants and values given by other processes shows the contribution of such a method in the field of material characterization.
Generating acoustic waves by laser: theoretical and experimental study of the emission source
Abstract A laser pulse incident on a material may generate acoustic waves by means of two different phenomena: by thermal expansion at low incident power density (thermoelastic effect); and by vaporizing surface material at high incident power density (ablation effect). A synthesis of various theoretical models representing the acoustic displacements generated by a point laser impact is given, and then validated by interferometer measurements of ultrasonic displacements generated both under ablation conditions and under thermoelastic conditions.
Aging live Antarctic fur seals and southern elephant seals
This study describes a method for extracting post-canine or incisor teeth from live antarctic fur seals (Arctocephalus gazella) and southern elephant seals (Mirounga leonina) respectively and their use to determine age in a field situation. Dental elevators were used to loosen the teeth from the alveolus and periodental ligament. Most teeth were removed within l-2 min and a total of 214 and 81 teeth were collected by this method from antarctic fur seals and southern elephant seals respectively. No seal recaptured at intervals up to a year after a tooth was extracted showed signs of infection or distress related to removal of the tooth. Teeth were thin-sectioned for the purpose of aging. In both species cementum growth layer groups were a more satisfactory indicator of age than dentinal growth layer groups. Estimates of age from cementum growth layers were confirmed for Antarctic fur seals using seals which had been tagged as pups up to 16 yr before sampling.