Year

2015

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronics Engineering

Abstract

Maximization of benefits in the oil and gas transportation industries requires an increase in the size and operating pressure of the pipeline, which in turn requires an increase in the strength of line pipe steel. To ensure reliable operation, public safety, and environmental protection, the design of pipelines must include strategies for controlling fractures. Identifying defect(s) where crack initiation is possible and predicting the crack propagation are of great importance when considering fracture control of pipelines.The current research is aimed at developing an improved method using the acoustic emission (AE) technique for determining fracture initiation and propagation in the line pipe steel during Charpy V-Notch (CVN) impact test and single edgenotched tension (SENT) test.

SENT tests on X70 and X80 line pipe steels were conducted, augmented with the AE measurements. It was found that the AE activity started before the yield point due to stress concentration at the crack tip and increased suddenly before the peak load was reached due to fracture initiation. Towards the end of the test, the AE hit density increased again. With an increase in the strain rate and a decrease in the test temperature, AE activity increased during the SENT test. It was found that the fracture initiation resulted in a „burst-type‟ AE signal with a 65- 75 dB amplitude and a 300-350 kHz average frequency, prior to the peak load point on the loaddisplacement curve. It was also found that the AE count rate increased as the crack growth rate increased and the AE average frequency decreased with the void size.

CVN tests on X70 line pipe steels were conducted at various temperatures. Tests with different specimen thicknesses were also performed at room temperature. The AE measurement technique was applied to all tests. It was found that the temperature and sample thickness affected the fracture mode (ductile or brittle) and the relative magnitude of the AE signals. The AE average frequency increased as the temperature decreased, while the AE average frequency decreased as the specimen thickness was reduced.There was a strong burst-type AE signal in a short time span between the yield point and the peak load point on the load-displacement curve, which is believed to be responsible for fracture initiation.

Finite Element Modelling (FEM) simulations confirmed that the fracture was initiated within a relatively short time period between the yield point and the peak load point on the loaddisplacement curve for both CVN test and SENT test, which is in good agreement with the observations from the AE monitoring.

The distinction between fracture initiation energy and fracture propagation energy is crucial in pipeline fracture control research. From the present study, it was found that using the peak load point to separate the two energy components is not accurate. The AE analysis conducted in the present thesis showed strong evidence of AE hits before the peak load, corresponding to the fracture initiation. Therefore, this thesis recommends that AE measurement be used to identify the fracture initiation, rather than the peak load point, in future pipeline research.

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