PSI - Issue 5

Takahide Sakagami et al. / Procedia Structural Integrity 5 (2017) 1370–1376 Takahide Sakagami/ Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Recently, fatigue crack propagation in aged structures has become a serious problem that can lead to their catastrophic failure. Inspection of fatigue damage is necessary to ensure safety and estimate the remaining strength of the steel bridges. Nondestructive testing (NDT) and nondestructive evaluation (NDE) techniques play an important role in the maintenance programs for steel bridges. Conventional NDT techniques employed for fatigue damage detection include visual testing, eddy current testing, magnetic-particle testing and ultrasonic testing. However these are time- and labor-intensive techniques; hence a high-performance NDT method is essential. Further for the effective maintenance of steel bridges structural integrity evaluation is essential for the fitness for service evaluation. Conventional stress- and strain-measurement techniques are insufficient in remote and full-field measurement. Thermoelastic stress analysis (TSA) using infrared thermography has been widely used in the industry as an effective, remote, noncontact and full-field stress measurement technique. Overview of the TSA technique was given by Dulieu-Barton et al. (1998), Pitarresi et al. (2003) or Greene et al. (2008). Innovative research works on the TSA technique are found in structural integrity evaluations for steel structures related with fracture mechanics evaluations. The developed techniques for analyzing crack tip stress fields were reviewed by Tomlinson et al. (1999), introducing the benefits of the technique, in which the fracture mechanics parameters such as stress intensity factor or J -integral were determined directly from the measured stress distribution around crack tips. The TSA technique has been successfully applied on the fatigue life assessment. Tomlinson et al. (2004) investigated fatigue crack propagation under the mixed mode loading. Diaz et al. (2004) applied their improved TSA technique for evaluating stress intensity factors in the fatigue test of weld specimen demonstrating the potential of TSA for crack growth analysis. Sakagami (2015) has been investigating the practicability of the TSA technique for the life cycle NDT and NDE for steel bridges. In this study the TSA technique was applied for on-site measurement of stress distributions around fatigue cracks, and the future crack propagation behavior was estimated by the fracture mechanics approach. First experimental studies were conducted for laboratory specimens which modeled a part of welded structure in steel bridges. The stress intensity factors were calculated from stress distributions measured by the TSA technique, and the relationship between stress intensity factor ranges and crack propagation rates were obtained. Further the TSA technique was applied to evaluate the effectiveness of repair or reinforcement for defective portions in steel bridge structures. Severity reduction in stress distribution around the fatigue crack after treatment was confirmed for actual steel bridge members by the TSA technique. Crack propagation rate was estimated from the stress intensity factor calculated from on-site stress measurement data. Dynamic stress change causes a very small temperature change under adiabatic conditions in a solid. This phenomenon is known as the thermoelastic effect and is described by Lord Kelvin’s equation , which relates the temperature change (  T ) to the sum of the changes in the principal stresses (  ) under cyclic variable loading as follows. ∆ = − ∆ = − ∆ (1)  : coefficient of thermal expansion  : mass density C p : specific heat at constant pressure 2. The TSA technique

T : absolute temperature k : thermoelastic constant

The sum of the changes in the principal stresses (  ) is obtained by measuring the temperature change (  T ) using high-performance infrared thermography.