Study On Some Key Issues In CTOD Test Technique

Toughness is a key mechanical parameter in the assessment of materials and structures' safety. There are many ways to test fracture toughness of materials. CTOD is being more widely applied in engineering, but some key issues must be studied in-depth. The test temperature, the appropriate number of samples, selection of appropriate acceptable value and assessment of pop-ins etc. in CTOD tests were discussed and analyzed in this thesis, and ideas and programs were also proposed.Materials at different temperatures show different properties whether ductile or brittle, which is used as a starting point here. Some different ways to test DBTT (Ductile to Brittle Transition Temperature) of materials are introduced. DBTT gotten by CTOD is higher than by series of Charpy impact test, so the CTOD test temperature should set higher properly than Charpy impact test. According to the relationship of DBTTs tested by different methods, whether the CTOD test temperature hangs in the interval of its DBTT can be forecast by Charpy impact tests. If the CTOD test temperature hangs in the interval of its DBTT, more specimens (≥6) should be prepared because the results will be scatter. Three specimens would be OK if not.CTOD test of high strength steel EQ70 were carried out at -10℃and -60℃. Results show that the fracture toughness is higher at -10℃, and the standard deviation is small; when at -60℃, the toughness is lower and the scatter is bigger. It is showed that more(≥6) samples should be prepared since -60℃is in the DBTT to reduce the dispersion.Characteristic value is often used to mark the fracture toughness level of materials during CTOD tests. The minimum value obtained from three tests or the equivalent to the minimum value of three is usually taken as the characteristic fracture toughness value. The number of specimens for an assessment depends of the scatter. The microstructure of base metal and weld position are uniform, but heat affected zone (HAZ) are heterogeneous, the scatter of latter is large.3 tests of base metal and weld position would be better, but not less than 6 tests should be prepared to insure its veracity samples of HAZ. When enough samples were tested, but the characteristic fracture toughness value is closed to acceptable value, then ECA (Engineering Critical Assessment) would be used together to ensure its security.Two factors affect the evaluation result of materials, one is acceptable value, the other is the calculation of CTOD. For base metal, the test results should not less the guideline of API; for weld metal, the acceptable value should be settled based on the importance degree and stresses of the servicing structure, as method of "the base metal toughness reduction" is proposed here,1/2～1/3 of it for base metal would be well. Pop-in which shows discontinuity in F-V curves is one of the considerable factors in the assessment of CTOD calculation. Lamellar tearing during tests will lead to discontinuity in F-V curves which looks like "pop-in". By analyzing the characteristics, it is suggest that the "pop-in" shall be seriously assessed according to BS7448-1 to ensure its safety.Method of "Control Experiment" is adopted, and problem existing in pre-cracking is studied in this thesis. Method of "high K ratio" is used during pre-cracking, half are pretreated by local compression before pre-cracking which is taken as "experimental group" and the others taken as "control group" are not. Results show that the fatigue cracks are close in shape to the straight front implied by the testing standard, which shows that when method of "high K ratio" is used, method of "local compression" is no longer needed. Firstly, method of "high K ratio" will ensure the fatigue crack front straightness, and we can get the CTOD value closer to the true value. Secondly, compared with method of "high K ratio" only during pre-cracking, after local compression, CTOD tested are more conservative. Different acceptable values should be set when local compression is used before pre-cracking (15% reduction is suggested here).