The present paper summarizes the main aspects studied in the Veritas Joint Industry research project entitled "Anchor Chain Vables Offshore -Material Properties-Reliability".The research project included experimental and numerical assessments of the strength and reliability of Chain of Oil Rig Quality. The study covered fracture, fatigue, stress analyses, NDE-evaluation of fabrication defects, the reliability of applied NDE method and an overall fracture and reliability analysis.
Crack propagation and arrest measurements have been made using six pressure vessel steels and a submerged-arc weldment. The base-metal crack arrest toughness, KIm, is found to increase linearly with temperature above nil-ductility transition (NDT), and between NDT and (NDT + 100°C) is substantially greater than KIR. NDT is found to be a more consistent reference temperature than room temperature NDT (RTNDT). It is suggested that the absolute value of KIm at any temperature is composed of cleavage and dimpled-rupture contributions, with regions of dimpled rupture being contained in shear walls normal to the crack plane and parallel to the direction of crack travel. Shear walls which do not completely fracture are the source of unbroken ligaments observed on the fracture surface. The arrest toughness of the weldment is significantly lower than that of the baseplate and displays little evidence of unbroken ligaments or shear walls.
The proposed test method is a continuation of a simple Jic-test method developed by Lereim and Embury [i]. The method is based on the application of a silicone impression technique for determination of the point of initiation of stable crack growth and the corresponding JIc value. See Fig. i. The replication technique was initially applied by Robinson [2,3] for measuring the CODi values. The technique has also been used by Garwood [4,5] for measuring the opening displacement at the original crack tip and the growing crack front with corresponding amounts of slow stable crack growth. Consequently, JIc, CODi, CODc, and COD a may be obtained from a single specimen. However, the infiltration method is dependent on having a reasonably short curing time for the silicone replica. Thus the application of the infiltration method is limited to testing at temperatures above 0 deg C as the curing time for the silicone rubber in general increases drastically with decreasing temperature. Therefore alternative test methods are required if the test temperature is below 0 deg C, and a single specimen test method is preferred.The test method is based on the use of a single specimen, silicon infiltration rubber, and measurements of load, load point displacement, and clip gauge displacement at the mouth of the notch tip. However, one of the specimens in a series has to be tested at room temperature in order to establish the actual relation between the crack tip opening displacement and load point displacement, which depends on the specimen's compliance. This is shown in Fig. 2, where data from testing at different temperatures are included for the size and geometry of the applied test samples. The specimens are tested at the actual temperature and loaded to instability which may be followed by rapid crack growth and possible arrest in a displacement controlled testing machine. Subsequently, unloading is performed and the sample is removed from the real test conditions without breaking the specimen into two halves. A typical record of the load deflection curve is shown in Fig. 3a. When the specimen is heated to room temperature, the silicone rubber is poured into the notch and crack tip. The casting is removed after hardening, sectioned, and COD at initiation of stable crack growth is measured from the profile, as shown in Fig. 3b. Further, the obtained COD. is applied in Fig. 2 for the determination of the corresponding loa~ point displacement at initiation, u i. Finally, u. is used in Fig. 3a for exact determination of the area below the ~oad -load point displacement trace up to initiation of stable crack growth. Then the work done prior to initiation is known, and JI is readily calculated through the well known expression JIc = 2U/Bb ~6].
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