Investigating the Fatigue Fracture Morphology of 0.28%C Pipeline Steel with Strain Induced Cavities.

Abstract

The study of fatigue is very important because it can lead to failure of line pipes, structural components and other engineering systems at load levels far below the original design. This intends to assess the effect of strain induced cavities on the fatigue fracture morphology of 0.28%C low alloy steel. The samples were machined into notched tensile samples. Some of the notched tensile samples were fractured to obtain three different prestrain levels. Strain level 0.77, 3.3 and 5.5 respectively. The notches were thereafter machined out and then converted to fatigue samples. The fatigue analysis was carried out and the fractured surfaces were examined to observe the effect of the strain levels on the fatigue fracture surface. The fatigue fracture surface was found to have multiple crack propagation origins. These origins increase with increased prestrain values. The fatigue striations at the fatigue fracture surfaces were found to become clearer with increased prestrain. The secondary cracks also increased with increased strain level too. It was observed that as a result of the high level of strain, the crack quickly propagated through the samples and there was no visible smooth zone that usually denotes the first stage and most times the beginning of the second stage of fatigue crack propagation. This study therefore shows that the cavities resulted in a quick spread of crack quickly through the material skipping the first stage to the beginning stage of the second stage of fatigue crack propagation. Therefore, adequate care should be taken to avoid too much deformation of the pipeline before they are put into use.