Fatigue Monitoring of a Dented Pipeline Specimen Using Infrared Thermography, DIC and Fiber Optic Strain Gages

Abstract

An investigation program has been launched with the objective of presenting combinations of analytical, experimental and numerical methods to predict and monitor fatigue initiation and fatigue damage progression in equipment such as pressure vessels, tanks, piping and pipelines with dents or shape anomalies. The present paper reports initial results from tests where these techniques were applied to a pipeline specimen containing a plain longitudinal dented subjected to hydrostatic cyclic loading. Some of the material’s fatigue properties assessment used validated rapid approaches based on infrared thermography. The monitoring of fatigue initiation and propagation in the actual specimen used nondestructive infrared inspection techniques. Thermoelasticity stress analysis (TSA) and three-dimensional digital image correlation (3D-DIC) were used to determine fatigue hot spots locations as well as strain concentrations. Full field TSA and fiber optic Bragg strain gages (FBSG) were used to determine the overall stress field (TSA) as well hot spot strain evolution (FBSG) along the loading cycles. Strain fields determined from the experimental measurements and from finite element analysis (FEA) were combined with the fatigue Coffin-Manson model to predict fatigue life (Nc). The tested 3 m long tubular specimen was fabricated with API 5L Gr. B 12.75″ OD with ¼″ thickness pipes. The excellent agreement among test and predicted results achieved up to now are commented in the paper.