레이저 스페클 간섭법에 의한 내부결함의 정량평가

Abstract

Nondestructive testing (NDT) method with the merits of rapid response, economic efficiency, and manifold applications has been an important role in industries from the safety diagnosis of large infra structure to the reliability evaluation of small products. There are many efficient NDT methods such as visual testing(VT), radiographic testing(RT), ultrasonic testing(UT), magnetic testing(MT), liquid penetrant testing(LT), eddy current testing(ET), acoustic emission(AE), thermography. Generally, VT, PT, MT, and ET are applied for the detection of surface defects and UT and RT are done for internal defects. In 1970's, a stress intensity factor in fracture mechanics had led to the need for quantitative nondestructive evaluation(QNDE) of a defect by NDT, which means that it is quantitatively determined with the characteristic of a defect such as position, size, shape and kind. Based on this factor, the reliability and life of a structure can be evaluated. With the advancement of science and technology, the demand for greater product quality and reliability has created a need for a highly efficient nondestructive testing method. Previous methods have been developing new techniques from contact to non-contact, from point by point to whole field, and with real-time. Among previous methods, UT and RT are well designed and rapidly developed for the inspection of inside defects. However, the limitation of UT, point to point scanning and contact inspection, results in time-consuming and the difficulty of application for small and complex structures and non-homogeneous structures with composite material. Although RT has the merit of whole field and non-contact inspection, but there are a few big problems of radiation hazard, the limitation of illumination direction and detectable thickness. NDT methods based on optical metrology such as Thermography, Holography, Electronic Speckle Pattern Interferometry (ESPI), Shearography etc can provide solution to the problems of previous methods. Laser application techniques have been applied for displacement measurement technique based on interferometer, laser based ultrasonic testing, Holography, speckle correlation interferometry etc. Particularly, speckle correlation interferometry can get the surface displacement on large area with real time, high resolution and non-contact basis, which gives the advantages in vibration analysis, deformation analysis and non-destructive testing. With computer science and image processing technology, film-based speckle correlation interferometry evolves into ESPI and Shearography, which substitute the film-based by the digitalized with CCD camera, frame grabber etc. Its basic idea was developed almost at the same time by Macovski, Ramasey and Scheafer in U.S.A. and Buttersand Leendertz. Phase shifting method developed by K. Creath makes ESPI more useful, which can automatically extract a phase from interference pattern and demodulate the surface displacement of a target. In speckle correlation interoferometry, the basic idea of comparison between two states of before and after deformation is a useful tool in NDT. The deformation concentration of inside or surface defect due to external loads such as vacuum, vibration, thermal, gravity etc appear as surface displacement, which is related to information of the defect. High-resolution speckle correlation interferometry is able to measure sub-micro deformation. Surface deformation on the defect appears as the distortion of fringe pattern. Shearography measures derivatives of surface displacements, contrary to ESPI which measures surface displacements. Therefore, Shearography allows strains to be determined without numerically differentiating displacement data. Strain concentrations, related to the position and size of a defect, are easily determined from Shearography result. However, these methods are improved from displacement resolution point of view. Research about quantitative evaluation of a defect, which bases the reliability evaluation of a product, is insufficient. Because there are difficulties in quantitative estimation of an inside defect with only surface information obtained by ESPI or Shearography. There are effective factors, which have influence on quantitative analysis of defects and correlated with each other. Shang, Ettemeyer, and Fulton estimated defect size in their experiment without any consideration of effective factors. Kang showed errors in quantitative evaluation of defect size by a setting of Shearography optical interferometer. We can think collectively from previous studies that NDT methods need quantitative evaluation for reliability analysis of a product. Effective factors in ESPI and Shearography, related to the setting of an optical interferometer and resolution of measurement system etc, have influence on the quantitative evaluation of a defect. However, correlations between these factors and defect size are not closely examined for getting the reliability of ESPI and Shearography in nondestructive testing. In order to determine a defect size quantitatively, this paper proposes "Critical shearing method", which is based on the findings of two maximum strain concentration points on the condition that a defect has a length. The effective factors in Shearography, external load, shearing amount, shearing direction, resolution of a measurement system, material properties of a target are classified, relations of which are examined by comparison of finite numerical analysis and experimental analysis. Also, relations of the effective factors in ESPI, external load, the resolution of a measurement system, material properties of a target are examined. Since experimental analysis verified these correlation in both methods. Hence ESPI and Shearography can determine the size and position of a defect on "Critical shearing method". The proposed method applies pressure pipeline, tire, semiconductor package and composite materials to inspect an inside defect with good agreements and high economic efficiency. Quantitative evaluation of a defect by ESPI or Shearography will lead reliable nondestructive testing method in industry.