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Sacrificial specimen is a small sized fatigue specimen which is designed so that fatigue damage may be produced in the specimen sufficiently earlier than in a structural member when it is secured to the structural member. In this study, a sacrificial specimen which is secured onto the structural member by using adhesive agent is developed. The main body of the sacrificial specimen is a thin plate which has an artificially formed crack at the central region thereof. A condition of fatigue crack growth at the artificially formed crack is monitored during service, and a timing at which fatigue damage will actually appear in a structural member is predicted based on the monitored condition. INTRODUCTION In order to keep safety of ships, bridges, iron towers, constructing machines and marine structures, it is important to predict fatigue damage of these structural members. Due to a recent progress in structural design method, fatigue designing has been improved day by day. However, there has not been established fatigue designing in which a portion at which fatigue damage will occur can be correctly or reliably predicted. This is due to a fact that fatigue mechanism is very complicated and there are existent various uncertain factors such as uncertainty of external force, precision of structural analysis, accuracy of fabrication and residual stress. Under the above circumstances, it would be effective to put an effort in safety management of the structural members during usage by investigating actual structural members as well as by a structural monitoring. Two kinds of methods are available for safety management during usage; crack detection method and fatigue prediction method (See Fig.1). In the former crack detection method, in-service inspection has been widely practiced (Goranson and Rogers, 1983). However, in the structures, there are many structural members which could hardly be inspected.
A fracture mechanics approach has been developed for predicting the fatigue lives of nonoverlapping, tubular K-joints. The approach is derived from experimental test results and can be generalized for other types of tubular joints or complex connections for which crack-growth fatigue data are available. Introduction A technique using fracture mechanics for predicting the fatigue life of axially loaded, nonoverlapping, tubular K-joints has been developed. This method relies on an experimentally determined stress-intensity factor, KI, as shown in Fig. 1. In this application, KI was derived from fatigue data and the corresponding crack growth records of six tubular-joint tests. The expression is general and has been verified by comparing predicted fatigue lives with the results of eight sets of published, nonoverlapping K-joint data that include more than 50 individual tests. This method for predicting fatigue life could be extended to other types of joints, if suitable experimental data are available for derivation of stress-intensity factors. This work has provided an improvement in offshore tubular-joint fatigue technology through the use of fracture mechanics techniques. Data on small-specimen fatigue-crack growth now can be used to estimate the fatigue-crack propagation life of full-scale, tubular K-joints. Effects such as random load histories, corrosive environment, and other important factors known to affect fatigue life can be incorporated in a more direct manner. Fracture mechanics principles also can benefit conventional fatigue analysis by providing a means of generating appropriate S-N curves for specific applications. This type of life prediction appears to be less prone to the large scatter that typically characterizes tubular-joint S-N results. For the application of this method to the fatigue problem for typical offshore platforms, stress-intensity factors problem for typical offshore platforms, stress-intensity factors are needed for overlapping and nonoverlapping tubular joints subjected to bending and axial cyclic loads. Progress is expected in this area as more tubular-joint data Progress is expected in this area as more tubular-joint data become available. Semisubmersible drilling vessels, deep-water platforms, and platforms in hostile environments experience platforms, and platforms in hostile environments experience more damaging stress cycles during their lifetime than do structures in the Gulf of Mexico. Thus, metal fatigue and the resulting fatigue cracks in highly stressed members and joints may be controlling design factors. Consequently, industry, certification authorities, and government agencies have directed increased attention to consideration of fatigue in offshore facilities. Thus, there has been consider-able effort toward developing the fatigue analysis techniques required for offshore design. In recent years, new technology based on fracture mechanics principles has received increasing attention in the fatigue analysis of tubular joints. But, because of complicated joint geometries, the analytical development of the stress-intensity factor, KI, has been protracted. This paper presents a fracture mechanics approach to predicting the fatigue lives of nonoverlapping, tubular predicting the fatigue lives of nonoverlapping, tubular K-joints that circumvents these difficulties. It is derived from experimental test results rather than purely analytical methods. JPT P. 461
