위상잠금 적외선열화상을 이용한 응력해석에 관한 연구

Abstract

Fatigue behavior is strongly affected by the environment, material, and loading conditions. The process of fatigue can be categorized into three stages: crack initiation, growth, and final fracture. Non-destruction evaluation (NDE) of fatigue damage is of critical importance for life assessments and structural integrity evaluations. Several NDE methods, including ultrasonics, acoustic emission, and thermography, have been used to monitor fatigue damage. This paper aims to describe thermal stress measurement using Lock-in thermography as a non-destructive testing method with real time and non-contact technology. Due to the first and second principles of thermodynamics, there is a relationship between temperature and mechanical behaviour laws. Lock-in thermography is classically used to measure linear thermo-elastic effect to evaluate stresses in structure under periodic, random or transient loading condition. For this study, the followings are peformed to analyse and to locate thermo-mechanical mechanism in fatigue fracture specimen materials and structures. This paper describes stress analysis of smooth plate and circle hole-notch plate by lock-in thermography. Temperature change of specimen under cyclic loading is negatively proportional to the sum of principle stress change and surface temperature measured by lock-in thermography is estimated to stress with thermoelastic equation. The infrared thermography test is conducted to measure the stress distribution resulted from the change of loading. The material considered in this paper is the SM 45C(AISI 1045). The fatigue tests are performed at Hz with a stress ratio equal to zero. and a maximum stress is 246.91 MPa on a smooth plate and a dynamic stress concentration factor is an 2.5 on a circle hole-notch plate in the theoretical stress value. Since this stress range is smaller than the static yield stress. A black paint layer has been applied to the specimens to obtain an emissivity close to 1. The paint must be uniformly applied. In case of the infinite iteration loading for material, surface temperature is more increase than resolution of infrared system and the thermal conduction can be neglected after about 20000 cycles on that specimens. At this time, stress is measured with thermography system and Temperature fluctuation. The theoretical stress values in the smooth plate specimens were evaluated 246.91 MPa and measured as 231.28 MPa in the Lock-in infrared thermography test. The theoretical stress concentration factor in the circle hole specimens is evaluated as 2.5 and measured as 2.08~2.48 in the Lock-in infrared thermography test. The average of test results is evaluated or measured as 2.26. In the case of the plate specimen, The error between infrared thermography test and FEM is evaluated as 4 %, which has good agreement. In the case of the circle hole specimen, The error between infrared thermography test and FEM is evaluated as 14 %, which has good agreement. In this paper, we measure the stress distribution of structure designed under the transient condition using the Lock-in infrared thermography and predict the stress value and the dynamic stress concentration factor based on the stress distribution. In the test, test results shows the change of stress distribution resulted from the frequency change measured. The stress value and the dynamic stress concentration factor resulted from the change of amplitude at frequency of 40~80 Hz are estimated and then the value is compared with results of finite element analysis. The relative measuring error is measured as 4 % because the spatial resolution of detection device is so limited and experimental system does not completely meet the thermal parallel condition of specimen. The many advantages of infrared thermography such as non contact, whole field, insensitivity of environmental vibration enables its user to measure the stress of small and micro structure, without shape restriction.