Lorentz force particle analyzer with an array probe based on speckle pattern interferometry

Abstract

A new contactless technique called Lorentz force particle analyzer (LFPA) with an array probe for detecting the flaws in metallic material is presented in this paper. Based on the principle of LFPA, the shape and size of the flaw or the direction of the crack can be obtained by analyzing the pulses of the force acting on the permanent magnet. In the LFPA system, the small Lorentz force on the magnet is measured by a laser-cantilever system with high sensitivity, which operates in a similar principle to that of an atomic force microscope. The traditional displacement detecting method in the LFPA is not suitable for the array probe presented in this paper due to its complex structure. Therefore, speckle pattern interferometry is introduced into the LPFA. The speckle pattern interferometry can measure not only the out-of-plane displacement of the multiple cantilever in the array probe, or of slopes of deformation, but also the in-plane displacement. Those advantages make the speckle pattern interferometry a useful tool in the LFPA for analysing the shapes of the flaws and the directions of the cracks. In this paper, a Michelson-type shear of graphic setup with enlarged angle of view is built to measure the displacement of the cantilever which is deformed by the flaws in the sample. Four frames of shear under several grams before and after the deformation are captured and recorded by a digital camera. The phase difference is processed for calculating the displacement with the software which is designed for the LFPA. A full-field measurement of the cantilever displacement is achieved and the relationship between the phase difference and the volume of the flaws is also obtained successfully. The utilization of the speckle pattern interferometry technique in the LFPA leads to the invention of a new real-time, online, in-situ contactless technique of detecting the shapes of the internal flaws and the directions of the cracks.