SPE Members Abstract The understanding and modelling of coiled tubing (CT) fatigue life has continued to be a major area of concern as CT diameters have increased, usage and variety of applications continue to grow, and tubing is subjected to higher pressures. This paper presents a method and testing apparatus which has been developed in an attempt to simplify and standardize CT fatigue testing. Preliminary results from this testing method are compared with full-scale tests and a fatigue life prediction model to demonstrate that this method is valid. Introduction CT is damaged each time it is spooled on and off the reel and over the gooseneck of the CT unit. This fatigue damage accumulates with each pass until the CT eventually fails. Since there is no non-destructive means of measuring the amount of damage accumulated in the CT, a life prediction model has been developed to track this damage accumulation and predict when the failure will occur. This model was developed based on full-scale CT fatigue testing using a CT unit and a test well. As new CT sizes and materials are developed this model must be expanded to encompass these developments. A group of companies (see Acknowledgment) are funding a project in which these model enhancements are being done. One objective of this project was to develop a simplified, standard CT fatigue testing method. This method was needed for the following reasons: * Full-scale fatigue testing is very expensive. Large numbers of fatigue tests are needed to draw reliable conclusions about this complex damage mechanism. A simplified method was needed to reduce the cost. * Fatigue testing to date was approached differently by each company, making it difficult or impossible to accurately compare the results. * No standard test was accepted for CT manufacturing companies to use as a quality control check for new CT. * A practical method was needed to test used samples of CT to determine if they were near the end of their fatigue life. P. 303^
ABSTRACT Two types of fatigue monitoring methods are experimentally examined for metal structures; fatigue crack detection and fatigue life prediction. As the detection method, five kinds of crack sensors, i.e., conductive film-sensor, conductive-paint sensor, plastic optical-fiber sensor, glass optical-fiber sensor and carbon-fiber sensor are made on an experimental basis. The sensors are bonded on the surface of compact specimen by adhesives or painting, and performances of each sensor are examined through the fatigue tests. As the prediction method, a fatigue coupon indicator of adhesive-type is developed. The main body of the coupon is a center cracked thin plate with 60mm-Iong, 10mm-width and 0.25mm-thickness, made from mild steel. The middle part in length of the plate is coated by Teflon film and the whole is sandwiched in between two thin epoxy resin plates. The coupon is bonded on a smooth specimen by adhesives, and the performance is investigated by pulsating tension and fully reversed fatigue tests. INTRODUCTION There is a possibility of fatigue failure in any repeatedly loaded structures. It would be useful to know when failure may occur. There are means of predicting this at the design stage, but in general the prediction can only be very approximate. In order to compensate the uncertainty, in-service inspections are carried out for most of the structures; Goranson (1983), Fujimoto (1991). While, inspections are carried out intermittently. Also, crack detection by inspection is usually probabilistic. If the present and former inspections fail to detect the initiated cracks, they are helpless for the resultant failure which may occur until the next inspection. The idea of structural monitoring was presented to cover such incompleteness of in-service inspections. There are two ways of monitoring for fatigue safety. One is to detect the initiated crack at early stage and the other is to predict crack initiation time in the future; Smith (1970), Gerardi (1990), Fujimoto (1994) (1995).
The approach includes both analytical efforts to the TAPS trade. Using a telescoping process of multiple models evaluate the relative effectiveness of different structural details, with increasing mesh density, global and regional ship forces are and in-service inspections to monitor performance. The calculated and applied to details of interest. At each transition analytical effort captures both the nominal fatigue damage from coarse to increasingly finer mesh, element forces (not throughout the ship, as well as the local effects of small displacements) are extracted and applied to the more detailed geometric stress risers from which fatigue fractures typically model in a manner similar to a free body diagram [2].
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- Energy > Oil & Gas > Upstream (0.68)
- Government > Regional Government > North America Government > United States Government (0.46)