| The fatigue and fracture of the structure have been driving the research on the fracture toughness and fatigue crack of the material,and it is more important to study the toughness and fatigue crack propagation problem by the fracture mechanics method.As the toughness index of the material,CTOD can fully reflect the cracking resistance ability and characterize the fatigue crack propagation law of the material.Its unique advantages make the rapid development of CTOD evaluation technology,and the key problems in the test become more prominent,these problems are increasingly need to be solved.At the same time,the fatigue crack propagation law of materials has a certain influence on the CTOD test.Combining the key problem of CTOD test with fatigue crack propagation to discuss can not only solve the corresponding problems in the test,but also study the fatigue crack propagation law.It is very important to promote the CTOD test technology and the anti-crack design of cracked components.In this paper,the base metal and welded joint of thick steel plate are studied for the problem of different sampling orientation in CTOD test and fatigue crack propagation under different sampling orientation.It is divided into four parts.In the first part of this paper,the CTOD test standards were compared.It was found that there is no clear requirement for the sampling orientation of thick steel plate in CTOD test.There are some limitations using a certain samples orientation in engineering practice.And some models of the fatigue crack propagation mechanism are studied,which provides theoretical support for the following study.In the second part,the CTOD test for API RP 2Z base metal and base metal with post weld heat treatment(PWHT)in Y-X and Y-Z sampling direction was conducted.The fatigue crack growth rate and CTOD test results were obtained for all samples in two sampling directions.The weaker cracking resistance direction in thick steel plate was determined.It provides the basis for rational selection of the orientation of the base material in the CTOD test and the rational use of the steel plate in the engineering practice.In the third part,the weld joint specimen was studied,and it was found that the horizontal residual stress is the main factors influencing the fatigue crack growth.The finite element analysis software was used to simulate the welding process of the thick plate and the CTOD test of NQ specimen was carried out.The fatigue crack propagation law of the NQ specimen was obtained.The results show that a qualified crack front straightness in the NQ specimen is available without the pretreatments.It provides a new idea for the selection of weld specimen orientation in CTOD test and the CTOD test value which is closer to the actual toughness of material.In the fourth part,the fatigue crack growth rate test of EH40 thick steel plate was studied.The relationship between the fatigue crack growth increment Aa and the plastic component of the crack opening Vp is analyzed.It is found that the two parameters maintain a good linear relationship only in the a/W ? 0.55,which modified the test data.The da/dN-?K curve was obtained,and the CTOD value ?was used as the parameter to study the fatigue crack growth rate.The expression of da/dN expressed by ?/2?a was found:da/dN =k(?/2?a)c.The formula is not only suitable for the linear elastic phase of materials,but also for the crack expansion of elastoplastic materials.It provides a new method for characterizing the fatigue crack growth rate and residual fatigue life of plastic materials. |
PDF Full Text Request