Novel ray-tracing algorithms in NDE: Application of Dijkstra and A⁎ algorithms to the inspection of an anisotropic weld

The degradation of ultrasonic array images due to propagation through an anisotropic material presents a significant inspection problem to the engineering industry. If the distribution of anisotropy is known, ray-tracing algorithms can be used to predict the path of sound through the material and hence correctly image anisotropic components. Conversely, ray-tracing can be used as part of an inversion procedure to infer the anisotropic properties from measured time-of-flight data. However, inversion methods often require thousands of ray-traces to map a single weld and as such, a rapid ray-tracing algorithm is essential for use. This paper explores the use of two path-finding algorithms as applied to a ray-tracing scenario: Dijkstra's algorithm and the A^⁎ algorithm. Although prevalent within computer science applications due to their low computation time, both algorithms have seen little use within the Non-Destructive Evaluation (NDE) field. This paper aims to both describe the algorithms and to demonstrate their relative merits for application in NDE. Dijkstra's algorithm was applied to an anisotropic weld inspection and the optimal parameters explored, drawing comparison to an equivalent inspection using a beam-bending algorithm to ray-trace. A comparison of accuracy and computation time between Dijkstra's algorithm and the A^⁎ algorithm shows them to maintain similar accuracy, but the A^⁎ algorithm to exhibit significant reductions in computation time.     

A closer look at the local responses of twin and grain boundaries in Cu to stress at the nanoscale with possible transition from the P–H to the inverse P–H relation

Nanocrystalline copper is considered to be a candidate for electrical contacts for dynamic systems because of its intrinsic conductivity and enhanced fretting resistance. However, the enhanced electron scattering at high-density grain boundaries significantly deteriorates the overall conductivity of nanocrystalline copper. Recent studies suggest that nanosized twin boundaries in copper might be a solution to such a dilemma. To better understand the general mechanical behavior of nanotwin boundaries, we conducted molecular dynamics simulation studies to investigate responses of both nanotwin and nanograin boundaries in copper to stress at the nanoscale, particularly in the critical range of 5–25 nm where the inverse Petch–Hall relation (P–H) may occur in nanocrystalline copper. The obtained results suggest that the twin boundary blocks dislocation movement more effectively and the degree of emitting dislocations under stress is considerably lower than that of grain boundary, leading to superior mechanical behavior. The inverse P–H relation is not applicable to the nanotwinned system. It is also demonstrated that the inverse P–H relation occurring in nanograined materials does not necessarily result from grain boundary sliding.