Three-dimensional Analysis of Crack in Centrally Perforated Photoelastic Cylinders under Internal Pressure

In this study, the frozen stress photoelastic method was used to investigate the three-dimensional effect on the crack growth behavior in a centrally perforated circular cylinder under internal pressure. The inner surface of the cylinder had a star shape, which consisted of six fins. The specimens were capped at the ends and pressurized internally above critical temperature after real cracks were introduced at the fin tip. After growing to a desired size, the pressure was reduced to stop crack growth and held through cooling. Two different types of cracks, part-through crack and long crack with the crack length nearly equal to the length of the cylinder, were considered. The experimental data were analyzed, and the results are discussed. INTRODUCTION An important engineering problem in structural design is evaluating structural integrity and reliability. It is well known that structural strength may be degraded by the presence of cracks in the material. In order to determine the severity of the crack or the service life of the structure, the failure criterion should include the crack propagation aspect of the localized failure. Since the crack growth behavior is controlled by the local stress at the crack tip, the distribution of the stress intensity factor along the crack front needs to be determined. The application of the frozen stress photoelastic method to the study of stresses in three-dimensional specimens is not new. An excellent discourse on this subject was given by Durelli [1]. Subsequently, Francis et al. [2] studied the pressurized crack behavior in a two-dimensional models of cylindrical specimen. Smith el at. [3] conducted photoelastic experiments on nozzle corner cracks in a pressurized vessel to determine the distribution of stress intensity factor along the crack front. In 1993, a systematic study was conducted by Smith et al. [4] to determine the stress intensity factor along the front of part-through cracks with different sizes in centrally perforated cylinders. Very recently, Leblond [5] provided a three-dimensional analytical framework for use in analyzing problems with linear elastic fracture mechanics constraints when the crack configuration is known. In this study, the frozen stress photoelastic method was used to investigate the three-dimensional effect on the crack growth behavior in a centrally perforated circular cylinder under internal pressure. The inner surface of the cylinder had a star shape, which consisted of six fins (Fig.1). The specimens were capped at the ends and pressurized internally above critical temperature after real cracks were introduced at the fin tip. After growing to a desired size, the pressure was reduced to stop crack growth and held through cooling. Two different types of cracks, a grown part-through crack and a machined long crack with the crack length nearly equal to the length of the cylinder (Fig.2), were considered. The experimental data were analyzed, and the results are discussed. THE EXPERIMENTS In order to obtain some insight into the three-dimensional effects on crack growth behavior under load, a series of experiments on pre-cracked centrally perforated cylindrical specimens, made of a photoelastic material, were conducted using the frozen stress method. The starter part-through crack was made by first drilling a small hole opposite the fin in which the crack was to be located, sliding a shaft with a tip blade into the hole, positioning the blade at the critical point on the fin surface, and then striking the shaft with a hammer. The starter crack then emanated from the blade tip into the materials as "a natural" or a real crack. For the long crack, the Report Documentation Page Form Approved