Study On Thickness Effect Of CTOD And Parameters Of Cohesive Zone Model

During the service of a steel structure,weak points of the structure?such as welded joints?are prone to cracks.Under the action of external loads,the cracks will gradually expand,seriously affecting the safe use of buildings.Therefore,in the design and material selection phase of the building,it must be ensured that the building material has sufficient crack resistance.There are four parameters that measure the resistance of a material to crack propagation.Crack tip opening displacement?CTOD?is one of them.CTOD is suitable for elasto-plastic materials,and has the characteristics of clear physical meaning and mature testing technology.It is often used in engineering to measure the fracture toughness of steel and welded joints.At present,CTOD parameters have the following problems in application:1)CTOD-K_I relationship does not reflect the effect of material thickness;2)statistical analysis of CTOD dispersion is not comprehensive;3)CTOD thickness effect formula has large error in data fitting;4)When using the cohesion model,a suitable cohesive energy?cannot be calculated directly from the CTOD value.In view of this,this article has carried out related research,the brief research process and conclusions are as follows:?1?Based on the influence mechanism of the reference thickness parameter on the G-K_I relation,three CTOD-K_I relations with thickness parameter T_h are proposed.The CTOD experiments of 92 mm thick API 2W Gr.60 steel,40 mm thick R3S-153mooring chain steel and 96 mm thick ADB790E steel were performed in accordance with the specification BS7448.After processing the experimental data and obtaining the CTOD and K_I values of the material under small yielding,the errors of the three CTOD-K_I relationships were tested.The results show that the proposed formula is reasonable and has certain generalization.?2?Based on the three types of distributions?normal distribution,lognormal distribution,and Weibull distribution?that are commonly used in probabilistic fracture mechanics,the two distribution models that also have a“peak”shape probability density function are supplemented.The experimental data of API 2W Gr.60 steel,R3S-153 mooring chain steel and ADB790E steel were statistically analyzed.Hypothesis testing and coefficient of determination were used to optimize the distribution model.The results showed that some materials are better described with supplemental distribution models.?3?The reasons for the poor fitting performance of the CTOD thickness effect formula are analyzed,and an improved formula is proposed.Then the CTOD experimental data of three different materials at the series thickness are used to verify the improved formula.The results show that the new formula has smaller error than the original formula.It can be used to predict the fracture toughness and obtain more accurate results.?4?The reason why the suitable cohesive energy?could not be calculated directly from the CTOD value was analyzed,and the new formula??_??was given.On the ABAQUS platform,a three-point bending cohesive zone model for a full-size X70 pipeline steel was established.After using the 9 coefficient combinations for trial calculation,compared with the experimental data,the results show that a suitable coefficient value can be found.Substituting it into the new formula to obtain the?value as the model parameters,the simulation results are in good agreement with the experimental values.