Toughness Evaluation For Welds Of Thick Steel Plate With Crack Tip Opening Displacement

The material fracture process and the welded structures integrity are the intersection research realms of rupture mechanics and material science. With the high-speed development of national economy, the application of larger engineering projects construction and the thicker wall welded structures (such as ocean structure, bridge structure, modern construction, hydropower and atomic-power station and so on) are the mega trend. But there are few theoretical research and engineering experiences in our country about welding craft and quality evaluation and control for welds in thick steel plate. Therefore, the toughness evaluation and control for welds in thick steel plate are of great importance and are concerned by mechanical scientists and material scientists and engineers. Being provided with many advantages, such as it's scientific and it's precise and it's appropriate expense and so on, the toughness evaluation with crack tip opening displacement (CTOD) for welds of thick steel plate will be applied wider and wider. However, there are some major problems to be further explored in this technology, including the problem of applicability, the problem of determining CTOD acceptable values for welds made of thick steel plate, the problem of lower qualified weld specimen ratio, and the problem about correlation between J and CTOD for welds in thick steel plates and so on.This dissertation has researched the problems listed above in theory and experiment and gotten five achievements as follows. 1).Demonstrated the applicability of the CTOD toughness evaluation for welds in thick steel plate; and made this technology possess the experimental infrastructure;2) Put forward two methods of determining the CTOD acceptable values in welds in thick steel plates("Parental Metal Method" and "Analogy Method"). They can determine the reasonable toughness in large thick wall structures;3) Developed "High K Ratio Method" which can increase the qualified weld specimens ratio greatly. This method can raise the qualified specimen ratio up to 93%. What is more, the toughness data measured from the specimen made by "High K Ratio Method" are real toughness. The "High K Ratio Method" also simplifies the CTOD test for welds, and then makes the CTOD test become more convenient and lower expense;4) Developed a "Double Gauge Method", which can measure J integral and CTOD values simultaneously from the same specimen and provide a reliable experiment exploring correlation between J and CTOD which has been being a theoretical problem in elastic-plastic fracture mechanics; 5) Discovered the linear correlation between J and CTOD in the heataffected zone of welds in thick steel plate, and these relationships may be not welding affected. These achievements not only improve CTOD test in welds and consummate the CTOD toughness evaluation system for welds in thick steel plate, but also provide the important technology support for integrity evaluation in large welded structures.Applied the achievements above, the dissertation evaluated the toughness for five welding crafts of DH steel, that made the welds in the offshore jackets possess enough toughness and the offshore jackets no longer needed heat treatment. Therefore the fabrication cost reduces obviously. The dissertation also evaluated the welds toughness for steel box beam which used in the back gulf cable staged bridge of Hong Kong-Shenzhen Western Corridor, and confirmed the qualified welding crafts which can ensure the welds possess enough toughness. Furthermore, the dissertation provided some clues which may be improve unqualified welding crafts and make them qualified. The work solved the key technology problem of toughness evaluation and control of steel box beam in bridge construction.This dissertation consists of seven chapters. First Chapter is an introduction, in which the principle of CTOD test and some it's standards have been introduced. Afterwards the main engineering applications of the welded structures made of thick steel plates have been presented. Then some key methods of determining toughness for welds similar to CTOD test have been presented and compared to the CTOD test. Finally, some problems to be further studied in CTOD toughness evaluating for welds in thick steel plate have been described. Of cause, the author's main works have been introduced briefly.In Chapter Two the tension tests have been conducted for the materials of welds of two welding crafts in thick steel plates. The experimental results illustrated that the tension curves of the materials in welds can all be considered as ideal elastic-plastic. Therefore, the applicability of the CTOD toughness evaluation in welds in thick steel plate has been demonstrated. Based on large amount of engineering tests and extensive literature summarization, the two methods of determining the CTOD acceptable values for welds in thick steel plates —"Parental Metal Method" and "Analogy Method" have been put forward.If the pretreatments (such as "local compression" and "reversed bending") are not conducted for welding position specimen, the pre-fatigue crack front straights in the specimens might be very low, and the qualified specimen ratio may also be very low. Whereas the pretreatment are conducted, although the qualified specimen ratio became high, but the toughness data measured from these specimen are not realtoughness. This is the key problem in the CTOD toughness evaluation for the welds in thick steel plate. Based on a large number of experiments and theoretical analysis of fracture mechanics, the "High K Ratio Method" has been developed to make fatigue crack in weld specimens in Chapter Three. It can improve greatly pre-fatigue crack front straights and increase the qualified specimen ratio to 93% without any pretreatment. Pre-fatigue cracking with "High K Ratio Method", the pretreatments are omitted. This not only simplifies the CTOD test in welds, but also can get the real value of CTOD toughness because it keeps original residual stresses in the specimens. Therefore, the key problem in the CTOD toughness evaluation for the welds in thick steel plate has been solved. What is more, the less test time and lower expense of CTOD test are needed.An experiment method, named by "Double Gauge Method", has been developed in Chapter Four. It can measure both J and CTOD values from one specimen simultaneously. Using the "Double Gauge Method" to study correlation between J and CTOD, the errors caused by different specimens can be vanished. This provides a reliable approach for exploring correlation between J and CTOD which has been being a theoretical problem of elastic plastic fracture mechanics. Besides, this method is very convenient to be conducted and the cost of specimens consumed is only the half of the cost of specimens consumed with the normal test method. Then the "Double Gauge Method" was used to research the correlation J and CTOD in weld heat affected zone made of thick steel plate. In weld heat affected zone made of thick steel plates, the research shows that, there are linear correlations between J and CTOD, and this kind of relationships is almost not welding related.In Chapter Five, five preliminary weld procedure specification (PWPS) used in offshore jackets fabrication have been evaluated with CTOD test. Especial the "High K Ratio Method" in chapter three had been applied for DH36 thick steel plate. The main procedures of CTOD toughness evaluation for welds in thick steel plate were all described, including machining CTOD specimen, determining the crack position in specimen, sparking the specimen crack, making fatigue crack, recording the load and displacement curves, additional fatigue cracking, crack measurement, and so on. It should be pointed out that, by means of CTOD evaluation, the five PWPS were permitted to be used in the offshore jackets fabrication. What is more, the offshore jackets no longer needed heat treatment and the fabrication efficiency increases and the productive cost reduces greatly.The contents of Chapter Six is an application of CTOD toughness evaluationsystem for welds in thick steel plate for steel box beam which used in the back gulf cable staged bridge of Hong Kong-Shenzhen Western Corridor. The applicability of CTOD toughness evaluation for welds of steel-box-beam has been demonstrated. The CTOD acceptable value in the welds of steel box beam has been determined with "Parental Metal Method" and "Analogy Method". The fatigue cracks in weld specimens have been made with "High K Ratio Method". Three PWPS have been evaluated under normal temperature (15°C) and low temperature (-20°C) . The fractography and chemical composition have been studied by means of scanning electron microscopy (SEM) for CTOD specimens of parental metal and weld bead of thick steel plate. The results show that there are some gas holes, micro-cracks and sulphur in welding position. This is the main causes which reduce the toughness in the welding position. One qualified PWPS was confirmed, and Two unqualified PWPS were abandoned. Therefore the quality of steel box beam has been ensured.In Chapter Severn, the dissertation has been summarized, and then some problems to be further explored have also been discussed.