ESPI를 이용한 하중조건에 따른 용접부의 잔류응력 정량평가

Abstract

Generally welding is a technique that joins two kinds of metal materials by heating. It has been recognized important technique that become an indispensable presence for joining of iron and nonferrous metals due to development of various welding methods with modern industry development. Adhesion Strength of welding joint is higher than the strength of rivet joint that is used in conventional structure joint. Also, a structure using welding joint could be made earlier than a structure using rivet joint. Welding plays a pivotal role in various industry fields such as shipbuilding, nuclear power plant, airplane, automobile and architecture industry because welding has many strong points like watertightness and airtightness. However, welding structures can be able to destroy unexpectedly due to the fact that quality of the material, residual stress, deformation, welding defects have an aptitude to happen easily in welding structures because welding is a kind of joint method by increasing heat. Especially, a main reason for leading to defects or destruction of structure can be residual stress. Residual stresses are stresses that remain after the original cause of the stresses (external forces, heat gradient) has been removed. In other words, stress could be occurred in the welding joints according to the change of temperature when external force does not act on the object. Especially, stress remains after cooling at room temperature completely because contraction stress is bigger when cooling The occurrence of welding residual stress caused by heat stress is influenced by external restriction of the parent metal, welding heat input, size and thickness of the parent metal when welding. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment. Heat from welding may cause localized expansion, which is taken up during welding by either the molten metal or the placement of parts being welded. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. The size and distribution of welding residual stress and welding deformation in welding structures have an effect on various sorts of damage like brittle failure, fatigue failure and stress corrosion cracking. So, research for this problem is necessary continuously. The study about residual stress has been started and realized the importance of the study of residual stress in view of the cause investigation of destruction after the destruction by residual stress at military equipment like fighters and battleship and at industrial facilities during and after the World War. Since then, as material of industry facilities are becoming more and more high strength with industrial development, the study for residual stress is accelerating because residual stress of fairly large size remain in the high strength material. There are several techniques that are used to measure the residual stress. They can be classified as theoretical and experimental methods. The theoretical method can be calculated by using physical constant like stress-strain and the modulus elasticity. However, the result of the theoretical method for measuring the residual stress sometimes can not be acquired correctly because of the dynamic complexity and diverse variables used in the process of calculation. Because of these reasons, data acquisition by experimental measuring method has been developed. These experimental methods can be classified as destructive and non-destructive methods. Measuring residual stress through destructive method has been established in various means but there is a fundamental problem that measuring objects has to be destroyed. So, nowadays non-destructive method has been developed rather than destructive one. There are lots of non-destructive method for measuring residual stress such as x-ray diffraction, neutron diffraction, ultrasonic method, and so on. X-ray diffraction is a non-destructive method which allows the measurement of residual stress in isolated spots spaced distances as small as 100 micrometers but it is easily affected by the surface condition. Neutron diffraction method can measure the internal stress of material but it needs very expensive neutron emission. Ultrasonic technique also has lots of merits, the short measurement time, simple equipment, measuring both the surface stress according to the sort of waves and internal stress. However, it can not easily measure the stress distribution that change in detail locally. Because of these existing problems in non-destructive method, this paper proposed the non-destructive and non-contact method using laser and optical interferometer as a method for measuring residual stress. Non-destructive measurement techniques with the merits of rapid response and manifold applications of measuring object has been and important role in industries from the safety diagnosis of infrastructures to the reliability evaluation of products. Non-destructive and non-contact measuring techniques applied with laser were established as laser application has been widely spread in various industry fields with the advancement of non-destructive measuring technique. These laser applied measurement methods can measure lots of objects in a short time with non-destructive, non-contact, real-time and high resolution. These are Thermography, Holography, Electronic Speckle Pattern Interferometry(ESPI), Shearography based on optical metrology. Application of these techniques in internal and external industry fields is gradually growing. Laser application techniques have been advanced from displacement measurement technique based on interferometer to laser based ultrasonic testing, Holography, speckle correlation interferometry, and so on. Particularly, holography and speckle correlation interferometry get the merits that it can the surface displacement on large area with real-time, high resolution and non-contactㆍnon- destructive method, so these are utilized in various industry field for vibration analysis, deformation analysis and non-destructive testing. With advanced computer science and image processing technology, speckle correlation interferometry for non-destructive measurement and inspection has advanced to ESPI and Shearography. Because of these technology development, the film of ESPI technique was changed to digitalized CCD camera and has evolved to digital technology that is united with image process technique. So, ESPI as a technology that can measure and analyse the whole area of object in real-time precisely has been utilized in various industry fields currently. In this study, therefore, the method for measuring residual stresses using ESPI technique that is one of the laser applied measurement technique excellent in the view of field application was proposed. Also, theoretical approaches was suggested and it was verified through experiments. The basic concept for measuring residual stresses is that there are difference of deformation and strain between the basic material part and welded part of a specimen when doing tensile and compression test for the welded specimen with equal force. So, the residual stress was measured through the proposed method with ESPI measurement system. The proposed measurement method as a basic study combined with theory and experiments applied for residual stresses prediction and developments of safety estimation technique will contribute to establishing a basis for improving the safety, integrity, conservativeness of weldment structures.