| In order to detect the fatigue crack initiation and propagation behavior of engineering structures, a dynamic damage detection approach based on acoustic emission (AE) and digital imaging correlation (DIC) technology is developed. An experimental technique and evaluation method to process fatigue crack development in structural steel and welds are constructed. Based on continuum damage mechanics and fundamental linear elastic fracture mechanics, a finite element numerical model of crack initiation and propagation behavior is built, and a theoretical prediction which agrees with the experimental results is derived.In the past decades, engineers using Non-Destructive Testing (NDT) methods such as visual inspection, eddy current, dye penetration, AE and other ultrasonic techniques to monitor the steel structures such as bridges, space framed structures, roof trusses mast, rail track and several others have been mentioned. Among those methods, AE has emerged as the most appropriate NDT method for studying fatigue crack growth in civil engineering steel structure because it can monitor their health in real time.Determination of the period of crack propagation between the initial and critical crack sizes requires knowledge of crack growth rates. In a prior study, AE parameters were correlated with material fatigue using compact tension specimen under different peak load.The test results showed that AE count rates, for small percentages of the applied load range close to the peak load, showed reasonable correlation with crack propagation rates. Based on the fatigue crack growth behavior of engineering steel structure which is monitored by AE,the quantitative relations are obtained in damage initiation and crack propagation. And then the remaining service life of fatigue damaged structures can be predicted.The single edge notched bending (SENB) specimens were designed from 16Mn sawn from low stressed areas of the steel bridge and fabricated according to ASTM E647 standards.Based on AE test, and DIC test, experimental setup capable of capturing crack initiation and propagation were developed. Fatigue tests were carried out on a servo-hydraulic testing machine with maximum load capacity of 250kN at ambient temperature (300 K).Innovative results obtained are as follows:Determine the accuracy of AE technology during fatigue crack propagation in both heterogeneous and homogeneous medium.Propose theoretical models for prediction crack extension using AE technology,continuum damage mechanics and fundamental linear elastic fracture mechanics.Proposes fatigue crack growth life assessment using AE, DIC and linear elastic fracture mechanics. Fatigue crack initiation conditions were also carefully studied experimentally using DIC and theoretically using continuum damage mechanics.Effective fatigue crack detection may lead to an early warning to avoid serious engineering accident. The fatigue crack characterization, initiation, propagation behavior and damage evolution have created key technical and scientific problems.The following conclusions were derived after the investigation:Generally, the quantitative AE damage characterization proved to be more effective as compared to the traditional qualitative analysis. A good correlation among the three theoretical fatigue life prediction models developed due to the fact that, they are all interconnected with the fundamental linear elastic fracture mechanics principles. It was evident that fatigue crack growth rates for the welded specimen are higher than the base metal. This was enhanced by the presence of inclusions and heterogeneous micro structure of the welds. This study provides theoretical and experimental foundation for fatigue life assessment and safety assessment of engineering structures. |
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