Patent Application
20160273997
This present disclosure relates to low cycle fatigue testing.
Low cycle fatigue testing can be used to evaluate the fatigue performance of small diameter tube butt welds at high stress range, high strain deformation, and low cycle numbers. It is commonly used for testing the fatigue properties of tubing welds present in the subsea control system. For example, during steel tube umbilical fabrication, the umbilical may experience large deformation when it passes through a radius controller, or stored on a reel. Such large deformation can be beyond the steel yield range, when the steel tube will go beyond the elastic deformation region, and experience certain degree of plastic deformation, which will not reverse, and thereby cause permanent elongation in the tube, and generate significant fatigue damage to the tube, which is called low cycle fatigue. Welds and connection joints are usually the weakest point for low cycle fatigue. During the test, the tube-shaped testing specimen is bent to the desired radius, and the smaller the radius, the higher the strain and stress, which sometimes can be beyond the yield stress. Next the tube-shaped testing specimen is straightened out to complete one cycle. After repeating this bending-straightening process for a certain number of cycles, the specimen will experience fatigue crack and fail eventually. Conventional testing method uses a pre-fabricated disc or cylinder, each with a defined radius, to bend the tube into the desired radius, then straighten the bent tube through a tube straightener or tension pull, which can be time-consuming, and sometimes produces biased testing results by introducing excessive tension loads. During steel tube umbilical fabrication, the steel tube usually bends into one direction first, and then bends into the opposite direction in the following cycle, which is not reflected in the conventional testing method. Apparatus and methods have been proposed for performing low cycle fatigue testing, which, among other applications, can be used for testing the fatigue performance of the umbilical or steel tube bundle), which can experience low cycle fatigue during the fabrication process.
In the drawings:
This document discloses apparatus and methods related to low cycle fatigue testing.
In some implementations, the testing specimen 11 has the butt weld located near its center region, and a strain gauge 18 is attached on the outer surface on or near the weld or joint connection area of the testing specimen 11. The strain gauge 18 is connected to the control box 17 through a signal cable 19. Control box 17 receives strain gauge reading during the low cycle fatigue test continuously. In some implementations, the control box can be a computer with suitable peripherals capable of receiving signals from the strain gauge 18, analyzing the signals to find out the bending status of the testing specimen and sending signals to hydraulic cylinders 14 to adjust the cylinder extension and retraction pattern, to control the testing automatically based on predefined testing purposes. In some implementations, the control box 17 allows for loading of new programs for new tests, and has a screen for displaying all controlled parameters useful for the low cycle fatigue testing, and an input device such as a keyboard for machine-human interaction.
In some implementations, the hydraulic cylinders 14 are connected to the control box 17 through control cables 19. The control box 17 sends signals to the hydraulic cylinders 14 to extend or retract the piston as needed during the low cycle fatigue test.
In some implementations, the tube-shaped testing specimen 11 may have a length from 0.5 meter to 5 meters.
In some implementations, the testing specimen 11 may be tensioned through end cap effect from internal pressure, or through external hydraulic cylinders 14, or through external clump weight, or through the clamps 15 located near its two ends.
In some implementations, the low cycle fatigue test can be carried out as follows: (i) fill up the testing tube 11 with water or other testing fluid, and pressurize the testing tube 11 to a predefined level; (ii) attach all clamps 15 and connect all components; (ii) set the minimum curvature and maximum curvature range in the control box 17; (iii) start the low cycle fatigue test, whereby the control box 17 will instruct the hydraulic cylinders 14 to bend the testing tube 11 until the maximum curvature is reached at the strain gauge 18, and then reverse to the minimum curvature; and (iv) repeat the same process until the pressure gauge 18 records a tube leakage failure, when the bending cycle number is recorded. A series of tests can be performed, after which data analysis is carried out to determine the fatigue curve parameters of the testing specimen 11.
Various other adaptations and combinations of features of the embodiments and implementations disclosed are within the scope of the present disclosure. It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.
