Differential geometry of ray surfaces in anisotropic solids and its contribution to NDE: modelling and experiment

Point-source/point-receiver techniques are one of the most widely used methods for nondestructive evaluation of anisotropic materials. The group velocities resulting from these techniques must be, for further inverse evaluation of elastic coefficients, geometrically converted into corresponding phase velocities. On the other hand, the phase velocities can be determined from a material's response to a line source. But, due to the anisotropy, the short line sources generated by cylindrical lenses are insufficient for reliable determination of the phase velocity. In this paper, a long line source is approximated by a set of linearly arranged point sources. As it follows from the differential geometry of ray surfaces, information obtained from such set of sources is sufficient for determination of phase velocities of both the quasi-transverse and the quasi-longitudinal modes of propagation. Moreover, this approach can be generalized for any arbitrary set of point sources only by employing a proper time-base transformation. The applicability of the presented approaches is illustrated on transversely isotropic and tetragonal fibrous composite materials.     

The segregation of As,Sb and Bi in Cr-Ni steels and alloys with different Ni contents investigated using AES

This paper reports on an AES study of grain boundary segregation in five kinds of Cr-Ni corrosion-resistant steels and alloys with different Ni contents that were enriched by As, Sb or Bi. The results show that all mentioned impurities segregate to the grain boundaries and that the segregation level depends on the type of the impurity. The highest amount of segregant atoms was found in the case of Bi, while it appeared to be substantially lower for As and Sb. The segregation level of each impurity grows also with the Ni content. This increase is more pronounced for Bi and Sb than for As. In particular the results demonstrate that the enrichment of grain boundaries by a given impurity is strongly connected to the solubility of this element in the matrix.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Sensitivity analysis of an inverse procedure for determination of elastic coefficients for strong anisotropy

The elastic coefficients of anisotropic solids are often evaluated from measurements of phase or group velocities of ultrasonic bulk waves by the usage of inverse optimizing procedures. This paper discusses the effects of various factors on such procedures results for transversely isotropic solids with considerably strong anisotropy. First, the inverse determination of all elastic coefficients of unidirectional CFRP composite is briefly outlined. Then the results of the optimization are treated as exact values and the sensitivity of the optimizing process versus main considered sources of inaccuracies is analyzed. Results of extensive simulations are presented to illustrate the effect of input data distortion, input data incompleteness, and geometrical conversion from experimentally obtained group velocities into corresponding phase velocities used as input data for the optimizing procedure. The paper takes note of how information about the elastic coefficients can be extracted from the different segments of the phase velocity surface. The stability versus input data distortion for inversion from group velocities and phase velocities is compared and the importance of reliable geometrical converting from group into phase velocities is illustrated. An novel method for geometrical conversion of distorted group velocity data into corresponding phase velocities based on affine combinations of low-order polynomials is presented and compared with piecewise or high-order polynomial fitting.     

AES study of underclad cracking

The present work deals with the occurrence of underclad cracks in three types of steels differing in the contents of the alloying elements (Cr, Ni, Mn) and in the impurities level (As, Sb, Sn). The presence of the intercrystalline fracture typical for the underclad cracks was proved to be dependent on the temperature and on the ageing time. It follows from the results of Auger electron spectroscopy, that the sulphur and phosphorus segregations accompanied by a higher number of carbide (possibly sulphide) particles cause a decrease of grain boundary strength. Segregations of other impurity elements were not detected at grain boundaries.