A comparative study between deflectometry and shearography for detection of subsurface defects

摘要

Nondestructive testing of objects is the basis for quality control in a production line. There exists a wide range of optical and tactile methods for the detection of surface defects. For hidden defects (below the surface) different methods like X-ray or ultrasound are state of the art; also, optical methods like thermography and interferometry can be used in combination with a load. This load can be mechanical, electrical or thermal and is used to produce a measurable signal (deviation of the surface, thermal signature) on the surface. Typically, the surface or the surface gradient of a specimen in a loaded and an unloaded state is measured and the two results are compared afterwards or in real time. The evaluation of shape differences is easier than measuring absolute shapes because systematic errors (e.g. calibration) cancel themselves out and the resolution mostly depends on the measurement system's sensitivity. In this paper we give an overview of the different parameters influencing the successful implementation of optical nondestructive testing (ONdT) methods. In a second step, we compare shearography and deflectometry, identify relevant parameters and show restrictions of both methods with regard to the systems used. We present measurements with different methods and show how these results can be compared. We discuss the feasibility of both methods and the applicability of the systems used in a production line with respect to parameters concerning the quality control of produced goods.